Purification of lactonic sophorolipids by crystallization.
Various experimental methods for purifying lactonic sophorolipids (SLs) via crystallization were studied. The commonly used solvent, ethanol, was found to have much higher solubility of the lactonic SLs than the acid SLs. Consequently, for purification of lactonic SLs, ethanol not only lacked the selectivity in removing acidic SLs but also resulted in significant loss of desired products. Aqueous buffers were subsequently studied as the solvent, based on the rationale that acidic SLs, having the free carboxylic acid group(s), are more hydrophilic than the lactonic SLs, especially at higher pH. Both phthalate and phosphate buffers were found more suitable for purifying lactonic SLs than ethanol. A practical and effective method for purifying lactonic SLs to about 99% purity using phosphate buffers was developed. The FTIR spectra also showed significantly less SL components with free acid groups in the purified SLs than in the crude SLs, confirming the removal of acidic SLs in the purification.