Solar food drying is a very simple, ancient skill. It requires a safe place to spread the food where dry air in large quantities can pass over and beside thin pieces. Sun is often used to provide the hot dry air. Dry, clean air including dry cold air from any source will dehydrate food.
Draping food over branches or spreading it on wide shallow baskets on the roof is an old widespread tradition still in use around the world. Many other arrangements have been used to support a thin spread of food pieces. Some options that have been used are to thread the pieces on a cord or a stick and hang it over a fire, wood stove or from the rafters. Or one can bundle herbs or strawflowers and suspend them from bushes or a door knob or nails in rooms with good ventilation. Screen doors placed across chairs or sheets hung between clothes lines or possibly on a quilting frame have also been used. Vans, clean garages, or backseats of cars can be safe places to spread trays of drying food just as well as specially constructed cabinets. In the pioneer tradition food might be spread in the attic or in an upstairs room with screened windows wide open.
Modern variations are to build special enclosed drying racks or cabinets to expose the food to a flow of dry air heated by electricity, propane or solar radiation are a modern variation. These are refinements not essential to the basic process but handy, particularly in the humid tropics or when the rainy season coincides with the harvest.
Natural ventilation may be used in dry areas such as the American southwest or the Arctic. If necessary, the drying capacity of the air can be increased by heating it, which lowers the relative humidity. While any source of heat may be used, solar energy is free and usually plentiful in season. A solar heating panel screened on both ends with air intake on one end and opening to the food at the other is universally used to solar heat air. Hot dry air may be moved over the food by use of natural convection or a solar chimney or a fan run on solar electricity.
Trays need not be bulky and in fact lightweight ones with open screening block less airflow and so are preferable. Screening may be woven out of local materials or may be commercial screen of non-toxic materials such as nylon and some plastics. Fiberglass window screening is not recommended as it is coated with vinyl that may contain flame retardants and other chemicals. Open weave organic fibers and nylon material works fairly well, but can be difficult to clean. The usual commercial bridal veil is too fragile to last as screening on the trays but may be spread over top to control insects. Avoid screen materials that may contain toxic chemicals or additives. Galvanized metal screens or aluminum or copper screens are not recommended as potentially toxic salts can migrate into the food. Top quality food drying screens are made from food-safe plastic screening, such as polypropylene, which is available from Living Food Dehydrators and Sunworks Technologies. (See Bibliography)
Trays, if used, are sized to comfortable dimensions (70 cm. (24") x 70 cm. (24") or 51 cm. (20") x 76 cm. (30"), for instance). Then a supporting rack is made to that size. Air flow is essential so it is important that trays be sufficiently far apart to ventilate properly -- 15 cm. (6") to 20 cm. (8") if using natural ventilation. Less for forced ventilation. Tray frames should be light, but strong -- 3 cm. (1.25") x 6 mm. (1/4") or 2 cm. (3/4") x 1.25 cm. (1/2") small wood strips are sufficient for most purposes. The wood strips for the tray frames are cut to the full length and the full width of the tray. They are overlaid at the corners, notched if they are very thick, glued and screwed. Or they may be nailed with small nails which are bent over on the under side and pounded flat. Screen is stapled on. It may be secured with silicon sealant or thin lightweight wood or both.
One or two heavier screens made from 2.5 cm. (1") x 5 cm. (2") pine and covered with galvanized hardware cloth, are useful for drying non-food items -- clothing, wool, kindling and so forth. For support of extra heavy loads, rigid galvanized trays may be used under food safe screen. Galvanized screens may also be used to make fruit leathers. The sauce is protected from the galvanized metal by a sheet of Tedlar or of regular kitchen plastic taped to the frame.
Indoors, it is easy to use screened trays placed around on chairs or saw horses. No further equipment may be needed. Outside, the food must be protected against insects and animals and moved or covered in case of rain or blowing dust. Exposed trays also must be carried in at night and out again in the morning to prevent rehydration from the dew even in the desert.
