Effect of Crystalline Structure on Oxidation of Fish Oil in Stearin:Fish Oil Mixtures

Abstract

Effects of incorporation of stearin fraction into fish oil on crystallisation behaviour and oxidative stability of fish oil were examined. The stearin fraction (S25) was obtained from step-wise dry fractionation of anhydrous milk fat at 25 °C. Analysis of fatty acid composition by gas chromatography showed that unsaturated fatty acids (89.3 %) were enriched in the fish oil whereas the S25 typically comprises of saturated fatty acids (79.4 %). A series of S25:fish oil mixtures (0:100, 25:75, 50:50, 75:25; 90:10 and 100:0) was prepared by mixing the S25 and fish oil at 50 °C under flushing of nitrogen gas. Crystallisation and thermal properties and crystal structures of the mixtures were investigated by differential scanning calorimetry and small- and wide-angle X-ray scattering, respectively. The crystallisation/melting temperatures and crystallisation/melting enthalpies of the mixtures increased with increasing ratios of S25 blended into fish oil. All mixtures showed similar crystal structures (double- and triple-chain length) and crystal polymorphs (α, β’ and β) at 4 °C. A decrease in S25 fraction in the mixtures caused a higher proportion of triple-chain length crystal packing and β polymorph. Measurement of peroxide values of the mixtures upon storage at 8 °C for 42 days showed that blending as low as 25 % S25 sufficiently delayed the lipid oxidation of fish oil by about two-fold and the rancidity threshold to 16 days instead of 9 days for fish oil only. This study demonstrates that mobility of oxygen and triacylglycerol (TAG) molecules in fish oil can be restricted by crystalline state of milk fat, in turn, limiting the propagation of oxidative reaction in fish oil.