Tuesday, May 31, 2011

Solar Food Dehydrating in a Humid Climate

[This post will also be published in the next issue of Greenzine, the official magazine of Transition Town Peterborough.]

Peterborough, Ontario does not have a desert climate; we're not likely to get one even with extreme climate change. So how does one dry food for storage using only the energy of the sun? With a keen eye on the weather forecast and a solar food dehydrator designed to function even if the humidity is 70 or 80 percent!

Sue Robishaw in Minnesota designed such a dehydrator – four feet by twelve feet! I didn't have room for such an installation so I adapted its principles for a humid climate solar dehydrator using the two glass panels from a storm door that I also use for my cold frame. I used cedar fence boards in the construction and made two units. I can easily move them from winter storage to any of several good drying locations in my yard.

The dehydrator works by forcing air to continuously move over the items to be dried. Outer glass captures the sunlight which warms the heat sink (black metal) resting on the drying trays; a shiny metal bottom reflects the heat up into the drying trays as well. The unit is roughly square and the drying trays are long rectangles; these are easier to handle than a large square tray. There is an inlet for air to enter the bottom front of the unit and an outlet for the air to exit the back top. There must also be a gap between the main body of the drying tray and the metal bottom. Using corrugated roofing tin provides such a gap, as do side bars on the bottom of the trays that rest on a flat metal surface such as aluminum flashing. I put detailed construction directions in an Instructable.

Any edible green can be dried in this dehydrator: I've successfully dried dandelion, garlic chives, herb leaves, lamb's quarters, spinach, and chard. Tender leaves work best; remove any ribs (such as in chard) to compost, use in a stir-fry, or preserve separately. Layer the leaves one leaf deep; feel free to combine different leaves on the tray (for a soup mix). If you start the drying early in the morning and there is a slight breeze with humidity less than 70%, you could have dried greens by sunset. I've gotten crumbly dry greens in as little as six hours. The leaves should be dry enough to crackle before putting them away in air-tight jars and storing in a cool, dark place. If you pulverize the leaves into a powder, it will take a lot to fill a jar. Use in any soup or stew during the winter for a hefty dose of vitamin A and minerals.

Herbs will dry to a nice dark green (not a brownish-green) because the heat sink keeps out the direct sunlight. For parsley, remove the heavier stalks and place the leaf bunches on the screen close together.
You can leave mint leaves on stems for ease of handling, but strip the leaves off to pack away. Sage is a “dry” leaf and will dry quickly. Garlic chives preserve their flavour as they dry; regular chives do not. Generally leave herb leaves on their stalks and strip them off the dried stems to pack away. It is best to fill a tray with one herb only (unless you're doing a “herbal blend”), as drying times will vary and it can be tedious to separate them for storage.

I've had success with fruit leathers and tomatoes, but they can take two or three days to dry. Be sure you're entering a fair weather spell if you want to solar dry these! Mary Bell's Complete Dehydrator Cookbook by Mary T. Bell has good notes on what things you should pretreat (usually by blanching) before drying. Dry It--You'll Like It! by Gen MacManiman is another good source book for recipes and food ideas.