C Sanchez

M Nigen

V Mejia Tamayo

T Doco

P Williams

C Amine

D Renard

Acacia gum: History of the Future

Keywords: Acacia gum, hydration, rheology, aggregation, coacervation, interfaces and emulsions

On behalf of the 90 th birthday of Professor Glyn O. Phillips, it is a great honor for authors of this publication to make a review on Acacia gum, one of the favorite polysaccharides extensively studied by Glyn and his collaborators all around the world during these last five decades. After remembering a synthetic historical perspective, the present critical review summarizes the main updated data of this complex polysaccharide from the chemical composition to the functional properties with a particular attention toward structure and bulk and interfacial properties. Biological properties of Acacia gums were not considered. Some of the main challenges in a near future for a better understanding of the functional properties of this polysaccharide concerns the detailed study of the gum maturation mechanism upon exudation, the structure and conformation of different molecular fractions, the role of minor components (minerals, polyphenols, lipids) on the structure and functionality of gums, the physicochemical properties of purified molecular fractions and the ways to modified them upon enzymatic modifications. In our opinion, the main challenges for a better understanding of the interfacial function of this polysaccharide (adhesion and stabilization at liquid and solid interfaces) will be to probe the interfacial induced conformational changes. This area of research seems to have been quite neglected during these last past years and fundamental questions arising from the adhesive and stabilizing properties of Acacia gum are still without answer today.

In addition, the amino-acid sequence contained in this complex polysaccharide are totally unknown today and future developments based on enzyme/chemical modifications and liquid chromatography coupled to on line mass spectrometry could unravel the sequence and decipher between the existence of one or more amino-acid sequences in Acacia senegal gum.

We sincerely hope Glyn, one of the "father of Arabic gum", will find some positive echo in this review.

It was used as a pigment binder and adhesive in paints for making hieroglyphs, and ancient inscription refers to it as kami. Furthermore, it was used as a binder in cosmetics and inks and as an adhering agent to make flaxen wrappings for embalming mummies. Herodotus, writing in the fifth century B.C., mentions its use in embalming in Egypt. Cardboard (?), a specific type of Egyptian material for cases enclosing or elements placed on, mummified bodies, were painted with gum-containing pigments [START_REF] Scott | An ancient Egyptian cartonnage broad collar -Technical examination of pigments and binding media[END_REF][START_REF] Scott | Examination of some pigments, grounds and media from Egyptian cartonnage fragments in the Petrie Museum, University College London[END_REF]. Ancient Greeks also mentioned the use of gum [START_REF] Diderot | Encyclopédie ou Dictionnaire Raisonné des Sciences, des Arts et des Métiers[END_REF]. Since the first century of the Christian era, the soluble gum provided by Sudan has been an article of commerce shipped to Arabian ports and hence to Europe [START_REF] Caius | The gum arabic of the bazaars and shops of Bombay[END_REF][START_REF] Parry | Gums and resins: Their occurrence, properties and uses (Pitman's common commodities and industries[END_REF]. It was called gum arabic after its place of origin [START_REF] Pomet | Histoire générale des drogues simples et composées, renfermant dans les 3 classes des végétaux, des animaux et des minéraux tout ce qui est l'objet de la physique, de la chimie, de la pharmacie, & des arts les plus utiles à la société des hommes[END_REF].

From Sudanese sources, AG was an article of commerce as early as the 12 th century B.C [START_REF] Cecil | Gum arabic[END_REF].

The therapeutic use of AG was already mentioned in Pliny, Discoridis and Theophrastus writings [START_REF] Amy | Contribution à l'étude des propriétés et de la structure des solutions et des gelées de gomme[END_REF][START_REF] Merat | Dictionnaire universel de matière médicale et de thérapeutique générale : contenant l'indication, la description et l'emploi de tous les médicaments connus dans les diverses parties du globe[END_REF]. However, the Ebers manuscript (a medicinal papyrus written in 1550 B.C.) already suggested to use AG as a contraceptive in association with dates. The famous queen Cleopatra requested the preparation of curative recipes based on AG [START_REF] Gramatica | La gomme arabique en oenologie (Partie 1)[END_REF]. In the ninth century of our era, the Arab physician Abu Zayd Hunayn ibn Ishaq al-Ibadi, writing in his Ten Treatises on the Eye, described AG as an ingredient in poultices or eye compresses. It was also used to relieve topical irritation and to protect in cases of superficial excoriation, ulcers, burns, sore nipples, etc. [START_REF] Caius | The gum arabic of the bazaars and shops of Bombay[END_REF]. The powdered gum is used in checking hemorrhage from leech bites. When blown up the nostrils, it stops severe nosebleed. The gum is also employed in catarrhal affections and irritation of the faces, by being held in the mouth and allowed to dissolve slowly. In Transvaal, a plaster made from capsicum fruit, Cape gum and strong Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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6 vinegar, is applied in acute inflammation of the bone marrow, and a mucilage of Cape gum to the mouth in thrush and sprue [START_REF] Caius | The gum arabic of the bazaars and shops of Bombay[END_REF].

By the Middle Age, AG was valued in Europe among scribes and illustrators. Following the gilding of letters in illuminated manuscripts, the application of color was the final stage. For this, illustrators mixed pigment in a binding medium. Between the 12 th and the 19 th centuries, AG was used in the composition of the metallo-gallic ink, the most used ink in Europe, for instance by painters like Rembrandt. The ink manufacturing was known at least two centuries before our era [START_REF] Flieder | Livres et documents d'archives: Sauvegarde et conservation[END_REF]. During the same period, AG trade was controlled by the Turkish Empire, giving rise to the name turkey gum. An export trade was also developed for a time around Bombay, hence the names "East Indian" or "Indian Gum".

In 1445, Prince Henry the Navigator set up a trading post on Arguin island (off the coast of modern Mauritania), which acquired AG and slaves for Portugal. Between the 14 th and the 19 th centuries, AG was an important trade associated to slave economy [START_REF] Cultru | Histoire du Sénégal du XVe siècle à 1870[END_REF]. From 1580, Portugal became the dominant influence along the coast. In 1638, however, they were replaced by the Dutch of the Occidental India Dutch Company, who were the first to begin exploiting the AG trade. Produced by the Acacia trees of Trarza and Brakna and used in textile pattern printing, this gum was considered superior to those previously obtained in Arabia. By 1678 the French had driven out the Dutch and established a permanent settlement at Saint Louis at the mouth of the Senegal River, where the French Company of the Senegal River (Compagnie Française du Sénégal) had been trading for more than fifty years [START_REF] Raffenel | Voyage dans l'Afrique Occidentale[END_REF]. In 1685, Frederick William of Brandeburg replaced the French and created a colonial domain. Successors left the trading post to the Dutch in 1717. Since the trading of gum was important for the European industry, France occupied Arguin after the 1721, 1723 and 1724 campaigns. Arguin was definitely left for good in 1728 because maintain a garrison was too expansive and the trade of the gum moved to the south.

During most of the 19 th century, AG was the major export product from the French and British trading colonies in modern Senegal and Mauritania. France in particular first started a conflict with inland African states over the supply of gum, providing an early spur for the conquest of French West Africa. As the Atlantic slave trade weakened in the early 19 th

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

ACCEPTED MANUSCRIPT

7 century, the Emirate of Trarza and its neighbors in what is today southern Mauritania collected taxes on trade, especially AG that the French were purchasing in ever-increasing quantities for its use in industrial fabric production. West Africa had become the sole supplier of world AG by the 18 th century, and its export from the French colony of Saint-Louis doubled in the decade of 1830 alone. Taxes, and a threat to bypass Saint-Louis by sending gum to the British traders at Portendick, eventually resulted in a direct conflict between the Emirate of Trarza and the French. In the 1820s, the French launched the Franco-Trarzan War of 1825 in order to avoid Arabs controlling the gum trade. The war incited the French to expand to the north of the Senegal River for the first time, heralding French direct involvement in the interior of West Africa. AG continued to be exported in large quantities from the Sahel areas of French West Africa (modern Senegal, Mauritania, Mali, Burkina Faso, and Niger) and French Equatorial Africa (modern Chad) until these nations gained their independence in 1959-61. In the beginning of the 20 th century, Europe consumed about 20 000 tons per year of AG [START_REF] Chevalier | Sur la production de la gomme arabique en Afrique Occidentale[END_REF].

Uses of Acacia gum

Acacia senegal/seyal trees are important for the ecology of arid and semi-arid areas where they naturally grow. They prevent soil degradation, fix atmospheric nitrogen and maintain soil moisture. Trees are resistant in period of drought. They act as wind barrier and are important for dune fixation. In addition, trees participate to soil fertilization and decomposition of dead leaves reinforces anti-erosive roots of trees [START_REF] Wickens | Role of Acacia species in the rural economy of dry Africa and the Near East[END_REF].

The tree has wide usage: the foliage and seed pods make excellent fodder for livestock, ropes can be made from the bark fibers of the roots, and the thorny branches are often used to make hedges to enclose cattle or protect agricultural farms. The tree can also be used for small-scale carpentry or for making agricultural tools. When it passes its gum-productive age, between 15 and 25 years old, its wood is used for both fuel and charcoal production [START_REF] Touré | Gum arabic[END_REF][START_REF] Wickens | Role of Acacia species in the rural economy of dry Africa and the Near East[END_REF] .

AG is unique among the natural gums because of its properties, including high solubility and it is widely used as a stabilizer, emulsifier, flavoring agent, thickener, or surface-finishing agent. It also activates turbidity or retards sugar crystallization. These properties make it a

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

ACCEPTED MANUSCRIPT

8 very interesting additive in the food industry, including for the production of beverages (including Coca-Cola®), confectionery, emulsions, flavor encapsulations, bakery products and brewing [START_REF] Touré | Gum arabic[END_REF][START_REF] Verbeken | Exudate gums: occurrence, production, and applications[END_REF][START_REF] Wickens | Role of Acacia species in the rural economy of dry Africa and the Near East[END_REF]. In wine production, AG prevents color pigment and protein precipitations, confers body and stabilizes the color. As mentioned above, AG has been also used for ages in non-food industries including pharmaceutical, printing, textile, and cosmetic industries [START_REF] Verbeken | Exudate gums: occurrence, production, and applications[END_REF].

The modern industrial era has produced an explosion of manufacturing uses for AG.

According to [START_REF] Cecil | Gum arabic[END_REF], gum was important in the 19 th century in early photography as an ingredient in gum bi-chromate prints. It is now used in lithography, where its ability to emulsify highly uniform thin liquid films makes it desirable as an antioxidant coating for photosensitive plates. The same quality also makes gum useful in sprayed glazes and hightech ceramics and as a flocculating agent. It is used as a binder for color pigments in crayons, a coating for papers and a key ingredient in the micro-encapsulating process for the production of carbonless copy paper, laundry detergents etc. It is used in textile sizing and finishing and for metal corrosion inhibition. Moisture-sensitive postage-stamp adhesives and matchsticks are also made with gum. [START_REF] Touré | Gum arabic[END_REF] adds that AG is used in the cosmetic industry as an adhesive when making face powders and masks and to render also creams and lotions smoother. New uses begin to emerge such as for instance the stabilization of carbon nanotubes [START_REF] Bandyopadhyaya | Stabilization of individual carbon nanotubes in aqueous solutions[END_REF].

Chemical composition and structure of Acacia senegal gum

Chemical composition

AG is a complex polysaccharide, either neutral or slightly acidic, found as a mixed calcium salt of a polysaccharide acid (Arabic acid). The backbone is composed of 1,3-linked β-Dgalactopyranosyl units. The side chains are composed of two or five 1,3-linked β-Dgalactopyranosyl units, joined to the main chain by 1,6-linkages. Both the main and the side chains contain units of -L-arabinofuranosyl, -L-rhamnopyranosyl, -D-glucuronopyranosyl and 4-O-methyl--D-glucuropyranosyl, the last two mostly as end units (D. M. W. [START_REF] Anderson | Studies on uronic acid materials[END_REF][START_REF] Islam | A review of recent developments on the regulatory, structural and functional aspects of gum arabic[END_REF][START_REF] Verbeken | Exudate gums: occurrence, production, and applications[END_REF]. [START_REF] Idris | Characterisation of gum from Acacia senegal trees of different age and location using multidetection gel permeation chromatography[END_REF] reported AG to be comprised of 39-42% galactose, 24-27% arabinose, 12-16% rhamnose, 15-16% glucuronic acid, 1.5-2.6% protein, 0.22-0.39% nitrogen, and 12.5-Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. [START_REF] Anderson | Specifications for gumarabic (Acacia-Senegal) -Analytical data for samples collected between 1904 and 1989[END_REF][START_REF] Idris | Characterisation of gum from Acacia senegal trees of different age and location using multidetection gel permeation chromatography[END_REF][START_REF] Karamalla | Analytical data for Acacia senegal var. senegal gum samples collected between 1993 and 1995 from Sudan[END_REF][START_REF] Verbeken | Exudate gums: occurrence, production, and applications[END_REF]) (Al-Assaf, Phillips, & Williams, 2005;[START_REF] Islam | A review of recent developments on the regulatory, structural and functional aspects of gum arabic[END_REF].

AG is a highly heterogeneous material that can be separated into three main fractions by hydrophobic interaction chromatography (HIC) [START_REF] Randall | Fractionation and characterization of gum from Acacia senegal[END_REF]. As an example, we showed on one sample that most of the gum (88.3% of total), an arabinogalactan-peptide (AGp, Fraction 1 or F1), had a very low protein content (1.1%) and a molecular weight of 2.910 5 g.mol -1 [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF].

The second fraction (10.4% of total), an arabinogalactan-protein complex (AGP, Fraction 2 or F2), contained 9% protein and had a molecular weight of 1.910 6 g.mol -1 . The third minor fraction (1.3% of total gum), referred as glycoproteins (GP, fraction 3 or F3), will consist of at least three glycoprotein populations with molecular weight ranging from 2.510 5 to 2.610 6 g.mol -1 . One of the GP had a molecular weight of 2.95 x 10 5 g.mol -1 and the highest protein content 24.6%, [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. These different values may change depending on gum origin, age, storage conditions, etc… (Al-Assaf, Andres-Brull, Cirre, & Phillips, 2012). [START_REF] Ray | Functionality of gum-arabic -Fractionation, characterization and evaluation of gum fractions in citrus oil-emulsions and model beverages[END_REF] fractionated AG by both HIC and gel permeation chromatography (GPC); their results were in broad agreement with those of [START_REF] Randall | Fractionation and characterization of gum from Acacia senegal[END_REF] and [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. The main amino acids present in the proteinaceous component of AG and AGP were hydroxyproline, serine and proline, whereas in GP, aspartic acid was the most abundant [START_REF] Islam | A review of recent developments on the regulatory, structural and functional aspects of gum arabic[END_REF]. [START_REF] Osman | The molecular characterization of the polysaccharide gum from Acacia-Senegal[END_REF] fractionated AG by HIC to yield four fractions, all of which had a similar carbohydrate composition, but differed in their content of protein, amino acid composition and molecular weight distribution. All four fractions reacted with an array of anti-arabinogalactan-protein monoclonal antibodies via anti-carbohydrate epitopes and were precipitated by Yariv's reagent, which indicated that all four fractions belonged to AGP's family.

AGP-type macromolecules represent about 94-96% of total compounds found in Acacia senegal or seyal gums. Minor components are mainly minerals (~3-5%) including Na, K, Ca,

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Mg, and trace metals such as Zn, Fe, Pb and Cu ions (D. M. W. [START_REF] Anderson | The chemical characterization of the test article used in toxicological studies of gum arabic (Acacia senegal (L) Willd)[END_REF][START_REF] Debon | In vitro binding of calcium, iron and zinc by non-starch polysaccharides[END_REF][START_REF] Kunkel | Magnesium binding by gum arabic, locust bean gum, and arabinogalactan[END_REF][START_REF] Mhinzi | Chemotaxonomic distinction of selected closely related Acacia species using chemical properties of their gum exudates[END_REF]. Small concentrations of tannins, around 0.4% [START_REF] Mhinzi | Chemotaxonomic distinction of selected closely related Acacia species using chemical properties of their gum exudates[END_REF], can be found giving more or less colored gums, which is especially remarkable with Acacia seyal. Variability in tannins content was reported both for A. senegal (0.3-0.6%) or A. seyal (0.6-1.2%) gums but only on five samples [START_REF] Minzhi | Intra-species variation of the properties of gum exudates from Acacia senegal var. senegal and Acacia seyal var. Fistula from Tanzania[END_REF]. Other emphasized that tannins can be found in AGs but not on Acacia senegal var. senegal gums (K.A. [START_REF] Karamalla | Gum Arabic -Quality and quantity assured[END_REF] which is clearly in contradiction with data from Minzhi (2002[START_REF] Minzhi | Intra-species variation of the properties of gum exudates from Acacia senegal var. senegal and Acacia seyal var. Fistula from Tanzania[END_REF]. [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF] identified traces of trans ferulic acid, ferulic acid and 8-5' non cyclic diferulic acid in AG and AGP and GP fractions. AGs also contain traces of lipids (M. P. [START_REF] Yadav | Chemical investigation of the structural basis of the emulsifying activity of gum arabic[END_REF]M.P. Yadav, Moreau, Johnston, & Hicks, 2012) and enzymes such as oxidases and peroxidases, diastases and pectinases [START_REF] Billaud | Substrates and carboxylic acid inhibitors of a partially purified polyphenol oxidase from gum Arabic[END_REF][START_REF] Fowler | An examination of some Gum-Enzymes. II. Chemical constitution of the gum from Boswellia serrata[END_REF][START_REF] Glicksman | Gum Arabic[END_REF][START_REF] Leo | Method of treating gum solutions[END_REF][START_REF] Reinitzer | The enzymes of gum-Acacia and certain other gums[END_REF].

