Acta Scientiarum: Agronomy (Jul 2011)
Modeling of the shrinkage kinetics of coffee berries during drying = Modelagem da cinética de contração de frutos de café durante a secagem
Abstract
The effects of drying conditions on the shrinkage of Coffea arabica L., cultivar Catuai Vermelho, were studied. An exponential model derived from the kinetic model was used to describe the shrinkage kinetics. In addition, an equation was proposed to describe the relationship between the superficial areas and weights of the coffee berries. Coffee berries were dried at temperatures (T) of 35, 45, 55 and 65oC and at relative humidities (ƒÓ) of 25, 35, 45 and 55%, forming 16 different sets of conditions, with an initial moisturecontent for all conditions of 2.27 dry basis (d.b.), dried until 0.11 d.b. Changes in the product dimensions were measured continuously during drying. Surface area-to-volume ratio values increased with moisture content decrease. The model was suitable for representing the shrinkage kinetics, as well the empirical equation for predicting surface area. Temperature had a greater influence over the constant rate of change of the characteristic dimension (k values) than ƒÓ. Activation energies were also determined to have magnitudes of 39.31, 37.32, 36.28 and 36.22 kJ mol-1 for relative humidities of 25, 35, 45and 55%, respectively. Neste trabalho foi estudado o efeito das condicoes de secagem sobre a contracao de frutos de cafe (Coffea arabica L.), cultivar Catuai Vermelho. Um modelo exponencial derivado do modelo cinetico foi utilizado para descrever a cinetica da contracao. Ademais, uma equacao foi proposta para descrever a relacao entre a area superficial e a massa dos frutos de cafe. Frutos de cafe foram secos as temperaturas de (T) de 35, 45, 55 e 65oC, juntamente com umidades relativas (ƒÓ) de 25, 35, 45 e 55%, totalizando 16 condicoesdiferentes, com um teor de agua inicial de 2,27 base seca (b.s.), secos ate 0,11 b.s. As alteracoes nas dimensoes do produto foram mensuradas continuamente durante a secagem. Os valores de relacao superficie-volume aumentaram com a diminuicao do teor de agua. Omodelo foi adequado para representar a cinetica da contracao, bem como a equacao empirica para predicao da area superficial. A temperatura teve maior influencia sobre a taxa constante de mudanca da dimensao (k) que ƒÓ. Energias de ativacao foram obtidas, commagnitudes de 39,31; 37,32; 36,28 e 36,22 kJ mol-1 para umidades relativas de 25, 35, 45 e 55%, respectivamente.
