The structured food systems (i.e. cellular tissues) are dissipative structures whose functionality mainly concerns their properties (physico-chemical properties, chemical and biochemical reactions), external interactions with surroundings (interactions with micro-organisms, heat and mass transport pathway) and especially, their interactions with consumers (nutritional value, quality, taste and flavour, texture, appearance: size, shape, colour). Dehydration or rehydration processes concern heat and mass transport phenomena (water, solutes) coupled with micro and macrostructure changes both producing important effects on food functionality. Control of these changes is the major concern in food product development. This control must be applied not only to the changes in physico-chemical properties but also to those related with consumers' issues. Food matrixengineering is a branch of food engineering which aims to apply the knowledge of the food matrixcomposition, structure and properties to promote and control adequate changes which can improve some sensorial and/or functional properties in the food. These changes, which are caused by some basic operations, are related to the phenomena of heat and mass transfer, vaporization-condensation, internal gas or liquid release, structure deformation-relaxation and phase transitions in matrixcomponents, and are usually coupled throughout the operation's progress. The final product may be a new product with improved composition and sensorial properties and/or more stability. All these concepts are discussed in this paper using several examples related to the application of combined food dehydration techniques.

Scarce use of physically based models for simulating foodstuff rehydration is related, inter alia, to difficulties in determining their hydraulic characteristic curve (water content vs. matric potential under equilibrium conditions). Its direct determination is not feasible for foodstuffs as it requires extended contact time with water to reach equilibrium that may cause microbial spoilage, swelling and physical destruction of the sample. To circumvent these difficulties, an alternative indirect method for determining the characteristic curve over the entire water-content range is proposed. It is based on the hypothesis that the end-parts of this curve, the air-entry value and saturated water content for the wet-end and water sorption isotherm for the dry-end, are relatively easily determined. The predicted characteristic curve was successfully verified for a model food material by comparing it with an independently measured values. Then, it was utilized for simulated rehydration by solving the Richards equation.

The state of water in foodstuffs is a guiding principle in food design, and the equilibrium concept of water activity (Aw) is ubiquitous. It is regarded as a primary variable or “hurdle” in preservation technology, and a key variable influencing chemical reaction during storage. However, the amount of water in any system differs as function of water activity depending whether it is determined by water sorption or desorption. Even though this hysteresis behaviour has already been described in the literature, no physical interpretation of its origin has yet been proposed with respect to detailed molecular organisation. This work shows, for two different food powders, gluten and a milk-based product that the hysteresis disappears when either go through their glass transition. A more complete DSC analysis for gluten during different sorption/desorption cycles demonstrates that the hysteresis is dependent on the ageing of the material, which evolves in the glassy state and is induced by structural relaxation.