Structure of Acacia senegal gum

AG is a highly branched polyanionic polysaccharide [START_REF] Fincher | Arabinogalactan-proteins -Structure, biosynthesis, and function[END_REF][START_REF] Keentok | On rod-like polymers[END_REF][START_REF] Swenson | Structure of gum arabic and its configuration in solution[END_REF][START_REF] Yomota | THE MOLAL OSMOTIC COEFFICIENTS AND COUNTERION ACTIVITY-COEFFICIENTS OF ARABATE WITH VARIOUS COUNTERIONS[END_REF] with a low charge density (Vandevelde & Fenyo, 1987). Recently, titration of AG and its fractions gave 223, 1259 and 1605 charges for AGp, AGP and GP, respectively [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. A maximum persistence length of about 3 nm was estimated for the AGp main fraction of AG [START_REF] Sanchez | The acacia gum arabinogalactan fraction is a thin oblate ellipsoid: A new model based on small-angle neutron scattering and ab initio calculation[END_REF].

Regarding the structure in solution, globular and close-packed shape of Acacia senegal gum molecules was suggested previously based on the low viscosity of gum solutions (D. M. W. [START_REF] Anderson | Recent advances in the chemistry of Acacia gums[END_REF][START_REF] Swenson | Structure of gum arabic and its configuration in solution[END_REF]. The globular or not-extended shape of AG molecules was also deduced from the relationships between the intrinsic viscosity [] or the radius of gyration R g or the ratio value between R g and R h and the molecular weight M w , [START_REF] Idris | Characterisation of gum from Acacia senegal trees of different age and location using multidetection gel permeation chromatography[END_REF]. For instance, the Mark-Houwink-Sakurada exponent , i.e. the slope of the log-log plot of [] vs M w , produced a slope of 0.54 (D. M. W. [START_REF] Anderson | Studies on uronic acid materials. 20. The viscosity molecular weight relationship for Acacia gums[END_REF] or 0.47 [START_REF] Idris | Characterisation of gum from Acacia senegal trees of different age and location using multidetection gel permeation chromatography[END_REF]. These values are mainly due to the AGp fraction as we found for this fraction a slope of 0.49 [START_REF] Sanchez | The acacia gum arabinogalactan fraction is a thin oblate ellipsoid: A new model based on small-angle neutron scattering and ab initio calculation[END_REF]. Based on a large number of structural data from literature, it appears that the [] vs M w relationship is not linear over the entire

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

M w range, indicating that the conformation is molecular weight dependent (Al-Assaf, [START_REF] Al-Assaf | Characterization and properties of Acacia senegal (L.) Willd. var. senegal with enhanced properties (Acacia (sen) SUPER GUM (TM)): Part 1 -Controlled maturation of Acacia senegal var. senegal to increase viscoelasticity, produce a hydrogel form and convert a poor into a good emulsifier[END_REF][START_REF] Al-Assaf | Studies on acacia exudate gums. Part I: the molecular weight of Acacia senegal gum exudate[END_REF][START_REF] Al-Assaf | Molecular associations in acacia gums[END_REF][START_REF] Anderson | The chemical characterization of the test article used in toxicological studies of gum arabic (Acacia senegal (L) Willd)[END_REF][START_REF] Anderson | Specifications for gumarabic (Acacia-Senegal) -Analytical data for samples collected between 1904 and 1989[END_REF]; D. M. W. [START_REF] Anderson | Studies on Uronic Acid Materials. Part XV. The use of Molecular-sieve Chromatography on Acacia senegal gum (Gum Arabic)[END_REF]; D. M. W. [START_REF] Anderson | The characterization of Acacia paolii gum and four commercial Acacia gums from Kenya[END_REF][START_REF] Chikamai | Gum-arabic from Acacia-Senegal (L) Willd in Kenya[END_REF][START_REF] Chikamai | An evalation of methods for characterizing and monitoring gum-arabic of commerce and related Acacia gums[END_REF][START_REF] Deeble | The treatment of aqueous gum arabic solutions with ultraviolet radiation[END_REF][START_REF] Duvallet | The characterization of gum-arabic from an experimental field of Ferlo (North Senegal)[END_REF][START_REF] Fang | Origin and thermodynamic properties of the instability of synthetic azo colorants in gum arabic solutions[END_REF][START_REF] Fang | Interaction of Gum Arabic with Fatty Acid Studied Using Electron Paramagnetic Resonance[END_REF][START_REF] Idris | Characterisation of gum from Acacia senegal trees of different age and location using multidetection gel permeation chromatography[END_REF][START_REF] Jurasek | The classification of natural gums. 3. Acacia-Senegal and related species (gum-arabic)[END_REF][START_REF] Karamalla | Analytical data for Acacia senegal var. senegal gum samples collected between 1993 and 1995 from Sudan[END_REF][START_REF] Kateyama | Estimation of concentration and performance of AGPs in emulsion systems using gum Arabic[END_REF][START_REF] Kuan | Effects of Ultraviolet Irradiation on the Physicochemical and Functional Properties of Gum Arabic[END_REF][START_REF] Li | Rheological study of gum arabic solutions: Interpretation based on molecular self-association[END_REF][START_REF] Li | Rheological properties of gum arabic solution: From Newtonianism to thixotropy[END_REF][START_REF] Mahendran | New insights into the structural characteristics of the arabinogalactan -Protein (AGP) fraction of gum Arabic[END_REF][START_REF] Mukherjee | Light scattering studies in solution of gum Arabic[END_REF][START_REF] Osman | The molecular characterization of the polysaccharide gum from Acacia-Senegal[END_REF][START_REF] Picton | Analysis of a complex polysaccharide (gum arabic) by multiangle laser light scattering coupled on-line to size exclusion chromatography and flow field flow fractionation[END_REF][START_REF] Randall | Fractionation and characterization of gum from Acacia senegal[END_REF][START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF][START_REF] Swenson | Structure of gum arabic and its configuration in solution[END_REF][START_REF] Veis | Light scattering in solutions of linear polyelectrolyte[END_REF][START_REF] Wang | Solution properties of conventional gum arabic and a matured gum arabic (Acacia (sen) SUPER GUM)[END_REF]. AG macromolecules with M w below 110 6 g.mol -1 are thus more spheroidal than macromolecules with M w above 110 6 g.mol -1 that appear more extended. These differences could be due only to differences in structures and/or alternatively to differences in the affinity for the aqueous solvent. Little information is available regarding the structure of Acacia seyal molecules. However, the few studies discussing about Acacia seyal gum structure evidenced that these molecules are more compact than those of Acacia senegal gum [START_REF] Al-Assaf | Studies on acacia exudate gums. Part I: the molecular weight of Acacia senegal gum exudate[END_REF][START_REF] Lopez-Torrez | Acacia senegal vs. Acacia seyal gums -Part 1: Composition and structure of hyperbranched plant exudates[END_REF]. For both A. gums, the anisotropy increases with the increase of molecular weight, with however more anisotropic conformations for A. senegal molecules. The conformation varies from spheres to oblate ellipsoids for A. seyal molecules, while it varies from oblate ellipsoids to more anisotropic conformations, such as oblate and prolate ellipsoids, for A. senegal molecules [START_REF] Lopez-Torrez | Acacia senegal vs. Acacia seyal gums -Part 1: Composition and structure of hyperbranched plant exudates[END_REF]. The ellipsoidal conformation of A. senegal molecules is also confirmed using hydrodynamic technics as sedimentation velocity analytical ultracentrifugation and size exclusion chromatography coupled to multi-angle light scattering (SEC MALS) and differential viscometry [START_REF] Gillis | A Novel Analytical Ultracentrifugation Based Approach to the Low Resolution Structure of Gum Arabic[END_REF].

Few studies focused on the structural properties of Acacia gums according to the origin of gums (i.e. the country). This missing information could raise some questions about the influence of the geographical area of harvest and the post-harvested treatment, especially from raw to spray-dried gums, on the conformations of molecules. Our group analysed the Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. To summarize AG is composed of a continuum of molecular species differing by their protein to sugar ratio, molecular weight and charges [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF], but also by different mesoscopic structures in solutions. The precise conformations of different molecular fractions remain uncertain from studies on total AG. The following section reports on the actual knowledge on the structures of the three molecular fractions of A. senegal gum isolated from HIC, i.e. the arabinogalactan-peptide (AGp), the arabinogalactan-protein (AGP)

and glycoproteins (GP) fractions.

Structure of the arabinogalactan-peptide fraction (AGp, fraction 1 or F1)

The first structural model for AGp was recently proposed [START_REF] Sanchez | The acacia gum arabinogalactan fraction is a thin oblate ellipsoid: A new model based on small-angle neutron scattering and ab initio calculation[END_REF]. From small angle neutron scattering (SANS) experiments in charge screening conditions and dynamic light scattering, AGp appeared to be a dispersion of two-dimensional structures with a R g of 6.5 nm, a R h of 9.1 nm and an inner dense branched structure [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF][START_REF] Sanchez | The acacia gum arabinogalactan fraction is a thin oblate ellipsoid: A new model based on small-angle neutron scattering and ab initio calculation[END_REF]. Data analysis and modeling of SANS experiments revealed a disk-like morphology with a diameter of 20 nm, a thickness of less than 2 nm and a central intricated ''network''. The structure of AGp could explain the low viscosity of AG solutions, and its ability to self-assemble and to interact with proteins. At the molecular level, no specific secondary structures could be detected using circular dichroism, which could be explained by the low amino-acid composition of the AGp fraction [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. However, Fourier transform infrared spectroscopy suggested the presence of extended -sheet and -

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Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

ACCEPTED MANUSCRIPT

13 turn structures but not -helix [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. The absence of -helix could explain partly the absence of secondary structures as revealed by circular dichroism.

Structure of the arabinogalactan-protein fraction (AGP, fraction 2 or F2)

The arabinogalactan-protein fraction from AG represents about 10-15% of total molecules and about 9% of the total protein concentration. Its M w is variable but generally comprised between 1 and 410 6 g.mol -1 [START_REF] Al-Assaf | Characterization and properties of Acacia senegal (L.) Willd. var. senegal with enhanced properties (Acacia (sen) SUPER GUM (TM)): Part 1 -Controlled maturation of Acacia senegal var. senegal to increase viscoelasticity, produce a hydrogel form and convert a poor into a good emulsifier[END_REF][START_REF] Al-Assaf | Studies on acacia exudate gums. Part I: the molecular weight of Acacia senegal gum exudate[END_REF][START_REF] Al-Assaf | Molecular associations in acacia gums[END_REF][START_REF] Castellani | Hydrocolloids with emulsifying capacity. Part 1-Emulsifying properties and interfacial characteristics of conventional (Acacia senegal (L.) Willd. var. senegal) and matured (Acacia (sen) SUPER GUM (TM)) Acacia senegal[END_REF][START_REF] Elmanan | Studies on Acacia exudate gums: Part VI. Interfacial rheology of Acacia senegal and Acacia seyal[END_REF][START_REF] Idris | Characterisation of gum from Acacia senegal trees of different age and location using multidetection gel permeation chromatography[END_REF][START_REF] Picton | Analysis of a complex polysaccharide (gum arabic) by multiangle laser light scattering coupled on-line to size exclusion chromatography and flow field flow fractionation[END_REF][START_REF] Randall | Fractionation and characterization of gum from Acacia senegal[END_REF][START_REF] Ray | Functionality of gum-arabic -Fractionation, characterization and evaluation of gum fractions in citrus oil-emulsions and model beverages[END_REF][START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF][START_REF] Vandevelde | Estimation of the charge-density of arabic acid by potentiometry and dye binding[END_REF] .

At the molecular level, AGP contains various secondary structures, including about 27% of polyproline II structures, -sheets, -turns and unordered structures, but not -helices [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. A wattle-blossom model was proposed to describe the structure of the AGP complex. It was postulated that the high molecular weight fraction of the gum is composed of large carbohydrate blocks with a molecular weight of approximately 210 5 g.mol -1 , these blocks being covalently linked to a polypeptide backbone [START_REF] Connolly | Heterogeneity and homogeneity of an arabinogalactan-protein: Acacia senegal gum[END_REF], 1988;[START_REF] Fincher | Arabinogalactan-proteins -Structure, biosynthesis, and function[END_REF]). An alternative model was suggested by [START_REF] Qi | Gum-arabic glycoprotein is a twisted hairy rope -A new model based on O-galactosylhydroxyproline as the polysaccharide attachment site[END_REF] in the form of a hairy twisted rope. This model would be comprised of a core rodlike protein (150 nm long) with a highly repetitive amino-acid sequence and the carbohydrate blocks (30 sugar residues) attached to hydroxyproline residues. However, as indicated previously, most studies strongly suggested that the molecules of the AGP complex have a spheroidal structure, which better supports the wattle-blossom model [START_REF] Connolly | Effect of a proteinase on the macromolecular distribution of Acacia-Senegal gum[END_REF][START_REF] Idris | Characterisation of gum from Acacia senegal trees of different age and location using multidetection gel permeation chromatography[END_REF][START_REF] Picton | Analysis of a complex polysaccharide (gum arabic) by multiangle laser light scattering coupled on-line to size exclusion chromatography and flow field flow fractionation[END_REF][START_REF] Vandevelde | Estimation of the charge-density of arabic acid by potentiometry and dye binding[END_REF].

From a study on total Acacia gum, a more detailed picture of the wattle-blossom structure of AGP was proposed recently [START_REF] Mahendran | New insights into the structural characteristics of the arabinogalactan -Protein (AGP) fraction of gum Arabic[END_REF]. Mild alkaline hydrolysis of the gum followed by GPC analysis indicated that AG consists of carbohydrate blocks of ~4.510 4 g.mol -1 , blocks much lower in mass than those previously reported, covalently linked to serine and hydroxyproline residues. Two folded polypeptide chains would be present in AG, one with a M w around 310 4 g.mol -1 corresponding to about 250 amino acids and the second one with a M w of about 510 3 g.mol -1 , corresponding to about 45 amino acids. A number of around 400 amino acids was calculated previously for the AGP fraction (Qi, Fong, & Lamport, Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. It was then assumed that carbohydrate blocks may have a thin oblate ellipsoid structure [START_REF] Mahendran | New insights into the structural characteristics of the arabinogalactan -Protein (AGP) fraction of gum Arabic[END_REF][START_REF] Sanchez | The acacia gum arabinogalactan fraction is a thin oblate ellipsoid: A new model based on small-angle neutron scattering and ab initio calculation[END_REF]. The model is interesting since it gives a clearer view of the possible spatial configuration of AGP. The steric arrangement of carbohydrate blocks in such a configuration is questionable and merits much more investigation, as well as the fine structure of macromolecules. One can imagine that AGP is a two-dimensional object with a folded protein network and interacting massive sugar blocks or an assembly of sugar blocks linked (covalently or not) between them by several polypeptide backbones.

Regarding the possible morphology of AGP in solution, HPSEC-MALLS provided some informative insights. AGP in solution has a weight average molecular weight of 1.8610 6 g.mol -1 and a radius of gyration of 30 nm [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. In addition, two exponent values are identified in the R g , [], R h vs M w relationships highlighting two types of conformations depending on the molecular weight range considered (Renard, Garnier, Lapp, Schmitt, & Sanchez, 2012). AGP would behave in solution as a branched or hyper-branched polymer with conformations ranging from globular to elongated shape depending on the size of the carbohydrate branches. SANS form factor revealed an elongated average conformation corresponding to a triaxial ellipsoid while inverse Fourier transform of the scattering form factor gave a maximum dimension for AGP of 64 nm (Renard, et al., 2012).

TEM highlighted the existence of isolated spheroidal particles (diameters ranging from about 10 to 40 nm) or more anisotropic morphologies (lengths from 20 up to about 60 nm) [START_REF] Renard | Structure of arabinogalactanprotein from Acacia gum: From porous ellipsoids to supramolecular architectures[END_REF]. Remarkably, all the particles were porous supramolecular assemblies of smaller structural subunits with dimensions of about 2-10 nm.

These building structural subunits were mainly branched chains and ring-like structures with diameters of about 1-5 nm.

It was recently suggested that AGP would be in fact a molecular association resulting from an aggregated fraction of AGp units stabilized by low molecular weight proteinaceous

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

components found in the GP fraction of AG [START_REF] Al-Assaf | Molecular associations in acacia gums[END_REF]. More accurately, maturation process would promote interactions between AGP, AGp and GP, inducing a molecular reorganization of the gum and the appearance of composite AGP architectures. In addition, spray-drying was found to increase the molecular weight of AGP due to selfaggregation [START_REF] Al-Assaf | Molecular associations in acacia gums[END_REF]. In summary, the so called AGP is in fact a heterogeneous population varying in anisotropy, chain density and porosity, and of all possible molecular combinations between AGP, AGp and GP. This structural heterogeneity likely depends on chemical composition of gum sample and its maturation process, natural or induced by processing.

