Studies on Water Evaporation Phenomena During Vacuum Cooling Process of Leafy Vegetables

在日本目前真空冷却的典型新鲜果蔬中，选取西兰花、胡萝卜和菠菜作为实验材料。测量了这些材料在不同操作条件下的真空冷却特性。本文介绍了一种利用显微切片-图像数据处理系统（MSIDS）测定西兰花水分蒸发表面积和体积的新方法，并利用图像数据分析仪测定了蔬菜和胡萝卜的水分蒸发表面积和体积。因此，可以根据这些材料的质量来预测它们的表面积。从比表面积和冷却速率之间的关系的调查，生菜的冷却速率被发现比西兰花的比表面积大于莴苣的冷却速率快。结果表明，冷却速率不仅与被冷却材料的比表面积有关，而且与蒸发量有关， ...更多信息 这表明水从材料中蒸发的容易程度。提出了一种测定被冷却材料水蒸发系数的方法，并应用蒸发-蒸发法测定了被冷却材料的水蒸发系数。结果表明，在闪点到控制燃烧室压力这段时间内，该系数呈递减趋势在最小压力或水蒸发的稳定状态期间。在稳定状态期间，该系数保持恒定值，比自由水面的系数低约4倍。较低的值被解释为考虑在表面上的水的蒸发，也在内部组织的叶，这导致了一个更大的阻力，蒸发水的传质速率。随着冷负荷的增加，各系数值减小，表明其与真空室内水蒸气分压有关。采用本文提出的水分蒸发模型对叶菜类蔬菜真空冷却过程进行了模拟。少

As experimental materials, broccoli, lettce and carrot were selected among typical fresh fruis and vegetables which are vacuum-cooled in present in Japan. The vacuum cooling characteristics of these materials were measred under various operatinal conditions. A new method was developed to measure the water-evaporaion surface area and volume of a broccoli by using a microslicer-image data processing system (MSIDS) and these of lettce as well as carrot were also measured with an image data anlysis apparatus for a wide use. Thus it became possible to predict the surface areas of these materials based on their masses. From the investigations in relationships between the specific surface areas and cooling rates, the cooling rates of lettuce were found to be faster than these of brocoli whose specific surface areas were greater than lettuces. These results showed that the cooling rate depended on not only the specific surface area of the materials to be cooled but also the water-evaporation c … More oefficient that indicated the degree of ease in the evaporation of water from the materials. The importance was recognized to develop a measuring method of the water-veaporation coefficients for the materials to be cooled.The water-evaporation coefficients of lettce were determined by applying an evaporation-boundary layr medel to the cooling characteristics data of it. The coefficient was found to show the tendency to decrease ecponentially during the period from the flash point to the point where the chamber pressure was sitrted to be controlled at the minimum pressure or the steady state period of water evaporation. During the steady state period the coefficient demonstrated to maintain the constant value indicating about 4 times lower one compared with that for a free surface of water. The lower values were explained by considering the evaporation of water on the surface and also at inner tissue of a leaf which led to a larger resistance to the mass transfer rate of evaporated water. The values of the coefficients decreased as increasing in the cooling load, indicationg it's dependence on the partial pressure of water-vapor in the vacuum chamber. A simulation of vacuum cooling process for leafy vegetables was sccessfully carried out by using a water-evaporation model presented in this study. Less