The main objective of freeze drying and dehydrating food is to remove moisture so that the food doesn't spoil, preserving it to consume later. Food preservation might seem like a modern invention, but it’s been practiced for centuries. Depending on the climate they lived in, ancient humans either dried their food in the heat of the sun or froze it on ice. Today’s methods are more efficient.
Turn up the Heat
The biggest difference between the two method starts with temperature. Food dehydrators work by increasing the temperature enough to dry out the food, but not enough to cook it. Home dehydrators are small appliances about the size of a rice cooker with stacked trays upon which you place your food in single layers. The heated air flows through the trays, removing the moisture slowly. It can take from several hours to several days to work, depending on the food. Common foods dehydrated using this method include vegetables, fruits and meats.
In freeze drying, first the food is frozen so that the moisture is in solid form. Then it’s placed in a vacuum chamber and the air is pumped out, decreasing the pressure. The temperature is increased, forcing a process called sublimation – which simply means that the moisture goes from solid to gas, without becoming liquid again. Freeze drying is generally not available for home food preservation. Instant coffee and pre-made meals for soldiers and astronauts are often freeze-dried.
Looks Aren't Everything
When foods are dehydrated, they can look completely different from the original. Grapes turn into raisins, for instance, and plums become prunes. Dehydrated banana chips are widely available in grocery stores, and these are thin, crispy slices that you eat out of the bag. Freeze-dried banana slices, on the other hand, are thicker and not as brittle, and with just a few minutes of soaking look and feel like real bananas. You can also eat them in the freeze-dried state. Freeze drying preserves the taste and texture of the original food more closely than dehydration.
Shelf Life and Storage
Dehydrated fruits and vegetables have a shelf life of 15 to 20 years, while freeze-dried foods and meals generally last 25 to 30 years. This is because freeze drying removes more moisture than dehydration, and moisture is the culprit in food spoilage. This also means that freeze-dried foods weigh less, making them easier to carry on family camping trips and more practical for soldiers.