Monday, September 30, 2013

Dehydrating food with a solar food dryer

I don’t know a gardener, myself included, who does not sometimes find himself or herself with more fresh food ripe and ready to eat than can be used even after striving for an extended harvest. We want to honor the food I grow and the efforts by many beings to make it available to us by eating all of it. I’ve already written about storing some crops indoors during the winter in this post. However, many crops cannot be stored without some processing. In this post I’ll discuss one of those methods, dehydration or drying, that we use to process some of the food we grow for storage and why and how we use a solar food dryer for this purpose.

I’ve arranged the post into sections. If you’ve not heard of preservation by dehydration before and want to know what it is and why and how to do it, read the whole post. If you know what dehydration is but wonder why anyone would use the sun as a dehydrator rather than buy one of the electrical appliances sold for the purpose, start at Solar Powered Food Dehydration. If you want to skip directly to the design we used and the modifications we made to it, start at Our Solar Food Dryer. And if you just want to know what foods we’ve dried and how we use them, skip to Foods We Dry.

One of the ways to preserve foods for a longer period of time than they normally last after harvest is dehydration or drying. Dehydration reduces the moisture content enough that spoilage organisms such as bacteria, yeasts, or molds cannot survive to attack the food during storage. Dehydration also concentrates the natural sugars in foods, making them as or in some cases more appealing to the palate as the fresh version. And dehydrated foods retain most of their nutrients, though if you choose to blanch them first vitamin C will be destroyed. Drying is well suited to fruits (including starchy fruits such as squash that we usually call vegetables), some green leafy vegetables, herbs, mushrooms, and seeds and grains. With care meats can be dried as well.

Seed and grain crops like beans and corn dry well enough in the range of temperatures and humidity found in a typical residence by spreading them in a thin layer on a screen and perhaps stirring them on occasion. They will dry sufficiently for long-term storage over a time span of a few days to a few weeks depending on their water content at harvest and the relative humidity in your residence. Sunflower seeds can be dried within the seedhead if the relative humidity of the surrounding air is low enough; otherwise remove them from the head and spread them on a screen as you would with beans or corn. I leave corn on its cob and beans in their shells while they are drying, removing the cob or shell after drying is complete. The dried seeds can be stored in glass or plastic containers (glass is preferable if you have a rodent problem).

Most herbs have a low enough water content that they can be dried successfully by the methods suggested in herb books: bundle the stems together and suspend the bundles in a location that does not receive direct sun, or put seedheads into a paper bag and remove the seeds when they and the seedheads are crispy dry. I suspend bundles of herb stems from a clothes drying rack located in a bedroom on the north end of the house, removing the leaves, the part generally used, when they are crispy dry and storing the leaves in a glass jar. I also use an apparatus called a Food PANtrie hanging from a homemade PVC pipe rack in our kitchen, as shown in the photo below, to dry herbs.

By cutting stems of herbs to a size that fits on the screens and piling the stems onto the screens, I can dry herbs successfully indoors through the summer and fall. As with the bundle method, I remove the leaves when they are crispy dry and store the dried leaves in a glass jar. Use your favorite search engine to locate a retail source for the Food PANtrie.

The problem with dehydrating wet foods like fruits and vegetables is that water is not easy to remove from the ripe food. You need to put a lot of heat into water to get it to evaporate out of the food that it’s in and into the air that surrounds the food, and you need to move that now water-laden air away from the food so that drier air can replace it, air that can now receive more water evaporated from the food. One way to do that is to use electricity to heat air and circulate the heated air around the food that you want to dry. The heated, moist air moves out of vents in the top or back of an electric dehydrator while drier, cooler air flows in at the front or bottom of the unit to continue the drying cycle. Countertop-sized and larger electric food dehydrators are available for purchase from brick-and-mortar stores and online. Used ones might be found cheap at a yard or estate sale or through Craigslist. While I don’t discourage anyone from using an electric dehydrator, especially if you don’t have a location suitable for a solar food dryer, I choose not to use an electric dehydrator because our electric usage is already higher than I’d like it to be. Plus during the summer when some of the fruits we dry are harvested it’s hot and humid in the house as it is. I don’t want to add any more heat and humidity to the inside atmosphere.

Solar Powered Food Dehydration

If you don’t want to use electricity to dry wet foods, what can you use? The sun is a source of heat that can be put to use to dry foods with the proper equipment, called a solar food dryer. If you live in an area that can count on several days in a row of hot, sunny, low humidity weather a solar food dryer could be as simple as a wooden frame with food-safe polypropylene screening tacked onto it and covered with finer food-safe screening to exclude insects. You’d place slices of the food you want to dry on the screen frame, cover it with the fine screening, and put it someplace where it will receive sunlight and air circulation for several days until the food is dry. The Food PANtrie I mentioned above was developed for this purpose. However, in the St. Louis region we don’t get this kind of weather during summer and fall, when we have fresh foods available for drying. In our humid climate, I cannot dry even thin-walled hot peppers properly in the Food PANtrie, much less any wetter foods.

