Combined nonenzymatic-enzymatic method for the synthesis of simple alkyl fatty acid esters from soapstock

Abstract

Soapstock (SS) is a by-product of the extraction of oilseeds to produce edible oils. Annual U.S. production exceeds one-half million tons. A representative sample of SS consists of 45.1% water, 10.0% free fatty acids, 10.1% triglycerides, 1.8% diglycerides, 3.6% phosphatidylethanolamine, 2.2% phosphatidylinositol, 2.7% phosphatidylcholine, 14.0% solvent-insolubles and 10.5% other material, which was not characterized. A process has been developed that sequentially employs a nonenzymatic and an enzymatic step to convert the lipid-linked and the free fatty acids of SS to the esters of monohydric alcohols. The first step of the process employed alcohol and potassium hydroxide to transesterify the glyceride-and phosphoglyceride-linked fatty acids of the substrate. Because water inhibited the reaction, it was necessary that the SS be dried before use. Nonetheless, even with some batches of SS with water contents below 1% (weight basis), ester hydrolysis accompanied esterification. Each of five examined simple primary alcohols participated effectively in the transesterification reaction, which proceeded rapidly at room temperature and was essentially complete within 1 h. The average ratio of transesterification to hydrolysis in four examined small primary alcohols was 4:1. However, in methanol this value was 99:1 due to the virtual absence of hydrolysis. Significant transesterification by a secondary alcohol (isopropanol) did not occur at room temperature. The minimum effective molar ratio of alcohol to lipid-linked fatty acids was 20:1. The minimum effective concentration of KOH was between 0.10 and 0.15N. The efficiency of the transesterification reaction exceeded 90% of theoretical maximum. The second step of the process involves lipase-mediated esterification of the free fatty acids in the preparation that are not esterified by the alkaline transesterfication. Of four lipase preparations examined (Novo Lipozyme IM 20 and SP435, and Amano PS30 and CE), only SP-435 catalyzed significant esterification of the free fatty acids. The reaction was not catalyzed by heat-denatured enzyme. In the pH range between 6 and 13.5, the enzyme reaction proceeded best at pH 6, although also well at pH 7. The optimal water concentration was 0.70% (vol/vol). At an enzyme dosage of 1.1% (weight basis, relative to the dry weight of SS present) under optimal conditions and at 42°C, 63% of the free fatty acids in a post-alcoholysis mixture were enzymatically esterified. The addition of molecular sieves did not increase esterification, which was probably retarded by the high viscosities of the reactions. Under the optimal conditions identified here, the degree of conversion of the fatty acids in SS to simple alkyl esters by the combined reaction scheme was 81%. Opportunities exist for further optimization of these reactions.