In this study, evening primrose oil (EPO), rich in γ-linoleic acid (GLA) in the sn-2 position was used as a substrate for the synthesis of structured triacylglycerols (sTAG) of the MLM-type (bearing a medium chain fatty acid in the sn-1- and -3-positions and a long chain fatty acid in the sn-2-position). The highest yield of sTAG by interesterification with tricapyrlin was 23 and 28% for Rhizopus delemar lipase (RDL) and Rhizomucor miehei lipase (RML), respectively. In an acidolysis using caprylic acid, methyl-tert-butyl ether (MTBE) was preferred over hexane and also the yield was higher than in interesterification and reached 32–38% for RDL and 25–28% for RML. Alternatively, a two-step method was used, in which first 2-MAG could be best obtained in MTBE at an initial water activity of aw = 0.11. In the ethanolysis of EPO catalyzed by RDL, 2-MAG was obtained, which contained about 78% of GLA. After isolation of the 2-MAG, the highest content of 79% sTAG of the MLM-type was obtained then by esterification of 2-MAG rich in GLA with caprylic acid. This reaction was catalyzed by RDL in n-hexane at a molar ratio of caprylic acid to 2-MAG of 3:1. All biocatalytic reactions were also performed with recycling of the lipase, but after several consecutive batches the activity of the lipase was decreased mostly due to mechanical instability of the carrier material.
Practical applications: The presented methods of medium engineering enable sTAGs to be synthesized using a natural substrate (EPO, Oenothera biennis L.). Especially the two-step method enables a high yield synthesis of sTAG containing γ-linoleic acid in the sn-2 position and the medium-chain caprylic acid in the sn-1 and sn-3 positions. This sTAG is rare in nature and the chemical synthesis is cumbersome. This sTAG could be applied in human nutrition as low-calorie fat with a beneficial effect on human health because of the high content of the polyenoic GLA.