Extraction of microalgal lipids and the influence of polar lipids on biodiesel production by lipase-catalyzed transesterification

Abstract

This work demonstrates the influence of polar lipids on the transformation of Nannochloropsis gaditana saponifiable lipids (SLs) to fatty acid methyl esters (FAMEs, biodiesel) by lipase-catalyzed transesterification. In order to obtain microalgal SL fractions containing different polar lipid (glycolipids and phospholipids) contents, lipids were extracted from wet microalgal biomass using seven extraction systems, and the polar lipid contents of some fractions were reduced by low temperature acetone crystallization. We observed that the polar lipid content in the extracted lipids depended on the polarity (log P value) of the first solvent used in the extraction system (ethanol, hexane, isopropanol or ethyl acetate). Lipid fractions with polar lipid contents between 75.1% and 15.3% were obtained. Some of these fractions were transformed into FAMEs by methanolysis, catalyzed by the lipases Novozym 435 and R. oryzae in tert-butanol medium. We observed that the reaction velocity was higher the lower the polar lipid content, and that the final FAME conversions achieved after using the same lipase batch to catalyze consecutive reactions decreased in relation to an increase in the polar lipid content. This indicates that these lipids affect transesterification velocity, FAME conversion and lipase stability - even in the presence of tert-butanol.