The present study aimed to assess the drying kinetics of black rice and fit different mathematical models (empirical and diffusive) to the experimental data, and evaluate the effect of drying air temperature on the physical-chemical and bioactive compounds quality of black rice. Drying air temperatures ranged from 40 to 80 ºC and the drying air speed was 1.5 m/s. Physical-chemical characterization of the product was based on the following parameters: moisture, water activity, ashes, total protein content, pH, total acidity, lipids, total carbohydrates, total anthocyanins, flavonoids, total phenolic compounds and antioxidant activity. Among the empirical models, Page showed the lowest mean squared deviations (MSD) and highest coefficients of determination (R2). For the diffusion model, the values of effective mass diffusivity and convective heat transfer coefficient increased with increasing drying air temperature, and the Biot number indicated that the first-type boundary condition would also describe well the drying process. Physical-chemical parameters and bioactive compounds differed between the temperatures used, and the temperature of 60 ºC led to the best relationship between drying time and preservation of product characteristics.

Many times, the thermal properties of a product are determined but their uncertainties (and, mainly, the covariance matrix) are not provided. Thus, in the simulations, it is not possible to establish a confidence band for a transient state described through the values obtained for these properties. In this article, a model was proposed to determine thermal diffusivity and convective heat transfer coefficient, providing the above-mentioned lack of information, for a product with spherical geometry during its cooling. The proposed model involved: 1) an experimental data set of the cooling kinetics in a point within the product; 2) a one-dimensional numerical solution of the heat conduction equation; 3) an optimizer based on the Levenberg-Marquardt algorithm to determine the thermal properties, their uncertainties, and the covariance between the parameters. Model was applied for determining thermal properties of strawberries, using an equivalent sphere to represent the geometry of the product, and the obtained results were compatible with the literature results.