Effect of water content on thermo-physical properties and freezing times of foods.

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

QUERETARO

Simposio; 11 International Symposium of the Properties of Water. ISOPOW 11; 2010

ISOPOW SCIENTIFIC COMMITTEE.

Numerical simulations of the heat transfer during freezing of foodstuffsrequire thermo-physical properties of the product as input data in order to obtainaccurate predictions. Thermo-physical properties, such as specific heat, thermalconductivity, initial freezing temperature, density, etc. are strongly affected bythe water content. When the process includes a phase change transition suchas in freezing, these properties undergo abrupt changes with temperature. Asthe amount of water in the foodstuff increases, the non-linearities of thefunctional relationships grow sharper and the specific heat becomes a quasiDelta Dirac function. The mathematical problem that represents the process istherefore highly non-linear.A finite element formulation was developed to numerically simulate thefreezing of foodstuffs with different water contents. The computational code wasimplemented using transformations of the variables in the heat conductionpartial differential equation, combining enthalpy and Kirchhoff formulation. Twoproducts were selected to simulate the freezing process: cooked minced meatand dough, due to their different humidities (66 and 33% water contentrespectively). The specific heat of these products were determined usingDifferential Scanning Calorimetry (DSC). The amount of bound water and theinitial freezing temperature were estimated from the endothermic DSC curve.Measured thermal properties were used as inputs in the numerical code in orderto check the rate of convergence of the method, and to calculate the freezingtime of foodstuffs. The numerical predictions were compared to experimentaltime-temperature curves during freezing of finite cylinders of cooked mincedmeat or dough. The interface heat transfer coefficient of the systems wereestimated using aluminium prototypes considering constant thermo-physicalproperties in the analyzed temperature range (-50 to 30 ºC). Thermocouplestype T were inserted into the finite cylinders of food materials in order to recordthe time- temperature curves during freezing in a tunnel. The experimental datasatisfactorily agreed with the numerical predictions. Furthermore the numericalcode was used to simulate the freezing of a dough product (croissant) whichhas an irregular three dimensional shape and a good agreement betweenexperimental temperatures and numerical predictions were obtained for thefreezing process.