Addition of purified exopolysaccharide isolates from S. thermophilus to milk and their impact on the rheology of acid gels

We present a new approach for investigating the impact of exopolysaccharides (EPS) from lactic acid bacteria on the rheological characteristics of acid milk gels by decoupling microbial EPS synthesis from milk acidification. Free and capsular EPS of Streptococcus thermophilus ST-143 were isolated and purified separately from batch fermentation media to obtain EPS preparations with high purity (>85% EPS). Defined amounts of EPS were added to milk prior to acidification, which was induced by a chemical acidulant or by S. thermophilus strains. For the first time it is shown that the stiffness of acid milk gels increases almost linearly with the concentration of free EPS, whereas those derived from capsular EPS did not affect rheology of acid gels when added at the same amount. Both EPS fractions were determined to be uncharged, but differed distinctly in molar mass, which was 2.6 × 106 Da for free EPS, and 1.4 × 105 and 7 × 103 Da for the two fractions that were derived from the capsular EPS (ratio, 1:3). This supports the hypothesis that it is molar mass rather than EPS charge, which is responsible for the contribution to gel stiffness. For stirred yogurt, apparent viscosity increased significantly with addition of EPS and storage time at 6 °C. Depletion interactions between uncharged EPS of high molar mass and negatively charged casein micelles can be regarded as main factor which is responsible for these effects.