Efficiency of modified skimmed milk base media to achieve high exopolysaccharide/cell ratios by Lactobacillus delbrueckii subsp bulgaricus SZ2 in optimised conditions defined by the response surface methodology

Exopolysaccharide (EPS) production by Lactobacillus delbrueckii subsp. bulgaricus SZ2 was optimised in modified MRS (M-MRS) using the response surface methodology (RSM). Maximum EPS production was 74.3 +/- 2 mg/L, and the optimised values of the three variables predicted for maximum EPS production included a temperature of 38.7 degrees C, Bacto-casitone and glucose concentrations of 24.5 and 29.6 g/L, respectively. To compare EPS production in MRS and skimmed milk (SM), the kinetics of EPS formation and growth were monitored in M-MRS, SM, skimmed milk plus 2% additional sucrose (Suc-SM) and skimmed milk containing Bacto-casitone (20 g/L) and yeast nitrogen base (5 g/L) (BY-SM). EPS production in all the media tested seemed to be growth-related. The EPS/cell ratios were determined to be 3.12 x 10-10, 1.43 x 10-10, 4.42 x 10-11 and 3.16 x 10-11 mg/cell, in Suc-SM, SM, M-MRS and BY-SM, respectively, clearly indicating the greater effect of C/N ratio when cell behaviour in EPS production is considered.