Effect of Low pH on the Susceptibility of Isolated Cod (Gadus morhua) Microsomes to Lipid Oxidation

During the extraction of muscle to produce protein isolates by acid or alkali solubilization, membranes are exposed to abnormally low or high pH. Low but not high pH treatment induces rapid oxidation of membrane phospholipids in the presence of hemoglobin. The goal of this research work was to study the oxidative stability of microsomes under the conditions met during acid solubilization. Isolated microsomes from cod muscle were used as a model system. At pH 5.3 or lower, 99% of isolated cod membranes sedimented at low centrifugation speeds. Isolated membranes that were exposed to pH 3.0 were less susceptible to hemoglobin-mediated lipid oxidation. Cod hemoglobin exposed to pH 3 was rendered less pro-oxidative than the untreated cod hemoglobin. However, when microsomes and hemoglobin were together exposed to low pH, oxidation was promoted. Citric acid and calcium chloride, as well as press juice isolated from cod muscle, were able to inhibit lipid oxidation of microsomal suspensions.