Incertezas na determinação de propriedades termofísicas de maçãs durante o resfriamento.

Abstract

The present work aimed to determine the thermophysical properties of apples during cooling, as well as to evaluate the effect of storage time on the maintenance of physical, chemical and bioactive compounds characteristics kept at room temperature and refrigerated. The Fuji apple was subjected to cooling kinetics in a natural convection household refrigerator and its thermophysical properties were determined assuming the geometry of an equivalent sphere. The position of the thermocouple inside the apple was not determined during the experiment and is therefore unknown and determined using analytical and numerical solutions. A heat conduction software (Solver) was developed to solve the inverse problem, using the spherical geometry of the diffusion equation and the boundary condition of the third type, obtaining the values of the thermal properties and their uncertainties. The direct problem was the solution of the partial differential equation, where from the developed solver it was possible to simulate the cooling kinetics of a previously specified point within the sphere. Finally, a storage study of apples at room temperature (25 °C) and refrigeration temperature (4 °C) was carried out. The physical and chemical parameters of bioactive compounds were evaluated at 7-day intervals until they were rotten. The mean values of diffusivity and convective coefficient of heat transfer were (@ = 1.42 ± 0.11) × 10-7 m2.s-1 and (@ = 3.196 ± 0.039) × 10-6 m2.s-1, respectively. Since the diffusivity value obtained by the proposed model was consistent with the value obtained from the Riedel correlation estimate ( = 1.38 x 10-7 m2.s-1). The adjustment of the simulated curve to the experimental points presented coefficient of determination (R2) greater than 0.99 and low values of the chi-square function ( @ 2 ). The thermocouple position in the equivalent sphere by numerical and analytical solution was r = 6.6 mm and r = 7.7 mm, respectively. Presenting a difference of 14.3% between the two methods used. It was observed during the storage under the two studied conditions an increase in the total soluble solids contents, total titratable acidity, water activity, humidity, anthocyanins, flavonoids and chromatic coordinates (a*, b* and C*); reduction in pH, firmness, color coordinate (L*), vitamin C, total phenolic compounds and chlorophyll (“a”, “b” and total). Therefore, it can be concluded that the spherical geometry and the boundary condition of the third type was adequate to describe the heat transfer process for the apple and cooling was a viable technique for postharvest conservation of apples, allowing a storage up to 42 days with greater preservation of bioactive compounds.