Moisture desorption isotherms of cucumber seeds: modeling and thermodynamic properties

Hygroscopic equilibrium curves or sorption isotherms are important in order to define dehydration limits of the product, estimate moisture content alterations under environment conditions and to acquire moisture content values for safe storage. This work aimed to determine desorption isotherms of cucumber seeds stored at different temperature (10, 20, 30, 40 and 50 ºC) and relative humidity (0.11 to 0.96) and the thermodynamic properties of this process. Mathematical models were fitted to experimental data in order to represent the agricultural products hygroscopicity. Net isosteric heat of desorption was performed based on the Clausius-Clapeyron thermodynamic relation and Gibbs free energy through the Van't Hoff equation. Based on statistical parameters, Modified Henderson model was the one that best represented hygroscopicity of cucumber seeds. Equilibrium moisture content of cucumber seeds decreased with temperature increment at a constant value of relative humidity. Differential enthalpy values, representing the energy required to remove moisture from the cucumber seeds, ranged from 2923 to 2469 kJ. kg-1. Values of differential entropy and Gibbs free energy decreased with moisture content increase. Enthalpy-entropy compensation theory is valid for the sorption of cucumber seeds, in which the water sorption mechanism in cucumber seeds can be considered to be enthalpy controlled.

吸湿平衡曲线或吸着等温线（sorption isotherms）对于确定产品的脱水极限、估算环境条件下的水分含量变化以及获取安全储存所需的水分含量值至关重要。本研究旨在测定不同温度（10、20、30、40和50 ℃）与相对湿度（0.11至0.96）下储存的黄瓜种子的解吸等温线（desorption isotherms），并分析该过程的热力学性质。为表征农产品的吸湿性特性，研究人员针对实验数据拟合了相应的数学模型。基于克劳修斯-克拉佩龙（Clausius-Clapeyron）热力学关系式以及通过范特霍夫（Van't Hoff）方程推导得到的吉布斯自由能，计算了黄瓜种子解吸过程的净等焓热。经统计参数对比验证，修正亨德森模型（Modified Henderson model）为最能表征黄瓜种子吸湿性的模型。在相对湿度恒定的条件下，黄瓜种子的平衡水分含量随温度升高而降低。用于脱除黄瓜种子中水分的微分焓值介于2923 至 2469 kJ·kg⁻¹之间。微分熵与吉布斯自由能的值随水分含量的升高而降低。焓-熵补偿理论适用于黄瓜种子的吸着过程，这表明黄瓜种子的水分吸着机制可被视为焓控过程。