Efficacy of vitamin E with or without probiotic, astaxanthin or rosemary extract on microbiological and chemical characteristics of fresh and frozen fillet of rainbow trout (Oncorhynchus mykiss)

Microbiological properties, proximate composition, total volatile base nitrogen (TVB-N), pH, fatty acid (FA) composition, peroxide value (PV), free fatty acids (FFA), histamine and water holding capacity (WHC) in fresh and frozen-stored (3, 6, and 12 months at −18 °C) fillets of rainbow trout (Oncorhynchus mykiss) (170.46 ± 1.16 g) fed diets containing 50 mg vitamin E (control) alone or with 200 mg vitamin E (VE), 500 mg probiotic (PR), 50 mg astaxanthin (AS) and 500 mg rosemary extract (RE) kg−1 for 4 months were assessed. Decreasing trends in fillet WHC (specially in control and AS groups) were observed in the 12-month storage. Although a low increase in fillet bacterial population occurred during the storage time in all groups, lower bacterial counts were observed in PR group. Fillet histamine, TVN and pH levels were in the accepted ranges in all treatments during the storage. Also, FFA level and proximate composition of fillets didn’t show any significant change during the storage time. The PR diet controlled microbiological activity and variations of pH and PV of fillet samples better than the other experimental diets. It also prepared an appropriate FA composition and retained the desired amounts of both n-3 and n-6 polyunsaturated fatty acids (PUFAs) in fish fillets until the end of storage period, while no significant differences were seen in values of TVN, pH, FFA and histamine among the treatments. However, AS and RE diets tended to be less functional maintaining the fillet quality during the storage time than the other experimental diets. According to the results, a diet containing 500 mg bacilli probiotic with 50 mg vitamin E kg−1 could be highly efficient in maintaining a good quality of trout fillet stored at − 18°C for 12 months