CONICET | Buscador de Institutos y Recursos Humanos

Numerical simulations of the heat transfer during freezing of foodstuffs require thermo-physical properties of the product as input data 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 in freezing, these properties undergo abrupt changes with temperature. As the amount of water in the foodstuff increases, the non-linearities of the functional relationships grow sharper and the specific heat becomes a quasi Delta Dirac function. 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. The computational code was implemented using transformations of the variables in the heat conduction partial differential equation, combining enthalpy and Kirchhoff formulation. Two products were selected to simulate the freezing process: cooked minced meat and dough, due to their different humidities (66 and 33% water content respectively). The specific heat of these products were determined using Differential Scanning Calorimetry (DSC). The amount of bound water and the initial freezing temperature were estimated from the endothermic DSC curve. Measured thermal properties were used as inputs in the numerical code in order to check the rate of convergence of the method, and to calculate the freezing time of foodstuffs. The numerical predictions were compared to experimental time-temperature curves during freezing of finite cylinders of cooked minced meat or dough. The interface heat transfer coefficient of the systems were estimated using aluminium prototypes considering constant thermo-physical properties in the analyzed temperature range (-50 to 30 ºC). Thermocouples type T were inserted into the finite cylinders of food materials in order to record the time- temperature curves during freezing in a tunnel. The experimental data satisfactorily agreed with the numerical predictions. Furthermore the numerical code was used to simulate the freezing of a dough product (croissant) which has an irregular three dimensional shape and a good agreement between experimental temperatures and numerical predictions were obtained for the freezing process.