Revised: Nov. 11, 2010
Contents:
- I. How It Works
II. Assembly
III. Testing The Stacked Dehydrator
IV. Operation
V. An Idea For Active Convection
VI. Apple Chips Recipe
The picture above shows two versions of my slow-cooker-powered food dehydrator. On the left is a single-tray dehydrator; the configuration on the right stacks trays in a 2-tier and could be expanded further. The heat source for both is a small 1.5-quart slow cooker (larger sizes can be adapted to the design). Those little gray boxes next to the cookers are DIY temperature controls (not absolutely necessary, except in some types of operation). They constrain the cookers to output a low steady heat. All of the other components are standard kitchen bakeware and accessories, and some of them could be fabricated from poster board or corrugated cardboard to cut costs. A stacked dehydrator could be assembled from scratch for less than $30, including a new cooker and the homemade temperature control. In the last section of this article is a recipe for apple chips, dried in the stacked dehydrator.
On VaporBaker, recipes refer to food dehydrators as dehydration ovens, because the word "oven" suggests a greater transformation in the food as the result of exposure to the low heat - it's baking with warm air. Rehydrated raisins do not turn back into grapes. Beyond camping food favorites like beef jerky and fruit roll-ups, dehydration ovens can produce a wide range bakery-style goods: from cookies to cakes to breads, although the preparation can be distinct to this type of baking. In some VaporBaker recipes, food must be baked in two (or more) stages with one of those stages being in a dehydration oven.
The aforementioned DIY temperature controls is not a thermostat. A good thermostat cycles (turns on and off) heat to keep the dehydrator within a set temperature range. The temperature control doesn't cycle. It tunes the slow cooker to output steady continuous heat, although the temperature can drift. Without a temperature control (or a thermostat), the cooker must be manually adjusted to steady the temperature. There are ways, though, to moderate a cooker's temperature rise (such as filling the crock with water).
A basic slow cooker by itself can dehydrate foods, but without a temperature control, it heats up too quickly (even on the LOW setting) and starts to cook the food. Less than an hour after being turned on, a cooker can burn close to 300°F/149°C. Raw foodists, who cook exclusively with a dehydrator, say that such high temperatures destroy the nutritional vitamins and enzymes in raw foods. They advise that dehydrators should operate in the temperature range of 105-120°F (40-49°C).
The dehydrators in this project can easily reach 120°F/49°C, which will safely dry all foods except meat. Meat that may be contaminated with salmonella or other pathogenic bacteria must first be heated to the sterilizing temperature of 160°F/71°C (165°F/74°C for chicken) and then dried at 130-140°F/54-60°C. The single tray dehydrator is suitable for drying meat. In the stacked configuration, the bottom tray can reach 140°F/60°C, but the top tray will be cooler - how much cooler depends on several factors such as the room temperature, which parts are metal (and therefore lose heat), the distance between the trays. Unless it is clear that a system can handle these heating requirements, I recommend dehydrating meat either inside the actual crock of a large slow cooker with a temperature control or in a commercial dehydrator.
I. How It Works:
These dehydrators operate with convection heat. Heat flows upward from the cooker, spreads out from the heat distributor, up through drying trays and out through the venting cover, carrying moisture with it. The above diagram shows the assembly of a 2-tier dehydrator. A 1-tier setup is minus the separator and second drying tray. Add more tiers by stacking separators and drying trays.
The components are standard kitchenware. All the pans should have the same diameter except that the heat distributor pan can be a little smaller (up to 1 inch) than the others to allow air to enter the bottom of the dehydrator and stoke the air flow upwards. Too much smaller and the gap would let in cold air that would adversely affect the dehydrator's operation.
My dehydrators were built with 9-inch pans. I think I could have enlarged the dehydrators up to 10 inches. Keep in mind that for a given-size cooker, the larger the dehydrator diameter, the lower the internal temperature will be, because of the larger volume of air that the cooker has to heat. A 4-quart cooker with a crock diameter of, say, 8 or 9 inches might be able to heat a larger dehydrator, in terms of tray size and number of trays.
