Kaidi Peng

Mohamed Koubaa
email: m.koubaa@escom.fr

Olivier Bals

Eugène Vorobiev

Recent insights in the impact of emerging technologies on lactic acid bacteria: A review

Keywords: Lactic acid bacteria, fermentation, emerging technologies, inactivation, microbial stimulation

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Lactic acid bacteria (LAB) have the ability to initiate rapid acidification of dairy products. They produce lactic acid in the medium as well as other molecules that include acetic acid, ethanol, aroma compounds, bacteriocins, exopolysaccharides, and enzymes [START_REF] Leroy | Lactic acid bacteria as functional starter cultures for the food fermentation industry[END_REF]. LAB are industrially important microbes as they are widely used in food fermentation and preservation, chemical synthesis, and pharmaceutical preparations [START_REF] D'aimmo | Antibiotic resistance of lactic acid bacteria and Bifidobacterium spp. isolated from dairy and pharmaceutical products[END_REF][START_REF] Esteban-Torres | Characterization of a halotolerant lipase from the lactic acid bacteria Lactobacillus plantarum useful in food fermentations[END_REF][START_REF] Hugenholtz | Metabolic engineering of lactic acid bacteria for the production of nutraceuticals[END_REF][START_REF] Sauer | The efficient clade: Lactic acid bacteria for industrial chemical production[END_REF]. Enhancing the multiplication rates of LAB cells as well as the performances of the fermentation processes (e.g. improved products' yields) are of great interest for different industrial sectors and are therefore increasingly explored. For instance, it has been reported that fructo-oligosaccharides and trehalose had a potential improvement in the production of bacteriocins by LAB [START_REF] Chen | Effects of prebiotic oligosaccharides and trehalose on growth and production of bacteriocins by lactic acid bacteria[END_REF]. The addition of dry Spirulina platensis to milk at a concentration of 6 mg.mL -1 was shown to improve the growth of Lactococcus lactis by 27% [START_REF] De Caire | Effect of Spirulina platensis biomass on the growth of lactic acid bacteria in milk[END_REF]. In another work, peptones obtained from the enzymatic hydrolysis of yellowfin tuna (Thunnus albacares) head waste were a better nitrogen source in enhancing the LAB growth compared to the standard peptones composing commercial MRS media [START_REF] Safari | Use of hydrolysates from yellowfin tuna (Thunnus albacares) heads as a complex nitrogen source for lactic acid bacteria[END_REF].

On the other side, some lactic acid bacteria are known to be very common bacterial contaminants in foods and fermentation industries. They include for example the strain Lactobacillus sake, which causes unpleasant odors and off-flavors in vacuum-packaged meat products, Lactobacillus brevis, which increases the turbidity of beer and wine, and lactobacilli, which inhibit the growth of yeast and limit the production of ethanol during biofuel production by fermentation [START_REF] Beckner | Microbial contamination of fuel ethanol fermentations[END_REF][START_REF] Björkroth | Evaluation of Lactobacillus sake contamination in vacuum-packaged sliced cooked meat products by ribotyping[END_REF][START_REF] Tribst | Ultra-high pressure homogenization treatment combined with lysozyme for controlling Lactobacillus brevis contamination in model system[END_REF]. Traditional food preservation technologies such as heat pasteurization, sterilization, drying, and evaporation can guarantee the microbial safety of food products; however, they are hampered by the shortcomings of high temperature that may induce the thermal degradation of food ingredients [START_REF] Choi | The effects of thermal and nonthermal processing methods on apple cider quality and consumer acceptability[END_REF][START_REF] Pereira | Environmental impact of novel thermal and non-thermal technologies in food processing[END_REF]. The addition of antimicrobial agents was also used in preventing the spoilage effect of LAB such as the addition of 2.5% to 3.3% Na-lactate in cooked cured ham products, which inhibits the growth of Lactobacillus curvatus compared to products prepared according to a standard recipe [START_REF] Stekelenburg | Effects of sodium lactate and other additives in a cooked ham product on sensory quality and development of a strain of Lactobacillus curvatus and Listeria monocytogenes[END_REF]. It is also well known that sulfur dioxide (SO2) is usually added to slow down the activity of LAB in wine [START_REF] Valera | Effect of chitosan and SO2 on viability of Acetobacter strains in wine[END_REF].

Recently, emerging technologies have been increasingly tested at lethal levels on LAB to modify the cell permeability, denaturing proteins and DNA, and changing the activity of enzymes [START_REF] Goodarzi | UV -Induced mutagenesis in lactic acid bacteria[END_REF][START_REF] Ulmer | Effects of pulsed electric fields on inactivation and metabolic activity of Lactobacillus plantarum in model beer[END_REF][START_REF] Ulmer | Effects of pressure-induced membrane phase transitions on inactivation of HorA, an ATP-dependent multidrug resistance transporter, in Lactobacillus plantarum[END_REF]. These actions appeared to be promising for the inactivation of contaminating microorganisms and to ensure microbial safety as well as keeping fresh-like characteristics of food products [START_REF] Cichoski | Ultrasound-assisted post-packaging pasteurization of sausages[END_REF][START_REF] González-Arenzana | Pulsed electric field treatment after malolactic fermentation of Tempranillo Rioja wines: Influence on microbial, physicochemical and sensorial quality[END_REF]. On the other hand, the application of these technologies at sub-lethal levels on LAB before or during the fermentation process was shown to modify the growth rate, and inducing the metabolic pathways leading to obtaining different production yields [START_REF] Dahroud | Low intensity ultrasound increases the fermentation efficiency of Lactobacillus casei subsp.casei ATTC 39392[END_REF][START_REF] Maeno | Introduction of bifunctional alcohol/acetaldehyde dehydrogenase gene (adhE) in Fructobacillus fructosus settled its fructophilic characteristics[END_REF]. Hence, in this review paper, the impacts of the emerging technologies of pulsed electric fields (PEF), power ultrasound (US), high-pressure processing (HPP), ultraviolet (UV), and microwave (MW) on LAB will be discussed. A special emphasis will be devoted to describing the recent achievements in applying these technologies on LAB at sub-lethal levels.

Lactic acid bacteria (LAB)

LAB are generally defined as a group of Gram-positive, non-spore forming, either cocci or rods, which produce lactic acid as the major metabolic end product of the carbohydrate fermentation [START_REF] Stiles | Lactic acid bacteria of foods and their current taxonomy[END_REF]. They are widespread in nature and typically exist in substrates with high carbohydrate content including plants, milk, fermented foods, and mucosal surfaces of the human body. The major genera of LAB are Lactococcus, Lactobacillus, Leuconostoc, Pediococcus, and Streptococcus (De Vuyst & Vandamme, 1994). LAB play an important role in the food industry, not only due to their ability to cause rapid acidification of the raw material but also due to their production of different molecules such as ethanol, aroma compounds, exopolysaccharides, and bacteriocins. This allows providing a diversity of products in the market with different nutritional, sensory and organoleptic properties [START_REF] Leroy | Lactic acid bacteria as functional starter cultures for the food fermentation industry[END_REF].

LAB are traditionally used as starter cultures and have a long history of applications in different industrial sectors. Spontaneous fermentation occurs due to the development of the natural microbiota present in the raw materials. In 1857, Louis Pasteur demonstrated for the first time the microbial origin for lactic fermentation in food. A few years later, in 1878, Joseph Lister successfully isolated a pure strain of lactic bacterium that was discovered to be responsible for milk acidification [START_REF] Santivarangkna | Alternative drying processes for the industrial preservation of lactic acid starter cultures[END_REF]. Nowadays, isolating LAB from the natural environment has become a routine operation. This promoted the production, since the beginning of the 20 th century, of a variety of commercial starter cultures for different industrial applications [START_REF] Hansen | Commercial bacterial starter cultures for fermented foods of the future[END_REF]. They are currently widely employed for the production of fermented foods, and contribute to the majority of the volume and value of the commercial starter culture [START_REF] Hansen | Commercial bacterial starter cultures for fermented foods of the future[END_REF]. Figure 1 shows the production process of LAB starter cultures.

LAB used as commercial starter cultures possess many metabolic properties, such as acidification activity, proteolytic activity, antagonistic activity, and the ability to produce exopolysaccharides (EPS) [START_REF] Ayad | Selection of wild lactic acid bacteria isolated from traditional Egyptian dairy products according to production and technological criteria[END_REF]. These features offer a variety of nutritional values and desirable organoleptic attributes to the products. For example, yogurt is one of the most popular fermented dairy foods and is formed by the milk fermentation by LAB. In this transformation, the bacteria may convert lactose into lactic acid, hydrolyze the milk proteins into individual peptides and amino acids, and degrade the milk lipids into free fatty acids. This leads to obtaining a diversity of products with different sensory, organoleptic, and rheological properties (e.g. sharp acid taste, smooth texture, viscous gel, etc.) [START_REF] Clark | The sensory evaluation of dairy products[END_REF]. Moreover, yogurt is an excellent source of proteins, calcium, riboflavin, folate, and vitamin B6, which make it a more nutritional product than milk and can be consumed by those who are lactose intolerant [START_REF] Das | Effects of new technology on the current manufacturing process of yogurt-to increase the overall marketability of yogurt[END_REF][START_REF] Tabasco | Selective enumeration and identification of mixed cultures of Streptococcus thermophilus, Lactobacillus delbrueckii subsp. bulgaricus, L. acidophilus, L. paracasei subsp. paracasei and Bifidobacterium lactis in fermented milk[END_REF]. Another example concerns the LAB fermentation of broccoli to increase the total content of glucosinolates and polyphenolic compounds, which constitutes a promising way to enhance the bioactivity and nutritional properties of products [START_REF] Ye | Fermentation-based biotransformation of glucosinolates, phenolics and sugars in retorted broccoli puree by lactic acid bacteria[END_REF].

