RESUMO
The biosurfactants have extensive applications in food and petroleum microbiology. The aims of this research were isolation and characterization of thermo-tolerant biosurfactants from highly producing yeast strains. The Bushnell Hass medium was used for screening the biosurfactant-producing yeasts. Biosurfactant presence was evaluated using oil displacement assay and surface tension test. The best biosurfactant-producing strain was named Candida keroseneae GBME-IAUF-2 and its 5.8s-rDNA sequence was deposited in GenBank, NCBI, under the accession number MT012957.1. The thin layer chromatography and Fourier-transform infrared spectroscopy analysis confirmed that the extracted biosurfactant was sophorolipid with a significant surface activity. The purified sophorolipid decreased the surface tension of water from 72 to 29.1 mN/m. Its maximum emulsification index, E24%, was recorded as 60% and preserved 92.06-97.25% of its original activity at 110-120°C. It also preserved 89.11% and 84.73% of its original activity in pH of 9.3 and 10.5, respectively. It preserved 96.66-100% of its original activity in saline extreme conditions. This is the first report of sophorolipid production by the yeast C. keroseneae. According to the high thermal, pH and saline stability, the sophorolipid produced by C. keroseneae GBME-IAUF-2 could be highly recommended for applications in microbial enhanced oil recovery as well as food industries as an excellent emulsifying agent.