OPTIMIZATION OF EXOPOLYSACCHARIDE PRODUCTION BY LACTOBACILLUS-DELBRUECKII SUBSP BULGARICUS RR GROWN IN A SEMIDEFINED MEDIUM

The optimal fermentation temperature, pH, and Bacto-casitone (Difco La boratories, Detroit, Mich.) concentration for production of exopolysac charide by Lactobacillus delbrueckii subsp. bulgaricus RR in a semidef ined medium were determined by using response surface methods. The des ign consisted of 20 experiments, 15 unique combinations, and five repl ications. All fermentations were conducted in a fermenter with a 2,5-l iter working volume and were terminated when 90% of the glucose in the medium had been consumed. The population of L. delbrueckii subsp. bul garicus RR and exopolysaccharide content were measured at the end of e ach fermentation. The optimum temperature, pH, and Bacto-casitone conc entration for exopolysaccharide production were 38 degrees C, 5, and 3 0 g/liter, respectively, with a predicted yield of 295 mg of exopolysa ccharide/liter. The actual yield under these conditions was 354 mg of exopolysaccharide/liter, which was within the 95% confidence interval (217 to 374 mg of exopolysaccharide/liter). An additional experiment c onducted under optimum conditions showed that exopolysaccharide produc tion was growth associated, with a specific production at the endpoint of 101.4 mg/g of dry cells. Finally, to obtain material for further c haracterization, a 100-liter fermentation was conducted under optimum conditions. Twenty-nine grams of exopolysaccharide was isolated from c entrifuged, ultrafiltered fermentation broth by ethanol precipitation.