Recently, LAB have gained further attention due to their great potential to be used as probiotics. Indeed, probiotics are defined according to the FAO/WHO as "live microorganisms which, when administered in adequate amounts, confer a health benefit on the host" [START_REF] Hotel | Health and nutritional properties of probiotics in food including powder milk with live lactic acid bacteria[END_REF]. Most of the probiotic microorganisms belong to the Lactobacillus genera. They are resistant to the gastrointestinal acidity and the bile conditions, able to adhere to the intestinal mucosa, improve the intestinal microbiota, and reduce the growth of undesirable bacteria [START_REF] Abushelaibi | Characterization of potential probiotic lactic acid bacteria isolated from camel milk[END_REF].

Besides, LAB have been investigated to possess many health benefits in therapeutic applications [START_REF] Vinusha | RETRACTED: Proteomic studies on lactic acid bacteria: A review[END_REF]. For example, three LAB strains identified as Lactobacillus plantarum, Lactobacillus helveticus, and Lactobacillus fermentum, have shown in vivo the ability to reduce the cholesterol [START_REF] Vorobiev | Selection of potential probiotic lactobacilli for cholesterol-lowering properties and their effect on cholesterol metabolism in rats fed a high-lipid diet[END_REF]. In another work, the EPS produced by the probiotic strain Lactobacillus acidophilus were shown to have positive effects against the CaCO2 colon cancer cells in a dose-dependent manner [START_REF] Deepak | Optimization of anticancer exopolysaccharide production from probiotic Lactobacillus acidophilus by response surface methodology[END_REF]. Besides, many LAB strains have been investigated and showed therapeutic benefits in diarrhea [START_REF] Ahmadi | Efficacy of probiotic use in acute rotavirus diarrhea in children: A systematic review and meta-analysis[END_REF][START_REF] Sazawal | Efficacy of probiotics in prevention of acute diarrhoea: a meta-analysis of masked, randomised, placebo-controlled trials[END_REF].

Fermentation is a method of food preservation known since ancient times. LAB fermentation, in particular, occupies an essential place in the food industry. For instance, LAB are reported to have the ability to control the fungal growth and remove the food-contaminating substances (e.g. mycotoxins) [START_REF] Perczak | The efficiency of lactic acid bacteria against pathogenic fungi and mycotoxins[END_REF].

Besides, lactic acid fermentation of legume seed sprouts by Lactobacillus casei 0979 significantly increased the content of isoflavonoids, and significantly decreased the mold number [START_REF] Budryn | Lactic acid fermentation of legume seed sprouts as a method of increasing the content of isoflavones and reducing microbial contamination[END_REF]. [START_REF] Prabhurajeshwar | Evaluation of antimicrobial properties and their substances against pathogenic bacteria in-vitro by probiotic Lactobacilli strains isolated from commercial yoghurt[END_REF] isolated 13 Lactobacillus strains from commercial yogurt, which showed different degrees of antagonistic effects against pathogens using modified agar-well diffusion method. The antimicrobial activity of LAB was explained in the literature by three mechanisms that include the metabolic production of organic acids providing an acidic environment, the competition for nutrients (depletion of essential nutrients), and the formation of antimicrobial compounds [START_REF] Nilsson | Growth control of Listeria monocytogenes on cold-smoked salmon using a competitive lactic acid bacteria flora[END_REF][START_REF] Schnürer | Antifungal lactic acid bacteria as biopreservatives[END_REF]. Bacteriocins produced by LAB have received considerable interest as natural antimicrobials. They are easily produced, non-toxic to eukaryotic cells, sensitive to protease enzymes, have no influence on the sensory quality of foods, and demonstrate an extreme potential against food spoilage microorganisms. Indeed, LAB bacteriocins are considered promising biopreservatives, which contributes to reducing the addition of synthetic preservatives, controlling the abuse of antibiotics, and decreasing the heat or other traditional sterilization treatments (Field et al., 2018;[START_REF] Yang | Influence of culture media, pH and temperature on growth and bacteriocin production of bacteriocinogenic lactic acid bacteria[END_REF].

Besides, many LAB strains possess the label "Generally Recognized As Safe (GRAS)" according to the US Food and Drug Administration agency.

LAB have become also increasingly important in the industrial manufacturing of biochemicals, cosmetics, and pharmaceuticals. As an example, producing lactic acid by LAB has received great attention. Indeed, lactic acid can be used in leather tanning, as a humectant, contributing to controlled drug release, and in the production of biodegradable and biocompatible polylactic polymers (e.g. polylactic acid (PLA)) [START_REF] Demirci | Lactic acid production in a mixed-culture biofilm reactor[END_REF][START_REF] Lasprilla | Poly-lactic acid synthesis for application in biomedical devices -A review[END_REF]. Both D(-) and L(+)-lactic acid isomers exist in nature, with the more occurrence of the L(+) form that is metabolized by human cells [START_REF] Vijayakumar | Recent trends in the production, purification and application of lactic acid[END_REF]. Fermentation and chemical synthesis are the two main manufacturing methods of lactic acid. Pure L(+)-lactic acid isomer is easy to obtain by fermentation compared to the chemical syntheses. Currently, many LAB strains have been reported to present the ability to produce pure L(+)-lactic acid [START_REF] Bernardo | L-(+)-Lactic acid production by Lactobacillus rhamnosus B103 from dairy industry waste[END_REF][START_REF] Cizeikiene | Use of wheat straw biomass in production of L-lactic acid applying biocatalysis and combined lactic acid bacteria strains belonging to the genus Lactobacillus[END_REF]. Furthermore, renewable resources and agri-food by-products such as starch, cellulose, and wheat straw can be used to produce lactic acid by fermentation, without carbon dioxide formation as a by-product of fermentation, which is another important advantage in contrast to oil-and fossil-fuel-based sources for the chemical syntheses [START_REF] Hofvendahl | Factors affecting the fermentative lactic acid production from renewable resources1[END_REF]. Lactic acid produced by fermentation is considered as an alternative for the production of PLA, which has bioresorbability and biocompatibility properties, and is becoming widely used in surgery as suture materials and bone fixation devices [START_REF] Lasprilla | Poly-lactic acid synthesis for application in biomedical devices -A review[END_REF][START_REF] Narayanan | L (+) lactic acid fermentation and its product polymerization[END_REF][START_REF] Vijayakumar | Recent trends in the production, purification and application of lactic acid[END_REF].

In recent years, more and more fermented food manufactures prefer to apply concentrated cultures in frozen or dried forms for direct inoculation to the food matrix, which has the advantages of reducing the fermentation time and decreasing the risk of bacteriophage infection, compared to the traditional fermentation requiring the inoculum preparation by successive subcultures until reaching the required volume for the fermentation process [START_REF] Peebles | Preparation of concentrated lactic Streptococcus Starters1[END_REF][START_REF] Pettersson | Studies on batch production of bacterial concentrates from mixed species lactic starters[END_REF][START_REF] Santivarangkna | Protection mechanisms of sugars during different stages of preparation process of dried lactic acid starter cultures[END_REF]. Commercial LAB starter cultures are commonly produced in large-scale fermentation (Figure 1). Hence, many attempts have been made to obtain high biomass yield and quality of LAB starter cultures by changing the fermentation parameters that include the production procedures (i.e. configuration of the fermentation equipment, fermentation process, medium composition, and growth conditions), downstream processing (i.e. concentration, drying and cryopreservation parameters, and addition of protective agents), and storage conditions [START_REF] Alonso | Novel preservation techniques for microbial cultures[END_REF][START_REF] Ambros | Temperature-controlled microwave-vacuum drying of lactic acid bacteria: Impact of drying conditions on process and product characteristics[END_REF][START_REF] Chen | Effects of prebiotic oligosaccharides and trehalose on growth and production of bacteriocins by lactic acid bacteria[END_REF][START_REF] Chisti | Sonobioreactors: using ultrasound for enhanced microbial productivity[END_REF][START_REF] Morgan | Preservation of micro-organisms by drying; A review[END_REF][START_REF] Pasupuleti | Applications of protein hydrolysates in biotechnology[END_REF].

LAB have diverse nutritional requirements for each specific species. In general, a growth medium contains the essential growth elements such as a carbon source, amino acids, peptides, vitamins, and minerals. Many research works were focused on selecting the right raw material and optimize the formulation of the medium to produce high biomass concentration and functional activity. For instance, replacing the nitrogen source in standard MRS media by egg white hydrolyzates had the positive impacts of promoting the growth of L. acidophilus NRRL B-449 and Lactobacillus reuteri B-14171 in terms of biomass production, maximum growth rate, and titratable acidity [START_REF] Mis Solval | Growth kinetics and lactic acid production of Lactobacillus plantarum NRRL B-4496, L. acidophilus NRRL B-4495, and L. reuteri B-14171 in media containing egg white hydrolysates[END_REF]. In another recent work, β-glucooligosaccharides (β-GOS) derived from barley β-glucan was used as a sole carbon source in M9 medium, resulting in a longer exponential phase and an approximately 25% increase in nisin Z (antimicrobial peptide) production of L. lactis subsp. lactis I2 compared with glucose [START_REF] Lee | β-glucooligosaccharides derived from barley β-glucan promote growth of lactic acid bacteria and enhance nisin Z secretion by Lactococcus lactis[END_REF].

