Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland; Department of Chemistry, University of Warmia and Mazury in Olsztyn, Olsztyn, Poland.
Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland.
Colloids Surf B Biointerfaces. 2023 Oct;230:113503. doi: 10.1016/j.colsurfb.2023.113503. Epub 2023 Aug 8.
The study discusses pitfalls in attempts to determine reliable surface tension values for the culture media and their extracts for two biosurfactant-producing yeast strains: Rhodotorula graminis and Rhodotorula babjevae. The values obtained from an Axisymmetric Drop Shape Analysis (ADSA) tensiometer showed systematically more and more shallow dynamic surface tension decays, suggesting a deterioration of their surface activity. The rate of this apparent surface activity loss was shown to depend on the sample history, with slower changes observed in vigorously shaken samples. On the other hand, the force-based Wilhelmy plate method provided apparently stable surface tension values of the order of 30 mN/m, in accordance with numerous previous literature reports on similar yeast biosurfactants. Both observations can be justified by the presence of an oil emulsified by biosurfactants produced by the yeast. We show that the odd (apparent) surface tension results are in fact the measurement artifacts resulting from slow demulsification and subsequent oil-spreading assisted by the yeast biosurfactants. The apparent surface tension reduction is thus indeed caused by the presence of biosurfactants, but its value does not represent their real adsorption in a thermodynamic sense. Consequently, the often reported in the literature very low surface tension values for the yeast culture media, of the order of 30 ± 5 mN/m, should be treated with caution, especially if the emulsion stabilized with the biosurfactant had not been fully destabilized prior to the measurement.
该研究讨论了尝试确定两种产生物表面活性剂酵母菌株（Rhodotorula graminis 和 Rhodotorula babjevae）的培养基及其提取物可靠表面张力值时的陷阱。从轴对称滴形状分析（ADSA）张力计获得的值显示出越来越浅的动态表面张力衰减，表明其表面活性恶化。这种明显表面活性损失的速率取决于样品的历史，剧烈摇动的样品中观察到的变化较慢。另一方面，基于力的Wilhelmy 板法提供了约 30 mN/m 的明显稳定的表面张力值，与类似酵母生物表面活性剂的许多先前文献报告一致。这两种观察结果都可以由油乳化来解释，这种油是由酵母产生的生物表面活性剂乳化的。我们表明，奇怪的（表观）表面张力结果实际上是由于缓慢的破乳以及随后由酵母生物表面活性剂辅助的油扩散而导致的测量伪影。因此，表面张力的表观降低确实是由生物表面活性剂的存在引起的，但从热力学意义上讲，其值并不代表它们的真实吸附。因此，文献中经常报道的酵母培养基的非常低的表面张力值，约为 30 ± 5 mN/m，应谨慎对待，特别是如果在测量之前未完全使与生物表面活性剂稳定的乳液失稳的话。