A single layer of trays outdoors may be covered with sheets of cotton, glass or plastic through which the sun falls on the food. Sunlight heats the food driving out moisture. The moisture-laden air falls down from the bottom of the screened trays. In this generic design, the food is usually exposed to direct sunlight. Direct sunlight destroys some of the more fragile vitamins and enzymes and the food loses color. The better quality food is produced by flat screen designs having a dark sheet of cloth or metal that shades the food. This metal shield slows the drying but these designs are still very productive. Their disadvantages are they spread out over a larger area of ground than the cabinets and they tend to blow over in gusty wind.
One simple, open-air dryer design that can be used indoors or outside is called the Kerr-Cole Z-dryer. This is a rack of trays 15 cm. (6") to 20 cm. (8") apart stacked in a frame. The frame of open racks is braced with a diagonal piece of wood forming a Z. (See the bibliography). The frame can be sized to fit an available space such as the back of a car, or some place in a well-ventilated room, etc. Also Z-dryers also can stand in the yard, optionally covered with a light cloth. Or one can be fitted to go into a solar heated, ventilated cabinet such as a downdraft solar food dryer. Without increasing drying time very much such an open rack may be covered with a lightweight cloth to protect from insects. Ants and other crawling insects may be blocked from the trays of food by placing the feet of a rack in containers of water. Such moats can be used with any of the stacked designs. If ants and some other insects invade the drying racks, it is difficult to remove them. However, they will go home at night. Protective measure instituted early the next morning can block their return.
UPDRAFT SOLAR DRYER designs are the most frequently seen cabinet form. In this design, the hot air flows upward through a solar heat collection trough and enters the bottom of a cabinet underneath the food. The dry air rises through the trays and around the food, exiting through a vent at the top or near the top of the shadowed side. (See the Bibliography -- Valdez) The theoretical basis for this design is that hot air rises and therefore when heated, the air flows naturally upward through the trays of food.
DIRECTLY HEATED SOLAR cabinet dryers allow the sun to directly heat and dry the food inside an enclosed one-piece cabinet. Direct heating tends to be very efficient and produces fast drying. Proper air flow is essential to achieve maximum performance.
Solar food dryers can be hybridized so they continue to dry during cloudy weather. Removing a tray or two from the bottom, a very small flame from a 16 oz. or picnic size propane burner can be placed on the bottom. Alternately a small electric heating unit may be used. Heat rises and triggers the same effect as the solar heat. It is important to avoid overheating the food, maintaining temperatures below 49°C 120°F.Food drying is not difficult, although some books give considerable details on handling each food differently. Special guidelines are needed for handling jerky and fish. (See bibliography and your own recipes.) General guidelines for fruits and vegetables follow:
- Good quality food cut in thin pieces, not more than about 6 mm. (1/4") to 1" (3/8") thick
- Spread thinly on trays initially so there is a third to a half of the screen area clear for the passage of air. Can be more compact after the first moisture is gone.
- Label all food on the trays and carry the label along through processing to storage.
Thick vegetables need to be blanched or lightly pre-cooked. Leafy vegetables may be wilted slightly with steam, or dried directly from the garden. Cook potatoes and green beans completely. Fruit is pretreated by dipping in quite sour lemon water, or Ascorbic Acid (Vit.C) 2000mg (.07 oz.)/1.1 litre (1 quart). After a short soak, the fruit is drained and spread on trays. This retards browning. (Later, the sour soak water flavored by the fruit can be used as a concentrate for making cool drinks.) Sugar, honey or salt are optional. Sulfuring is no longer used in most homes due to the possibility of breathing the hazardous fumes and allergic reactions to sulfur compounds. It has been found to be unnecessary anyway.
To test if food is sufficiently dried, remove a piece and let it cool. Vegetables should be brittle. Fruits, because of their sugar content, may never get beyond a firm bend or leather quality and it is okay if they become a little brittle.They just need a little more soaking or chewing time for full flavor to develop.