Besides modifying the culture conditions, various emerging technologies were tested to be implemented for LAB processing. These technologies include the application of pulsed electric fields (PEF), power ultrasound (US), high-pressure processing (HPP), ultraviolet (UV), and microwave (MW).

Impact of emerging technologies on lactic acid bacteria (LAB)

Impact of pulsed electric fields on LAB

Pulsed electric fields (PEF) technology consists of applying an electrical treatment of short time (i.e. from several ns to several ms), with a field intensity ranging from 100-300 V.cm -1 to 20-80 kV.cm -1 , to a product placed between two electrodes [START_REF] Barba | Current applications and new opportunities for the use of pulsed electric fields in food science and industry[END_REF][START_REF] Barba | Green alternative methods for the extraction of antioxidant bioactive compounds from winery wastes and by-products: A review[END_REF]Lebovka & Vorobiev, 2010;[START_REF] Miklavcic | Handbook of electroporation[END_REF][START_REF] Vorobiev | Selection of potential probiotic lactobacilli for cholesterol-lowering properties and their effect on cholesterol metabolism in rats fed a high-lipid diet[END_REF]. PEF treatment is usually carried out at ambient temperature or slightly higher than the ambient temperature. The application of short electrical pulses at high voltage enables the minimal thermal changes during the process, and avoids or reduces the detrimental effects on the sensory and physical properties of foods. PEF treatment can also induce the breakdown of cell membranes, the formation of temporary (reversible) or permanent (irreversible) pores, which is known as electroporation. These characteristics make PEF able to be widely implemented in the food-processing industries, which mainly include the applications for non-thermal microbial inactivation, valuable compounds recovery from plant matrices, and as a pre-treatment method to enhance freezing and drying processes [START_REF] Ben Ammar | Effect of a pulsed electric field and osmotic treatment on freezing of potato tissue[END_REF][START_REF] El Kantar | Pulsed electric field treatment of citrus fruits: Improvement of juice and polyphenols extraction[END_REF][START_REF] Parniakov | Pulsed electric field assisted vacuum freeze-drying of apple tissue[END_REF]Roohinejad, Koubaa, et al., 2018).

The exposure of microorganisms to PEF induces the formation of pores inside the membrane and increases its permeability (Figure 2). The degree of electroporation depends on the electrical field intensity, the pulse duration, the number of pulses, the medium conductivity, as well as the cell structure and size [START_REF] Barbosa-Cánovas | Preservation of foods with pulsed electric fields[END_REF][START_REF] Rols | Control of electric field induced cell membrane permeabilization by membrane order[END_REF][START_REF] Vernhes | Chinese hamster ovary cells sensitivity to localized electrical stresses[END_REF][START_REF] Vorobiev | Pulsed electric field-assisted extraction of juice from food plants[END_REF]. The electroporation process has been described to occur in different phases (Blaustein et al., 2006;[START_REF] Zimmermann | Electrical breakdown, electropermeabilization and electrofusion[END_REF]. Pore formation results from a super threshold membrane voltage when the cells are exposed to an external electric field. The charges are redistributed on both sides of the membrane and are accompanied by the increase of the membrane potential. The compression of the cell membranes occurs due to the attraction of the opposite sign charges, leading to increasing the electric field beyond a certain critical value, and thereby the formation of pores across the cell membrane [START_REF] Miklavcic | Handbook of electroporation[END_REF][START_REF] Neumann | Permeability changes induced by electric impulses in vesicular membranes[END_REF][START_REF] Toepfl | Chapter 6 -Overview of pulsed electric fields processing for food[END_REF][START_REF] Zimmermann | Dielectric breakdown of cell membranes[END_REF]. Pore formation is a dynamic process and can be reversible or irreversible depending on the treatment parameters. At low or mild electric field intensity, when the pore size is small compared to the total membrane surface area, the cells may reseal and repair their membranes after suspending the treatment and the electroporation is then qualified as reversible [START_REF] Emanuel | Influence of the current density in moderate pulsed electric fields on P. putida F1 eradication[END_REF][START_REF] Soliva-Fortuny | Effects of pulsed electric fields on bioactive compounds in foods: a review[END_REF]. Further increasing the electric field intensity with longer treatment time causes the formation of large pores, and thereby leads to irreversible breakdown and cell death.

A considerable increase in the cell membrane permeability to ions, molecules, and even macromolecules occurs during the period after the pores formation until their closure [START_REF] Chen | Membrane electroporation theories: a review[END_REF][START_REF] Kandušer | Cell membrane fluidity related to electroporation and resealing[END_REF][START_REF] Puc | Quantitative model of small molecules uptake after in vitro cell electropermeabilization[END_REF].

Electroporation allows the incorporation of molecules inside the cells, which makes it an effective technique to introduce drugs and genes into living cells, especially in biotechnology and medicine fields [START_REF] Kotnik | Electroporation-based applications in biotechnology[END_REF][START_REF] Mir | Therapeutic perspectives of in vivo cell electropermeabilization[END_REF]. Electroporation was employed to develop genetic engineering techniques for LAB vector construction and has gained great interest in research development. For instance, the recombinant pNZ8148-BSH (expression vector -bile salt hydrolase) gene was inserted into the LAB strain NZ9000 with electrotransformation method [START_REF] Bin | Cloning of bile salt hydrolase gene and its expression in lactic acid bacteria[END_REF]. Results showed that the BSH gene was completely expressed in the LAB. A second example consists of transforming fructophilic lactic acid bacteria (FLAB) that have the unique fructophilic substrate, meaning that they prefer D-fructose over D-glucose as main carbon source [START_REF] Maeno | Introduction of bifunctional alcohol/acetaldehyde dehydrogenase gene (adhE) in Fructobacillus fructosus settled its fructophilic characteristics[END_REF]. FLAB were reported to be able to grow efficiently on D-glucose without external electron acceptors after transformation with the adhE (bifunctional alcohol/acetaldehyde dehydrogenase) gene by electroporation. LAB could be also genetically modified by electrotransformation to obtain desirable metabolic characteristics and novel phenotypes, or also to eliminate unwanted traits [START_REF] Shareck | Cloning vectors based on cryptic plasmids isolated from lactic acid bacteria: Their characteristics and potential applications in biotechnology[END_REF].

The application of PEF needs to be controlled to induce reversible pore formation as well as maintaining the cell viability [START_REF] Kumar | Pulsed electric field processing in food technology[END_REF]. The optimum electroporation parameters are strain-dependent [START_REF] Lu | A critical combination of operating parameters can significantly increase the electrotransformation efficiency of a gram-positive Dietzia strain[END_REF]). An improved electrotransformation method of 12 LAB strains was proposed by [START_REF] Landete | An improved method for the electrotransformation of lactic acid bacteria: A comparative survey[END_REF]. It was based on the electrotransformation protocol described by [START_REF] Holo | High-frequency transformation, by electroporation, of Lactococcus lactis subsp. cremoris grown with glycine in osmotically stabilized media[END_REF]. The modified procedure consisted of adding 5 mM KH2PO4 pH 7.4 and 2 mM MgCl2 into the washing solution (0.5 M sucrose, 10% glycerol) and enhancing the voltage from 2.0 kV to 2.5 kV for Lactococcus strain.

Nowadays, new interesting effects of PEF on LAB have been discovered.

Especially, the positive influence of PEF treatment on the growth and fermentation characteristics. For example, the LAB strains of Lactobacillus acidophilus LA-K and Lactobacillus delbrueckii ssp. bulgaricus LB-12 were treated by mild PEF with the electrical field strength of 1 kV.cm -1 , pulse width of 3 µs, and pulse period of 0.5 s.

The results obtained showed that the mild PEF-treated culture reached the logarithmic phase of growth an hour earlier than the untreated cells. Meanwhile, the mild PEF-treated L. acidophilus LA-K had longer exponential phase (11 h) of growth compared to the control (8 h) without PEF, the L. delbrueckii ssp. bulgaricus LB-12 subjected to mild PEF had extended logarithmic phase (7 h) of growth in contrast to the untreated cells (6 h) [START_REF] Najim | A mild pulsed electric field condition that improves acid tolerance, growth, and protease activity of Lactobacillus acidophilus LA-K and Lactobacillus delbrueckii subspecies bulgaricus LB-12[END_REF]. A similar result was obtained for the treatment of lactobacilli cells by an electric field strength of 7.5 kV.cm -1 for 3.5 ms, which increased the growth rate by 4.49-21.25% (Lye et al., 2012). PEF treatment also enhanced the β-glucosidase activity for the bioconversion of isoflavones glucosidase to aglycones in biotin-soymilk (Ewe, Wan-Abdullah, et al., 2012b). It was suggested that these effects could be explained by the formation of pores in the cell membrane by PEF treatment, which facilitates the uptake of nutrients from the growth medium and the excretion of metabolites from the cells [START_REF] Mattar | Electro-stimulation of Saccharomyces cerevisiae wine yeasts by pulsed electric field and its effect on fermentation performance[END_REF][START_REF] Tryfona | Impact of pulsed electric fields on Corynebacterium glutamicum cell membrane permeabilization[END_REF]. Moderate PEF treatment has been also suggested to be a promising engineering technique to enhance the production of EPS by L. lactis subsp. cremoris during fermentation [START_REF] Ohba | Moderate pulsed electric field treatment enhances exopolysaccharide production by Lactococcus lactis subspecies cremoris[END_REF].

