Indication that the nitrogen source influences both amount and size of exopolysaccharides produced by Streptococcus thermophilus LY03 and modelling of the bacterial growth and exopolysaccharide production in a complex medium

Streptococcus thermophilus LY03 is a yogurt strain producing the same exopo lysaccharide material in both milk and MRS broth. Actually, two types of ex opolysaccharides are produced simultaneously. The two exopolysaccharides ar e identical in monomer composition (galactose and glucose in a 4:1 ratio) b ut differ in molecular size. Gel permeation chromatography revealed a high- molecular-mass exopolysaccharide (1.8 x 10(6)) and a low-molecular-mass exo polysaccharide (4.1 x 10(5)). Both exopolysaccharides can be isolated from the fermentation broth separately. The proportion in which they are produce d is strongly dependent on the carbon/nitrogen ratio of the fermentation br oth. A shift from a high-molecular-mass exopolysaccharide to a low-molecula r-mass exopolysaccharide was observed with increasing initial complex nitro gen concentrations. All necessary biokinetic parameters to study the kineti cs of S. thermophilus LY03 fermentations were obtained from a mathematical model which describes both S. thermophilus LY03 growth and exopolysaccharid e production and degradation. The model is valid with various initial compl ex nitrogen concentrations and can be applied to simulate exopolysaccharide production in a milk medium.