I have done a test of emulsion activity (and surface tension) to characterize the biosurfactant produced by bacteria. The emulsion activity reduced after 24 hours, but no more than 10%. How can I say whether the biosurfactant was stable or not? Is there any standard of how many (%) reduction of emulsion activity to say that it is stable or not?
Emulsification test is a criteria to see the emulsification property of the biosurfactant. If you want to check the stability, then treat the surfactant at different pH, temperature, salt etc before testing its emulsion stability. At the end you will get different values and compare them to find the optimum levels for the surfatcnt. From the optimum values you can decide how stable the surfactant is.
Emulsion stability will help you to understand how good the surfactant is in maintaining emulsion. Example. The orange cold drinks has orange oil in it and emulsified with tween and the emulsion remains stable for months.
Also when you do the experiment other than the variable you want to test, all other factors should be maintained constantly. E.g., if you want to see the stability of the surfactant at different pH make sure the temperature of the system stays stable.
In my opinion you need to use DLS method (dynamic light scattering)
more stable = larger structures (micelle) for a long time
Would say it is not possible to get a clear picture from these tests unless you have a method to extract and purify the biosurfactant. Then it would be possible to quantify its concentration during storage/aging.
The problem with the simple tests mentioned is that results will be strongly impacted by other compounds in the cell broth. Therefore the need for isolation.
Emulsification test is a criteria to see the emulsification property of the biosurfactant. If you want to check the stability, then treat the surfactant at different pH, temperature, salt etc before testing its emulsion stability. At the end you will get different values and compare them to find the optimum levels for the surfatcnt. From the optimum values you can decide how stable the surfactant is.
Emulsion stability will help you to understand how good the surfactant is in maintaining emulsion. Example. The orange cold drinks has orange oil in it and emulsified with tween and the emulsion remains stable for months.
Also when you do the experiment other than the variable you want to test, all other factors should be maintained constantly. E.g., if you want to see the stability of the surfactant at different pH make sure the temperature of the system stays stable.
I once assayed surfactant (perhaps SDS perhaps Triton X-100) by measuring drop size from a burette. The drop volume of pure water was 30 uL (measured gravimetrically or volumetrically) but sank to half that with small amounts of surfactant. Of course you measure quite a few drops, and average. A very simply and sensitive assay that focuses explicitly on the surface tension; but I take the point (above) about other variables in the mixture (temperature, pH, viscosity, etc).
to repeat, whether you measure emulsification efficiency or only simply surface tension the results are effected by a multicomponent system. If test results do not change during storage you may state that likely your system is stable. A change in what ever direction maybe associated with production or degradation of biosurfactant or associated with another change in composition.
I think the right way to determine the stability of biosurfactant is measurement of BS concentration. But emulsification activity (EA) and surface tension (ST) measurements are realy used for biosurfactant stability studies. You can draw a graph with the storage time of samples on Y-axis and EA on Y-axis. The same with ST. In theory there must be correspondence between EA and ST decreasing. It’ll be possible to create approximation (linear or logarithmic or hyperbolic etc.) and to count rate of decreasing. I don’t remember storage stability researches, but you can check this papers with thermal, pH and salinity stability studies and maybe compare with your data: Pornsunthorntawee et al. Bioresour Technol. 2008;99(6):1589-95; Niea et al. Biotechnology Advances. 2010;28: 635–643.
Emulsion activity using E24 test is not a reliable test. It is just for seeing if the produced biosurfactant has activity qualitatively. I think you have to find another analysis like what Marcin Staśko mentioned above
For crude sample, the surface activity of the samples stored for different time periods may indicate relative stability. In case of pure or almost pure sample, it may be possible to compare with HPLC standard.
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