The aim of the present study was to investigate important factors contributing to the high oxidative stabilityof fish-oil-enriched yoghurt, with particular emphasis on the possible antioxidative effects of peptidesreleased during yoghurt fermentation. Yoghurt samples were stripped from sugars and lactic acidand subsequently fractionated by ultrafiltration using membranes with cut off sizes of 30 kDa, 10 kDaand 3 kDa. The fractions were tested for antioxidant activity by investigating the inhibition of oxidationin liposome model system, 1,1-diphenyl-2-picrylhydrazyl radical-scavenging activity, iron-chelatingactivity, and reducing power. The lower molecular weight fractions were found to be more effective antioxidantsthan higher molecular weight fractions. The lower molecular fractions were further tested asantioxidants in fish-oil-enriched milk. On the basis of peroxide value, volatiles, tocopherol and sensorycharacteristics, the lower molecular weight fractions 3–10 kDa and <3 kDa showed protection againstoxidation of fish oil to the same extent as caseinophosphopeptides. The oxygen content of the yoghurtwas also found to be lower than that of milk. Thus our findings suggests that the higher oxidative stabilityof yoghurt might be due to antioxidant peptides released during the fermentation of milk by lactic acidbacteria and/or by the lower oxygen content of yoghurt, which subsequently reduces the oxidative stressof fish oil incorporated in the yoghurt. The results show that antioxidant peptides may be used as aningredient in foods enriched with fish oils to increase their oxidative stability.
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