Caroline Hamon
email: caroline.hamon@cnrs.fr

Solenn Réguer

Ludovic Bellot

Solenn Reguer
email: solenn.reguer@synchrotron-soleil.fr

Ludovic Bellot-Gurlet
email: ludovic.bellot-gurlet@sorbonne-universite.fr

Yvan Pailler
email: yvan.paillier@inrap.fr

Verane Brisotto
email: verane.brisotto@inrap.fr

Tracking the first bronze metallurgists of Western Europe: combined use-wear analysis and X-ray fluorescence synchrotron spectroscopy of a stone toolkit from Ploneour-Lanvern (Brittany)

Keywords: Early Bronze Age, stone tools, use-wear analysis, X-ray fluorescence imaging, synchrotron source

Tracking the first bronze metallurgists of Western

Europe: combined use-wear analysis and X-ray fluorescence synchrotron spectroscopy of a stone toolkit from Ploneour-Lanvern (Brittany)

Introduction

The emergence and development of copper and bronze metallurgy is a key step in the evolution of protohistoric societies. At the turn of the second millennium BC in Europe, copper and Early Bronze Age societies were characterized by major economic shifts. Emerging powerful elites enriched themselves by controlling the exploitation of ore sources (copper, tin, silver, and gold), mastering metal production, and developing extensive diffusion networks. The intensification of long distance exchanges of valuable objects and know-how stimulated the emergence of new cultural complexes.

Along the Atlantic coast, the rise of copper alloy metallurgy has been dated to the middle of the third millennium BC, within Late Neolithic to Early Bronze Age contexts, two millennia after its emergence in southeastern Europe [START_REF] Radivojevi | On the origins of extractive metallurgy: new evidence from Europe[END_REF]. [START_REF] Hamon | Proceedings of the international congress[END_REF] Archaeological and Anthropological Sciences, 12, 14. https://doi.org/10.1007/s12520-019-00999-x Bell-Beaker populations appear to have been instrumental in stimulating the spread of these new technologies which accompanied their phenomenal cultural expansion throughout continental and littoral Europe between 2800 and 1900 BC. In these contexts, the deposition of stone implements within so-called metallurgists' graves throughout Europe highlights the intrinsic mobility, craft skill, increasing specialization, and particular social status of these craftsmen [START_REF] Bátora | Contribution to the problem of "craftsmen" graves at the end of Aeneolthic and in the early bronze age in central, Western and Eastern Europe[END_REF][START_REF] Butler | Bell Beakers and early metal-working in the Netherlands[END_REF][START_REF] Drenth | Prehistoric stone tools for metalworking from the Netherlands: an overview, in from copper to bronze[END_REF][START_REF] Fitzpatrick | The Amesbury archer': a well-furnished bronze age burial in southern England[END_REF]. Between 2100 and 1800 BC, in Western Europe, the Armorican Tumulus culture in Brittany and the Wessex culture in southwest England developed an intense and novel interregional exchange network across the Channel based on metallurgical innovations [START_REF] Marcigny | Rythmes et contours de la géographie culturelle sur le littoral de la Manche entre le IIIe et le Ier millénaire. In: Lehoerff A, Talon M Movement[END_REF], often termed a system of "maritories" [START_REF] Needham | Encompassing the Sea: "Maritories" and Bronze Age maritime interactions[END_REF].

However, the exact organization, techniques, processes, and equipment employed in bronze metallurgy still have to be identified within these cultures (Pernot 1998). In fact, these workshops generally left only ephemeral archeological traces, such as hearths, rare crucible fragments, or various kinds of stone tools [START_REF] Armbruster | (ed) lithic technology in metal using societies[END_REF][START_REF] Boutoille | Les techniques du dinandier : formes et fonction des outils lithiques spécifiques à la déformation plastique des métaux[END_REF][START_REF] Freudenberg | Stone age or bronze age? Cushion stones and other stone tools used for early metalworking in Schleswig-Holstein[END_REF][START_REF] Webb | Identifying stone tools used in mining, smelting, and casting in middle bronze age Cyprus[END_REF]. Consequently, the reliable identification of metallurgists' stone tools on different types of archeological sites (open-air settlements, specialized workshops, enclosures) appears to be a promising and challenging new approach to identifying the practice of bronze metallurgy.

To date, most of the archeometric analyses of copper and bronze metallurgy have focused on the characterization of the composition and structure of metal objects (axes, daggers, swords, etc.), in order to answer questions regarding ore origin, places of production, and circulation of metal. However, archeometric studies have rarely been applied in order to identify the possible role of stone implements within the metallurgy of copper, bronze, silver, or gold on the European continent. But as a matter of fact, stone tools undoubtedly played a major role at several stages of these metallurgical processes, from the extraction of ores to their processing by crushing and grinding, the casting/forging of metal objects, and finally their sharpening/finishing. In southern France, a combination of microscopic use-wear analysis and X-ray fluorescence spectroscopy (XRF) of stone tools has been used to identify metallurgists' stone toolkits which were involved in processing copper ores at La Capitelle du Broum in Cabrières (Hérault, France), one of the oldest metal processing sites in the western Mediterranean Basin dating to 2900 cal BC [START_REF] Hamon | Les outils à cupules, marqueurs de la métallurgie du district de Cabrières-Péret (Hérault) au Chalcolithique[END_REF]). In the Iberian Peninsula, combined use-wear analysis and synchrotron-based XRF have recently identified stone tools involved in copper and silver processing in the Middle Bronze Age culture of El Argar [START_REF] Delgado-Raack | Lithic perspectives on metallurgy: an example from copper and bronze age south-East Iberia[END_REF][START_REF] Delgado-Raack | Die Silberschmiede von Tira del Lienzo, 2014. Totana, Prov. Murcia im Kontext der El Argar Metallurgie[END_REF]. On the Atlantic coast, microscopic use-wear analysis conducted on two Middleand Late Bronze Age series from Brittany has led to the identification of a complete percussion and abrasion toolkit which was involved in different steps of the production of metal objects [START_REF] Hamon | Le macro-outillage lithique sur les sites de l'âge du Bronze armoricain. Quelques hypothèses fonctionnelles pour aborder la notion d'artisanat[END_REF]. This paper presents a methodology which combines microscopic use-wear analysis at low and high magnification and Synchrotron XRF analysis using fast imaging, in order to identify a metallurgist's stone toolkit on a recently excavated Early Bronze Age site at Kersulec in Ploneour-Lanvern (Brittany, France, 1950-1690 BC). The goal of this combined approach was to obtain detailed information regarding the functioning of the macrolithic tools (kinetics, hafting, use-gesture) and the material being transformed (nature, state, texture, composition). Ultimately, this would allow us to (1) identify the tools involved in metallurgical processes, (2) identify the material that they had been in contact with, and finally (3) determine in which stage of the chaîne operatoire of metallic production they were used. This case study aims to present the most suitable combination of archeometric methods in order to identify the metallurgical stone tools within larger macrolithic assemblages found on Bronze Age sites. The generalization of such functional studies of stone tools offers an opportunity to identify the equipment used in the different stages of metal production (mining, processing, finishing) and to highlight the existence of copper and bronze production workshops within the various kinds of Early Bronze Age archeological sites (open-air settlements, specialized workshops, enclosures) known throughout Europe.

Materials and method

The site of Kersulec in Ploneour-Lanvern (Finistère)

The Early Bronze Age occupation site of Kersulec in Ploneour-Lanvern (Finistère; Fig. 1) is located in a flat area on the northwestern slope dominating the valley which ends in the Trunvel Lake. The substratum of the northern part of the site consists of Trunvel micaschists, while to the south, it consists of Ploneour orthogneiss. These formations are overlain by clayish silts in which the archeological remains were discovered in the course of a rescue excavation carried out in 2015 by the Institut National de Recherches Archéologiques Préventives (INRAP) [START_REF] Pailler | Un dépôt en fosse du Néolithique moyen 2 et une occupation de l'âge du Bronze ancien, Ploneour-Lanvern, Kersulec, (Finistère) : rapport de fouille, INRAP Grand Ouest[END_REF].

Despite the fact that dug features tended to be scarce and poorly preserved, it was possible to identify several structures. A cist burial and a pit burial, which had been cut by a large waste pit, were revealed. These burial types are well-known in the Early Bronze Age of lower Brittany. The relative chronology indicates that these burials predated the domestic occupation. The occupation itself is composed of a small building with a surface area of approximately 10 m 2 , two storage pits and two light structures each consisting of four posts; the latter were interpreted as possible workshops or granaries. [START_REF] Hamon | Proceedings of the international congress[END_REF] Archaeological and Anthropological Sciences, 12, 14. https://doi.org/10.1007/s12520-019-00999-x Several isolated structures, 2 small wells and 13 hearths complete the settlement. All of these structures suggest a rather small-scale domestic occupation of short duration. This is confirmed by the homogeneity of the ceramic assemblage and by two C14 dates which date the occupation to the first third of the second millennium cal. BC (1955-1775cal BC and 1885-1690 cal. BC). In this context, the discovery of a crucible used for smelting copper or bronze, together with a series of stone tools, under one of the possible post-built structures in the northern part of the site raised the possibility of the existence of a metallurgical workshop. Such structures remain difficult to identify on Early Bronze Age sites as they generally leave little or no traces, except for some preserved stone and clay tools. Thus, the site of Kersulec in Ploneour-Lanvern offers a rare opportunity to explore evidence for metallurgical production and related craft activities dating back to the Early Bronze Age in western France.

Sampling and use-wear analysis of macrolithic tools

Following their discovery, the macrolithic tools from Kersulec were analyzed according to the principles of technological analysis, i.e., in terms of their raw materials (types, cohesion, granulometry, and mechanical properties), typology (morphometry and morphology of the tools), and technology (shaping processes and schemes, possible hafting, localization of the active surfaces). Macro-and microwear analysis was conducted at the UMR 8215 Trajectoires laboratory using a stereomicroscope SMZ 800 (up to 80 x) and an Olympus BX51M microscope (up to 200 x).

