Stabilization of cod liver oil with a quaternary combination of α-tocopherol and synergists: Method of assessment

This study aimed to identify the concentrations of a quaternary combination of α-tocopherol (α-TOH, 1200-2000ppm), ascorbyl palmitate (AP, 100-900ppm), phosphatidylcholine (PC, 1000-9000ppm), and L-lysine (1000-9000ppm) optimum for the protection of cod liver oil against oxidation at 30°C. The control oil contained 1100ppm α-TOH. The evaluation of antioxidant potency considered the rates of formation of conjugated dienes (CD) and thiobarbituric acid reactive substances (TBARS) and rate of loss of α-TOH between Day 1 and Day 4 of oxidation. The trials were designed and analyzed by RSM. Mathematical models of second order polynomial with good confidence (p≤0.05) fitted the formation of TBARS and loss of α-TOH but not the length of the induction period (IP) and the rate of formation of CD. The optimal levels of the four additives (in ppm) were found to be 1200 for α-TOH, 100 for AP, 9000 for PC, and 1000 for L-lysine. Higher concentrations of α-TOH and AP either did not give better protections or resulted in a loss of antioxidant efficacy compared to when these additives were added at lower levels. PC was effective at the wide range of high concentrations while L-lysine was most protective at levels up to 5000ppm. The rate of formation of TBARS was found to provide a very good model for evaluating the stabilizing effects of additives (R²=0.8187, p-value=0.012) while the IP and the rate of formation of CD provided unsatisfactory models (p values 0.557 and 0.112, respectively). Practical applications: This paper considers the different ways to use oxidation kinetic data to evaluate the effectiveness of synergistic combinations of antioxidants and synergists in stabilizing fish oils. It finds that the commonly used induction period provides a poor model for assessment while the rate of formation of TBARS during the induction period provides a good model, which is an important methodological consideration.