Freeze drying, also called lyophilization, is a dehydration process that freezes food and then removes the ice by sublimation under a vacuum. The process preserves the original shape, texture, and nutritional content of the food better than any other preservation method. Freeze-dried food rehydrates quickly and has a shelf life of 10 to 25 years when properly packaged.
The food is frozen to minus 20 to minus 40 degrees Celsius (minus 4 to minus 40 degrees Fahrenheit). The frozen food is placed in a vacuum chamber, and the pressure is reduced below the triple point of water (611 pascals). Ice sublimes directly to water vapor without passing through a liquid phase. The water vapor is condensed on a cold coil and removed. The primary drying phase removes 95 percent of the water. A secondary drying phase at a slightly higher temperature removes the remaining bound water.
Home freeze dryers are available from Harvest Right and other manufacturers. A countertop model costs $2,000 to $4,000 and processes 4 to 10 kilograms of food per batch. The drying cycle takes 20 to 40 hours, depending on the food type and moisture content. The equipment includes a freeze dryer chamber, a vacuum pump, and oil filters. The vacuum pump requires periodic oil changes.
Freeze drying works well for fruits, vegetables, cooked meats, dairy products, eggs, and complete meals. High-fat foods such as nuts and avocados do not freeze dry well because the fat can become rancid during the long drying cycle. foods with high sugar content, such as fruit syrups, may not fully freeze due to Freezing point depression.
Freeze-dried food is packaged in mylar bags with oxygen absorbers to remove residual oxygen. A vacuum sealer is used to remove air from the bags. The packaged food is stored in a cool, dark location. Oxygen and moisture barrier properties of the packaging determine the shelf life. Mylar bags with 5-mil thickness provide adequate protection for long-term storage.