Isolation and characterization of a Saccharomycopsis lipolytica mutant showing increased production of citric acid from canola oil

A process for the production of citric acid from canola (rapeseed) oil using the yeast Saccharomycopsis lipolytica was examined. A citrate nonutilizing strain, designated NTG9, which had an improved citric to isocitric acid ratio, was isolated after mutagenesis of S. lipolytica ATCC 20228 with nitrosoguanidine. Although the mutant grew well on canola oil, unlike the parent strain or a spontaneous revertant (JF2), it did not grow on glycolytic intermediates below glycerate-3-phosphate, amino acids, hexadecane, or yellow kerosene. The mutant was shown to be impaired in the gluconeogenic pathway because of a loss of phosphoenolpyruvate carboxykinase activity. A preliminary study of the effect of micronutrients on citric acid production by S. lipolytica NTG9 showed that manganese had a stimulatory effect on the process whereas zinc and iron were inhibitory. A revised growth medium was tested and found to increase citric acid production while decreasing that of isocitric acid.