Abstract
This article compares diffusion models used to describe seedless grape drying at low temperature. The models were analyzed, assuming the following characteristics of the drying process: boundary conditions of the first and the third kind; constant and variable volume, V; constant and variable effective mass diffusivity, D; constant convective mass transfer coefficient, h. Solutions of the diffusion equation (analytical and numerical) were used to determine D and h for experimental data of seedless grape drying. Comparison of simulations of drying kinetics indicates that the best model should consider: 1) shrinkage; 2) convective boundary condition; 3) variable effective mass diffusivity. For the analyzed experimental dataset, the best function to represent the effective mass diffusivity is a hyperbolic cosine. In this case, the statistical indicators of the simulation can be considered excellent (the determination coefficient is R2 = 0.9999 and the chi-square is χ2 = 3.241 × 10–4).
Highlights
After harvest, the time of fruit conservation under natural conditions is limited to only a few days
The mass transfer coefficient was kept constant at h = 1 × 10+10 m·s–1, which corresponds to a boundary condition of the first kind
An analysis of the all results indicates that the liquid diffusion model well describes drying of seedless grapes, with pretreatment through dipping in hot water, using air in low temperatures
Summary
The time of fruit conservation under natural conditions is limited to only a few days. For instance, belong to the most perishable fruits. They are very susceptible to microbial decay and moisture loss. The life time of the product is much higher than in the first case, beyond resulting in a food very appreciated in several parts of the world: raisins. The drying process of grapes under natural conditions [1] is generally slow due to the resistance of its skin. The demand for natural foods, without adding chemicals, is increasing in many parts of the world. Alternatives to chemical additives are used as pretreatment for the production of raisins, for instance: abrasion method [7] and dipping in hot water [6, 8]
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