Very recently, AGP and AGp molecular fractions were degraded enzymatically using acidic and alkaline proteases in order to probe the conformation and structure of the two main fractions of Acacia senegal gum. While AGp fraction kept intact whatever the enzymes and conditions used, AGP was found to be degraded only in alkaline conditions. The absence of degradation in acidic conditions questioned about the potential modification of the structure of AGP molecular fraction at low pHs. The decrease in molecular weight of AGP after enzymatic treatment confirmed the accessibility of enzymes toward polypeptide cleavages and papain was found to be the most efficient protease with a decrease of M w from 1.79 × 10 6 to 1.68 × 10 5 g mol -1 . The molecular structure of control and enzyme-treated AGPs surprisingly predicted similar secondary structures content. The similar conformations adopted by control and enzyme-cleaved AGPs probed at the molecular and mesoscopic scale (by SANS) would be in favor of a high flexibility of the polypeptide backbone before and after enzymatic treatment in accordance with the repetitive and palindromic nature of peptide sequence and the overall symmetry of the carbohydrate moieties along the protein backbone [START_REF] Goodrum | Gum arabic glycoprotein contains glycomodules of both extensin and arabinogalactan-glycoproteins[END_REF][START_REF] Kieliszewski | The latest hype on Hyp-O-glycosylation codes[END_REF][START_REF] Kieliszewski | Extensin -Repetitive motifs, functional sites, posttranslational codes, and phylogeny[END_REF]. It was finally suggested that a self-similarity driven-process would be at the origin of the assembly of AGP from a consensus glycopeptide building block with a symmetrical distribution of arabinosides and polysaccharide substituents (Figure 2) [START_REF] Renard | Enzymatic hydrolysis studies of arabinogalactan-protein structure from Acacia gum: The self-similarity hypothesis of assembly from a common building block[END_REF].

Structure of the glycoprotein fraction (GP, fraction 3 or F3)

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

The glycoprotein fraction is a minor component of AG (< 2%) but is rich in proteins (25-50%).

Regarding the amino acid composition, GP fraction is less rich in hydroxyproline and serine but richer in asparagine and aspartic acid, but also in tyrosine and phenylalanine residues [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. Using HIC, at least three different fractions were identified in GP with M W ranging from 310 5 to 310 6 g.mol -1 [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. Following HIC and SEC, the three fractions were purified and displayed each three molecular populations, with a clear continuum of species. It appears obvious that a deeper study of GP is needed to better understand the complexity of this minor fraction. Like AGP, the different glycoproteins are characterized by the presence of polyproline II conformation (9%), -helix (9%), -sheet (38%), -turns (23%) and unordered structures (18%) [START_REF] Renard | Structure of glycoproteins from Acacia gum: An assembly of ring-like glycoproteins modules[END_REF].

However, no mesoscopic models have been proposed to date for these glycoproteins. As well, physical chemical properties of GP fraction are almost unknown, except its very active surface properties [START_REF] Castellani | Hydrocolloids with emulsifying capacity. Part 3-Adsorption and structural properties at the air-water surface[END_REF].

Very recently, Renard et al. (2014) studied the structure of one glycoprotein (GP) fraction of AG isolated from HIC and SEC, which revealed a mixture of spheroidal monomers and more anisotropic oligomers in GP solution as suggested by the two exponent values found in the R g vs. M w relationship and TEM observations [START_REF] Renard | Structure of glycoproteins from Acacia gum: An assembly of ring-like glycoproteins modules[END_REF]. The GP conformation probed by SAXS was ascribed to a thin object with a triaxial ellipsoid morphology, certainly attributed to GP oligomers. A 9 nm diameter particle was also identified by SAXS in agreement with the dimensions (diameters of 8 to 11 nm) identified by TEM on single isolated ring-like structures. All the identified isolated particles had a spheroidal shape while slight anisotropy appeared when ring-like structures self-associated.

Contrary to what was previously observed on AGp and AGP, no outer structure combined to an inner porous network of interspersed chains was observed in the spheroidal particles morphology. These spheroidal particles were structurally made of an inhomogeneous outer thick shell and a central hole giving rise to the particles a typical ring-like morphology with a 8 to 11 nm diameters [START_REF] Renard | Structure of glycoproteins from Acacia gum: An assembly of ring-like glycoproteins modules[END_REF].

In summary, the GP fraction from AG would be an assembly of ring-like glycoproteins modules. These ring-like structures were certainly due to hydroxyproline (Hyp)arabinogalactan (AG) subunits, as suggested by the secondary structures content of GP [START_REF] Renard | Structure of arabinogalactanprotein from Acacia gum: From porous ellipsoids to supramolecular architectures[END_REF]. GP monomer, with a rather globular shape and homogeneous long-

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

chain branches, would be able to self-associate giving rise to small oligomers with a rather compact conformation and bigger oligomers with a more extended conformation closely related to the self-association mode.

Physico-chemical properties of Acacia senegal gum

Solubility in polar and non polar solvents

The ability of AG to easily dissolve in water is not new [START_REF] Diderot | Encyclopédie ou Dictionnaire Raisonné des Sciences, des Arts et des Métiers[END_REF][START_REF] Pomet | Histoire générale des drogues simples et composées, renfermant dans les 3 classes des végétaux, des animaux et des minéraux tout ce qui est l'objet de la physique, de la chimie, de la pharmacie, & des arts les plus utiles à la société des hommes[END_REF], and it is said that AG is soluble in cold and hot water up to concentrations of about 50-55%

and insoluble in alcohol [START_REF] Erni | Interfacial rheology of surface-active blopolymers: Acacia senegal gum versus hydrophobically modifed starch[END_REF][START_REF] Ewart | Identification of Stabilizing Agents[END_REF][START_REF] Izydorczyk | Polysaccharide gums: Structure, functional properties, and applications[END_REF][START_REF] Turner | Elements of chemistry : including the recent discoveries and doctrines of the science[END_REF][START_REF] Verbeken | Exudate gums: occurrence, production, and applications[END_REF][START_REF] Waters | Some qualitative tests for gum arabic and its quantitative determination[END_REF]. However it is not so easy to prepare a dispersion of gum at 50% concentration and for instance a gum solubility of 37% was experimentally determined at 25°C [START_REF] Taft | Solvents for gum Arabic[END_REF]. Despite its high solubility in water, the mineral composition of the bulk can induce the precipitation of AG. The aqueous solution of AG forms a white jelly with basic acetate while it is soluble with neutral lead acetate. The AG solution also precipitates using potassium or sodium silicate, borax, ammonium oxalate, mercuric chloride and ferric salts [START_REF] Parry | Gums and resins: Their occurrence, properties and uses (Pitman's common commodities and industries[END_REF]. In addition, it was actually demonstrated using a great number of solvents that AG is poorly soluble in solvents other than water [START_REF] Taft | Solvents for gum Arabic[END_REF][START_REF] Taft | Solvents for gum Arabic II Transactions of the[END_REF].

Solubility of Acacia gum in alcohol solutions

AG is soluble in dilute alcohol solution and precipitates when the alcohol concentration in water is of about 50%, with a complete precipitation of AG macromolecules with 60% alcohol [START_REF] Norman | The chemical constitution of the gums. Part I. The nature of gum arabic and the biochemical classification of the gums[END_REF][START_REF] Parry | Gums and resins: Their occurrence, properties and uses (Pitman's common commodities and industries[END_REF]. With 30% alcohol, no precipitate occurs, and with 40% alcohol an opalescent/turbid ("faint") precipitate is obtained [START_REF] Waters | Some qualitative tests for gum arabic and its quantitative determination[END_REF]. These results clearly depend on gum concentration. Alcohol precipitation has been long used to purify AG [START_REF] Mukherjee | Light scattering studies in solution of gum Arabic[END_REF][START_REF] Mukherjee | Influence of concentration on the properties of arabic solution[END_REF][START_REF] Nelson | Heterogeneity of gum arabic and its salts[END_REF][START_REF] Oakley | A new type of osmometer for low pressures, with some preliminary results for gum arabic[END_REF][START_REF] Taft | Solvents for gum Arabic II Transactions of the[END_REF][START_REF] Thomas | A physico-chemical study of gum arabic[END_REF][START_REF] Veis | Light scattering in solutions of linear polyelectrolyte[END_REF]. However, it has been reported that prolonged contact with alcohol decreases the solubility of AG [START_REF] Van Beek | Purification and viscosity experiments on gum arabic[END_REF].

The nature of interactions and mechanism of demixing in AG-ethanol-water ternary system have not been studied in great details. However the ability of alcohols (and some salts) to induce phase separation/precipitation in polysaccharides, and more generally biopolymers

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

dispersions is not new and is called simple coacervation [START_REF] Bamford | The theory of coacervation[END_REF]H.B. Bohidar, 2008;Bungenberg de Jong, 1949a;[START_REF] Gupta | Free-energy landscape of alcohol driven coacervation transition in aqueous gelatin solutions[END_REF][START_REF] Jamieson | Studies of elastin coacervation by quasi-elastic light-scattering Faraday Discussions[END_REF][START_REF] Koets | Phase separation in aqueous colloidal systems[END_REF][START_REF] Mohanty | Systematic of alcohol-induced simple coacervation in aqueous gelatin solutions[END_REF][START_REF] Nixon | Phase relationships in simple coacervating system isoelectric gelatin -ethanol -water[END_REF][START_REF] Van Oss | Coacervation, Complex-coacervation and flocculation[END_REF][START_REF] Veis | A review of the early development of the thermodynamics of the complex coacervation phase separation[END_REF]. Simple coacervation is basically a liquid-liquid phase separation of biopolymers experiencing a change in the solvent quality. This demixing mechanism will be discussed in the section « Assembly properties of AG ».

It would be useful to determine phase diagrams of AG-alcohol-water systems and to characterize in each phase the macromolecular composition, i.e. the relative compositions of AGp, AGP and GP. As these three fractions display or are supposed to display different physical chemical properties, it is likely that simple ways to obtain AG enriched in one or another of the three fractions could be identified.

Hydration properties of Acacia gum macromolecules

The observed hydrophilic nature of AG has probably not motivated many studies on the hydration properties. Few papers can be found in literature. In one example, it was shown that AG is not hydrated to a great extent as has been claimed by many authors [START_REF] Grollman | The vapour pressure of aqueous solutions with special reference to the problem of the state of water in biological fluids[END_REF]. Using vapor pressure measurements, no hydration of the gum was measured with NaCl (0.05M) or KCl (0.07M), which would indicate that ions were preferentially hydrated.

With 0.18M sucrose, 0.7g water/g of gum was measured. In a subsequent study, a hydration of 0.9g water/g for Ca gum and 1.1g water/g for Na gum was measured using a membrane equilibrium method [START_REF] Oakley | The hydration of gum arabic and glycogen[END_REF]. A minimal value of 0.6-0.7 g water/g of gum was found by a cryoscopic method and no hydration of the gum in presence of KCl or KBr was found, which seems to confirm previous results [START_REF] Gortner | The cryoscopic method for the determination of "bound water[END_REF]). An interesting point was that when AG concentration in studied dispersions increased from 3% to 10%, the amount of bound water decreased from 1.2g to 0.6g/g of gum [START_REF] Newton | A method for estimating hydrophilic colloid content of expressed plant tissue fluids[END_REF]. It is possible that some self-association (i.e. aggregation) of AG macromolecules occurred with increasing concentration, leading to the decrease of water accessibility towards AG macromolecules and to the release of bound water from AG macromolecules during self-association process.

When analyzing interactions of biopolymers with water, one can thus distinguish free and bound water [START_REF] Chandler | Nature of bound water in colloidal systems[END_REF][START_REF] Gortner | The cryoscopic method for the determination of "bound water[END_REF]. Free or freezing water is the water where melting/crystallization temperature and enthalpy of melting/crystallization are not significantly different from those of bulk water [START_REF] Hatakeyama | Interaction between water and hydrophilic polymers[END_REF]. Bound

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

water gathers non-freezing water that is very closely associated with the macromolecules and freezing bound water that is less closely associated but displays different physical properties than free water [START_REF] Hatakeyama | Interaction between water and hydrophilic polymers[END_REF]. Non-freezing water is made of a monolayer, possibly multilayers, of water molecules in very close interaction with the biopolymer. The monolayer is generally calculated from sorption isotherms (gravimetric method) following the use of an appropriate model, the most often used being the BET (Brunauer-Emmet-Teller) or the GAB (Guggenheim-Anderson-de Boer) model [START_REF] Blahovec | Sorption isotherms in materials of biological origin mathematical and physical approach[END_REF]. This type of hydration water has to be considered as an integrating part of the native biopolymer structures [START_REF] Luschermattli | Thermodynamic functions of bio-polymer hydration. 1. Their determination by vapor-pressure studies, discussed in an analysis of the primary hydration process[END_REF]. The total amount of water (W c )

interacting with a biopolymer is then the sum of free and bound water. Another parameter dealing with the interaction of biopolymers and especially fibers with water is the waterholding capacity (WHC). The WHC is a measure of the ability of a fiber source to immobilize water within its matrix [START_REF] Robertson | A method to measure the water-holding properties of dietary fiber using suction pressure[END_REF]. WHC encompasses both adsorbed water and possibly water trapped within macromolecules.

Generally, water (W c or WHC) in AG amounts to about 3-6 g water/g gum, which is a range previously mentioned [START_REF] Takigami | Effect of preparation method on the hydration characteristics of hylan and comparison with another highly cross-linked polysaccharide, gum-arabic[END_REF]. The saturation W c value is in the range of values found for other polysaccharides such as xanthan and hyaluronan [START_REF] Phillips | Hydration characteristics of the gum exudate from Acacia senegal[END_REF]. The bound water (W b ) is around 1 g water/g gum and the non-freezing water (W nf ) is within 0.4-0.7 g water/g gum. In addition, the second virial coefficient A 2 (mL.mol.g -2 , also called B2 or B22), an indirect way to define the affinity for solvent, can be calculated from light scattering measurements. Negative A 2 values indicate a bad affinity for the solvent and attractive interactions between biopolymers while positive A 2 values indicate preferential interactions with the solvent and repulsions between macromolecules. Values of 5.10 -5 [START_REF] Picton | Analysis of a complex polysaccharide (gum arabic) by multiangle laser light scattering coupled on-line to size exclusion chromatography and flow field flow fractionation[END_REF] or 4.2.10 -5 mlmolg -2 [START_REF] Veis | Light scattering in solutions of linear polyelectrolyte[END_REF] were found for AG. The values were low but positive, under the experimental conditions used, indicating a preferential interaction with water. An interesting result concerns the effect of temperature on the water monolayer (X m ). It was found that increasing the temperature from 25 to 45°C resulted to an increase of X m from 0.08 to 0.11 g water/g gum. As an increase in temperature lowers the energy of hydrogen bonding, and as hydrogen bonding is at the basis of polysaccharide hydration (Q. Wang & Cui, 2005), one could expect a concomitant decrease of the structural water, as determined for instance for starch [START_REF] Al-Muhtaseb | Water sorption isotherms of starch powders -Part 1: mathematical description of experimental data[END_REF] or chitosan [START_REF] Rosa | Moisture sorption properties of chitosan[END_REF],

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

ACCEPTED MANUSCRIPT 20 but also for soy proteins [START_REF] Cassini | Water adsorption isotherms of texturized soy protein[END_REF]. It is also well known that by increasing temperatures there is an increase of energy of adsorbed water molecules, which allow the leaving of some adsorbed water molecules from the active centers of the adsorbent. As a result, the amount of adsorbed moisture decreases. The results reported here for AG are somewhat counterintuitive. However, these results are not unique as similar trends were demonstrated for microcrystalline cellulose [START_REF] Cadden | Moisture sorption characteristics of several food fibers[END_REF] cited by Vernon-Carter et al., 2006) or myosin [START_REF] Das | Analysis of moisture sorption characteristics of fish protein myosin[END_REF] cited by Vernon-Carter et al., 2006).

All these results converge on the same conclusion. The affinity of AG for water provides an extremely favorable environment for binding water, which is probably mainly due to the carbohydrate component of AG and its highly branched characteristic [START_REF] Phillips | Hydration characteristics of the gum exudate from Acacia senegal[END_REF].

The polypeptide component also interacts with water since it contains a significant number of hydrophilic aminoacids [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. The sugars units would first bind water at the hydroxyl (OH) groups associated with the uronic acids, forming the non-freezing water [START_REF] Phillips | Hydration characteristics of the gum exudate from Acacia senegal[END_REF]. Freezing-bound water is also tightly associated with carbohydrate chains, which could form intra-molecular hydrogen bonds within the highly cross-linked gum structure [START_REF] Phillips | Hydration characteristics of the gum exudate from Acacia senegal[END_REF]. Thereafter, there would be large intra-molecular and intermolecular voids which could be occupied by water in a variety of metastable states, preventing the formation of the ideal ice structure.

It is interesting to note that when AG in solution is dried, either with alcohol or by heating (vacuum distillation), it becomes practically insoluble [START_REF] Thomas | A physico-chemical study of gum arabic[END_REF]. When it is thus dried, the gum swells in water to a jelly-like mass which does not dissolve except on long standing. AG in powder form heated above 100°C, and especially at 170 °C, when immersed in water, also swells up to a great extent but does not dissolve and the gel thus formed is non-sticky [START_REF] Moorjani | Influence of heat on the chemical composition and physical properties of gum arabic[END_REF]. This insolubility is explained by the complete dehydration of the gum. However, it is likely that aggregation of AG macromolecules occurs as heating from 100 to 170 °C results in an increase of the viscosity of solutions. Protein degradation also occurs at temperatures above 100 °C that may affect the gum solubility [START_REF] Cozic | La gomme arabique : Variabilité des propriétés physicochimiques et contribution des "ArabinoGalactanes-Protéines[END_REF]. The insolubilization of the gum by heating at high temperature (150 °C) is known for a long time [START_REF] Fremy | Sur la composition et le mode de production des gommes dans l'organisation végétale[END_REF]. Boiling the gum or adding alkali at cold temperature dissolves the gum again [START_REF] Fremy | Sur la composition et le mode de production des gommes dans l'organisation végétale[END_REF]. The ability of AG to re-bind water which has been

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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21 released by increasing the temperature is of great value in confectionary and jellies applications [START_REF] Phillips | Hydration characteristics of the gum exudate from Acacia senegal[END_REF].

Rheological properties of Acacia gum

Viscosity (at zero shear rate) of Acacia gum dispersions

The molecular interactions of AG with the solvent determine in part the viscosity of dispersions. Viscosity is also governed by the shape, molecular size and concentration of molecules and is affected by temperature and pressure. Since AG macromolecules are weak polyelectrolytes, it is expected that pH, ionic strength and type of ions (according to the Hofmeister serie) must have a significant effect on the viscosity [START_REF] Stephen | Gums and Mucilage[END_REF].

