What's the Best Way to Dehydrate Fruits and Vegetables?

Whether you're dehydrating your own raw material fruits and vegetables or buying them dried, it's worth considering how they got that way.

By Dave Fusaro, Editor in Chief

Dehydration as a method of food preservation has been around for eons. Because microorganisms need the water in foods to propagate, drying renders food and ingredients safe from bacteria, yeast, mold and other forms of microbial contamination.

It also creates some interesting textures. Raisins, prunes and fruit "leather" are not just dried versions of their originals, they have their own legions of fans.

There are three commons forms of dehydration: freeze drying, drum drying and sugar infusion. If you're going to dehydrate on-site -- either key ingredients or your final product – deciding which is best for your product and process requires a dialog between both your plant operations people and your product developers.

Freeze drying involves the removal of water from a frozen product without thawing it. Sublimation, if you remember your high school chemistry, is the process of passing directly from the solid phase to the gaseous phase without passing through an intermediate liquid phase. In freeze drying, ice changes immediately from a solid to a vapor, so it can be removed. But it requires quite a bit of energy as well as pressure.

"The cell walls, because they are frozen, are rigid. So a high percentage of nutritional value is retained," says Irv Dorn, sales manager for Van Drunen Farms (www.vandrunenfarms.com), a Momence, Ill., supplier of dried fruit and vegetable ingredients. "The product goes from maybe 90 percent water to less than 3 percent, often 1 percent or less. You get a fruit or vegetable with a beautiful look, taste and nutritional value – plus it's shelf stable, with a long shelf life."

Freeze-dried fruits are best in cereals, snack foods, confectionery and trail mixes, and the vegetables are for dried soups and seasoning mixes.

Van Drunen Farms employs all three forms of dehydration. "Drum drying involves the removal of water from purees," Dorn continues. "That slurry is spread on two counter-rotating, heated cylinders less than a sheet of paper apart. In one rotation, it's dried."

But this process only creates powder or very fine flakes, at best very small particulates, nothing close to whole pieces. The resulting product is also shelf stable, typically less than 5 percent water, and the process is less expensive than freeze-drying.

Drum-dried products are suitable as a base for snacks, soups, baked chips and some bakery applications, and can stand alone in some grain cereals (when the product being dehydrated is rice or wheat).

The third method employed by Van Drunen replaces the water in a product with some infused medium, typically fructose or sucrose. "You put the fruits in a tank, add the infusion medium, and the water diffuses out of the product and the medium diffuses in," explains Dorn. "You drain the vat and repeat with new medium. It takes several exchanges. You end up with a product that's 30 percent moisture. Often, we'll dehydrate it further, maybe to about 15-20 percent moisture. The result is like a raisin."

There are two downfalls to sugar infusion. The diffusion removes a lot of the nutrients from the fruit or vegetable. And it replaces them with sugar, which can be a liability. But the result is a good-looking, semi-dried product that yields the cheapest product per pound of those processes (although also the fewest pieces per pound).

"We do cold or passive infusion," says Dorn. "We think the fruits retain more of their shape and color." But infusion can be speeded up with heat.
Infused products are tasty when eaten whole and do well in snack mixes, trail mixes, baked products and hot and cold cereals.

EnWave Corp. (www.enwave.net), Vancouver, British Columbia, has developed a new form of dehydration. Its nutraREV process, which uses radiant energy under vacuum, won an Innovation Award at the 2009 Institute of Food Technologists Food Expo.

The process is similar to freeze-drying but is quicker and at a lower cost, claims Tim Durance, chairman, co-CEO and co-inventor of the company's radiant energy vacuum (REV) technology. "We use microwave energy for the energy transfer, without creating any heat in the process," he says.

The result is a dehydrated product that appears to be a "puffed" version of the original, retaining the ingredient's color, shape and nutrients.

EnWave is an equipment builder, interested in selling its machines direct to food product development departments.

In addition to nutraREV, EnWave has three processes it is commercializing. PowderREV creates powdered ingredients for both food and pharmaceutical uses. QuantaREV is a large-scale belt version of nutraREV, for producing higher quantities. BioREV is strictly for pharmaceutical applications.

EnWave also markets MiVAP machines, tray-based technology it licenses from German company Hans Binder