花椒吸附等温线及热力学性质

Adsorption Isotherms and Thermodynamic Properties of Zanthoxylum bungeanum Seeds

采用静态称质量法,测定花椒在20、30、40℃,水分活度为0.005~0.982条件下的吸湿特性,绘制出花椒的吸附等温线。结果表明,花椒吸附等温线属于Ⅲ型等温线,在同一温度下,花椒的平衡含水率随水分活度的增加而增加。用8种经典吸附模型对其进行拟合比较,Oswin模型效果最优,其次是GAB模型。Oswin模型的决定系数R2达到了0.999,平均相对误差E值为4.11%~8.13%。根据上述模型得到花椒的相对安全含水率与绝对安全含水率约为0.06 kg/kg与0.045 kg/kg。热力学性质显示,花椒属于毛细管多孔特性;当花椒的含水率超过0.08 kg/kg时,水同物料的结合能及净等量吸附热较低,花椒中的水分较易去除;花椒的平均单位质容量的范围为13~18 mol/J,可以根据平均单位质容量来选择最合理的预处理方法和干燥方法。

Hygroscopic property is one of the important parameters to study the drying and storage of Zanthoxylum bungeanum seeds. In this work, the adsorption isotherms of Zanthoxylum bungeanum seeds were determined at 20, 30 and 40 ℃ over a water activity range of 0.005–0.982 using the static gravimetric method. These isotherms showed that the moisture adsorption isotherm of Zanthoxylum bungeanum seeds belonged to type III isotherm, suggesting that at a certain temperature, the equilibrium moisture content increases with increasing water activity. Among 8 classical mathematical models, Oswin model was the best model to describe the isothermal moisture adsorption process with coefficient of determination（R2） of 0.999 and average relative error（E） of 4.11%–8.13%, followed by GAB model. Based on these models, the relatively safe and the absolutely safe moisture content in Zanthoxylum bungeanum seeds were 0.06 and 0.045 kg/kg, respectively. The thermodynamic properties showed that Zanthoxylum bungeanum seeds had capillary porosity. The binding energy of water to the seeds with a moisture content of more than 0.08 kg/kg and the net isosteric heat of adsorption were low so that the water was easy to remove. The average capacity per unit mass of the seeds ranged from 13 to 18 mol/J, and the most suitable pretreatment method and drying method could be achieved according to this parameter.