All dried foods may be pasteurized after drying except for the greens and herbs. Spread dried food no more than 2.5 cm. (1") thick on a metal tray and cover it with another dark metal tray. A solar oven is excellent for this. Place it in an oven at temperature between 93°C (200°F) and 105°C (220°F) for 10 minutes. Stir and leave another 10 minutes. Do not overtreat. Cool and package immediately. Store in a dry location as cool as possible.
For people on the move or with limited storage space, plastic freezer bags are safe, durable and easily transported. Food should be put in small clean bags, labeled and dated. The smaller bags can be grouped into larger freezer bags, giving larvae two layers to penetrate if they attempt to invade. For those who avoid plastic, glass jars or metal containers with tight lids do well. Pack to eliminate air.
Indians in the American Southwest sometimes stored dried food in large earthen jars packed very tightly and covered with leather tied on tightly. These jars were kept on the roof and so were subject to low temperatures at night. Others stored dried food in hay-lined pits lined with flat rocks to deter rodents. These pits were so deep a person had to be assisted to get out. These were covered with leather or boards as a rain protection.
All stored food should be checked periodically for weevils. Weevils are a small, relatively clean insect. Infestations come from eggs hatched in your storage area. They grow to about 13 mm. (1/2") long and then go into a small webbed cocoon. The mature form is a thin, gray-brown moth about 13 mm. (1/2") long. Infestation can be controlled by eliminating the adult moths before they lay their eggs. Once hatched, the larvae feed only on your clean food. The form most prevalent in the USA is found as white, soft-bodied active "worms" with dark heads. Since they feed only in the stored food, they do not usually carry disease or toxic contamination. Even if the larvae themselves are not visible, weevil infestation is easily recognized. Small brown granules in the bottom of the packages or 13 mm. (1/2") bits of white webs indicate infestation. The same pasteurization method can be used to kill adult and immature weevils. Treat and then sift out the residue. Pasteurized dried goods rarely show infestation if stored in airtight containers. If stored in bread bags, they almost always will.
Cooked weevil infested food is usually safe to serve and eat if food is scarce. Some cooks on seeing larva facetiously say "Just a little clean protein ... God's gift to vegetarians." But prevention is the best policy. Clean, quick handling and good packaging is the key along with storing at the lowest available room temperature. Below 70 degrees there is little or no weevil activity.
So drying and storing food is a simple process. Using dried food can be equally simple. Fruits or vegetables may be eaten out of hand. Or fruit may be rinsed with water, drained briefly, and placed in a closed jar in the refrigerator to soften. Any of the dried produce may be covered with boiling water to slightly above the food level. Food should then be tossed to insure all parts of the dried bits are in contact with water. Most foods are allowed to stand for 15 minutes (equal parts packed food and water) before being added to standard recipes. Heavier pieces may require more time to rehydrate to the center. Easier yet, they may be just thrown in dry by the handful into soups or casseroles. Crisp dried foods may be pulverized in a blender and added by the spoonful to recipes for breads, soups, casseroles, sauces, etc.
For instance, greens can be used as a regular dish by pouring boiled water over them and then continuing steaming for a few additional minutes depending on the type of green. Dried squash and small pieces of potatoes can be covered with boiling water plus about 13 mm. (1/2") depth. They are then allowed to stand for 20 to 30 minutes to become moistened to the center before starting to cook. Larger pieces of potatoes require soaking for several hours.
Nutritionally, dried food is ranked by the USDA as better than canning, just under freezing. The tastes are related to the food, but there is some uniqueness in their flavor and texture. This is similar to the differences between fresh, frozen, and canned foods...another variation in taste.
In conclusion, select good food, wash, slice, dip or blanch, spread on trays or other drying arrangement and allow to dry. Pasteurize, package with a label showing the date and anything special about the food and store in a cool dark dry location as much as possible.. Use in rotation, oldest packages first. If there is excess, it can be pulverized in a blender and added to many foods: breads, casseroles, soups, gravies, sauces.
Stored dried food is prepared every growing season. Dried food should be used within the next months as the nutritional value slowly drops over time. It is customarily eaten the following winter and then any remainder composed as soon as new foods become available. As you enjoy solar food drying, you are joining generations who have preserved foods in this manner over the ages. Please pass this simple basic knowledge long to your children or community.
