Abstract: Fermentation experiments in a 7 L bioreactor were carried out to study fermentation characteristics of ammonia-tolerant Rhizopus oryzae mutant ST50-2 and L-lactic acid production using aqueous ammonia as the neutralizer. Compared with its parental strain Rhizopus oryzae AS3.189, the mutant strain resulted in a reduction in fermentation period from 72 h to 56 h, in alcohol content by 22.22% and in ADH specific activity by 16.06% and an increase in L-lactic acid production by 12.30%, in LDH specific activity by 83.09% and in reducing sugar utilization rate and biomass accumulation. CaCO3 influenced mycelial pellet formation in the 7 L-scale fermentation of the mutant strain, and agitation speed and amount of aqueous ammonia addition had influence on mycelial growth, the size of mycelial pellets and L-lactic acid production, and L-lactic acid production and fermentation period were affected by pH. The optimal fermentation conditions were determined as follows: agitation speed of 400 r/min, aqueous ammonia as the neutralizer at a 15% addition and pH of smaller than 6.0 and the resulting mycelial pellet diameter, fermentation period and L-lactic acid yield were 1.0－1.5 mm, 56 h and 93.32 g/L, respectively.

Key words: ammonia-tolerant Rhizopus oryzae, L-lactic acid, lactate dehydrogenase(LDH), alcohol dehydrogenase (ADH)