Production of the exopolysacchzrides by Streptococcus thermophilus: Effect of growth conditions on fermentation kinetics and intrinsic viscosity

Production of exopolysacharides (EPS) by a commercial Streptococcus thermophilus strain was evaluated at different growth conditions [temperature (32–45 °C), carbon source and initial nitrogen (N) content]. Lactose from deproteinized whey and sucrose allowed to obtain EPS yields higher than 1200 mg/mM of the consumed carbon source. Intrinsic viscosity of the EPS was significantly reduced by ionic strength indicating a polyelectrolyte behavior. Growth conditions used for the production of the EPS had a significant effect (p < 0.05) on the intrinsic viscosity. This was attributed to the effect of growth conditions on the molecular properties of the EPS [stiffness and molecular weight (MW)]. High MW EPS were produced when the bacteria grew at a high specific growth rate; however MW of the EPS and specific growth rate were not linearly associated. In the lactose fermentations carried out at different temperatures specific EPS synthesis rate was positive and linearly associated with the specific lactose consumption rate (R2 = 0.967) and specific galactose production rate (R2 = 0.967). Critical coil overlap parameter, [η]C⁎, for the EPS produced in the lactose fermentations carried out at 43 and 45 °C was determined to be approximately 7.6, and their critical overlap concentrations (C⁎) were 0.45 and 0.87 g/dL, respectively.