Fourteen stone tools were analyzed in order to better understand their function and how they were actually used within a wide range of possible food preparation and craft activities. The toolkit consists of 14 tools including hammerstones made from quartz fragments and cobbles, rare grinding tools used in food production, and burnishers and polishers involved in ceramic production. Only five of these tools displayed possible use-wear related to metal production. Among these, three were selected for residue analysis because of the good state of preservation of their surfaces.

Since its development in the 1990s, use-wear analysis on macrolithic tools has focused on the development of a suitable terminology adapted to the diversity of rocks employed, particularly for coarse and heterogeneous rocks, and on the need for highly integrated low and high power observation of the traces. The terminology employed in this paper follows the generally accepted framework for use-wear analysis of macrolithic tools. At a macroscopic level [START_REF] Hamon | Functional analysis of stone grinding and polishing tools from the earliest Neolithic of North-Western Europe[END_REF][START_REF] Adams | Functional analysis of macro-lithic artefacts[END_REF], the topography and microtopography of the surfaces are described in terms of their morphology (flat, sinuous, rough) and regularity. The leveling of the surfaces is characterized by its distribution (loose, covering, or concentrated) and density (pattern of leveling, i.e., separated, close, or connected), incidence, morphology (flat, sinuous or rounded), and texture (rough or smooth). [START_REF] Hamon | Proceedings of the international congress[END_REF] Archaeological and Anthropological Sciences, 12, 14. https://doi.org/10.1007/s12520-019-00999-x Linear traces follow more or less the same guidelines for distribution and density, but orientation and dimensions are also considered. Grain edge rounding, grain extraction, and microfractures are also described at this scale of observation. At a microscopic level, descriptions of micropolish follow the terminology that is generally used, with some adaptations to take account of the heterogeneous character of the raw material and its granular structure (Clemente [START_REF] Conte | Los instrumentos líticos de Túnel VII: una aproximación etnoarqueológica[END_REF][START_REF] Hayes | Sandstone grinding/pounding tools: use-trace reference libraries and Australian archaeological applications[END_REF].

In parallel, experimental reference data has been compiled focusing on a variety of technical fields, from food processing (cereals, nuts, plants, etc.) to craft production including the shaping of pottery, the manufacture of ornaments, the production of stone or bone tools, the treatment of skins and vegetable fibers, the grinding of pigments, etc. (for a general overview, see [START_REF] Dubreuil | Ground stones: a synthesis of the use-wear approach[END_REF]. In this field, very few studies describe specific use-wear traces left on experimental stone tools involved in metal processing as most of the experiments that focus on ore processing or the production of metallic tools tend to be aimed at describing the products obtained [START_REF] Timberlake | The use of experimental archaeology/archaeometallurgy for the understanding and reconstruction of early bronze age mining and smelting technologies[END_REF]) and the traces left on metallic objects (Gutiérrez [START_REF] Sáez | Traceology on metal. Use-Wear Marks on copper-based tools and weapons[END_REF]. Our own reference data includes more than 130 experiments encompassing all fields of food processing and craft production including food grinding (cereals, plants, nuts), pottery production, the manufacture of ornaments, the manufacture of stone and bone tools, the treatment of skins and vegetable fibers treatment, the shaping of metal objects, and the crushing of pigments [START_REF] Hamon | Functional analysis of stone grinding and polishing tools from the earliest Neolithic of North-Western Europe[END_REF][START_REF] Hamon | Which analytical framework for the functional analysis of grinding stones ? The blind test contribution[END_REF]. Recently, experimental tests were conducted on the crushing of copper ore and on the shaping of copper and bronze objects by percussion and abrasion using 18 stone tools (Fig. 2). A summary of the related use-wear traces at macro-and microscopic scales is presented in Fig. 2. Contact with metal is mainly characterized by rough leveling of the surfaces, microbreakage of the individual grains, and shiny and compact to micropitted micropolish. These observations corroborate the description of "a shiny surface" on hammers and anvils surfaces employed for the manufacture of metallic objects (e.g., Delgado-Raack and Risch 2008, p. 239).

X-ray fluorescence imaging using the DiffAbs beamline at Soleil Synchrotron

Use-wear analysis led us isolate three tools (n°13, 15, 18) from among the stone tool assemblage; these displayed specific use-wear suggesting possible processing of metallic ores and metal objects. These three tools were mapped through X-ray fluorescence using the DiffAbs beamline at the Soleil Synchrotron facility (France). The equipment used allows rapid XRF imaging and yields elemental mapping over large surfaces of the tools in quite a short time span (approx. 1 h) and allows us to identify specific areas of interest, e.g., metallic residues. It seeks to identify and estimate the concentrations and distributions of copper, arsenic, iron, zinc, and lead residues which are used in different bronze alloys.

The DiffAbs beamline of the Soleil Synchrotron facility (France) is a versatile setup providing multi-technique and multi-scale characterization tools capable of analyzing a large variety of materials. X-ray diffraction, absorption, and fluorescence spectroscopies can be applied in local probe or imaging (raster maps) modes to reveal chemical and structural information.

The so-called macrobeam setup at DiffAbs consists of two cylindrical, vertically-focusing mirrors, and a Si (111) double crystal monochromator, which allows monochromatization and focusing of the beam to about 285 μm × 216 μm (horizontal × vertical, full width at half maximum FWHM).

The acquisition of pluricentrimetric elemental maps with a resolution of several hundreds of microns is possible, thanks to the combination of the focused X-ray beam, the efficiency of the detectors used, and the new continuous Flyscan acquisition mode [START_REF] Leclercq | FLYSCAN: A Fast and Multi Technique Data Acquisition Platform for the SOLEIL Beamlines[END_REF]. This fast, efficient scanning of the samples leads to a reduction in the acquisition time per map (with frequencies of up to several hundreds of Hz) allowing much more data to be collected from bigger samples and from a larger corpus [START_REF] Esteve | Flyscan opportunities in medicine: the case of quantum rattle based on gold quantum dots[END_REF]. In this work, a spatial resolution of 300 μm and a dwell time of 30 ms were chosen. This spatial resolution was adopted to allow the analysis of large surfaces within a reasonable time compatible with the available beam time and the number of samples to be analyzed.

A major advantage of this setup is that no sampling or sample preparation is required. The samples are directly positioned on the beam trajectory with an orientation of about 25°. All of the measurements were performed using an incident beam energy of 18 keV. XRF spectra were recorded with a four-element silicon drift detector (SDD, Vortex-ME4®, Hitachi, with a total active area of 120 mm 2 -200 mm 2 ), placed in the same horizontal plane at 90° from the beam to minimize the X-ray scattering contribution.

All XRF experiments were performed at room temperature and atmospheric pressure (Ar K lines from ambient air are visible in the XRF spectra) blocking. Due to air absorption, the sensitivity to the detection of fluorescence from light chemical elements such as Si, P, S, etc. is limited.

Analysis of each recorded XRF spectrum was performed using the PyMCA software suite [START_REF] Solé | A multiplatform code for the analysis of energy-dispersive X-ray fluorescence spectra[END_REF]. Energy calibration and a fitting procedure of the fluorescence lines for each spectrum allowed us to identify and evaluate semiquantitatively the contribution of each element. The images of elemental distribution correspond to the plotting of the calculated area of the main fluorescence lines fitted by Gaussian shapes. The detection of trace elements must take into account the inherent limitations of the method: the possible overlapping of the Sn L peak (3.44 keV) and the major Ca Kα peak (3.6 keV) and the possible overlapping of the Ag peak and the Ar Kα peak (2.96 keV). For this reason, Sn and Ag peaks are difficult, if not impossible, to detect when quantities of residues and preserved signals are very low, and at the quite high energy required for the detection of the Cu and Zn peaks. Metallic trace elements of Pb residues were also sought. The mapping of the complete surface of the stone tools allows us to isolate elements involved in rock composition on unused areas and elements composing residues related to the processed matter. Black and white images clearly highlight the elemental distribution, especially that of copper residues.

For each area imaged, the same data interpretation process was followed. Whole maps and distributions of main elements are observed and described in light of the areas pinpointed by use-wear analysis. Then, a merging of the XRF spectra is carried out on selected zones, i.e., on areas representative of the original raw material on the one hand and on zones showing clear use-wear traces according to the microscopic analysis on the other. This is done so as to highlight the chemical contrasts between the rock itself and possible residues arising from the use of the artifact. When evidence for copper working activities is being sought, then a particular focus is placed on the intensity of Cu Kα peak.

Results

The analyzed tools correspond to a restricted spectrum, in terms of action modes as well as activities represented. Among the stone tool assemblage, the combination of particular raw materials and specific macroscopic use-wear traces on three tools led us to interpret them as metallurgical tools which were used with different kinematics and actions on the material.

