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In this study, gelatin was first extracted by alkaline and acidic treatment including 0.19 N NaOH and 0.12 N acetic acid solution by ratio of skin of rainbow trout (Oncorhynchus mykiss (to solution of 1 to 7 and then heat treatment in 50 °C. Then, hydrolysed by alcalase enzyme for 4 hours with the ratio of enzyme to the substrate 1 to 100 and the degree of hydrolysis were measured after 4 hours. DPPH and ABTS free radical scavenging activity, as well as reducing power assay of gelatin hydrolysate were measured. The results showed that the degree of hydrolysis after 4 hours was 46/7%. Also the highest DPPH and ABTS free radical scavenging and reducing power at concentration of 10 mg/ml were 39/8%, 50/7%, and 0/123, respectively. The skin from fish filleting can be a suitable raw material for extraction of peptides with biological activities. The results showed that peptides derived from rainbow trout fish skin gelatin can be considered as a natural antioxidant.

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 Fish protein hydrolysates from whole kilka, using alcalase enzyme (ratio 1: 100) under optimal temperature (55°C) and pH (8.5) was evaluated for its hydrolysis degree and antioxidant activity. Results of the hydrolysis degree recorded at time intervals of 1, 2, 3 and 4 hours indicated the hydrolysis degree increased with increase in the hydrolysis time. The evaluation of FPH antioxidant activity, using DPPH, ABTS and reducing power assay tests at 3 concentrations (1, 2 and 5 mg/ml indicated the highest inhibitory effect at 5 mg/ml was 74.4%, 72.3% and 1.8 absorbance in 700 nm for DPPH, ABTS and reducing power assay, respectively. Generally, the findings of this research confirmed the potential of kilka as a source of natural antioxidants for food applications.