This can be clearly observed with arabic acid where a maximum of viscosity was reached at a pH in the range 5.5-6.3 [START_REF] Thomas | A physico-chemical study of gum arabic[END_REF]. The lower viscosity at acidic pH can be explained by the fact that, due to neutralization of carboxyl groups at low pH, electrostatic repulsions decrease. This leads to a decrease in the hydrodynamic volume of the carboxylbearing polysaccharides, and hence, in the viscosity of the polysaccharide solution [START_REF] Vanderreijden | Rheological properties of commercially available polysaccharides with potential use in saliva substitutes[END_REF]. Acid-induced hydrolysis of polysaccharide can also contribute to the lower measured viscosity. The decrease in viscosity observed at pH larger than 8-10 could be explained by a strong weakening of hydrogen bonds, resulting in less efficient interactions with water or, alternatively, to conformational changes resulting in a reduction of charged groups-induced electrostatic repulsions. It can also be noticed that the viscosity was almost steady between pH 5 and 9. A steady viscosity of AG dispersions was also measured between pH 6.2 and 8.5 by Riddell & Davies (1931).

The increase in NaCl concentration induced a decrease in viscosity. For a salt-free solution, electrostatic repulsions due to the charges on the macromolecule favor a stretched chain conformation as a result of long-range electrostatic effects. This behavior results in higher viscosity. Addition of a simple electrolyte screens these intermolecular electrostatic repulsions and allows the molecules to compact towards the volume of an uncharged polymer with the same number of residues linked in the same way [START_REF] Giannouli | Effect of polymeric cosolutes on calcium pectinate gelation. Part 3. Gum arabic and overview[END_REF], resulting in a lower viscosity [START_REF] Smidsrod | Estimation of relative stiffness of molecular chain in polyelectrolytes from measurements of viscosity at different ionic strengths[END_REF][START_REF] Tinland | Dependence of the stiffness of the xabthan chain on the external salt concentration[END_REF]. Similar results were obtained by other authors [START_REF] Amy | Contribution à l'étude des propriétés et de la structure des solutions et des gelées de gomme[END_REF]Williams, Phillips, & Randall, 1990).

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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22

Like for other common biopolymers, viscosity increased with AG concentration (Williams et al., 1990) and decreased with the increase in temperature [START_REF] Stephen | Gums and Mucilage[END_REF]. Regarding the effect of AG concentration, we collected a great number of data from literature and the relationship between the relative viscosity / 0 and the AG concentration was exponential over a 0.13-56 wt% concentration range. However, what is important to notice, irrespective of the geographical origin of AG, various chemical compositions and physical chemical conditions of sample preparation for viscosity measurements, practically all data are described by a single exponential (Figure 3). Such an exponential delimitates two regions, one region where the viscosity gently increases with concentration and the other region where viscosity sharply increases with concentration.

Flow and viscoelastic properties of Acacia gum dispersions

Unlike most polysaccharides, used for their thickening or gelling properties (Wand & Cui, 2005), AG dispersions display low viscosity even at quite high concentration and do not gel (except when AG powder is thermally treated at high T then rehydrated, as noted above).

The observed low viscosity of AG dispersions probably can explain the belief that their flow behavior is newtonian, i.e. the relationship between the shear stress (s, N.m -2 ) and the shear rate ( , s -1 ) is linear (D. M. W. [START_REF] Anderson | Studies on uronic acid materials. 20. The viscosity molecular weight relationship for Acacia gums[END_REF][START_REF] Bemiller | Plant gums[END_REF][START_REF] Izydorczyk | Polysaccharide gums: Structure, functional properties, and applications[END_REF]. Some experimental results seem to confirm this behavior (Salazar-Montoya, Jimenez-Avalos, & Ramos-Ramirez, 2012). This depends on gum concentration. Shear-thinning flow of AG dispersions is observed at high AG concentrations, typically 15-30% and above [START_REF] Araujo | Effects of certain preservatives on aging characteristics of Acacia[END_REF][START_REF] Dunstan | The rheology of engineered polysaccharides[END_REF][START_REF] Gomez-Diaz | Intrinsic Viscosity and Flow Behaviour of Arabic Gum Aqueous Solutions[END_REF][START_REF] Nussinovitch | Exudate gums[END_REF][START_REF] Williams | Structure-function relationships of gum arabic[END_REF].

More recently, it has been shown that AG dispersions also display shear-thinning flow behavior, even at AG concentrations as low as 1-4% [START_REF] Li | Rheological study of gum arabic solutions: Interpretation based on molecular self-association[END_REF][START_REF] Mothe | Rheological behavior of aqueous dispersions of cashew gum and gum arabic: effect of concentration and blending[END_REF][START_REF] Sanchez | Structure and rheological properties of acacia gum dispersions[END_REF][START_REF] Weinbreck | Rheological properties of whey protein/gum arabic coacervates[END_REF]. It was hypothesized that the presence of AG aggregates could explain such an unusual flow behavior [START_REF] Li | Rheological study of gum arabic solutions: Interpretation based on molecular self-association[END_REF][START_REF] Mothe | Rheological behavior of aqueous dispersions of cashew gum and gum arabic: effect of concentration and blending[END_REF]. Hydrogen bonding could partly explain the formation of these hypothetical aggregates [START_REF] Li | Rheological study of gum arabic solutions: Interpretation based on molecular self-association[END_REF]. In addition, time-dependent or thixotropic flow behavior was also observed [START_REF] Li | Rheological study of gum arabic solutions: Interpretation based on molecular self-association[END_REF]Sanchez, Renard, et al., 2002). It was suggested that the aggregation of AGP component was at the origin of this behavior (Li, Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008 et al., 2011). Time-dependent thickening flow behavior at low shear rates and low biopolymer concentrations have been previously reported for colloidal globular protein dispersions [START_REF] Giordano | Stochastic order in macromolecular solutions -A SANS experiment[END_REF][START_REF] Lefebvre | Rheological evidence for large scale structures in moderately concentrated isoionic protein solutions[END_REF][START_REF] Matsumoto | Rheological and small-angle X-ray-scattering investigations on the shape and ordered arrangement of native ovalbumin molecules in aqueous colloids[END_REF][START_REF] Renard | Small angle neutron scattering and viscoelasticity study of the colloidal structure of aqueous solutions and gels of a globular protein[END_REF][START_REF] Renard | Rheological properties of mixed gels made of microparticulated whey proteins and beta-lactoglobulin[END_REF]. It is generally supposed that bulk aggregation between globular proteins or protein aggregates is responsible for the observed rheological properties [START_REF] Lefebvre | Rheological evidence for large scale structures in moderately concentrated isoionic protein solutions[END_REF][START_REF] Renard | Rheological properties of mixed gels made of microparticulated whey proteins and beta-lactoglobulin[END_REF]. However, time-dependent rheological properties of diluted biopolymer dispersions can also be caused by surface properties of macromolecules during rheological measurements. In this case, an equilibrium between surface and bulk rheological properties may occur. These features were clearly demonstrated with AG dispersions (Sanchez, Renard, et al., 2002).

Viscoelastic properties of AG dispersions were also characterized and revealed, as expected, a predominant liquid-like behavior [START_REF] Goycoolea | Solution rheology of mesquite gum in comparison with gum-arabic[END_REF][START_REF] Matsumura | Interaction of gum arabic, maltodextrin and pullulan with lipids in emulsions[END_REF]Sanchez, Renard, et al., 2002). Indeed, mechanical spectra obtained at 6wt% AG concentration by oscillatory testing revealed that the viscous or loss modulus (G", N.m -2 ) was higher than the elastic or storage modulus (G', N.m -2 ) throughout a wide frequency range but G' became larger than G" at the highest frequencies (Sanchez, Renard, et al., 2002). Similar behavior was recorded at 18wt% [START_REF] Matsumura | Interaction of gum arabic, maltodextrin and pullulan with lipids in emulsions[END_REF] or 50wt% [START_REF] Goycoolea | Solution rheology of mesquite gum in comparison with gum-arabic[END_REF] AG concentration. Interestingly, the evolution of G' and G" as a function of frequency followed a power law behavior with exponents of 1.4 and 0.8, respectively, smaller than the exponents 2 and 1 classically found for viscoelastic liquids (Sanchez, Renard, et al., 2002). It was then concluded that AG dispersions were structured liquids. Surface effects also have an impact on measured viscoelastic properties. Dynamic mechanical spectra after 120 min rest of AG samples at 6wt% gum concentration in the rheometer therefore showed a typical gel-like behavior with G' larger than G" over the entire range of selected frequencies (Sanchez, Renard, et al., 2002). The building-up with time of a predominantly elastic interfacial structure was demonstrated.

In summary, AG dispersions display newtonian flow behavior at gum concentrations below about 20wt% and shear-thinning above. However, in practical industrial situations where applied shear rates are usually above 100 s -1 , flow behavior of AG can be considered as newtonian. Sometimes, the measured flow behavior is non-newtonian and even thixotropic

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

at low AG concentrations. This unusual behavior seems to be mainly due to surface effects and reversible shear-induced aggregation of AG macromolecules. In this case, further studies are needed to clarify the situation, by using for instance rheology coupled to small angle xray or neutron scattering.

Assembly properties of Acacia gum

Molecular associations and assembly of biopolymers depend strongly on the extent of macromolecule-solvent and macromolecule-macromolecule interactions. following different mechanisms such as aggregation or coacervation (simple or complex).

Self-association and aggregation properties of Acacia gum

Aggregation of AG is apparently a natural mechanism depending on the physiology of trees, and particularly the ageing [START_REF] Idris | Characterisation of gum from Acacia senegal trees of different age and location using multidetection gel permeation chromatography[END_REF]. The characterization of AG harvested on trees of different ages, from 5 to 15 years old, showed both the increase in the mean radius of gyration of AG and the proportion of aggregates in solution. This aggregation mechanism occurs when the trees grow older up to about 15 years.

Aggregation of AG in aqueous solution was also evidenced in several studies using size exclusion chromatography (SEC) that showed an elution peak in the void volume of the column, in addition to the peaks corresponding to the three main fractions of AG [START_REF] Idris | Characterisation of gum from Acacia senegal trees of different age and location using multidetection gel permeation chromatography[END_REF][START_REF] Mukherjee | Influence of concentration on the properties of arabic solution[END_REF][START_REF] Ray | Functionality of gum-arabic -Fractionation, characterization and evaluation of gum fractions in citrus oil-emulsions and model beverages[END_REF]Sanchez, Renard, et al., 2002). This peak, which was removed from SEC chromatograms after filtration of AG solution on 0.45 µm filters, was attributed to the elution of aggregates [START_REF] Al-Assaf | Molecular associations in acacia gums[END_REF]. Association and aggregation properties were also highlighted over a large concentration range in AG using scattering and microscopy experiments. Using static light scattering measurements, Wang et al. showed the increase of R g from 20 to 50 nm on filtered AG samples as the concentration

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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25 of AG increased from 0.0413 to 5.21 g•L -1 , respectively (Q. [START_REF] Wang | Solution properties of conventional gum arabic and a matured gum arabic (Acacia (sen) SUPER GUM)[END_REF]. Aggregates were also evidenced in more concentrated AG solution, ranging from 5 to 300 g•L -1 , by SAXS and SANS measurements, and observed using cryo-TEM [START_REF] Dror | Structure of gum arabic in aqueous solution[END_REF]. All these experiments confirm the self-association behavior of AG in aqueous solution over a large range of concentration.

The apparent contradiction between the high solubility of AG in water and its propensity to self-associate could origin from the chemical composition of the molecular fractions isolated from AG. Indeed, as described above, AG is composed of arabinogalactan-protein type macromolecules [START_REF] Akiyama | Gum arabic is a kind of arabinogalactan protein[END_REF]. In plant kingdom, the association property of AGPs is well established: these macromolecules have specific functions in interaction and recognition cellular mechanism [START_REF] Showalter | Arabinogalactan-proteins: structure, expression and function[END_REF]. It is also well known that AGPs can selfassemble and aggregate both in vitro and in vivo [START_REF] Baldwin | A novel hydroxyproline-deficient arabinogalactan protein secreted by suspension-cultured cells of Daucus-Carota -Purification and partial characterization[END_REF] Capataz-Tafur, Trejo-Tapia, Rodriguez-Monroy, & Sepulveda-Jimenez, 2011). Hence, the selfassociation property of AG in solution is consistent with the adhesive nature of AGPs. Very recently, it was proved that the capacity of adventitious roots of English ivy (Hedera helix) to climb vertical surfaces was due to AGP assembled in nanospheres that were the key component of the high-strength adhesive secreted by this plant (Y. J. [START_REF] Huang | Nanospherical arabinogalactan proteins are a key component of the high-strength adhesive secreted by English ivy[END_REF].

Recently, studies devoted to the characterization of the three dimensional structure of isolated AG, AGP and GP fractions, also evidenced the self-assembly behavior of these isolated macromolecules (Renard, et al., 2012[START_REF] Renard | Structure of arabinogalactanprotein from Acacia gum: From porous ellipsoids to supramolecular architectures[END_REF][START_REF] Renard | Enzymatic hydrolysis studies of arabinogalactan-protein structure from Acacia gum: The self-similarity hypothesis of assembly from a common building block[END_REF][START_REF] Sanchez | The acacia gum arabinogalactan fraction is a thin oblate ellipsoid: A new model based on small-angle neutron scattering and ab initio calculation[END_REF]. These studies highlighted some differences between the three main fractions towards their affinity for aqueous solvent and of course their ability to selfassemble and aggregate in aqueous solution. The study focusing on GP fraction showed first of all that it was not easy to rehydrate GP powder. A significant proportion of GP macromolecules aggregated with the formation of a substantial undissolved material after centrifugation [START_REF] Ray | Functionality of gum-arabic -Fractionation, characterization and evaluation of gum fractions in citrus oil-emulsions and model beverages[END_REF][START_REF] Renard | Structure of arabinogalactanprotein from Acacia gum: From porous ellipsoids to supramolecular architectures[END_REF]. On the contrary, the rehydration of AGp and AGP powders were complete in aqueous solution without the formation of undissolved material (Renard, et al., 2012;[START_REF] Sanchez | The acacia gum arabinogalactan fraction is a thin oblate ellipsoid: A new model based on small-angle neutron scattering and ab initio calculation[END_REF]. Hence, GP fraction contains some macromolecules with a lowest affinity towards aqueous solvent compared to those included in AGp and AGP fractions, in agreement with the delayed elution of this fraction by HIC. The self-aggregation behavior of GP fraction during its rehydration was attributed to the Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008 more pronounced hydrophobic nature of the GP macromolecules [START_REF] Renard | Structure of arabinogalactanprotein from Acacia gum: From porous ellipsoids to supramolecular architectures[END_REF][START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. Transmission Electron Microscopy (TEM) experiments performed on each fraction also revealed that the self-aggregation behavior and the morphology of formed aggregates depend clearly on the fraction studied. It is likely that drying of samples for TEM experiments could impact the observed morphologies. Self-assembled or aggregated macromolecules were observed with AGP and GP fractions (Renard, et al., 2012[START_REF] Renard | Structure of arabinogalactanprotein from Acacia gum: From porous ellipsoids to supramolecular architectures[END_REF], but not with AGp fraction [START_REF] Sanchez | The acacia gum arabinogalactan fraction is a thin oblate ellipsoid: A new model based on small-angle neutron scattering and ab initio calculation[END_REF]. The aggregates evidenced with AGP and GP macromolecules in aqueous solution could be promoted by several intermolecular attractive weak forces such as hydrogen bond between saccharidic residues as suggested by [START_REF] Mahendran | New insights into the structural characteristics of the arabinogalactan -Protein (AGP) fraction of gum Arabic[END_REF] and/or hydrophobic interaction between polypeptide backbones as suggested by Renard et al. (2012).

Differences between AGp, AGP and GP fractions towards aggregation in aqueous solution could be explained by their differences in chemical composition and particularly their protein content and amino-acids composition. The protein content in AGp, AGP and GP fractions is therefore around 1.1%, 9% and 24.6%, respectively [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF] and GP fraction contains more hydrophobic amino-acids such as glycine, valine, isoleucine, leucine, phenylalanine than AGp and AGP fractions [START_REF] Renard | Acacia senegal gum: Continuum of molecular species differing by their protein to sugar ratio, molecular weight, and charges[END_REF]. Hence, both the high protein content and the presence of hydrophobic amino-acids in higher proportion in GP fractions could explain the highest sensitivity of GP macromolecules towards self-assembly and aggregation in aqueous solution.

After harvesting, AG can be submitted to several physical treatments such as heating, spraydrying or irradiation before to be used. These treatments can also influence the extent of the natural aggregation process of AG. Aggregation behavior is enhanced when AG is heated or Interestingly, aggregating AG through controlled Maillard reaction can improve its functional properties [START_REF] Al-Assaf | Characterization and properties of Acacia senegal (L.) Willd. var. senegal with enhanced properties (Acacia (sen) SUPER GUM (TM)): Part 1 -Controlled maturation of Acacia senegal var. senegal to increase viscoelasticity, produce a hydrogel form and convert a poor into a good emulsifier[END_REF]. Consequently, the natural functional properties of AG could be improved by its self-assembly properties.

