Abstract
New Zealand mutton bird (Puffinus griseus) is the most abundant marine bird in the Southern Hemisphere. The bird can be used for food, there is also sufficient quantities of oil that is present in the proventriculus, which is produced from the prey that the parent birds feed to their juveniles. Diet of mutton bird includes small fishes and marine zooplankton (mainly krill) that are omega-3 (n-3) rich. The lipid composition, fatty acid positional distribution, colour and thermal degradation properties of New Zealand mutton bird stomach oil (MBSO) samples that were obtained in 2015 at various collection dates (designated as SO15) and in 2017 (designated as SO17) harvesting season were investigated in this study.
Investigation of colour differences, using a UV/Visible spectrophotometer, amongst the oils indicated that the reddish colour of the oil is likely to be due to the presence of astaxanthin. The SO15 samples had a significant (p<0.05) red colour compare to sample SO17, whereas the L* and b* values of SO17 were significantly higher (p<0.05) than SO15. Wax esters (WE, 47%-67%) and triacylglycerol (TAG, 19%-42%) were major lipid classes in MBSO, as determined by capillary chromatography with flame ionisation detector (Iatroscan). Thin layer chromatography (TLC) was employed to separate lipid classes and gas chromatography (GC) was utilized to determine fatty acid profile of MBSO. Results indicated that monounsaturated fatty acids (MUFA) were the most dominant in the fatty acid profile of MBSO, with oleic acid (C18:1) consisted more than one fourth of the total fatty acid (%). Palmitic acid (C16:0, 9%-13%) was the most abundant saturated fatty acid (SFA), while major n-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) contributed around 14%-16% and 13%-15% of the total fatty acid (%), respectively. Higher MUFA content (49- 50%) was observed in the WE fraction compared to TAG (34-41%) whereas three times more SFA (25-26%) were esterified to the TAG backbones than those in WE. PUFA content in SO17 sample was significantly (p<0.05) different from the SO15 samples with only 22% in TAG and 46% in WE fractions compared to 40% in TAG and 39% in WE of SO15 samples.
Positional distribution of major long-chain polyunsaturated fatty acids (LC-PUFA) in MBSO were investigated by carbon nuclear magnetic resonance (13C-NMR). Stearidonic acid (SDA), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) were all distributed more at sn-1,3 position in SO17 sample compared to SO15 samples. Signal responding to carbonyl and methylene regions showed that DHA was predominantly located at sn-2, whereas SDA was evenly distributed and EPA was slightly dominated at the b-position but more evenly distributed compare to DHA.
Thermal properties of MBSO were evaluated using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) under air atmosphere. Melting and crystallization temperatures were presented as ranges due to the effect of polymorphism of TAG crystals and effect of ii melting/crystallization points of individual fatty acids attached to TAG and WE. Time for the maximum reaction (tmax) occurred earlier in the DSC curve than the TGA, indicating that the sample was being rapidly oxidised by thermal decomposition prior to weight gain due to peroxide formation. MBSO samples were rapidly oxidized thus their onset time for oxidation were difficult to detect. Melting, crystallization and decomposition profiles show distinctive thermal stages, suggesting a progressive influence on thermal degradation of polyunsaturated fatty acids (PUFA), MUFA, and SFA. Differences in thermal stability of MBSO from the two harvesting seasons were mainly due to variation in their fatty acid composition and positional distribution of the PUFA content, and these differences are link to a combination of factors including environmental water temperature, harvest location and feed availability to the birds.