中文摘要：

为可视化果蔬干燥过程中内部水分传递现象,利用低场核磁共振成像技术（magnetic resonance imaging,MRI）研究了圆柱状胡萝卜40、70℃热风干燥过程中内部水分传递过程,获得了物料收缩状态下水分廓线特征和变化规律。研究表明,干燥过程中,胡萝卜样品的水分廓线沿径向、轴向同时向中心不规则收缩,其内部的水分传递是一个多维、非稳态传递过程,并具有non-Fickian传递特征;干燥初始,水分梯度在物料表面迅速形成。随着干燥的进行,物料干基含水率低于7.33kg/kg时,其湿区直径收缩比率大于实测直径收缩比率,表面成为＂干区＂,干湿界面退缩到物料内部。圆柱状胡萝卜的热风干燥过程可以用Henderson-Pabis模型进行描述,所建水分传递模型可以很好地模拟70℃干燥试验结果,最大相对误差为7.69%,出现在干燥最后阶段,其他时刻相对误差低于4%;物料中心层水分传递模型可以很好地预测40℃干燥试验过程。研究结果可以为干燥工艺的选择以及物料收缩状态下水分传递过程的理论模拟提供支持。

英文摘要：

The drying process is one of the most important processes in food engineering.Heat and moisture transfer in fruits and vegetables during drying is a complex process,and knowledge of the moisture profile is fundamentally important for industrial processes,because the quality aspects of dried foodstuffs,such as nutrient content,safety and weight,are related to moisture content.A better understanding of the mechanism of moisture transfer should help improve product quality and the efficiency of the drying process.Such information will also help produce realistic computer simulations of drying processes and Fickian and non-Fickian models of moisture transport.Moisture profiles in foodstuffs can be measured using destructive and non-destructive methods.Slicing and freezing is not an accurate method because of low precision and moisture loss during cutting process,although it could give inside moisture distribution.In recent years,the applicability of nuclear magnetic resonance imaging（MRI） to measure mass transport phenomena in porous systems,especially in foodstuffs and biological materials,including drying processes,has been demonstrated.NMR imaging as a non-destructive,non-invasive,promising technique has been used to obtain moisture profiles during drying of vegetables and fruits.In this paper,the moisture transport in a cylindrical carrot sample was visualized and studied using nuclear magnetic resonance images obtained from the drying experiments.The transient moisture profile distributions in carrot were measured during the hot-air drying process with air temperatures of 40 and 70℃,respectively.Experimental results showed that the moisture profiles moved irregularly toward the center of the carrot sample in both the axial and radial directions,indicating a muti-dimensional and unsteady-state mass transfer process that has non-Fickian moisture transport characteristics.In the initial drying stage,a sharp moisture gradient was found indicating significant moisture flux at the surface of the carrot.With the