Bacterial exopolysaccharides (EPSs) have diverse and complex chemical structures that strongly influence their functional properties and biological functions. They can be divided into homopolysaccharides (HoPS), which contain a single type of monosaccharide, and heteropolysaccharides (HePS), which consist of two or more types of monosaccharides ^[1][2][3][16,17,18]. Most of the EPS produced by LAB are HePS and are synthesized intracellularly, while some LAB species/strains produce HoPS by extracellular enzymes ^[4][5][10,19]. The HoPS produced by LAB (Table 1) can be classified as glucans, fructans, or galactans, which consist of D-glucose, D-fructose, or D-galactose, respectively ^[4][6][10,20]. Glucans consist of glucose residues as the main backbone structure with different degrees of branching and binding sites that vary from bacterial strain to bacterial strain. Glucans can be classified as either α-glucans or β-glucans, and are produced by a variety of LAB species in the genera Leuconostoc, Lactobacillus, Streptococcus, and Weissella ^[7][21]. Alpha-glucans can be divided into four groups: (i) dextran is water-soluble and mostly has α-(l→6) bonds, although some branching may occur at α-(l→2), α-(l→3) or α-(l→4) ^[8][9][22,23]; (ii) mutan is generally water-insoluble and contains mainly α-(l→3) bonds with branching at α-(l→6) ^[7][21]; (iii) reuteran is a water-soluble branched α-glucan consisting of α-(l→4)-linear fragments linked by α-(l→6) bonds ^[10][24]; and (iv) alternan exhibits alternating α-(l→3) and α-(l→6) bonds and shows lower viscosity and higher solubility in water ^[11][25]. Beta-glucans are also HoPS, produced by Pediococcus and Streptococcus spp., and consist of D-glucose linked by β-(l→3) bonds together with β-(l→2) branches ^[5][12][19,26]. Fructans are water-soluble fructose polymers produced by strains of Streptococcus salivarius, Leuconostoc mesenteroides, Limosilactobacillus reuteri (formerly Lactobacillus reuteri), Lactobacillus johnsonii and Fructilactobacillus sanfranciscensis (formerly Lactobacillus sanfranciscensis). They can be divided into (i) levan with β-2,6 link(ages and (ii) inulin with β-2,1 linkages ^[13][27]. The water-soluble galactans are less abundant, have α-(1→6)-linked galactose units, and are produced by a few LAB strains belonging to Weissella confusa, Lactococcus lactis subsp. lactis and Lactobacillus delbrueckii subsp. bulgaricus ^[5][14][19,28].

EPS produced by LAB can be used to thicken and stabilize fermented dairy products. Lowering the pH during the fermentation process of yoghourt can lead to syneresis resulting from destabilisation of casein micelles. In situ production of EPS has been used to overcome this problem, as it leads to better rheological properties than when EPS is added as one of the components ^[75][76][92,123]. The starter cultures used for yoghourt production, Lactobacillus delbruecki subsp. bulgaricus and Streptococcus thermophilus, were selected to produce exopolysaccharides, generally 60–150 mg/L and 30–890 mg/L, respectively ^[46][77][8,124]. In recent years, studies have been conducted on new EPS-producing starter or adjunct cultures to be used in yoghourt production ^[46][78][8,125]. For example, Han et al. ^[79][126] evaluated 19 high EPS-producing Str. thermophilus strains isolated from traditional Chinese fermented milk products and used in yoghourt production. They selected a starter with high EPS production (SH-1), which exhibited lower syneresis, better texture, and better sensory evaluation than the samples fermented with a commercial yoghourt starter culture ^[79][126]. Fermented dairy products with low fat content can also be improved by using starters with high EPS production, as milk fat plays an important role on the taste, texture, and overall rheological properties of these products ^[80][127]. For example, the performance of EPS-producing Limosilactobacillus mucosae (formerly Lactobacillus mucosae) DPC 6426 as an adjunct culture in the production of low-fat yoghourt has been evaluated and shown to reduce syneresis and improve the functional properties of yoghourt ^[81][128].

