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La thèse portait  sur la :

Stabilité des microconstituants végétaux aux températures intermédiaires : devenir des vitamines et autres microconstituants dans les produits à base de fruits et légumes.

Stability of vegetable micro-constituents at intermediate temperatures: fate of vitamins and other micro-components in products based on fruits and vegetables.

The European project « Optimized Products for Elderly Populations (OPTIFEL) » was launched to ameliorate elderlies’ nutritional status. Since this population often suffers from malnutrition, it was envisaged to conceive food products based on fruit and vegetables, with a dense nutritive value. Therefore, products were enriched with important nutrients, among vitamin C and folates. To comply with the intention of supplementations consisting in an increased intake of nutrients, the study of their stability, especially of easy degradable molecules, is of utmost importance. The present work in particular, was dedicated to the stability of vitamin C and 5-methyltetrahydrofolate when food is warmed-up that is heated at an intermediate temperature range (60-80°C).
It turned out that the deterioration pace of vitamin C is principally influenced by the filling volume of recipients on a lab-scale. A negligible effect was found for the food matrix meaning that products based on apples and carrots can interchangeably be used for fortifications. Concentration adaptions are easy to control as the degradation loss per time in the concentration range 2-5 mmol/kg, is independent of the initial concentration. Increasing temperature in the range 60-80°C, does not have an impact in a real food matrix either. The latter indicates that another factor, probably oxygen, becomes limiting as enhancing the supply of energy does not increase degradation rates anymore. Thus, by heating products at 80°C, the microbial safety margin can be increased while the nutritional value is kept as if heated at 60°C.
From literature it is known that degradation at this temperature range only proceeds via the aerobic degradation pathway. It has been shown in the present work that in food products, the oxygen availability decreases down to anaerobic conditions, also near the surface, during heat treatments at 80°C while oxygen in model solutions stays abundant. Hence, the headspace gains in importance during long warm holding of real food products and dynamics of oxygen and ascorbic acid might determine degradation paces. However, oxygen is not alone responsible for the degradation initiation since ascorbic acid in ultrapure water does not degrade at 80°C during 8 h, even if oxygen is abundant during the whole length of time. An additional trigger, as Fe3+ ions or maybe also other constituents in food matrices, must be present. An interaction between oxygen and the trigger might result in the generation of reactive oxygen species that finally deteriorate the vitamin.
For complete stabilization of 5-methyltetrahydrofolate, the amount of ascorbic acid is crucial in contrast to the food matrix that is used for supplementation. The protective effect of ascorbic acid is however time-limited even if it remains in excess. The duration of complete stability can be prolonged by increasing the initial ascorbic acid concentration.
Heat treatments under real conditions that is when food products are warmed-up by a microwave, an Actifry ® device or held warm by a water bath, lead to minor to negligible vitamin losses.
These are good news for the project since the vitamin amount that is added, is preserved during warming-up of products and must not be controlled. The results indicate the complexity of vitamin degradation since the stability depends crucially on the experimental set-up. It can be concluded that predictive modeling should be performed under real conditions. Vitamin C and 5-methyltetrahydrofolate, which are generally referred to be very susceptible to oxygen and heat, are fairly stable under reheating conditions in real food products.

Le jury était composé de  :

Mr Oliver Dangles, Professeur, Université d’Avignon, Examinateur
Mr Schmitt-Kopplin, Professeur,  Université Technique de Munich, Rapporteur
Mr Eric Marchioni, Professeur, Université de Strasbourg, Rapporteur
Mme Catherine Renard, Directrice de Recherche Inra, Directrice de thèse
Mr Michael Rychlik, Professeur, Université Technique de Munich, Co-directeur