Monday, August 11, 2008
P52

Citric acid production by yeast species grown on glycerol from soy biodiesel processing

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

Selected citric acid-producing yeast species were screened for their ability to use raw glycerol from the processing of soy biodiesel for citric acid production. During the transesterification of soybean oil during biodiesel processing, glycerol is formed. A number of commercial applications exist for citric acid such as its utilization in foods, beverages and pharmaceuticals. The selected species screened were Candida tropicalis ATCC 20240, Candida parapsilosis ATCC 7330, Candida guilliermondii ATCC 9058 and Yarrowia lipolytica ATCC 20177. A medium (pH 5.2) containing the raw glycerol was devised that supported the growth of the yeast species.  The yeast strains were grown in the medium at 30oC for 168 hours with aeration following inoculation with yeast cells from cultures grown for 48 hours in the same medium. After centrifuging the medium to pellet the yeast cells, the supernatant was assayed for its citric acid content using a coupled enzyme assay involving the enzymes citrate lyase, malate dehydrogenase and lactate dehydrogenase. The assay follows the decrease in absorbance at 340 nm which is proportional to the citric acid concentration in the sample. After 168 hours of growth, the highest citric acid concentration was produced by C. tropicalis while lower levels of citric acid were synthesized by the other yeast strains. Overall, citric acid was found to be produced by all the yeast species tested from the raw glycerol but the Candida species appeared to be more capable of synthesizing citric acid.

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