Determination of Nϵ-carboxymethyllysine in milk products by a modified reversed-phase HPLC method

A modified reversed-phase-HPLC method with o-phthalaldehyde pre-column-derivatisation for determination of Nϵ-carboxymethyllysine in food samples is presented. It is shown, that the method has to be modified if applied to milk products, including specific modifications in sample preparation and chromatographic separation conditions. The increased selectivity of a double endcapped RP 18 phase is necessary for reliable separation of Nϵ-carboxymethyllysine in hydrolysates of complex products like cheese. With a detection limit of 0.5 pMol the method shows high sensitivity and a very good reproducibility (s=2.81%). In total, several different milk products (n=50) as well as fresh, processed and ripened cheese samples (n=50) were analysed. The highest amounts of Nϵ-carboxymethyllysine were found in a whey cheese (1016 mg/kg protein), evaporated milk (1691 mg CML/kg protein), coffee cream (613 mg CML/kg protein) and cocoa milk (413 mg CML/kg protein). Nϵ-carboxymethyllysine could not be detected in UHT milk, fresh, processed and ripened cheese. The results show that Nϵ-carboxymethyllysine can give valuable information on lysine damage in severely heat-treated milk products and in products, with added sugar, pre-damaged constituents or stabilising agents. ©