The first (tool n°13), a microgranite cobble, broken lengthwise, presents numerous zones of percussion on several of its faces (Fig. 3). Both ends of the artifact display fine percussion zones on beveled flattened surfaces. Two percussion cupules occupy the center of face 1, while the opposite face, face 2, features a single percussion cupule. They are surrounded by highly shiny areas. Use-wear analysis has shown that the cupules correspond to a series of coarse and transversal and deep linear percussion impacts. The peripheries of the cupules present concentrated, close, smooth leveling with shiny aspect, together with covering, oblique, parallel striations that attest to the grinding of hard mineral material in fine fractions (fragments, powder) (Fig. 3a). An open, smoothed, and rough micropolish, with strong metalized shine, is visible on these surfaces. The main cupule (Fig. 3b) associates fine percussion impacts and covering and smooth leveling on its low topography. Besides the three shiny zones localized between the percussion zones and the sides of the tool, covering, surface, smooth leveling at the bottom of the cupules along with fine percussion impacts, further demonstrate the simultaneity of crushing and grinding actions. The sides of the tool exhibit covering, close, smooth leveling associated with short individual striations on the faces, all orientated following a transverse 3c). Based on this association of traces, we can say that this tool would have been used alternately for reducing hard mineral matterusing its ends and the center of its faceand then for finer grinding of the mineral matter in smaller fraction. For this tool, XRF maps were acquired in order to analyze the two faces bearing visible cupules. The distribution of the main chemical elements is displayed in Figs. 4 and5. These results clearly show that Fe, Sr, Ca, Rb, and Ti are constitutive elements of the rock itself. Edges of higher intensity are visible on the Ca and Sr maps, and these two elements are collocated. Two other elements (Ti, Rb) seem related to the distribution of some specific minerals composing the rock: Ti appears as small spots or stars and Rb as diffuse stains. The distribution of these five different elements does not show any correspondence with the distribution of use-wear traces and seems more related to the structure of the rock itself. Nevertheless, the XRF map draws attention to two other specific elements distributed on a specific zone (Fig. 4): Cu and Zn are collocated on the periphery of the cupule where smooth leveling has been detected. Furthermore, the XRF spectra show a difference between the measurements for these specific zones and the rock itself, where Cu is clearly less present (Fig. 6). Cu is distributed mostly as very small dots at the periphery of the cupules and not over their entire periphery.

The second tool (tool 15; Fig. 7) was made from a mesial fragment of a polished eclogite ax. Trapezoidal in shape, the object displays use-wear on its ends and sides. The microscopic observations indicate that the faces show neither distortion of their morphology nor use-wear traces: the uniform polishing, as well as the shiny coating, is solely the result of the final shaping and polishing of the initial ax. Both ends were used in the same manner: their plano-convex surfaces show a covered close smoothing associated at its periphery with a group of short oblique to transverse striations (Fig. 7a right). The peripheral edges of these surfaces display overlapping close, smooth leveling, with shiny aspect, as well as covered, short, transversal, and parallel microstriations, generally associated with longitudinal facets (Fig. 7a left). One of these facets, on one side, bears clear longitudinal striations. Although end 1 does not display visible micropolish, end 2 bears considerably bright, semi-closed, rough, and micropitted micropolishes in places (Fig. 7b). Edges and facets on the ends and sides of the tool constitute the most solicited zones of use, consistent with longitudinal abrasive actions on flat metallic surfaces (Fig. 7c). No traces of percussion have been observed. A similar combination of traces has already been reported by Delgado-Raack and Risch (2008:239), which confirms that the tool probably functioned as a hammer. XRF maps were acquired for both ends of the artifact (Figs. 8 and9). They present a very particular elemental distribution: Fe, Mn, Ti, and Ca are most likely related to the rock composition of the tool. Ca and Ti are quite homogeneously distributed within the volcanic paste, while Fe and Mn show a more "granular" appearance in relation to specific minerals. On the two ends of this tool, the XRF Kα lines of Fe, Mn, and Ti are less intense, and their distribution seems to be the exact inverse of that observed on the polished zone, suggesting that these elements relate instead to the rock itself (Fig. 10). In contrast, this polished zone seems to be enriched by Cu and Zn. Despite the low quantity measured, this particularity is noteworthy as the distribution of Cu and Zn is clearly correlated with the polished facets visible on end (2). The third implement is made out of a small ovoid cobble of diorite whose shape has been completely modified by use (tool 18; Fig. 11). Intensively used, its ends and sides show facets of percussion, and there are two percussion cupules on each of its faces. The use-wear analysis highlighted four groups of working zones on the tool. The two pointed ends show covering smooth leveling with a transversal orientation, a high degree of leveling of the surface, important microbreakage of the grains, and a bright, semi-closed, rough, and micropitted micropolish (Fig. 11d). Three facets, directly related to the smoothed ends, show evidence of fine percussion: the impacts display very regular distribution and diameters together with considerable rounding of the grains (Fig. 11a-c ande). Four narrower, circular zones occupy one side of the tool in particular: they bear coarser, lozenge-shaped impacts. The two opposite faces exhibit impacts or cupules created by strong percussion (Fig. 11f). In between these zones, natural areas present covering, smooth leveling, and random percussion impacts, which attest to the simultaneous, rather than successive, use of these zones. Each of these four zones is related to a specific action mode on the material being transformed: percussion of the material by strong impacts on the faces and sides, regularization by fine percussion on oblique areas, and finishing by abrasion with the ends. All of these use-wear traces are compatible with the shaping of metallic objects, whereby percussion and abrasion were carried out alternately. An XRF map was acquired in order to reveal the distribution of elements on face (1) of the tool, where a cupule is visible. The distributions of the main elements are presented in Fig. 12. The XRF maps clearly show that Fe, Ti, Ca, and Sr are constitutive elements of the rock itself. In general, these elements are homogenously distributed; Ti has a more granular aspect. Two Cu spots correlated with Zn that are visible on each side of the cupule are of particular interest. Their localizations correspond to that of a well-developed, shiny, smooth leveling. This presence of Cu is underlined by the XRF spectra in Fig. 13 which highlight the difference in the intensity of the Cu Kα peak between the spots containing Cu and the rock itself.

These three tools share several notable features. First, they are all made of hard, cohesive, heat-resistant raw materials that are particularly suitable for the treatment of ores or metallic items. Secondly, they have all been intensively used as indicated by the significant deformation of the morphology of several of their faces. The macroscopic traces clearly suggest diversity in the handling of these tools, with evidence for crushing, shaping, and abrading actions. The microscopic use-wear analysis points to the processing of a semi-hard mineral material, and comparisons with the experimental data suggest that this material was metallic. The elemental XRF imaging underlines a correlation between the localization of these use-wear traces and the distinctive XRF lines of copper and zinc, two elements known to be involved in copper metallurgy and in bronze alloys. However, it remains difficult to distinguish whether these elements are the residues of copper production (with Zn possibly present naturally) or whether they are residues of bronze alloy production. Despite the spectral interferences that prevent the detection of tin traces, the processing of copper ore and copper-based alloys (i.e., bronze) is demonstrated. The combination of these four levels of analysis allows us to propose the following interpretation of the functions of these stone tools: one crushing-grinding tool [START_REF] Hamon | Proceedings of the international congress[END_REF] Archaeological and Anthropological Sciences, 12, 14. https://doi.org/10.1007/s12520-019-00999-x (n°13) used for the processing of copper ore, a small hammering and polishing tool (n°15) used for the shaping of metallic objects, and one tool (n°18) used for fine percussion and abrasion in the manufacture of metallic objects.

Discussion

A comprehensive methodology based on microscopic use-wear analysis and XRF imaging for the detection of stone tools used for metallurgical activities

The results clearly demonstrate the very close association between copper residues and metallic micropolishes on the working faces of the three stone tools selected. Consequently, a number of points can be made regarding the methodology.

In view of previous published tests [START_REF] Hamon | Les outils à cupules, marqueurs de la métallurgie du district de Cabrières-Péret (Hérault) au Chalcolithique[END_REF][START_REF] Delgado-Raack | Die Silberschmiede von Tira del Lienzo, 2014. Totana, Prov. Murcia im Kontext der El Argar Metallurgie[END_REF], the amount of metallic residues detectable using XRF measurements was expected to be particularly low. Several combined taphonomic and alteration processes can explain the generally poor preservation of copper and bronze residues on the surfaces of stone tools. In particular, this can be due to their high tendency to migrate into the soil or surroundings objects. For this reason, copper and bronze residues are generally difficult to identify on archeological artifacts using standard technologies, such as portable XRF techniques, even XRF measurements based on the previous optical detection of localized macroscopic residues. The use of the XRF raster map technology on the DiffAbs beamline of Soleil Synchrotron offered several advantages for optimizing the detection of copper residues. First, the ability to choose the most suitable X-ray microbeam energy allows the measurement settings to be adjusted to achieve the best possible level of detection. Secondly, the use of the continuous Flyscan mode makes it possible to analyze and map the complete working surface of the tools and to get comparable elemental spectra on a large grid including natural surfaces and use-wear surfaces. The common distribution of micropolishes (characterized by microscopic analysis) and copper residues (highlighted by the XRF Flyscan imaging) appears to exclude the possibility that these phenomena are the result of external contamination or microvariations in the mineral composition of the rock itself. [START_REF] Hamon | Proceedings of the international congress[END_REF] Archaeological and Anthropological Sciences, 12, 14. https://doi.org/10.1007/s12520-019-00999-x Despite the tiny amounts of copper detected, their codistribution with micropolishes definitely links their presence to the use of the tools for metallurgical operations. This common distribution suggests that the copper residues were trapped in the interstices of the rock and then covered by the micropolish or that the copper micro-residues were included within the micropolish itself during its formation. Our integrated analysis has clearly shown that only some of the areas bearing use-wear traces also display copper residues. In particular, the cupules did not contain any residues of copper although the location of these percussion impacts at the center of the main area of micropolish and copper micro-residues indicates that they were almost certainly used for the crushing of ore. For this reason, our observations would suggest that good preservation of the micropolishes has favored the good preservation of the copper residues and that micropolish formation integratesat least partlymicro-residues of the transformed material. This assertion is entirely in line with previous observations that highlighted the presence of cereal phytoliths within the micropolishes of flint sickles [START_REF]Préhistoire de l'agriculture : nouvelles approches expérimentales et ethnographique[END_REF] or of apatite residues incorporated within micropolishes observed on bone sharpeners [START_REF] Christensen | Pour relancer un débat : que sont les pierres à rainure du Natoufien Proche-Oriental ?[END_REF]. Our results support the idea of a common origin for the two kinds of traces, micropolishes, and residues, linked to the purpose for which the tool was used. [START_REF] Hamon | Proceedings of the international congress[END_REF] Archaeological and Anthropological Sciences, 12, 14. https://doi.org/10.1007/s12520-019-00999-x In this sense, our observations provide new evidence for discussing the nature and mechanisms at the origin of the formation of micropolishes, which was the subject of robust exchanges between use-wear analysis specialists in the 1980s and 1990s [START_REF] Costa | A few comments concerning residue analysis of stone plant-processing tools[END_REF][START_REF] Christensen | Processus de formation et caractérisation physicochimique des polis d'utilisation des outils en silex. Application à la technologie préhistorique de l'ivoire[END_REF][START_REF] Unger-Hamilton | Methods in microwear analysis. Prehistoric sickles and other stone tools from Arjoune[END_REF][START_REF] Yamada | The formation process of "use-wear polishes[END_REF].

