Water and Physical Properties of Foods

Abstract

Two topics related to the physical properties of foods and associated water effects were reviewed: the physical properties of hydrocolloids and care foods for dysphagic patients, and the water sorption and glass transition of foods. The percolation model effectively described the behavior of the rheological properties of food hydrocolloids near the sol-gel transition point relating to polymer network structure. The rheological properties of food hydrocolloids influence the velocity of the bolus. For thickener solutions, viscosity (μ), dynamic viscosity (η'), and complex viscosity (η^*) are well correlated with the maximum velocity through the pharynx, V_max, and are thus considered to be suitable indices of liquid-type foods for patients with dysphagia. For gels, the decrease in V_max was slightly above the concentration of the sol-gel transition point. Water sorption isotherms provide information on the interaction between water and solid components in foods. Water sorption isotherms of foods can be fitted by the GAB equation. Using solution thermodynamics, the partial molar quantities of both water and solutes can be estimated from water sorption isotherms. The concept of glass transition can explain many phenomena such as the reaction rate of non-enzymatic browning and powder caking. Glassy materials change to the rubbery state at T_g. The value of T_g for those materials decreases with increasing water content due to the plasticizing effect of water. The difference in the chemical potential of solid (adsorbent) between a solution and a pure solid, ΔG_a^s, which was obtained from water sorption isotherms by solution thermodynamics, was correlated with the depression of the glass transition point ΔT_g.