Effect of dietary thyme on the oxidative stability of egg yolk.

摘要

Lohmann laying hens (n=10 per group) were fed ad libitum on a maize-soyabean layer ration with or without 3.0% ground thyme (Thymus vulgaris) plant material. Egg sample collection commenced 4 weeks after the feeding trial began and lasted for 2 weeks. The effect of dietary thyme on the oxidative stability of shell eggs over a 60-day refrigerated storage was 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 was investigated. Yolk lipid oxidation was determined by monitoring malonaldehyde formation through use of third-order derivative spectrophotometry. Although malonaldehyde was not produced during storage of shell eggs, it was present in the yolk of fresh eggs. Thyme feeding 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. It is suggested that there may be additional thyme components with antioxidant activity that pass into egg yolk providing antioxidant properties.