To preserve foods, the moisture contained within must be removed through a process called
lyophilisation. If it isn't, micro-organisms (like bacteria) will thrive and feed, leading to food decomposition, mould growth and inedibility.
Freeze-drying is a relatively modern process. Food is placed on large racks inside a vacuum chamber where the temperature is lowered to below freezing, and then slowly raised. The water in the food transforms from a solid state to a gas, thereby maintaining the food's structure and preserving its all-important nutrients. Freeze-drying removes about 98-99%. The lower the moisture content, the longer the shelf life.
Freeze-drying is akin to keeping the food in a state of suspended animation and so once rehydrated the food is as fresh and nutritious as it was the moment it was frozen. Dehydrating and canning involves the heating of food to temperatures which can impair its nutritional value by breaking down the vitamin and mineral content (vitamins A and C, thiamine, riboflavin and niacin). The taste can also be affected and heat can denature and break food fibres which change the texture.