Coacervation of Acacia gum

Coacervation has generally been defined as a liquid-liquid phase separation occurring in (bio)-polymers solutions under suitable conditions (Bungenberg de Jong, 1949b). It is well accepted that coacervation corresponds to a dehydration process of (bio)-polymers. The phase separation gives rise to two incompatible and immiscible liquid phases: a (bio)polymer dense phase, called the "coacervate" phase, coexisting with a very dilute colloidal phase, the supernatant [START_REF] Bamford | The theory of coacervation[END_REF]Bungenberg de Jong, 1949a;[START_REF] Menger | Anatomy of a coacervate[END_REF]. The dilute liquid phase remains in equilibrium with the coacervate phase. Depending on the number of (bio)-polymers involved in the phase separation, coacervation can be classified as simple or complex (Bungenberg de Jong, 1949a). In the following sections, we will discuss about the assembly of AG according to simple and complex coacervation mechanism.

Simple coacervation

Simple coacervation is a liquid-liquid phase separation where only one (bio)-polymer is involved (H.B. [START_REF] Bohidar | Coacervates: A novel state of soft matter -An overview[END_REF]Bungenberg de Jong, 1949a). It occurs as a result of a decrease in the solubility of (bio)-polymers through water competition caused by modifying the physical chemical properties of the solvent. In aqueous solutions, simple coacervation can be promoted by the action of salts (sodium sulphate, sodium chloride), the addition of a watermiscible non-solvent (ethanol, methanol, acetone, etc…), by modifying pH or increasing/decreasing temperature, thus turning the aqueous solvent medium, good for the (bio)-polymer, into a marginal one (H.B. [START_REF] Bohidar | Coacervates: A novel state of soft matter -An overview[END_REF]Bungenberg de Jong, 1949a;[START_REF] Ezpeleta | Gliadin nanoparticles for the controlled release of all-trans-retinoic acid[END_REF][START_REF] Mauguet | Gliadin matrices for microencapsulation processes by simple coacervation method[END_REF][START_REF] Mohanty | Systematic of alcohol-induced simple coacervation in aqueous gelatin solutions[END_REF].

The addition of ethanol to AG/water dispersion led to the formation of a new dense phase containing liquid droplets, called coacervates, dispersed in a continuous phase. AG macromolecules will tend to spontaneously self-associate according to a coacervation mechanism once a critical ethanol percentage has been reached. Whatever the AG Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008 concentration, the coacervation mechanism occurs for ethanol concentration below the precipitation point (unpublished data). Simple coacervation induces the formation of coacervated and diluted phases that both contain concentrated and diluted AG molecules, respectively. The binodal curve, delimiting the one-phase and two-phases regions, was determined by our group for A. senegal gum in water/ethanol solution (Figure 4, unpublished data). Upon the increase in ethanol percentage and AG concentration, the solubility/affinity of AG molecules for water/ethanol solution decreases.

In this ternary water/ethanol/AG system, ethanol acts as a suitable dehydrating agent which shifts the energetic balance in favor of the attraction between AG macromolecules [START_REF] Koets | Phase separation in aqueous colloidal systems[END_REF]. When ethanol is added, the quality of the solvent decreases, becoming a poor solvent for the solubility of AG macromolecules, by both modifying the hydrogen bond network and the polarity of the medium (H. B. [START_REF] Bohidar | Anomalous self-assembly of gelatin in ethanol-water marginal solvent[END_REF][START_REF] Mohanty | Systematic of alcohol-induced simple coacervation in aqueous gelatin solutions[END_REF]. In addition to the role of ethanol in the disturbance of hydrogen bonds network, ethanol also decreases the dielectric constant and consequently the polarity of the solvent that could favor the self-association of macromolecules [START_REF] Mohanty | Systematic of alcohol-induced simple coacervation in aqueous gelatin solutions[END_REF]. Consequently, induced coacervation of AG macromolecules by the addition of a non-solvent such as ethanol corresponds to a dehydration process due to the modifications of solvent physical chemical properties.

Complex coacervation

Complex coacervation is based on the ability of two oppositely charged (bio)-polymers to interact and associate involving a liquid-liquid phase separation with the formation of liquid [START_REF] Tiebackx | Gleichzeitige ausflockung zweier kolloide[END_REF].

AG is a weak polyelectrolyte negatively charged extensively used for the elaboration of complex coacervates by mixing it with positively charged proteins extracted from both animal (gelatin, bovine serum albumin, β-lactoglobulin, sodium caseinate, etc) and plant (wheat, pea, soybean and lentil proteins) kingdoms [START_REF] Aryee | Formation of electrostatic complexes involving mixtures of lentil protein isolates and gum Arabic polysaccharides[END_REF]Bungenberg de Jong, 1949a, 1949b;[START_REF] Burgess | Spontaneous formation of small-sized albumin Acacia coacervate particles[END_REF][START_REF] Dong | Charge compensation, phase diagram, and protein aggregation in soy protein-gum arabic complex formation[END_REF][START_REF] Ducel | Evidence and characterization of complex coacervates containing plant proteins: application to the microencapsulation of oil droplets[END_REF][START_REF] Liu | Effect of pH, Salt, and Biopolymer Ratio on the Formation of Pea Protein Isolate-Gum Arabic Complexes[END_REF][START_REF] Niu | Phase separation behavior and structural analysis of ovalbumin-gum arabic complex coacervation[END_REF][START_REF] Schmitt | Structure and technofunctional properties of protein-polysaccharide complexes: A review[END_REF][START_REF] Schmitt | Complex coacervation between betalactoglobulin and acacia gum in aqueous medium[END_REF][START_REF] Weinbreck | Complex coacervation of whey proteins and gum arabic[END_REF];

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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29 [START_REF] Ye | Formation of stable nanoparticles via electrostatic complexation between sodium caseinate and gum arabic[END_REF]. Several review papers well summarized the AG-protein complex coacervation mechanism describing the physico-chemical, structural and functional features of AG-protein complexes and coacervates (de Kruif, [START_REF] De Kruif | Complex coacervation of proteins and anionic polysaccharides[END_REF][START_REF] Kizilay | Complexation and coacervation of polyelectrolytes with oppositely charged colloids[END_REF][START_REF] Sanchez | Self-assembly of beta-lactoglobulin and acacia gum in aqueous solvent: Structure and phase-ordering kinetics[END_REF][START_REF] Schmitt | Structure and technofunctional properties of protein-polysaccharide complexes: A review[END_REF][START_REF] Schmitt | Protein/polysaccharide complexes and coacervates in food systems[END_REF][START_REF] Turgeon | Protein-polysaccharide interactions: phase-ordering kinetics, thermodynamic and structural aspects[END_REF] .

Complex coacervation between AG and proteins occurs by charge neutralization with the involvement of non-specific electrostatic interactions between de-protonated carboxyl groups of AG macromolecules and the protonated amino groups of proteins. As complex coacervation mechanism mainly occurs by the involvement of weak electrostatic interactions, this assembly mechanism is substantially influenced by the physical chemical properties of the solvent (pH, ionic strength, nature of salts and temperature) and the structural and physical chemical properties (global charge, charge distribution, flexibility and concentration) of biopolymers (polysaccharides and proteins) [START_REF] Schmitt | Structure and technofunctional properties of protein-polysaccharide complexes: A review[END_REF][START_REF] Ye | Complexation between milk proteins and polysaccharides via electrostatic interaction: principles and applications -a review[END_REF]. The identification of the specific conditions resulting in a two phase system, named phase diagram and where binodal curve is determined, is a tedious work that requires time and large quantities of raw material. Trying to overcome these major drawbacks, phase diagram of a β-lactoglobulin -AG mixture has been recently determined through an innovative miniaturized approach based on millifluidic (Amine et al., submitted). In this work authors proved that by using turbidity measurements based on image analysis within only 2 µl biopolymers droplets, binodal curve was able to be determined with a good agreement with those obtained in bulk. This method should find applications for the screening of numerous protein-polysaccharide mixtures for industrial issues.

Indeed, the formation of complex coacervates between AG and proteins is of industrial interest to value them, by enhancing their functional properties and developing novel biopolymer assemblies. One of the first and the most important industrial applications of complex coacervation is microencapsulation. Polysaccharide/protein microcapsules are used in many industries (food, pharmaceutical, cosmetics, agricultural, etc…) to entrap and protect sensitive molecules (aroma compounds, bioactives, drugs, enzymes) against processing (heat, shear, redox potential, etc…) and storage conditions (oxygen, temperature and moisture). The use of microcapsules is also a mean to deliver the encapsulated interest

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

molecules to the specific target with the optimal kinetic by changing the physical chemical conditions (pH), mechanical process (chewing) or enzymatic action [START_REF] Schmitt | Protein/polysaccharide complexes and coacervates in food systems[END_REF].

Several AG/protein (gelatin, whey proteins, gliadins, pea globulin) microcapsules were developed and elaborated to encapsulate different molecules such as lemon and orange flavors, oil, pesticides and flavonoid compounds [START_REF] Aberkane | Structuration mechanism of beta-lactoglobulin -acacia gum assemblies in presence of quercetin[END_REF][START_REF] Bosnea | Complex coacervation as a novel microencapsulation technique to improve viability of probiotics under different stresses[END_REF][START_REF] Ducel | Evidence and characterization of complex coacervates containing plant proteins: application to the microencapsulation of oil droplets[END_REF][START_REF] Eratte | Complex coacervation with whey protein isolate and gum arabic for the microencapsulation of omega-3 rich tuna oil[END_REF][START_REF] Gao | Controlled release effect of insecticide microcapsules and their results in common household insect pest control[END_REF] ). The preparation of microparticles by complex coacervation was similarly performed on AG/polysaccharide mixtures such as AG and chitosan [START_REF] Butstraen | Preparation of microcapsules by complex coacervation of gum Arabic and chitosan[END_REF].

The rheological properties of solutions could also be modified by the formation of complex coacervates between AG and proteins. The rheological properties of protein and polysaccharide in associative conditions result in different behaviors compare to each individual biopolymer. It is expected that the bulk viscosity of the system is improved with the formation of microstructures as complex coacervates [START_REF] Schmitt | Protein/polysaccharide complexes and coacervates in food systems[END_REF]. The characterization of the rheological properties of AG/whey protein coacervates evidenced that these assemblies displayed a viscous character [START_REF] Weinbreck | Rheological properties of whey protein/gum arabic coacervates[END_REF]. As electrostatic interactions mainly stabilize complex coacervates, protein/polysaccharide molar ratio, pH and ionic strength are key parameters for the rheological properties of complex coacervates.

The maximum of viscosity is obtained for mixing conditions leading to the complete charge neutralization of the two biopolymers. This viscous behavior was similarly evidenced on AG/Chitosan complex coacervates [START_REF] Espinosa-Andrews | Interrelationship between the zeta potential and viscoelastic properties in coacervates complexes[END_REF]. In their work, they highlighted an interrelationship between the biopolymers mass ratio, the charge density and the viscoelastic properties of the coacervated phase.

The complex coacervates formed between AG and proteins gather the surface properties of each biopolymer. Hence, complex coacervates can also be used to stabilize air/water or oil/water interfaces in a variety of foamed and emulsified products [START_REF] Dickinson | Interfacial structure and stability of food emulsions as affected by proteinpolysaccharide interactions[END_REF].

Schmitt et al. reported that the surface activity of AG/-lactoglobulin complexes formed at

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008 charge neutralization ratio (pH 4.2) was as high as the pure adsorbed protein in the same condition [START_REF] Schmitt | Effect of time on the interfacial and foaming properties of beta-lactoglobulin/acacia gum electrostatic complexes and coacervates at pH 4.2[END_REF]. However, the viscoelastic properties of the film formed by AG/-lactoglobulin complexes were stronger than those of the pure protein and the gas permeability of the film was significantly reduced compare to pure -lactoglobulin. Similar results were obtained in chilled dairy foams using whey protein isolate/AG complexes [START_REF] Schmitt | Protein -polysaccharide complexes from basics to food applications[END_REF]. Consequently, the stability of the foam can be improved by the adsorption of AG/protein complexes at the air bubble interfaces. Emulsions can also be stabilized by AG/protein complexes or coacervates. Ducel et al. evidenced that AG/pea globulin and AG/α-gliadin complexes or coacervates tend to decrease more strongly the oil/water interfacial tension than the pure protein [START_REF] Ducel | Plant protein-polysaccharide interactions in solutions: application of soft particle analysis and light scattering measurements[END_REF].

The coacervates films were characterized by a long relaxation time and a high surface elasticity. In addition, the authors reported that charged complex coacervates were more efficient to stabilize oil droplets.

Surface properties: adsorption at solid-liquid and liquid-liquid interfaces

Surface properties of AG, and of a number of plant gum exudates, are unique in the polysaccharide world. By surface properties, we mean both the ability of AG to decrease interfacial tension between gas-water, liquid-liquid or solid-liquid interfaces, and to stabilize these interfaces through steric and electrostatic interactions and hydration forces [START_REF] Adamson | Physical-Chemistry of Surfaces[END_REF]. These properties can be used to form and stabilize foams [START_REF] Redgwell | Hydrocolloids from coffee: physicochemical and functional properties of an arabinogalactan-protein fraction from green beans[END_REF], emulsions and to stabilize solid nanoparticles. It can be noticed that studies on foaming properties of AG are rare as compared to studies on stabilization of liquid or solid particles, especially nanoparticles.

Surface properties: adsorption at solid-liquid interfaces

Surface properties of Acacia senegal gum can be used to stabilize solid nanoparticles such as carbon nanotubes [START_REF] Amiri | Highly Dispersed Multiwalled Carbon Nanotubes Decorated with Ag Nanoparticles in Water and Experimental Investigation of the Thermophysical Properties[END_REF][START_REF] Bandyopadhyaya | Stabilization of individual carbon nanotubes in aqueous solutions[END_REF][START_REF] Dror | Conformation of polymers dispersing singlewalled carbon nanotubes in water: A small-angle neutron scattering study[END_REF][START_REF] Edri | Cryo-staining techniques in cryo-TEM studies of dispersed nanotubes[END_REF]; M. T. [START_REF] Kim | Carbon Nanotube Modification Using Gum Arabic and Its Effect on the Dispersion and Tensile Properties of Carbon Nanotubes/Epoxy Nanocomposites[END_REF][START_REF] Kumar | Exfoliated single-walled carbon nanotubebased hydrogen sensor[END_REF][START_REF] Najeeb | The effect of surface modifications of carbon nanotubes on the electrical properties of inkjet-printed SWNT/PEDOT-PSS composite line patterns[END_REF] , gold, silver, magnetic or bioceramic nanoparticles [START_REF] Batalha | Gum Arabic coated magnetic nanoparticles with affinity ligands specific for antibodies[END_REF][START_REF] Gils | Designing of silver nanoparticles in gum arabic based semi-IPN hydrogel[END_REF][START_REF] Kannan | Functionalized radioactive gold nanoparticles in tumor therapy[END_REF][START_REF] Kattumuri | Gum arabic as a phytochemical construct for the stabilization of gold nanoparticles: In vivo pharmacokinetics and X-raycontrast-imaging studies[END_REF][START_REF] Ma | Size-Controlled Dissolution of Organic-Coated Silver Nanoparticles[END_REF][START_REF] Roque | Adsorption of gum Arabic on bioceramic nanoparticles[END_REF][START_REF] Song | Hydrophobic Interactions Increase Attachment of Gum Arabic-and PVP-Coated Ag Nanoparticles to Hydrophobic Surfaces[END_REF] ; and flavonoids nanoparticles [START_REF] Aberkane | Structuration mechanism of beta-lactoglobulin -acacia gum assemblies in presence of quercetin[END_REF]).

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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32

Obviously, the ability of AG to stabilize solid interfaces was at the basis of ink and paint manufacturing [START_REF] Balantrapu | Silver nanoparticles for printable electronics and biological applications[END_REF][START_REF] Song | Preparation and properties of electrophoretic microcapsules for electronic paper[END_REF][START_REF] Wang | Microencapsulated electric ink using gelatin/gum arabic[END_REF] .

AG was also used to stabilize latex nanoparticles as a model system of interface with a surface coverage of about 0.5 -5 mg/m 2 depending on solvent conditions and initial AG concentration [START_REF] Gashua | Molecular characteristics, association and interfacial properties of gum Arabic harvested from both Acacia senegal and Acacia seyal[END_REF][START_REF] Snowden | Functional characteristics of gum arabic[END_REF]. The surface coverage was found to be similar at liquid-liquid interfaces in O/W emulsions [START_REF] Randall | The role of the proteinaceous component on the emulsifying properties of gum arabic[END_REF] . The high M w fraction of AG, AGP, was found to be the most effective to be adsorbed at the interface after only 15 min while AGp fraction was ineffective in the stabilization of the latex dispersions (M. L. [START_REF] Snowden | Functional characteristics of gum arabic[END_REF].

Electronic spin resonance data indicated that AG adsorbed at the solid-liquid interface with approximately half of its segments close to the surface in trains and the other half in loops and tails extending away from the surface into solution (M. L. [START_REF] Snowden | Functional characteristics of gum arabic[END_REF]). An alternative model of end-on or multilayer adsorption was recently proposed to explain the high layer thickness after adsorption of AG on latex particles [START_REF] Gashua | Molecular characteristics, association and interfacial properties of gum Arabic harvested from both Acacia senegal and Acacia seyal[END_REF]. As noted above, AG has also been explored as coating agent of nanomaterials for biomedical applications, namely magnetic nanoparticles [START_REF] Ali | Biological effects of gum arabic: A review of some recent research[END_REF][START_REF] Banerjee | Multifunctional pH-sensitive magnetic nanoparticles for simultaneous imaging, sensing and targeted intracellular anticancer drug delivery[END_REF], 2010;[START_REF] Palma | Covalent coupling of gum arabic onto superparamagnetic iron oxide 57 nanoparticles for MRI cell labeling: physicochemical and in vitro characterization[END_REF] . AG coupled magnetic nanosystem could therefore find applications as a MRI contrast agent for cell-labeling applications. Other applications concerned the use of AG as a nontoxic material in the production of readily administrable biocompatible gold nanoparticles for diagnostic and therapeutic applications in nanomedicine [START_REF] Kattumuri | Gum arabic as a phytochemical construct for the stabilization of gold nanoparticles: In vivo pharmacokinetics and X-raycontrast-imaging studies[END_REF] (Kattumuri, Katti, Bhaskaran, Boote, Casteel, Fent, Roberton, Chandrasekhar, Kannan & Katti, 2007) or to improve antioxydant properties of nanoparticles [START_REF] Kong | Synthesis and antioxidant properties of gum arabic-stabilized selenium nanoparticles[END_REF]. Surprisingly, studies of the adsorption of AG on 2D solid surfaces and their related interfacial properties have never been performed to the best of our knowledge while novel and efficient techniques such as ellipsometry coupled or not to quartz crystal microbalance exist today. In addition, there are no information in the literature on the testing of AG adhesion or any appropriate standard adhesion method used for a similar material. Future prospects in this important area of adhesion properties on solid surfaces could be very interesting and useful to unravel the role and function of AG at solidliquid and liquid-interfaces.