News and recent developments
- November 2012: Solar Food Dehydrator - Stan Cajdler of Brisbane, Australia has developed a solar food dehydrator, which significantly reduces the formation of mold spores caused by the nightly cooling of food. Typically, the drying process can last for several days for successful solar food drying. In this design several water containers (for thermal mass) are positioned in the lower chamber of the dehydrator. As the collector plate heats up, it heats up the surrounding air, which rises through the drying chamber. During the day, radiant heat from the collection plates also heats the water containers, which slowly release heat overnight. This overnight thermal air movement through the upper chamber dramatically reduces the formation of mold spores. For more information on Stan’s solar cooker designs check out: Sizzling Solar Systems
- October 2012: Foresight Technology has developed a lightweight, large capacity, portable, solar food dehydrator. It is highly effective. Its unique four sided absorber design allows for an extended daily drying period. It is available for shipping.
- February 2011: A group from the UC Davis Program for International Energy Technologies installed a solar box dryer for drying fruit in Nicaragua. They worked with the local organizations Grupo Fenix and the Solar Women of Totogalpa. They also connected with students and faculty at the Alternative Energy Program at Nicaragua's National Engineering University, and the directors of the new dried fruit export company SolSimple. They plan to send a follow-up trip the summer of 2011. More Information...
- May 2006: John Maina of Kenya wins the Energy Globe Award for 2006. Employing solar energy for drying food & gaining income security In Kenya, 30-40% of vegetables and fruits are lost due to poor post-harvest handling. The lack of firewood, which is necessary for drying and treatment of durable goods, is one of the major reasons for the loss. Since 2002 SCODE (Sustainable Community Development Services) has employed a solar dryer in Kenya for the drying of harvesting produce. The advantages are obvious: solar energy is free and available virtually everywhere. The fresh produce can be made durable in a cheap way and market value will be increased. Through the fast drying process the farmers can raise harvesting production and therefore able to generate additional income. This means up to 50% more productivity. The time needed for collecting firewood before, can now be used for various other activities and deforestation is reduced as well. Currently 30 solar dryers have been installed and 920 farmers have been trained in the use of these. Thirty craftsmen have also been trained in the construction and installation of these solar dryers. The project contributes to an overall improvement of living conditions, family nutrition, environment protection and income generation. At the moment the project is implemented in the Rift Valley in Kenya and has the potential to be duplicated in other areas.
- March 2011: Analysis of the Drying Kinetics of S. Bartolomeu Pears for Different Drying Systems - Electronic Journal of Environmental, Agricultural and Food Chemistry
- January 2009: Drying Studies of Single Layer Thompson Seedless Grapes - R.L. Sawhney, D.R. Pangavhane, and P.N. Sarsavadia
- January 2009: Micro-enterprises in Solar Food Processing Technology - Case Study - M. Ramakrishna Rao, D.J. Rao, S.L. Kumar
- January 2009: Solar drying of fruits, vegetables, spices, medicinal plants and fish: Developments and Potentials - B. K. Bala & Serm Janjai
- January 2009: Solar drying of mushroom using solar tunnel dryer - B. K. Bala, M. A. Morshed, and M. F. Rahman
- January 2009: Studies on Fortification of Solar Dried Fruit bars - G. Sarojini, V. Veena, M. Ramakrishna Rao
- January 2009: Thematic studies for processing and preservation of food supplement, chilies and ginger by drying through solar energy - Ranjita Bezbaruah Sharma
- July 2006: The Granada conference paper covering the construction and use of a solar coffee bean dryer in Chiapas, Mexico - Maria Cristia Martinez Sosa and Cesar Estrada Aguilar
- November 2000: International Conference of Solar Cooking: Solar Drying - David Whitfield
- November 1984: Understanding Solar Food Dryers - Roger G. Gregoire, P.E.
