l-Lactic acid fermentation by culture of Rhizopus oryzae using ammonia as neutralizing agent

• New neutralizing agents for submerged culture of fungal cells were surveyed.
• Neutralizing agents composed of ammonium bicarbonate, urea and ammonia increased the productivity of lactic acid by 15%.
• The productivity and yield of R. oryzae BCRC 3371 are higher than R. oryzae ATCC 9363 at the optimal pH.

In order to avoid the formation of calcium sulfate solid waste in the downstream processing, the traditional neutralizing agent calcium carbonate used in Rhizopus lactic acid fermentation was replaced by ammonia water. However, the yield of lactic acid is hampered because of the toxicity of ammonia to the microbial cells. It is imperative to surmount the dilemma for how to maintain the lactic acid production but reduce the damage of cells by ammonia. In this work, ammonium bicarbonate and urea were added to ammonia water to reduce the chemical damage of ammonia on fungal cells. l-Lactic acid production in an air-lift bioreactor by cultures of R. oryzae BCRC 33071 and ATCC 9363 were compared. The results showed that the highest productivity (3.01 ± 0.05 g/h l) and yield (85.3 ± 1.3%) was obtained by culture of R. oryzae BCRC 33071, using 120 g/l of glucose as substrate and a mixture of ammonium bicarbonate (2%), urea (1%) and ammonia (10%) as neutralizing agent.