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Optimization of medium composition and pH for lactic acid production byLactobacillus plantarum ATCC21028 were evaluated using statistical methods. Ethanol stillage, a liquid waste from refinery, was utilized as the sole carbon source. Complex interactions among concentrations of stillage (50-100%), yeast extract as a nitrogen source (0-5 g/L) and pH (4-9) were studied using central composite design (CCD) experiments. Using 50% of stillage in the presence of 2.5 g/L of yeast extract at pH 6.5 yield the highest lactic acid (94.68±0.006 mg/L), productivity (1.97 mg/L/h) and yield (0.128 mg/mg initial sugar). The optimum cultivation for the lactic acid production consisted of 50.003% of stillage and2.433 g/L of yeast extract at pH 6.537 (R²=0.999). The model was confirmed by culturing L. plantarumunder predicted conditions in a 5-L fermenter. The amount of lactic acid (94.24±0.018 mg/L) was not significantly different from the predicted value. [ABSTRACT FROM AUTHOR]