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Solar Dehydrators and Food Preservation

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Solar dehydrator

Drying food is one of the most ancient and traditionally widespread methods of preserving food.  In this article I’ll look at the modern up-date to that tradition by way of details on the construction and use of a solar food dehydrator.

Before I do that I’d like to pass on a recipe that has been a real attitude-saver in the summer heat.  As I write this we’re into our second week of 90-degree (F) temperatures and soaking humidity.  Field work has been particularly difficult at a time when most of the planting needs to be done so at the end of a shift in the garden we have been rewarding ourselves with a thirst-quenching and refreshing bottle of home-made ginger beer.  Here’s how it’s made:

Home-made Ginger Beer

4 1/2 ounces ginger root
9 quarts water
1 pound 2 ounces sugar
4 lemons
approx. 1 pint “ginger bug”

The last item in the list needs to be addressed first since it is key to the whole process.  The “ginger bug” is captured wild yeast and the way this is done is quite simple.  Place 2 teaspoons of grated ginger and 2 teaspoons of sugar in a pint canning jar with a cup of water.  Stir and cover with a piece of cheesecloth to allow free circulation of air and put in a warm place.  On top of the warming oven is great if you have a wood cook stove, or on the counter next to the range in a modern kitchen.  Add ginger and sugar every couple of days until the “bug” begins to ferment.  This will happen in 2 to 5 days typically.  Once you see a little foam on top and bubbles around the edge the bug is ready to use.  I culture it further adding water, ginger and sugar to get near a pint for the above recipe but the quantity is not critical.  Note that ginger has powerful antimicrobial properties and here keeps spoiling organisms from growing in the bug while creating an environment where wild yeasts can proliferate.  Ginger also carries on its skin a fungus of the aspergillus family that produces enzymes to convert starch into sugar.  In old recipes for liquid yeast you will often see ginger added with flour to culture the yeast.  There the ginger was not only antimicrobial but also worked to break down the starches in the flour to provide a steady food supply for the yeast.  But I digress.  Once the “ginger bug” is ready proceed as follows:

Grate 4 ½ ounces ginger into 3 quarts of water.  Add 1 pound 2 ounces sugar and the juice from 4 lemons (seeds and pulp, ok; no skin).  Bring to a boil and simmer covered for 15 minutes.  Remove from heat and strain liquid into fermentation vessel with remaining 6 quarts of water.  (For fermentation vessel a 4 or 5 gallon food-grade plastic bucket covered with plastic is perfectly adequate.  I use a 5-gallon carboy with an airlock.  The airlock is not necessary but it makes it easy to monitor the progress of the fermentation).  Allow the liquid to cool to body-temperature before adding the “ginger bug” (this should also be strained saving a small amount in the mason jar to keep the “bug” going.  Add fresh water, sugar and ginger to the jar and you’ll be ready for your next batch).  Set the fermenter in a warm place to ferment for 5 to 10 days.  When fermentation is complete the beer is ready to bottle.  Before bottling add 5 tablespoons of priming sugar and mix thoroughly (this will make your beer bubbly as the yeasts enjoy their final snack).  Bottle in glass beer bottles if you have a capper or use plastic soda or water bottles with tight-fitting screw lids.  Set in a warm place to develop carbonation.  After one week the brew is ready to drink.  In lieu of the “ginger bug” any ale yeast can be used.  These are readily available from homebrewing supply houses. 

Variations on this recipe are endless.  Sweeteners such as molasses, honey, malt extract, etc. can be substituted for the sugar.  More or less ginger can be used.  I recently made a batch using grapefruit instead of lemon, which came out delicious.  If you would like more information on traditionally-brewed ginger beers, many fine recipes are included in Stephen Buhner’s book, “Sacred and Herbal Healing Beers: The Secrets of Ancient Fermentation” (Siris Books, 1998, ISBN 0-937381-66-7) The instructions for the “ginger bug” are taken from “Wild Fermentations: the Flavor, Nutrition and Craft of Live-Culture Foods” by Sandor Ellix Katz (Chelsea Green, 2003 ISBN 1-931498-23-7).  I have praised this book often for its exuberant treatment of cultured foods but here I must add a caveat.  In his ginger beer recipe Sandor recommends bottling the beer directly without benefit of a primary fermenter.  Don’t do this!  The bottles will explode.  Even those which don’t will be impossible to open without making a mess.

Now, about that solar food dehydrator.  Laurie and I first became interested in solar dehydrators as a result of an article in HomePower Magazine (www.homepower.com).  The dehydrator we are using today is essentially the one described in that article with some minor modifications.  To anyone contemplating building their own I would recommend some further modifications, but first let me describe in general terms how ours was built. 