At the bottom is the heat source, a small slow cooker. The pictures show a 1.5-quart cooker with a 7-1/2 inch diameter ceramic crock and base unit (the base unit has been plugged into a temperature control). The cooker can function as a heat source with or without the crock. Without the crock, the dehydrator heats up faster and hotter, and the temperature fluctuates more with changes in ambient temperature. Thus, when the cooker's base heats the dehydrator directly, the cooker MUST be connected to a temperature control to stabilize the temperature. Direct heating also doesn't produce water vapor that could impact the dehydration process as vapor heating does.
In vapor heating, the base unit heats a water-filled crock, which then heats the dehydrator. The dehydrator heats up slowly, the temperature is more stable and the water limits the maximum temperature. The water vapor conducts heat to the distributor more efficiently than air. It also helps stabilize the temperature inside the dehydrator and limits the maximum temperature to the boiling point of water (212°F/100°C). I did try operating the dehydrator with the crock empty. Then, the dehydrator heated up VERY slowly. After an hour, the top tray had not reached 90°F.
The heat distributor spreads the heat energy out to maximize the drying area and catches any drips from the drying trays. In my configurations, the distributors are 9-inch metal cake or pie pans. The pans must completely cover and sit flat on the cooker's base unit or on the crock to maximize heat transfer and, in the case of a vapor-heated dehydrator, to prevent water vapor escaping from the crock. Sometimes, drops of water will occasionally condense at points where the crock and heat distributor pan meet and drip down the sides of the base unit. So long as the drops are few and don't seem to be affecting the dehydration, I merely wipe them up with a sponge. Larger puddles should be dealt with either by reducing the heat or by placing a heat-proof gasket between the crock rim and the heat distributor pan.
The drying trays are metal splatter screens I got in the bakeware aisle of a local market. They cost less than $5 US each (less than $4 when on sale) and were 11 inches in diameter. I've seen them as large as 14 inches. In general, the splatter screens must be larger than the diameter of the heat distributor (eg, larger than the cake pan). Heat from the distributor radiates upward through the mesh and around the food, taking moisture with it. Other fine mesh products could substitute, including screen-door mesh or needlepoint canvas mounted in a cardboard frame. However, splatter screens are already food safe and heat safe.
The separator forms the drying chamber over the first drying tray and holds the drying trays apart. In my 2-tier dehydrator, the band from a 9-inch springform pan serves as a separator. It stands at about 2-1/2 inches tall, which is a bit too high, in my opinion for the majority of foods. The drying temperature drops as the distance between the heat distributor increases. Plus, the metal band itself loses heat. A 1-inch or 1-1/2 band from, for example, a cake pan with removable bottom would reduce the temperature differential and still fit most of the foods that I dehydrate. Lining it with craft foam (sold in 2mm thick sheets) or even plastic food wrap as insulation would reduce it more.
In my 2-tier dehydrator without any insulation, the temperature at the top tray ranges between 15 to 20 degrees (F) cooler than the temperature at the bottom tray. However, when I line the separator with plastic wrap, the temperature differential narrows to 8 to 10 degrees (F). Separators don't need to be metal though. A strip of thick cardboard taped or stapled into a ring would be fine as a separator, and thick cardboard or corrugated cardboard is a better insulator than bare metal.
The venting cover forms the drying chamber for the top drying tray, but also must allow the water vapor to exit the dehydrator. Stacking another separator ring and topping it with a perforated pizza pan could work (a splatter screen might be too open). If the perforations vent too much, block off some of the holes (with a rolled up kitchen towel, for example) to help build up the heat in the top level. In fact, since it won't be holding food, the vent could be a disk of perforated cardboard, cut to fit on the separator. Cardboard also insulates better than metal.
I turned my 9-inch stainless steamer insert upside-down to serve as a 1-piece venting cover. However, the insert is bare stainless steel and 4 inches tall. That large, uninsulated volume contributes to the temperature difference between the top and bottom trays. It would have been a better venting cover if the insert had been only 2 inches tall and lined with insulation like craft foam or plastic wrap. However, the extra space is much appreciated for rising bread dough in the dehydrator.
