Freeze-drying is a method of preserving substances by removing water from them. The food industry freeze-dries such heat-sensitive products as coffee, tea, shrimp, mushrooms, and strawberries. Biologists use freeze-drying to preserve delicate or small organisms for display or microscopic study. Drug companies use it to prepare many medicines. Freeze-drying is also called lyophilization << ly `of` uh luh ZAY shuhn >> .
In freeze-drying, the item is frozen. Most commonly, the frozen product is placed in a chamber. Air is pumped out to create a vacuum. Then, the product is gently heated. In vacuum conditions, the ice in the substance does not melt into liquid water. Instead, it turns directly into water vapor in a process called sublimation.
As the ice sublimates, the substance dries and becomes porous (full of tiny holes). The pores allow easy access to water. Rehydrating (adding water to) freeze-dried food can thus restore it nearly to its original form.
Freeze-drying is a gentle drying method. It preserves food’s texture, color, and nutrients. But it is more expensive than other drying methods.
