The consequences of reactions between
protein and oxidizing
lipids on the nutritional quality of food
proteins have been investigated using a
whey protein-
methyl linolenate-water model system. In rat assays, significant reductions were observed in
protein efficiency ratio, net
protein ratio, net
protein utilization,
biological value and true
nitrogen digestibility, especially when the reaction had taken place at high moisture content, high temperature and in the presence of excess
oxygen. The losses of bioavailable
lysine and
tryptophan as measured by rat assays followed a similar pattern. The chemical value of each
amino acid multiplied by the true N digestibility closely resembled the rat assay value. In general, the reaction products of
lysine and
tryptophan formed during
lipid oxidation were biologically unavailable. The bioavailabilities of
methionine and of '
methionine plus
cyst(e)ine' were determined in separate assays.
Cyst(e)ine was calculated as '
methionine plus
cyst(e)ine' minus
methionine. In
whey protein which had reacted with oxidizing
methyl linolenate, the bioavailable
methionine content was not significantly reduced even though 82% of the
methionine residues were present as
methionine sulphoxide. In
hydrogen peroxide-treated
casein in which all
methionine residues were oxidized to the
sulphoxide,
methionine sulphoxide was found to be 96% as utilizable as a
methionine source to the rat. Free
methionine sulphoxide was 87% utilizable.
Cyst(e)ine appeared to be as sensitive as
lysine to reactions with
lipid oxidation products. In
whey protein which had reacted with oxidizing
methyl linolenate, the bioavailabilities of
cyst(e)ine,
lysine,
tryptophan and
methionine were reduced by 28, 24, 11 and 8% respectively and true N digestibility by 9%. These results are discussed in relation to food products.