2.6. Surface tension and biosurfactant stability determination
The surface tension (ST) was performed to assess the surface
activities of the biosurfactant produced by the strain USTBa. A Du-
Nouy Tensiometer (VWR International, Radnor, PA, USA) was used to determine the surface tension of the cell-free supernatant. The
thermostability was evaluated by incubating the culture broth
supernatant at different temperatures (15–100 C) for 15 min and
ST measurement. Similarly, the effect of pH was determined by
varying the pH (2.0–12.0) of the supernatant. The effect of salinity
(2.0–12.0% NaCl w/v) on ST was determined by adding various concentrations
of NaCl to the supernatant and determining the ST after
30 min at room temperature. The results were further confirmed
by the oil displacement test.