All modern, reputable home food preservation sources recommend that almost all home food drying be done with a dedicated, purpose-made electric dehydrator.
The recommendation is for overall quality reasons, rather than safety.
The Ball Blue Book says, “Commercial or homemade electric dehydrators provide the most reliable and consistent results.” [1]Ball Blue Book. Muncie, Indiana: Healthmark LLC / Jarden Home Brands. Edition 37. 2014. Page 162.
The University of Minnesota Extension says that electric dehydrators “produce the best quality product as compared to other methods of drying.” [2]Jopp, LouAnn. Methods for drying food at home. University of Minnesota Extension. Accessed January 2018 at https://www.extension.umn.edu/food/food-safety/Preserving/drying/methods-for-drying-food-at-home/
That being said, here is an overview of all the possible methods of drying food at home.
(To be clear, this is drying, not freeze drying.)
Air drying
This method is also referred to as “room drying.”
Michigan State Extension says,
While Michigan State University Extension does not normally suggest air drying, it can be done under certain conditions. Air drying can be done but it needs to be done on a hot, dry (low humidity), day. Or in a room that is hot and dry with an air current.” [3]Venema, Christine. Drying herbs can be done using one of four techniques. Michigan State Extension. 2 January 2013. Accessed January 2018 at https://msue.anr.msu.edu/news/drying_herbs_can_be_done_using_one_of_four_techniques
So Easy To Preserve says,
Herbs and peppers can be strung on a string or tied in bundles and suspended from overhead racks in the air until dry. Enclosing them in paper bags, with openings for air circulation, protects them from dust, loose insulation and other pollutants. Nuts are spread on papers, a single layer thick.” [4]Andress, Elizabeth L. and Judy A. Harrison. So Easy to Preserve. University of Georgia Cooperative Extension. Bulletin 989. Sixth Edition. 2014. Page 336.
Penn State Extension says,
Room drying at room temperature works only if heat, humidity, and air movement are adequate. Today’s air-conditioned homes may be too cool to dry foods quickly enough. While sun drying works in dry climates, the high humidity in Pennsylvania makes this method impractical here.” [5]Drying Fruits and Vegetables (Dehydration). LaBorde, Luke and Martha Zepp. PennState Extension. Code EE0065. August 2017.
Sun drying
Sun drying is just spreading things out in the sun and letting them dry. It’s how some raisin producers in California’s San Joaquin Valley still dry their grapes into raisins.
The electric dehydrator book, Preserve it Naturally, acknowledges the upsides to sun drying:
This is the original dehydration method…It is the least expensive way to dehydrated food and it can accommodate large quantities at one time.” [6]Excalibur. Preserve it naturally. Sacramento, California. 4th edition, 2012. Page 9.
It then goes on to highlight why sun drying works for so few people nowadays:
… trying it at home is more trouble than it’s worth. Sun drying demands near-perfect low humidity conditions and temperatures in the high 80s F to assure a reasonable amount of success. And even if you do manage to meet these rather difficult requirements, food dried in the sun will take several days as compared to several hours in a dehydrator. Because sun drying takes so long, the food produced is of lower quality and nutritional value. Food is at the mercy of insects, dirt, and the elements.” [7]Preserve it naturally. Page 9.
The Ball Blue Book says
Natural methods, such as sun and room drying, require warm days of 90 F or more, low humidity, little air pollution and control of insects for a quality finished product.” [8]Ball Blue Book. Edition 37. 2014. Page 162.
So Easy to Preserve says,
The high sugar and acid content of fruits make them safe to dry out-of-doors when conditions are favorable for drying. Vegetables (with the exception of vine dried beans) and meats are not recommended for out-of-doors drying. Vegetables are low in sugar and acid. This increases the risks for food spoilage. Meats are high in protein, making them ideal for microbial growth when heat and humidity cannot be controlled. It is best to dry meats and vegetables indoors using controlled conditions of an oven or food dehydrator.
To dry fruits out-of-doors hot, dry, breezy days are best. A minimum temperature of 85ºF is needed with higher temperatures being better. It takes several days to dry foods out-of-doors. Because the weather is uncontrollable, drying fruits out-of-doors can be risky. If it rains in California while the grapes are drying, the entire supply of raisins can be destroyed.
