Semicontinuous Fermentation of Cassava by Streptococcus bovis Using Membrane Bioreactor to Produce Lactic Acid

The main objective of this study is to compare the production of lactic acid from cassava by Streptococcus bovis in semicontinuous system with and without membrane separation. Fermentation was run for 72 h, and membrane filtration was performed using micro filtration membrane (MF). The results obtained demonstrate that higher productivity of lactic acid was achieved using fermentation coupled with membrane separation. It was also found that dilution rate (D) of 0.02 h-1 led to slight increase in the lactic acid productivity. Despite this small increase, this investigation shows that membrane separation has the potential to improve the performance of semicontinuous fermentation system.

Application of metabolic controls for the maximization of lipid production in semicontinuous fermentation

Acetic acid can be generated through syngas fermentation, lignocellulosic biomass degradation, and organic waste anaerobic digestion. Microbial conversion of acetate into triacylglycerols for biofuel production has many advantages, including low-cost or even negative-cost feedstock and environmental benefits. The main issue stems from the dilute nature of acetate produced in such systems, which is costly to be processed on an industrial scale. To tackle this problem, we established an efficient bioprocess for converting dilute acetate into lipids, using the oleaginous yeast Yarrowia lipolytica in a semicontinuous system. The implemented design used low-strength acetic acid in both salt and acid forms as carbon substrate and a cross-filtration module for cell recycling. Feed controls for acetic acid and nitrogen based on metabolic models and online measurement of the respiratory quotient were used. The optimized process was able to sustain high-density cell culture using acetic acid of only 3% and achieved a lipid titer, yield, and productivity of 115 g/L, 0.16 g/g, and 0.8 g⋅L−1⋅h−1, respectively. No carbon substrate was detected in the effluent stream, indicating complete utilization of acetate. These results represent a more than twofold increase in lipid production metrics compared with the current best-performing results using concentrated acetic acid as carbon feed.