ORIGINAL PAPER Ethanol production by Saccharomyces cerevisiae and Kluyveromyces marxianus in the presence of orange-peel oil Mark R. Wilkins Æ Lilis Suryawati Æ Niels O. Maness Æ Donna Chrz Received: 7 August 2006 / Accepted: 14 December 2006 / Published online: 24 January 2007 Ó Springer Science+Business Media B.V. 2007 Summary Two ethanologenic yeasts, Saccharomyces cerevisiae and Kluyveromyces marxianus, were used to ferment sugar solutions modeling hydrolyzed Valencia orange peel waste at 37°C. Orange stripper oil pro- duced from orange peel was added in various amounts to determine its effect on ethanol production. The minimum peel oil concentration that inhibited ethanol production was determined after 24, 48 and 72 h and the two yeasts were compared to one another in terms of ethanol yield. Minimum inhibitory peel oil concen- trations for ethanol production were 0.05% at 24 h, 0.10% at 48 h, and 0.15% at 72 h for both yeasts. S. cerevisiae produced more ethanol than K. marxianus at each time point. Keywords Bioprocessing Á Citrus Á Ethanol Á Fermentation Á Limonene Á Yeast Introduction From 2000 to 2005, approximately 9.5 million metric tons of oranges and grapefruit/year were processed into juice and fruit sections in the US (Pollack and Perez 2005). Approximately 50–60% of the processed fruit becomes citrus peel waste, a waste product con- sisting of peels, seeds and membranes left over after juice extraction. Citrus fruit processors generally dry and pelletize this waste to produce a low value cattle feed called citrus pulp pellets. Some small processors cannot afford to invest capital in the equipment needed to produce citrus pulp pellets and must pay haulers to take citrus peel waste away from their facility and dispose of it in a landfill. Demand for fuels produced from renewable re- sources has increased in recent years due to increased prices for oil, concerns about greenhouse gas produc- tion, and increasing reliability on foreign sources of energy in the US and Europe. Development of energy from renewable resources can provide domestic energy supplies while reducing net greenhouse gas emissions and developing a more favorable energy balance than traditional petroleum fuel production (Farrell et al. 2006). Utilization of waste products like citrus peel waste to produce liquid transportation fuels, such as ethanol, would reduce dependence on petroleum while turning current liabilities into valuable assets for processors. Previous researchers have successfully hydrolyzed both orange and grapefruit peel waste using cellulase and pectinase enzymes to glucose, galactose, fructose, arabinose, xylose, rhamnose, and galacturonic acid (Grohmann and Baldwin 1992; Grohmann et al. 1994; Wilkins et al. 2005, 2006). Glucose, fructose and galactose from hydrolyzed citrus peel waste can be fermented to ethanol by Saccharomyces cerevisiae yeast (Grohmann et al. 1994). In order to ferment these sugars, orange peel oil concentration in the hydrolysate must be reduced prior to fermentation (Grohmann et al. 1994). Numerous studies have M. R. Wilkins (&) Á L. Suryawati Department of Biosystems and Agricultural Engineering, Oklahoma State University, 111 Agricultural Hall, Stillwater, OK 74078, USA e-mail: mark.wilkins@okstate.edu N. O. Maness Á D. Chrz Department of Horticulture and Landscape Architecture, Oklahoma State University, 360 Agricultural Hall, Stillwater, OK 74078, USA 123 World J Microbiol Biotechnol (2007) 23:1161–1168 DOI 10.1007/s11274-007-9346-2