Secagem de frutos verdes de café em diferentes temperaturas de bulbo seco e temperatura de ponto de orvalho

Abstract

The coffee harvest in Brazil is predominantly non-selective, by the complete stripping method, and due to the inequality of maturation, it results in a large portion of green fruits. One of the main stages of the post-harvest process is drying and this must be well established for this type of fruit, as they are more sensitive to this stage of processing. The general objective of this work was to evaluate the effect of drying at different dry bulb temperatures in combination with different dew point temperatures on green coffee fruits and on the quality of the grains. The unripe fruits were harvested manually and selectively, and had an average initial water content of 2.50 ± 0,08 kg.kg -1 (b.s.). Drying was carried out in a fixed layer dryer coupled to a composite air conditioning system in which the drying air was controlled with an air flow of 20 m3min-1m-2, at temperatures used were three dry bulb temperatures (30 °C, 35 ° C and 40 °C) in combination of 4 dew point temperatures (7.5 °C, 11.2 °C, 16.2 °C and 20.4 °C). The unripe fruits were continuously dried to an average water content of 0.139 dry basis (ds). Models of drying kinetics were tested on the experimental data, evaluated the endosperms of the grains with scanning electron microscopy, analyzed the number of black-green defects, changes in color, electrical conductivity of the grain exudates and quantification of trigonelline, chlorogenic acids and caffeine. The Andrade model showed the best fit, except for treatments with Tbs of 40°C and Tpo of 7.5°C and Tpo of 11.2°C, which were the Valcam and Midilli models respectively. It was verified by means of scanning electron microscopy (SEM) that damage occurred in the green beans with the increase of temperature and in the black-green beans their cell structure is always well degraded regardless of the drying treatments performed. The smallest amounts of black-green defects were found at the lowest drying temperatures, with these grains presenting the highest values of L* and b*, and the lowest values of a*; also, lower values of electrical conductivity and chlorogenic acids.