Antioxidants in Chicken Egg Yolk: Effects of Cooking, Storage and Gastrointestinal Digestion

• Author / Creator

  Nimalaratne, Liyana Arachchilage Chamila K.

• Eggs are one of the most nutritious and economically affordable foods. They contain high quality proteins and lipids and serve as an important source of vitamins and minerals. Despite the nutritional importance, the egg industry is vulnerable to the consumers’ perception about egg cholesterol and associated risk of cardiovascular diseases. A correlation between egg cholesterol and risk of cardiovascular diseases is not supported by many clinical studies; findings from recent research, however, suggested that egg contains many bioactive components that may provide health benefits beyond basic nutrition. Oxidative stress is associated with onset and development of various diseases including cancers, and cardiovascular and neurodegenerative diseases etc. Antioxidants in the diet are thought to play a protective role against oxidative damage. In comparison to extensive research on antioxidants from plants (fruits, vegetables, cereals, herbs etc), there is limited research on egg antioxidants. The Canadian egg industry uses corn and wheat as their main feed ingredients. It was not known if the phenolic compounds in these cereals could be transferred from feed into the egg yolk. Moreover, the potential of chicken eggs as an antioxidant food commodity has not been fully explored. We aimed to test two hypotheses: 1) antioxidant compounds present in hen’s feed may contribute to antioxidant activity of egg yolk, and 2) domestic cooking, storage and gastrointestinal digestion would affect the antioxidant activity of egg yolk. In the first study, our results showed that phenolic acids in feed could not be transferred into eggs; for the first time, we identified that two aromatic amino acids, tryptophan and tyrosine, are the major contributors to the antioxidant property of egg. The antioxidant activity, based on oxygen radical absorbance capacity (ORAC), DPPH (2, 2-diphenyl-1-picrylhydrazyl) radical scavenging and ABTS (2,2′-azino-bis-3-ethylbenzthiazoline-6-sulphonic acid) radical scavenging assays, and the contents of aromatic amino acids were significantly reduced by all cooking methods (boiling, frying and microwaving). The effect of cooking methods on yolk carotenoids, a well-known group of antioxidants, was evaluated. A LC-(APCI)-MS/MS method was used to identify and quantify all-E- and Z- isomers of lutein, zeaxanthin, canthaxanthin and β-apo-8’-carotenoic acid ethyl ester in fresh and cooked egg yolks. Both fresh and cooked yolks showed similar carotenoid profiles except the content of Z-isomers increased while that of E-lutein reduced in cooked samples; the total loss of carotenoid ranged from 6 to 18% in different cooking methods. A stepwise extraction protocol was developed to determine the total antioxidant activity of egg yolk. Samples from raw and cooked egg yolks were extracted to obtain four fractions containing carotenoids, vitamin E, free amino acids (FAAs), and phosvitin. The antioxidant activity of each fraction was determined by ORAC and ABTS assays. FAAs contributed to approximately two thirds of total antioxidant activity. The effects of simulated retail storage conditions and cooking on the antioxidant activity as well as the aromatic amino acids and carotenoids in general table eggs, omega 3/lutein enriched eggs, and eggs from heritage chicken breeds were evaluated. Different types of eggs did not differ in their FAA, ORAC and malondialdehyde (MDA) contents. Six weeks of storage at refrigerated temperature did not change the ORAC value and the contents of FAAs, carotenoids, and MDA in egg yolk. Boiling and frying significantly reduced the ORAC value and the contents of FAAs and carotenoids while the MDA level increased. An in vitro model (TIM-1) was used to investigate the effect of simulated gastrointestinal digestion on antioxidant activity and carotenoid profile of egg yolk. Bioaccessibility, but not digestive stability was significantly affected by the method of cooking. No trans-cis isomerization of carotenoids was observed while scrambled eggs showed a significantly lower bioaccessibility compared to that of boiled eggs. Gastrointestinal digestion significantly increased the ORAC value of eggs. FAA analysis revealed that digestion resulted in a 17-fold increase in FAAs in whole egg and 4.5 to 6-fold in egg yolk. Free aromatic amino acids contribute to the antioxidant activity; however, they were not the major contributing factor. The findings of this thesis establish the primary evidence of egg as an antioxidant food commodity; the antioxidant activity of egg is not affected by storage, reduced during cooking but significantly increased during gastrointestinal digestion.

• Subjects / Keywords

  • Egg carotenoids
  • Antioxidants
  • Egg Yolk
  • Domestic cooking methods
  • Gastrointestinal digestion
  • Free amino acids

• Graduation date

  Spring 2015

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3BC3T83V

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.