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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Surface properties: adsorption at liquid-liquid interfaces

A great number of studies dealt with emulsification properties of AG [START_REF] Bai | Fabrication of oil-in-water nanoemulsions by dual-channel microfluidization using natural emulsifiers: Saponins, phospholipids, proteins, and polysaccharides[END_REF][START_REF] Briggs | Experiments on emulsions. II. Emulsions of water and benzene[END_REF]R.A. Buffo & Reineccius, 2000;[START_REF] Buffo | Factors affecting the emulsifying and rheological properties of gum acacia in beverage emulsions[END_REF][START_REF] Castel | Droplet size distribution, rheological behavior and stability of corn oil emulsions stabilized by a novel hydrocolloid (Brea gum) compared with gum arabic[END_REF][START_REF] Castellani | Hydrocolloids with emulsifying capacity. Part 1-Emulsifying properties and interfacial characteristics of conventional (Acacia senegal (L.) Willd. var. senegal) and matured (Acacia (sen) SUPER GUM (TM)) Acacia senegal[END_REF][START_REF] Charoen | Influence of interfacial composition on oxidative stability of oil-in-water emulsions stabilized by biopolymer emulsifiers[END_REF][START_REF] Charoen | Influence of Biopolymer Emulsifier Type on Formation and Stability of Rice Bran Oil-in-Water Emulsions: Whey Protein, Gum Arabic, and Modified Starch[END_REF][START_REF] Chivero | Assessment of soy soluble polysaccharide, gum arabic and OSA-Starch as emulsifiers for mayonnaise-like emulsions[END_REF][START_REF] Desplanques | Impact of chemical composition of xanthan and acacia gums on the emulsification and stability of oil-in-water emulsions[END_REF][START_REF] Dickinson | The role of hydrocolloids in stabilising particulate dispersions and emulsions[END_REF][START_REF] Dickinson | Hydrocolloids at interfaces and the influence on the properties of dispersed systems[END_REF][START_REF] Dickinson | On the film-forming and emulsion-stabilizing properties of gum arabic: dilution and flocculation aspects[END_REF]Dickinson, Galazka, & Anderson, 1991a, 1991b;[START_REF] Dickinson | Surface activity and emulsifying behaviour of some Acacia gums[END_REF][START_REF] Djordjevic | Chemical and physical stability of protein-and gum arabic-stabilized oil-in-water emulsions containing limonene[END_REF][START_REF] Dluzewska | Effect of homogenisation pressure and some hydrocolloids on beverage emulsion stability[END_REF][START_REF] Gashua | Molecular characteristics, association and interfacial properties of gum Arabic harvested from both Acacia senegal and Acacia seyal[END_REF][START_REF] Gharibzahedi | Optimization and characterization of walnut beverage emulsions in relation to their composition and structure[END_REF][START_REF] Huang | Hydrocolloids in emulsions: particle size distribution and interfacial activity[END_REF][START_REF] Jayme | Zeta potentials of gum arabic stabilised oil in water emulsions[END_REF][START_REF] Jin | Comparative viscoelasticity studies: Corn fiber gum versus commercial polysaccharide emulsifiers in bulk and at air/liquid interfaces[END_REF][START_REF] Kim | Microencapsulation properties of gum Arabic and several food proteins: Liquid orange oil emulsion particles[END_REF][START_REF] Klein | Enhanced stabilization of cloudy emulsions with gum Arabic and whey protein isolate[END_REF][START_REF] Ma | Size-Controlled Dissolution of Organic-Coated Silver Nanoparticles[END_REF][START_REF] Mahfoudhi | Assessment of emulsifying ability of almond gum in comparison with gum arabic using response surface methodology[END_REF][START_REF] Matsumura | Interaction of gum arabic, maltodextrin and pullulan with lipids in emulsions[END_REF][START_REF] Mcnamee | Emulsification and microencapsulation properties of gum arabic[END_REF][START_REF] Mirhosseini | Discrimination of orange beverage emulsions with different formulations using multivariate analysis[END_REF][START_REF] Nakamura | Stabilization of emulsion by natural polymer. Study of gum Arabic[END_REF][START_REF] Nakauma | Comparison of sugar beet pectin, soybean soluble polysaccharide, and gum arabic as food emulsifiers. 1. Effect of concentration, pH, and salts on the emulsifying properties[END_REF][START_REF] Ozturk | Formation and stabilization of nanoemulsion-based vitamin E delivery systems using natural biopolymers: Whey protein isolate and gum arabic[END_REF][START_REF] Padala | Adsorption of Gum Arabic, Egg White Protein, and Their Mixtures at the Oil-Water Interface in Limonene Oil-in-Water Emulsions[END_REF][START_REF] Piorkowski | Beverage emulsions: Recent developments in formulation, production, and applications[END_REF][START_REF] Pomet | Histoire générale des drogues simples et composées, renfermant dans les 3 classes des végétaux, des animaux et des minéraux tout ce qui est l'objet de la physique, de la chimie, de la pharmacie, & des arts les plus utiles à la société des hommes[END_REF][START_REF] Ray | Functionality of gum-arabic -Fractionation, characterization and evaluation of gum fractions in citrus oil-emulsions and model beverages[END_REF][START_REF] Reiner | A Comparison of the Stability of Beverage Cloud Emulsions Formulated with Different Gum Acacia-and Starch-Based Emulsifiers[END_REF][START_REF] Seifriz | Studies in emulsions. VI. The effect of acidity on type and reversibility of emulsions[END_REF][START_REF] Shi | Characterization and emulsifying properties of octenyl succinate anhydride modified Acacia seyal gum (gum arabic)[END_REF][START_REF] Shotton | Effect of emulsifier concentration on rheological properties of Acacia emulsions[END_REF][START_REF] Shotton | The emulsifying properties of gum Acacia[END_REF]; M. J. [START_REF] Snowden | Adsorption and stabilising properties of gum arabic samples of different origin[END_REF][START_REF] Snowden | Functional characteristics of gum arabic[END_REF][START_REF] Vasile | Physicochemical, interfacial and emulsifying properties of a non-conventional exudate gum (Prosopis alba) in comparison with gum arabic[END_REF][START_REF] Vernon-Carter | Color degradation and coalescence kinetics of Aztec marigold oleoresin-in-water emulsions stabilized by mesquite or Arabic gums and their blends[END_REF][START_REF] Vernon-Carter | Stability of capsicum annuum of oleoresin-in-water emulsions containing prosopis and Acacia gums[END_REF][START_REF] Xiang | Gum Arabic-stabilized conjugated linoleic acid emulsions: Emulsion properties in relation to interfacial adsorption behaviors[END_REF][START_REF] Yadav | Chemical investigation of the structural basis of the emulsifying activity of gum arabic[END_REF][START_REF] Yao | Effect of Gum Arabic, Gum Ghatti and Sugar Beet Pectin as Interfacial Layer on Lipid Digestibility in Oil-in-Water Emulsions[END_REF][START_REF] Yao | Physical and Chemical Stability of Gum Arabic-Stabilized Conjugated Linoleic Acid Oil-in-Water Emulsions[END_REF][START_REF] Zhang | Effect of Arabic Gum and Xanthan Gum on the Stability of Pesticide in Water Emulsion[END_REF]. The listing is far from being exhaustive but shows clearly the abundant literature on the subject. We know that physical chemical parameters (pH, ionic strength, type of ions, temperature, homogenization pressure, etc.) all influence the structure of biopolymer-stabilized emulsions and their stability. Here we wish to focus on the composition and structural aspects at the expected origin of surface properties of the gum.

It is today widely accepted that it is the protein-rich high-molecular weight fraction of AG, the AGP complex, which mainly provides the surface properties of gum. A fair non-linear relationship between AGP concentration and emulsion stability was recently shown with a seemingly AGP critical concentration of about 10% [START_REF] Nishino | Effect of AGP on emulsifying properties of gum Arabic[END_REF]. Here emulsion stability was checked following heating at 60°C for 3 weeks and we may wonder whether surface properties of AGP were highlighted or heat-induced

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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34 aggregation of AGP at the interfaces, in line with the process of "Super Gum" formation described below [START_REF] Al-Assaf | Characterization and properties of Acacia senegal (L.) Willd. var. senegal with enhanced properties (Acacia (sen) SUPER GUM (TM)): Part 1 -Controlled maturation of Acacia senegal var. senegal to increase viscoelasticity, produce a hydrogel form and convert a poor into a good emulsifier[END_REF][START_REF] Aoki | Characterization and properties of Acacia senegal (L.) Willd. var. senegal with enhanced properties (Acacia (sen) SUPER GUM (TM)): Part 2 -Mechanism of the maturation process[END_REF]Aoki, Katayama, et al., 2007). We remember here that "AGP" molecular fraction both encompasses the so-called AGP and the high M w GP fractions as well as minor concentrations of the AGp fraction. One direct consequence is that companies using gum in their products, for instance to make stable oil-in-water (o/w) emulsions, want to obtain gum samples with high proportions of AGP, e.g. above 12%. This explains also why modified gum with higher content in high-molecular weight fractions was developed recently through controlled Maillard reaction [START_REF] Al-Assaf | Characterization and properties of Acacia senegal (L.) Willd. var. senegal with enhanced properties (Acacia (sen) SUPER GUM (TM)): Part 1 -Controlled maturation of Acacia senegal var. senegal to increase viscoelasticity, produce a hydrogel form and convert a poor into a good emulsifier[END_REF]. The modified gum contains about 20% of AGP and displays better o/w emulsion stabilizing properties than unmodified gum, reinforcing the current opinion on the role of AGP on gum surface properties. However, the reality seems a bit more complex since AG samples containing small amounts of AGP, e.g. 8%, can produce sometimes o/w emulsions with better stability than gums with higher levels of AGP. A detailed analysis of experimental facts reported in literature can then be useful to clarify the effect of gum molecular composition on its surface properties. The discussion will

benefit from what we know on the surface properties of individual molecular fractions (AGp, AGP and GP).

Despite its good surface properties, AG is far from being as efficient as proteins to form and stabilize oil-in-water emulsions. An oil to emulsifier ratio of about 1:1 is therefore needed for AG while a lower ratio of 1:10 is common for proteins. It is then not surprising that proteinrich macromolecules play an important role in the emulsifying/stabilizing properties of AGs [START_REF] Randall | The role of the proteinaceous component on the emulsifying properties of gum arabic[END_REF][START_REF] Ray | Functionality of gum-arabic -Fractionation, characterization and evaluation of gum fractions in citrus oil-emulsions and model beverages[END_REF]. It was then reported with samples of various AG species, having nitrogen contents in the range from 0.1% to 7.5%, a good correlation between the nitrogen content of the gum and its limiting long-time interfacial tension and between the emulsifying capacity and the initial rate of change of tension with time (Dickinson, et al., 1988). It was also shown by SEC on supernatants after removing oil droplets that protein-rich fractions adsorb strongly at the oil-water interface [START_REF] Randall | The role of the proteinaceous component on the emulsifying properties of gum arabic[END_REF]. Similar results using the same methodological approach were obtained by others [START_REF] Alftren | Comparison of molecular and emulsifying properties of gum arabic and mesquite gum using asymmetrical flow field-flow fractionation[END_REF][START_REF] Flindt | Studies on acacia exudate gums. Part V. Structural features of Acacia seyal[END_REF][START_REF] Padala | Adsorption of Gum Arabic, Egg White Protein, and Their Mixtures at the Oil-Water Interface in Limonene Oil-in-Water Emulsions[END_REF] . However, a careful examination of results reveal that while a preferential adsorption onto oil-water interfaces of protein-rich fractions occurred, all

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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35 molecular fractions seems to be present at the interfaces with however different adsorption kinetics. In the frame of the wattle-blossom model, it is supposed that, in analogy with block co-polymers, the more hydrophobic protein chain anchors at the interface while the protruding hydrophilic carbohydrate blocks attached to this chain provide a strong steric barrier towards flocculation and coalescence [START_REF] Islam | A review of recent developments on the regulatory, structural and functional aspects of gum arabic[END_REF][START_REF] Randall | Fractionation and characterization of gum from Acacia senegal[END_REF].

However, numerous hydrophilic hydroxyamino acid residues are present in the polypeptide chain, and no structural data today exist about the spatial position (buried or in periphery) of the polypeptide chain, casting some doubt on the model proposed by Islam et al. and Randall et al. In fact, the efficiency of AG is better related to the way it adsorbs onto interfaces and the mechanical properties provided by interfacial films rather than to a low interfacial tension [START_REF] Shotton | Interfacial properties of solutions of salts and arabic acid[END_REF]. Surface charged groups provide the basis for some electrostatic contribution to the colloidal stabilization, however the relative low value of the (negative) zeta potential, 10-20 mV under beverage emulsion conditions [START_REF] Jayme | Zeta potentials of gum arabic stabilised oil in water emulsions[END_REF][START_REF] Ray | Functionality of gum-arabic -Fractionation, characterization and evaluation of gum fractions in citrus oil-emulsions and model beverages[END_REF] , and the pH-independent destabilization mechanism (coalescence),

indicates that the primary mechanism is steric stabilization [START_REF] Dickinson | Hydrocolloids at interfaces and the influence on the properties of dispersed systems[END_REF][START_REF] Trindade | Aging mechanisms of oil-in-water emulsions based on a bioemulsifier produced by Comment citer ce document[END_REF]. It is better to say electro-steric stabilization mechanism as suggested by Jayme et al.

(1999) as both minerals (R. A. [START_REF] Buffo | Factors affecting the emulsifying and rheological properties of gum acacia in beverage emulsions[END_REF] and low pH (R. A. [START_REF] Buffo | Factors affecting the emulsifying and rheological properties of gum acacia in beverage emulsions[END_REF][START_REF] Djordjevic | Chemical and physical stability of protein-and gum arabic-stabilized oil-in-water emulsions containing limonene[END_REF][START_REF] Vernon-Carter | Stability of capsicum annuum of oleoresin-in-water emulsions containing prosopis and Acacia gums[END_REF] are unfavorable to emulsion stability.

Another very important parameter defining emulsion stability is interfacial rheology. The ability of AG to form highly cohesive viscoelastic interfacial multilayer (gel-like) films is not new [START_REF] Briggs | Experiments on emulsions. II. Emulsions of water and benzene[END_REF][START_REF] Serrallach | Strength of emulsifier films at liquid-liquid interfaces[END_REF][START_REF] Shotton | The interfacial tension of solutions of arabic acid[END_REF][START_REF] Shotton | Interfacial films between benzene and solutions of salts of arabic acid[END_REF][START_REF] Shotton | Interfacial properties of solutions of salts and arabic acid[END_REF][START_REF] Wibberley | Some physical properties of interfacial films of potassium arabate[END_REF]. AG interfaces are jammed, very strongly shear-elastic and exhibit non-linear interfacial shear rheology indicative of 2D soft solid behavior [START_REF] Erni | Interfacial viscoelasticity controls buckling, wrinkling and arrest in emulsion drops undergoing mass transfer[END_REF]Erni & Parker, 2012;[START_REF] Erni | Interfacial rheology of surface-active blopolymers: Acacia senegal gum versus hydrophobically modifed starch[END_REF].

Electro-steric forces, probably including hydration forces, and elastic interfaces are at the origin of emulsion stabilization by AG, especially against coalescence (Dickinson, et al., 1988).

Since the presence of proteins and steric effects are important in the formation and stabilization of emulsions, structural modifications of Acacia senegal gum was proposed as a mean to convert it into a good emulsifier [START_REF] Al-Assaf | Characterization and properties of Acacia senegal (L.) Willd. var. senegal with enhanced properties (Acacia (sen) SUPER GUM (TM)): Part 1 -Controlled maturation of Acacia senegal var. senegal to increase viscoelasticity, produce a hydrogel form and convert a poor into a good emulsifier[END_REF]. Structural modifications

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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36 were induced through controlled Maillard reaction incubating kibbled gum many weeks at 60°C under controlled moisture mimicking natural maturation process according to the authors (Al-Assaf, [START_REF] Al-Assaf | Modified gum Arabic[END_REF]. This process resulted in an increase of M w of AG from about 4.2x10 5 g.mol -1 up to about 20x10 5 g.mol -1 (Al- Assaf, et al., 2007).

