The effects of Ca(OH)(2), NH4OH and NaOH as neutralizing agents on the efficiency of L-lactic acid production by Lactobacillus paracasei were investigated in this study. Fermentation performance with Ca(OH)(2) was superior to NH4OH and NaOH because it had the highest oxygen transfer rate (OTR) and lowest environmental osmotic pressure. Much smaller bubbles were generated using a calcium lactate solution compared with those generated using ammonium lactate and sodium lactate solutions, indicating that Ca(OH)(2) had the highest OTR. Moreover, experiments demonstrated that ammonium lactate and sodium lactate caused more severe osmotic stress on cell growth than calcium lactate. In conclusion, the effects of neutralizing agents on L-lactic acid production efficiency could be ascribed to the contribution of lactates to OTR and environmental osmotic stress. (C) 2014 Published by Elsevier Ltd.