High humidity in the South is a problem for drying fruits out-of-doors. A humidity below 60 percent is best. Often these ideal conditions are not available when the fruit ripens and other alternatives to dry the food are needed.
Fruits dried out-of-doors must be covered or brought under shelter at night. The cool night air condenses and could add moisture back to the food, thus slowing down the drying process.” [See full entry here: Sun drying] [9]So Easy to Preserve. 2014. Page 330.
So Easy to Preserve goes on to advise against trying to sun dry herbs,
Sun drying is not recommended because the herbs can lose flavor and color.” [10]So Easy to Preserve. Page 359.
Cornell Cooperative Extension essentially says don’t even think about sun drying in New York State:
Although drying foods in the sun is a popular method in some areas of the United States and the world, New York State does not have a suitable climate for it. Successful outdoor drying requires several days of at least 85°F with humidity below 60%. Typically, in New York State when the temperature is high enough, the humidity is too high for drying food successfully.” [11]Katherine J. T. Humphrey and Judy L. Price. Drying Foods in New York State. Cornell Cooperative Extension. 2011. Accessed January 2018 at https://blogs.cornell.edu/cceherkimer/files/2009/04/2011_Drying_Foods_in_New_York_State.pdf
Pasteurization for sun drying
When you sun-dry anything, there is a second step you must do, which is to pasteurize the produce so that it does not become infested with insects during storage.
So Easy to Preserve says,
“Sun dried fruits and vine dried beans need treatment to kill insects and their eggs.
Freezer Method – Seal the food in freezer-type plastic bags. Place the bags in a freezer set at 0ºF [-17 C] or below and leave them at least 48 hours.
Oven Method – Place the food in a single layer on a tray or in a shallow pan. Place in an oven preheated to 160ºF [71 C] for 30 minutes.” [12]So Easy to Preserve. Page 332.
Note: Cornell State Extension seems to suggest pasteurization for any dried food not blanched or cooked first.
Pasteurizing: Dried food not previously heat-treated by blanching or cooking should be pasteurized to insure destruction of insects and insect eggs. Pasteurization is the last step before packaging for long storage.” [13]Katherine J. T. Humphrey et al. Drying Foods in New York State.
Vine drying
So Easy to Preserve says,
To dry beans (navy, kidney, butter, great northern, lima, lentils and soybeans) leave bean pods on the vine in the garden until the beans inside rattle. When the vines and pods are dry and shriveled, pick the beans and shell them. No pretreatment is necessary. If beans are still moist, the drying process is not complete and the beans will mold if not more thoroughly dried. If needed, drying can be completed in the sun, oven or a dehydrator.” [14]So Easy to Preserve. Page 332.
The University of Kentucky Extension says,
[Vine drying] is used for mature beans and peas such as limas, pintos, lentils, and blackeyed peas. Dry these in their pods on the vine until they rattle when shaken. The vines will be shriveled. Pick and shell the beans. Test for dryness (they should shatter when hit with a hammer). If the test beans are still moist, leave the rest on the vine to further dry them. You will need to pasteurize food that has been vine dried.” [15]Daly-Koziel, Kathy and Fudeko Maruyama. Drying Food at Home. University of Kentucky Cooperative Extension Service. H.E.3-501. 1997. Accessed January 2018 at https://johnson.ca.uky.edu/files/drying_food_at_home_0.pdf
The University of California Extension Master Food Preservers emphasized in a slide presentation what vine drying can be used for: “Vine drying is for beans only!” [16]Mosbacher, Sue. UCCE Central Sierra Program Representative. Master Food Preservers & Master Gardeners. Drying Fruits and Vegetables. PowerPoint Presentation. Slide 5. Accessed January 2018 at https://ucanr.edu/sites/cottagefoods/files/209273.pdf
[Note: some commercial producers vine-dry raisins (see: Calraisins), but that is beyond the scope of this home survey piece.]
Solar drying
Solar drying involves using a contraption designed to capture and magnify the heat from the sun, as well as to help protect the food from the elements and pests.
So Easy to Preserve says,
Solar drying uses the sun as the heat source, but a specially designed dehydrator increases the temperature and air current to speed up the drying time… Solar dryers use a reflectant such as aluminum foil or glass to increase the sun’s temperature by 20 to 30 F . Air current is improved by using air vents at each end. Cooler air enters the dryer, crosses the food, removes moisture and escapes. Plastic covers the frame and prevents rain or condensation from dampening the food. Screens over the events keep insects and birds off the food.” [17]So Easy to Preserve. Page 331.
