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The effect of glycerol as a sole and secondary substrate on the growth and fatty acid composition of Rhodotorula glutinis
Emily R. Easterling, W. Todd French *, Rafael Hernandez, Margarita Licha
Dave C. Swalm School of ChemicalEngineering, Box 9595, Mississippi State University, MS 39762, USA
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Rhodotorula glutinis is a yeast that produces copious quantities of lipids in the form of triacylglycerols (TAG) and can be used to make biodiesel via a transesteriﬁcation process. The ester bonds in the TAG are broken leaving behind two products: fatty acid methyl esters and glycerolthat could provide an inexpensive carbon source to grow oleaginous yeast R. glutinis. Described here are the effects of different growth substrates on TAG accumulation and fatty acids produced by R. glutinis. Yeast cultured 24 h on medium containing dextrose, xylose, glycerol, dextrose and xylose, xylose and glycerol, or dextrose and glycerol accumulated 16, 12, 25, 10, 21, and 34% TAG on a dryweight basis, respectively. Lipids were extracted from R. glutinis culture and transesteriﬁed to form fatty acid methyl esters. The results show a difference in the degree of saturation for the carbon sources tested. Cells cultivated on glycerol alone had the highest degree of unsaturated fatty acids at 53% while xylose had the lowest at 25%. R. glutinis can be cultivated on all sugars tested assingle carbon substrates or in mixtures. Glycerol may be used as secondary or primary carbon substrate. Ó 2008 Elsevier Ltd. All rights reserved.
Article history: Received 26 November 2006 Received in revised form 8 May 2008 Accepted 8 May 2008 Available online 9 July 2008 Keywords: Glycerol Lipid Oleaginous Yeast Biodiesel
1. Introduction Biodiesel is one of the alternative fuels currentlybeing produced in the United States and elsewhere around the World (Van Gerpen, 2004; Sheehan et al., 1998). A chemical process called transesteriﬁcation is used to make biodiesel, this is a process in which the glycerol is separated from the triacylglycerides in fats or vegetable oils. (http://www.biodiesel.org/resources/biodiesel_basics/default.shtm). Glycerol is 10% of the product output(Fortenbery, 2005), or 1 lb of glycerol for each gallon of biodiesel fuel. The future supplies and usage of glycerol are expected to increase as biodiesel plants increase production, and the output will greatly outpace demand. Biodiesel production has already had a signiﬁcant impact on the price of reﬁned glycerol (http:// www.virent.come/whitepapers/Biodiesel%20Whitepaper.pd). A major concern of glycerolproducers is the reduced price of glycerol resulting from the increased production of biodiesel. Some alternative uses for this glycerol that have been investigated are substrates for fermentation process or the production of biosurfactants (Ashby et al., 2006; Solaiman et al., 2006). Another alternative use for glycerol is as a growth substrate for the cultivation of oleaginous yeasts. Oleaginousyeasts are single-celled fungi deﬁned as having at least 20% of their dry weight made up of lipids (Ratledge, 1977). Not only do these yeasts contain membrane lipids, but they accumulate lipid in
* Corresponding author. Tel.: +1 662 325 4308; fax: +1 662 325 2482. E-mail address: French@che.msstate.edu (W.T. French). 0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2008.05.030
the form of triacylglycerol (TAG) (Gill et al., 1977; Davoli et al., 2004). Rhodotorula glutinis is an oleaginous yeast which is able to activate non-esteriﬁed fatty acids for the synthesis of triacylglycerol (Gangar et al., 2001). In R. glutinis, fatty acids are activated in an ATP dependent manner prior to being used. Gangar et al. (2002) have demonstrated that...