BACKGROUNDProducts with high concentrations of polyunsaturated fatty acids (PUFAs) are now in high demand for health care and medicine. In this work, a novel lipase-catalyzed ethanolysis of low-grade fish oil was developed with combined use of two commercial free and immobilized lipases (triacylglycerol acylhydrolase, EC 3.1.1.3), namely Novozyme NS81006 and 435, to flexibly produce fatty acid ethyl esters (FAEEs) and concentrated PUFAs. RESULTSIn a first step, most of the fatty acid glycerides (FAGCs) were converted to ethyl esters with conversion rate of 70-80% but only a minor part of docosahexaenoic fatty acids (DHAs) was converted (<20%). Using molecular distillation to evaporate FAEEs, the content of PUFAs in the resulting heavy phase, primarily as glycerides and secondarily as ethyl esters, increased from approximate to 18% of crude fish oil to 34%. A second ethanolysis step under the catalysis of Novozyme 435 was used to further convert the residual FAGCs, especially DHA glyceride, to ethyl esters with final conversion rates of 80-100%. PUFAs content was further enhanced to 41% in the form of PUFA ethyl esters after a second-stage moleulcar distillation. CONCLUSIONSThis two-step process combined the merits of free and immobilized lipases for ethanolysis of fish oil, achieving high conversion of PUFAs to corresponding ethyl esters with significant improvement of catalyst reusability in the second step. This process also provides flexible production of enriched PUFAs in the forms of glycerides or ethyl esters from low-grade fish oil feedstocks. (c) 2018 Society of Chemical Industry