Dehydration (food preservation method)
A process that removes water from food to stop microbial growth, one of the oldest preservation methods.
Dehydration means removing water from a food until micro-organisms can no longer grow. It is one of the oldest preservation techniques, long predating refrigeration and canning.
The deciding factor is not total water content but water activity, meaning the share of water that is actually available to micro-organisms. That is why a very sugary or very salty food keeps well despite a fair amount of water: sugar and salt bind the water.
Freeze-drying is the process that best preserves nutritional value. The food is frozen and then the water is sublimated under vacuum, passing directly from solid to gas, with no liquid phase and no significant heating. Vitamins, aromas and structure are far better preserved than with conventional hot drying, which notably degrades vitamin C.
Take care when reading labels, though: the nutritional values of a dehydrated food are given per 100 g of dry product, so they are concentrated. One hundred grams of dried apricots is not equivalent to one hundred grams of fresh apricots but to roughly five times as much fruit, sugars included. This is the main source of misunderstanding about dried fruit.