A bronze metallurgist's toolkit in Kersulec

The comprehensive techno-functional approach developed for the macrolithic tool assemblage has allowed us to identify new evidence for the existence of a metallurgical workshop, which up to now was only suspected on the basis of the presence of a single crucible fragment. It also demonstrates the diversity of the types of tools and actions involved in the treatment of metallic objects. Crushing, hammering, abrading, and sharpening of tools all occurred on the Kersulec settlement. The large spectrum of actions represented suggests the use of stone tools at different stages of the chaine operatoire of metal processing (Fig. 2). These observations fit with previous observations and use-wear analyses conducted on different types of metal objects (axes, daggers, points) produced during the Copper and Bronze Ages in western and southern Europe [START_REF] Dolfini | The function of chalcolithic metalwork in Italy: an assessment based on use-wear analysis[END_REF][START_REF] Sáez | Puntas de Palmela: procesos tecnológicos y experimentación[END_REF][START_REF] Sáez | Traceology on metal. Use-Wear Marks on copper-based tools and weapons[END_REF][START_REF] Quilliec | L'épée atlantique : échanges et prestige au Bronze final, Mémoire de la Société Préhistorique Française n°XLII[END_REF]. These studies reveal that hammering, abrasion, and polishing actions were an integral part of the shaping and sharpening of metal objects prior to their use and deposition.

In Kersulec, crushing tools were used in ore processing and in the reduction of ore into powder before casting. Hammers were used for the shaping of metallic objects by plastic deformation, especially for the thinning and elongation of metal sheets. Finally, abraders and polishers were used to regularize the surface of metal objects up to their finishing. Hence, despite the relatively low number of tools potentially associated with the metallurgical process, all stages of the production of metal objects are represented on the Early Bronze Age settlement of Kersulec. [START_REF] Hamon | Proceedings of the international congress[END_REF] Archaeological and Anthropological Sciences, 12, 14. https://doi.org/10.1007/s12520-019-00999-x These tools must have been used alongside metal and wooden tools, as has been suggested in various other archeological contexts [START_REF] Thevenot | Un outillage de bronzier : le dépôt de la Petite Laugère, à Génelard (Saône-et-Loire, France)[END_REF].

Furthermore, these tools were all manufactured from dense, fine-grained rocks, specifically chosen for their hardness and resistance to impacts and to the effects of heat. The blanks selected were generally cobbles, offering naturally homogeneous surfaces, a low internal fissuring rate and smooth edges. The crushing tools are also of quite small dimensions, which would suggest precision crushing activities on small quantities of precious metallic compounds. It is also remarkable that the recycled Neolithic ax is probably related to the metallurgical activity. These choices indicate that raw materials with very particular mechanical properties were being deliberately selected for finishing operations requiring high precision. The Kersulec tools attest to the intensive use of all faces of the tools and an optimization of particular blanks. This is evident in the combination of several action modes (up to three) on one single tool and by the intensive use of almost all of the available faces of the tools.

Implications for the emergence of metallurgy in Northwestern Europe

The identification of crushing tools used for copper ore processing in Kersulec attests to metal production in Brittany, a question that has long been the subject of intense debate between specialists in the Bronze Age of Western Europe.

Based on the evidence for copper-based metallurgy in Kersulec, which is one of very few sites to produce such evidence, it would seem that, in Brittany, this early metallurgy was not organized around structured workshops or specialized sites. Apart from mining contexts, it is generally difficult to identify these metallurgical sites as they do not involve complex organization, complicated logistics, or structured production areas: a hearth, some charcoal, a handful of stone tools, and some ore, which generally disappears after it has been smelted, are the only remains that can be expected from this activity. [START_REF] Hamon | Proceedings of the international congress[END_REF] Archaeological and Anthropological Sciences, 12, 14. https://doi.org/10.1007/s12520-019-00999-x Kersulec perfectly illustrates this situation, and the fact that that the metalurgists' stone tools were not found in a dedicated area, or within a well-organized workshop, is not exceptional for the period. Indeed, the low visibility of this activity on the Early Bronze Age sites of Brittany and its association with domestic settlements suggest that such workshops existed in different kinds of contexts: open-air settlements, domestic contexts, enclosures, etc. Such an organization of the production clearly contradicts the notion of specialized sites dedicated to metal production.

Although the very discrete copper outcrops in Brittany have not yet revealed any clear evidence for extraction, this discovery of copper ore processing tools in Kersulec fuels the hypothesis of widespread small-scale copper or bronze production workshops in the region. This small-scale production at local level would fit with the exploitation of the small copper outcrops known in Brittany, especially in Finistère. The possible exploitation of these copper deposits has long been a subject of debate because of the relatively poor quality and low volumes of the ores present and their limited accessibility (Briard 1984, p. 93). This production could have been controlled by the elites buried in the Armorican Tumuli, elites who would have contributed to the development of an economy of prestige goods based on privileged exchanges with the British Isles, the Channel, and the North Sea coasts between 2150 and 1950 BC. As in many other contemporaneous funerary contexts, the deposition of stone tools related to metallurgy highlights the particular status of metallurgical crafts and of the people who mastered these technologies [START_REF] Nicolas | L'artisanat à l'âge du Bronze ancien en basse Bretagne[END_REF][START_REF] Turek | Craft symbolism in the bell beaker burial customs: resources, production and social structure at the end of the Eneolithic period[END_REF]).

Conclusion

Our identification of a metallurgical stone tool kit through usewear analysis conducted on objects from the Early Bronze age site of Kersulec appears to represent a promising and challenging new approach to understanding the earliest copper and bronze working in the area. Our comprehensive analysis is based on the best combination of techniques so as to limit the biases intrinsic to each individual methodology. It allows us to propose highly reliable functional interpretations that could not have been reached by applying each methoduse-wear and synchrotron micro-XRF analysisseparately. It underlines the potential of macrolithic tool use-wear analysis to reveal the existence of metal production on different types of archeological sites where structures associated with metallurgical production are barely discernible or difficult to recognize and where metallic objects are rare due to intensive recycling and diffusion. Although copper and bronze residues remain difficult to detect due to taphonomic factors, the multiplication of use-wear studies on macrolithic tools in Copper and Bronze Age contexts completes our knowledge of the chaine operatoire for the production of metal objects.

By allowing us to identify the metallurgist's stone tools and the different stages in the production of metal objects, this approach offers us the possibility to study in detail the techniques and processes respectively involved in the various regional productions of metal objects. It also offers us an opportunity to identify the contexts in which copper and bronze were produced, those who mastered these crafts and those who controlled the production, and thus it can help us to understand the major changes that occurred in the Early Bronze Age societies of Europe at the turn of the second millennium BC.

Fig. 1 :

 1 Fig. 1: Localization of the Early Bronze Age site of Ploneour-Lanvern (Brittany), among the Atlantic coast.

  Comparison of spectra from the rock and working zones allows us to confirm the intensity difference of the Cu Kα peaks and thus to confirm the presence of Cu linked specifically to the use-wear traces. Hamon et al. (2020) Archaeological and Anthropological Sciences, 12, 14. https://doi.org/10.1007/s12520-019-00999-x

Fig. 3 :

 3 Fig. 3: Ploneour-Lanvern «Kersulec», crushing tool on microgranite cobble, n°13; in blue, localization of two cupules on face 1: (a) interface between cupule 1 (with percussion impacts) on the right and reflexive smooth leveled zone on its periphery (with detailed of

Fig. 4 :

 4 Fig. 4: Tool n°13, face 1. XRF elemental maps at 18 KeV with acquisition step of 300 μm, showing distribution of use-wear and major rock elements.

Fig. 5 :

 5 Fig. 5: Tool n°13, face 2. XRF elemental maps at 18 KeV with acquisition step of 300 μm, showing distribution of use-wear and major rock elements.

Fig. 6 :

 6 Fig. 6: Tool n°13, XRF spectra -18 KeV; (a) fit from PyMCA to deconvolute the contribution of the different elements; (b) comparison of spectra from used and non-used (natural) surfaces of the tool.

Fig. 7 :

 7 Fig. 7: Ploneour-Lanvern «Kersulec», ax fragment in eclogite reused as hammering and polishing tool, n°15 (a) macrotraces on end 1 (left, micropolish visible on the edges; right, transversal microstriations); (b) macro-and microtraces on end 2 (with characteristic micropolish); (c) macro-and microtraces on side 1 (with characteristic micropolish) (photo: E. Collado, drawing Y. Pailler, microphotos: C. Hamon).

Fig. 8 :

 8 Fig. 8: Tool n°15, end 1. XRF elemental maps at 18 KeV with acquisition step of 300 μm, showing distribution of use-wear and major rock elements.

Fig. 9 :

 9 Fig. 9: Tool n°15, end 2. XRF elemental maps at 18 KeV with acquisition step of 300 μm, showing distribution of use-wear and major rock elements.

Fig. 10 :

 10 Fig. 10: Tool n°15, XRF spectra -18 KeV (a) fit from PyMCA to deconvolute the contribution of the different elements; (b) comparison of spectra from used and nonused (natural) surfaces of the tool.

