Vacuum cooler through boiling of some water in fresh produce to removes heat.

Vacuum cooling removes heat from vegetables by boiling off some of the water they contain.

Produce is loaded into a sealed container and the air is pumped out. This reduces the pressure from normal air (approximately 100 KPa) to a virtual vacuum (<1KPa). Under these conditions water boils at <7°C.

Fresh produce loaded in the sealed chamber room. When

As water inside the vegetables changes from liquid into gas it absorbs heat energy from the product, cooling it. This vapour is removed by drawing it past refrigeration coils, which condenses it back into liquid water.

For vacuum cooling to cool vegetables quickly, they must be able to lose moisture easily. For this reason vacuum cooling is very well suited to leafy products, such as lettuces, Asian greens and silverbeet. Products such as broccoli, celery and sweet corn can also be cooled effectively using this method. Vacuum cooling is not suitable for products with waxy skins, or low surface area compared to their volume, e.g. carrots, potatoes or zucchini.

For every 5°C reduction in temperature, approximately 1% of the produce weight needs to be turned into water vapour. However, modern hydro-vacuum coolers address this issue by spraying water over the produce during the vacuum process. This can reduce moisture loss to negligible levels.

For suitable products, vacuum cooling is the fastest of all cooling methods. Typically, only 20 – 40 minutes is needed to reduce temperature of leafy products from 30°C to 4°C. In the example shown below, vacuum cooling reduced the temperature of harvested broccoli by 11°C in 15 minutes. Large vacuum coolers can cool many pallets or bins of product simultaneously, reducing demand on cool room systems. The process can even be used on packed cartons, so long as there is sufficient venting to allow air and water vapour to escape quickly. 

Post time: Jan-25-2021