Effect of Water Content on Thermo-Physical Properties and Freezing Times of Foods

Abstract

For the prediction of temperature change in different foodstuffs during the freezing and thawing processes accurate estimation of the thermo-physical properties of the product is necessary such as specific heat, density, and freezable water content, enthalpy, initial freezing temperature, etc. These data allow the adequate design and optimization of equipment and processes. Water is a main component in all foods and greatly influences the behaviour of these properties depending on its concentration. During the freezing process, which involves the phase change of water into ice, the specific heat, thermal conductivity, and density undergo abrupt changes due to the latent heat release. This complex process doesn?t have an analytical solution and it can be described as a highly non-linear mathematical problem. Many difficulties arise when trying to numerically simulate the freezing process especially when using the finite element method (fem) which is especially useful when dealing with irregular shaped foodstuffs. Several techniques were applied to consider the large latent heat release when using the fem. One of the traditional methods is the use of the apparent specific heat, where the sensible heat is merged with the latent heat to produce a specific heat curve with a large peak around the freezing point, that can be considered a quasi delta-Dirac function with temperature (depending on the amount of water in the food product) (Pham, 2008). However this method usually destabilizes the numerical solution. The implementation of the enthalpy method, which can be obtained through the integration of the specific heat with temperature (Comini, 1990, Pham, 2008, Fikiin, 1996, Santos et al, 2010), and the Kirchhoff function, which is the integral of the thermal conductivity, allows the reformulation of the heat transfer differential equation into a transformed partial differential system with two mutually related dependent variables H (enthalpy) and E (Kirchhoff function) (Sheerlinck et al., 2001). These functions H and E versus temperature are smoother mathematical functions compared to the specific heat, thermal conductivity and density versus temperature, avoiding inaccuracies and/or divergence of the numerical method. Even though it brings great advantage to the resolution of the problem, with the simultaneous enhancement of the computational speed of the program, this transformation of variables is not widely used in literature. Unleavened dough and cooked minced meat were selected due to their significant difference in water content in order to explore the performance of the computational code written using the enthalpy-Kirchhoff formulation. Another important reason is because cooked minced meat and dough are both present in several ready to eat meals, therefore contributing valuable information to food processors interested in optimizing cooling and freezing operating conditions of semi- or fully processed goods. Moreover the called ?Bake Off Technology? (BOT) which consist of dough refrigeration and or freezing and then transportation to the small shops where the product is baked. The objectives of the present chapter were: 1) to experimentally determine by Differential Scanning Calorimetry the thermo-physical properties of dough and cooked minced meat in the freezing range: specific heat as a function of temperature, bound water, heat of melting, initial freezing temperature, etc. 2) to develop and validate a finite element algorithm to simulate the freezing process in regular and irregular shaped foodstuffs 3) to introduce appropriate equations of the thermo-physical properties in the numerical program to assess the effect of total water content, bound water, and surface heat transfer coefficient on freezing times in an irregular food system