What are some methods for Dehydrating Fruits and Vegetables at home?
Dehydration concentrates the fruit sugars and other nutrients by removing the water that dilutes these substances in the plant's natural state. By drying slowly, much of the medicinal properties of the plant can be preserved and even amplified, and much of the flavor remains. However, some of the nutritional characteristics of the food products are volatile, and evaporate together with the water. Vitamin C is lost in some proportion but it can also concentrate other nutrients, particularly fibre in plant foods (source:betterhealth.vic.gov.au), so diets that require more of vitamin C should contain more fresh fruit than dried fruit. Industrial techniques do exist for keeping this vitamin content high in dried products, but these methods are not always practical for dehydrating fruits and vegetables at home.
The theory behind dehydration is highly technical but simple to understand. Heat provokes evaporation of water from the material being dried. It stimulates the movement of water from the center of the portion being dried to its exterior, where the humidity passes to the air. This process must occur slowly, and each specific food product has an ideal temperature for adequate dehydration. Also, the size of the pieces being dehydrated is a significant factor. Slices, cubes or whole fruits and vegetables will dry at different rates. The temperature can't be too hot, or the outer layers of the target food will form a crust that traps humidity inside the pieces, which causes rotting after being dried.
Conversely, too little heat will not remove enough water, and the residual humidity will also provoke rotting. Heat rises, so the source of the drying heat should usually be located below the material being dried. Circulating air in the drier removes this evaporated humidity from the surrounding space, keeping the water from returning to the heated food. Again, because the hot air that passes over the drying material is rising, it can carry the evaporated water away through a ventilation system above the drying material. The same circulating air that delivers dry heat to the target carries the humid heat away from this target. Thus, scientific dehydration balances temperature, air circulation and air humidity to acquire the correct drying conditions for each different type of food product.
Drying techniques vary according to heat sources that are available. Originally, many food products were dried in the sun, but this allows no control over the temperature, and the sunlight alters the color of some products. The home oven is an adequate drier, but the temperature must be kept low, and the door must be kept partially open to allow the evaporated humidity to leave. This consumes gas or electricity. Drying boxes can be constructed of wood or metal, with an electric light bulb in the bottom, gratings for shelves above this, and holes in the top for ventilation.
Remember that wood is flammable, so the heat source can't be too close to the box. Also, the wood used should not be resinous or aromatic, which will affect the drying food. A thermometer installed in the top of the box, with its bulb inside the box and the numbered scale outside, allows for monitoring the temperature. Solar dehydrators (watch solar deydrators on youtube) use black tubes or covered black panels to collect the heat of sunlight and conduct it into the bottom of the drying box, where it rises to perform its dehydrating function. This technique uses no electrical energy and reduces the cost of home dehydration to almost nothing.
In preparing fruits and vegetables for dehydrating at home, choose the best of the produce. Remove leaves and any rotten spots, wash it well, and cut into thick cubes or slices for larger produce, or leave smaller varieties whole. Some products need to be blanched in steam or boiling water, to deactivate the internal enzymes that are an additional source of rotting. Scatter the pieces on the shelves above the heat source, leaving space for the heated air to circulate in the oven or box. Turn on the heat source, and guarantee that the heated air has an outlet.
The size of this outlet (box holes or the oven door) can be regulated by opening more or less, or by blocking partially, to balance the temperature in the drier with the air flow coming out. Research into the drying specifications for each food product, as well as your own experience, will teach you the correct temperature, slice thickness, blanching technique and shelf density (how much material on each shelf of the drier)for the specific food you have available to dry.
After dehydration, dried foods should be stored to protect them from light, heat and especially humidity. This can be done in glass jars, plastic food boxes, or even plastic bags (they must have no holes). Remove as much air as possible by filling the container completely. Plastic bags can be squeezed to remove nearly all the air. Store these in a place that is colder than room temperature, if possible. Most dehydrated products last a year or more, with adequate care during the drying and packaging stages.
Dehydrated fruits and vegetables can be re-hydrated by cooking, steaming, boiling, or in cold liquids such as water, juice, milk, or even yogurt. They can be mixed with grains and nuts to make granola, or eaten by themselves like raisins. With experience, they can provide an economic and gratifying alternative to canning, pickling, freezing, or industrialized preserved foods, aiding the family's health as well as the pocketbook.
More useful resources
How To Build a Solar Food Dehydrator
6 Simple Food Preservation Methods
The Secret to Making Great Dehydrated Food
Does the dehydrated foods lose Their Nutritional Value?
Dirty Secrets of the food processing industry
How Does a Food Dehydrator Work?
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Edited by: Rajesh Bihani ( Find me on Google+ )