Effect of dietary thyme on the oxidative stability of egg yolk

The effect of dietary thyme on the oxidative stability of shell eggs over a 60-day refrigerated storage has been evaluated. Also, the influence of dietary thyme and storage time on the oxidative stability of liquid yolks adjusted to various pH values and agitated in the absence or presence of light has been investigated. Yolk lipid oxidation was determined by monitoring malonaldehyde formation through use of third-order derivative spectrophotometry. Results showed that although malonaldehyde is not produced during storage of shell eggs, it is present in the yolk of fresh eggs. It is also evident that thyme treatment reduced oxidation of liquid yolk, which significantly increased with light and increased acidity at pH 3 and declined thereafter as acidity increased to pH 2. A comparative examination of the antioxidant activity of various synthetic and natural antioxidants added to yolk suggested that thymol, the main antioxidant constituent of thyme, cannot be considered totally responsible for the oxidative resistance of the thyme-treated yolk. There may be additional thyme components with antioxidant activity that pass into egg yolk providing antioxidant properties.