Chapter 14
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Contribution of Pyrrole Formation and Polymerization to Non-Enzymatic Browning during Chicken Roasting Francisco J. Hidalgo, Manuel Alaiz, and Rosario Zamora Instituto de la Grasa, Avenida Padre Garcia Tejero 4, 41012 Sevilla, Spain
Protein pyrrolization produced during chicken roasting was studied to better understand the molecular mechanisms involved in the resulting browning. Chicken legs were roasted at 200 °C for different periods of time and their colors determined using a chromameter. In addition, the skins of roasted legs were homogenized in sodium phosphate buffer and studied for browning, fluorescence, lipid oxidation, protein pyrrolization, and amino acid losses. Chicken roasting resulted in both peroxidation of lipids and losses of some of the amino acid residues of skin proteins, including lysine. In addition, there was a very good correlation between these two measurements, suggesting that the formation of oxidized lipid/amino acid products was occurring and pyrrole formation was a consequence o f these reactions. Furthermore, there were very good correlations among browning, fluorescence and pyrrolization produced during chicken roasting, which suggested, in accordance with previous studies, that pyrrole formation and polymerization were contributing to the formation of both browning and fluorescence in this system.
The processing of foods for consumption often causes changes in their functional properties, nutritive value, flavor and color. The magnitude of these changes is related to both the kind of food and the type of processing. Food roasting has been used by man from ancient times to alter sensory properties of foods, to improve palatability and to extend the range of tastes, aromas and textures in the diet. In addition, it destroys enzymes and micro-organisms and lowers the water activity of the food to some extent, thereby increasing its shelf-life (/). When meats are roasted, the production of aromas is observed in addition to the characteristic golden brown color. © 2001 American Chemical Society
Ames and Hofmann; Chemistry and Physiology of Selected Food Colorants ACS Symposium Series; American Chemical Society: Washington, DC, 2001.
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202 This color is believed to be a consequence of the Maillard reaction, the caramelization of sugars and dextrins (either present in the food or produced by hydrolysis of starches) to furfural and hydroxymethyl furfural, and the carbonization of sugars, fats and proteins (/). However, the molecular mechanisms by which this color is produced are mostly unknown at present, and most investigations have been carried out using model systems. In an attempt to investigate browning reaction mechanisms in real food systems, the posent study was undertaken to analyze the production of brown color in chicken skins during roasting. Chicken skins were selected because they are mostly composed of lipids and proteins, and carbohydrates are almost absent. In fact, the proximate composition described for chicken skins, which is related to the diet and the age of the birds, is 47.9-52.6 % moisture, 37-45 % lipid, 7-9 % protein, and 0.4-0.5 % ash (2-5). In addition, poultry lipids generally exhibit a higher degree of unsaturation compared to red meats, thus being more prone to oxidation (4). Therefore, in this system oxidation of lipids is likely to occur, and the formed lipid oxidation products may react with the reactive amino acid residues of proteins to produce oxidized lipids/amino acid reaction products (OLAARPs). Previous research from this laboratory has shown that, in model systems, O L A A R P s are able to contribute to the browning and fluorescence produced in lipid/protein systems by a mechanism which implies the formation and polymerization of pyrrole derivatives at the ε-amino group of lysine residues (5). The key intermediates in these reactions seem to be the JV-substituted 2-(lhydroxyalkyl)pyrroles produced by reaction of epoxyalkenals and the ε-amino group of lysine residues. A general scheme for the reactions produced between epoxyalkenals and other products of lipid peroxidation, including oxidized fatty acids with the structure of 4,5-epoxy-l-oxo-2-pentene, with protein reactive groups is shown in Figure 1. This mechanism not only produces color and fluorescence but it has also been related to the production of volatile heterocyclic compounds in oxidized lipid/protein mixtures (6) and short chain aldehydes (7). The production of N-substituted 2-(l-hydroxyalkyl)pyrroles is always accompanied by formation of N-substituted pyrroles, which are much more stable and have been found in more than twenty fresh food products, including meats, fishes, vegetables and nuts (
