EFFECT OF WATER CONTENT ON THERMO-PHYSICAL PROPERTIES AND FREEZING TIMES OF FOODS

M. V. SANTOS; V. VAMPA; A. CALIFANO; N. ZARITZKY

Water Stress in Biological, Chemical,Pharmaceutical and Food Systems

Springer

Año: 2012;

Numerical simulations of the heat transfer during freezing of foodstuffs require thermo-physical properties of the product in order to obtain accurate predictions. Thermo-physical properties, such as specific heat, thermal conductivity, initial freezing temperature, density, etc. are strongly affected by the water content. When the process includes a phase change transition such as freezing, these properties undergo abrupt changes with temperature; the mathematical problem that represents the process is therefore highly non-linear. A finite element formulation was developed to numerically simulate the freezing of foodstuffs with different water contents in an irregular geometry. The computational code was implemented using transformations of the variables in the heat conduction partial differential equation, applying an enthalpy and Kirchhoff formulation and validated with experimental data. Two products were selected to simulate the freezing process: cooked minced meat and dough, due to their different water contents. The specific heat of these products, the amount of bound water and the initial freezing temperature were measured from the Differential Scanning Calorimetry (DSC) endothermic thermograms. Thermal properties were used as inputs in the numerical code in order to calculate the freezing time of foodstuffs. The numerical code was used to simulate the freezing of dough and meat products with different water contents using an irregular geometry. The effect surface heat transfer coefficients and moisture content in the foodstuff on freezing times, was analyzed. It was observed that at low h values the heat transfer control is located in the fluid and freezing times increased with the freezable water contents. In contrast, for high h values, there is an internal heat transfer control and the thermal conductivity (k) of the solid (that is proportional to the ice content of the sample) governs the freezing rate.  As a consequence large amount of water in the food will not necessarily require longer freezing times. The freezing times are greatly dependent on the unfreezable water present and the surface heat transfer coefficient, as well as the total moisture content.