Fig. 11 :

 11 Fig. 11: Plonéour-Lanvern «Kersulec», small multifaceted hammering and crushing tool on a diorite cobble, n°18: (a-c and e) small percussion impacts on some facets in the ends; (d and g) combined smooth leveling and fine percussion on the ends; (f) percussion impacts in main cupule on face 1 (photo: E. Collado, drawing V. Brisotto, microphotos: C. Hamon).

Fig. 12 :

 12 Fig. 12: Tool n°18, face 1. XRF elemental maps at 18 KeV with acquisition step of 300 μm, showing distribution of use-wear and major rock elements.

Fig. 13 :

 13 Fig. 13: Tool n°18 face 1, XRF spectra -18 KeV (a) fit from PyMCA to deconvolute the contribution of the different elements; (b) comparison of spectra from used and non-used (natural) surfaces of the tool.

Acknowledgments -We acknowledge Soleil for providing access to their synchrotron radiation facilities, and we would like to thank the whole team for their assistance in using the "DiffAbs" beamline. The experimental tests have been undertaken in collaboration with Henri Gandois (UMR 8215, Trajectoires) with the participation Aurélien Burlot; a great thanks to them for accepting us to use photos and results.


Cécile Joseph

Raphaëlle Savoire

Christelle Harscoat-Schiavo

Didier Pintori

Julien Monteil

Chrystel Faure

Fernando Leal-Calderon

Redispersible dry emulsions stabilized by plant material: rapeseed press-cake or cocoa powder

Keywords: emulsions, spray-drying, freeze-drying, plant material

This work aimed at valorizing by-products of the agrifood industry: powders deriving from rapeseed press-cakes and cocoa. Novel oil-in-water emulsions stabilized by these vegetal powders were formulated and were dehydrated by freeze-drying and spray-drying. Dry emulsions of at least 40% oil content and with a moisture content below 2.6 wt.% were obtained. Dry-emulsions did not exhibit any clumping even after several months of storage at room temperature, and were redispersible in water. To compare the ability of the drying process to preserve the state of dispersion, the size distributions of the oil droplets were compared before dehydration and after redispersion of the dry emulsions in an aqueous phase.

Several oil-to-vegetal powder weight ratios were probed, and the impact of a solid fillerlactose-was investigated.

Introduction

Vegetable oils are oils or fats extracted from plant seeds. Under conventional conditions, seeds are crushed and pressed, thus generating liquid oil and a solid press-cake. The increase in oil production worldwide implies a proportional increase in oil mill wastes. Therefore, immediate attention is required to manage mill waste materials in order to minimize their environmental impact and to recycle them as value-added ingredients or functional materials for various domains including the food industry [START_REF] Chandrasekaran | Oils seeds[END_REF]. Press-cakes are very often exploited for composting or animal feeding. While valuable, such type of valorization has low technological and economical value. Most biomass waste is a complex and variable mixture of molecules, and separation is a key issue. A complication comes from the fact that both the bio-waste and the materials to be separated are solid. Therefore, separation frequently involves organic solvents and/or harsh processing conditions. In the current regulatory climate, the chances of introducing new stabilizers into foods that rely on the use of harsh chemicals or large quantities of organic solvents are very small. For obvious economic, regulatory and environmental reasons, applications requiring a minimum level of bio refinery are highly sought after. Here, we highlight the possibility to valorize press cakes as inexpensive, renewable, and abundantly available stabilizers for the fabrication of dry emulsions.

Emulsions are liquid-liquid (oil-water) metastable colloids generally obtained in the presence of surface-active molecules like surfactants, amphiphilic polymers or proteins. Solid amphiphilic particles can also be used as emulsion stabilizers. Following the seminal studies by Pickering and Ramsden [START_REF] Pickering | CXCVI.-Emulsions[END_REF][START_REF] Ramsden | Separation of Solids in the Surface-Layers of Solutions and "Suspensions" (Observations on Surface-Membranes, Bubbles, Emulsions, and Mechanical Coagulation). --Preliminary Account[END_REF], solid-stabilized emulsions are commonly referred to as "Pickering emulsions". In this kind of system, large solid particles (from 1 nm to several microns) are irreversibly anchored at the oil-water interface [START_REF] Binks | Particles as surfactants-similarities and differences[END_REF]. Their kinetic stability is generally enhanced, compared to conventional surfactantstabilized emulsions, owing to the formation of a thick and rigid interfacial layer. To adsorb at the interface, particles are required to be wetted by both immiscible liquids, i.e. they must exhibit amphiphilic properties. As a matter of fact, the emulsion type (oil-in-water (O/W) or water-in-oil (W/O)) is mainly determined by the relative wettability of the particles, as stated by the empirical Finkle's rule [START_REF] Finkle | The theory of emulsification[END_REF]: hydrophilic (resp. hydrophobic) particles will preferentially stabilize oil (resp. aqueous) drops dispersed in water (resp. oil).

Pickering emulsions based on solid particles of biological origin are of great interest. Using these biomass-based particles to stabilize emulsions meets consumers' demand for a replacement of synthetic surface-active agents by environmentally friendly compounds. For instance, many studies have been directed towards emulsions stabilized by food-grade particles. These include fat and wax crystals, starches (Yusoff & Murray, 2011), chitin nanocrystals, cellulose fibers [START_REF] Campbell | Scalable fabrication of anisotropic micro-rods from food-grade materials using an in shear flow dispersionsolvent attrition technique[END_REF], flavonoids [START_REF] Luo | Particle-Stabilizing Effects of Flavonoids at the Oil-Water Interface[END_REF] and protein microgels [START_REF] Destribats | Emulsions stabilised by whey protein microgel particles: towards food-grade Pickering emulsions[END_REF]. [START_REF] Gould | Cocoa particles for food emulsion stabilisation[END_REF] and [START_REF] Joseph | O/W Pickering emulsions stabilized by cocoa powder: Role of the emulsification process and of composition parameters[END_REF] explored the functionality of cocoa particles as emulsion stabilizers through the mechanism of Pickering stabilization.

Compared to conventional emulsions, dry emulsions have many technological assets: increased shelf life, lack of bacteriological growth owing to the very low water activities, facilitated transport, and improved oil stabilization against oxidation [START_REF] Gu | Influence of pH and iota-carrageenan concentration on physicochemical properties and stability of beta-lactoglobulinstabilized oil-in-water emulsions[END_REF][START_REF] Klinkesorn | Stability of spray-dried tuna oil emulsions encapsulated with two-layered interfacial membranes[END_REF][START_REF] Shaw | Spray-dried multilayered emulsions as a delivery method for omega-3 fatty acids into food systems[END_REF]. To avoid oil droplets from merging into a macroscopic separated oil phase, either during the drying process or during the storage of the dry powder, a hydrophilic wall material is generally added to the aqueous phase. Typical examples of species used as wall materials are glucose, lactose, maltodextrin, starch, and cellulose. Once water is evaporated, this wall material, together with the emulsifier used to stabilize the emulsion, forms a solid phase that immobilizes the droplets, thus avoiding their recombination (coalescence). The amount of solid hydrophilic wall material needed ranges in general between 30 and 80 wt.% of the total final powder [START_REF] Gu | Influence of pH and iota-carrageenan concentration on physicochemical properties and stability of beta-lactoglobulinstabilized oil-in-water emulsions[END_REF][START_REF] Jost | Heat-Set Gels Based on Oil/Water Emulsions: An Application of Whey Protein Functionality[END_REF][START_REF] Klinkesorn | Stability of spray-dried tuna oil emulsions encapsulated with two-layered interfacial membranes[END_REF]. Alternative strategies to the introduction of this solid wall materials have been explored. These include the formation of viscoelastic layers based on polyelectrolyte complexes [START_REF] Gu | Influence of pH and iota-carrageenan concentration on physicochemical properties and stability of beta-lactoglobulinstabilized oil-in-water emulsions[END_REF][START_REF] Klinkesorn | Stability of spray-dried tuna oil emulsions encapsulated with two-layered interfacial membranes[END_REF][START_REF] Moreau | Production and characterization of oil-in-water emulsions containing droplets stabilized by betalactoglobulin-pectin membranes[END_REF][START_REF] Mun | Influence of Droplet Characteristics on the Formation of Oil-in-Water Emulsions Stabilized by Surfactant-Chitosan Layers[END_REF], cross-linked whey proteins [START_REF] Cho | Encapsulation of Fish Oil by an Enzymatic Gelation Process Using Transglutaminase Cross-linked Proteins[END_REF][START_REF] Jost | Heat-Set Gels Based on Oil/Water Emulsions: An Application of Whey Protein Functionality[END_REF][START_REF] Kellerby | Lipid oxidation in a menhaden oil-in-water emulsion stabilized by sodium caseinate cross-linked with transglutaminase[END_REF][START_REF] Romoscanu | Cross linking and rheological characterization of adsorbed protein layers at the oil-water interface[END_REF][START_REF] Sarkar | Microstructure and long-term stability of spray dried emulsions with ultra-high oil content[END_REF], proteinpolysaccharide complexes induced by Maillard reaction [START_REF] Li | Enhancing the stability of proteinpolysaccharides emulsions via Maillard reaction for better oil encapsulation in spraydried powders by pH adjustment[END_REF].

The intrinsic stiffness of the interfacial layers, possibly combined with the excess polymeric material efficiently protects droplets against recombination.

In the present work, we propose a facile and cost-effective solution to valorize mill wastes and/or by-products of the agrifood industry. We fabricated O/W emulsions based on an oil currently used in the food industry, sunflower oil, stabilized by delipidated and finely crushed powders of vegetal origin, cocoa powder and rapeseed press-cake. These latter were selected because they are abundant and because of their large content in surface-active species like proteins [START_REF] Lecumberri | Dietary fibre composition, antioxidant capacity and physico-chemical properties of a fibre-rich product from cocoa (Theobroma cacao L.)[END_REF][START_REF] Leming | Chemical composition of expeller-extracted and coldpressed rapeseed cake[END_REF]. Two processes were implemented to dehydrate the emulsions and to produce fine powders: freeze-drying and spray-drying. The redispersability of the powders in water was examined. To compare the ability of the drying process to preserve the state of dispersion, the size distributions of the oil droplets were compared before dehydration and after redispersion of the powders in an aqueous phase.