An inexpensive analog probe thermometer stuck through a hole in the top continuously monitors the dehydrator's temperature. The thermometer is accurate only for the top level; the temperature for the bottom tray must be extrapolated (as explained later).
II. Assembly:
The following instructions apply to the 2-tier dehydrator. For the 1-tier, omit steps 6 and 7.
Parts For 9-Inch Stacking Dehydrator As Shown (see text for substitutions):
- 1 1.5-quart ROUND slow cooker (if used, the crock having flat top surface)
- 1 metal cake or pie pan, 9-inch top diameter and bottom large enough to cover the crock or base unit
- 1 springform or removable bottom pan, 9 x 2-1/2 inches
- 2 metal splatter screens, 11-inch diameter
- 1 steamer insert, 9 inch diameter, 2 to 4 inches tall
- 1 analog instant-read thermometer (0-220°F)
- wax paper for the tray liners
- plastic wrap for insulation, if necessary
A Note About Insulation: I recommend insulating the inside walls of the separators and venting cover for the best heat retention, especially if they are made of metal. The easiest way to do this is to line the insides of each of those components with plastic wrap. I have found that insulating the separator alone in my setup (see picture above) can reduce the temperature difference between trays by 10°F.
The dehydrator will operate fine without insulation even if all the parts are made of metal. In the case of an uninsulated stacked dehydrator, I recommend swapping the trays every few hours so that all the food is exposed to the same amount of heat.
1. Heat source option 1 - base unit direct heating: Remove the crock to heat the dehydrator from the cooker's base unit directly. Skip step 2. With this option, the cooker's base unit MUST be connected to a temperature control.
Heat source option 2 - vapor heating: Fill the cooker's crock with 1/4 to 1 inch of water. Less water generates a higher temperature, but risks running the crock dry sooner, if there are any leaks in the seam between the crock and the heat distributor pan.
2. Optional for vapor heating: During testing, if significant amounts of water and water vapor appear to be leaking from the seam between the crock and the heat distributor, try turning down the heat to reduce the steam pressure inside the crock. If that fails, try placing a gasket between the top of the crock and the heat distributor. The gasket can be cut from a heat-resistant material such as a foam or rubber sheet.
In the picture above, the blue gasket is shown only as an example. My setup does not leak significantly so I have NOT actually tested the gasket with the cooker turned on.
3. Center the heat distributor (cake or pie pan) on the cooker's base or on the crock's rim or optional gasket. If desired, place shims of aluminum foil along top edge of pan to enhance the air flow into the dehydrator (see text above). My testing suggests that the shims are not absolutely necessary.
4. Put a drying tray (splatter shield) on top of the heat distributor and center it.
5. Put the separator (band of the springform pan or removable-bottom pan or a substitute) on the drying tray and center it (take out the bottom first).
6. Place the second drying tray (splatter shield) over the separator and center.
7. Put the venting cover (inverted steamer insert) on the second drying tray and center.
8. Insert thermometer through an opening in the venting cover. In the above picture, a rolled up kitchen towel blocks air holes to maintain the temperature in the top section. The strongest heat flow is along the sides, so the towel is arranged on the cover to block air from the outside inward. Do not block all the vent holes.
III. Testing The Stacked Dehydrator:
Set the dehydrator in a draft-free area. In my stacked setup with an insulated separator, there is an 8 to 10 degree temperature difference between the second and first drying trays. The upper tray is always cooler. I recommend testing the temperature difference in any stacked setup as follows.
1. Assemble the dehydrator, including the separator. Rest the probe of a digital thermometer on the first drying tray, at or near the center. A curved probe works best. A digital thermometer probe will give the most accurate reading because they measure the temperature close to the tip of the probe, where the sensor is located.
2. Complete the assembly of the dehydrator. Turn on the cooker to HIGH and monitor the temperature on the bottom probe thermometer until it reaches 120°F (49°C).
3. Turn the cooker to WARM until the temperature stabilizes for at least 10 minutes. Record the temperatures in both thermometers.
4. In use, the actual temperature differential may vary with the type and amount of food being dried. It may be helpful to run this test one or two more times with the trays filled with food.
