The effects Of D-limonene concentration, enzyme loading, and pH on ethanol production from simultaneous saccharification and fermentation (SSF) of citrus peel waste by Saccharomyces cerevisiae were studied at 37 degrees C. Prior to SSF, citrus peel waste underwent a steam explosion process to remove more than 90% of the initial D-limonene present in the peel waste. D-Limonene is known to inhibit yeast growth and experiments were performed where D-limonene was added back to peel to determine threshold inhibition amounts. Ethanol concentrations after 24 h were reduced in fermentations with initial D-limonene concentrations greater than or equal to 0.33% (v/v) and final (24 h) D-limonene concentrations greater than or equal to 0.14% (v/v). Ethanol production was reduced when enzyme loadings were (IU or FPU/g peel dry solids) less than 25, pectinase; 0.02, cellulase; and 13, beta-glucosidase. Ethanol production was greatest when the initial pH of the peel waste was adjusted to 6.0. (c) 2007 Elsevier Ltd. All rights reserved.