ARS | Publication request: Simultaneous saccharification and fermentation of citrus peel waste by Saccharomyces cerevisiae to produce ethanol

Submitted to: Process Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 30, 2007
Publication Date: December 1, 2007
Citation: Wilkins, M.R., Widmer, W.W., Grohmann, K. 2007. Simultaneous saccharification and fermentation of citrus peel waste by Saccharomyces cerevisiae to produce ethanol. Process Biochemistry. 42(12):1614-1619.

Interpretive Summary: Prior to fermentation, citrus peel waste was pretreated with a process to remove peel oil which inhibits microbial fermentation of sugars in this material into produce ethanol. Experiments were then performed where limonene was added back to the peel in order to determine the threshold amounts of limonene which cause inhibition of fermentation using Saccharomyces cerevisiae. Enzyme loadings of hydrolysis enzymes, necessary to liquefy the citrus waste to facilitate handling, were also varied to determine their effect on ethanol yields. Ethanol yield was significantly reduced when limonene concentrations were greater than 0.33% (v/v). Ethanol production was also significantly reduced when enzyme loadings (IU or FPU/g peel dry solids) were below 24.9, pectinase; 0.022, cellulase; and 13, beta-glucosidase.

Technical Abstract: The effects of 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 combined with flash cooling and condensation of volatilized peel oil to remove more than 90% of the initial limonene present in the peel waste. Limonene is known to inhibit yeast growth and experiments were performed where limonene was added back to peel to determine threshold inhibition amounts. Ethanol concentrations after 24 h were reduced in fermentations with initial limonene concentrations greater than or equal to 0.33% (v/v) and final (24 h) 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 24.9, pectinase; 0.022, cellulase; and 13, beta-glucosidase. Ethanol production was greatest when the initial pH of the peel waste was adjusted to 6.0.