Impact of starch and exopolysaccharide-producing lactic acid bacteria on the properties of set and stirred yoghurts

The impact of exopolysaccharide (EPS)-producing lactic acid bacteria with well-known structures and starch (0.75%) on the rheological properties (apparent viscosity and elastic modulus) and physical properties (syneresis) of set and stirred yoghurts was studied. Three EPS-producing strains with different structural characteristics were studied: Streptococcus thermophilus ST1 (anionic, stiff and linear EPS), Lactobacillus delbrueckii subsp. bulgaricus LB1 (neutral, stiff and ramified EPS) and Lb. delbrueckii subsp. bulgaricus LB2 (neutral, flexible and highly ramified EPS). The presence of linear, stiff, and anionic EPS from ST1 increased the elastic modulus in all yoghurt conditions, possibly owing to electrostatic interactions with caseins. Higher viscosity values were obtained with stiff and linear or slightly branched EPS from the ST1 and LB1 for all yoghurt conditions. Starch addition increased the values of the rheological and physical properties of all stirred yoghurts, probably due to the repulsion between proteins and polysaccharides favouring thermodynamic incompatibility.