5. Vacuum pre-cooling
Vacuum pre-cooling is to place fruits and vegetables in a vacuum chamber and quickly extract air to a certain degree of vacuum. The heat is converted into hot air under vacuum and is pumped away, so as to achieve the purpose of rapid cooling. The evaporation of water is accelerated when the pressure is reduced. For example, when the pressure is reduced to 533.29Pa, water can boil at 0°C. From the simple point of view, the product temperature in the vacuum chamber will be reduced to 0 degrees after the heat of the product in the vacuum chamber is exhausted. The low temperature point of vacuum pre-cooling is also at 0 degrees, because it can only pump out the cooling capacity but not produce it.
The effect of vacuum cooling depends to a large extent on the surface of fruits and vegetables, the difficulty of tissue loss and the speed of vacuuming the vacuum chamber. Therefore, the effects of vacuum cooling of different types of fruits and vegetables vary greatly. Lettuce, spinach, lettuce and other leafy vegetables are suitable for vacuum cooling. Lettuce packaged in a carton is vacuum pre-cooled, which can drop from 35°C to 2°C in 25-30 minutes, and lettuce without a tight wrap takes only 15 minutes. Some vegetables such as asparagus, cauliflower, cabbage, celery, spring onions, mushrooms and sweet corn can also be cooled by vacuum. However, some products with a smaller surface area such as a variety of fruits, root vegetables, tomatoes and other fruits and vegetables are slow due to slow heat dissipation. Vacuum cooling is not suitable. The packaging of vacuum cold products cannot be completely sealed.