Here at the homestead we have a rule: “Work with what you have.”  It’s like, “Play with the hand you’re dealt,” and if we were poker players rather than homesteaders we’d probably have sheds full of aces rather than building materials.  As it was, our first job was to sort through the glass collection in search of a piece of glass appropriate to the task.  We quickly found a large piece of plate glass measuring 42 inches by 68 inches so we proceeded to design our version of the dehydrator around that piece of glass.  This makes a very big dehydrator.  Smaller is ok.  (Tip #1: You don’t need to use a single piece of glass.  You can use 2 or 3 or more pieces laid up like shingles; i.e. laid from bottom to top of the collector area of the dehydrator with about an inch or so of overlap between pieces of glass.  A longitudinal support down the center (technically called a muntin) would make it possible to use even smaller pieces.  Be creative.  Work with what you have.)  I should mention now that we insulated both the collector box and the drying chamber of our dehydrator, however we are convinced now that this was totally unnecessary.  As with most solar-powered devices our dehydrator tends to produce too much heat.  Temperatures between 90 degrees F. and 130 degrees F are ideal.  More than that and you’re doing a bad job of cooking the food rather than a good job of drying.

In designing the collector box then, the width is determined by the glass configuration you use and the depth should be about 4 inches to accommodate the heart of the collector, the heat exchanger.  Remember that the goal is to convert sunlight, which is radiant heat, into hot air, which is convective heat.  To do that we need a heat exchanger.  The method we used (direct from the HomePower article) was to insert 5 layers of black anodized aluminum window screen into the collector box in such a way that the air entering the bottom of the box has to pass through the layers of screen to get to the drying chamber.  In order to get the screen to behave itself inside the box we used strips of wood lattice ¼ inch thick by 1 inch wide cut to lengths that would frame rectangles equal to the interior dimensions of the collector box (in our case 42” x 68”).  Laying a single set of strips loosely on the floor we stapled 2 pieces of screen to them.  We turned the assembly over and stapled a single layer of screen to the other side.  We fastened a second layer of lattice to the first using carpet tacks, then stapled two more layers of screen to that.  Strips of 1 inch by 1 inch batten fastened to the sides of the collector box support the assembly in its proper orientation.  Notice that air enters the open lower end of the collector below the screens and rises through them to reach the drying chamber.  The bottom (or back) of the collector box can be any material.  We used sheet metal painted black, which absorbs any sunlight that makes it through the screens and acts as an additional surface for heat-exchange with the air.

Notice in the pictures that the angle of the collector box relative to the drying chamber is fixed.  This angle is not particularly critical but should be set equal to the degrees latitude of the location where the dryer is to be used.  Thus we are at 43 degrees north latitude, our dryer is angled at 43 degrees.  This optimizes the unit for drying during the late September early October harvest period and helps prevent overheating during intense midsummer sun.  An angle of 45 degrees simplifies construction and would work well for all of New York State.  The top end of the collector box opens directly into the drying chamber, which I will describe next.

The drying chamber is about 12 inches deep and about 28 inches high at the back.  The roof of the chamber slopes toward the back and is hinged so it can be opened to exhaust the heated air from the collector.  Inside the chamber on either side we fastened 14 1 inch by 1 inch strips with about ¾ inch between them.  These support the screens on which the foodstuffs to be dried are placed.  We are unlikely to ever put 14 screens in the dryer at the same time, but having that many slots gives us lots of flexibility in our spacing.  Access to the screens is by way of a large door at the back, virtually the entire back of the drying chamber.  The door is hinged at the bottom and opens downward.  In the open position it has chains at either side which support it horizontally so it can be used as a shelf when loading or unloading the dehydrator.  We made our own screens, the frames being 1 inch by 1 inch wood screwed together.  On the table saw we cut a shallow 1/8 inch groove longitudinally in one face of the pieces so that we could install nylon screening with a splining tool.  Stapling the screen in place works just as well but takes a little more effort.  Alternatively it is possible to find ready-made screens and build the dehydrator to accommodate them.  We recently went to a building supply auction where for a few dollars we got a large quantity of screens for Andersen casement windows.  The screening is nylon and the frames are rugged anodized aluminum.  We plan to build a future dehydrator around these screens.  Note that it is generally agreed that you should avoid using aluminum screen in direct contact with drying foods.  Especially high acid foods like tomatoes will corrode the screens and may incorporate the resultant metallic salts into the food.

We have had great success drying a variety of foods in our dehydrator.  Herbs and medicinal plants such as burdock root and dandelion root occupy the early season from April through June.  We have dried cherries, blueberries, elderberries and apples as well as summer squash, garlic scapes, sweet corn and lots of tomatoes.  When it comes to drying tomatoes, Romas are one of the best varieties.  For the last few years we’ve been growing Principe Bourghese (Prince of the City), the preferred drying tomato in Italy.  The book, “Stocking Up” by Carol Hupping and the staff of the Rodale Food Center (3rd Edition, Simon and Shuster, 1990) is an excellent guide to drying practically every food imaginable with solid information on what works and what doesn’t.  There are also included plans for an alternative dehydrator model with its own interesting features.  For anyone looking to go further down the solar-drying road this book is a must-read.

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