Question

In the dry-freezing process of dehumidifying foods, the pressure around the food is lowered by means of a vacuum pump so that the water will evaporate. The residue left behind will often be frozen. Why?

Answer 1

When the vacuum pump to force air out of the chamber is on, the heating units are also applying a small amount of heat to the shelves. Since the pressure is low, the ice sublimates rather than evaporates, means it turns into water vapor directly .

The water vapor condenses onto the freezing coil in solid ice form, after flowing out of the freeze-drying chamber, through the freezing coil. This process continues till the material is dried , so, The residue left behind will often be frozen.