The effect of freshness of heads of rainbow trout ( Onchorhynchus mykiss ) for hydrolysing fish protein was investigated. To simulate storage and transportation, hydrogen peroxide and iron was added to minced heads to induce oxidation in the raw material prior to one week of storage. The effect of antioxidant in retarding oxidative changes during hydrolysis or to the raw material was investigated by adding butylated hydroxy toluene (BHT) prior to hydrolysis or storage. Enzymatic hydrolysis was carried out using bromelain and papain. The oil fraction was separated from the water soluble proteins, and the soluble phase was freeze dried. Both the oil fraction and protein fraction from enzymatic hydrolysis was affected by oxidative state of raw material. FFA was significantly higher in those FPH made from raw material added pro-oxidants, addition of antioxidant did not affect the level of FFA. The solubility of proteins in dried fish protein hydrolysates (FPH) decreased significantly when using oxidized raw material. Although addition of antioxidant improved the solubility, it was still significantly lower compared to those FPHs not added pro-oxidants. The FPH with decreased solubility also had higher levels of carbonyl groups which indicate protein oxidation. However, the oxidative state of raw material did not affect fatty acid composition in oil fraction or the amino acid composition in the FPH.

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Exploring and making use of underutilized marine resources can be a sustainable approach to achieve future demands of fish consumption by the ever-growing population. Five species, namely European plaice (Pleuronectes platessa), European flounder (Platichthys flesus), lemon sole (Microstomus kitt), megrim (Lepidorhombus whiffiagonis), and thornback ray (Raja clavate), often captured as by-catch in Norway, were characterized for their nutritional value and potential accumulation of hazardous components. The proximate composition, protein profile, fatty acid profile as well as essential and toxic trace elements and polychlorinated biphenyls (PCBs) were analyzed. Digestible indispensable amino acid (DIAA) ratios and scores (DIAAS) and contributions of omega-3 fatty acids to the diet were calculated. Analysis on proximate composition revealed low fat contents of 0.74 to 1.25% and sufficient protein contents between 16.9 and 24% in the five species. Results of DIAA indicate a profitable distribution, with contributions exceeding the daily intake recommendations for an adult person related to a 200 g fillet. Moreover, findings on the distribution of eicosapentaenoic (EPA) and docosahexaenoic acid (DHA) showed remarkable results, considering that the investigated species are lean fish. All five investigated fish exceed the recommended average daily intake level (AI) of EPA + DHA in a 200 g portion. As to toxic trace elements and PCBs, no significantly elevated levels were found considering a portion size of 200 g. Consequently, the nutritional quality of the investigated fish can be regarded as profitable with overall low potential health risks.

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