Résumé
Infant formulae are dairy based products carefully designed to complete all nutritional needs of non-breast-fed new-borns. Like most of the industrial milk products, infant formulae are submitted to thermal treatments from pasteurization to sterilization. But the nutritional properties as well as the biophysical and organoleptic properties are affected by the intensity of the thermal treatment. The main degradation reactions causing nutritional damage to the products are protein denaturation, Maillard reaction and lactose isomerisation. These reactions are greatly enhanced by the presence of minerals and vitamins (mostly iron and ascorbic acid). The objective of this study is to quantify the effect of the time and temperature variables of a UHT treatment on the damages caused by Maillard reaction in milk based infant formulae. This work is part of the European project ICARE (6th framework programme). In this study, the degree of advancement of the Maillard reaction during ohmic heating of a model infant formula was determined by quantifying various chemical markers, by means of chemical and physical methods. 1/ In the early steps of the Maillard reaction, a reducing sugar condenses with an amine base to form Amadori products which were quantified by measuring furosine. 2/ Then Amadori products turn into advanced glycated end products (AGEs), an interesting property of which is to be fluorescent. AGEs were globally measured by means of a new spectrometric method called FAST (Fluorescence of Advanced Maillard products and Soluble Tryptophane) which was developed in recent works to quantify and discriminate the intensity of heat treatments applied to milk and milk based products. Carboxymethyllysine is another interesting AGE that was followed: it is a terminal reaction product which accumulates during thermal treatment. 3/ During the end Maillard reaction steps, some AGEs turn into melanoidins, brown terminal nitrogenous compounds which could be monitored by colorimetry.Ohmic heating was chosen to achieve UHT levels, in a lab scale reactor specifically designed for the study. The principle relies on Ohm’s law: the circulation of an electric current through a resistant food matrix produces heat, proportionally to the electric field and to the electrical conductivity of the product. Ohmic heating is a volumetric method offering the great advantage versus classical UHT techniques, of avoiding thermal gradients inside the product.