Microbial exopolysaccharides are biothickeners that can be added to a wide variety of food products, where they serve as viscosifying, stabilizing, emulsifying or gelling agents. Numerous exopolysaccharides with different composition, size and structure are synthesized by lactic acid bacteria. The heteropolysaccharides from both mesophilic and thermophilic lactic acid bacteria have received renewed interest recently. Structural analysis combined with rheological studies revealed that there is considerable variation among the different exopolysaccharides; some of them exhibit remarkable thickening and shear-thinning properties and display high intrinsic viscosities. Hence, several slime-producing lactic acid bacterium strains and their biopolymers have interesting functional and technological properties, which may be exploited towards different products, in particular, natural fermented milks. However, information on the biosynthesis, molecular organization and fermentation conditions is rather scarce, and the kinetics of exopolysaccharide formation are poorly described. Moreover, the production of exopolysaccharides is low and often unstable, and their downstream processing is difficult. This review particularly deals with microbiological, biochemical and technological aspects of heteropolysaccharides from, and their production by, lactic acid bacteria. The chemical composition and structure, the biosynthesis, genetics and molecular organization, the nutritional and physiological aspects, the process technology, and both food additive and in situ applications (in particular in yogurt) of heterotype exopolysaccharides from lactic acid bacteria are described. Where appropriate, suggestions are made for strain improvement, enhanced productivities and advanced modification and production processes (involving enzyme and/or fermentation technology) that may contribute to the economic soundness of applications with this promising group of biomolecules.

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The effects of different carbohydrates or mixtures of carbohydrates as substrates on bacterial growth and exopolysaccharide (EPS) production were studied for the yoghurt starter culture Streptococcus thermophilus LY03. This strain produces two heteropolysaccharides with the same monomeric composition (galactose and glucose in the ratio 4:1) but with different molecular masses. Lactose and glucose were fermented by S. thermophilus LY03 only when they were used as sole energy and carbohydrate sources. Fructose was also fermented when it was applied in combination with lactose or glucose. Both the amount of EPS produced and the carbohydrate source consumption rates were clearly influenced by the type of energy and carbohydrate source used, while the EPS monomeric composition remained constant (galactose-glucose, 4:1) under all circumstances. A combination of lactose and glucose resulted in the largest amounts of EPS. Measurements of the activities of enzymes involved in EPS biosynthesis, and of those involved in sugar nucleotide biosynthesis and the Embden-Meyerhof-Parnas pathway, demonstrated that the levels of activity of ␣-phosphoglucomutase, UDP-galactose 4-epimerase, and UDP-glucose pyrophosphorylase are highly correlated with the amount of EPS produced. Furthermore, a weaker relationship or no relationship between the amounts of EPS and the enzymes involved in either the rhamnose nucleotide synthetic branch of the EPS biosynthesis or the pathway leading to glycolysis was observed for S. thermophilus LY03.Food hydrocolloids play an important role in the rheological properties of food products (29,34). Examples are plant carbohydrates such as modified starch and guar gum, animal proteins like gelatin and casein, and microbial exopolysaccharides (EPS) such as xanthan and gellan. The addition of food additives is not always allowed, so natural yoghurts and fermented milks are dependent on the EPS-producing capacities of the starter strains used. EPS are microbial polysaccharides that are secreted extracellularly and are either associated with the cell surface in the form of capsules or released into the medium in the form of slimes (11). The thermophilic yoghurt strains Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus produce heteropolysaccharides that consist of a repeating unit of neutral sugars (glucose, galactose, and rhamnose) in different ratios (1,3,6,11,13,15,(17)(18)(19)(20)(21)24, 32; G. T. Lamothe, F. Stingele, J.-R. Neeser, and B. Mollet, Abstr. Am. Soc. Microbiol. Conf. Streptococcal Genet., abstr. no. 2A-16, p. 67, 1998). Some of these polysaccharides display pseudoplastic and thixotropic properties (4, 5, 6, 13). It has been postulated that their rheological properties are influenced not only by the amount of EPS but also by its structure. Recently, the structures of the repeating units of some EPS produced by L. delbrueckii subsp. bulgaricus (21) and S. thermophilus (3,13,15,24,32) have been elucidated. However, there is still a lack of knowledge of the physiological asp...

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