Origem de defeito preto-verde na secagem artificial de cafés imaturos para diferentes condições de temperatura de bulbo seco e temperatura de ponto de orvalho

Abstract

The unevenness of maturation of coffee makes a great part of green fruits in the harvest period. The postharvest seeks management solutions so as to minimize defects during the drying process, in this case, black-green defects, from those fruits harvested immature. The objective of this work was to investigate the minimal quantification of the occurrence of black-green defects in the drying of thin-layer immature coffees, subjected to different temperature conditions and relative humidity of the drying air. In addition to adjusting mathematical models to the experimental data, as well as determining the effective diffusion coefficient, the water reduction rate (WRR) and the colorimetry of the grains after drying. A total of 9 coffee crops (Coffea arabica L.) of the Topázio Amarelo variety were harvested manually and selectively at the green maturation stage, with an initial water content of 2,106 ± 0,05 kg.kg-1 (bs). After the dry processing, coffee from each of these harvests was subjected to a drying treatment resulting from the combination of dry bulb temperatures (35, 40, 45 °C) and dew point temperatures (2.6, 10.8, 16.2 °C), up to the final water content of 0.124 ± 0.05 kg.kg-1 (bs), in a fixed layer dryer conjugated to a Laboratory Air Conditioning System (SCAL). In addition to drying kinetics, WRR and effective diffusivity were evaluated. After drying, the samples were separated into two parts, one with black-green and sour defects, the other with the other grain. The lowest percentage of black-green defects occurred at a temperature of 35 °C, combined with a dew point temperature of 2.6 °C (11.00%). This is the most suitable treatment for the drying of natural green coffees. On the other hand, the highest percentage of this defect occurred in the combination of Tbs of 35 °C with Tpo of 16.2 °C (14.17%). The combination of Tbs 35 °C and Tpo 16.2 °C had the lowest effective diffusion coefficient of 0.551 x 10-11 m2 s -1. The Midilli model presented the best adjustments to the experimental data for all drying combinations. The lowest WRR was 0.063 kg.kg-1.h-1 and occurred in the combination of Tbs of 35 °C with Tpo of 16.2 °C. In the portion of coffee without defects, among the treatments carried out in the Tbs of 35 °C, the combination with the Tpo of 2,6 °C showed the lowest values of "L". For the "a" coordinate, the lowest values are found in the combination of Tbs of 35 ° C with Tpo of 2.6 °C.