Secagem a vácuo de cafés descascados: cinética de secagem e efeitos fisiológicos

Abstract

The drying process is a very important step for a large part of the grains, because, in general, these products are harvested with high moisture contents, which favors rapid deterioration. When not properly performed, drying can cause damage to the products, consequently causing quality decrease. Several physical, physiological and biochemical changes can occur in the grains during drying. Physiological analyzes have been used as indicators of this quality. Traditionally, mechanical drying with heated air is used for coffee grains. However, due to its low efficiency, alternatives that allow reducing the drying time without changing the quality of the coffee are required. Considering that vacuum drying provides smaller exposure times by reducing the vapor pressure, this study aimed at assessing the drying kinetics and the physiological effects caused in peeled coffee beans, when subjected to vacuum drying. Coffee fruits were studied at the maturation stage and processed by wet conventional methods (parchment, peeled). After processing, the grains were dried in a vacuum oven under absolute pressures of 147, 447 and 747 mmHg (at local atmospheric pressure of 747 mmHg) at 40 °C, until reaching 11 kg of water / kg of material. After drying, the samples were stored under controlled conditions of refrigerated air (10 ºC and 50% relative humidity) for 30 days in order to obtain uniform moisture content of the fruits. Coffee grains were evaluated by electrical conductivity, potassium leaching, germination, scanning electron microscopy and drying kinetics. Multivariate exploratory principal component analysis (PCA) was used for the physiological analyzes, with the objective of correlating the effects of the physiological analyzes with the used vacuum pressures. For the adjustment of the mathematical models, a non-linear regression analysis was performed using the Quasi-Newton method. It was concluded that the use of vacuum significantly reduced the drying time. The model that better adjusted was the Page Modified. Drying under absolute pressure of 447 mmHg was an interesting alternative to the conventional one because it showed no damage to the cellular structures of the grain, which was similar to the obtained in the local pressure of 747 mmHg. However, the lowest pressure, 147 mmHg, led to significant alterations in the grain quality.