Modified gums allow the production of oil-in-water emulsions with smaller droplets, improved interfacial viscoelasticity and emulsion stability as compared to the unmodified control gum. The effect of AG molecular weight (M w ) on emulsion stability (Dickinson, et al., 1991a) or surface rheological properties [START_REF] Nakamura | Stabilization of emulsion by natural polymer. Study of gum Arabic[END_REF]) was previously demonstrated. In this case, the initial size of emulsion droplets was not impacted by M w but emulsion stability was improved with high M w . Similar results using matured gums were obtained in other studies (Aoki, Katayama, et al., 2007;Castellani, Al-Assaf, Axelos, Phillips, & Anton, 2010;[START_REF] Castellani | Hydrocolloids with emulsifying capacity. Part 3-Adsorption and structural properties at the air-water surface[END_REF][START_REF] Yao | Physical and Chemical Stability of Gum Arabic-Stabilized Conjugated Linoleic Acid Oil-in-Water Emulsions[END_REF]. In addition, it was shown that modified gums have the ability to decrease interfacial tension faster than unmodified gums, which is an important parameter in determining emulsifying capacity as noted above (Aoki, Katayama, et al., 2007;Castellani, Al-Assaf, et al., 2010;[START_REF] Castellani | Hydrocolloids with emulsifying capacity. Part 1-Emulsifying properties and interfacial characteristics of conventional (Acacia senegal (L.) Willd. var. senegal) and matured (Acacia (sen) SUPER GUM (TM)) Acacia senegal[END_REF][START_REF] Yao | Physical and Chemical Stability of Gum Arabic-Stabilized Conjugated Linoleic Acid Oil-in-Water Emulsions[END_REF]. The improved stability of produced emulsions could be due partly to the higher viscoelasticity of interfaces formed from matured gums (Castellani, Al-Assaf, et al., 2010;[START_REF] Castellani | Hydrocolloids with emulsifying capacity. Part 1-Emulsifying properties and interfacial characteristics of conventional (Acacia senegal (L.) Willd. var. senegal) and matured (Acacia (sen) SUPER GUM (TM)) Acacia senegal[END_REF]. It is interesting to note that surface properties of matured gums are very close to that of the AGP fraction as obtained by HIC chromatography [START_REF] Castellani | Hydrocolloids with emulsifying capacity. Part 3-Adsorption and structural properties at the air-water surface[END_REF].

Although these results using matured AGs seem conclusive on the interfacial properties of high M w protein-rich macromolecules, some comments may be of interest. The first remark is that few studies have been done to definitely conclude on the benefit of matured gums regarding initial emulsion droplet size and emulsion stability. In addition, emulsions have never been prepared under the same experimental conditions (homogenizing conditions, gum concentration, oil to emulsifier ratio, dispersion pH, etc…), rendering the comparison of results difficult. For instance, it was shown that initial emulsion droplet size was significantly lower for oil to emulsifier ratios above 1 but was not different for ratios below 1 [START_REF] Kateyama | Estimation of concentration and performance of AGPs in emulsion systems using gum Arabic[END_REF]. Another concern deals with the nature of supramolecular structures produced by Maillard reaction. Not only the AGP M w (and their concentration) increases but the composition of supramolecular structures change too with the incorporation of AGp and low

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

M w GP fractions (Aoki, Al-Assaf, et al., 2007). Obviously it can be anticipated that the global architecture of supramolecular structures is also modified. In these conditions, how to compare matured high M w macromolecules with a smaller AGP fraction of control gum?

Another approach to demonstrate the role of protein-rich high M w macromolecules in AG surface properties was to hydrolyze it with protease-type enzymes. In this case, loss of emulsification properties was observed [START_REF] Chikamai | Processing of gum arabic and some new opportunities[END_REF].

Acacia senegal gum, hydrolyzed by pronase (a mixture of proteases) for 24h at 37 °C, did therefore form interfacial films less viscoelastic than with native AG [START_REF] Elmanan | Studies on Acacia exudate gums: Part VI. Interfacial rheology of Acacia senegal and Acacia seyal[END_REF], in relation with the significant decrease of protein-rich high M w AGP concentration (Aoki, Al-Assaf, et al., 2007;[START_REF] Connolly | Heterogeneity and homogeneity of an arabinogalactan-protein: Acacia senegal gum[END_REF][START_REF] Connolly | Effect of a proteinase on the macromolecular distribution of Acacia-Senegal gum[END_REF][START_REF] Elmanan | Studies on Acacia exudate gums: Part VI. Interfacial rheology of Acacia senegal and Acacia seyal[END_REF][START_REF] Randall | The role of the proteinaceous component on the emulsifying properties of gum arabic[END_REF].

However, by hydrolyzing AG while measuring interfacial viscoelasticity showed that, after 510 min hydrolysis, interfacial viscoelasticity decreased but remained high. It is possible that hydrolyzing the gum in the bulk or at an interface did not produce the same results. One can note that high M w macromolecules present in Acacia seyal gum are also hydrolyzed by pronase but to a lesser extent, which can be partly due to its more compact conformation [START_REF] Elmanan | Studies on Acacia exudate gums: Part VI. Interfacial rheology of Acacia senegal and Acacia seyal[END_REF][START_REF] Flindt | Studies on acacia exudate gums. Part V. Structural features of Acacia seyal[END_REF]. As AGP architecture is modified after hydrolysis, one may wonder whether the loss in surface properties is due to a decrease in M w or to a change in macromolecular conformation. In fact, both chemical structure of the polysaccharide component [START_REF] Connolly | Heterogeneity and homogeneity of an arabinogalactan-protein: Acacia senegal gum[END_REF][START_REF] Mahendran | New insights into the structural characteristics of the arabinogalactan -Protein (AGP) fraction of gum Arabic[END_REF][START_REF] Randall | The role of the proteinaceous component on the emulsifying properties of gum arabic[END_REF] and AGP conformation remain largely unaffected by proteases, suggesting a selfsimilar structure for the AGP component [START_REF] Renard | Enzymatic hydrolysis studies of arabinogalactan-protein structure from Acacia gum: The self-similarity hypothesis of assembly from a common building block[END_REF].

Owing to the expected surface properties of protein-rich molecular fractions of AG, some scarce studies tried to unravel the specific properties of individual macromolecular components. Unlike one study where four fractions were obtained by SEC [START_REF] Ray | Functionality of gum-arabic -Fractionation, characterization and evaluation of gum fractions in citrus oil-emulsions and model beverages[END_REF], the remaining studies were classically concerned by HIC fractions, i.e. AGp, AGP and GP (Castellani, Al-Assaf, et al., 2010;[START_REF] Castellani | Hydrocolloids with emulsifying capacity. Part 3-Adsorption and structural properties at the air-water surface[END_REF][START_REF] Fauconnier | Characterization of two Acacia gums and their fractions using a Langmuir film balance[END_REF][START_REF] Lopez-Franco | Macromolecular dimensions and mechanical properties of monolayer films of Sonorean mesquite gum[END_REF][START_REF] Ray | Functionality of gum-arabic -Fractionation, characterization and evaluation of gum fractions in citrus oil-emulsions and model beverages[END_REF]. Among these studies, one of them studied the effect of fractions on oil-in-water emulsions characteristics [START_REF] Ray | Functionality of gum-arabic -Fractionation, characterization and evaluation of gum fractions in citrus oil-emulsions and model beverages[END_REF] and three studies focused on interfacial properties of fractions as determined by interfacial tension measurements and Langmuir-Blodgett films (Castellani, Al-Assaf, et al., 2010;[START_REF] Castellani | Hydrocolloids with emulsifying capacity. Part 3-Adsorption and structural properties at the air-water surface[END_REF][START_REF] Fauconnier | Characterization of two Acacia gums and their fractions using a Langmuir film balance[END_REF][START_REF] Lopez-Franco | Macromolecular dimensions and mechanical properties of monolayer films of Sonorean mesquite gum[END_REF]. It was clear from

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Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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38 these studies that GP fraction was the more efficient to decrease interfacial tension with values at equilibrium of about 23 mN.m -1 as compared to values of 30 mN.m -1 for AGP and about 45 mN.m -1 for AGp (Castellani, Al-Assaf, et al., 2010). It is important to note that experiments were done at 0.05wt% gum concentration and that similar experiments at higher gum concentration should be instructive. In terms of surface pressure and limiting area, GP fraction was again the more efficient and AGp fraction the less efficient [START_REF] Castellani | Hydrocolloids with emulsifying capacity. Part 3-Adsorption and structural properties at the air-water surface[END_REF][START_REF] Fauconnier | Characterization of two Acacia gums and their fractions using a Langmuir film balance[END_REF][START_REF] Lopez-Franco | Macromolecular dimensions and mechanical properties of monolayer films of Sonorean mesquite gum[END_REF]. Results were unclear in terms of surface elasticity as determined from Langmuir-Blodgett film experiments. One study showed that the whole gum produced more elastic films than fractions which formed films with similar elasticity [START_REF] Lopez-Franco | Macromolecular dimensions and mechanical properties of monolayer films of Sonorean mesquite gum[END_REF]. Another study showed that films from AGp fractions were more elastic than films made from AGP or GP fractions, the latter being the less elastic [START_REF] Fauconnier | Characterization of two Acacia gums and their fractions using a Langmuir film balance[END_REF]. These results were obtained using the same Acacia senegal gum concentration (10wt%) but different subphase composition, which may explain in part the observed differences.

Although there is no doubt about the important surface activity of AG protein-rich molecular fractions, and of the important role of high M w fractions in the stabilization of emulsions, nitrogen content alone cannot be used to predict performance of AGs for emulsification.

Gums with similar protein content may exhibit significant differences in emulsifying capacity and emulsion stability (Dickinson, et al., 1991b), confirming the view that it is the nature and distribution of the proteinaceous component of the gum which is important, not just its overall amount. Gums with higher protein content do not also necessarily produce more stable emulsions and some Acacia seyal gum samples with a much lower protein content (0.8%) have been found to give better emulsion stability than Acacia senegal gum (R. A. [START_REF] Buffo | Factors affecting the emulsifying and rheological properties of gum acacia in beverage emulsions[END_REF]. More surprisingly, it was shown recently that high M w arabinogalactan macromolecules extracted from Peach exudate and not containing protein displayed better emulsifying and emulsion stabilizing properties than Acacia senegal gum [START_REF] Qian | Fractionation and physicochemical characterization of peach gum polysaccharides[END_REF]. It has been suggested that AGP surface properties depend considerably on the polysaccharide component [START_REF] Goodrum | Gum arabic glycoprotein contains glycomodules of both extensin and arabinogalactan-glycoproteins[END_REF]. [START_REF] Anderson | Chemotaxonomic aspects of the chemistry of Acacia gum exudates[END_REF] suggested that the superior emulsifying power of gum Arabic may be related to the significant proportions (< 10 mol%) of terminal Rhap groups, which possess hydrophobic centers (D. M. W. [START_REF] Anderson | Chemotaxonomic aspects of the chemistry of Acacia gum exudates[END_REF]. In addition, the -1,3 galactan backbone should form an

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008 amphipathic helix, and recently confirmed by [START_REF] Kitazawa | beta-Galactosyl Yariv Reagent Binds to the beta-1,3-Galactan of Arabinogalactan Proteins[END_REF], the inside of the helix comprising a hydrophobic surface with the bulk of the galactan hydroxyl groups oriented toward the outer surface of the helix [START_REF] Goodrum | Gum arabic glycoprotein contains glycomodules of both extensin and arabinogalactan-glycoproteins[END_REF]. The "hydrophilic" carbohydrate blocks attached to the polypeptide chain could therefore have a substantial impact on the adsorption of AG onto hydrophobic surfaces.

The presence of surface active components in AG implies that the nature of dispersed phase is important to determine emulsion formation and stability. Based on the varying degree of stability against washing of emulsions made with different oils, it was suggested that the efficiency with which AG is adsorbed depends at least in part on the nature and polarity of oil (Dickinson, et al., 1991b;[START_REF] Shotton | The emulsifying properties of gum Acacia[END_REF]. A non polar dispersed phase will therefore display a higher interfacial tension with water and will favor adsorption of the more tensioactive components. This is the basis of the extensive use of AG to stabilize orange oil emulsions, limonene, the main orange oil component, being a non polar molecule. This also may explain why the hydrophobic GP fraction adsorbs faster at interfaces than the AGP fraction when the non polar n-hexadecane is used [START_REF] Castellani | Hydrocolloids with emulsifying capacity. Part 3-Adsorption and structural properties at the air-water surface[END_REF]. For instance, hexadecane-in-water emulsions made with AG are more stable than decanol-inwater emulsions [START_REF] Chanamai | Comparison of gum arabic, modified starch, and whey protein isolate as emulsifiers: Influence of pH, CaCl(2) and temperature[END_REF]. In addition, both polarity and water solubility of the dispersed phase influence the destabilization mechanism. With a high polarity and high water-soluble oil (e.g. decanol), oil-in-water emulsions are unstable to both Ostwald ripening and coalescence when stabilized by a weakly adsorbing biopolymer such as AG [START_REF] Chanamai | Comparison of gum arabic, modified starch, and whey protein isolate as emulsifiers: Influence of pH, CaCl(2) and temperature[END_REF]. When a low polarity and high water-soluble oil is used (e.g. decane), emulsions are stable to coalescence, but unstable to Ostwald ripening. Finally, with low polarity and low water-soluble oil (e.g. hexadecane), emulsions are stable to both Ostwald ripening and coalescence [START_REF] Chanamai | Comparison of gum arabic, modified starch, and whey protein isolate as emulsifiers: Influence of pH, CaCl(2) and temperature[END_REF].

Finally, it is important to mention that trace levels of lipids, probably attached to the "AGP" macromolecules, would improve the surface properties of AG (M. P. [START_REF] Yadav | Chemical investigation of the structural basis of the emulsifying activity of gum arabic[END_REF]M.P. Yadav, et al., 2012). This points out the potential role of minor components present in AGs on their surface properties, e.g. free proteins, peptides, oleoresin, feruclic acids. These low M w components could explain that some gums with low AGP concentrations, for instance lower than 10%, still display excellent ability to stabilize oil-in-water emulsions or that gums

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Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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40 with similar protein content and AGP concentration produce emulsions with significant differences in stability. This subject should deserve much more attention in the future.

Enzymatic modifications of Acacia gum

Modifications of AGs with enzymes first occurred during its maturation process. The gum from the earliest exudation, called "green gum", is not entirely soluble yielding a glairy mucus-like fluid from which a perfect solution separates after a certain time. After storage during several months, a change is observed probably due to enzymes, so that gum becomes fluid and entirely soluble [START_REF] Reinitzer | The enzymes of gum-Acacia and certain other gums[END_REF]. Neither the nature of these enzymes nor their origin is known. A recent Ph.D. student has studied these changes during gum maturation and it appeared that green gum displayed a peculiar rheological behaviour and was highly heterogeneous with part of kibbles containing very high M w AGP [START_REF] Cozic | La gomme arabique : Variabilité des propriétés physicochimiques et contribution des "ArabinoGalactanes-Protéines[END_REF]. Upon storage, the gum recovered a classical distribution of molecular fractions.

Enzymatic modifications of AG, especially protein hydrolysis, were used primarily to demonstrate that most of proteins were associated with the higher molecular weight component of the gum and contribute to the elaboration of the wattle-blossom model [START_REF] Connolly | Heterogeneity and homogeneity of an arabinogalactan-protein: Acacia senegal gum[END_REF][START_REF] Connolly | Effect of a proteinase on the macromolecular distribution of Acacia-Senegal gum[END_REF][START_REF] Randall | The role of the proteinaceous component on the emulsifying properties of gum arabic[END_REF]. It was then demonstrated that protease treatment of Acacia senegal gum mostly degraded the protein-rich high M w component of the gum, i.e. both AGP and high M w GP fractions, but did not degrade or marginally AGp and low M w GP components (Aoki, Al-Assaf, et al., 2007;[START_REF] Connolly | Heterogeneity and homogeneity of an arabinogalactan-protein: Acacia senegal gum[END_REF][START_REF] Connolly | Effect of a proteinase on the macromolecular distribution of Acacia-Senegal gum[END_REF][START_REF] Flindt | Studies on acacia exudate gums. Part V. Structural features of Acacia seyal[END_REF][START_REF] Mahendran | New insights into the structural characteristics of the arabinogalactan -Protein (AGP) fraction of gum Arabic[END_REF][START_REF] Osman | The molecular characterization of the polysaccharide gum from Acacia-Senegal[END_REF][START_REF] Randall | The role of the proteinaceous component on the emulsifying properties of gum arabic[END_REF][START_REF] Randall | Fractionation and characterization of gum from Acacia senegal[END_REF][START_REF] Renard | Enzymatic hydrolysis studies of arabinogalactan-protein structure from Acacia gum: The self-similarity hypothesis of assembly from a common building block[END_REF]. The lower M w obtained after hydrolysis was in the range 1.7-2x10 5 g.mol -1 , depending on the enzymes and conditions used with the lower values obtained with papain (Renard et al., 2014). This can be considered as the nominal building block of the gum as obtained by protein enzymatic hydrolysis. The limiting intrinsic viscosity values obtained were in the range 12-15 mL.g -1 [START_REF] Chikamai | Processing of gum arabic and some new opportunities[END_REF][START_REF] Connolly | Heterogeneity and homogeneity of an arabinogalactan-protein: Acacia senegal gum[END_REF][START_REF] Connolly | Effect of a proteinase on the macromolecular distribution of Acacia-Senegal gum[END_REF][START_REF] Renard | Enzymatic hydrolysis studies of arabinogalactan-protein structure from Acacia gum: The self-similarity hypothesis of assembly from a common building block[END_REF]. The decrease of gum viscosity after enzyme hydrolysis has been suggested as a way to improve their processing [START_REF] Chikamai | Processing of gum arabic and some new opportunities[END_REF].

