GeoPathfinder

Energy Efficient Food Preservation

Once you've gotten started with gardening, you'll soon have to find a way to deal with the surplus (hopefully you've planned for this!). Reducing the amount of energy you use for food storage will further shrink the carbon footprint that you have reduced by eating locally.

The following information is from workshops presented by Larisa every year at the Midwest Renewable Energy and Sustainable Living Fair. If you missed the fair, or didn't get a handout, the printed materials are here along with some visual elaboration. Topics covered at the workshops included the solar food dryer we designed, steam-canning, steam-juicing, and root-cellaring. Larisa's book dealing with all of these topics (A Pantry Full of Sunshine) can be found on our Home Page.

Of course, food that is fresh is best, but our Minnesota Winters challenge that option. The next best thing to harvesting just before your meal is to have food that is "live stored" - food that keeps itself. All you have to do is provide the right microclimate for the veggies that have this hibernation factor built into their biological cycle. I recommend reading "Root Cellaring" by Nancy Bubel for inspiration on building and using a root cellar. My own methods rely heavily on insulated picnic coolers (owned by most folks but unused in the Winter months) which are moved from the north side of the house, to the sauna (not in use in the early Fall), and eventually to an insulated pit in the shed when Winter weather threatens. The handout from the fair shows temperature and humidity conditions for a variety of vegetables/fruits. "Root cellaring" is for more than roots! For a free Adobe PDF file download of this hand-out, just Click here.

A Solar Food Dryer That Works! (even in the humid upper-Midwest)

Over the years I've tried about every solar dryer design imaginable. The only common factor in all those attempts was their very limited usefulness here in the humid upper Midwest. None of them could reliably turn food into a non-moldy finished product, unlike the many successful electric models I had built for myself and friends. Some didn't work at all if not tracked periodically during the day. Others relied on electric "backup" at night. It was with this backround that the "idea light" came on in my head.

This is our finished solar dryer, 12 by 4 feet, with 2x2 foot stainless steel screens framed in 2x2 inch cedar, and one of the heat generating cover panels raised for access. Sunlight shines through the clear glazing, lowering its frequency to make more infrared (heat). It then hits black-painted aluminum, heating the metal. The black aluminum re-radiates heat onto the food below, causing it to heat and lose moisture. The moisture passively flows up the air channels under the food screens. And the galvanized steel roofing, whose ribs form the air channels, reflects heat back up toward the food.

This photo shows a side view of a model solar dryer (only 2 by 2 feet) that we show folks for construction details. You can see the loose-pin hinge connecting the solar collector with the base. The stainless steel food screen is at the bottom of the loose, 2x2" cedar-rimmed tray. And you can see the air space formed below it by the ribs in the galvanized roofing. More below!

The Hot Tin Roof Theory. One day (in 1985) I needed to dry a bunch of greens and the current solar dryer was full (a couple of handfuls was all it could handle). I had an old window screen laying around and a corrugated metal roof built over our old mobile home. Using a ladder to get on the roof, I put the screen down first and put the food on it. I wanted to warm the food while keeping the sunlight out so I covered it with a piece of black cloth. Then to keep everything from blowing away or being bothered by flies, I covered it with the storm window that was laying around with the screen.

Later that afternoon I thought I'd check up on the experiment. The greens in the old dryer were still quite limp so I crawled up the ladder to take a look at the stuff on the roof. Much to my surprise, the roof-top greens were crispy dry! It looked like I had finally stumbled on something that worked. I tried several other foods on the roof before I was convinced enough of the design to build a unit at ground level for easier access.

Basic Design Principles.

I found through experimenting that the primary ingredients for this idea were: * glazing (glass or greenhouse plastics) * black surface over the food (fabric or metal) * screen to hold food * corrugated, galvanized metal roofing tilted toward the sun

The sun shines through the clear glazing onto the black surface, causing it to heat up. This heat is radiated from the black surface and onto the food screen below it. The shiny, sloped metal roofing that everything rests upon reflects heat back up toward the food. Also, its corrugations provide an airspace under the screen for moisture-laden air to circulate up the slope and out of the dryer by natural convection. The food is not exposed to sunlight and retains its color. This combination of design factors also met the criteria of what I thought would be the ideal solar dryer: * utilizes passive solar energy * has no moving parts * no tracking required to follow the sun * food not exposed to sunlight * spacious enough to accommodate large pickings * moderate temperatures to dry quickly but not overheat * easy to use and clean * absolutely must work reliably, even in high humidity * not necessary to remove partially dry food from the dryer overnight * stable in windy locations * "critter" and bug-proof, for the most part

Building the Deluxe Super Dryer.

