Effects of Low Temperature Solvent Fractionation on the Thermal Oxidative Stability and Antioxidant Activity of Refined Hoki Oil and its Derived Fractions
Authors: Tengku‐Rozaina Tengku Mohamad , Birch Edward John 
Published: September 2018

Thermal oxidative stability of refined hoki oil (RHO) and its fractions obtained via low temperature solvent crystallization (LTSC) are carried out using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) by isothermal and programmed heating. In the LTSC method, the sample is dissolved with hexane at a ratio of 1:4 (w/v). Results show the induction time for thermal change occurrs earlier in the TGA than the DSC as the oil gains weight due to oxygen uptake prior to heat release during decomposition. The onset time for oxidation of the liquid fraction (RHOL) occurs earlier than its parent oil (RHO) and solid fraction (RHOS) during isothermal heating due to the high amount of polyunsaturated fatty acid (PUFA) content. Thermal decomposition of RHO and its fractions occur at three stages which indicates the progressive degradation of fatty acids, followed by volatilization of polymerization and pyrolysis products. Thermal decomposition pattern of RHOL is similar to RHO and RHOS, however, the starting temperature of thermal decomposition of RHOL is significantly lower (120.60 °C) than RHO (139.91 °C). The vitamins A and E in RHOL are higher than RHOS but the antioxidant activity (ABTS and DPPH) of these two oils is not significantly different since the increased amount of PUFA in RHOL made it more susceptible to oxidation.

Practical Applications: Fish oil has been used as a raw material for omega‐3 fatty acid concentrates production that can be incorporated into food and pharmaceutical products. LTSC method is used to concentrate the omega‐3 fatty acids in the fish oils. The liquid fraction obtained from the LTSC method contains more unsaturated fatty acids compared to the parent oil and its solid fraction. However, it is more prone to oxidation and can affect the acceptability of food products fortified with fish oil. Information on the oxidative stability of fish oil is useful to food manufacturers to optimize processing conditions and for prediction of shelf life of food products fortified with fish oil. DSC and TGA have been used as instrumental methods for measuring the oxidative stability of oils. These methods offer a simple and rapid measurement for oxidative stability of oils compared to traditional chemical and physical oxidative stability methods.

Oxidative stability of refined hoki oil (RHO) and its fractions obtained via low temperature solvent crystallization are measured using differential scanning calorimetry (top part of the figure) and themogravimetric analysis (bottom part of the figure). The oxidation onset time of the liquid and solid fractions of RHO are earlier than the parent oil. The liquid fraction of RHO is less stable to heat compared to RHO parent oil and its solid fraction due to its high PUFA content. In contrast, the solid fraction of RHO is less stable than RHO parent oil because it contains less vitamins A and E compared to RHO parent oil and its liquid fraction.