Enhanced production of L(+)-lactic acid from corn starch in a culture of Rhizopus oryzae using an air-lift bioreactor

L(+)-Lactic acid production was investigated using 8 Rhizopus strains and various culture conditions. Rhizopus oryzae NRRL395 showed the highest yield of L(+)-lactic acid among various Rhizopus strains. The optimum production medium contained the following: corn starch, 120 g/l; ammonium sulfate, 1.35 g/l; small amounts of mineral salts (potassium phosphate, magnesium sulfate and zinc sulfate). The data obtained for the flask culture were successfully reproduced using a 3 l air-lift bioreactor under the optimum conditions. The yield (based on initial carbon source concentration) and final concentration of L(+)-lactic acid were 85% and 102 g/l, respectively, in the 3 l air-lift bioreactor. The lactic acid was recovered with a yield (based on lactic acid containing in broth) of 90% using 4 steps, i.e., filtration, active carbon treatment, ultra-filtration and concentration by vacuum evaporation. The lactic acid purified was 100% L(+)-form as judged by HPLC.