- Handbook of Solar Air Heating for Drying and Space Heating - Planters Energy Network
- Detailed Project Report on Solar Fruits and Vegetables Dehydration - Planters Energy Network
- Detailed Project Report on Solar Air Heating for Paddy Drying - Planters Energy Network
- Dissemination of Solar Drying Technology for Industrial Sector Including Agro-Industries in the Country - Planters Energy Network
- Development and Market Analysis for Integration of a PCM Storage in Solar Drying Systems - Planters Energy Network
- Solar Food Dehydrator - Sunkeep: lightweight, effective, large capacity solar food dehydrator
- Country Lore: Super-easy, Low-cost Solar Dehydrator
- Chris's ENGR305 Solar Food Dehydrator (made from cardboard) - Appropedia
- The Solar Food Dryer: How to Make and Use Your Own High-Performance, Sun-Powered Food Dehydrator, by Eben Fodor (2006)
- Solar Drying - presented by David Whitfield at the International Conference on Solar Cooking in Kimberly, South Africa in 2000 (PDF version here)
- El Paso Solar Energy Association materials on solar drying
- Solar food dryers at Build It Solar
- Guia de Uso de secaderos solares - Martin Almada (English version)
- Full text of book: Solar and Energy Conserving Food Technologies: A Training Manual - (US Peace Corps, 1984, 175 p.)
- Full text of book: Solar Drying in Morocco (GTZ)
- Solar Drying - Practical Action
- August 2012: Argentino desenvolve fogão solar e invenções ambientais, no ES - G1 ES (English version)
Solar food drying bibliography
- Erickson, Duane, produces a small, hanging, screened food dryer. Duane Erickson Enterprises, 1170 Elgin Avenue, Salt Lake City, Utah. 84106.
- Fodor, Eben, The Solar Food Dryer: How to Make and Use Your Own High-Performance, Sun-Powered Food Dehydrator, 2006 by New Society Publishers.
- Jagadeesh, A. Director, Shri AMM Murugappa Chettiar Research Centre, Tharamani, Madra - 600 113 INDIA. Designs of simple basket dryers.
- Kerr, Barbara. The Sustainable Living Center. 3310 Paper Mill Road, Taylor, Arizona 85939 USA. Schematic of downdraft design.
- LIVING FOODS DEHYDRATORS, 3023 352nd SE, Fall City, WA 98024. A source of food-safe plastic screening. They also supply an electric heating rack that might be used for hybridizing a solar drying cabinet.
- MacManiman, Gene, DRY IT -- YOU'LL LIKE IT. MacManiman, Inc., P. O. Box 546, Fall City, WA. 98024. 1973. (Non-solar as of 1992, but a good source of drying literature, materials and recipes. Their very fine design of drying cabinet depends on electricity but the trays can be placed in racks for solar drying if desired, returning them to the electric cabinet only when solar heat is not sufficient.)
- Susan McClure and the staff of the Rodale Food Center. PRESERVING SUMMER'S BOUNTY, a Quick and Easy Guide to Freezing, Canning, Preserving and Drying What You Grow. Rodale Press, Book Readers' Service, 33 East Minor Street, Emmaus, PA 18098. 1998 ISBN 0-87596-979-8
- Rodale Plans, SOLAR FOOD DRYER edited by Ray Wolf. Rodale Press, 33 East Minor Street,
- Emmaus, PA. 18049. ISBN 0-87857-333-X paperback. 1981. Out-of-print. If you can get an old copy, this is a good source of how to prepare and use dried foods. Also contains plans for a downdraft solar food dryer and information on scientifically knowing when food is dry enough.
- Valdez, Arnold and Maria, "A Cookbook for Building a Solar Food Dryer." A good updraft design and instruction of use of dried foods. Sangre de Cristo Printing. 924 Main Street, Alamosa, Colorado. 81101.
- Z-DRYER, Kerr-Cole Solar Box Cookers, P.O.Box 27417, Tempe, AZ, 85281. $1 plus large stamped envelope.
[The original text for this article was provided by Barbara Kerr on November 2, 1998.]
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