基于自由体积理论分析面团膜中的水分扩散特性

Analyzing Water Diffusion Properties in Dough Film Based on the Free Volume Theory

以面团膜为对象,采用动态水分吸附分析仪测定其在20、40℃时9个相对湿度水平下的吸湿过程。采用Vrentas&Duda自由体积理论确定扩散系数与水分质量分数和温度的关系,根据水分含量随时间的变化,反演求解自由体积模型的参数。研究表明,自由体积理论确定的扩散系数在大部分水分质量浓度范围内能够很好模拟面团膜对水分的吸湿过程,但在低水分区域的预测值明显偏低,为此,在Vrentas&Duda模型中增加一个考虑水分含量的自由体积项,以对该模型进行改进。随水分含量增加扩散系数表现出先略有降低、然后增大再快速增大3个阶段。最后,从β-转变和自由体积变化的角度深入讨论了在低水分含量时水分对扩散系数产生影响的机理。

The water adsorption process by dough films at 20 and 40℃was measured using a dynamic vapor sorption system under nine relative humidity(RH)levels.Water diffusivity was expressed as a function of water content and temperature according to the Vrentas&Duda free volume theory.The parameters of the free volume model were obtained by inversion of water content as a function of time.The results showed that the water diffusion coefficient determined based on the free volume theory could well simulate the water adsorption process by dough films at most water contents tested,but the predicted values were significantly lower than the actual ones at low water contents.Therefore,the Vrentas&Duda was improved by adding a free volume term as a function of water content.As the water content increased,the water diffusion coefficient slightly decreased initially,then increased,and finally increased rapidly.Finally,the mechanism of water diffusion at low water contents was discussed from the perspectives ofβ-transition and free volume changes.