Batch fermentations were performed to investigate the effect of manganese addition, in the form of MnSO4.H2O, on the performance of Lactobacillus casei for producing L-lactic acid from whey permeate supplemented with yeast extract. There was a particular emphasis on evaluating how little yeast extract and MnSO4 . H2O is required while still obtaining high sugar conversion and lactic acid yield, as nutrient supplementation is a raw material cost and can lead to extra residual impurities remaining after fermentation. The addition of MnSO4 . H2O had a significant beneficial affect with the fermentation time being reduced from 120 to 24 h for permeate supplemented with 0.50%w/v yeast extract. Fermentations were performed with MnSO4 . H2O concentrations in the range of 0.001-0.03 g/l. From 0.005 to 0.03 g/l, the fermentation performance was very similar, however at the low concentration of 0.001 g/l, the fermentation was significantly slower. With MnSO4 . H2O addition. the yeast extract concentration was reduced to 0.30%w/v while still maintaining high sugar conversion and lactic acid yield, however the fermentation was slower at 37 h. At 0.1%w/v yeast extract supplementation. the fermentation performance was poor with only 67% sugar conversion after 150 h of fermentation. (C) 2001 Elsevier Science Ltd. All rights reserved.