Antioxidant activity of Cod (Gadus morhua) protein hydrolysates: in vitro assays and evaluation in 5% fish oil-in-water emulsion.

Cod protein hydrolysates were fractionated according to the molecular mass into three fractions of >5 kDa, 3-5 kDa and <3 kDa using an ultrafiltration membrane system. The antioxidative activity of the crude hydrolysates and the fractions were investigated, both in in vitro assays (DPPH radical scavenging activity, reducing power, Fe²⁺ chelating activity and inhibition of lipid oxidation in liposome model system), and in 5% fish oil-in-water emulsions. The <3 kDa fractions had very good radical scavenging activity, Fe²⁺ chelating activity and reducing power while the fraction 3-5 kDa resulted in higher protection against oxidation in the liposome model system. When tested in 5% oil-in-water emulsions, all the fractions, including the crude protein hydrolysate, were able to protect fish oil against oxidation in an iron induced oxidation system. However, none of the peptide fractions were effective in preventing tocopherol loss and showed no tocopherol sparing property. Volatile oxidation products showed an interaction between the aldehydes and the protein/peptides added in the emulsions, and this needs further investigation.