Microbial synthesis of short-chain sophorolipid esters using yeasts of the Starmerella clade

Acetylated sophorolipid (SL) esters exhibit surface-active as well as bioactive properties. Their synthesis requires a three-step route comprising microbial production of the SL lactone followed by chemical transesterification and enzymatic re-acetylation. Here, a direct microbial synthesis of SL esters was investigated using three-component feeding of glucose, oleic acid and a wide variety of alcohols. Straight-chain primary alcohols with chain lengths of C3-C12 impaired SL production of Starmerella bombicola severely, whereas toxicity of short-chain primary, secondary and branched-chain alcohols was significantly lower, and total SL production of up to 60 g l^-1 was obtained in initial shake-flask cultures. Besides lactonic, acidic and bolaform SLs, acetylated SL esters were detected in small quantities in the fermentation broth. Upon continuous alcohol feeding the SL ester fraction could be increased to 10-40 g l^-1 of methyl, ethyl, isopropyl and isobutyl SL esters under optimized conditions. Following chromatographic purification, the ester structures were verified by LC-MS and NMR spectroscopy. The synthesis of the isopropyl SL ester was scaled up successfully to a 2 l bioreactor, yielding 100 g l^-1 total SL including 30 g l^-1 of the isopropyl ester. To elucidate the SL ester formation capabilities within the Starmerella clade, five additional species were screened. SL synthesis was shown for the first time with S. caucasica and S. davenportii. With all strains, including ω-hydroxylating open-chain SL producing S. kuoi and S. batistae, SL ester formation was detected. Thus, the ability for SL ester synthesis seems to be ubiquitous among SL producing Starmerella strains.