Analysis of Physical Properties of Foods Relating to Water and Their Application

Abstract

Some topics related to physical properties of foods and effect of water on them were reviewed. The freeze-drying of liquid foods can be analyzed by a modified URIF (Uniformly Retreating Ice Front) model below the sample temperature of T_g', the glass transition temperature for CAS (Concentrated Amorphous Solution) in frozen low-concentration material. If sample temperature increases above T_g' during freeze-drying, CAS in the frozen sample is in the rubbery state. The CAS in the rubbery state may collapse during freeze-drying and block the path of vapor, which makes it difficult to remove water from the sample. Water sorption isotherms give information on the interaction between water and solid components in foods. Using solution thermodynamics, the thermodynamic parameters of both water and solutes can be estimated. The integral Gibbs free energy, ΔG^s, is considered to be a suitable parameter for evaluating the interaction between solid and water from the thermodynamic point of view. The thermodynamic parameters obtained from water sorption isotherms relate to the degree of reduction in the glass transition temperature (T_g), accompanied by water sorption. The difference between the chemical potential of solid (adsorbent) between a solution and a pure solid, ΔG_a^s, which is obtained from water sorption isotherms by solution thermodynamics, is correlated with ΔT_g (≡T_g − T_g0; where T_g0 is T_g of dry material). This indicates that plasticizing effect of water on foods can be evaluated through the parameter ΔG_a^s. The relaxation time τ obtained from the dielectric relaxation is also considered to be suitable parameters for the characterization of the plasticizing effect of water on a glassy material. The percolation model effectively described behavior of the rheological properties of food hydrocolloids near the sol-gel transition point. The maximum velocity through the human pharynx, V_max, which is obtained by the ultrasonic pulse Doppler method, is considered to be a good predictor of aspiration risk for patients with dysphagia. For a gelling agent, V_max decreased with the increase in concentration; the decrease was slight above the concentration of the sol-gel transition point.