Fish oils are very sensitive to oxidation as they contain fatty acids with four to six double bonds, therefore it is rather difficult to stabilize them against rancidification. The most common natural antioxidants are tocopherols. At low concentrations (about 100mg/kg), a-tocopherol is more active than 7- and S-tocopherols while at high concentrations (about 1000mg/kg) it is the reverse, and S- and 7-tocopherols are more active than a-tocopherol. The same could be said in the case of vegetable oils (see below).
Lecithin and ascorbyl palmitate are synergists in fish oils enriched with tocopherols. Both choline and ethanolamine bound in phospholipids inhibit the accumulation of hydroperoxides in sardine oil. Different flavonoids, especially quercetin, stabilize fish oils against rancidification, shown as synergism with a-tocopherol under conditions of the Schaal Oven Test at 60°C and in glass bottles at 2.50C. The most efficient derivative was 5,3',4'-trihydroxy-7-methyl-(O)-flavanone. Flavonoids, such as myricetin, quercetin and morin, were found to be more active than synthetic antioxidants in seal blubber and menhaden oils. Extracts from herbs, such as rosemary and oregano, were found to be very active in mackerel oil (Tsimidou et al., 1995).
The stabilization of fish muscle against rancidification is still more important than the stabilization of oils as unpleasant off-flavours are rapidly produced by interaction of fish lipid oxidation products with proteins. Viscera lipids are more susceptible to oxidation than muscle or skin lipids (Ohshima et al., 1993). The chloroform-methanol extracts of sardine meal possessed antioxidant activity, mainly due to phospholipids, particularly phosphatidylcholine. Phosphatidyl-ethanolamine showed synergistic activity with a-tocopherol. Metal chelating agents, such as sodium phosphate, glutamate or citric acid actively protected refrigerated herring against flavour deterioration. Rosemary extract was also efficient as a synergist in stabilizing frozen-crushed fish meat against rancidification. Similar activity was observed in dried sardine (Wada and Fang, 1994). Rosemary oleoresin stabilized lipids of rainbow trout during refrigeration at 40C or frozen storage (at —20oC) for several months (Akhtar et al., 1998). Other ground spices have antioxidant effects in sardine muscle, such as garlic, basil, oregano, parsley, rosemary, sage and thyme, at both 40°C and at 0°C. The addition of ground spices is more acceptable than that of plant extracts as the additives are cheaper and are not objectionable from the standpoint of safety.
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