Physical Properties of Foods and Effect of Water on Them V Rheology and Food Engineering

Abstract

Basic knowledge of dynamic properties, i.e., viscosity and elasticity, and their application to food engineering were reviewed. Viscosity relates shear stress τ to shear rate for substances; and elasticity, shear stress τ to shear strain γ. Steady flow viscosity μ of liquid can be measured by a cone-and-plate viscometer, as a function of . Viscoelasticity values are obtained by static or dynamic methods. In particular, from dynamic viscoelastic measurements, several viscoelastic parameters, which reflect inner structure and state of material, can be obtained: the storage modulus G’, the loss modulus G”, the complex modulus G^＊(＝G’＋iG”; i, imaginary unit), the dynamic viscosity η’, and the complex viscosity η^＊(＝η’−iη”). Food hydrocolloids are rheologically classified into 4 groups according to the angular frequency (ω) dependence of G’ and G”: true gel, weak gel, true polymer solution, and dilute polymer solution. According to Cox-Merzs’ empirical law, η^＊＝μ at ω＝. The velocity of thickener solutions through the human pharynx was measured by the ultrasonic pulse Doppler method to give an index for thickener solutions as foods for dysphagic patients. The value of V_max of the solutions approached that of yogurt, which is rarely aspirated by dysphagic patients, as the concentrations (μ) increased. The parameter V_max of the thickener solutions correlated well with μ.