In order to improve the purity of lactic acid isomers, the effects of pH, temperature, fermentation time and their interactions on L(+) or D(-)-lactic acid production were evaluated during lactic acid fermentation of the non-sterile kitchen wastes. The results showed that L(+)-lactic acid was the main isomeric form. The isomer purity was much higher at acidic or alkalic pH (non-controlled pH, pH 5 and pH 8) than neutral pH (pH 6 and pH 7). Increasing the fermentation temperature from 35 degrees C to 45 degrees C at pH 7 enhanced the isomer purity from 60:40 to 83:17. The optimal fermentation time for the purity of lactic acid isomers was found to depend on the corresponding pH and temperature. From the response surface analysis, the optimized combination of pH and temperature could obviously increase the L(+)-isomer concentration. It is confirmed that the variation of the isomer purity with pH, temperature and fermentation time change resulted from the substitution of microbial community composition. The lactic acid bacteria and Clostridium sp. dominated the fermentation of non-sterile kitchen wastes, and the emergence and disappearance of lactic acid bacteria which produced L(+)-isomer and Clostridium sp. resulted in the variations of the isomer purity. (c) 2007 Elsevier Ltd. All rights reserved.