Freeze drying, also known as lyophilisation, is the best and highest-quality fruit or vegetable drying process. The fruit is first frozen at an exceedingly low temperature, upon which humidity is extracted via a vacuum system in the chamber. Depending on the product and the recipe, the drying process can take up to 25 hours. In the course of the process, sublimation transforms ice into water vapor. In that way, dehydration is carried out, where more than 95% of the water evaporates from the fruit or vegetable, which retains all the nutritional values, color, smell and taste. Fruit and vegetable produced with the lyophilisation method are considered one of the exceedingly high-quality, healthy and natural food items, which, in their different forms, can be consumed either individually, or as an addition to muesli, biscuits, protein bars, cakes and similar things.

Freeze dried process the contamination of produc is avoided due to the fact that 95% of the moisture has been isolated and most of the bacterias and mold are not able to survive this process. 

Freeze dried process has been developed during 60's when NASA was looking for new technologies, etc. space food. Food that will keep taste, color and nutritions, cooked meal that is easier for use. They wanted astronauts feels like eating home...

That is how they came with freeze dried process where meals have been cooked, then frozen and dried in vakum chambers in a way to isolate moisture and still keep the 95% of nutritions with only 20% of real weight of food. 

1969 freeze dried food was introduced on Appollo 2 mission to moon and its been official food of astronauts since then.