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Aims: Bioactive peptides with antioxidant properties derived from fish by-products and wastes by enzymatic hydrolysis have become a topic of great interest for pharmaceutical, health food, and processing/preservation industries. Materials & Methods This study aimed to characterize peptide fractions with antioxidative activity recovered from rainbow trout gelatin hydrolysate. Four peptide fractions (<3 kDa, 3–10 kDa, 10–30 kDa, and >30 kDa) were obtained from gelatin hydrolysate by subjecting them to centrifugal ultrafiltration using successively a 30 kDa, 10 kDa, and a 3 kDa membrane. The
10–30 kDa fraction was characterized in terms of amino acid composition. The antioxidant activity of all fractions was monitored by 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2-azino bis(3-ethylbenzthiazoline)-6-sulfonic acid (ABTS) radical scavenging as well as reducing power activity.
Findings: The free amino acids in 10–30 kDa fraction were dominated by Gly, Pro, Ala, and Hyp; the total hydrophobic amino acid of 10–30 kDa fraction was also 61.5%. All hydrolysate and peptidic fractions demonstrated high antioxidant activities. Moreover, 10–30 kDa fraction exhibited the highest DPPH and ABTS radical scavenging activity (2, 5, and 10 mg ml⁻¹) and reducing power (10 mg ml⁻¹) compared to other fractions (p<0.05).
Conclusion: These results revealed the potential of peptide fractions recovered from rainbow trout skin gelatin as source of natural antioxidants for use in food products.

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The aim of this study was to produce protein isolates from rainbow trout (Oncorhynchus mykiss) and to investigate its nutritional, structural, functional and sensory properties. Fish protein isolates were produced by pH change method using alkaline and acidic pHs (2.5, 3.5, 11 and 11.5). The extracted proteins were evaluated for nutritional value, and functional properties, sensory properties, color, and structural changes of proteins. The extracted proteins showed relatively high nutritional value in the amount of essential amino acids. The solubility, gelling and foaming capacity of the proteins depended significantly on its extraction pH. So that the percentage of solubility and foaming capacity of proteins extracted under alkaline conditions showed a higher rate than acidic conditions while the least gelation concentration was related to pH 11.5. Examination and comparison of color (L, a, b and whiteness) and sensory properties of isolated proteins showed that proteins at acidic pH had a brighter and whiter color and better sensory properties than those from alkaline pH. The change in pH caused extensive protein degradation as shown by FTIR and SEM. The above results showed that the protein isolated from rainbow trout waste has good functional and nutritional properties in alkaline conditions, while the sensory properties and color of acidic conditions showed better results.

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Fat oxidation during the storage period is one of the important factors in the deterioration of food quality. Hydrolyzed fish waste is one of the most important sources of bioactive peptides as a natural antioxidant. The purpose of this research is adding bioactive peptide that obtained from the enzymatic hydrolysis of rainbow trout waste (viscera) to the cooked hamburger formulation and measure the characteristics of the hamburger. Fish wastes were hydrolyzed in optimum conditions (temperature 59°C, time 118 minutes and concentration of 2% alcalase enzyme and then, 0.5, 1, 1.5 and 2% by weight, were added to the hamburger samples. A hamburger is also as a blank. The tests were performed in 3 repetitions and the averages were compared with Duncan's test to check the significance of the variables at P < 0.05 and the data were reported as mean ± standard deviation. The results showed that the cooked hamburger formulation contains 2% viscera by weight, has the highest percentage of cooked yield, fat and moisture retention, and the lowest amount of hardness, cooking loss and shrinkage. Using of a waste source, in order to turn it into a valuable product with antioxidant properties, lead to reduce the amount of fish waste and helps the environment. With this method, natural antioxidants can be used instead of synthetic sources.