Spectrophotometric assay using o-phthaldialdehyde for determination of reactive lysine in dairy products

The applicability of a simple and rapid spectrophotometric assay using o-phthaldialdehyde (OPA) to determine reactive lysine in dairy products was evaluated. For this purpose the values of reactive lysine of model systems and dairy products obtained by this procedure were compared with those obtained by the 1-fluoro-2,4-dinitrobenzene (FDNB) method in dialyzed and non-dialyzed samples. As a reference the latter method was applied to dialyzed samples to avoid interference of sugars. The results obtained showed that the OPA procedure is reliable for use even in the presence of high levels of sugars and can also be used to evaluate lysine damage after Maillard-type reactions.