Preserve it Naturally says,
Solar drying is somewhat more efficient than traditional sun drying because of the increased temperatures. It also refines the drying process by (a) a tracking system to follow the sun, (b) a venting system to control the temperature, (c) enough space for construction and efficient operation, and (d) a back-up system to provide an alternative heat source and a fan to circulate air. However…. it is still unpredictable, slow, time-consuming and offers no assurance of food quality.” [18]Preserve it naturally. Page 9.
Microwave Drying
Michigan State Extension says,
Microwave drying can be used when only a small amount of herbs is to be dried. Follow the instructions that come with the microwave. When the leaves are crisp and crumbly between the fingers the herbs are ready for storage. ” [19]Venema, Christine. Drying herbs can be done using one of four techniques.
So Easy to Preserve says,
Microwave ovens are recommended only for drying herbs, because there is no way to create enough air flow in them.” [20]So Easy to Preserve. Page 333.
Clemson Cooperative Extension says, “Microwave ovens are recommended only for drying herbs, because there is no way to create enough air flow to dry denser foods.” [21]Schmutz, P.H. Drying Foods. Clemson Cooperative Extension. HGIC 3080. June 1999.
Cornell Cooperative Extension says, “Because of ineffective air circulation, food cooks before it dries in a microwave oven. Only herbs can be successfully dried in a microwave oven. Follow the manufacturer’s directions for specific ovens.” [22]Katherine J. T. Humphrey et al. Drying Foods in New York State.
The University of Kentucky Extension cautions you first to make sure that you aren’t going to damage your microwave:
There is some debate as to the advisability of drying herbs in the microwave ….. The microwave oven is not designed for drying fruits, vegetables, or meat. Some people have dried fresh herbs in the microwave, but doing so may result in a burned out magnetron or a fire. Read your manufacturer’s directions for information concerning microwave drying. Combination microwave/convection ovens use the convection cycle for drying. Again, read the manufacturer’s instructions.” [23]Daly-Koziel, Kathy et al. Drying Food at Home.
Oven drying
To be used for food drying, the oven needs to be able to go down at least to 60 C / 140 F.
If you try to dry food at a temperature much higher than that, you may end up cooking it. So Easy to Preserve says, “First, check the dial and see if it can register as low as 140°F [60 C]. If your oven does not go this low, then your food will cook instead of dry.” [24] So Easy to Preserve. 2014. Page 335.
Penn State Extension says,
Oven drying works well if you can set your oven to a temperature of 140 to 150°F [60 – 65 C]. Open the oven door 2 to 3 inches (5 to 8 cm) to allow moisture to escape. A convection oven works well because it combines low heat with a fan to move the air.” [25]Laborde, Luke et al. Drying Fruits and Vegetables.
The Ball Blue Book says,
Oven drying is a good choice only for small quantities of food because the energy costs of operating a gas or an electric oven are high compared to the cost of operating an electric food dehydrator. Also, a conventional oven only heats food and does not carry away moisture.” [26]Ball Blue Book. Edition 37. 2014. Page 162.
Preserve it Naturally says,
Oven drying, on the whole, is very ‘iffy’…. scorching is usually a major problem. Also, food frequently comes out more brittle, darker in texture, and less tasty…. the energy cost is usually substantially greater than an electric food dehydrator. Standard ovens … were manufactured for purposes other than dehydrating, therefore, oven drying is usually unsuccessful.” [27]Preserve it naturally. Page 9.
So Easy to Preserve says,
Everyone who has an oven has a food dehydrator. By combining the factors of heat, low humidity and air current, an oven can be used as a dehydrator. An oven is ideal for occasional drying of meat jerkies, fruit leathers, banana chips or for preserving excess produce like celery or mushrooms. Because the oven may also be needed for every day cooking, it may not be satisfactory for preserving abundant garden produce. Oven drying is slower than dehydrators because it does not have a built-in fan for the air movement. (However, some convection ovens do have a fan). It takes about two times longer to dry food in an oven than it does in a dehydrator. Thus, the oven is not as efficient as a dehydrator and uses more energy.” [28]So Easy to Preserve. Page 334.
