Adsorption isotherms and isosteric heat in Coffea arabica beans

ABSTRACT The knowledge of the adsorption hygroscopic equilibrium in coffee is pertinent for the adequacy of its storage, thus ensuring the quality of the product to the end consumer. The present study aimed to determine the adsorption isotherms of Arabica coffee (Acauã Novo cultivar) beans at the temperatures of 30 ºC, 40 ºC, 50 ºC and 60 ºC, as well as the integral adsorption isosteric heat. The equilibrium moisture contents were obtained by the gravimetric static method, using water activities between 0.10 and 0.83 (decimal). From the obtained data, several mathematical models were fitted to represent the adsorption isotherms. The statistical criteria for selecting the best model were the coefficient of determination, estimated mean standard error, relative mean error and distribution of residues. The Peleg model was the one that best represented the adsorption isotherms of the Arabica coffee beans under the studied conditions. The isotherms showed a sigmoidal type II shape. For a constant water activity, an increase in the temperature promotes a decrease in the equilibrium moisture content of Arabica coffee beans. In a moisture content range between 0.0191 and 0.2534 (dry base), the isosteric heat values ranged from 4,042.19 kJ kg-1 to 2,395.48 kJ kg-1.

摘要
咖啡吸附吸湿平衡（adsorption hygroscopic equilibrium）的相关知识对于其仓储优化至关重要，由此可保障终端消费者所购产品的品质。本研究旨在测定30℃、40℃、50℃及60℃条件下，阿拉比卡（Arabica）咖啡（Acauã Novo品种）生豆的吸附等温线（adsorption isotherms）与积分吸附等排热（integral adsorption isosteric heat）。本研究采用重量静态法（gravimetric static method），在0.10至0.83（十进制）的水活度（water activities）范围内测定平衡含水率（equilibrium moisture contents）。基于所得实验数据，本研究拟合多款数学模型（mathematical models）以表征吸附等温线。筛选最优模型的统计准则包括决定系数（coefficient of determination）、估计平均标准误差（estimated mean standard error）、相对平均误差（relative mean error）及残差分布（distribution of residues）。在本研究的实验条件下，Peleg模型为最优拟合阿拉比卡咖啡生豆吸附等温线的模型。所得吸附等温线呈现S型II型曲线形态。当水活度固定时，温度升高会导致阿拉比卡咖啡生豆的平衡含水率降低。在干基（dry base）含水率0.0191至0.2534的范围内，吸附等排热数值介于4042.19 kJ·kg⁻¹至2395.48 kJ·kg⁻¹之间。