Freeze-drying (aka lyophilization) is a dehydration process based on the the sublimation of water in a food product.

This means essentially that the product’s water content transitions from a solid state to a gaseous state (i.e. from ice to vapor) without going through the liquid state.

Freeze-drying is widely considered a super high quality dehydration technique for many reasons including:

  • freeze drying operates at very low temperatures, which helps to preserve a product’s nutritional value as high as 97%, taste and appearance
  • freezing inhibits the microbiological and chemical processes that can significantly delay the product’s deterioration
  • extended shelf life of freeze dried foods compared to dehydrating or canning

How does it work?

Water rich and raw foods are comprised of roughly 80%–95% water, which can be subdivided into the categories “free” and “bound” water. Free water freezes but bound water does not.

All free water and as well some bound water is removed during the freeze-drying process while free water must remain frozen at all times. The most significant difference between freeze drying and vacuum-drying.

The usual freeze-drying process is comprised of the basic steps:

  1. Freezing: The food is often frozen under atmospheric pressure.
  2. Primary drying: The sublimation step where the frozen “free” water is removed.
  3. Secondary drying: The food is dried to the desired humidity by removing any remaining “bound” water.

Freeze-drying, including home freeze drying, should be performed in controlled conditions where the water does not melt. This would lead to the most favorable result and shelf life of the product.

The remaining product develops a highly porous structure after being freeze dried. Freeze dried food can be rehydrated simply by adding water.

The water in the products can be free water or water bound to the matrix by various forces. Free water freezes, but bound water does not freeze. In the freeze-drying process, all ice water and some bound water must be removed.1

SUMMARY

Freeze-drying is a process of food dehydration whereby the water is removed by turning it from ice to vapor. A three-step process ensures that the food is preserved retaining most of its nutritional value, taste, and appearance while also extending its shelf life.