Water mediated phenomena in some multi-functional food processes

[Departement_IRSTEA]GEAPA [TR1_IRSTEA]42 - ALITECH / IRMA

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anglais. 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 .