Protein-bound Nε-carboxymethyllysine and Nε-carboxyethyllysine in raw and heat treated whites and yolks of hen eggs

Little is known about N-epsilon-carboxymethyllysine (CML) and N-epsilon-carboxyethyllysine (CEL), two typical advanced glycation end-products, in raw and cooked egg whites and yolks. This study was aimed to evaluate the levels of protein-bound CML and CEL in hen eggs as affected by heating (80 degrees C 6 min, 100 degrees C 30 min) and the source of eggs based upon a validated HPLC-MS/MS approach. A large biological variation in AGEs among individual raw eggs was found (e.g., CEL in yolks 1.26 - 10.57 mg/kg, n = 16). The yolk had higher levels of CML (125% more in average) and CEL (82% more in average) than the white for each raw egg, attributing to the higher protein levels and higher susceptibility of egg yolks to lipid oxidation. Heating led to significant increases (p < 0.05) of CML in egg whites and yolks, but resulted in an average of 17 - 18% decrease of CEL in egg yolks. Whether eggs were cage-free or not had little effect on their CML and CEL levels, but raw and heated eggs from two different companies contained significantly different amounts of CML and CEL. Overall, egg whites are more susceptible to heating than egg yolks and severe thermal treatment should be avoided to reduce the contents of AGEs in egg whites.