Evaluation of tracers for the authentication of thermal treatments of milks

For regulatory purposes, several methods have been proposed for defining biochemical tracers to identify heat treatments to which milks may be submitted. However, none of these methods is universal whereas they are usually restricted to one or two categories of treated milks and are not able to discriminate any unknown sample arriving in a control laboratory. Therefore, Arilait-Recherches and Syndilait (Syndicat National des Laits de Consommation) decided to undertake a study in order to define an efficient discrimination approach applicable to commercial milk samples. About 200 commercial samples were tested, produced in several plants where they received the standard heat treatments here-applied. Five types of heat treatments were studied: pasteurisation, high pasteurisation, direct UHT, indirect UHT, and sterilisation. In order to estimate the effect of storage on long-term storage milks, samples were submitted to two storage conditions: 90 days at 25 °C and 90 days at 35 °C. The selected tracers were: furosine; lactulose; native α-lactalbumin; denaturised α-lactalbumin; percent of denaturised α-lactalbumin; FAST index, tryptophan fluorescence, β-lactoglobulin, and lactoperoxidase: altogether 5000 measurements were collected. Analysis of variance clearly demonstrates that no tracer can be selected to universally discriminate milks from all these technologies. The only possible authentication technique must be based on a multivariate approach. The application of factorial discriminant analysis by combining at least five tracers, gives a possible solution to this question. Most discriminative tracers are those which globally measure the structural modifications of the milk protein rather than those which specifically quantify the metabolites of the Maillard reaction.