Comparison of the performance of ascorbic acid and α-tocopherol in combination with whey protein to increase the oxidative stability of Vanami shrimp fillet (Litopenaeus vannamei)

Shrimps are highly sensitive to oxidation at refrigerator temperature. On the other hand, storage of shrimp in freezing conditions leads to a decrease in product quality after thawing. It should be noted that shrimp oxidation also occurs in freezing conditions, but the oxidation rate in these conditions is much slower than storage in refrigerated conditions. Therefore, it seems necessary to use a method that can control the oxidation of shrimp in both freezing and refrigerating conditions. The aim of this study was to evaluate the feasibility of controlling shrimp oxidation (at refrigerator temperature) using whey protein coating containing ascorbic acid or α-tocopherol, and to compare the efficacy of these antioxidants (in combination with whey protein).  In order to advance the purpose of the research, shrimp fillets were stored in four treatments, including treatments No. 1 (control), 2 (shrimp fillet coated by whey protein), 3 (shrimp fillet coated by whey protein + ascorbic acid) and 4 (shrimp fillet coated by whey protein+ α-tocopherol) at refrigerator temperature for 9 days.  In order to evaluate the oxidation intensity and also the stability of the treatments against oxidative damage, peroxide indices, free fatty acids, anisidine and thiobarbituric acid of the treatments were determined on days 0, 3, 6 and 9. This study was implemented in form of completely randomized design and data were analyzed by one-way ANOVA. Significant differences among means were tested by Duncan's test at 95 confidence level.  The results showed that whey protein alone (treatment 2) as shrimp coating can partially control the oxidation process of fillet fats compared to control. But when whey protein was combined with ascorbic acid (treatment 3) and α-tocopherol (treatment 4), the coatings' strength against oxidative deterioration significantly increased (p<0.05). According to our findings, during the storage period, the lowest amount of peroxide, free fatty acids, anisidine and thiobarbituric acid indices were related to treatment 3 (p<0.05). During the storage period, all the mentioned indicators (in all treatments) had an increasing trend, but the slope of this trend was different and the lowest slope was related to treatment 3. Comparison of fresh shrimp fillet fatty acid profile with fatty acid profile of treatments at day 9 showed that the whey protein coating combined with ascorbic acid (treatment 3) had the most protective effect on the structure of fatty acids. Overall, according to the results of the present study, it can be claimed that whey protein- ascorbic acid coating is more effective than whey protein-α-tocopherol coating to increase the oxidative stability of shrimp fillet. Therefore, the ascorbic acid is more efficacious than α -tocopherol (in combination with whey protein) in controlling the oxidation of shrimp fillets.