When using specific enzymes of polysaccharide such a -galactosidase, a limited 8% decrease in galactose content was observed in parallel to a 27% increase in protein content [START_REF] Chikamai | Processing of gum arabic and some new opportunities[END_REF]. Analysis by SEC showed a broadening of the AGP fraction and a

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Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

narrowing of the AGp fraction, GP fraction being unchanged. Interestingly, a recent patent reported the use of -galactosidase to produce modified AGs with improved emulsifying properties [START_REF] Heidebach | Enzymatic treatment of gum Arabic[END_REF]. Very recently, a glucuronosyltransferase was isolated from Arabidopsis thaliana and used with AG. It was found that glucuronic acids were incorporated up to 1/3 of AG total weight. Oil-in-water emulsions made by the enzymemodified gum arabic were slightly smaller in droplets size and remarkably more stable compare to emulsions made with native AG [START_REF] Dilokpimol | Enzymatically modified-Gum arabic using Arabidopsis betaglucuronosyltransferase has altered emulsion property[END_REF].

Conclusions and future prospects

Acacia gum is a plant exudate mainly produced in sub-sahalian regions of Africa. It is a natural ingredient of geo-political and economic importance. Two gums are authorized for uses, Acacia senegal gum and Acacia seyal gum. The first one was in the past the most used and studied whereas a growing demand of low cost natural gum can explain the growing part of Acacia seyal sales in recent years. Acacia gums are used by humans since prehistoric times and continue to be widely used today, the World demand having risen by 25% since the last ten years. Gums are mainly used in Food industry (confectionary, drinking industry) but also in non-food applications (pharmacy, cosmetics, materials).

Acacia senegal gums are composed of arabinogalactan-proteins (AGP) type biopolymers. It contains a continuum of hyperbranched amphiphilic charged polysaccharide-protein complexes differing by the amount of protein, type of sugars, sugar to amino-acid ratios, degree of branching, conformation and physicochemical properties. It also contains minor components such as minerals, polyphenols and traces of lipids. Arabinogalactan-proteins in Acacia senegal gums have generally anisotropic shapes and can be described as highly porous ellipsoidal objects. In fact, these macromolecules are kinds of sponges, which can explain their ability to interact with different kinds of entities (e.g. proteins, minerals, polyphenols, etc...). Besides their known biological properties, Acacia gum biopolymers display interesting functional properties such as high affinity for water, low newtonian viscosity even at quite high gum concentrations and the ability to adsorb and stabilize gazliquid, liquid-liquid and solid-liquid interfaces. Surface properties of gums are strongly related to the presence of protein-rich high molecular weight species.

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Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Based on the literature survey, socio-economic challenges and our own work, some important issues and bottlenecks can be identified both on scientific and technological point of views. Some of the most relevant points, in our opinion, are described in the following.

Maturation mechanisms of Acacia gums: once exuded, gums evolve with time through sun drying, oxidation, (enzymatic) hydrolysis and interactions between AGP and between AGP and minor components upon storage and processing. The consequences are a change in biopolymer composition and distribution, structure and functional properties. Probably one of the most challenging questions is the mechanism leading to the known gum from the "green gum". Initial glairy gum is composed by very high M w AGP that imparts specific rheological behaviour. With time, even in the dry state, the M w of AGP decreases, which is may be caused by the action of some glycosidases and/or proteases. The presence of these enzymes has never been reported. Apart from hydrolysis, gum can also be oxidized which can have significant functional consequences, especially when it is rich in polyphenols.

Composition, structure and functional properties of Acacia seyal gum: for economic reasons, Acacia seyal gum represents actually 50-75% of sales. However, it has been much less studied than Acacia senegal gum. The differences between both gums in terms of sugars and amino-acid compositions are mostly known, as well as the major differences in M w distribution. However, Acacia seyal gum is richer in minerals and polyphenols, less rich in proteins, more compact, more unstable in solution, less charged, less surface active, less hydrolysable by enzymes. The structure and conformation of AGP from Acacia seyal are unknown and their functional properties badly known. We do not know the reasons for the molecule compactness and whether such a compactness can be modified changing the solvent polarity or temperature. We know that assemblies with proteins are possible but stabilization of assemblies appear difficult. We do not know the structure of oil-in-water interfaces covered by this gum. However, we know that stabilization of emulsions or suspensions can be achieved using high gum concentrations. The presence of polyphenol can be detrimental as far as solubility is concerned, however this could open the possibility to crosslink AGP molecules by oxidative enzymes.

Nature and concentration of minor components: Acacia gums contained 3-6% of minor components which deserve much more attention. Part of variability in gum functional

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Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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43

properties must originate from these minor compounds. Among these components, the major part is composed by minerals (3-5%). The concentration and nature of ions are important since they are supposed to impact the charge density of AGP, which in turn plays an important role in the hydration, solubility and compactness of AGP as well as in the stabilization of colloidal suspensions. Polyphenol type molecules are present in colored gum nodules depending on the gum type (Acacia senegal vs Acacia seyal gum) and origin. The nature of these polyphenols and their concentration need to be determined; they can result in oxidation phenomena and modify functional properties of AGP. Traces of lipids (the GPI anchor) linked to high M w AGP has been reported in Acacia senegal gum. It is obvious that the presence of lipids impacts the gum surface properties. We need to know whether lipids are present, at which concentration and whether they are limited to GPI or other kinds of lipids. When we perform SEC-MALLS experiments on gums, UV profiles show invariably the presence of highly rich-protein molecules with low M w (long elution times) at very small concentrations. These molecules could be free proteins or oligosaccharide-proteins issued from AGP degradation. It cannot be ruled out that these molecules could be some of the enzymes mentioned in the literature, e.g. oxidases, or may be hydrolase-type enzymes responsible of the time-dependent maturation of gum. We know that these enzymes are present but we do not know their nature or origin.

Structure of arabinogalactan-proteins (AGP): all biopolymers in Acacia senegal and seyal gums are hyperbanched arabinogalactan-proteins with various M w , size, sugar or amino-acid composition, sugar to amino-acid molar ratio, charge density and hydrophobicity. We know that protein-rich high molecular weight fractions have important surface properties and the ability to stabilize colloidal suspensions. We know that these fractions display anisotropic conformations. However, results were obtained on mixtures containing fractions in various proportions including gum aggregates. Highly purified molecules are then needed to provide new structural insights through the use of combined approaches, for instance using Hydrophobic Interaction Chromatography (HIC) then Size Exclusion Chromatography (SEC).

Other possibilities are to combine controlled alcohol or salt-induced precipitation and HIC, SEC or Ion-Exchange Chromatography. The architecture of fractions remains unclear despite recent advances. The organization of sugar blocks onto the polypeptide or polypeptides chains is unknown, e.g. nature of interactions between sugar chains and between sugar

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Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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44 chains and polypeptides, steric constraints, possible secondary structures of sugar blocks (e.g. helix of the  1,3-galactan backbone). In addition, the global structural organization of the molecules is directly related to the amino-acid sequences of polypeptides that determine the linking of sugar blocks. Theses sequences are unknown for Acacia gum.

Determine the amino-acid sequences is challenging because sugar blocks have to be degraded either by chemical or enzymatic treatments or both. Please note that the charge density of AGP should be determined as well, both to better understand the structure and the macromolecular functional properties.

Functional properties of Acacia gums and their arabinogalactan-proteins (AGP) fractions:

functional properties (hydration, assembly, rheological and surface properties) of Acacia senegal gum have been characterized in more or less details. This is not the case for Acacia seyal gum. A number of issues remains (solvent affinity, formation of aggregates, unusual rheological behavior, etc..) but the most important one concerns the functional properties of the different AGP fractions that have been rarely studied so far. Only one paper reports emulsions made with the two major fractions purified from HIC (HIC-F1 and HIC-F2). One can find in the literature some conflicting data on interfacial properties of F1, F2 and F3 but fractions were not highly purified and no systematic investigation of experimental conditions was tried (gum concentration, pH, ionic strength, nature of oil, temperature). It is impossible to understand and control gum functional properties without a precise knowledge of fraction functional properties. Today, we have no idea about their hydration properties, their rheological properties, their solution properties (phase behavior, assembly) depending on the solvent polarity, their surface properties. In addition, the question of functional synergism between fractions deserves some attention.

Solution properties: solvent affinity, phase behavior, formation and dynamics of aggregates:

we have noted recently aggregation-disaggregation phenomena involving protein-rich AGP of Acacia senegal gum depending on gum concentration. This suggests that some molecular fractions display different solvent affinities. This can be seen as well through the huge hydration properties of the soluble part of F3 fraction as compared to total gum, F1 or F2 fractions. The effect of solvent polarity on the dynamics of intra-and inter-molecular interactions between AGP should be studied in relation with the ionic strength, the chaotropic solvent and temperature, then varying the strength of hydrogen bonding,

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Nano/microparticles-based on AGP assembly with other biopolymers: formation, stabilization, industrial scaling: the formation of nano/microparticles based on electrostatic interactions between Acacia senegal gum and proteins is not a difficult task at the lab scale.

On the other hand, the stabilization of these assemblies against changes in pH or ionic strength is a bottleneck that has limited up to now their industrial applications. Since

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

assemblies are obtained through weak interactions, cross-linking stabilization without chemical treatments imply either the use of enzymes or physical treatments. Using enzymes to stabilize Acacia gum-protein microparticles has been reported, this is a possible but expensive way. Another way is protein denaturation by heating, which has been proven to be efficient at the lab scale. An important challenge is the formation and stabilization of Acacia gum-protein nano/microparticles at the industrial scale. Issues about mixing conditions and heating in large volumes need to be considered.

Enzymatic modification of Acacia gum: the possibility to modify the structure and functional properties of Acacia gums clearly represents the best future way of innovation. Actually, the only way to propose new ingredients based on Acacia gums is i) to enrich them with AGP, ii)

to purify AGP, iii) to assemble them with proteins, iv) to graft lipids by chemical reaction.

Enzymes can potentially expand the modification possibilities of gums. Two ways should be explored. The first one concerns hydrolysis of gums. This can be achieved either by the use of proteases or/and glycosidases. Proteases have demonstrated their ability to degrade Acacia gums and to significantly decrease their viscosity, which is of particular use in industry. However, protein-rich fractions are degraded, which impairs their surface properties. A major bottleneck is then to decrease the gum viscosity while maintaining high surface properties. The use of glycosidases may be a solution, however their efficiency is impaired by the hyperbranched characteristic of gums. Fortunately, a number of enzymes or enzymatic cocktails are actually available and must be screened. Enzymes with more specific activities could be found but the questions of their availability and costs should be questioned. In particular, the possibility to increase the concentration of charged carboxylate groups removing methyl groups appears interesting. The second way, probably the most promising, is to graft onto the gum specific molecules or chemical groups. Then one can imagine to graft lipids or oligopeptides to improve amphipathic properties of gums or to graft carboxylate groups to increase the gum charge density. It remains to find efficient enzymes compatible with an industrial use then to optimize grafting processes. Finally, one major bottleneck is to crosslink gum molecules by enzymes to form 100% gum nano/microparticles. This should open a lot of new applications as texturing agents and stabilized microcapsules for aroma protection.

Literature

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008 the overall symmetry, or self-ordering, of carbohydrate moieties and arabinoside oligomers attached to it. The self-similarity approach was then used to build the AGP macromolecular assembly from the elementary building blocks 1 and 2 (adapted from Renard et al., 2014).

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Figure 3: Effect of concentration on the relative newtonian viscosity (/ 0 ) of Acacia senegal gum dispersions. Data were taken from [START_REF] Taft | Solvents for gum Arabic II Transactions of the[END_REF], Riddell and Davies (1931), Briggs (1941), Schleif et al. (1951), [START_REF] Williams | Structure-function relationships of gum arabic[END_REF], [START_REF] Goycoolea | Solution rheology of mesquite gum in comparison with gum-arabic[END_REF], [START_REF] Mothe | Rheological behavior of aqueous dispersions of cashew gum and gum arabic: effect of concentration and blending[END_REF], Sanchez et al. (2002) and [START_REF] Li | Rheological study of gum arabic solutions: Interpretation based on molecular self-association[END_REF]. Gum samples were from different geographical origins. Measurements corresponded to different pH (4-8), temperature (20-30°C) and ionic strengths. Solid line (this paper), dashed lines (Gooycoolea et al., 1995) and dot lines (Gaïa et al., 1981) are exponential fits of data (unpublished data). Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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List of figures

Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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Comment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C., Renard, D. (2018).

Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

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  irradiated. Al Assaf et al. showed that R g increased from 33 to 73 nm when dried AG was stored at 110°C during two days under control humidity[START_REF] Al-Assaf | Characterization and properties of Acacia senegal (L.) Willd. var. senegal with enhanced properties (Acacia (sen) SUPER GUM (TM)): Part 1 -Controlled maturation of Acacia senegal var. senegal to increase viscoelasticity, produce a hydrogel form and convert a poor into a good emulsifier[END_REF]. Aggregation was also revealed in spray-dried AG samples, displaying a large proportion of aggregates in solution compare to raw material. Aggregation in spray-dried AG has been attributed to the pasteurization step included in the process[START_REF] Al-Assaf | Molecular associations in acacia gums[END_REF]. During heat treatment, aggregates are formed between AGp and GP fractions involving hydrophobic association of the proteinaceous components. Irradiation of AG in the presence of acetylene also enhances aggregation with an increase of R g from 25 to 67 nm for AG irradiated at 6 kGy (Al-Assaf, etComment citer ce document : Sanchez, C., Nigen, M., Mejia Tamayo, V., Doco, T., Williams, P., Amine, C.,Renard, D. (2018).Acacia gum: History of the future. Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008 ACCEPTED MANUSCRIPT 27 al., 2009). In this case, aggregates were formed through C-C covalent bonds between the carbohydrate moieties.

  structure and mechanical properties of interfaces: a huge number of studies have reported the formation and stabilization of oil-in-water emulsions based on Acacia senegal gum. No clear picture emerged from such studies about the intricate relationships between gum concentration, nature of oil phase (hydrophobicity), protein-rich AGP concentration and homogenization energy. The important role of protein-rich fractions makes no doubt, however it seems to depend on gum concentration, hydrophobicity of dispersed phase and quantity of interfaces. The relative contributions of the different fractions during the full emulsification process need to be clarified. Solid-like viscoelastic properties of interfaces and electrosteric repulsion mechanisms stabilize emulsions. Depending on process conditions, the structure of interfaces remains unknown, especially in terms of thickness (monolayers vs multilayers), surface gum concentration and topology of fractions. These characteristics will affect emulsion stability and for instance permeability to hydrophobic molecules such as aroma.

Figure 1 :

 1 Figure 1: R h conformation plots of spray-dried (black) and raw (red) Acacia senegal (A) and seyal (B) gums. 202 spray-dried and 100 raw A. senegal gums, 28 spray-dried and 6 raw A. seyal gums were analysed (unpublished data).

Figure 2 :

 2 Figure 2: (left) Elementary building blocks 1 and 2 used to build arabinogalactan-protein (AGP) macromolecular assembly from Acacia Senegal gum. Building block 2 was built by a linear arrangement of five building blocks 1. (right) Three-dimensional model of AGP made of the assembly of 6 building blocks 1 and 8 building blocks 2 by covalent and/or noncovalent interactions. The elementary building blocks were constructed taking into consideration the quasi-palindromic nature of polypeptide backbones and as a consequence

Figure 4 :

 4 Figure 4: Phase diagram of Acacia senegal gum-ethanol-water system determined by the cloud point method (pH 5 citrate buffer, 25°C). Appearance of turbidity was checked by UV-Vis spectroscopy. The solubility curve appears in red and delineates the monophasic (I) from the biphasic (II) region. Inset : Micrographs of Acacia senegal gum-ethanol-water system with concentrations of 45% ethanol and 20 g.L -1 gum. The arrow in the phase diagram indicates the composition of the mixture. Scale bar: 5 m (unpublished data).

  conformation plots (log-log plot of R h vs. M w ) after SEC MALS experiments of raw and spray-dried Acacia senegal and seyal gums harvested in several countries as Sudan, Chad, Senegal, Eritrea, Mali, Mauritania and Burkina Faso (Figure1, unpublished data). These analyses concerned 202 spray-dried and 100 raw A. senegal gums, 28 spray-dried and 6 raw A. seyal gums. The hydrodynamic coefficient,  h , is constant for each specie with values ranging from 0.53 to 0.55 for spray-dried and raw A. senegal gum and 0.43 to 0.44 for sparydried and raw A. seyal gum. Hence, the conformations of Acacia gum molecules depend on the Acacia gum specie, but not on the geographical area of harvest or the post harvested treatment (from raw to spray-dried gums). These  h values confirm again the more compact structure of Acacia seyal molecules.

Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008 ACCEPTED MANUSCRIPT 12 R h

  Several factors such as solvent chemical properties, physical chemical treatments and macromolecules physical chemical properties can influence the assembly pathway and of course the functional properties of the assemblies. It is well known that AG can associate with itself or other biopolymers, such as proteins. Its association and assembly properties are often used in several areas (food, pharmaceutical, medicine, etc.) to elaborate AG-based assemblies with specific functional properties. According to physical chemical conditions and the presence or not of other macromolecules as partner of the assembly, AG can assemble

  Rheological properties: unusual flow behaviour and viscoelastic properties: shear-thinning and time dependent flow of diluted Acacia senegal gum is probably due to surface effects and may be to AGP-induced aggregation. Performing rheo-SAXS experiments will help to clarify the situation. At high concentrations, gum solution display viscoelastic properties seemingly dominated by the viscous component. We do not know whether highly concentrated solutions can be characterized as soft colloidal gels or polymeric gels and what is the role of AGP fractions on viscoelastic properties.

	electrostatic and hydrophobic interactions. Building phase diagrams should allow predicting
	stability of gum solutions depending on environmental conditions. The formation of a small
	concentration (1-2%) of AGP-based aggregates is mostly unavoidable and participates to the
	variability in gum functionality.

Food Hydrocolloids, 78, 140-160. , DOI : 10.1016/j.foodhyd.2017.04.008

Emulsifying-stabilization properties of Acacia gum: processing conditions, role of AGP,