About the origin of asclepic acid derived from crude homo- and heterolipids during successive solvent extraction of rapeseeds

Abstract

To improve oil yield at the final stage of process, relation among solvent extraction mechanism and the removal of homo- and heterolipid classes with different acyl chain domains was studied. Selected technological parameter was decreasing quality of crude oil in the last successively extracted fractions. The composition of fatty acids, originally linked as lipid building-blocks, indicated no differences up to 94 wt% of hexane extractable oil from the rapeseed material. During the same time interval, it was released 95.8 cumulative wt% of oleic acid and only 76.9 cumulative wt% of (Z)-11-octadecenoic (asclepic, cis-vaccenic) acid. Steep decrease of oleic acid content was correlated with the down-trend of total monounsaturated fatty acid concentrations in the simple glycerolipid fractions (from 57.0 –59.0 to 26.7 wt%) and free fatty acid fractions (from 59.0–60.0 to 40.1 wt%). On the other hand, we have observed 7.14-fold increase of asclepic acid concentration derived from the simple glycerolipids (mono-, di- and triacylglycerols), wax and sterol esters. The increment of asclepic acid was 4.78-fold in the fraction of the complex glycerolipids and acylate steryl glycosides. Lipidomic and proteomic analyses of isolated oil bodies, rapeseed hull and dehulled seeds proved an origin of (Z)-11-octadecenoic acid. Furthermore, we determined similar fatty acid composition of lipid fractions derived from pure oil bodies and from dehulled seeds. In both cases, the low content of total asclepic acid: 3.13–3.15 wt% in aleurone cells was observed. Low amount of (Z)-11-octadecenoic acid is novel finding, which indicated that (Z)-11-octadecenoic acid originated from seed coat species that are less accessible to the hexane. It was confirmed that the main portion of asclepic acid moieties were derived from wax and sterol esters, triacylglycerols and heterolipids originally present in rapeseed coat (epidermal layer, testa and aleurone layer). Therefore, increasing amounts of asclepic acid and its precursor, palmitoleic acid, released from seed coats was significant indicator of solvent extraction process efficiency.