Lactic acid production by Bacillus coagulans—kinetic studies and optimization of culture medium for batch and continuous fermentations

Abstract

Bacillus coagulans is an atypical strain for lactic acid production; the thermophile character of this strain (growth at 52°C) proves that it is particularly adapted for industrial production of lactate without sterile conditions. In the first step, continuous culture was performed to define the experimental domain of aeration and nitrogen supplementation. The aerobic condition showed a positive influence on growth and a negative effect on lactate production. At steady state for pH 6.4, the concentrations of biomass and lactic acid were 3.9 and 19.5 g l −1, respectively, without aeration and 4.6 and 11 g l −1 with aeration. The nitrogen source is essential for the fermentation process. Pulses of different types of yeast extract (liquid and powder) were added into the fermentor at steady state. After pulses, biomass concentration increased two and three times with powdered yeast extract and liquid yeast extract, respectively. Liquid yeast extract was more efficient for growth than powdered yeast extract probably due to degradation of vitamins in a spray dryer. Secondly, a factorial fractional experimental design was performed to optimize batch fermentation. Temperature and pH control, the initial concentration of sugar, and the nitrogen source were optimized. For the initial sucrose concentration of 60 g l −1, productions of biomass and lactic acid were 3.1 and 55 g l −1, respectively. The maximal specific production rate of lactic acid is high (6.1 ± 0.3 g/l · h/g/l cell) in comparison with mesophilic lactic acid bacteria.