Stability of biosurfactant produced by native strain of Bacillus subtilis in variation of environmental conditions

Biosurfactants are amphipathic compounds produced by a wide variety of microorganisms and due to their unique properties are used by many industries. Hence، maintenance and stability of their functional properties against various environmental conditions is a grave concern. To achieve the goal of production and application of biosurfactant in petroleum industry، a native strain of Bacillus subtilis was isolated from hydrocarbon contaminated soil using microbial screening procedures. Biosurfactant production was carried out in shaker incubator during a nine-day period of fermentation at 30°C and 200 rpm. After separating microbial biomass، the influence of salt treatments (concentration of 1-10% NaCl)، pH variations (1-13) and also heat treatments (room temperature up to 120°C for 30 minutes) on function of crude biosurfactant existing in fermentation broth were investigated using tensiometery. According to this study، biosurfactant produced by native strain of B. subtilis was resistant to heat treatment and comparing to untreated control (23. 3 mN/m) did not show noticeable difference. There was a slight variation in surface tension in pH 5-9; however، it seemed that acidic pH had more severe impact on biosurfactant function and subsequently annihilated its function in pH below 5. Biosurfactant، which exists in culture filtrate، was relatively resistant to the salt concentration of 1-10% and maximum surface tension was at 10% salt concentration (28. 6 mN/m) which indicates only a minor difference comparing to that of control.