Vacuum drying of peeled coffee cherry beans: Drying kinetics and physiological effects

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. Several physical, physiological and biochemical changes can occur in the grains during drying. Physiological analyzes have been used as indicators of this quality. 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. The fruits were harvested at their maximum maturation potential and submitted to the removal of the peel by a mechanical process. The peeled fruits were dried in a vacuum oven under absolute pressures of 147, 447 and 747 mmHg (at the local atmospheric pressure of 747 mmHg) at 40 °C, until reaching 11 kg of water/kg of material. After drying, the grains were stored under controlled conditions of refrigerated air (10 °C and 50% relative humidity) for 30 days, standardizing drying. Coffee grains were evaluated by scanning electron microscopy and drying kinetics. 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 best fit was Page Modified. Drying under absolute pressure of 447 mmHg was an interesting alternative to the conventional one, as it did not damage the cellular structures of the grain, which was similar to that obtained at the local pressure of 747 mmHg. However, the lower pressure, 147 mmHg, led to significant changes in grain quality.