How Dehydrated Food Works

Quick: You've got a doggie bag of leftover pizza and you don't want it to spoil. What do you do with it? The first thought in your head was probably "refrigerator." Certainly, that has been the go-to method of food preservation for most of the past 100 years. But before that, people had to do a little more than open and close a door to make their food last longer. The techniques they relied on — fermenting, pickling, curing and canning — are still around today, though their uses have evolved. The same can be said about dehydration, one of the oldest methods of food preservation.

Food dehydration was probably more of a discovery than an invention. Some unknown food pioneer likely noticed fallen fruit desiccated by the hot sun and realized it was still edible long after it dropped from the tree. The technique had caught on by 12,000 B.C.E., when evidence suggests that Egyptians were using the desert heat to dry fish and poultry [source: Shephard]. The sun continued to be the main dryer until the Middle Ages, when residents of cooler, wetter Europe began constructing buildings specifically designed to dehydrate food, which came to be known as stillhouses. Here, fruits, vegetables, herbs, and other foods were strung across a room and dried by the heat of a fire.

The next big development was mechanized dehydration, and for that we have French inventors Masson and Chollet to thank. In the mid-1800s, these old-school foodies developed a process by which vegetables were dried with air heated to 105 degrees Fahrenheit (40.5 degrees Celsius), then compressed into vegetable cakes [source: Prescott and Proctor]. That might not sound great to us, but to sailors of the time, the dried veggies were a welcome source of nutrition.

Since then, dehydrated food has fallen in and out of favor with the American palate. It got a boost in World War II, when the need for small and lightweight rations led to widespread use among combat troops. Back home, however, American housewives would have none of it. Aside from dried soups, potatoes and pudding, little of what was served on the front lines made it back home. Dehydrated food didn't make a comeback until the 1960s, when back-to-the-land campers and hikers once again discovered its excellent transportability [source: Lovegren].

The concept behind food dehydration is simple: Remove moisture to make it last longer. But why does that help? To understand this phenomenon, first you need to know a little bit about why food spoils.

If you've ever let something get buried in your refrigerator, you've seen the effects of one factor: microorganisms. Bacteria, yeasts and molds love to feed on your leftovers, a process that eventually makes your food look and smell terrible. Another cause of spoilage is the food's natural enzymes. They're what cause ripening in fruits and vegetables, turning your bananas from a firm, yellow fruit to a squishy, brown one. Reducing the moisture content in food hampers both of these processes, allowing it to last up to a year when properly stored [sources: UNL, Boyer and Huff, U of M Extension].

Longevity does have some costs, however. See, when fruits and vegetables are dried, they lose as much as 80 and 90 percent of their moisture, respectively. That means that they're much smaller and lighter — and much easier to eat. So while you might never think of eating 20 fresh apricots in one sitting, it's very easy to do when they're dehydrated.

What's the problem with that? For one, it means more calories: One cup of fresh apricot halves is 74 calories, while one cup dried is 212. There's more fiber, too: 3.1 grams versus 6.5 grams in the apricot comparison [source: Ray]. While fiber is actually good for you in small doses, the high levels found in dried fruit could quickly lead to bowel problems like gas, bloating, cramping, constipation and even diarrhea. A final issue is the amount of fructose. This naturally occurring sugar is more concentrated in dried fruit, potentially causing tooth decay and, in some people, digestive issues [sources: Anne, Kirchheimer].

At least you'd get more vitamins, right? Not necessarily. Depending on how the food is prepared and dehydrated, losses of heat-sensitive vitamins like vitamin C and thiamin can be significant. Fruits like apples, apricots, peaches, and prunes lose about 6 percent of their vitamin A, 55 percent of thiamin, 10 percent of niacin and 56 percent of vitamin C [source: Francis]. Unfortunately, there aren't any perfect solutions to this problem. Techniques like blanching (briefly scalding food in hot water) and pretreatment with sulfites can slow the loss of some nutrients, but they also increase the loss of others [source: U of M Extension].

The ancient Egyptians who laid their food out to dry in the sun wouldn't recognize the way food is dehydrated commercially today. Huge facilities outfitted with shiny steel equipment churn out fruits, vegetables, meats, herbs, and dairy at a quality and efficiency that would have been unthinkable back then.

Like the stillhouses before them, the oldest mechanized dehydrators relied on good ol' hot, dry air to remove water from the surface of the food. This first generation of equipment — which includes bin, tray, conveyor, kiln and tunnel dryers — has three basic components in common:

While these types of dehydrators have been around for a while, they still work great for drying grains, sliced fruits and vegetables, even today.

The next big thing in dehydration was technology that could turn liquid and liquefied food into powders and flakes. One example of this method is spray drying, which is exactly what it sounds like: Fluid foods are forced through a nozzle, transformed into airborne droplets and then introduced to hot, dry air. Milk, egg, ice cream, cheese and fruit juice powder are created this way. Drum dryers create powders and flakes from fluid food particles that are too large for spray drying. They consist of rotating drums that are heated by steam from the inside. Liquefied foods are applied to the outside of the drums to create products like potato flakes, pre-cooked cereals and dried soup.

A third innovation is probably one you've heard of: freeze drying. Like the phrase suggests, this process consists of freezing the food and then drying it. But this isn't just any drying process. The moisture, which is actually ice, is removed from the food through a process called sublimation, meaning it goes from ice to water vapor without ever becoming water. Basically, the ice evaporates. This process, executed under a vacuum and at temperatures as low as -50 degrees Fahrenheit (-45 Celsius), helps food maintain its structure, making it taste, smell and feel much like fresh food when it's rehydrated [source: Mountain House]. Almost any food can be freeze dried, including that astronaut ice cream you get at the science museum gift shop [source: Vega-Mercado et al.].

You don't need big, fancy machinery to dehydrate your own food at home. In fact, most people already have everything they need to get started. The only three things that are required are heat to push out moisture, dry air to absorb it and air movement to carry it off.

A simple way to dehydrate food is to take a page from our ancestors' book and dry it in the sun. This technique is basically free: All you need is about three to four days of direct sunlight and humidity levels below 20 percent [source: Jopp]. Unfortunately, this precludes much of the United States (with the exception of the desert Southwest), where cool and moist conditions could cause mold growth before the dehydration process is complete.

Food can also be air dried, which is done indoors. Great candidates for this method are herbs, hot peppers and mushrooms, which should be threaded on string or tied in bunches and hung, like in the stillhouses of medieval times. Slap on a paper bag for dust protection, and you're ready to go.

If you want to kick the technology up a notch, but not the cost, try dehydrating with your oven. Heat the oven to 140 degrees Fahrenheit (60 degrees Celsius), prop the door open a couple of inches and set up a fan to blow across the opening. Place the food in a single layer on a cookie sheet. Drying times can range from one hour for herbs to 48 hours for juicy, meaty fruits like peaches [source: Boyer and Huff]. Microwave ovens can be used as well, but only for herbs. Place several branches between paper towels, and you should have dry, brittle herbs in just two or three minutes.

Finally, you can purchase a home dehydrator, which generally consists of a heating element, a fan for air circulation and trays on which you arrange the food. Like oven drying, this method produces results ranging from a couple of hours to a couple of days, depending on the food. Dehydrators usually produce the best results, but they can cost you anywhere from $40 to $350.

With any of these methods, proper preparation is essential. For instance, some foods are better blanched before they are dehydrated. And meat is particularly tricky, as it must be trimmed of all fat and cooked before dehydrating. There are plenty of great websites with preparation tips and drying times, so be sure to check those out if you get serious about trying dehydration.