Propriedades termofísicas e descrição do resfriamento de produtos agrícolas com geometria cilíndrica.

Abstract

Because it is necessary to extend the shelf life of agricultural products after harvesting, several techniques are available in the literature for food preservation. Before applying one of these techniques, sometimes these products are submitted to a minimum processing. The minimal processing consists of a set of operations that eliminate the unusually consumed parts of agricultural products, reducing them to smaller portions by cutting so that they are ready for immediate consumption or subsequent preparation, and at the same time maintain their sensory characteristics as in the fresh state. Despite the advantages inherent to this practice, for example, to bring practicality to the consumer and to minimize losses, this is responsible for the first physical alterations of the food tissue, which contribute to its perishability and consequent reduction of its useful life. However, by cooling the product after minimal processing, its shelf life can be significantly increased. In other cases, some agricultural products requires the extension of durability keeping their characteristics fresh as much as possible, and in these situations the method of cooling can also be used. The design of refrigeration equipment and the calculation of the cost associated with the cooling of agricultural products requires that some thermophysical properties of these products be known. This also makes it possible to describe the cooling kinetics of the least favorable point in the interior, which is the point that reaches the desired temperature last. Therefore, the objective of this work is to develop an experimental methodology and a computer program that enables the determination of all thermophysical parameters required to describe the cooling kinetics of products with cylindrical geometry. Specifically, the program was applied in the study of the cooling of carrot (Daucus carota L.) cuts, cassava (Manihot esculenta Crantz) cuts, whole bananas (Musa spp) with peel and cucumber (Cucumis sativus L.), with previously defined dimensions. A solver was developed for the direct problem, using an analytical solution of the two-dimensional diffusion equation for the finite cylinder, with boundary condition of the third kind. For the inverse problem, an experimental dataset of the temperature over time at the center of products; and a ready-to-use optimizer software were used. The solver should also be able to predict spatial temperature distribution at any time during cooling and estimate the time of this process for cylinders with other dimensions. In addition, the solver can also be used to describe product pasteurization processes. The values obtained for the convective heat transfer coefficient and the thermal diffusivity of the products are consistent with the values obtained from the literature and from the Riedel estimate. This work also proposes an experimental methodology to determine the specific heat of carrot, using two curve fittings for the function that describes Newton's law of cooling, with small extrapolations, obtaining the average value of the quantity and its uncertainty; by propagation of errors.