COMBINED NONENZYMATIC-ENZYMATIC METHOD FOR THE SYNTHESIS OF SIMPLE ALKYL FATTY-ACID ESTERS FROM SOAPSTOCK

Soapstock (SS) is a by-product of the extraction of oilseeds to produc e 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% phosphatidylethan olamine, 2.2% phosphatidylinositol, 2.7% phosphatidylcholine, 14.0% so lvent-insolubles and 10.5% other material, which was not characterized . A process has been developed that sequentially employs a nonenzymati c and an enzymatic step to convert the lipid-linked and the free fatty acids oi SS to the esters of monohydric alcohols. The first step of t he process employed alcohol and potassium hydroxide to transesterify t he glyceride- and phosphoglyceride-linked fatty acids of the substrate . Because water inhibited the reaction, it was necessary that the SS b e dried before use. Nonetheless, even with some batches of SS with wat er contents below 1% (weight basis), ester hydrolysis accompanied este rification. Each of five examined simple primary alcohols participated effectively in the transesterification reaction, which proceeded rapi dly at room temperature and was essentially complete within 1 h. The a verage ratio of transesterification to hydrolysis in four examined sma ll primary alcohols was 4:1. However, in methanol this value was 99:1 due to the virtual absence of hydrolysis. Significant transesterificat ion by a secondary alcohol (isopropanol) did not occur at room tempera ture. The minimum effective molar ratio of alcohol to lipid-linked fat ty acids was 20:1. The minimum effective concentration of KOH was betw een 0.10 and 0.15N. The efficiency of the transesterification reaction exceeded 90% of theoretical maximum. The second step of the process i nvolves lipase-mediated esterification of the free fatty acids in the preparation that are not esterified by the alkaline transesterificatio n. Of four lipase preparations examined (Novo Lipozyme IM 20 and SP435 , and Amano PS30 and CE), only SP-435 catalyzed significant esterifica tion of the free fatty acids. The reaction was not catalyzed by heat-d enatured enzyme. In the pH range between 6 and 13.5, the enzyme reacti on proceeded best at pH 6, although also well at pH 7. The optimal wat er concentration was 0.70% (vol/vol). At an enzyme dosage of 1.1% (wei ght basis, relative to the dry weight of SS present) under optimal con ditions and at 42 degrees C, 63% of the free fatty acids in a post-alc oholysis mixture were enzymatically esterified. The addition of molecu lar sieves did not increase esterification, which was probably retarde d by the high viscosities of the reactions. Under the optimal conditio ns identified here, the degree of conversion of the fatty acids in SS to simple alkyl esters by the combined reaction scheme was 81%. Opport unities exist for further optimization of these reactions.