The permeability of the cell membrane increases during the PEF treatment, which may enhance the accumulation of ions inside the bacteria. For example, zinc and magnesium ions are important micronutrients for the proper function of the human body, they are cofactors of many essential enzymes, participate in the cell metabolic processes, as well as in gene expression [START_REF] Macdiarmid | Zinc transporters that regulate vacuolar zinc storage in Saccharomyces cerevisiae[END_REF]. Their deficiency increases the risk of anemia, depression, loss of appetite, heart disease, and immune system defects. The appropriate concentration of both ions is recommended daily to maintain human health [START_REF] Derom | Magnesium and depression: a systematic review[END_REF][START_REF] Góral | Magnesium enriched lactic acid bacteria as a carrier for probiotic ice cream production[END_REF]. Nonetheless, recent studies have reported that zinc and magnesium deficiency is one of the most frequently diagnosed electrolytic disorders in humans [START_REF] Pankiewicz | Effect of pulse electric fields (PEF) on accumulation of magnesium and zinc ions in Saccharomyces cerevisiae cells[END_REF]. Many authors discovered that metal ions accumulated in the microbial cells and consumed in the form of metalloproteins can be an efficient solution to this deficiency [START_REF] Góral | Effect of pulsed electric fields (PEF) on accumulation of magnesium in Lactobacillus rhamnosus B 442 Cells[END_REF]. LAB have been widely studied and proved to be able to accumulate zinc and magnesium ions [START_REF] Góral | Magnesium enriched lactic acid bacteria as a carrier for probiotic ice cream production[END_REF][START_REF] Mrvčić | Interaction of lactic acid bacteria with metal ions: opportunities for improving food safety and quality[END_REF].

Moreover, PEF treatment has been demonstrated to greatly promote the enrichment of these ions in LAB. For instance, it was demonstrated that the maximum bioaccumulation of zinc and magnesium ions in Lactobacillus cells under optimized PEF parameters were higher than the control sample (i.e. without PEF treatment) by 164% and 220%, respectively [START_REF] Góral | Bioaccumulation of zinc ions in Lactobacillus rhamnosus B 442 cells under treatment of the culture with pulsed electric field[END_REF][START_REF] Góral | Effect of pulsed electric fields (PEF) on accumulation of magnesium in Lactobacillus rhamnosus B 442 Cells[END_REF].

Furthermore, LAB enriched with magnesium ions by PEF treatment was investigated for addition into ice cream, and was displayed to be a potential carrier for probiotic ice cream production [START_REF] Góral | Magnesium enriched lactic acid bacteria as a carrier for probiotic ice cream production[END_REF]. PEF technology was thereby used to enrich LAB in zinc and magnesium ions for their supplementation to foodstuffs, which makes metal ions easily absorbed with high bioavailability, and contributes to reducing their deficiency [START_REF] Góral | Magnesium enriched lactic acid bacteria as a carrier for probiotic ice cream production[END_REF][START_REF] Góral | Bioaccumulation of zinc ions in Lactobacillus rhamnosus B 442 cells under treatment of the culture with pulsed electric field[END_REF][START_REF] Góral | Effect of pulsed electric fields (PEF) on accumulation of magnesium in Lactobacillus rhamnosus B 442 Cells[END_REF].

Besides, PEF treatment was used to increase the intracellular trehalose content, the cholesterol assimilation activity, and to improve the acid tolerance and protease activity of Lactobacillus [START_REF] Najim | A mild pulsed electric field condition that improves acid tolerance, growth, and protease activity of Lactobacillus acidophilus LA-K and Lactobacillus delbrueckii subspecies bulgaricus LB-12[END_REF][START_REF] Vaessen | Pulsed electric field for increasing intracellular trehalose content in Lactobacillus plantarum WCFS1[END_REF]. Several studies demonstrated that trehalose can protect organisms against various environmental stresses such as heat, cold, acid, desiccation, and osmotic pressure [START_REF] Elbein | New insights on trehalose: a multifunctional molecule[END_REF][START_REF] Lin | Production and characterization of a recombinant thermophilic trehalose synthase from Thermus antranikianii[END_REF][START_REF] Mahmud | Differential importance of trehalose accumulation in Saccharomyces cerevisiae in response to various environmental stresses[END_REF][START_REF] Yoshiyama | Trehalose accumulation enhances tolerance of Saccharomyces cerevisiae to acetic acid[END_REF].

Especially, it could be used as a cryoprotective agent as its addition promoted high microorganism viability during freezing and freeze-drying processes [START_REF] Stefanello | Trehalose as a cryoprotectant in freeze-dried wheat sourdough production[END_REF]. In the same line, PEF treatment with an electric field strength of 7.5 kV.cm -1 increased the intracellular trehalose content in L. plantarum WCFS1 to ≈100 mM, which could enhance the robustness of LAB during the processing steps (e.g. protecting the membrane structure against stress conditions by preventing the intracellular and extracellular formation of ice crystals during cryopreservation) [START_REF] Vaessen | Pulsed electric field for increasing intracellular trehalose content in Lactobacillus plantarum WCFS1[END_REF]. The fluorescence staining method revealed that PEF treatment resulted in a small fraction (23%) of LAB having a permeated membrane. It was shown that membrane pore resealing for propidium iodide (PI) uptake was very fast (in the order of seconds), and the treatment caused irreversible permeated membrane with a 75% survival rate of cells. Further research studies are required to optimize the cell viability as well as the trehalose content [START_REF] Vaessen | Pulsed electric field for increasing intracellular trehalose content in Lactobacillus plantarum WCFS1[END_REF].

PEF offers also the ability to inactivate microorganisms at near ambient temperatures with minimal effects on the sensory, nutritional, and health-promoting attributes of the food products. It has been considered as an alternative non-thermal technology and received much attention in the food preservation field [START_REF] Elez-Martínez | Inactivation of Lactobacillus brevis in orange juice by high-intensity pulsed electric fields[END_REF][START_REF] Pereira | Environmental impact of novel thermal and non-thermal technologies in food processing[END_REF][START_REF] Timmermans | Moderate intensity pulsed electric fields (PEF) as alternative mild preservation technology for fruit juice[END_REF]. Inactivating LAB by PEF has been also largely documented. For instance, PEF treatment combined with manothermosonication (MTS) was applied during milk processing [START_REF] Halpin | Combined treatment with mild heat, manothermosonication and pulsed electric fields reduces microbial growth in milk[END_REF]. The milk samples treated with MTS/PEF contained lower LAB compared to raw milk at each time point of post-processing over a 21-days period. The survival of LAB after PEF treatment depended mainly on the total energy input, the field strength of PEF, and the environmental parameters. In another work, L. plantarum cells in model beer were inactivated by PEF treatment above the critical values of 13 kV.cm -1 and 64 kJ.kg -1 [START_REF] Ulmer | Effects of pulsed electric fields on inactivation and metabolic activity of Lactobacillus plantarum in model beer[END_REF]. It was shown that below these intensities, the cell membrane damage was reversible, and the metabolic activity and the membrane integrity were reduced without loss of viability. Inactivating LAB by PEF in various media was assessed by [START_REF] Jaeger | Protective effect of milk constituents and sublethal injuries limiting process effectiveness during PEF inactivation of Lb. rhamnosus[END_REF]. The study showed that the same treatment parameters (30 kV.cm -1 and 116 kJ.kg -1 ) resulted in a 5.5 log-cycle reduction of Lactobacillus rhamnosus in Ringer solution, but allowed reaching only 2 log-cycle reduction in milk. The protective effect of milk against PEF treatment could be related to the presence of micellar casein and whey proteins [START_REF] Jaeger | Protective effect of milk constituents and sublethal injuries limiting process effectiveness during PEF inactivation of Lb. rhamnosus[END_REF].

When considering PEF effectiveness on microbial inactivation as a method for food preservation, it was demonstrated that 4 ms of PEF treatment at 20 kV.cm -1 was sufficient to inactivate all microorganisms (5 log reduction) present in wine samples, while it has no negative effect on the color and phenolic compounds of red wine [START_REF] Delsart | Impact of pulsed-electric field and high-voltage electrical discharges on red wine microbial stabilization and quality characteristics[END_REF].

Although efficient in most of the cases, the effects of food composition on the resistance of microorganisms to PEF are complex and the cellular basis of this phenomenon still remains unclear. LAB like Leuconostoc and L. brevis in fruit juice, L. plantarum and Lactobacillus hilgardii in wine, and some LAB strains in raw milk can be harmful and cause food poisoning. Therefore, more investigations on the effectiveness and mechanism of action of PEF on LAB in complex foods are required to obtain higher product's safety and quality [START_REF] Elez-Martínez | Inactivation of Lactobacillus brevis in orange juice by high-intensity pulsed electric fields[END_REF][START_REF] Marsellés-Fontanet | Optimising the inactivation of grape juice spoilage organisms by pulse electric fields[END_REF][START_REF] Puértolas | Pulsed electric fields inactivation of wine spoilage yeast and bacteria[END_REF].

