Gel formation and rheological properties of fermented milk with in situ exopolysaccharide production by lactic acid bacteria

Exopolysaccharides (EPS) produced by lactic acid bacteria can be used as natural stabilizers in fermented milk. The ability of EPS to modulate viscosity has not been correlated to their concentration but rather to their structure and their interactions with other milk compounds. The development of a product with desirable characteristics requires knowledge of the EPS structure-function relationship. The aim of this study was to compare the influence of various EPS structures on the gel formation and rheological/physical properties (firmness, apparent viscosity, elastic modulus, syneresis) of fermented milk. Three Streptococcus thermophilus-HC15 (control); 0131 (neutral, flexible, branched EPS); 2104 (anionic, stiff, linear EPS)-and four Lactobacillus delbrueckii subsp. bulgaricus-210R (control); 11842 (neutral, flexible, branched EPS); 702074 (neutral, flexible, branched EPS); 291 (neutral, stiff, branched EPS)-were compared. Strains were grown at 42 A degrees C in skim milk until pH reached 4.6. Gel formation and elastic modulus were modified by the anionic and linear EPS from strain 2104 as compared with the other strains. Higher values for apparent viscosity, firmness and whey retention were obtained with strains 0131, 2104 and 291 producing EPS with linear or few branching and high molecular weight. This work showed that the gel formation and rheological/physical properties of fermented milk are modified by the structural characteristics of EPS, especially negative charge, flexibility, degree of branching and molecular weight. This study contributes to a better understanding of the EPS structure-function relationship in view to provide tools to develop fermented products with desirable characteristics.