Simultaneous saccharification and fermentation of citrus peel waste by Saccharomyces cerevisiae to produce ethanol

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 °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.