A variety of LAB cultures are used in cheese-making as starters and/or adjunct cultures ^[82][129]. Nowadays, customers are looking for healthier, low-fat cheeses, and this is where bacterial strains that can produce EPS come into play. As with any other dairy product, the amount of fat is critical to the texture and flavor of the cheese. Therefore, EPS produced by starter/adjuvant cultures can be used as a fat substitute and texturizer in the production of low-fat cheese ^[46][8]. Several studies have shown that EPS improves the texture and quality of low-fat cheese, resulting in a product that has similar properties to its full-fat counterpart. For example, EPS-producing strains of the genus Lactobacillus have been shown to increase moisture content and improve the melting of low-fat mozzarella cheese ^[46][83][8,130]. In a study by Costa et al. ^[84][131], an EPS-producing strain of Lactococcus lactis was used on semi-fat cheddar cheese. Several positive effects of the exopolysaccharide were observed, such as an increase in cheese yield. There was no negative interaction between the starter strain or the ripening strain and the exopolysaccharide-producing strain. After 3 months of ripening, some tests showed that the addition of the EPS-producing strain resulted in a semi-fat cheddar cheese with similar characteristics to a full-fat cheddar cheese without any change in taste ^[84][131]. For low-fat cheddar cheese, the addition of the EPS-producing culture of Lb. plantarum resulted in cheeses with higher moisture content, higher proteolysis and better sensory values, as well as lower hardness and cohesion compared to the control cheese ^[85][132]. In addition, the use of a mixed starter culture containing EPS-producing strains in the production of low-fat Kasar cheese improved the textural properties by producing a less compact protein matrix and a spongy structure ^[86][133]. The type of EPS was also shown to affect the rheological properties of the cheese. It was shown that the lower branching ropy EPS reduced protein particle size and decreased creaminess in a model of a low-fat fresh cheese, while the capsular EPS mainly contributed to the reduction of syneresis ^[87][134]. The use of EPS-producing LAB in full-fat cheeses has also been studied by several authors. In the production of Prato cheese, the use of exopolysaccharide-producing cultures was shown to improve yield and increase moisture content without affecting proteolysis, pH, melting ability, and sensory acceptability ^[88][135]. Rehman et al. ^[89][136] used a high EPS-producing strain of Lactobacillus kefiranofaciens isolated from Tibetan kefir grain to improve the chewiness and hardness of mozzarella cheese. In a recent study, in situ production of EPS was used to improve the production yield and rheological properties (hardness, elasticity and adhesiveness) of sour whey cheese-requesón ^[90][137].

Kefir is a self-carbonated, slightly alcoholic beverage made from fermented milk, traditionally from Eastern Europe. Kefir preparation requires kefir grains, which are composed of proteins and polysaccharides and contain a symbiotic association of homofermentative and heterofermentative lactic acid bacteria, acetic acid bacteria, and yeasts ^[80][127]. During the fermentation process, exopolysaccharides known as kefiran are produced and act as viscosity regulators. Kefiran is a branched water-soluble glucogalactan composed of equal parts glucose and galactose, and its production is mainly due to Lactobacillus kefiranofaciens ^[46][80][8,127]. Within the complex community of kefir grains, other species of LAB have been isolated from kefir and found to produce EPS, such as Lactiplantibacillus plantarum, Lacticaseibacillus paracasei, Lactobacillus helveticus, Lactiplantibacillus pentosus, Lactococcus lactis subsp. Lactis, and Leuconostoc mesenteroides ^{[91][92][93][94]}[138,139,140,141]. In the last decade, selected strains of mesophilic LAB have been investigated for their ability to ferment milk and produce EPS to improve the rheological properties of fermented milk-based foods ^{[70][91][92][95]}[83,138,139,142]. In addition, much attention has also been paid to non-dairy products fermented with kefir grains. Due to the wide variety of sugars and subtracts used in the fermentation process, the microbial diversity of non-dairy kefir grains may be greater than that of traditional kefir grains. Several authors reported 45 different bacterial species and 23 yeasts ^[96][143]. However, due to its great complexity, the relative composition of bacteria and yeasts may vary during the kefir fermentation process, making it difficult to control and obtain uniform products for industrial use ^[96][143].

