EFFECT OF PH, PROTEIN DENATURATION, AND PROTEOLYSIS ON THE STABILITY OF NATIVE EGG YOLK TO LIPID OXIDATION (LIPOPROTEIN, CHOLESTEROL)
Abstract
An abundance of unsaturated lipids and the presence of metal catalysts suggests that egg yolk is prone to lipid oxidation. Research was conducted to determine the stability of shell egg and liquid yolk to lipid oxidation, and to investigate factors influencing native yolk lipid stability. During 18 months refrigerated storage, shell egg did not undergo lipid oxidation as measured by increases in thiobarbituric acid reactive substances (TBARS) and did not develop oxidized cholesterol derivatives. Liquid yolk did not undergo oxidation during 3 weeks of storage at room temperature, but did increase in TBARS at 45 and 55(DEGREES)C. Adjustment of normal yolk pH 6 to pH 5 did not cause TBARS formation, but the rate of oxidation increased in yolk and yolk plasma adjusted to more acidic conditions, with pH 3 >> pH 2 > pH 4. Protein denaturation using 6M guanidinium chloride resulted in an increased rate of TBARS production in yolk and yolk plasma. Papain hydrolysis of yolk plasma in the presence of added iron produced an increase in conjugated dienes and trienes. It is concluded that shell egg and liquid yolk manifest a stability to lipid oxidation that is unusual in light of the abundance of susceptible lipids and metal catalysts. This lipid stability is impaired when yolk is acidified. Increases in lipid oxidation upon denaturation or hydrolysis of the protein moiety suggests that the ordered structure or compartmentalization of lipid in the form of lipoprotein may partially account for the inherent oxidative stability of native yolk.
Degree
Ph.D.
Subject Area
Food science
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