Several oil-to-vegetal powder weight ratios were probed, and the impact of the added wall material -lactose-was investigated.

Materials and methods

Materials

The oil phase of the emulsions consisted of sunflower oil (Rustica, France, density: 920 kg.m 3 ) purchased from a local supermarket. Cocoa powder is generally obtained after grounding the cocoa beans and crushing the obtained paste by a hydraulic press to extract cocoa butter. In that respect, cocoa powder is reminiscent of a press-cake.

Preparation of parent emulsions stabilized by plant material

The aqueous phase containing cocoa powder or rapeseed press-cake and lactose (several proportions were probed, see Table 1), was first ground using a bead mill (Dyno ® -Mill Multi Lab, Willy A Bachofen-WAB, France), equipped with chrome accelerators (DYNO-Accelerators, Willy A Bachofen-WAB, France). The grinding chamber (0.6 L) was previously filled with water and 810 -4 m zirconium beads (Zyrmill Y, Willy A Bachofen-WAB, France) at the maximum level of bead filling (65% v/v). Milling was run in continuous mode in one single pass. The liquid was pumped at a flow rate of 0.1 kg/min (1.6710 -3 kg/s) with a rotational speed for the grinding media of 10 m/s. The dispersion was cooled with water circulating in the jacket of the grinding chamber.

The aqueous dispersion deriving from the bead milling step was then agitated using an Ultra-Turrax® mixer (rotor-stator system, T25 digital, dispersing element S25 N-25F, IKA, Germany) working at 12 000 rpm (1257 rad/s) for 2 min. The stirring speed was then increased up to 16 000 rpm and the required amount of sunflower oil was slowly added to the water phase under agitation (around 5 min). Mixing was then extended for 10 min at 20 000 rpm (2095 rad/s). The system was finally homogenized using a single-chamber high-pressure homogenizer (ALM, Pierre Guérin, FRANCE), operating at a pressure of 265 bars. The whole emulsion volume was submitted to 2 passes through the homogenizing chamber.

Parent emulsions characterization

Optical microscopy

An Olympus BX51 optical microscope (Olympus, Japan) fitted with a digital color camera (Color View, Olympus, Japan) was used to gain qualitative information about the emulsions in terms of characteristic droplet size and aggregation state. All the emulsions prepared in this study were observed and characteristic images with be provided in subsequent sections. To facilitate the observation, emulsions were always diluted by dispersing 200µL (210 -7 m 3 ) of emulsion in 1 500µL (1.510 -6 m 3 ) of deionized water. Samples were deposited on a glass slide and were covered by a cover slip. A specific configuration was adopted in order to avoid sample damage. Indeed, we observed that the shear applied upon spreading of the confined liquid sometimes provoked droplet coalescence on the glass substrate. To avoid this phenomenon, cavity microscopy slides (well slides) with single ground and polished spherical cavities (Marienfeld, Germany) were adopted for observations. The samples were placed in the cavity zone before the cover slip was carefully deposited.

Droplet size measurements

Surfactant-stabilized emulsions droplets are commonly analyzed using light-scattering. In solid-stabilized emulsions, both particles and droplets are detected and consequently the scattering signal is not directly exploitable. A specific treatment was applied to our emulsions to get rid of particles present both in the bulk phase and at the oil/water interface, following the method presented by [START_REF] Joseph | O/W Pickering emulsions stabilized by cocoa powder: Role of the emulsification process and of composition parameters[END_REF].

The emulsions were submitted to a 4-fold dilution in a 10 wt.% SDS solution. The system was then stirred with a magnetic bar for 10h. The emulsions diluted in the SDS solution were then centrifuged for 5 minutes at 220g (Rotanta 460 RF Centrifuge, Hettich Lab Technology, Germany), g being the earth gravity constant, in order to separate the particles (sediment) from the oil droplets (cream). The resulting creams were milky and colorless, indicating that cocoa or rapeseed particles were not present anymore. In their seminal work, [START_REF] Joseph | O/W Pickering emulsions stabilized by cocoa powder: Role of the emulsification process and of composition parameters[END_REF] proved that this protocol led to droplets separation/disaggregation. After careful inspection of the droplets under the microscope before and after SDS treatment, they concluded that the droplets sizes were apparently preserved. They also observed the emulsions by fluorescence microscopy and particles were not detected anymore at the interfaces after SDS treatment.

This low molecular weight surfactant is known to displace species with high interfacial adsorption energies like proteins [START_REF] Mackie | Competitive displacement of β-lactoglobulin from the air/water interface by sodium dodecyl sulfate[END_REF][START_REF] Pradines | Interfacial properties of mixed β-lactoglobulin-SDS layers at the water/air and water/oil interface[END_REF].

The creams were collected, diluted 50 times in deionized water and analyzed. The size distributions were measured using the Mastersizer 2000 apparatus (Malvern Instruments, France). Static light-scattering data were transformed into size distribution using Mie Theory.

The droplet refractive index was 1.47 and that of the aqueous phase was 1.33. A small volume of sample was introduced under stirring at 1 600 rpm (168 rad/s) in the dispersion unit containing a solution of SDS at 0.2 wt.%. Each measurement was performed in triplicate. The droplets distributions were described in terms of their volume-averaged diameter, Dv, surfaceaveraged diameter, Ds, and polydispersity, P, defined as:

𝐷 = ∑ ∑ ; 𝐷 = ∑ ∑ ; 𝑃 = ∑ ( ) ∑ ( 1
)
where 𝐷 is the median diameter, i.e. the value for which the cumulative undersized volume fraction is equal to 50%, and 𝑁 is the total number of particles with diameter 𝐷 .

Analysis of soluble compounds

Hereafter, the term "particles" will refer to the water-insoluble fraction of the cocoa powder or of the rapeseed press-cake. Being denser than water, such fraction can be separated from the soluble one upon centrifugation.

A 10 wt.% dispersion of vegetal powder dispersed in deionized water was centrifuged for 1h at 100 000 g (Aventi J-301 Centrifuge, Beckman Coulter, USA). The supernatant containing the soluble fraction was collected and freeze-dried for mass measurement. The same procedure was performed on a 10 wt. % dispersion of vegetal powder processed in the bead mill. After re-dissolution of the freeze-dried supernatant, water-soluble carbohydrates, proteins and polyphenols were quantified. Carbohydrates were titrated using the anthrone-sulfuric acid assay [START_REF] Leyva | Rapid and sensitive anthrone-sulfuric acid assay in microplate format to quantify carbohydrate in biopharmaceutical products: method development and validation[END_REF]. Briefly, carbohydrates were hydrolyzed (150µL (1.510 -7 m 3 ) of reagent at 1g/L (1 kg/m 3 ) for 50µL (510 -8 m 3 ) of sample, 30 s of stirring, 20 min at 100 °C)

and the resulting furfuralic compounds were assayed at 620 nm using a glucose calibration curve from 0 to 100 mg/L (0 to 0.1 kg/m 3 ). Proteins were quantified using Bradford reagent (or Coomasie brilliant Blue) (150µL (1.510 -7 m 3 ) for 5µL (510 -9 m 3 ) of sample), which is known to form light-absorbing complexes in the presence of proteins. Complexes were titrated at 595 nm using a Bovine Serum Albumin (BSA) calibration curve from 0 to 500 mg/L (0 to 0.5 kg/m 3 ) [START_REF] Noble | Quantitation of Protein Chapter 8[END_REF]. Polyphenols were quantified using Folin-Ciocalteu reagent (100µL (10 - 7 m 3 ) for 20µL (210 -8 m 3 ) of sample). This yellow-colored acid turns to blue upon phenol oxidation. Polyphenol quantification was performed using a gallic acid calibration curve from 0 to 100 mg/L (0 to 0.1 kg/m 3 ) [START_REF] Zhang | A simple 96well microplate method for estimation of total polyphenol content in seaweeds[END_REF].

Water-soluble compounds within the emulsions were also analyzed and quantified.

Emulsions were centrifuged for 1h at 100 000 g (Aventi J-301 Centrifuge, Beckman Coulter, USA), in order to induce particles sedimentation and droplets creaming. The intermediate aqueous phases were carefully sampled and centrifuged a second time for 5 min at 4600 g to remove residual droplets prior to freeze-drying and analysis.

Emulsion drying

The dehydration of O/W emulsions is a process employed in the pharmaceutical, cosmetic, and food arenas to obtain solid-like powders. The most frequently methods employed to evaporate water rapidly are spray-drying and freeze-drying (Hernandez Sanchez, Cuvelier, & Turchiuli, 2015; Marefati, Rayner, Timgren, Dejmek, & Sjöö, 2013). Spray-drying is a unit operation in which the emulsion is pumped, atomized and dried in contact with a hot air current. Freeze-drying is a process where water is crystallized at low temperature and then sublimated directly from the frozen state into vapor by decreasing the pressure. Parent emulsions were dried just after fabrication using 2 different techniques.

Freeze drying: emulsions were stored at least 12h at -80 °C and then introduced into a freezedryer (FTS Systems, Dura-Dry, USA) at a pressure of 0.1 mbar for 24h.

Spray-Drying: the emulsions were dried in a pilot-scale spray dryer (Niro Mobile Minor, GEA, Denmark) equipped with a rotary atomizer. The emulsions were fed into the spray tower at a feed flow rate of 1.5L/h (4.210 -7 m 3 /s). Inlet and outlet air temperatures were 220 °C and 90 °C, respectively.

The obtained powders were stored at room temperature.

2.5.

