Hemoglobin-mediated lipid oxidation of protein isolates obtained from cod and haddock white muscle as affected by citric acid, calcium chloride and pH

Acid or alkali solubilization followed by isoelectric precipitation can be used to isolate proteins with good functional properties from muscle tissue. Both acid (pH 3.0) and alkali (pH 10.5) treatment of the muscle decreased lipid oxidation catalyzed by hemoglobin. Citric acid and calcium chloride improved the oxidative stability of both acid- and alkali-solubilized muscle protein isolates when added to the homogenized muscle before separating the membrane or directly to the isolated membranes in the assay. Citric acid may have functioned in part by lowering the pH and destroying preformed peroxides. Exposing the muscle and the hemoglobin together at pH 3 promoted lipid oxidation, while addition of citric acid/calcium chloride or press juice to washed cod prior to solubilization inhibited lipid oxidation even when the tissue and hemoglobin were acidified together.