Many are the marvels of today's small appliances. You can zap a hot dog in a microwave in 30 seconds. You can make a milkshake in the buzz of a blender. It takes only a minute for a food processor to shred a carrot or a chunk of cheese.

And it takes only a few hours - while you're away or sleeping - for a food dehydrator to dry fruits and vegetables, turning them into handy snacks or easily stored ingredients.More and more gardeners, backpackers, natural-foods enthusiasts and budget watchers are finding that dehydrating food is easy and economical. Automatic dehydrators make it possible to dry different foods simultaneously and to set and forget the machine, knowing it will turn itself off.

"If it wasn't easy, I wouldn't be doing it," said Marilyn Gordhamer of Lakeville, Minn., a mother of seven who uses her dehydrator almost daily. She saw the dryer demonstrated at a church meeting and knew immediately that she wanted one.

The demonstrator was Dave Dornbush, co-founder of Alternative Pioneering Systems Incup, which manufactures the Harvest Maid dehydrator - the top-seller nationally.

"Before I got the dehydrator, my kids ate way too much candy," said Gordhamer. Now, a year and a half after getting the machine, the young Gordhamers eat beef jerky, fruit "leather" and granola bars, snacks their mother turns out with her food-dryer.

Besides snacks, Gordhamer dries herbs - dill, parsley, even celery tops - homemade noodles and tomatoes for use in casseroles and soups. "You know how leftover cooked rice just dies in the refrigerator?" she asked. "I put it in the dehydrator, make my own instant rice. I just add water to reconstitute it.

"You can dry fruits and vegetables as much or as little as you want," Gordhamer said. "The drying cuts down on the volume so terrifically. With tomatoes, for example, I dry them, then chop them and freeze them, ready to use in cooking."

The shelf life of home-dried vegetables and fruits, according to the folder, is four to six months. Dried vegetables need about two hours soaking time before cooking. Most dried fruit can be eaten or used in recipes as it is.

"How to Dry Foods" by Deanna DeLong (HPBooks, Tucson, Ariz., 1979, 160 pages) is a good source of information on this process. This book includes design plans for building your own home food dehydrator, directions for making fruit leathers and candied fruits, charts on drying herbs and spices and 100 recipes. The recipe selection includes old-timers such as Leather Britches, the pioneers' name for dried green beans with ham.

Gardeners load their dehydrators with sliced beans, zucchini and tomatoes and whole kernel corn. No more steamy kitchens for these cooks; no more watching the clock while tomatoes were processed. And no more jars and canners to pack and unpack.

Those who prefer freezing for fruits and many vegetables turn to drying for tomatoes and zucchini, two vegetables which come out of the deep-freeze collapsed and waterlogged.

Other cooks find dryers an easy way to turn excess food into treats for picnics and camping expeditions. When the refrigerator simply won't hold the watermelon that you got at such a good buy, you can slice it and dry it. The result is a slab of chewy sweetness that's an unbelievable hot pink in color. Drying intensifies a food's flavor - and color.

Mushrooms are another food that can be dried quickly and easily when you have an oversupply, said Gordhamer. Backpackers, hikers and canoeists can make their own homemade dry mixes, even adding seasonings before sealing the mixtures in waterproof plastic bags. (Keeping moisture out of dehydrated foods is important. If wrapped, rather than sealed, dried foods will readily absorb moisture, then deteriorate.)

Dehydrators are available in a variety of sizes and shapes; prices range from $60 on up to $300. Food dryers are sold in hardware stores and discount department stores.