11-24: Ability of Candida species to produce citric acid from 25% soy biodiesel-based glycerol

The major coproduct resulting from the processing of soybean oil to soy biodiesel is raw glycerol. Although considered a low value coproduct, raw glycerol can serve as a substrate for yeast in the fermentation of the specialty chemical citric acid. Citric acid has a number of applications in the beverage, food and pharmaceutical industries. In this study, the ability of selected Candida species to produce citric acid from soy-based raw glycerol was investigated. The yeast species screened were Candida parapsilosis ATCC 7330, Candida guilliermondii ATCC 9058, Candida catenulata ATCC 18821, Candida tropicalis ATCC 20240 and Candida rugosa ATCC 20306. A 25% (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 30^oC for 168 hours with aeration. After using centrifugation to pellet the yeast cells, the supernatant was assayed for its citric acid concentration. The citric acid level was determined 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. tropicalis while C. rugosa synthesized the lowest citric acid level. The levels of citric acid produced by the other Candida species were similar. In conclusion, all of the yeast species screened produced citric acid from 25% raw glycerol with the most capable citric acid-producing species being C. tropicalis ATCC 20240.