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

1996 Fiscal Year Final Research Report Summary

• Project/Area Number: 07660346
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Single-year Grants
• Section: 一般
• Research Field: 生物環境
• Research Institution: The University of Tokyo
• Principal Investigator: SAGARA Yasuyuki Graduate School of Agricultural and Life Sciences, Lecture, 大学院・農学生命科学研究科, 講師 (30012024)
• Co-Investigator(Kenkyū-buntansha): KAWAGOE Yoshinori Gradate School of Agricultural and Life Sciences, Assistant, 大学院・農学生命科学研究科, 助手 (80234053)
• Project Period (FY): 1995 – 1996
• Keywords: Fresh fruits and vegetables / Pre-cooling / Specific surface / Vacuum cooling / Evaporation

Research Abstract

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 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.