Practical methods to reduce impurities for gram-scale amounts of acidic sophorolipid biosurfactants

Raw sophorolipids, a variety of bioderived glycolipid biosurfactants produced by the yeast Candida bombicola, generally present themselves as a mixture of acetylated acidic and lactonic forms mixed with residual fatty acids. To recover the acidic form alone in large amounts, e.g., tens of grams, which is interesting for various applications in cleaning technologies, material science, medicine, one must either purify the mixture through column chromatography, thus tremendously reducing the final yield, or remove the fatty acids via hot hexane extraction followed by alkaline hydrolysis and a liquid-liquid extraction process; in the latter, pentanol, used as extraction medium, is a high boiling point solvent and, consequently, difficult to remove under standard vacuum extraction conditions. Here, we compare, quantify, and draw the positive and negative aspects of the typical hexane/pentanol extraction route compared to two additional ones, never reported for this system: a cold extraction from a pentanol/alkane mixture and a consecutive filtration on inverse C18-modified silica phase and silica gel in, respectively, water/acetonitrile and methanol/dichloromethane mixtures. The first one generates highly pure products but quantitative extraction is quite long (up to several days) while the second is an extremely handy and rapid (few hours) way but less quantitative as it provides a product with not less than 6% impurities.