Abstract

Flaxseed (Linum usitatissimum L.) is rich in health-promoting bioactive compounds. Among plant foods, flaxseed has the highest content of lignans, mainly in the form of secoisolariciresinol diglucoside (SDG). Flaxseed oil also has a very high concentration of the essential omega-3 fatty acid alpha-linolenic acid (ALA). This thesis presents studies on both SDG and ALA. An HPLC method for quantification of SDG in hydrolysed flaxseed extracts was developed and used to compare the SDG content in 29 flaxseed cultivars (6.1-13.3 mg/g seed, dry weight). Full spectroscopic (NMR) data were obtained for two compounds, 4-O-β-D-glucopyranosyl-p-coumaric acid (CoAG) and 4-O-β-D-glucopyranosyl-ferulic acid, eluting before SDG in the HPLC system developed. NMR analyses provided the first evidence that SDG in flaxseed is bound in a linear oligomeric structure with an average composition of five SDG residues interlinked by four 3-hydroxy-3-methyl glutaric acid residues (average size 4 kDa). Chromatographic (UHPLC, GPC, SPE) and spectroscopic (MALDI-TOF) analyses of seven different flaxseed cultivars showed that the size of the SDG oligomers differed among cultivars depending on seed SDG/CoAG ratios. In attempts to decrease the use of fish-based feeds in aquaculture, vegetable oils are being assessed as replacements for fish oil. However, this has hitherto resulted in decreased levels of the long-chain omega-3 fatty acids (n-3 HUFA) that make fish a healthy food choice for humans. Previous studies have shown that sesamin, a lignan from sesame seed, is able to increase conversion of ALA to n-3 HUFA in rainbow trout (Oncorhynchus mykiss) but less efficiently than expected when flaxseed oil, given its high ALA content, was used. In order to investigate whether high levels of ALA or possibly minor compounds in flaxseed oil could be responsible for the low n-3 HUFA formation, rainbow trout were fed diets containing purified flaxseed triacylglycerols (PFSO) with and without sesamin. However, it was found that PFSO was unable to increase ALA conversion compared with unaltered flaxseed oil and that PSFO with or without sesamin addition had the opposite effect to unaltered flaxseed oil with or without sesamin addition, respectively. The reasons for these effects are discussed.