Characterization of dry emulsions 2.5.1. Moisture content

The residual moisture content of dry emulsions was determined gravimetrically with a moisture analyzer (Moisture Series, Fischer Scientific, US) by drying the samples at 90 °C until a constant weight was reached. Values were expressed on a wet basis.

Oil encapsulation efficiency

During the drying process oil can be macroscopically released. The oil encapsulation efficiency is defined as the fraction of oil which remains embedded in the grains matrix after the drying process. The oil content of spray-dried emulsions was determined using the normalized method of Soxhlet extraction with hexane (NF EN ISO 734-1), and was compared to the theoretical oil content assuming that only water was eliminated during the drying process (no oil leakage).

Bulk density

To determine the average bulk density, a 6mL-glass test tube was totally filled with the dry emulsion powders. The tube was tapped vertically against a bottom solid surface to pack the powder and eliminate voids. The initial height was restored by adding powder and the process was repeated until no further variation of the volume was noticed after tapping.

Optical microscopy

Microscope observations were performed in order to identify potential differences in grain morphologies, depending on the drying technique (Olympus BX51 optical microscope, Olympus, Japan). Dry emulsion powders were directly spread on a glass slide and observed in air, without cover slip, to avoid compression of the grains that would produce oil leakage.

2.6. Rehydrated emulsions

Redispersion of dry emulsions in water

The powders were redispersed by mixing them with the amount of distilled water required to restore the same composition as that of the parent emulsions. The amount of water mw, to be added to a mass md of dry emulsion is given by equation ( 2):

𝑚 = 𝑚 -1 (2)
where DE0 is the dry extract (wt.%) of the parent emulsion, i.e. the summation of the vegetal powder, lactose and oil contents indicated in Table 1. The redispersion was made the same day the emulsions were dehydrated and after an 8-month storage period at room temperature (20  3 °C).

The powders were gently dispersed by manual shaking using a spatula until the samples became homogeneous. In all cases, the redispersion process took less than 1 min.

Optical microscopy and droplet size measurements

Rehydrated emulsions were observed following the method described in section 2.3.1.

Droplet size distributions were measured using exactly the same protocols as for their parent emulsions, after SDS treatment (section 2.3.2).

We define Rv -respectively Rs-, as the ratio between the droplet volume-averaged diameter -respectively the surface-averaged-of the rehydrated emulsion and of the parent emulsion.

Provided oil leakage after the drying process is insignificant, the ratios are expected to be larger than unity if coalescence between the droplets occurs during the drying process. The smaller these ratios (the closer to unity), the better the protection against coalescence. Rs reveals the relative evolution of the oil/water interfacial area. Indeed, the total surface area of the droplets is given by 𝑆 = , where Vd is the volume of the dispersed phase and Ds is defined by Eq.1. Rv is a good indicator for assessing the extent of droplet coalescence, as the volume-averaged diameter provides pronounced statistical significance to large droplets (see Eq. 1).

Statistical analysis

All the above-mentioned analyses (average droplet size measurements, soluble compounds, moisture content, oil encapsulation efficiency, bulk density of dry emulsions) were performed at least in triplicate and the values are expressed as means  mean deviations.

Statistical analyses based on ANOVA + test post hoc Tukey HSD were performed using R freeware through the Rstudio interface.

Results and discussion

Parent emulsions

The compositions of parent emulsions were selected so as to find the best compromise between two antagonistic constraints: (i) good kinetic stability upon drying, which is facilitated by a large amount of vegetal powder and wall material, and (ii) low viscosity of the emulsions to be sprayed in the atomizer, which is generally obtained at low dried extracts. Table 1 indicates the selected compositions of all parent emulsions as well as their main droplet size features deduced from light scattering (Ds, Dv and P). The powder content was kept constant in the experiments, while the powder origin (cocoa or rapeseed), the lactose content and the oil content were varied. In all cases, the average diameters were in the micrometer range. Vegetal particles being relatively large (from 0.1 to several tens of microns), they act as "collectors" likely to attach two of more oil droplets on their periphery. This bridging configuration favors the formation of large droplet flocs. The emulsions were kinetically stable since no macroscopic oil leakage was observed at the top of the recipients and the mean diameters did not evolve after several months of storage at 4 °C (data not shown). Due to the strong aggregated state, the emulsions were highly viscous, even gelled in some cases, which delayed gravity driven phenomena. Only a slight creaming phenomenon was observed after a few days of storage but it had no influence on the size distribution. We would like to stress here that the measured size distributions are relative to individual droplets, and not to aggregates, since the adopted sizing protocol ensured full detachment of the particles from the interfaces and disaggregation of the droplets (see § 2.3.2).

The emulsions are stabilized by the particles (insoluble fraction of the powder, [START_REF] Joseph | O/W Pickering emulsions stabilized by cocoa powder: Role of the emulsification process and of composition parameters[END_REF] and also potentially by surface-active species initially dissolved in the continuous phase. One of the most important roles of emulsifiers is to facilitate the formation of small emulsion droplets during homogenization. An effective emulsifier should rapidly adsorb to the freshly formed droplet surfaces, reduce the interfacial tension to facilitate droplet break-up under shear and provide a protective coating that prevents the droplets from aggregating with their neighbors [START_REF] Leal-Calderon | Emulsion science: basic principles[END_REF]. The phospholipids of cocoa beans are important as surface-active compounds in chocolate products. It is generally admitted that the phospholipid level in the common varieties of cocoa beans is relatively constant at 0.3 to 0.5 wt.% [START_REF] Parsons | Identification and quantitative analysis of phospholipids in cocoa beans[END_REF]. In rapeseeds, the phospholipid content is of the order of 1 wt.% (Ambrosewicz- [START_REF] Ambrosewicz-Walacik | Phospholipids of Rapeseeds and Rapeseed Oils: Factors Determining Their Content and Technological Significance-A Review[END_REF]. Proteins are also very efficient stabilizers due to their amphiphilic nature, their ability to adsorb at the oil/water interface at relatively fast rates and to protect droplets against coalescence under different conditions [START_REF] Mcclements | Protein-stabilized emulsions[END_REF]. Rapeseed powder has a significantly larger overall protein content than cocoa powder: ~35 wt.% vs. ~20 wt.% [START_REF] Lecumberri | Dietary fibre composition, antioxidant capacity and physico-chemical properties of a fibre-rich product from cocoa (Theobroma cacao L.)[END_REF][START_REF] Leming | Chemical composition of expeller-extracted and coldpressed rapeseed cake[END_REF]. The composition of soluble compounds in terms of carbohydrates, polyphenols and proteins was measured for both powders and the results are reported on Table 2. Proteins contained in both powders are mostly insoluble. It is likely that particles are adsorbed at the oil/water interfaces via surface-active moieties deriving from insoluble proteins (attached to the particles), protein aggregates, polyphenol-protein or polyphenol-polysaccharide complexes [START_REF] Foegeding | Proteinpolyphenol particles for delivering structural and health functionality[END_REF][START_REF] Liu | Food-Grade Covalent Complexes and Their Application as Nutraceutical Delivery Systems: A Review[END_REF] and phospholipids. Owing to their small size (and consequently their intrinsically large mobility), soluble proteins may rapidly adsorb at the interfaces and may also significantly contribute to emulsion stabilization.

Properties of dry emulsions

Parent emulsions were dried following the two techniques described in section 2.4. Table 3 indicates the theoretical composition of dehydrated emulsions. This latter was calculated assuming that the total amount of water was evaporated and that all other components were fully recovered. Dry emulsions were characterized right after their preparation in terms of oil content, residual moisture content and bulk density, and the results are reported in Table 4.

The measured values of the oil content (Table 4) are very close to the theoretical ones reported in Table 3 (the differences are within the error bars), indicating that the oil loss was insignificant during the drying process.

In emulsified formulations meant to be spray-dried, the oil content relative to dried matter generally remains below 33 wt.% [START_REF] Carneiro | Encapsulation efficiency and oxidative stability of flaxseed oil microencapsulated by spray drying using different combinations of wall materials[END_REF][START_REF] Ixtaina | Physicochemical characterization and stability of chia oil microencapsulated with sodium caseinate and lactose by spray-drying[END_REF][START_REF] Shamaei | Microencapsulation of walnut oil by spray drying: Effects of wall material and drying conditions on physicochemical properties of microcapsules[END_REF]. A few studies reported significantly higher oil contents [START_REF] Mezzenga | Spray-dried oil powder with ultrahigh oil content[END_REF][START_REF] Sarkar | Microstructure and long-term stability of spray dried emulsions with ultra-high oil content[END_REF]. Our systems containing between 40 and 68 wt.% oil are thus in a relatively high range compared to current investigations. The powders remained kinetically stable under quiescent conditions, with no apparent oil leakage even after several months of storage at room temperature. However, upon shearing between fingers, oil was progressively released providing a smooth feel. This simple experiment reveals that oil delivery can be produced by applying a mechanical stress. Irrespective of the drying technique, the moisture contents are between 0.9 and 2.6 wt.%, within the prescribed regulations for dried powder in the food industry (<3-4 wt.%) [START_REF] Klinkesorn | Stability of spray-dried tuna oil emulsions encapsulated with two-layered interfacial membranes[END_REF].

Bulk density is an important property as it determines the ability of the powder to be packed. This parameter is influenced by the grain size distribution, the grain shape, the state of aggregation of the grains and the moisture content [START_REF] Goula | Influence of spray drying conditions on tomato powder properties[END_REF]).

On Table 4, it appears that for each formulation, powders deriving from the freeze-drying process have lower bulk densities than those obtained by spray-drying. Fig. S1 (supplementary material) shows the visual (macroscopic) aspect, as well as typical microscope images of the systems. Spray-dried emulsions are made of well-defined large spheroidal grains, whose diameter is of the order of several tens of micrometers. On the contrary, powders deriving from freeze drying are coarse. They are made of irregularly shaped and strongly aggregated grains.

