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

以西兰花、生菜和胡萝卜为实验材料，从日本目前真空冷藏的典型新鲜水果和蔬菜中选取。在不同的操作条件下，测量了这些材料的真空冷却特性。本文提出了一种利用显微切片图像数据处理系统测量青花菜水分蒸发表面积和体积的新方法，并用广泛应用的图像数据分析仪对生菜和胡萝卜的水分蒸发表面积和体积进行了测量。因此，根据这些材料的质量来预测它们的表面积成为可能。从比表面积与冷却速度关系的研究中发现，生菜的冷却速度比比表面积大于生菜的西兰花快。这些结果表明，冷却速度不仅与被冷却材料的比表面积有关，而且与水分蒸发c…有关。更高的效率，这表明了材料中水分蒸发的容易程度。认识到开发被冷却材料水分蒸发系数的测量方法的重要性，将蒸发边界层模型应用于生菜的冷却特性数据，确定了生菜水分蒸发系数。该系数在闪点至腔室压力控制在最小压力点或蒸发的稳态期之间呈递减趋势。在稳态期，该系数保持恒定值，比自由水面的系数低约4倍。考虑到叶片表面和内部组织的水分蒸发，导致对蒸发水传质速率的阻力较大，从而解释了较低的值。这些系数随冷负荷的增加而减小，表明它与真空室中水蒸气的分压有关。利用本研究提出的水分蒸发模型，成功地模拟了叶菜类蔬菜的真空冷却过程。较少

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