Using the basic principles and design criteria established from our experimentation, we built a 4-by-12 foot, waist-high "shed". The metal roofing on this shed has corrugations that run north to south. The roof angle is approximately 12-15 degrees from horizontal and slopes toward the south (in the northern hemisphere). This gives enough slant so the warm air will rise but not so much that the food will slide downhill. The mini "shed" space underneath can be used to store firewood or garden accessories. The 4-foot width enables food to be reached from either side yet is wide enough to achieve sufficient hot air flow. The legs are 2-by-2 inch treated wood and stick into the ground about 6-12 inches. Metal fence T-posts attached to the wooden frame with U-bolts are a good alternative if you don't like treated wood. Additional leg bracing may be necessary if your dryer is large. A good "starter" size is 4 by 4 feet with four 2-by-2 foot trays. You can make more modules of this size later if you need to add capacity.

Our dryer holds twelve 2-by-2 foot screens made from mitered cedar 2-by-2's, with a deck screw in each corner. The lower inside edge of the frame is cut away so that the screen is recessed. This eliminates the possibility of screen edges snagging clothes when you lean against the dryer. The screen is stainless steel which, although costly, is easy to clean, provides a non-toxic surface for the food, and should last a lifetime. A bead of high-temp, food-grade, silicone caulk keeps food particles from getting stuck between the wood frame and the screen. A less costly screen option is to use food-grade polypropylene screening (available from www.dryit.com). You will need to provide support under this material to prevent sagging. You could use galvanized metal fence wire or hardware cloth for this as the food would not come into direct contact with the metal. My original experiments used fiberglass window screen, but I have since discovered that it is coated with polyvinyl chloride that is stabilized with several substances, one of which may be lead. Please do not place food in direct contact with any material that is not absolutely food safe. This would include plastic garbage bags/cans, galvanized metal, and aluminum screening or cookware.

The cover framework can also be constructed from 2-by-2 inch mitered cedar. The glazing is Kalwall brand, 40 mil, fiberglass reinforced polyester. It holds up better than glass in hail storms and weighs much less. My neighbor built a dryer but used acrylic glazing. The acrylic was much cheaper initially but needed replacement after three years of use due to the formation of numerous cracks. New dryers made by friends and neighbors use locally available clear, corrugated fiberglass greenhouse glazing. It is much less expensive than Kalwall while appearing to be just as durable.

Instead of the black cloth used in the original experiments we now use a layer of thin aluminum sheet, painted with "Barbeque Black" paint on both sides, stapled to the underside of the cover framework. The top is black to absorb the sun's heat and the bottom is black to re-radiate that heat onto the food. You could use steel sheets (28 gauge or lighter - not galvanized so paint will adhere better) or used "printer plates", but aluminum "flashing" is easy to find in 2-foot widths. The black metal heat collector is more convenient to use, less messy, and probably less expensive in the long run than black cloth.

The cover framework is attached to the dryer base with T-strap hinges on both the north and south sides. These were made into loose-pin hinges so you can open the dryer from either side by pulling the pins and lifting the lid on the opposing side. A bent steel rod can be inserted into the hinges to prop the lid open. Right angle brackets are fastened to the south edge of the roofing below each screen (where there is a hinge placed) to keep the trays of food from sliding off the downhill side.

All of the cedar is coated with a homemade sealant: melt 1 pound of paraffin wax, remove from heat and vigorously stir in 3 quarts of linseed oil and 1 cup of gum spirits turpentine. When cool it looks like crystallized honey. Apply liberally with a brush. This is also great stuff for windowsills or exterior trim where you want water repellancy. Recoat in subsequent years when needed. The legs and under-framework need no other sealants if you use the treated wood option. ACQ treated lumber is the newest option and supposedly more environmentally safe.

Improvements and Alternatives.

* a shelf or table nearby as a handy work space * removeable legs to facilitate winter storage * 1-inch square aluminum framework for the collector instead of wood * adjustable slope to accommodate seasonal sun angle changes or for high latitudes * white wall on north side to increase solar gain at high latitudes * brown or blue paint on collector metal sheet to avoid over-heating near equator * east and west sides enclosed to prevent excessive cooling by cross-flow winds * covers can also serve as coldframe covers if the black, heat generating material is removable or separate

Some illustrations:

These are sliced red sweet peppers ready for a day of sunshine. They can also be cut using a potato "frencher", which makes 1/2-inch pieces instead of strips.

These are "drying" tomatoes. The variety is "Principe Borghese" which is available commercially or through Seed Saver's Exchange. We cut them in half and squeeze out the juicy seed cavity (you can drink the "innards"), then place them "skin side" down on a stainless steel cookie sheet until somewhat dry, then move them to the stainless screen.

Ah, the middle of Summer and we're up to our eyeballs in sweet corn! Even growing an open-pollinated variety that doesn't mature equally, we still have a hard time eating more than a dozen ears per day when they really start maturing quickly. Here we see about 150 ears, picked the night before and steam-blanched, cooled overnight, and cut off the cobs the next morning. The overnight cooling actually starts the drying process. And if we tried to do the whole process in the morning, we wouldn't have it out into the dryer until noon. That would be a terrible waste of sunshine!