The University of Kentucky Cooperative Extension gives some practical advice on assessing how well your oven would do for food drying:
Oven drying is a lengthy process that takes 2 to 3 times longer than an electric dehydrator. It also consumes more energy. If, however, you do not own a dehydrator, your kitchen oven can possibly be used to dry food. First, check your oven: (a) Does it have a vent? All gas ovens and most (not all) electric ovens do. (b) What is the lowest heat setting? You will need 140 F [60 C]. Very few newer ovens can be maintained below 200 F [ 90 C]. To determine the lowest setting place an oven thermometer in the rear of the oven. Prop the door open 4 inches (10 cm). Set the oven thermostat for its lowest setting and place a fan out and to the side of the oven door. Run the fan so that the air current flows through the oven and out the other side. Monitor the oven thermometer’s temperature reading. If the temperature runs greater than 160 F [Ed: sic], you cannot use the oven. This heat will slow cook, not dry, the food.” [29]Daly-Koziel, Kathy et al. Drying Food at Home.
The following points about oven drying are partly drawn from a PowerPoint presentation given by the UCCE Central Sierra Master Food Preservers. [30]Mosbacher, Sue. Drying Fruits and Vegetables.
Ovens compared to dedicated electric food dehydrators
- Ovens are slower (convection ovens less so): It takes at least twice as long to dry in a regular oven as it does in a dehydrator. Fruit leathers, for instance, take 6-8 hours to dry in a dehydrator, up to 18 in an oven.
- Ovens use more energy so therefore are mostly costly to use as dehydrators. Depending on the cost of cooking fuel where you are and how much drying you do, you could over time end up spending more than you would purchasing an actual dehydrator.
- Low capacity: few North American ovens come with more than two oven racks. That, and the amount of empty rack space around the trays recommended, allow for very little actual drying room.
To use an oven as a dehydrator
- Oven door must be left open 2 to 6 inches (5 to 15 cm) [Ed: see CAUTION below]. This allows moisture to escape the oven more easily.
- Ideally, place a fan outside the oven door off to one side, blowing air through that opening space. That can improve air circulation inside the oven.
- You can’t trust the temperature gauge on stoves in general to accurately reflect the temperatures actually being achieved in ovens at the best of times, and particularly with the door open, so it’s best to place a separate actual oven thermometer in the oven near the food to monitor the temperature. Check the temperature every few hours.
- So Easy to Preserve says, “For air circulation, leave the oven door propped open two to six inches. Circulation can be improved by placing a fan outside the oven near the door. CAUTION: This is not a safe practice for a home with small children. Because the door is left open, the temperature will vary. An oven thermometer placed near the food gives an accurate reading. Adjust the temperature dial to achieve the needed 140°F.” [31]So Easy to Preserve. Page 335
- Leave at least 2 to 3 inches (5 to 8 cm) of space between oven racks.
- When placing drying trays on the oven racks, ensure that the trays have 3 to 4 inches (7 to 10 cm) of space on each side of them away from the oven side walls, and 2 to 3 (5 to 8 cm) inches of space between oven front and back.
- The University of Kentucky Extension advises, “Remember to turn the food over approximately every half hour.”
Dedicated, purpose-built electric food dehydrator
The Ball Blue Book says,
Commercial or homemade electric dehydrators provide the most reliable and consistent results, often without pretreatment, because of the controlled temperature and air flow. Food dehydrated by this method dries quickly and evenly. The quality of the finished product can be excellent. Food can be dried 24 hours a day, summer or winter, rain or shine.” [32]Ball Blue Book. Edition 37. Page 162.
Michigan State Extension says, “The best technique for drying herbs is a food dehydrator. It is the fastest, easiest, way to produce high-quality dried herbs.” [33]Venema, Christine. Drying herbs can be done using one of four techniques.
Virginia Tech says, “Electric dehydrators take less time to dry foods and are more cost efficient than an oven.” [34]Boyer, Rene. Using Dehydration to Preserve Fruits, Vegetables, and Meats. Virginia Cooperative Extension. Pub # 348-597. June 1999. https://pubs.ext.vt.edu/348/348-597/348-597.html
See dedicated Electric Food Dehydrator page for full discussion.
References