In recent years, interest in vegetarian and vegan diets has increased for many reasons (e.g., health concerns, ethical concerns, sustainability). The challenge for manufacturers of plant-based milk alternatives is to produce products with acceptable taste and texture for customers ^[75][97][92,144]. The application of EPS-producing LAB has emerged to improve the sensory and organoleptic analysis of such products, as EPS can positively influence texture, mouthfeel, and syneresis ^[75][98][92,145]. Several studies have focused on the use of in situ production of EPS in fermented plant-based beverages, as it is possible to develop a large number of products with improved sensory properties. Li et al. ^[99][14] studied the fermentation process of soy milk with an EPS-producing L. plantarum strain over a period of 21 days. The fermented soy milk maintained the apparent viscosity and EPS content, exhibited satisfactory technological properties, and improved the taste of soy milk ^[99][14]. In another study, Hickisch et al. ^[97][144] attempted to produce a vegetable yoghourt alternative from lupins, which proved to be a good plant choice. A different strain of EPS-producing Lactiplantibacillus plantarum (TMW 1.460) was used for the fermentation process, which achieved better sensory values, such as appearance, texture, aroma, and taste ^[97][144]. Beverages mimicking the characteristics of cow’s milk yoghourt were also developed using an aqueous extract of quinoa flour and Weissella spp. as EPS producers and showed high acceptability in terms of acidity, sweetness, texture, and overall appearance ^[100][101][146,147].

EPS produced by LAB has been proposed as a promising alternative to replace the use of polysaccharides in bakeries as an additive for sourdough because they have thickening properties. EPS-producing LAB can be incorporated into fermented sourdough to have a positive effect on the techno-functional properties of baked goods ^{[102][103][104]}[148,149,150]. The in situ production of EPS has been shown to improve the handling and stability of the dough, but also to increase the technological properties of the final product, such as the texture and volume of the bread ^[104][105][150,151]. Moreover, the use of selected EPS-producing LAB strains in sourdoughs has been investigated as a novel technological approach to compensate for quality losses in the functional properties of reduced-sugar products or to replace the added fat in bakery products ^[103][106][149,152]. In addition, some studies have shown that the use of EPS-producing LAB has a cold-protective effect and overcomes the quality loss of frozen dough products ^[107][153]. The use of polysaccharides has gained interest in recent years with the increasing production of gluten-free products. Not only are customers looking for healthier products, but people with a chronic autoimmune disease—celiac disease—are also looking to bakeries for better-quality gluten-free products ^{[46][75][108]}[8,92,154]. The beneficial effect of EPS produced by LAB is to improve the structure and volume of gluten-free or gluten-containing bread, as they are able to bind water and form a high-quality network with other dough components. This later leads to softer bread crumb and longer shelf life ^[4][109][10,155]. The in situ production of EPS was demonstrated in a recent study by Zheng et al. ^[31][45], in which the EPS-producing Limosilactobacillus reuteri was used during the fermentation process. Its ability to produce fructans and glucans resulted in better-quality gluten-free bread.

While EPS-producing LAB are widely used in the dairy industry, their use in the meat industry is still a relatively new area of research ^[61][74]. Meat production dates back many millennia, and the use of lactic acid bacteria has given us an enormous variety of traditional foods around the world. By producing lactic acid or acetic acid in raw meat, we can control food safety, texture, color of meat, and much more ^[75][110][92,94]. Meat products contain many important nutrients (protein, iron, zinc, vitamins, etc.), but consumption of high-fat meat products may correlate with diseases such as diabetes or cardiovascular disease. Customer demand for low-fat meat products challenges the meat industry to develop new products to serve this market. This poses many problems for the meat industry because low-fat processed meat products may be deficient in terms of technological and sensory properties ^{[61][75][110][111]}[74,92,94,156]. Hydrocolloids are widely used as food additives in the food industry. In the meat industry, they are known to improve the water-holding capacity and gelling properties of meat proteins and can improve the texture of low-fat meat products, but nowadays customers reject food additives on the ingredient list ^{[112][113][114][115]}[157,158,159,160]. Therefore, in situ production of EPS by LAB during meat processing could be an interesting alternative. Few studies have been conducted on the in situ production of EPS by LAB in meat products. Meat products evaluated for the use of LAB for in situ production of EPS included cooked ham, raw fermented sausages (sucuk and salami), and spreadable raw fermented sausages ^[61][74]. Although the processing conditions for ham production showed a negative effect on EPS production, the use of in situ EPS-forming LAB was more promising for the production of fermented sausages ^[61][116][74,161]. In the study by Hilbig et al. ^[32][46], selected LAB strains (L. plantarum and L. curvatus strains) were able to produce sufficient amounts of EPS during sausage fermentation to allow reduction of the high fat content of fermented raw sausages ^[32][46]. Moreover, Dertli et al. ^[117][162] used EPS-forming LAB cultures in a Turkish fermented sausage to improve its textural properties, which became harder, less sticky, and tougher.