EFFECTS OF PH AND AERATION ON GAMMA-POLY(GLUTAMIC ACID) FORMATION BY BACILLUS-LICHENIFORMIS IN CONTROLLED BATCH FERMENTER CULTURES

Bacillus licheniformis ATCC 9945A was grown on Medium E in batch ferme ntations in which the pH was maintained at 5.5, 6.5, 7.4, and 8.25. Th e effects of pH on cell growth, carbon source utilization, and gamma-p olyglutamic acid (gamma-PGA) production, molecular weight, and polymer stereochemistry were determined. The gamma-PGA yield was highest (15 g/L, 96 h growth time) at pH 6.5. The increase in gamma-PGA formation at pH 6.5 corresponded with a relatively high specific production rate at high gamma-PGA concentration (0.09 h(-1), similar to 15 g/L gamma- PGA). In contrast, the specific gamma-PGA production rates at fermente r pH values of 5.5 and 7.4 decreased significantly for gamma-PGA ferme nter yields > similar to 5 g/L. Interestingly, alteration of the mediu m pH had little to no significant effects on the product quality as me asured by stereochemical composition and molecular weight. While gluta mate and glycerol utilization were similar as a function of pH, citrat e consumption increased at pH 6.5, indicating that the formation of ga mma-PGA from citrate at pH 6.5 was of increased importance. The effect of aeration was evaluated by increasing the agitation speed (250 to 8 00 rpm) and aeration rate (0.5 to 2.0 L/min) at pH 6.5, the pH of maxi mal gamma-PGA production. Increased aeration resulted in doubling of t he cell dry weights (2 to 4 g/L), increasing gamma-PGA yields (6.3 to 23 g/L by 48 h) and increasing in the maximum gamma-PGA-specific produ ction rate (0.09 to 0.11 h(-1)). Other effects of increased agitation included a rapid depletion of glutamate and citrate (by 50 h) and a de crease in product molecular weight. Despite the increase in agitation and aeration, oxygen limitation of the culture was not avoided, becaus e the partial pressure decreased to < 1.0% by 29 h. (C) 1995 John Wile y & Sons, Inc.