Various food processes aim at controlling the content and distribution of water in solid foodstuffs (at a macroscopic and molecular level) in order to increase their stability and shelf-life. The objective is generally to reduce the availability of water for undesirable reactions or transfers. This can be achieved most notably through partial removal of water, addition of water activity lowering agents, or change in the state of water. The various physical, physico-chemicaI and bio-chemical phenomena (transfer, reactions, transformations, deformations...) induced by or accompanying the migration or phase change of water are defined in this paper as "water mediated phenomena", which may influence the evolution and further behaviour of the food material during the process. The objective of the present paper is to show how the increased understanding of water mediated phenomena makes it possible to improve process control and food quality. This is illustrated by the analysis of the four following examples: i) control of thawing phenomena or solute concentration in the food outer layer in osmotic dehydration and immersion freezing, related to the evolution of inner concentration gradients andlor crystallisation phenomena; ii) control of cracker colour and texture, related to water departure mechanisms and solute entrainment by water during baking; iii) control of oil absorption related to the creation of heterogenous porous structure and internal pressure evolution during frying and cooling. (Résumé d'auteur)

Dispersions of commercial casein and whey protein and laboratory-prepared soybean protein were studied in mixed dispersants of water with various aliphatic alcohols, methanol, ethanol, n-propanol and 2-propanol. Supernatant and protein sediments were separated by centrifugation in two steps: 1800 rpm 10 min, followed by centrifugation of the supernatant at 50000 rpm for 60 min (125000xg). A gel-like protein sediment obtained at low alcohol concentration by high-g centrifugation increased in amounts as a function of the alcohol concentration until it progressively transformed, with higher alcohol concentrations, into an opaque flock (precipitate), sedimenting at 1800 rpm. It was concluded that the sediment obtained by ultracentrifugation was a protein of increased density which was produced by partial and progressive dehydration and alcohol binding. The conversion of the sediment into a flock or precipitate is discussed in terms of the hydrophilic-lipophilic balance of the protein and of the polar-nonpolar character of the dispersant.

[Departement_IRSTEA]GEAPA [TR1_IRSTEA]42 - ALITECH / IRMA | Extrait de document | Various food processes aim at controlling water in solid foodstuffs in order to increase their stability and shelf-life. In most cases, the main objective is to reduce water content and/or change the state and activity of water in the food matrix. This can be achieved throughout partial removing of water, addition of water activity lowering agents, or freezing . Processes to do so include air drying (conventional or super-heated steam), cooking, deep-fat frying, candying, salting, osmotic dehydration, air-blast or immersion freezing (1, 2, 3, 4). During such processes, water may migrate throughout the food matrix as well as at the interface between the solid food and the surrounding fluid (gaz, liquid). Water migration can be induced by different mechanisms, including diffusion, osmosis, or flowing in the matrix pores. Water may also undergo phase change (evaporation, freezing). Depending on the way water migrates or changes phase, the evolution in the multi-component and multiphase food are different, and can be classified into three types: i) evolution of inner concentration, pressure or structure fields ; ii) evolution of physical and physico-chemical state ; iii) evolution of microbiological, biochemical, thermal or enzymatic reaction kinetics. The objective of the present paper is to show how the further understanding of the "water mediated " phenomena makes it possible to improve process control and food quality. This is illustrated by various examples, such as: i) control of oil absorption related to the creation of pores and structural heterogeneity during frying, induced by water migration mechanisms and pressure fields; ii) control of thawing phenomena or solute concentration in the food outer layer in osmotic dehydration and immersion freezing, related to the evolution of inner concentration fields and /or crystallisation phenomena ; iii) control of cracker colour, related to solute entrainment by water during baking .

L'eau intervient à de multiples niveaux dans la qualité et la stabilité des aliments. L'eau établit des interactions de différentes natures avec les ingrédients alimentaires, et c'est l'équilibre entre ces interactions et les interactions, ou liaisons, entre solutés de petite taille et/ou macromolécules qui contrôle la structure et la stabilité des ingrédients et des colloïdes alimentaires. Solvant des réactants, plastifiant du milieu de réaction et réactant, l'eau participe aux réactions se produisant pendant la transformation et la conservation des aliments. Il est bien connu que le développement des microorganismes responsables de l'altération des aliments est fonction des propriétés thermodynamiques de l'eau (activité de l'eau, pression osmotique). Les mécanismes qui contrôlent la vitesse des réactions chimiques ou de transformation physiques dans les milieux concentrés, voir solides, sont complexes et les modèles de prédiction sont rares et parfois controversés. Le concept de transition vitreuse de plus en plus utilisé en technologie des aliments est présenté succinctement dans cet article. Des exemples montrent l'intérêt de connaître le diagramme d'état de constituants alimentaires, c'est à dire l'état physique en fonction des conditions de température et d'hydratation, pour la maîtrise des opérations de transformation et de conservation des aliments