Impact of power ultrasound on LAB

Ultrasound generally refers to sound waves whose frequency is beyond the threshold of human hearing (≈20 kHz). It can be classified into three types based on frequency range: (i) power ultrasound (16-100 kHz), (ii) high-frequency ultrasound (100 kHz-1 MHz), and (iii) diagnostic ultrasound (1-10 MHz) [START_REF] Patist | Ultrasonic innovations in the food industry: From the laboratory to commercial production[END_REF][START_REF] Zhang | Non-thermal technologies and its current and future application in the food industry: a review[END_REF]. Low-frequency ultrasound delivers high energy that induces cavitation in liquid medium and is generally used for industrial processing and sonochemistry [START_REF] Mason | Practical Sonochemistry: Power Ultrasound Uses and Applications[END_REF]. The energy provided by an ultrasound generator is emitted via either an ultrasonic bath or a probe-based system and generates intense sound waves that propagate through a medium and creates compression and rarefaction regions. If the energy applied is high enough, sufficient intensity voids within the liquid could be produced, which are known as cavities (Figure 3). In the succeeding cycles, these cavities grow and become unstable, the consequent collapse violently can cause a rapid rise in temperature up to 5000 °C and the pressure increase up to 50 MPa, resulting in extreme shear and mechanical forces. This cavitation collapse generates various physical and chemical effects during food processing and contributes to the wide applications of power ultrasound such as improvement of mass transfer, emulsification, microbial inactivation, freezing, drying, and degradation of organic pollutants [START_REF] Khadhraoui | Ultrasound technology for food processing, preservation, and extraction[END_REF][START_REF] Mason | Practical Sonochemistry: Power Ultrasound Uses and Applications[END_REF][START_REF] Paniwnyk | Applications of ultrasound in processing of liquid foods: A review[END_REF]Roohinejad, Koubaa, et al., 2018). High-frequency low power ultrasound as a rapid and non-destructive method in food analysis is commonly applied for food quality monitoring and diagnostic purposes [START_REF] Awad | Applications of ultrasound in analysis, processing and quality control of food: A review[END_REF][START_REF] Noci | Chapter 6 -Dairy products processed with ultrasound[END_REF]. Based on the measurements of ultrasonic signal attenuation and ultrasound velocity through the food material, low power ultrasound is successfully applied to evaluate the composition, structure and physical state of dairy food products. For instance, ultrasound evaluation was demonstrated as a quick and cheap method to determine the state of milk being curdled during the renneting process and to measure the mechanical properties of noodle dough during production [START_REF] Jiménez | Contributions to ultrasound monitoring of the process of milk curdling[END_REF]20 Kerhervé et al., 2019).

The application of power ultrasound technology in food fermentation processes has been explored in recent years and has demonstrated its capacity to enhance the growth and metabolic activity of several microorganisms, as well as for the production of commercially valuable compounds [START_REF] Ojha | Effect of high intensity ultrasound on the fermentation profile of Lactobacillus sakei in a meat model system[END_REF]. The application of power ultrasound in dairy fermentation mainly includes milk fermentation by LAB [START_REF] Abesinghe | Effects of ultrasound on the fermentation profile of fermented milk products incorporated with lactic acid bacteria[END_REF][START_REF] Ojha | Effect of high intensity ultrasound on the fermentation profile of Lactobacillus sakei in a meat model system[END_REF]. Power ultrasound applied to assist fermentation or as a pre-treatment of the inoculum showed beneficial effects on LAB fermentation. For example, power ultrasound pre-treatment of wasted bread stillage used as a substrate for fermentation by L. rhamnosus ATCC 7469 resulted in a 15% increase of lactic acid concentration, and higher viable cells compared to the control samples without pre-treatment. It was reported that power ultrasound pre-treatment could be an alternative to sterilize the raw materials for LAB fermentation [START_REF] Aleksandra | Ultrasound as a physical treatment of stillage for lactic acid fermentation[END_REF]. In another work, power ultrasound pre-treatment of LAB before their use to ferment soymilk induced stress response, which was demonstrated by a better β-glucosidase activity and an increase of the fraction of aglycones in the total soymilk isoflavones [START_REF] Liu | Strategic ultrasound-induced stress response of lactic acid bacteria on enhancement of β-glucosidase activity for bioconversion of isoflavones in soymilk[END_REF]. Likewise, power ultrasound-treated cells (30 kHz, 60 W, and 3 min) showed better tolerance toward acidic conditions (pH 2 and pH 3) as well as promoted antimicrobial activity and cell adhesion ability [START_REF] Yeo | Effect of ultrasound on bioconversion of isoflavones and probiotic properties of parent organisms and subsequent passages of Lactobacillus[END_REF]. Nevertheless, it is worthy to note that the growth of L. fermentum FTDC 1311 was significantly decreased immediately upon power ultrasound (30 kHz, 100 W, and 2 min) application, with less intestinal adherence ability (Lye et al., 2012). This behavior could be explained by the local heating of power ultrasound, which causes the cell protein denaturation.

Ultrasound-assisted fermentation was reported as a promising method to enhance microbial productivity. Some current and future concepts of sonobioreactors were designed and described in the literature [START_REF] Chisti | Sonobioreactors: using ultrasound for enhanced microbial productivity[END_REF]. Typically, power ultrasound applied during fermentation brings a faster acid development of yogurt starter cultures (e.g. Streptococcus thermophilus, L. bulgaricus, Bifidobacterium, and L. acidophilus), along with reducing the total fermentation time of milk, as well as enhancing the rates of cell growth and sugar consumption [START_REF] Dahroud | Low intensity ultrasound increases the fermentation efficiency of Lactobacillus casei subsp.casei ATTC 39392[END_REF][START_REF] Hashemi | Effect of ultrasound on lactic acid production by Lactobacillus strains in date (Phoenix dactylifera var. Kabkab) syrup[END_REF][START_REF] Nguyen | Effect of high intensity ultrasound on carbohydrate metabolism of bifidobacteria in milk fermentation[END_REF][START_REF] Ojha | Effect of high intensity ultrasound on the fermentation profile of Lactobacillus sakei in a meat model system[END_REF]. These observations were attributed in the literature to several mechanisms that include (i) an induced lipid peroxidation of the cell membrane by power ultrasound and a promoted rotational diffusion of fatty acyl chains in the membrane, which leads to increased membrane permeability that induces facilitated nutrient internalization and subsequent growth improvement, (ii) the generation of pressure fluctuation during power ultrasound treatments that can induce stress on cells, possibly promoting cell growth and proliferation, together with changes in the metabolic processes; and (iii) the cell membrane rupture during power ultrasound treatment, and therefore the release of growth-promoting factors (e.g. vitamins, nucleotides, amino acids, and enzymes) that could stimulate the growth of the intact cells (Ewe, Wan Abdullah, et al., 2012;[START_REF] Gaudreau | The use of crude cellular extracts of Lactobacillus delbrueckii ssp. bulgaricus 11842 to stimulate growth of a probiotic Lactobacillus rhamnosus culture in milk[END_REF][START_REF] Hashemi | Effect of ultrasound on lactic acid production by Lactobacillus strains in date (Phoenix dactylifera var. Kabkab) syrup[END_REF][START_REF] Wu | Effects of ultrasound on milk homogenization and fermentation with yogurt starter[END_REF]. SEM images of LAB cells before and after power ultrasound treatment are shown in figure 3.

Likewise, a recent review paper reported the impact of power ultrasound as a potential technology applied to fermented dairy products. The benefits include the shortening of the processing time, the increase of the probiotic viability, and the generation of products with low lactose content, higher oligosaccharides concentration, less undesirable taste, and less addition of prebiotic or β-galactosidase [START_REF] Guimarães | High-intensity ultrasound: A novel technology for the development of probiotic and prebiotic dairy products[END_REF].

The effects of power ultrasound on LAB largely depend on the process parameters (e.g. amplitude, time, temperature, and frequency). High-intensity power ultrasound can induce the microbial inactivation to arise from the acoustic cavitation phenomenon, while it has some advantages such as the cost efficiency and the minimization of the loss of products' quality. Hence, power ultrasound has been regarded as a non-thermal process technology and attracted great interest in food preservation and safety. LAB are widely used to produce functional products by fermentation despite some strains that have drawbacks or are considered as spoilage microorganisms in foods, producing excessive acidity during the storage of fermented foods [START_REF] Wouters | Effects of high pressure on inactivation kinetics and events related to proton efflux in Lactobacillus plantarum[END_REF]). They include for example those producing lactic acid in organic rice beverage and which cause the so-called post-acidification (i.e. the decrease of pH during the storage) [START_REF] Bevilacqua | Using physical approaches for the attenuation of lactic acid bacteria in an organic rice beverage[END_REF], or the fermentation of the residual sugar in wine that generates acetic acid and increases the volatile acidity of wine [START_REF] Lonvaud-Funel | Lactic acid bacteria in the quality improvement and depreciation of wine[END_REF]. Power ultrasound has been reported with good results to relieve these defects. For instance, power ultrasound (20 kHz frequency, 130 W power, 40-80% amplitude, 2-6 min time exposure, and 2 s pulse time) was applied to avoid the post-acidification of L. plantarum L12 and L. reuteri DSM 20016 [START_REF] Racioppo | Ultrasound attenuation of lactobacilli and bifidobacteria: Effect on some technological and probiotic properties[END_REF]. The results obtained showed that the power ultrasound conditions of 80% amplitude and 4-6 min exposure time caused a significant reduction of the cell count (1-2 log cfu.mL -1 ), however, the power ultrasound conditions of 60% amplitude and 6 min exposure time decreased the acidification with no effect on the cell viability.

