Synthesis of structured lipids by two enzymatic steps: Ethanolysis of fish oils and esterification of 2-monoacylglycerols

This paper studies the synthesis of structured triacylglycerols (STAGs), rich in polyunsaturated fatty acids (PUFAs) by a two-step enzymatic process: (i) alcoholysis of fish oils (cod liver and tuna oils) with ethanol to obtain 2-monoacylglycerols (2-MAGs), catalyzed by 1,3 specific lipases and (ii) esterification of these 2-MAGs with caprylic acid (CA, 8:0), also catalyzed by a 1,3 specific lipase, to produce STAGs of structure CA–PUFA–CA. As regards the alcoholysis reaction, three factors have been studied: the influence of the type of lipase used (lipase D from Rhizopus oryzae, immobilized on Accurel MP1000, and Novozym 435 from Candida antarctica), the operational mode of a stirred tank reactor (STR operating in discontinuous and continuous mode) and the intensity of treatment (IOT = lipase amount × reaction time/oil amount). Although higher 2-MAG yields were obtained with lipase D, Novozym 435 was selected due to its greater stability in the operational conditions. The highest 2-MAG yield (63%) was attained in the STR operating in discontinuous mode at an IOT of 1 g lipase × h g oil^?1 (at higher IOT the 2-MAGs were degraded to glycerol). This system was scaled up to 100 times the initial volume, achieving a similar yield (65%) at the same IOT. The 2-MAGs in the final alcoholysis reaction mixture were separated from ethyl esters by solvent extraction using solvents of low toxicity (ethanol and hexane); the 2-MAG recovery yield was over 90% and the purity was approximately 87–90%. Regarding the esterification of the 2-MAGs, the following factors were studied: the influence of the lipase type used, the presence or absence of solvent (hexane) and the reaction time or intensity of treatment (IOT = lipase amount × reaction time/2-MAG amount). Of the five lipases tested, the highest STAG percentages (over 90%) were attained with lipases D and DF, immobilized on Accurel MP1000. These STAGs contain 64% CA, of which 98% is at positions 1 and 3. Position 2 contains 5% CA and 45% PUFAs, which means that all the PUFAs that were located at position 2 in the original oil remain in that position in the final STAGs. The lipase D immobilized on Accurel MP1000 is stable in the operational conditions used in the esterification reaction. Finally the purification of STAGs was carried out by neutralization of free fatty acids with hydroethanolic solution of KOH and extraction of STAGs with hexane. By this method purity was over 95% and separation yields were about 80%.