To dry them I need a solar food dryer that can collect sufficient solar heat to heat air to at least 120F, preferably hotter, and move the air over the foods to be dried. John Michael Green in his book Green Wizardry points out that the 1970s proved to be a fertile time for the development of solar food dryer designs. The basic design is as follows. A collector (a large, thin box with a black metal bottom covered with a transparent material and provided with vents at the bottom and top) is placed so that sunlight falls on it to heat up the metal. Air that enters at the bottom passes over the heated metal and heats up, reducing its relative humidity. That heated air rises and the rising, dry air is channeled through the top vent and over foods arranged on screens, with the now moist air finally exiting above the screens. A number of individuals and groups developed successful designs. You can find plans in appropriate technology books from the 1970s and by using your favorite search engine to see what the web has on offer.

Our Solar Food Dryer

The solar food dryers designed during the 1970s have a high capacity but are large and difficult to move. They are meant to remain outside for months at a time, which subjects them to the forces of wind and rot. I wanted a smaller version that I can move at will by myself to keep it out of the heavy rains and severe storms that we are prone to receive. The design that Mike used to to make our solar food dryer is found in the book The Solar Food Dryer: How to Make and Use Your Own High-Performance, Sun-Powered Food Dehydrator by Eben Fodor. I recommend this book to anyone who wants to make a portable solar food dryer with a capacity of up to six pounds.

Our dryer is shown in action in the photo at the beginning of this post. Mike made ours to the dimensions specified in the book. You can purchase a kit containing the materials needed to construct this solar food dryer from SunWorks or buy or find materials locally as we did. SunWorks also sells the screen material separately in the size needed for making the dryer to the dimensions in the book. We purchased the screen material from SunWorks.

Mike’s basic carpentry skills and tool set were sufficient to the task, though he made some modifications to the plan to adjust to materials we had on hand and to make aspects of the project easier. He didn’t include the two small side vents in the plan, an oversight on his part. The dryer works fine without them, however. He didn’t include the friction lid support in the plan that holds the vent door open. In the photo below you’ll see that in its place I tied a piece of twine to one of the knobs on the vent door.

The twine can be positioned underneath the window or the brick weighing the window down so that the vent door is held open to the desired amount. That is determined by checking the temperature within the dryer with a probe-type thermometer. The photo above shows ours in action.

Mike didn’t include light bulbs for supplementary heating at night so we either leave the food in the dryer overnight or, when we are concerned that doing so will allow for contamination, take the food out of the dryer in late afternoon and refrigerate it until we put it back into the dryer for further drying. Nor did he include the foam weatherstripping along the top edge of the dryer; it did not prove to be necessary. After attempting three times to cut salvaged storm windows to size and succeeding only in breaking the windows, we gave up and simply laid a storm window on top of the finished dryer, as shown in the photo at the top of the post. I use a brick to weigh the glass down to prevent wind from blowing the glass off the dryer. That’s good enough for the usual breezes we have and I have added another half brick on windier days, but I don’t use the solar food dryer on very windy days to avoid the risk of the glass sheet blowing off. If you live in an area where sunny days are also consistently windy, you’ll need to use the attachment system in the book to keep the glazing on the dryer. If you prefer not to use glass for glazing, polycarbonate sheet is an excellent substitute. It’s easier to cut and lighter than glass, plus it’s unbreakable.

Until the dryer was finished I did not realize that our house doors are too narrow to carry the dryer through the door in its usual orientation. In order to move the dryer in and out of the house, I had to tip it on its side and ease first one pair of legs, then the body of the dryer, and then the other pair of legs through the door. Because of this added difficulty I did not use the dryer as much as I could have. Mike cut the legs shorter to ease this problem, but it was still awkward to get the dryer through the door opening. Once we got our garden shed with its four foot wide door opening, I could carry the dryer through the shed door in its normal orientation. The shed is also much closer to the sunny area where I put the dryer than the house is. Now that I store the dryer in the shed I use it more often. I recommend you measure your door first and modify the plan in the book if needed so your dryer will fit through the door you’ll carry it through.

Not only is the dryer too wide to fit through our house doors but the screens are also too wide to fit through them so I cannot load the screens in the kitchen. Instead I load the screens by placing them on a plastic table outside that is wide enough to hold the screens. I weigh the food before putting it on the screen to make sure I do not put more than 3 pounds of food on a screen, as recommended. Leave enough room between pieces of food for sunlight to strike the collector and for air to pass through the screens, as shown below. The back door is open in the bottom photo, showing two loaded screens in the dryer sitting on wood tracks.

You’ll need to put the dryer someplace where it gets several hours of morning and afternoon sun. If you’re putting the dryer on a lawn with an uneven surface, I suggest putting a sheet of wood on the ground first and placing the dryer on the sheet of wood for added stability.

