Free fatty acids in sunflower oil soap stock progressed the sophorolipid production by Candida catenulata

Sophorolipids are a group of surface-active glycolipid compounds that can be synthesized by several yeasts, especially the Candida genera. Soap stock is a by-product of vegetable oils refinery that enriches mono-unsaturated fatty acids, and its application is interesting as a low-cost industrial residue in sophorolipid production. In this study, the sophorolipid production was evaluated by Candida catenulate KP324968, using sunflower oil soap stock and glucose as the hydrophobic and hydrophilic carbon sources, respectively. The results indicated the ratio of hydrophobic to a hydrophilic carbon source, initial pH, and type of nitrogen source had significant roles in the performance of the cells. The highest specific biomass growth rate was about 0.0447 +/- 0.0013 h(-1) and a sophorolipid production rate of 49.96 +/- 0.33 mg L-1 h(-1) was obtained at a soap stock-to-glucose ratio of 1, initial pH of 5.5, and temperature of 30 degrees C. The addition of organic nitrogen sources such as yeast extract and urea obtained a higher sophorolipid production in comparison with ammonium nitrate and potassium nitrate as the mineral nitrogen sources. Usage of both yeast extract and urea increased the sophorolipid production rate by 4.4% and 6.9%, in comparison with separate usage of yeast extract and urea, respectively. The characterization of the sophorolipid confirmed an emulsification index of about 30 +/- 2.9% for diesel-in-water and 50 +/- 3.5% for sunflower oil-in-water systems. The critical micelle concentration was determined at 280 mg L-1. The produced sophorolipid at a concentration of 500 mg L-1 showed an antibacterial capacity about of 95.4 +/- 1.1%, and 44.7 +/- 0.8% against Bacillus subtilis (Gram-positive) and Escherichia coli (Gram-negative) bacteria, respectively.