Quality of fillet from trout (Oncorhynchus mykiss) fed diets supplemented with olive pomace at two different levels

The increasing importance of sustainable aquaculture productions has generated a rising interest for the development of functional diets with benefits both for fish and quality of fish products. Rainbow trout (Oncorhynchus mykiss) can be fed diets rich in pigments, such as astaxanthin, which gives the reddish color to fish flesh. However, the oxidation of both astaxanthin and polyunsaturated fatty acids (PUFA), fish products’ flagship, can spoil feed or fillets. The present trial aimed to evaluate the antioxidant benefits of dietary olive pomace in terms of trout growth performance and quality of fresh and frozen fillets. To reach these goals, 600 rainbow trout (311.3 ± 48.8 g) were randomly distributed in 12 tanks and fed one of either 4 experimental diets formulated to be isoproteic (41.4 g/100 g feed), isolipidic (23.75 g/100 g feed) and isoenergetic (22.4 MJ/100 g feed): (i) control diet (CTRL); (ii) CTRL added with olive pomace at 1% (T1); (iii) CTRL added with olive pomace at 1.6% (T1.6) on the feed as is; (iv) CTRL added with a commercial blend of polyphenols (CTRL+). Polyphenols and astaxanthin feed contents were analyzed. After 10 weeks, feed conversion ratio (FCR) and specific growth rate (SGR) were calculated, and 8 fish/tank (24 fish per diet) were slaughtered. Flesh color, fatty acid (FA) profile and lipid oxidation (thiobarbituric acid reactive substances, TBARS) of 15 fish/diet were analyzed immediately and after 5 months at −20 °C. In addition, other 9 fish/diet were allotted to a descriptive sensory evaluation. Data were analyzed with the PROC GLM of SAS with diet as fixed effect. FCR and SGR did not differ among groups. Diets did not significantly modify fillet color values (lightness, yellowness, and redness) in line with the muscle content of astaxanthin (from 6 mg/kg in CTRL to 7.2 mg/kg in T1.6). However, the trained panelist described the color of cooked fillets as more intense (p< 0.05) in T1.6 and CTRL group than in the others. Besides, the panelist described the aroma of T1.6 fillets as more intense (p < 0.05) than the other groups, with algae, damp/earth and acidity attributes. Despite T1.6 diet was richer than the others in OH-tyrosol, tyrosol, and ferulic acid, lipid oxidation tended to increase after storage, being TBARS values 0.16, 0.15, 0.16, and 0.19 mg MDA-eq./100 g in CTRL, CTRL+, T1, and T1.6, respectively. Interestingly, at the end of the storage, the highest content of the saturated FAs was found in T1.6 and CTRL + groups (p < 0.05).