Likewise, power ultrasound (25 kHz, 200 W, 10.53 min, and 74 °C) was efficient in inhibiting the growth of LAB in packaged sausages [START_REF] Cichoski | Ultrasound-assisted post-packaging pasteurization of sausages[END_REF]. Less colony number of LAB was observed in the power ultrasound-treated samples after 60 days of storage, compared to the untreated samples or those conventionally pasteurized.

Impact of high-pressure on LAB

High-pressure processing (HPP), also known as high hydrostatic pressure (HHP) processing, is a non-thermal technique of food preservation that inactivates pathogenic and spoilage microorganisms by applying high-pressure (100-1000 MPa) to food products (Figure 4) for a short time (from few seconds to several minutes) and at low temperature [START_REF] Li | A review on recent development in non-conventional food sterilization technologies[END_REF][START_REF] Oliveira | Natural antimicrobials as additional hurdles to preservation of foods by high pressure processing[END_REF]. High pressure treatment has been reported to induce changes in the cell morphology (e.g. separation of the cell wall from the cytoplasmic membrane), denaturation of proteins and nucleic acids, inactivation of enzymes and cell membrane damaging, which lead to the death of microorganisms [START_REF] Knorr | High pressure application for food biopolymers[END_REF][START_REF] Mor-Mur | Chapter 5 -Microbiological aspects of high-pressure processing[END_REF]Roohinejad, Koubaa, et al., 2018;[START_REF] Tsevdou | High hydrostatic pressure processing of foods[END_REF]. Besides, pressure treatment does not break covalent bonds and involves mild temperatures that preserve most of the sensory and nutritional food properties [START_REF] Muntean | High pressure processing in food industry -Characteristics and applications[END_REF]. The application of HPP in the food industry has attracted widespread interest in their ability to improve the food quality and extend its shelf life.

Most of the studies related to HPP treatment are dealing with the inactivation of microorganisms. As stated above, some LAB strains are recognized as spoilage microorganisms of acidic products, and could, therefore, be inactivated by HPP. For example, it was reported that the application of pressure at 200-300 MPa for 10 min and at ambient temperature to yogurt prevented the post-acidification and maintained the initial number of viable LAB [START_REF] Tanaka | Application of hydrostatic pressure to yoghurt to prevent its after-acidification[END_REF]. Increasing the pressure over 300 MPa, prevented the post-acidification process and decreased the viable LAB number. In the same way, the high-pressure treatment within the range of 200-600 MPa for 5 min and at ambient temperature (≤22°C) significantly reduced the viable count of Lactococcus in a commercially manufactured fresh lactic curd cheeses [START_REF] Daryaei | Effect of high pressure treatment on starter bacteria and spoilage yeasts in fresh lactic curd cheese of bovine milk[END_REF].

The effectiveness of HPP treatment on microbial inactivation depends on the pressure, holding time, type of microorganisms, and food matrix [START_REF] Rendueles | Microbiological food safety assessment of high hydrostatic pressure processing: A review[END_REF]. For example, L. lactis subsp. cremoris MG1363 and SK11 were completely inactivated by HPP at a pressure of 400 to 800 MPa but not significantly affected at 100 and 200 MPa with the same treatment time (5 min) and temperature (25 °C). The population of MG1363 decreased by 7.3 log cycles while the viable cell number of SK11 was reduced by 2.5 log cycles after HPP treatment at a pressure of 300 MPa [START_REF] Malone | Effects of high pressure on the viability, morphology, lysis, and cell wall hydrolase activity of Lactococcus lactis subsp. cremoris[END_REF]. Several authors studied the inactivation of LAB in different substrates by HPP. [START_REF] Molina-Höppner | Protective effect of sucrose and sodium chloride for Lactococcus lactis during sublethal and lethal high-pressure treatments[END_REF] found that L. lactis had less survival in milk buffer after HPP treatment compared to the cells in milk buffer supplemented with 0.5 M sucrose or 4 M NaCl. Similarly, it was demonstrated that the survival of Listeria monocytogenes after high-pressure treatment was greater in a PBS emulsion contained olive oil compared to cells in PBS solution alone [START_REF] Simpson | The effect of high hydrostatic pressure on Listeria monocytogenes in phosphate-buffered saline and model food systems[END_REF].

HPP treatment can induce the injury of food-borne bacteria in foods when applying high-pressure values. It was reported that the primary pressure damage of cells occurs at HPP pressures of 400 MPa or higher [START_REF] Alpas | Evaluation of high hydrostatic pressure sensitivity of Staphylococcus aureus and Escherichia coli O157:H7 by differential scanning calorimetry[END_REF]. Some industries apply pressure over 600 MPa to food products to guarantee the elimination of pathogens in ready-to-eat foods [START_REF] Nikparvar | A dynamic model of membrane recovery mechanisms in bacteria following high pressure processing[END_REF]. Applying these high values of pressure is not always profitable on account of the operation costs and it is only suitable for limited foods. It was shown that HPP treatment at 450 MPa reduced the L. monocytogenes (KCTC 3569) count by 6-8 log cfu.g -1 for 7-14 days in chicken breast fillet, but also significantly reduced the flavor pleasantness and aroma strength of the fillet [START_REF] Kruk | The effect of high pressure on microbial population, meat quality and sensory characteristics of chicken breast fillet[END_REF]. In addition, the damaged cells after HPP processing have been observed to possess the capacity to recover and restart the growth after suspending the treatment [START_REF] Bozoğlu | Injury recovery of foodborne pathogens in high hydrostatic pressure treated milk during storage[END_REF]. Some reports proposed the combination of pressure and antimicrobial agents to increase the death rate of food-borne bacteria, which is related to the impact on bacteriocins on the injured cells by HPP treatment [START_REF] Garriga | Bactericidal synergism through bacteriocins and high pressure in a meat model system during storage[END_REF][START_REF] Kalchayanand | Interaction of hydrostatic pressure, time and temperature of pressurization and pediocin AcH on inactivation of foodborne bacteria[END_REF]. [START_REF] Arqués | Effect of combinations of high-pressure treatment and bacteriocin-producing lactic acid bacteria on the survival of Listeria monocytogenes in raw milk cheese[END_REF] demonstrated that the combined use of HPP treatment and bacteriocin was more effective compared to the individual effects on reducing the counts of L. monocytogenes Scott A during the ripening of raw milk cheeses. Gradually, the recovery of the sub-lethally injured cells after HPP treatment created widespread concern in food safety [START_REF] Nikparvar | A dynamic model of membrane recovery mechanisms in bacteria following high pressure processing[END_REF]. Some authors also studied the HPP application on the fermentation of LAB. For instance, HPP was employed on the glycerol/glucose co-fermentation process of L. reuteri [START_REF] Mota | Lactobacillus reuteri growth and fermentation under high pressure towards the production of 1,3-propanediol[END_REF]. The pressure-treated cells at 10 MPa produced higher 1,3-propanediol (1,3-PDO) titer with yield and productivity improvements of ≈11% and 12%, compared to the same samples at 0.1 MPa.

Nonetheless, the application of HPP was shown to decrease the biomass concentration of cells with a more pronounced effect at higher pressure levels, according to the growth profile of L. reuteri during fermentation under 0.1, 10, 25, 32 MPa [START_REF] Mota | Lactobacillus reuteri growth and fermentation under high pressure towards the production of 1,3-propanediol[END_REF]. In another work, HPP treatment at 100 MPa applied to the lactic acid fermentation for 180 min during yogurt production was shown to inhibit the growth of L. bulgaricus, but the cells revealed normal metabolic activity when returned back to atmospheric pressure [START_REF] Mota | Probiotic yogurt production under high pressure and the possible use of pressure as an on/off switch to stop/start fermentation[END_REF]. HPP treatment showed the great possibility to be used as an on/off switch to stop/start fermentation of LAB.

High-pressure homogenization (HPH) is another interesting technology usually used to inactivate microbial cells in different matrices. Equipment for HPH are mainly used todays in dairy-or fruit-based processing, paints and fuels, as well as recovering of high-added value compounds for cosmetic and pharmaceutical industries [START_REF] Koubaa | Current insights in yeast cell disruption technologies for oil recovery: A review[END_REF]. The working principle consists of forcing a suspension to pass at high pressure and high velocity through a narrow gap, which causes the creation of high shear forces that lead to cell wall rupture and the inactivation of the microbial cells.

