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August 11, 2009

Quality for Keeps: Drying Foods Part 1

Here's an exceptional article about dehydrating foods. This article was written by Karla Vollmar Hughes and Barbara J. Willenberg from the University of Missouri. Dehydrating foods is a great way to store what comes out of your garden or foods you purchase. Dehydrated fruits are a great snack, and dried vegetables go great in a soup or stew. Make sure you have some dehydrated foods in your food storage because these are quick to prepare, and would go great in a 3 month food supply.

Of all food preservation methods, that of drying foods has received the most widespread and enthusiastic publicity in recent years. Actually, drying is one of the oldest methods of food preservation. Techniques have been passed from one generation to another based on what worked and what didn't. Methods used for drying food have become sophisticated over time. Initially, salting and drying in the sun, an open room or on stove tops were the accepted methods. It wasn't until 1795 that the first dehydrator was introduced, in France, for the purpose of drying fruits and vegetables. Today, the variety of dried foods in the marketplace has created a multimillion dollar industry. For many people, drying food at home is a convenient way to preserve foods.


Molds, yeast and bacteria need water to grow. When foods are sufficiently dehydrated (dried), microorganisms cannot grow and foods will not spoil. Dried fruits and fruit leathers may be used as snack foods; dried vegetables may be added to soups, stews or casseroles. Campers and hikers value dried foods for their light weight, keeping qualities and ease of preparation.

Nutritional value

The nutritive value of food is affected by the dehydration process. Vitamins A and C are destroyed by heat and air. Using a sulfite treatment prevents the loss of some vitamins, but causes the destruction of thiamin. Blanching vegetables before drying (to destroy enzymes) results in some loss of Vitamin C, B-complex vitamins and some minerals because these are all water soluble. On the other hand, blanching does reduce loss of vitamins A, C and thiamin during dehydration and storage.

There are more calories in dried foods on a weight-for-weight basis because of the concentration of nutrients. For example, 100 grams of fresh apricots have 51 calories, while 100 grams of dried apricots have 260 calories. In general, dried foods are not a major part of the American diet and nutrient loss is, therefore, not a concern. Nutritive value, as well as flavor and appearance, is best protected by low temperature and low humidity during storage.

Drying methods

Foods can be dehydrated by various means: the sun, a conventional oven, an electric dehydrator or a microwave oven (for herbs only). Drying, like other preservation methods, requires energy. Unless sun drying is possible, the energy cost of dehydrating foods at home is higher than for canning, and in some cases more expensive than freezing.

Solar drying is a modification of sun drying in which the sun's rays are collected inside a specially designed unit with adequate ventilation for removal of moist air. The temperature in the unit is usually 20 to 30 degrees higher than in open sunlight, which results in a shorter drying time. While solar drying has many advantages over sun drying, lack of control over the weather is the main problem with both methods. Missouri weather is not suitable for sun or solar drying because there are few consecutive days of high temperatures and low humidity. It is likely that the food will sour or mold before drying is completed.

Oven drying is the most practical way to experiment with dehydration. It requires little initial investment, protects foods from insects and dust, and does not depend on the weather. Continual use of an oven for drying is not recommended because ovens are less energy efficient than dehydrators, and energy costs tend to be high. Also, it is difficult to maintain a low drying temperature in the oven, and foods are more susceptible to scorching at the end of the drying period. Oven-dried foods usually are darker, more brittle and less flavorful than foods dried by a dehydrator.

Foods can be dried on trays in an electric dehydrator, a self-contained unit with a heat source and ventilation system. Electric dehydrators are used to dry foods indoors. Such dryers can be purchased or made at home and vary in sophistication and efficiency. Although the initial investment is fairly high for an electric dehydrator, it maintains low temperatures and uses less energy than an oven. The quality of the product is better than with any other method of drying. As with oven drying, there is no dependence on weather conditions.

It is not recommended that microwave ovens be used for drying foods, because the food will partially cook before it dries, imparting an overcooked flavor. Microwave ovens can be used to dry some herbs quickly — but watch them carefully to prevent them from catching on fire. Check the owner's manual for drying recommendations.

Drying times in conventional ovens or dehydrators vary considerably depending on the amount of food dried, its moisture content, and room temperature and humidity (and the use of fans, for oven drying). Some foods require several hours and others may take more than a day. Prolonging drying time (by using lower temperatures) or interrupting drying time may result in spoilage.

It is important to control air temperature and circulation during the drying process. If the temperature is too low or the humidity too high (resulting in poor circulation of moist air) the food will dry more slowly than it should and microbial growth can occur. Watch temperatures closely at the beginning and end of the drying period. If the temperature is too high at first a hard shell may develop on the outside, trapping moisture on the inside. This is known as case hardening. Temperatures that are too high at the end of the drying period may cause food to scorch. Temperatures between 120 degrees Fahrenheit to 140 degrees Fahrenheit are recommended for drying fruits and vegetables. Temperatures up to 150 degrees Fahrenheit may be used at the beginning, but should be lowered as food begins to dry. For at least the last hour of the drying period, the temperature should not exceed 130 degrees Fahrenheit.