Charles Taylor Jr., age 10, of Dayton, Ohio, for his question:

HOW DOES FREEZE DRYING WORK?

Freeze drying is a process that removes water from a substance to preserve the substance for future use. Drug companies use freeze drying in the preparation of many medicines and the process is also used to preserve some foods and beverages.

Freeze drying differs from other drying methods because the substance is frozen before being dried. The item is then placed in a refrigerated vacuum chamber. There, any water in the product changes from ice to water vapor.

Unlike other drying processes, freeze drying does not shrink the substance or lessen its ability to dissolve in a liquid. For this reason, freeze drying is useful for coffee and tea and some drugs which must dissolve rapidly. The low temperature at which the process takes place allows serums and other drug solutions to retain their original characteristics.

Hikers often carry freeze dried foods because these products are light and compact. But freeze drying is not widely used for food preservation because the difficulties of freeze drying animals and plant cells make it uneconomical.