Structures and Properties of Sophorolipids in Dependence of Microbial Strain, Lipid Substrate and Post-Modification

Abstract

Abstract Starmerella bombicola and Candida kuoi are known to secrete structurally divergent sophorolipid type glycolipids (SLs) under nitrogen limitation. In the present work SLs were produced in titers of 3.9–78.6 g L−1 with the two yeast strains utilizing stearic, oleic and linoleic acid as substrates. HPLC-ELSD combined with HPLC-MS and NMR spectroscopy was used for qualitative and quantitative analysis of the SL mixtures. While S. bombicola almost exclusively produced lactonic diacetylated SLs with a preference for subterminal fatty acid hydroxylation, C. kuoi synthesized diacetylated, terminally hydroxylated open chain SLs with up to 25% of dimeric and trimeric products. Surface tension measurements showed a higher surface and interface activity of the lactonic products from S. bombicola in comparison to open chain C. kuoi based SLs. The lowest CMC of 5.4 mg L−1 and minimum surface tension at the CMC of 35.9 mN m−1 were obtained for the stearic acid based lactones. Similar tendencies were observed in interfacial tension analysis with 3.6 mN m−1 for oleic acid based lactonic SLs at the interface water/paraffin oil in comparison to 9.4 mN m−1 for the corresponding open-chain SL. The acidic C. kuoi SL mixtures directly exhibited foaming properties whereas the S. bombicola SLs needed alkaline deacetylation and ring opening to display foaming comparable to that of the structurally related alkyl polyglycosides.