<p>The aim of this study was to determine the main thermal properties of the granular mass of coffee (specific heat, thermal conductivity, and thermal diffusivity) for different degrees of roasting, as well as to model and simulate thermal conductivity at different degrees of roasting. For determination of specific heat, the mixing method was used, and for thermal conductivity, the theoretically infinite cylinder method with a central heating source. Thermal diffusivity was simulated algebraically using the results of the properties cited above and of the apparent specific mass of the product. Thermal conductivity was also simulated and optimized through finite element analysis software. At darker roasting, there was an increase in specific heat and a reduction in thermal conductivity and thermal diffusivity. Comparing thermal conductivity determined in relation to simulated and optimized conductivity, the mean relative error was 1.02%, on average.</p>

This study evaluated colour variation as a function of the drying kinetics of immature coffee subjected to different drying conditions. Additionally, mathematical models were fitted to experimental data for green coffee berries dried in a thin layer under different drying conditions, besides the water reduction rate (WRR) was determined. Each of the drying environments was determined by the combination of three different dry bulb temperatures (Tdb) (35, 40 and 45 °C) and three different dew point temperatures (Tdp) (2.6, 10.8 and 16.2 °C). Coffee (Coffea arabica L.) berries of the Topázio Amarelo variety were collected manually and selectively with initial water content of 2.106 ± 0.05 kg.kg-1 (dry basis, db); they were processed by the dry-processing method and dried to a final water content of 0.124 ± 0.05 kg.kg-1 (db) in a fixed-bed dryer combined with a laboratory air conditioning system (LACS). Following this process, the samples were separated into two parts, one containing beans with black-green and sour defects and the other containing the remaining beans, and the colour was read for each group. For all drying combinations, the Midilli model best fit the experimental data. The lowest WRR was 0.063 kg.kg-1.h-1 and occurred in the combination with the Tdb of 35 °C and Tdp of 16.2 °C. In the portion of coffee without defects, among the treatments performed at the Tdb of 35 °C, the combination with the Tdp of 2.6 °C had the lowest luminance values. For the “a” coordinate, the lowest values were found in the combination of the Tdb of 35 °C and Tdp of 2.6 °C.

The inequality of coffee maturation leads to a large portion of green berries in the harvest. Post-harvest management techniques seek to minimize defects during the drying process, such as black-green defects in harvested immature berries. The present study aimed to investigate the minimum occurrence of black-green defects in the drying of immature coffee berries subjected to different temperature conditions and relative humidity values. In addition to fitting mathematical models to the experimental data, the effective diffusion coefficient and the water reduction rate (WRR) were determined. Nine coffee crops (Coffea arabica L.) of the Topázio Amarelo variety were harvested manually and selectively during the green maturation stage, with an initial water content of 2.106 ± 0.05 kg.kg -1 (dry basis, d.b.). After drying, the coffee was subjected to a drying treatment in a fixed-layer dryer with combined dry bulb temperatures (Dbt) of 35, 40 and 45 °C and dew point temperatures (Dpt) of 2.6, 10.8 and 16.2 °C until a final water content of 0.124 ± 0.05 kg.kg -1 (db) was reached. After drying, black-green defects were quantified as percentages. In addition to the drying kinetics, the WRR and effective diffusivity were evaluated. The lowest percentage of black-green defects occurred at a temperature of 35 °C and a Dpt of 2.6 °C (11.00%), which is the most suitable treatment for drying natural green coffees. The highest percentage of defects occurred when a Dbt of 35 °C was combined with a Dpt of 16.2 °C (14.17%). This combination showed the lowest effective diffusion coefficient of 0.551 x 10 -11 m 2 .s -1 . The Midilli model had the best fit to the experimental data for all drying combinations. The lowest WRR was 0.063 kg.kg -1 .h -1 and was observed when a Dbt of 35 °C was combined with a Dpt of 16.2 °C.