In this study, the possibility of tri-n-decylamine (TDA) was investigated as the extractant in extractive lactic acid fermentation with a recombinant yeast being used. Extractive fermentation with TDA did not provide high L-lactic acid production relative to fermentation without extraction. Through determination of trace components of the TDA, it was found that the TDA contained a high concentration of 1-decylaldehyde, which was confirmed to be toxic to yeast and to have an inhibitory effect on cell growth even at low concentration (40 ppm). When 1-decylaldehyde in TDA was reduced from 700 ppm to 33 ppm, the productivity and total concentration of lactic acid increased by 1.8 times and 2.5 times, respectively. (C) 2008 Elsevier Inc. All rights reserved.