Capsular Exopolysaccharides from Two Streptococcus thermophilus Strains Differ in Their Moisture Sorption Behavior.

Streptococcus thermophilus is a species frequently used in the manufacture of fermented milk. Apart from acid production, some strains additionally synthesize exopolysaccharides (EPS) which contribute to texture improvement and syneresis reduction, both being attributable to the EPS's high water binding capacity. There are two different types of EPS that may be produced, namely free exopolysaccharides (fEPS) which are secreted into the medium, and capsular EPS (cEPS) which remain attached to the bacterial cell wall. This study aims to analyze their individual contribution to techno-functional properties of fermented milk by determining the moisture sorption behavior of isolated fEPS and cell-attached cEPS from two S. thermophilus strains separately: ST-1G, a producer of non-ropy fEPS and cEPS, and ST-2E, a producer of ropy fEPS and cEPS. Differences in moisture load and sorption kinetics, determined for the first time for microbial EPS, were related to structural and macromolecular properties. The observed data are discussed by using previously published data on the physical properties of stirred fermented milk produced with these two strains. ST-1G EPS showed a higher cEPS fraction, a higher moisture load and slower moisture desorption than EPS produced by ST-2E, thus contributing to lower syneresis in fermented milk. For ST-2E, higher gel viscosity was related to a higher intrinsic viscosity and molecular mass of the ropy fEPS. Both strains produced complex EPS or EPS mixtures with clearly different molecular structures.

Structural and technological characterization of ropy exopolysaccharides produced by Lactobacillus plantarum strains isolated from Tarhana.

Exopolysaccharide producing starter cultures enable manufacturing "clean labeled" foods with improved textural and nutritional properties. The structural and technological analyses were performed on the ropy exopolysaccharides of six Lactobacillus plantarum. The incubation temperature, time and pH affected the exopolysaccharide production and high exopolysaccharide was produced in the presence of sucrose and maltose. The viscosity of exopolysaccharide was high at acidic conditions except PFC311E that showed viscous at neutral pH. Lactobacillus plantarum strains produced between 120 and 400 mg/L exopolysaccharide in which the highest was observed at L. plantarum PFC311. Exoploysaccharides were degraded over 300 °C except PFC311E that degraded at 295.7 °C. The NMR analyses revealed that the exopolysaccharides were synthesized by α1-6, α1-3 and α1-4 bonds with glucose, galactose and fructose moieties. In conclusion, L. plantarum PFC311 produced ropy exopolysaccharide with different structural, rheological and thermal properties and reveals potential to be used in food industry.

Shear and extensional rheology of acid milk gel suspensions with varying ropiness.

Exopolysaccharides (EPS) synthesized by lactic acid bacteria during fermentation significantly affect the rheology of set-style acid milk gels and that of acid gel suspensions, produced from the gels by stirring. In this study, shear and uniaxial extensional flow of acid gel suspensions made with seven single strains of Streptococcus thermophilus or Lactococcus lactis was investigated. Six strains produced ropy EPS, and maximum filament length determined by using a continuous stretching method was up to four times higher than filament length of an EPS-negative control strain. The systems containing ropy EPS showed a different response to shear and extensional deformation. In shear rheology, higher apparent viscosities and an enhanced structural breakdown was observed for acid gel suspensions with more pronounced ropiness. Breakup time and extensional viscosity, determined by using a Capillary Breakup Extensional Rheometer (CaBER™), significantly increased with increasing ropiness. The increase of extensional viscosity with increasing ropiness was, however, much higher than the effects of ropiness on shear viscosity. As relaxation times also depended on ropiness, it is concluded that ropiness is caused by EPS-EPS interactions that can be better discriminated in extensional rheology. PRACTICAL APPLICATIONS: To improve the texture of fermented milk, lactic acid bacteria that are able to produce ropy exopolysaccharides (EPS) are increasingly used in the dairy industry. The EPS exhibit a significant influence on processing properties and sensory characteristics of the resulting products, which can be estimated by means of shear and extensional rheology. The current work provides information on these respective properties of acid gel suspensions, which facilitate product design by supporting the selection of appropriate starter cultures.

Production and molecular structure of heteropolysaccharides from two lactic acid bacteria.

In the dairy industry, exopolysaccharides (EPS) produced in situ from lactic acid bacteria are of great interest because of their contribution to product texture. Some EPS cause ropiness which might be linked to specific physical and chemical EPS properties. EPS show a broad variety of chemical structures and, because analysis is rather complex, it is still a major challenge to establish structure-function relationships. The aim of this study was to produce EPS with different degree of ropiness, perform in-depth structural elucidations and relate this information to their behaviour in aqueous solutions. After cultivation of Streptococcus thermophilus DGCC7919 and Lactococcus lactis LL-2A and subsequent EPS isolation, both EPS showed similar macromolecular properties, but pronounced differences in monosaccharide composition and glycosidic linkages. Our data suggests that mainly the side chains in the EPS from LL-2A might be responsible for a higher ropiness than that observed for EPS from DGCC7919.

Heterofermentative Lactobacilli Causing Ropiness in Basque Country Ciders.

Ropy strains of heterolactic Lactobacillus spp. were isolated from bottled spoiled ciders produced in the Basque Country. The isolates were able to produce slime experimentally on glucose containing medium and apple must. The production of ropy slime occurred during exponential growth and the increase in viscosity was correlated with production of polysaccharide. They were characterized with regards to growth at different pH, sulfur dioxide (SO2), ethanol and temperatures and showed similar behavior. All ropy strains grew in the presence of high ethanol concentrations (15%) and at low temperatures (10 and 15°C). In addition, these were tolerant to 10, 25 and 50 mg/l of total SO2 (pH 3.8), but with longer lag times as the concentration of SO2, increased. However, the ropy lactobacilli studied were affected by the pH of the medium and in general pH below 3.7 limited their growth seriously.