Mechanism Of Heat And Mass Transfer In Food During The Vacuum Cooling Process

Vacuum cooling is a very efficient rapid cooling technology in the food cold chain. Recently, it has been widely applied to postharvest and heated foods such as lettuce, cabbage, cut flowers, fruits, baked foods, cooked rice, cooked meat, aquatic products, boiled egg, bean curb, cut fruits, flour products, glue pudding and so on. To promote the development and application of the vacuum cooling technology, many optimized technologies have been designed by domestic and foreign researchers, such as multistage drop pressure, spraying water, immersion cooling and so on. However, some negative phenomenon as high weight loss and nonuniform temperature distribution often occur. Based on a comprehensively summarization of the research status on vacuum cooling technology, the following works were carried out in the aim of studying the heat and mass mechanism in the food during the vacuum cooling process:1)A new type of vacuum cooling machine was designed after analyzing the classical vacuum cooling equipment. Dropping the pressure uninterruptedly is a noticeable feature for classical vacuum cooling machine, which means the vacuum pump has to work with its total power until the chamber pressure reaches to the set value. Although this scheme can ensure the chamber pressure dropping to the certain value in the shortest period, it may cause excessive boiling or food structure injury. In order to overcome these defects, the new vacuum cooling machine works with a flexible scheme–––dropping the pressure according to the time, temperature or pressure. The flexible scheme can not only ensure the vacuum pump running with a moderate power but it can also adjust the whole cooling process according to the structural properties of the food. Otherwise, temperature difference between different parts of the same sample during vacuum cooling process is a critical evaluation factor for cooling effect. An external infrared thermal imaging system was designed in this paper to detect the surface temperature in real time for the vegetable, soup and cooked meat in a vacuum cooling process. To improve the production efficiency of the new vacuum cooling machine, other suggestions are given in this paper such as shortening the delayed turn–on time of vacuum pump and replacement of mechanical refrigerator.2)Weight loss has been paid most attention by the commercial circles since it affects their investment return directly. So, how to reduce the weight loss of food during the vacuum cooling process is always a hot issue. To find a method that can reduce the weight loss of food during the vacuum cooling process, the mechanism of the water losing was studied with the help of a high speed camera. The results show that the violent explosion of bubble in food is the key factor causing the invalid water loss. It was found that reducing the pressure drop rate near the "flash point" can inhibit the explosion of bubble effectively after comparing different variable speed schemes.3)Temperature distribution in food can affect its chewiness after vacuum cooling, especially for meat product. So, the real–time temperature distribution on the ham section was measured using the infrared thermal imaging system. The mechanism of heat and mass transfer of moisture in ham during a vacuum cooling process was explored. The results show that the temperature changing in ham does not always follow the linear rule. The hottest point does not always locate at the geometric center; the junction zone of crust and core is the key factor that affects the temperature distribution in ham. Properly varying the evacuation rate during the cooling process can not only uniformize the temperature distribution across the ham section but also reduce both the weight loss and energy consumption. Consequently, a reasonable varying evacuation rate can help improve the vacuum cooling effect.4)To further study the mechanism of heat and mass transfer of moisture in meat product, a numerical model was built for boiled pork during vacuum cooling process. In this model, the vapor–liquid phase change, migration of water and vapor in porous food, heat of phase change, external convective heat and radiation heat were coupled together. It can not only predict the temperature distribution in boiled pork during vacuum cooling process but also predict the weight loss rate and the porosity change of matrix.5) Optimizing the vacuum cooling process of leafy vegetable is meaningful because it is one of the most suitable foods for vacuum cooling. Shanghaiqing, a type of classical Chinese leafy vegetable, was studied in this paper to guide the optimization of vacuum cooling technology for leafy vegetables. The results show that the leaf was cooled more easily than the petiole if the whole vegetable was intact, especially if the petiole was undamaged, because the leaf had a bigger evaporative area due to the presence of many stomas on the lower epidermis. If a part of the petiole was cut off, the temperature of the end nearest the cut decreased more quickly than the leaf. In addition, the cooling rate of the leaf was reduced because the free water used for refrigeration was drawn away by the damaged petiole. Temperature differences were also observed between different parts of the leaf. The leaf margin cooled most quickly at the beginning of the vacuum cooling process. However, this advantage disappeared after a short time, as the water was too limited to be transported to the margin. At the end of the vacuum cooling process, the final temperature of each part had risen with the increase in distance from the main vein. Therefore, obtaining a uniform temperature distribution of the leaf during vacuum cooling requires adjusting the cooling scheme, for example, using the optimal volumetric displacement.6) A numerical model was built for leafy vegetable during vacuum cooling process. this model coupled not only different physical fields but also the structural characteristics of leafy vegetable especially the structural difference between leaf and petiole. It can roughly predict the surface temperature distribution of leafy vegetable during vacuum cooling process.