The physical and chemical properties of Trachurus trachurus (horse mackerel) as well as a food product in the presence and absence of natural antioxidants, were investigated. The rate of lipid oxidation in the freeze-dried horse mackerel product stored at room temperature (22 degrees C) was investigated, using peroxide value (PV), thiobarbituric acid reactive substances (TBARS) and hexanal concentration to determine primary, secondary and tertiary oxidation products. Natural antioxidants were used to slow down oxidation processes and to improve nutritional and eating quality. These included vitamin E + vitamin C + citric acid (250, 250, 100 mg kg(-1), respectively), vitamin E + vitamin C + citric acid + rosemary (250, 250, 100, 250 mg kg(-1), respectively) and rosemary (250 mg kg(-1)). Biochemical analysis such as PV and TBARS measurements showed that the combination of vitamin E + vitamin C + citric acid (250, 250, 100 mg kg(-1)) were found to be the most significant (P <= 0.01) in controlling the rate of lipid oxidation in freeze-dried food followed by rosemary (250 mg kg(-1)). There was an initial rise in the PV of all samples, which dropped after 8 weeks, because of formation of secondary oxidation products. The rise in PV was much higher in the control samples with no antioxidants added, compared with the antioxidant-treated samples. Combinations of E + C + citric acid treatment group followed by rosemary alone were the most significant in lowering PV, hence, reducing formation of primary oxidation products (P <= 0.01). Once the primary oxidation products were lowered, TBARS levels (secondary oxidation products) increased in control samples after 12 weeks; however, in E + C + citric acid treatment group and rosemary-treated samples, this increase was significantly lower (P <= 0.01). There was a rise in hexanal concentration for up to 8 weeks, which dropped only in the antioxidant-treated groups and continued to rise in the control samples.
