Production of optically pure lactic acid by microbial fermentation: a review

Biotransformation of organic wastes into value-added products is gaining interest owing to waste management issues, exhaustion of fossil fuels and the demand for biodegradable plastics. Lactic acid is widely used for polymers, foods, beverages, medicines, cosmetics and clothing. However, the major obstacle in large-scale fermentation of lactic acid is achieving enhanced yield, productivity and optical purity with cheap resources. Therefore, we review methods and recovery techniques for production of microbial lactic acid using cheap fermentative substrates. New strategies allow to alleviate limitations associated with substrate inhibition, product inhibition, undesirable by-products, sensitivity to toxic compounds, inefficient utilization of mixed sugars and overuse of neutralizing agents. Efficient utilization of mixed sugars can be achieved with simultaneous saccharification and fermentation using mixed cultures, isolating carbon catabolic repression-negative strains and altering the metabolic pathway. Lactic acid productivity can be improved by co-culture, maintaining high cell density and periodically removing end-products accumulated in the fermentation medium. Inhibition by toxic compounds can be eliminated by using engineered feedstock which releases less inhibitors, by using inhibitor-tolerant microbes and by development of genetically engineered strains. Fed-batch fermentation was found to be better than other operation modes due to less substrate inhibition.