We found that ants are able to get into the dryer and eat the food when the dryer is not hot enough to kill them, usually when I leave food in the dryer overnight. If you have light bulbs for supplementary heating you may not have problems with ants getting into the food overnight. I solved the ant problem by putting a reused metal can under each leg of the dryer and filling the metal can with water. Ants cannot cross the moat of water in the can. As long as no plant stems contact the solar dryer anywhere else and you refill the cans as the water dries, the dryer will remain ant-free. The other way to solve the ant problem is to remove the food in late afternoon and store in in the refrigerator overnight, returning the food to the dryer the next morning to finish drying.

Foods We Dry

Generally speaking, foods are left to dry until they are leathery or crisp. In July or August foods will dry enough in two to three days of clear to partly cloudy skies. By now the days are short enough and the sun is low enough that it takes four or five days to dry zucchini. If cloudy skies or rain occurs before the food is dry, I store the food in the fridge till sunny weather returns. Also by this time of year the air coming in the dryer is cooler, reducing the drying efficiency further, and we start to get a higher proportion of cloudy to clear days in our region. Solar drying is not practical here after the end of October. In late spring the weather warms enough to start using the dryer.

The foods we have dried so far in our solar food dryer are as follows. I wash and cut ripe food to size before loading it onto the screens. I don’t blanch foods; it doesn’t seem to be necessary.
    Shiitake mushrooms: these dry quickly, in two days. We remove the stems and dry the caps gills up the first day, gills down the second. Other mushrooms would likely dry similarly.
    Herbs: when we have more herbs than we can dry in the house, we dry them in the solar food dryer. Herbs should dry at lower temperatures, under 120F, so I cut down on sunlight entering the dryer by covering the window with a white dish towel and leaving the vent wide open. Herbs dry in a day or two.
    Beef: Mike has dried thin strips of marinated beef into jerky. He only does this in summer, when the temperature inside the dryer goes to 150F and above for at least a few hours, and he removes the beef in late afternoon and keeps it in the fridge overnight if it is not yet dry. It takes two or three days to dry the beef to the jerky state.
    Tomatoes: Any tomato will dry successfully if it is cut into 1/8-1/4” slices and the slices laid on the screens. Since they are watery they need three or more days to dry fully.
    Hot peppers: I tried drying them whole but it took many days to dry them. It worked better to cut them in half and lay the halves on the screens. The time it takes depends on how thick the walls are. Sweet peppers should dry well but I expect they will take longer than hot peppers and would probably dry best if cut into strips.
    Zucchini: I dry the variety ‘Costata Romanesca’ which remains tender even at 4 pounds in size, cutting them into 1/8” think rounds. It’s a great way to make use of the zucchinis you don’t find until they are huge. You don’t need to remove the seeds or skin. Dry the rounds till they are crisp and you have an excellent substitute for tortilla or potato chips!
    Peaches: Halve and pit them, then slice them about 1/8 to 1/4” thick and dry the slices. The sweet concentrated peach flavor is delicious!
    Strawberries: Depending on the size of the berries you can halve or quarter them. As with peaches and tomatoes, dry them thoroughly so they won’t mold in storage. Another concentrated flavor delight!

Just because I haven’t listed it here doesn’t mean it won’t dry well; it just means I haven’t had enough of it at the right time to dry it. Kale, for instance, should dry nicely and I plan to try it when I have enough excess kale.


  1. Hi Claire-

    Great post! We've had great success drying kale and chard every harvest season. It's a great addition to soups in the winter. Also, if you end up with a bumper crop from your plum or apricot trees, they dry well (they start drier than peaches) and several varieties are improved by drying. I never would've expected to eat so many prunes until much later in life, but they're convincingly delicious!

    Thanks for the good design review of the SunWorks kit. The door size issue is a great consideration that I wouldn't have noticed if you hadn't mentioned it, so thanks for that. How long do you typically keep foods that you've dried, and do you have issues with them "going bad" after a certain number of weeks/months?

    On another note, the New Society site also has fixed links available for each of the books you mentioned if you'd like to use them. They are:

    Cheers, and I hope your preserved foods and late harvest are bountiful and delicious.

  2. Thanks, Steve! I like kale chips a lot but haven't dried kale because I haven't grown it in the spring or summer for the last few years. That in turn is because I've had problems with harlequin bugs attacking mature kale plants during the summer. I'm considering growing kale again in spring and summer next year, removing and drying mature plants before the harlequin bugs get bad. Then we could have dried kale for use in the winter and spring.

    Thanks for the fixed links too! I'll edit the post to include them.

  3. Yes, a great post, very informative. Thanks for posting it. I've reposted a link over on the Green Wizards site.

  4. Food dehydrators are perfect for people who want to make nutritious food that can be easily stored to be used in meals and as snacks later. Cooking healthy meals can be a challenge because they typically require special appliances, time and a lot of kitchen space. Website