超声强化真空干燥全蛋液的干燥特性与动力学模型

Drying Characteristics and Kinetic Model of Liquid Whole Egg during Ultrasound-Reinforced Vacuum Drying

为研究超声对真空干燥黏稠食品物料的强化效应,搭建了一套真空超声干燥设备。以全蛋液为研究对象,进行超声强化真空干燥实验,探讨超声声能密度、超声作用时间、干燥温度对全蛋液干燥特性及微观结构的影响,并建立动力学模型。结果表明:超声波作用可强化物料内部传质过程,提高干燥速率,且超声强化效应随着声能密度的增大而增强。此外,超声处理时间不宜过长,当干燥温度为50℃,超声声能密度为2.0 W/g持续作用2.5 h之后,进一步延长超声作用时间对全蛋液干燥过程的强化效果不明显。扫描电子显微镜结果发现,超声处理会使物料组织间隙增大、连通性增强,同时形成更多的微细孔道,降低水分扩散阻力。对9种薄层干燥数学模型进行实验数据的非线性拟合分析,结果显示:Page模型的决定系数R2均大于0.99,均方根误差和残差平方和均小于0.01,拟合效果最好。因此,Page模型可用来描述全蛋液超声真空干燥过程中水分比的变化规律。以Fick扩散定律为依据,确定全蛋液干燥传热传质有效水分扩散系数(D_(eff))的变化范围为:1.645 6×10^(-9)~6.549 7×10^(-9) m^2/s,且随着温度及超声声能密度的增大而增大。由Arrhenius方程建立有效水分扩散系数与温度的关系,得到全蛋液水分活化能(E_a)为16.151 2 k J/mol。实验结果可为全蛋液真空超声干燥工艺参数优化及生产控制提供理论依据。

A new ultrasound-reinforced vacuum drying device was built to investigate the enhancing effect of ultrasound on the vacuum drying of viscous food. Ultrasound-reinforced vacuum drying of liquid whole egg was carried out to explore the influence of drying temperature, ultrasound energy density and irradiation time on the drying characteristics and microstructure. Furthermore, a kinetic model was established. The results showed that the internal mass transfer process could be enhanced by applying ultrasound to vacuum drying. Meanwhile, the drying rate was improved with the increase of ultrasound energy density. The drying process of liquid egg at 50 ℃ was no longer affected by ultrasound treatment at an energy density of 2.0 W/g after 2.5 h. The results of scanning electron microscopy（SEM） showed that many microtunnels were observed in the microstructure of powdered eggs, which was beneficial to enhancing the connectivity and consequently reducing water migration resistance. Nine different thin layer drying models were fitted nonlinearly with the experimental data. Comparison of correlation coefficient（R^2）, root mean square error（RMSE） and reduced chisquare（χ~2） and verification experiments showed that R^2 of the Page model was greater than 0.99, RMSE and χ~2 were less than 0.01. Then, the Page model was the most suitable to describe the relationship between liquid egg moisture and drying time. The effective moisture diffusivity（Deff） was calculated according to Fick＇s second law to range from 1.645 6 × 10^（-9） to 6.549 7 × 10^（-9） m^2/s, which was increased with the increase of ultrasonic energy density. The linear relationship between the natural logarithm of effective moisture diffusivity and the reciprocal temperature was described by the Arrhenius empirical formula. The activation energy（Ea） was 16.151 2 k J/mol. These results can provide a valuable basis for optimizing and controlling ultrasound-reinforced vacuum drying of liquid whole egg.