Improved Optical Purity of Lactic Acid Through Mixed Microbial Fermentation

Lactic acid, used for the production of polylactic acid (PLA) biopolymers, is produced naturally in two forms, L(+) or D(-). The form, known as the optical purity, of the lactic acid used impacts the characteristics of the resultant polymer; typically > 90% L(+)-lactic acid is desired. Previous work has demonstrated that mixed microbial codigestion of foodwaste and primary sludge can achieve 97% of theoretical lactic acid yield in less than 24 h, however the resultant optical purity was low at 57% L(+)-lactic acid [1]. Lactobaciluss plantarum, an organism which has been found to dominate the digestion of food waste to lactate, produces L(+)-lactic acid preferentially and releases lactate racemase to convert the product to D(-)-lactic acid under high lactic acid concentrations. Lactate racemase is oxygen sensitive. To improve optical purity of the fermentation, dissolved oxygen was injected into the digestor at controlled concentrations varying from 0 to 1.5 mg L^-1. Results indicate that minor amounts of oxygen are effective at improving optical purity of the resultant acid.

1.      R. RedCorn and A.S. Engelberth, Identifying conditions to optimize lactic acid production from food waste co-digested with primary sludge Biochemical Engineering Journal, 105, Part A(2016) 205-213.