Starmerella bombicola is known to produce sub-terminally hydroxylated lactonic sophorolipids (SLs), while Candida kuoi synthesizes acidic open chain SLs with terminally hydroxylated fatty acids. Upon feeding glucose and fatty alcohols both strains form long-chain nonionic SLs. According to structure elucidation the SLs consist of a hydroxylated fatty acid esterified with fatty alcohol and linked via a glycoside bond to the diacetylated sophorose unit. Palmityl, stearyl, and oleyl alcohols lead to products with lipid chain lengths of C32 or C36. Oleyl alcohol is the preferred substrate leading to 45 g L −1 of the double unsaturated C36 SL with S. bombicola and 20 g L −1 with C. kuoi. Scale up from shake flask to 1.5 L fermentations is possible and 65 g L −1 long-chain SLs are obtained with S. bombicola within 7 days. Mixed feeding of oleic acid and a variety of fatty alcohols leads to new long-chain SLs. In the presence of oleic acid the yeasts do not oxidize the fatty alcohol and thus the production of biosurfactants with tailored chain length is possible. The long-chain SLs show good emulsification ability of water/paraffin oil mixtures at low energy input and reduced interfacial tension significantly. Practical Applications: Sophorolipids are produced by fermentation on industrial scale focusing on cleaning and detergent applications. Mainly lactonic or anionic open-chain forms are used today. The new long-chain SLs presented in this manuscript are accessible with existing production technology and can be produced with high titers from cost-efficient renewable raw materials. In contrast to the commercial products the long-chain SLs are more hydrophobic and exhibit a strong emulsification behavior. Therefore they have the potential to broaden the application range of SLs in future. They may be useful as novel emulsifiers for cosmetic creams and lotions, pharmaceutical ointments and food products or may find application in oil spill remediation.
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.
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