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Free aromatic amino acids in egg yolk show antioxidant properties
Chamila Nimalaratne, Daise Lopes-Lutz, Andreas Schieber, Jianping Wu ⇑
Department of Agricultural, Food and Nutritional Science (AFNS), 4-10 Ag/For Centre, University of Alberta,Edmonton, Alberta, Canada T6G 2P5
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Laying hens are known to be able to ‘‘bio-convert’’ health-promoting components from the diet into eggs. Wheat and corn are important sources of antioxidant phenolics in the human diet. Given the fact that wheat and corn are major feed ingredients of laying hens in Canada, the objectives of this study were tocharacterise the presence of novel phenolic compounds in egg yolk and to determine the effect of cooking methods on their content and activity. The total phenolic content of yolk extracts was 3.83 and 3.49 lmol gallic acid equivalents/g dry yolk for wheat-based and corn-based yolks, respectively. Antioxidant properties of yolk extracts were measured by the ORAC, DPPH and ABTS assays. ORAC values were151.1 ± 26.1 and 155.9 ± 9.5 lmol TE/g for wheat-based and corn-based yolks, respectively. All cooking methods signiﬁcantly reduced (p < 0.05) the antioxidant values. Ferulic acid was detected in trace amounts and no other phenolic compounds were found. However, tryptophan and tyrosine were found to be two main contributors to the antioxidant property of egg yolk. The contents of total free aminoacids were 10081.0 and 10009.5 lg/g in wheat-based and corn-based yolks, respectively; cooking methods were found to reduce signiﬁcantly the amount of free amino acids. Ó 2011 Elsevier Ltd. All rights reserved.
Article history: Received 4 March 2011 Received in revised form 26 March 2011 Accepted 19 April 2011 Available online 1 May 2011 Keywords: Egg yolk Total phenolic content Antioxidantproperties Domestic cooking methods Aromatic amino acids Tryptophan Tyrosine
1. Introduction Eggs are considered as one of nature’s perfect foods that have been consumed for centuries all over the world. Although they contain all the necessary nutrients for a new life, consumption of eggs in many developed countries has declined due to the public perception on its high content of cholesterol.However, current evidence suggests that there is no direct link between egg consumption and blood cholesterol levels (Lee & Grifﬁn, 2006; Qureshi et al., 2007). Egg yolk is a rich source of both nutritive and non-nutritive compounds important to human health. It is well known that hens’ diet inﬂuences yolk composition. Through dietary manipulation, certain phytochemicals with important health beneﬁts canbe enriched in egg yolk (Surai & Sparks, 2001). Over the past few years, many research studies have demonstrated that bioactive feed compounds may be transferred from hens’ feed into the yolk. Lutein and zeaxanthin are the most extensively studied phytochemicals in egg yolk. They can help prevent age-related macular degeneration (AMD) and eye cataract either as antioxidants and/ or by ﬁlteringharmful blue light (Chung, Rasmussen, & Johnson, 2004; Moeller, Jacques, & Blumberg, 2000). Phytoestrogens with potential health beneﬁts such as soy isoﬂavones have also been reported to be present in egg yolk (Kuhnle et al., 2008; Saitoh, Sato, Harada, & Matsuda, 2004).
⇑ Corresponding author. Tel.: +1 780 492 6885; fax: +1 780 492 4265.
E-mail address: email@example.com (J. Wu). 0308-8146/$ -see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodchem.2011.04.058
Phenolic compounds commonly found in cereal grains, fruits and vegetables are important antioxidants that are suggested to play a preventive role in the development of many chronic human diseases such as cancer and cardiovascular diseases (Adom, Sorrells, & Liu, 2005; Li, Pickard, & Beta, 2007; Liu,...