Thus, it can be stated that the two drying techniques generate different grain morphologies and aggregation states, which impacts their average bulk densities.

For both drying techniques, powders had the same macroscopic appearance after several months of storage. Neither oil leakage nor clumping were noticed, reflecting efficient stabilization of the encapsulated oil droplets by the solid components.

Rehydrated emulsions

Powders were redispersed the same day they were dehydrated (t0) using pure water in order to restore the composition of the parent emulsions. We never observed any oil layer residing at the top of the emulsions, confirming the absence of macroscopic oil leakage. We first compare formulations A and D which are identical, albeit based on two different vegetal sources. Table 5 indicates the values of the size ratios Rs and Rv and Fig. 1 (a2, a3, d2, d3) shows selected micrographs of the rehydrated emulsions. The corresponding size distributions are given in Fig. 2. Rehydrated emulsions are aggregated, as the parent emulsion. Formulation A based on rapeseed press cake provides very good performances. The size ratios Rs and Rv are close to unity for the powder deriving from spray-drying (Table 5). By comparing Fig. 1-a1 and a2, we deduce that parent and rehydrated emulsions are almost identical and this is confirmed by the size distributions in Fig. 2. For the system processed by freeze-drying, the size distribution becomes broader, with a new mode at large diameters, around 20 m (Fig.

2), reflecting droplet coalescence. This agrees with the observations under the microscope (Fig. 1-a3). One can notice in the image of formulation D on Fig. 1-d3 that droplets larger than the average are present, thus confirming the existence of the supplementary mode.

Powders based on cocoa could also be redispersed in pure water but both drying pathways induced substantial droplet coalescence, as revealed by the size ratios Rs and Rv, which were significantly larger than unity (Table 5). Interestingly, the emulsion deriving from spray-drying had a size distribution narrower than that deriving from freeze-drying (Fig. 2).

Redispersion in water was also achieved after a storage period of 8 months at room temperature for the systems deriving from spray-drying. In Figure S2 (supplementary material)

we compare the volume-averaged diameter of the parent emulsions, the dry emulsions rehydrated at t0 and the dry emulsions rehydrated after 8 months. The average diameters of the rehydrated emulsions measured at t0 and at t = 8 months are comparable, which confirms that dry emulsions remain kinetically stable over a long period of time.

To sum up, rapeseed press cake provides better performances than cocoa powder. This could be explained by the overall largest content in amphiphilic species in the rapeseed source (see section 3.1), providing better anchoring and coverage capabilities. These species include phospholipids, polyphenols complexes and proteins, these latter being the most abundant ones. Our results also reveal that emulsions processed by spray-drying are less prone to coalescence than those submitted to freeze-drying. This latter technique induces cryoconcentration of the droplets, likely to compress them and to provoke coalescence events [START_REF] Marefati | Freezing and freezedrying of Pickering emulsions stabilized by starch granules[END_REF].

Influence of the lactose content

Lactose is currently used as an edible filler [START_REF] Ixtaina | Physicochemical characterization and stability of chia oil microencapsulated with sodium caseinate and lactose by spray-drying[END_REF][START_REF] Li | Characterization of mechanical and encapsulation properties of lactose/maltodextrin/WPI matrix[END_REF][START_REF] Maher | Microstructure and lactose crystallization properties in spray dried nanoemulsions[END_REF]. The positive effect of lactose is generally attributed to the formation of a protective (against coalescence) continuous glass phase in which the droplets are embedded [START_REF] Rosenberg | Microencapsulation of volatiles by spray-drying in whey protein-based wall systems[END_REF].

Rapeseed-stabilized emulsions

The highly performing formulation A containing 7.5% lactose was compared to formulation B devoid of lactose. The macroscopic aspects of the powders obtained from formulations A and B are identical (see S1, supplementary material). Table 5 indicates the characteristic size ratios and some typical micrographs of the rehydrated emulsions (at t0) are reported in Fig 1 indicating that the grains were not fully dispersed in the rehydrated emulsion.

These results underline the ability of lactose to protect droplets against coalescence during the drying process. During the spray-drying process, the implementation of elevated air temperatures accelerates water evaporation from the aerosol particles surface. This results in the fast formation of a hard crust which provides a semi-permeable membrane around the particles, allowing further evaporation, while preventing leaching of oil droplets to the surface [START_REF] Gaiani | How surface composition of high milk proteins powders is influenced by spray-drying temperature[END_REF]. In addition, protein-carbohydrate conjugates formed as a result of the Maillard reaction between the amino groups of the proteins and the carbonyl group of lactose might have a significant effect on the crust formation [START_REF] Aghbashlo | Influence of wall material and inlet drying air temperature on the microencapsulation of fish oil by spray drying[END_REF].

Cocoa-stabilized emulsions

Because of the rather poor performance of formulation D (see section 3.2.1), the amount of lactose was raised up to 15.5 wt.% in formulation E. The macroscopic aspect of the powders deriving from formulation E is shown in S1 (e2, e4) (supplementary material). The powder obtained by freeze-drying is again coarser than that deriving from spray-drying. Emulsions were rehydrated at t0 and the main characteristics are given in Table 5 (size ratios), Fig. 1 (e2, e3) (typical micrographs) and Fig. 4 (size distributions). The droplets form dense aggregates whose size exceeds several hundred microns. By comparing these data with those corresponding to formulation D (Table 5, Fig. 1 (d2, d3), Fig. 2), it turns out that the increase in lactose concentration has a protective effect against coalescence.

Influence of the oil/vegetal powder ratio

Dry solid powders are available today mainly in formulations that comprise a relatively high weight percentage of non-oily compounds. The minimum amount of solid hydrophilic carrier needed is in general between 30 and 80 wt.% of the total final powder [START_REF] Gu | Influence of pH and iota-carrageenan concentration on physicochemical properties and stability of beta-lactoglobulinstabilized oil-in-water emulsions[END_REF][START_REF] Jost | Heat-Set Gels Based on Oil/Water Emulsions: An Application of Whey Protein Functionality[END_REF][START_REF] Klinkesorn | Stability of spray-dried tuna oil emulsions encapsulated with two-layered interfacial membranes[END_REF]. Dry solid powders comprising a high content of oil compounds would bring several significant advantages and are highly sought after.

In formulation C, the fraction of oil is twice as large as in formulation A. The increase in the oil/solid ratio is generally detrimental to the stabilization, since there is proportionally fewer wall material. Actually, the powder obtained by spray-drying exhibits good resistance to coalescence. Indeed, the size ratios Rs and Rv have low values, close to 1.5 (Table 5), and the evolution of the size distributions is moderate, with only a slight widening towards higher sizes (Fig. 5). The powder deriving from freeze-drying is more prone to coalescence. The corresponding rehydrated emulsion (at t0) becomes polydisperse, with two main modes centered on 5 and 50 microns. Thus, a dry powder comprising two thirds of oil (Table 4) can be obtained, preferentially by spray-drying. The powder exhibits no clumping or oil leakage (see S1, supplementary material) and its rehydratation leads to an emulsion without oil leakage as well.

Conclusion

It this paper, we investigated the possibility to produce novel redispersible powders based on Pickering oil-in-water emulsions stabilized by plant material, and processed by freeze-drying and spray-drying. Powders of at least 40% oil content and with a moisture content below 2.6 wt.% were easily achieved, without any loss. Fine and easily handleable dry powders were obtained. The study investigated the impact of the drying process and of several formulation parameters, including the filler/wall (lactose) content, the oil content, the vegetal origin of the stabilizing material (rapeseed and cocoa), on the overall properties of rehydrated emulsions.

Compared to their parent emulsions, rehydrated emulsions exhibited larger average droplet sizes due to the occurrence of coalescence phenomena during the drying process. Yet, the size distribution remained generally in the colloidal range (<100 m), and macroscopic release was not observed. Formulations based on rapeseed press-cake provided excellent performances, since after spray-drying, the redispersed emulsion had almost the same features as the parent emulsion in terms of size distribution. Strictly speaking, no additional hydrophilic filler compound was required. However, the stability against coalescence was significantly enhanced in presence of lactose. Spray-drying had less impact than freeze-drying on the evolution of the size distribution and rapeseed material was more performant than cocoa powder.

Pickering emulsions based on plant material represent a versatile and promising route to formulate powders that can find applications in the food industry. Such powders are a complete foodstuff as they contain lipids, proteins, fibers and polyphenols. They could be used as intermediary products to be incorporated in aqueous formulations (instant drinks, vegetal milks, soups).

  Fig. 1 (a1, b1, c1, d1, e1) show characteristic microscope images of the emulsions, revealing their strong aggregated state. Droplets form large and dense flocs, in which they are totally immobilized (they are not submitted to Brownian motion).[START_REF] Joseph | O/W Pickering emulsions stabilized by cocoa powder: Role of the emulsification process and of composition parameters[END_REF] described emulsion stabilization as being due to surface-active moieties of particles adsorbed at the interface, and network stabilization due to physical trapping of droplets in the particles network.

(

  a2,a3,b2,b3). Unlike formulation A, rehydrated emulsions deriving from formulation B exhibit significantly larger droplet average diameters than their parent emulsion for both drying techniques. This is confirmed by the size distributions of Fig.3. Once again, among the two techniques, spray-drying provides better performances, reflected by smaller Rs and Rv values.Spherical aggregates can be clearly distinguished in Fig.1-b2. These latter are reminiscent of the grains (i.e. aerosol particles) observed in the dry powder (S1-b1, supplementary material),

Acknowledgements

This work was performed, in partnership with the SAS PIVERT, within the frame of the French Institute for the Energy Transition (Institut pour la Transition Energétique (ITE)) P.I.V.E.R.T (www.institut-pivert.com) selected as an investment for the Future (« Investissement d'Avenir »). This work was supported, as a part of the Investments for the Future, by the French Government under the reference ANR-001-01.