This is a close-up of two trays of sweet corn. The top one is freshly loaded in, sitting at the top of the dryer so it gets the maximum warmth on its first day. The bottom tray has been drying for one day and is almost done. The variety we're growing is called Tru Gold, and it's available commercially as Organic seed. We use it in soups, stews, and just rehydrated with water. As with all processed foods, it's not as nutritious as fresh but Summer doesn't last long up here!

This shows part of the solar dryer filled with apple chunks, from wild apple trees growing near our home. They've been cut in half, cored, and run through the "French-fry" cone on the food chopping attachment of a Kitchen Aid Mixer.

This is a close-up of the apple pieces on the stainless steel screen, ready to dry. In late September, with our lowering sun angle and very cool nights, they take 2 or 3 sunny days to fully dry.

This is a typical daily harvest of raspberries from our 50-foot long patch. The variety is Autumn Bliss. This is what's left after we've eaten what we can handle fresh! What to do with the rest when you're maxed out on "raspberry this" and "raspberry that"?

We put two day's harvest into our stainless steel steam juicer. This will remove the Pasteurized juice for bottling, leaving the pulp and seeds behind. The pulp-seed mix can be solar dried (on cookie sheets as a "fruit leather") or canned in the steam canner, depending on the weather. Blackberries really need their seeds removed from the pulp before canning or drying as they are larger, more numerous, and more "gritty".

This is the fully (and obviously well used) assembled steam juicer. The bottom pan holds about an inch or so of water (instead of gallons of water!) that boils to make steam. The steam rises through the next layer, which is where the juice collects. The third layer, most of which is nested in the second, is the perforated basket of berries. The top layer is, surprise!, just a top. The silicone hose hanging down the front has a clamp on it to retain the juice until you're ready to tap it off. And to reduce distillation of water into the juice pan (from the cold stainless top), especially before the berries get hot enough to release juice, we normally run it with a folded towel on top for insulation.

And this is the result. Seven, 16-ounce, recycled Grolsch beer bottles hold the product of the process (actually there was one more that I removed from the juicer and heated to boiling separately after the juicer had cooled and the pulp dripped totally "dry"). The bottles get filled to the top with hot juice, straight out of the hose, the top is put on with the wire bails, and you're done! Well, actually just a little bit of rinse to do on the bottles from the overflowed juice.

Stainless Steel Dryer Screens:

Hopefully, you may be inspired to try building your own solar food dryer. If you need some stainless steel dryer screen, in previous years we've ordered it for dryer workshops organized in our area, where a group of 10 folks got together to build their own solar dryers. We don't currently have any in stock and we don't keep a stock of it. But if someone reading this gets inspired to order enough to organize a dryer building workshop in their area we'd be happy to post a link here to your website/e-mail address so that others could obtain your leftover screen. We aren't interested in becoming retailers, but if you, dear reader, wish to help others gain access to materials, that's terrific!

It comes in 4-foot wide, 100-foot long rolls (as well as shorter rolls at a much higher price) and we have been getting it from Cambridge International (formerly Cambridge Wire Cloth, in Massachusetts). Clicking their link takes you to a table of stainless mesh sizes, showing wire diameter, open space percentage, etc. To get the most recent price quote, you'll have to e-mail them (sales @camwire.com). Someone who recently got a quote (early July, 2008) reported a $250 minimum order. Type 304, .023", 12-mesh, 2x2-foot chunks, 24 pieces minimum was $10.50 per piece. The product we bought was type 304 stainless with 12 wires per inch and a wire size of .018". Some friends of ours used .023" with 10 wires per inch, which is a little more "open" and slightly easier to clean. Cambridge's page with the 10-mesh screen can be seen by Clicking Here. Thanks, Pete!

Another company, in California for those closer to the West coast, is TWP, Inc. Their site is a bit more user-friendly, with direct links to buy the screening. And an e-mail correspondent found another site, City Wire Cloth, with a minimum order of $100. They offer a 4 x 10-foot piece of type 304, .018", 12 mesh. Either way, don't expect low prices! This is a life tool, eminently recyclable, and will no doubt be usable for 7 generations. If you can find a local metal-working shop that has leftover stainless screen from some industrial contract job, or if you can find some in your local metal "scrapyard", the recycling price will be probably be much lower.

And if you're interests lie more in first producing more of your own food, instead of preserving it, have a look at our Eating Year-Round from the Garden page. If you'd like to know why you might want to grow your own food, or how to supply the nutrition you need from those foods, try the Vegetarian Homesteading page. And if you're really radical about nutrients, see how to get the most from your soil on our Living Soil Dynamics page.