For example, [START_REF] Bury | Disruption of Lactobacillus delbrueckii ssp. bulgaricus 11842 cells for lactose hydrolysis in dairy products: a comparison of sonication, high-pressure homogenization and bead milling[END_REF] studied the impact of sonication, HPH, and bead milling on the disruption of Lactobacillus delbrueckii ssp. bulgaricus 11842 cells for lactose hydrolysis in dairy products. The authors demonstrated that the maximum release of intracellular β-galactosidase was observed after 2-3 min of bead milling and after three passages through the high-pressure homogenizer (135 MPa). They also reported that one passage at 200 MPa was comparable to that released between two or three passages at 135 MPa. More recently, Franchi et al., (2013) investigated the impact of HPH to inactivate spoilage microorganisms in beer. They reported that Lactobacillus delbrueckii was the most resistant microorganism tested, requiring 250 MPa to reach a six decimal reduction.

HPH was not only applied for LAB cell disruption but also at sub-lethal levels to improve the functional properties of dairy products. For example, [START_REF] Tabanelli | Effect of a non-lethal High Pressure Homogenization treatment on the in vivo response of probiotic lactobacilli[END_REF] studied the effect of a non-lethal HPH treatment on the in vivo response of probiotic lactobacilli. They showed that this treatment could increase some in vivo probiotic characteristics, modify the interaction of lactobacilli with the small intestine, and induce a higher IgA response compared to the untreated cells. In another work, the same research group showed that the application of HPH at sub-lethal levels could modulate hydrophobicity and auto-aggregation without modification of viability and decarboxylase activity [START_REF] Tabanelli | Effect of sub-lethal high pressure homogenization treatments on the in vitro functional and biological properties of lactic acid bacteria[END_REF].

Impact of ultraviolet on LAB

Ultraviolet radiation is a widely used technology for the sterilization of drinking water, air disinfection, and surface decontamination [START_REF] Donsingha | Kinetics model of microbial degradation by UV radiation and shelf life of coconut water[END_REF]Forney & Moraru, 2009;[START_REF] Ochoa-Velasco | Effect of UV-C light on Lactobacillus rhamnosus, Salmonella Typhimurium, and Saccharomyces cerevisiae kinetics in inoculated coconut water: Survival and residual effect[END_REF]. Ultraviolet ray covers the non-ionizing region of the electromagnetic spectrum with the wavelength range of 200-400 nm, which is further subdivided into three regions, UV-A (315-400 nm), UV-B (280-315 nm), and UV-C (200-280 nm) (Ewe et al., 2013;Forney & Moraru, 2009;[START_REF] Koutchma | Advances in ultraviolet light technology for non-thermal processing of liquid foods[END_REF]. The most effective ultraviolet light to inactivate microorganisms is located in the range of UV-C 200-280 nm. The germicidal effect of UV occurs due to the absorption of UV light by the cells and the subsequent disruption of DNA (preventing the cells from DNA transcription and replication) (Figure 5), which causes the cell death [START_REF] Choudhary | Ultraviolet pasteurization for food industry[END_REF][START_REF] Donsingha | Kinetics model of microbial degradation by UV radiation and shelf life of coconut water[END_REF]Falguera et al., 2011;[START_REF] Ochoa-Velasco | Effect of UV-C light on Lactobacillus rhamnosus, Salmonella Typhimurium, and Saccharomyces cerevisiae kinetics in inoculated coconut water: Survival and residual effect[END_REF]. Most of the published studies are focused on studying the efficiency of UV to inactivate LAB in foods, such as cantaloupe melon, coconut water, and fruit juices [START_REF] Donsingha | Kinetics model of microbial degradation by UV radiation and shelf life of coconut water[END_REF][START_REF] Fredericks | Efficacy of ultraviolet radiation as an alternative technology to inactivate microorganisms in grape juices and wines[END_REF][START_REF] Lamikanra | Effect of processing under ultraviolet light on the shelf life of fresh-cut cantaloupe melon[END_REF][START_REF] Lu | Inactivation of microorganisms in apple juice using an ultraviolet silica-fiber optical device[END_REF].

The model brine (7.9% w/w or 13.2% w/w) chiller system inoculated with ≈6.0 log10 cfu.mL -1 LAB appeared to have far less microbial populations than the control after exposure to ultraviolet light (UV-C) for 5 min [START_REF] Gailunas | Use of Ultraviolet Light for the Inactivation of Listeria monocytogenes and Lactic Acid Bacteria Species in Recycled Chill Brines[END_REF]. In another work, small thin-film reactor, delivering UV (wavelength ranging from 230 to 320 nm) by a well-distributed silica optical fibers, was designed to treat apple juice [START_REF] Lu | Inactivation of microorganisms in apple juice using an ultraviolet silica-fiber optical device[END_REF].

Results showed that the L. brevis bacteria were reduced by up to 6 log orders with the UV dose of 2.37 mJ.cm -2 , an optical-fiber distribution density of 15 fiber.cm -2 , and a film thickness of sample of 2 mm. Besides, some interesting results were also revealed concerning the impact of UV on LAB. For example, it was reported that the 0.81,2.44,4.07,and 8.14 kJ.m -2 ) combined with modified atmosphere packaging (MAP) treatments had no significant effect on LAB in fresh processed "Red Oak Leaf" lettuce, and even stimulated the growth of the cells during storage [START_REF] Allende | Combined ultraviolet-C and modified atmosphere packaging treatments for reducing microbial growth of fresh processed lettuce[END_REF]. The influence of UV on microorganisms is mainly related to the species of bacteria, the type and dose of UV, and the growth medium of cells (Ewe et al., 2013). Bacteria possess diverse protection mechanisms against UV stress via the production of photo-protective substances, or adapting their response toward UV radiation by having a growth delay and resuming their growth after the radiation is removed [START_REF] Berney | Adaptation to UVA radiation of E. coli growing in continuous culture[END_REF][START_REF] Hung | Effects of ultraviolet light irradiation in biotreatment of organophosphates[END_REF]. The behaviors of four L. acidophilus BT 1088, L. fermentum BT 8219, L. acidophilus FTDC 8633, and Lactobacillus gasseri FTDC 8131 cells after UV treatment with various wavelength (UV-A, UV-B, UV-C) at the dosage of 30, 60, 90 J.cm -2 were investigated (Ewe et al., 2013). The results demonstrated that all the treated cells were able to proliferate well in bio-supplemented milk at 37 °C, the UV-B treated L. fermentum BT 8219 and L.

gasseri FTDC 8131 at the low dosage of 30 J.cm -2 reached 1-2 log higher counts compared to the untreated controls after fermentation for 24 h, whereas the UV-B treated L. acidophilus BT 1088 and L. fermentum BT 8219 at the mid-range of 60 J.cm -2 have higher β-glucosidase activities with 67.9% and 31.7% increase in contrast to the untreated cells. The stimulation mechanism could be explained by the low UV intensity that causes the lipid peroxidation in the cell membrane of lactobacilli and thereby results in increased membrane permeability, and subsequently enhancing the growth of cells by increasing the influx of nutrient and transport of oxygen, along with improving the excretion of metabolic waste out of the cells [START_REF] Bose | Correlation between UVA-induced changes in microviscosity, permeability and malondialdehyde formation in liposomal membrane[END_REF].

In addition, the application of UV to induce mutations of LAB and improve their activities has been reported by some authors. For example, L. gasseri 1131-M8 was generated from Lactobacillus gasser JCM1131 T by UV irradiation [START_REF] Arihara | UV-induced Lactobacillus gasseri mutants resisting sodium chloride and sodium nitrite for meat fermentation[END_REF]. A better tolerance against sodium chloride and sodium nitrite was demonstrated, compared to the original strain during meat fermentation. The mutants of Streptococcus lactis subspecies diacetylactis S1 produced by the UV light had enhanced acid or diacetyl + acetoin-producing abilities [START_REF] Kuila | Ultraviolet light-induced mutants of Streptococcus lactis Subspecies diacetylactis with enhanced acid-or flavor-producing abilities[END_REF].

It was also reported that the UV application (254 nm, 20 s) induced the yield of streptomycin-resistant mutants of lactobacilli and Lactococcus, which were 1-2 log higher than the control [START_REF] Goodarzi | UV -Induced mutagenesis in lactic acid bacteria[END_REF].

