What Is Freeze-Drying? Ask Paul

There’s a millennia-old tradition of drying food. Removing moisture slows down microbial activity, so meat and fish can be kept for long periods without going bad. It also removes all that water weight, so the food becomes more portable, perfect for one’s prehistoric nomadic lifestyle.

Classic drying is still popular, although nowadays electric dehydrators are used rather than sun and wind, to create jerkies, raisins, and other familiar dried delicacies. Drying food in that way is great, but it does have a couple of limitations. It requires exposing the food to a modest amount of heat (home dehydrators work around 120°-140°F) for at least a few hours, which gives the food a cooked rather than fresh flavor, collapses its cellular structure, and induces enzymatic browning in fruits and vegetables.

The method also is unable to remove all the moisture—dried fruit is still 15 to 20 percent water—which gives it that chewy, leathery texture.

Enter freeze-drying. Freeze-drying uses cold temperature and vacuum to pull water out of food without heat damage, leaving behind a structurally intact, bone-dry, fresh-tasting, crisply brittle product.

Freeze-drying takes place in two basic steps. First comes your freezing; then comes your drying. 

The freezing needs to be rapid, because fast freezing minimizes the formation of large ice crystals that can damage cell walls and harm the integrity of the food. The freezing also needs to be extra-cold, to ensure there’s no liquid water left in the food. (In frozen foods in your home freezer, especially sugary or salty ones, some of the water remains liquid, which is why sorbet,  to name just one example, is scoopable.)

The frozen-solid food is then subjected to vacuum, which causes the ice in it to sublimate, turning directly into vapor without ever becoming liquid. The food becomes slightly porous, with empty space throughout, where its water used to be.

Freeze-drying isn’t just a modern high-tech phenomenon; for centuries, the naturally frigid, rarefied peaks of the Andes have been used for freeze-drying whole potatoes into a portable food called chuño. The cold weather freezes the potatoes' water into ice; and the frigid, dry, high-altitude dawn winds allow the ice to sublimate. Available in South American markets and online, chuños have a funky, earthy flavor that’s very welcome in cold-weather stews.

The first freeze-dried food many of us encounter is astronaut ice cream, the bizarrely crumbly-yet-rich foamlike treat sold in museum gift shops. Camping food is often freeze-dried too; the removal of water can make entire meals weigh only a few ounces, ideal for trips into the back country or outer space. It's also used to make instant coffee powder, drying out the brew without cooking off the flavor.

Supermarkets now carry a lovely variety of freeze-dried fruits and vegetables. These items are highly snackable in their own right, but they’re fun to cook with too. Because of their very low moisture content, they easily add intense flavor to recipes. Whole freeze-dried fruit is the best fruit to include in homemade ice cream, because the flavorful chunks don’t turn into ice chunks. Crushed into powder, they’re even more versatile: Try subtly enhancing a vanilla cake with the sweetness of powdered fresh corn, covered with the strawberriest frosting ever, or top a batch of bruschetta with ricotta and brillilant green pea powder.

Ask Paul Adams, senior science research editor, about culinary ambiguities, terms of art, and useful distinctions: paul@americastestkitchen.com