Study of the kinetics of solution concentration versus the browning of foodstuffs

Laboratoire de Recherches sur la Réactivité des Solides, (LA. 23), Faculté des Sciences Mirande, BP 138, 21004 Dijon Cedex, France.

Abstract

This paper is at the convergence of two concerns related by common physicochemical features : on the one hand drying operations applied in the processes of foodstuffs (juice concentration, solid drying and lyophilisation) ; on the other hand the physicochemical studies of phenomena of evaporation and dehydration of pure liquids or mineral solids. In this respect fundamental research on non-enzymatic browning is often done in equilibrium conditions of evaporation. On the contrary the present paper is an attempt to detect the possible activation of browning by evaporation.

This study is conducted on the concentration of a commercial grape juice.

• a)

  The basic phenomenon found in each experiment is the slowing down of the transformation by interface cooling, where the endothermic process of evaporation occurs. This slowing down whose establishment is followed from initial time to stationary working conditions results in a characteristic feature i.e. the Smith-Topley effect in that the reation rate depends on water vapour pressure. The discussion results in a general view of the problem of slowing down by the endothermic effects of the transformation.

• b)

  The phenomenon of evaporation has some relation with that of browning which it accompanies in foodstuffs. It seems to play a part of activation depending closely on two parameters : evaporation time and temperature. It may impart its kinetic factors to the kinetics of browning. The profile of the Smith-Topley effect is found in the curves relating the rate of browning to the water vapour pressure. This effect, however, which only occurs in relation with evaporation, has an origin that is basically different from the effect usually obtained for a particular humidity in a closed system.

Résumé

Cet article cherche à déceler l’éventuelle activation du brunissement non enzymatique par l’évaporation, à partir d’une étude conduite sur la concentration de jus de raisin.

En ce qui concerne le phénomène d’évaporation de la solution, le caractère permanent retrouvé dans chaque expérience est le ralentissement de la transformation par les effets endothermiques à l’interface évaporative. La façon dont la vitesse dépend du temps et de la pression de vapeur d’eau est étudiée.

L’évaporation joue un rôle activateur dans le brunissement de substance alimentaires. Elle peut imprimer à la cinétique de brunissement ses propres caractères cinétiques, en dépendance étroite avec la durée de l’évaporation et la température.