The freeze-drying process makes use of a useful characteristic of the water that is contained within all food. When water is cooled at low pressure, it actually evaporates, in a process known as sublimation
. To freeze-dry a fruit, it is placed within a low-pressure container and cooled to just above freezing point. The water escapes from the food, and we are left with dry fruit which is nutritionally intact. It is simply dehydrated.
Sublimation can often be seen when you open your freezer: visible clouds of water vapour often emerge. This is the same principle upon which freeze-drying works.
Freeze-dried fruit can be anything from one-fifth to one-tenth of the original weight of the fruit. It is thus a very compact source of nutrition.