Secagem de cafés despolpados em pressões sub-atmosféicas

Abstract

Drying is a process of great importance in the final quality of the coffee beverage. Despite being a routine operation, conditions such as exposure of the product to high temperatures can damage its quality. Moreover, it is the stage of highest energy demand, therefore a process with high costs. Among the types of drying of agricultural products, vacuum drying allows evaporation of water at low temperatures with increasing water reduction rate and drying time reduction. Moreover, the deteriorating media decrease due to the low oxygen concentration in the atmosphere of the drying process. Thus, the objective was to evaluate the kinetics and physiological effects caused by vacuum drying of pulped coffee, as well as to propose a drying model for the vacuum process. The present work was carried out at the Agricultural Product Processing Laboratory of the Engineering Department of the Federal University of Lavras. The coffee fruits (Coffea arabica L. cv. Topázio MG 1190) were harvested in the UFLA experimental field manually and selectively. These were mechanically peeled and subsequently pulped. After this process, the parchment coffee was dried in a vacuum oven under absolute pressures of 747 (atmospheric pressure), 447, 347, 247 and 147 mmHg and drying air temperature of 35 ± 1 and 40 ± 1oC. For the adjustment of the mathematical models, nonlinear regression analyzes were performed by the Gauss-Newton method, using the STATISTICA 5.0® software (Statsoft, Tulsa, USA). The choice of the best model was a function of the statistical parameters: estimate standard deviation (SE), relative mean error (P) and coefficient of determination (R2

). To analyze the physiological effects, the coffees were evaluated by scanning electron microscopy. The Midilli model was one of the best fit for the pulped coffee for the two temperatures under study at the local (atmospheric) pressure condition. After selecting the best model, it was modified to suit the proposed vacuum drying conditions. An Arrhenius equation was used for its modification. The results shown confirm the satisfactory fit of this model to describe the vacuum drying kinetics. Through the micrographs it was possible to observe the difference of the grain cell wall according to the vacuum reduction. For the absolute pressure of 347 mmHg, little damage to the cell membranes was noted, thus providing a suitable condition for the drying of pulped coffee, reducing up to 19% in the drying time in relation to the local (atmospheric) pressure. It was observed a reduction of approximately 50% in the drying time to the lowest absolute pressure used, however, this was where the greatest cell damage was observed.