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: 2015; p. 383 - 392

For the prediction of temperature change in different foodstuffs duringfreezing and thawing processes, accurate estimation of the thermophysicalproperties of the product is necessary, such as specificheat, density, freezable water content, enthalpy, and initial freezingtemperature. These data allow the adequate design and optimization ofequipment and processes. Water is a main component in all foods andgreatly influences the behavior of these properties, depending on itsconcentration. During the freezing process, which involves the phasechange of water into ice, the specific heat, thermal conductivity, anddensity undergo abrupt changes due to the latent heat release. Thiscomplex process does not have an analytical solution and it can bedescribed as a highly nonlinear mathematical problem. Many difficultiesarise when trying to numerically simulate the freezing process, especiallywhen using the finite element method (FEM), which is especially usefulwhen dealing with irregular-shaped foodstuffs. Several techniques havebeen applied to consider the large latent heat release when using FEM.One traditional method is the use of the apparent specific heat, wherethe sensible heat is merged with the latent heat to produce a specificheat curve with a large peak around the freezing point, which can beconsidered a quasi-delta-Dirac function with temperature (depending onthe amount of water in the food product) (Pham 2008). However, thismethod usually destabilizes the numerical solution. Implementation ofthe enthalpy method, which can be obtained through the integration ofthe specific heat with temperature (Fikiin 1996; Comini et al. 1990;Pham 2008; Santos et al. 2010), and the Kirchhoff function, which isthe integral of the thermal conductivity, allows the reformulation of theheat transfer differential equation into a transformed partial differentialsystem with two mutually related dependent variables H (enthalpy)and E (Kirchhoff function) (Scheerlinck et al. 2001). These functions,H and E versus temperature, are smoother mathematical functionscompared to the specific heat, thermal conductivity, and density versustemperature, avoiding inaccuracies and/or divergence of the numericalmethod. Even though it brings great advantage to the resolution ofthe problem, with the simultaneous enhancement of the computationalspeed of the program, this transformation of variables is not widelyused in the literature. Unleavened dough and cooked minced meat wereselected due to their significant difference in water content in order toexplore the performance of the computational code written using theenthalpy-Kirchhoff formulation. Another important reason is becausecooked minced meat and dough are both present in several ready-toeatmeals, therefore contributing valuable information to food processorsinterested in optimizing cooling and freezing operating conditions ofsemi- or fully processed goods. The objectives of this work are (1) toexperimentally determine by differential scanning calorimetry (DSC)the thermo-physical properties of dough and cooked minced meat inthe freezing range: specific heat as a function of temperature, boundwater, heat of melting, initial freezing temperature, etc.; (2) to developand validate a finite element algorithm to simulate the freezing processin regular and irregularly shaped foodstuffs; and (3) to introduceappropriate equations of the thermo-physical properties in the numericalprogram to assess the effect of total water content, bound water, andsurface heat transfer coefficient on freezing times in an irregular foodsystem.