Production of L-lactic acid by Rhizopus oryzae using semicontinuous fermentation in bioreactor

Abstract

Semicontinuous fermentation using pellets of Rhizopus oryzae has been recognized as a promising technology for L-lactic acid production. In this work, semicontinuous fermentation of R. oryzae AS 3.819 for L-lactic acid production has been developed with high L-lactic acid yield and volumetric productivity. The effects of factors such as inoculations, CaCO3 addition time, and temperature on L-lactic acid yield and R. oryzae morphology were researched in detail. The results showed that optimal fermentation conditions for the first cycle were: inoculation with 4% spore suspension, CaCO3 added to the culture medium at the beginning of culture, and culture temperature of 32-34°C. In orthogonal experiments, high L-lactic acid yield was achieved when the feeding medium was (g/l): glucose, 100; (NH4) 2SO4, 2; KH2PO4, 0.1; ZnSO 4·7H2O, 0.33; MgSO4·7H 2O, 0.15; CaCO3, 50. Twenty cycles of semicontinuous fermentation were carried out in flask culture. L-lactic acid yield was 78.75% for the first cycle and 80-90% for the repeated cycles; the activities of lactate dehydrogenases (LDH) were 7.2-9.2 U/mg; fermentation was completed in 24 h for each repeated cycle. In a 7-l magnetically stirred fermentor, semicontinuous fermentation lasted for 25 cycles using pellets of R. oryzae AS 3.819 under the optimal conditions determined from flask cultures. The final L-lactic acid concentration (LLAC) reached 103.7 g/l, and the volumetric productivity was 2.16 g/(l•h) for the first cycle; in the following 19 repeated cycles, the final LLAC reached 81-95 g/l, and the volumetric productivities were 3.40-3.85 g/(l•h). © Society for Industrial Microbiology 2010.