Impact of microwave on LAB

Microwaves (MW) are electromagnetic waves whose frequencies ranged from 300 MHz to 300 GHz [START_REF] Mishra | Effect of low power microwave radiation on microorganisms and other life forms[END_REF]. Typically, the microwaves at frequencies of 915 MHz and 2.45 GHz are largely employed in food processing that includes cooking, drying, pasteurization and sterilization, and material pre-treatment [START_REF] Datta | Microwave and radio frequency processing[END_REF][START_REF] Liu | Effects of pretreatment on the microbial community and l-lactic acid production in vinasse fermentation[END_REF]. The domestic ovens are generally operating at a frequency of 2.45 GHz [START_REF] Datta | Microwave and radio frequency processing[END_REF]. The mechanism of microwave sterilization has been discussed in the literature [START_REF] Chandrasekaran | Microwave food processing-A review[END_REF][START_REF] Mishra | Effect of low power microwave radiation on microorganisms and other life forms[END_REF]. It was largely approved that microbial inactivation is mainly due to the thermal effect of microwave irradiation, while the non-thermal effect of microwaves was controversial [START_REF] Tahir | Simple method to study the mechanism of thermal and non thermal bactericidal action of microwave radiations on different bacterial species[END_REF][START_REF] Vela | Mechanism of lethal action of 2,450-MHz radiation on microorganisms[END_REF]. Microwave heating was proved to be more effective to inactivate L. plantarum, compared to the conventional heating method [START_REF] Shin | Inactivation of Lactobacillus plantarum by pulsed-microwave irradiation[END_REF][START_REF] Tajchakavit | Enhanced destruction of spoilage microorganisms in apple juice during continuous flow microwave heating[END_REF]. Microwave-alkali coupled pre-treatment was employed on vinasse for the fermentation of L. casei [START_REF] Liu | Effects of pretreatment on the microbial community and l-lactic acid production in vinasse fermentation[END_REF]. The indigenous bacterial load was effectively inactivated after microwave-alkali coupled treatment at 700 W with 0.06 g NaOH g -1 vinasse for 8 min, which allowed the inoculated L. casei strain becoming the dominant genus after 24 h fermentation. Moreover, the microwave-alkali coupled pre-treatment allowed releasing more reducing sugars, and therefore an increased yield and optical purity of L-lactic acid by 49.8% and 57.2%, respectively, compared to the steam-alkali coupled pre-treatment (0.06 g NaOH g -1 vinasse, 120 °C, 20 min).

Microwave-assisted drying has emerged as a potential method for the dehydration of fruits and vegetables [START_REF] Funebo | Microwave-assisted air dehydration of apple and mushroom[END_REF][START_REF] Maskan | Microwave/air and microwave finish drying of banana[END_REF][START_REF] Soysal | Microwave drying characteristics of parsley[END_REF]. It allows reducing the drying time and providing high-quality products [START_REF] Prabhanjan | Microwave-assisted convective air drying of thin layer carrots[END_REF]. Drying is a commonly employed technique for long-term microbial cell preservation as well as for LAB starter culture [START_REF] Alonso | Novel preservation techniques for microbial cultures[END_REF]. The effect of microwaves on the drying efficiency of LAB was investigated [START_REF] Ambros | Temperature-controlled microwave-vacuum drying of lactic acid bacteria: Impact of drying conditions on process and product characteristics[END_REF][START_REF] Ambros | Microwave-freeze drying of lactic acid bacteria: Influence of process parameters on drying behavior and viability[END_REF][START_REF] Ambros | Structural basis of the impact of microwave drying on survival and shelf life of Lactobacillus paracasei[END_REF]. For example, shorter drying times of 3 to 5 h for Bifidobacterium animalis ssp. lactis INL1 was achieved by microwave-freeze drying, in contrast to ≈24 h by the conventional freeze-drying. Similar cell viability was observed for both techniques. The impacts of microwave-assisted freezing on the drying kinetics of LAB depend primarily on the process parameters that include mainly the power input and chamber pressure. For instance, different microwave power inputs (1.5 W.g -1 to 3 W.g -1 ) and chamber pressure levels (0.6, 1, 2 mbar) were tested on the freeze-drying process of two LAB strains. The results showed that the drying time was reduced with the increase of the power input and showed a linear relationship. Within the set of microwave parameters, the drying time was shortened with the decrease of pressure [START_REF] Ambros | Microwave-freeze drying of lactic acid bacteria: Influence of process parameters on drying behavior and viability[END_REF]. Furthermore, [START_REF] Ambros | Structural basis of the impact of microwave drying on survival and shelf life of Lactobacillus paracasei[END_REF] investigated the impact of microwave application during drying (freeze-drying and vacuum-drying) on viability, shelf life, and microstructure of the model strain Lactobacillus paracasei F19. The results showed that the freeze-dried and microwave-assisted freeze-dried L. paracasei F19 had 40% and 35% survival, respectively. Vacuum-drying and microwave-assisted vacuum-drying of L. paracasei F19 had similar viability of ≈20%. Meanwhile, it was concluded that the implementation of microwave energy did not cause a difference in the storage ability and structure of dried cells [START_REF] Ambros | Structural basis of the impact of microwave drying on survival and shelf life of Lactobacillus paracasei[END_REF].

Microwave-assisted drying contributed to shortening the drying time of the cells, and therefore to saving energy. Microwave-assisted freeze-drying was considered a great alternative to conventional freeze-drying for future industrial applications [START_REF] Ambros | Structural basis of the impact of microwave drying on survival and shelf life of Lactobacillus paracasei[END_REF].

Advantages and limitations of emerging technologies

Emerging technologies including pulsed electric fields (PEF), power ultrasound (US), high-pressure processing (HPP), and ultraviolet (UV) are promising non-thermal technologies that have been shown to effectively inactivate microorganisms in food matrices at near-ambient temperatures, with no detrimental effect on the nutritional and organoleptic quality of food ("fresh like" characteristics).

Microwave (MW) heating is regarded as a novel thermal technology, providing volumetric heating in which thermal energy is generated directly inside the food, which reduces the considerable heat losses during its transfer into the food product, compared to the conventional heating technologies (e.g. by fuel combustion or using an electric resistive heater generating heat outside the product). Emerging technologies have also the advantage of being environmentally friendly food processing. On the other side, the degree of cell damage when applying emerging technologies strongly depends on the processing parameters (e.g. intensity, treatment time, and energy input). The treated cells were induced to have different metabolic activities, intend to increase and improve the production of beneficial products and bioactive compounds. However, the application of these technologies in industry is actually rather limited. The food processing equipment and processing conditions need to be explored at industrial scale rather than at laboratory conditions. The high investment costs, as well as the cost-intensive maintenance, obstruct the wide implementation of these technologies in the industry. The influence of the above-mentioned emerging technologies (PEF, US, HPP, UV, MW) on LAB is outlined in table 1.

Conclusions

LAB have received extensive attention for a long time due to their wide applications in various industries, in particular in food fermentation and preservation.

Emerging technologies have been applied to LAB and showed some positive effects such as microbial inactivation, improved biomass production, raised fermentation efficiency, and increased yields of the useful metabolic products. Besides, it was demonstrated that the stimulation or inactivation effects of emerging technologies on LAB are highly dependent on the cell type, and processing parameters and conditions.

Nonetheless, more research work is required to ensure that these technologies could provide suitable and meaningful impacts on LAB before their implementation at industrial scale. Reprinted with permission from [START_REF] Roohinejad | Energy saving food processing[END_REF]. [START_REF] Barba | Mild processing applied to the inactivation of the main foodborne bacterial pathogens: A review[END_REF]. Increased the sensitivity to acid (pH 2.5) and bile salts. Increased the hydrophobicity of L. reuteri by 20%. [START_REF] Racioppo | Ultrasound attenuation of lactobacilli and bifidobacteria: Effect on some technological and probiotic properties[END_REF] Inactivation of lactic acid Wine 183-227 W, acoustic 71.8-99.3% cells reduction. [START_REF] Gracin | Influence of high power ultrasound on Brettanomyces and lactic acid bacteria in wine in continuous flow treatment[END_REF]

  growth of lactobacilli and bioconversion of isoflavones in biotin-supplemented soymilk upon ultrasound-treatment. Ultrasonics Sonochemistry, 19(1), 160-173. Ewe, J.-A., Wan-Abdullah, W.-N., Alias, A. K., & Liong, M.-T. (2012a). Bioconversion of isoflavones and the probiotic properties of the electroporated parent and subsequent three subcultures of Lactobacillus fermentum BT 8219 in biotin-soymilk. J Microbiol Biotechnol, 22, 947-959. Ewe, J.-A., Wan-Abdullah, W.-N., Alias, A. K., & Liong, M.-T. (2012b). Enhanced growth of lactobacilli and bioconversion of isoflavones in biotin-supplemented soymilk by electroporation. International Journal of Food Sciences and Nutrition, 63(5), 580-596. Ewe, J.-A., Wan-Abdullah, W.-N., Alias, A. K., & Liong, M.-T. (2013). Ultraviolet radiation enhanced growth of lactobacilli and their bioconversion of isoflavones in biotin-supplemented soymilk. LWT -Food Science and Technology, 50(1), 25-31. Falguera, V., Pagán, J., Garza, S., Garvín, A., & Ibarz, A. (2011). Ultraviolet processing of liquid food: A review: Part 2: Effects on microorganisms and on food components and properties. Food Research International, 44(6), 1580-1588. Field, D., Ross, R. P., & Hill, C. (2018). Developing bacteriocins of lactic acid bacteria into next generation biopreservatives. Current Opinion in Food Science, 20, 1-6. Forney, L. J., & Moraru, C. I. (2009). Ultraviolet light in food technology: principles and applications. CRC press.Franchi, M. A.,Tribst, A. A. L., & Cristianini, M. (2013). High-pressure homogenization: a non-thermal process applied for inactivation of spoilage microorganisms in beer.

Figure captions Figure 1 .

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Figure 2 .

 2 Figure 2. Schematic diagram of reversible and irreversible breakdown of cell

Figure 3 .

 3 Figure 3. a. Schematic representation of a micro-jet formation through collapsing

Figure 4 .

 4 Figure 4. A typical high-pressure processing system for treating pre-packed foods.

Figure 5 .

 5 Figure 5. Mechanism of action of UV-light for microbial inactivation. Reprinted with

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Acknowledgements

This work was supported by the China Scholarship Council and by "Université de technologie de Compiègne" (France).