Tuesday, April 20, 2010
11-58

Citric acid production by Candida species and Yarrowia lipolytica grown on glycerol from soy biodiesel processing

Thomas P. West, Biology and Microbiology, South Dakota State University, Box 2104, Brookings, SD 57007

During soy biodiesel production, raw glycerol is the major coproduct formed during the processing of soybean oil. Although raw glycerol is a low value coproduct, it is suitable as a substrate for yeast fermentation of the specialty chemical citric acid. Citric acid has a variety of uses in the beverage, food and pharmaceutical industries. In this investigation, the ability of selected Candida species and Yarrowia lipolytica to convert soy-based raw glycerol to citric acid was analyzed. The yeast species screened were Candida tropicalis ATCC 20240, Candida rugosa ATCC 20306, Candida parapsilosis ATCC 7330, Candida catenulata ATCC 18821 and Yarrowia lipolytica ATCC 20177. A 6% (v/v) raw glycerol-containing medium (pH 5.2) was developed that sustained the growth of the yeast species. The species were grown in the medium at 30oC for 168 hours with aeration. After collecting the yeast cells by centrifugation, the supernatant was assayed for its citric acid concentration. The citric acid level was measured by following at 340 nanometers the reaction of a coupled enzyme assay that included the enzymes citrate lyase, malate dehydrogenase and lactate dehydrogenase. After 168 hours of growth, the highest citric acid concentration was produced by C. parapsilosis while C. rugosa synthesized the lowest citric acid level. The levels of citric acid produced by the other Candida species and Y. lipolytica were comparable. Overall, the five yeast species screened produced citric acid from 6% raw glycerol with C. parapsilosis being identified as being the most capable citric acid-producing species.

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