Abstract

Ethanol is a main by-product in the fermentation broth of Rhizopus oryzae during the production of high-optical purity L-lactic acid. By screening the lower activity of alcohol dehydrogenase (ADH) mutant, thus decreasing the flux of pyruvic acid to ethanol may be a virtual method for increasing the conversion rate of glucose to L-lactic acid. Mutagenesis of R. oryzae As3.3461 was conducted in this study, and a high-producing lactic acid mutant HBF-12 was screened out with selective medium of Yeast-Peptone-Dextrose (YPD) containing allyl alcohol. This mutant showed higher conversion rate of 88.4 g/l to lactate in a 7 L bioreactor, which was an increase of 30.1% compared to original strain of 67.95 g/l, and the conversion rate to ethanol decreased 73.6%. The zymological analysis indicated HBF-12 had a lower ADH specific activity, but an appreciably higher lactate dehydrogenase (LDH) specific activity. The HBF-12 mutant also had a higher consumption rate of glucose and higher biomass collection.

Key words: Alcohol dehydrogenase, ethanol, L-lactic acid, Rhizopus oryzae.