IV. Operation:
1. In a stacked dehydrator, the top tray is always cooler than the lower or bottom tray. If the temperature difference is known, then add that number onto the reading to get the temperature in the bottom tray. Drying at a lower temperature does less harm to nutrients in food, so the temperature of the bottom tray should take precedence.
2. Assemble the dehydrator up to the first drying tray or if the dehydrator is already assembled, disassemble it down to the first drying tray.
If the food is liquid or very wet like a granola or fruit leather paste, place a liner on the drying tray and center it. Then pour or arrange the food on the liner. Other foods may not require the trays be lined and will dehydrate faster without a liner. When arranging food on a tray, leave a 1/2 space along the edge so that the heat can flow upward. Solid foods can be placed close together, but try not to let them overlap too much. Some items, like cookie dough, shouldn't touch at all.
3. If the second level is needed, assemble the separator and second drying tray. Then place the food on the drying tray as in step 2.
4. Complete assembly of the dehydrator.
5. Set the heat selector on the cooker to "HIGH" and the temperature control (if used) should be set to maximum.
6. Monitor the thermometer until it reaches about 10°F/5°C below the target temperature (in the stacked dehydrator, estimate the temperature of the bottom tray, which is warmer). Stabilize the heat by adjusting the temperature control (see How to Make and Use a Slow Cooker Temperature Control) or by alternately switching the cooker's heat selector between "LOW" and "WARM".
The temperature control steadies the heat more precisely than manually adjustments of the cooker's heat selector. With a temperature control, the dehydrator can run overnight without being monitored. For overnight operation, first stabilize the heat so that temperature stands still or drifts barely 1 degree over the course of an hour. Then turn temperature control down a little. The system will cool a little, but continue to operate with a higher heat safety margin.
7. Halfway through the drying process, swap the top and bottom trays, so that they are exposed to the same amount of heat overall.
8. Check for water vapor leaks on the crock rim. The vapor may condense into water drops and slide down the side of the cooker. Sponge up any water collected at the base of the cooker. Turning down the heat may stop the leaks. Otherwise, installing a gasket between the crock and the heat distributor may seal the leaks.
V. An Idea For Active Convection:
The dehydrator in this project operates on the principle of passive convection. Heat rises from the bottom of the dehydrator and carries moisture with it up and out the vent at the top. In active convection, a fan accelerates the flow of hot air and the rate of dehydration. In other words, foods dry faster.
It's not practical to install a fan inside this type of make-shift dehydrator, but another idea is to set the fan on top of the venting cover and angle it so that it blows ACROSS (not into) or UP AND AWAY FROM the vent holes. If it blows across the vent holes, the slight pressure difference will draw air up from inside the dehydrator, like smoke pulled up a chimney by the wind. If it blows up, the fan will suck moist air from the dehydrator directly.
I haven't tried active convection on my dehydrator, because I don't have the fan yet. One of those small computer fans might be ideal for this application. The fan should be set on low and blow a very gentle breeze. If the breeze is too strong, the temperatures inside the dehydrator could fall fast.
VI. Apple Chips Recipe:
Apple slices dehydrate in a few hours, much less time than, say, blueberries, which, even when cut in half, will take at least a full day. They are one of my favorite snacks, and very inexpensive if made at home. Other recipes sprinkle them with cinnamon (or other spices) and powdered sugar before dehydrating. I like them plain.
In the pictures, the slices are from a large apple. I arranged them in a spoke pattern for the pictures and could barely fit 10 slices to a tray. That was not the most efficient arrangement. If I had placed the slices in concentric circles, like a bulls-eye, or if I had cut the tray liner with a smaller center hole (nearly 2 inches in diameter in the pictures), I would have been able to fit more. The chips shrink as they dry, so a little overlap is not a problem either.
Makes 20 to 40 chips (depending on apple size and slice thickness)
- 3 to 6 calories per chip
- Oven Temperature: 105-120°F (40-49°C)
Ingredients:
Method:
1. Assemble dehydrator to level of first drying tray. Center a liner of wax or parchment paper (not aluminum foil) on the drying tray.
2. Mix water and lemon juice in a small bowl.
3. Wash and quarter the apple. Cut out the core and seeds from each quarter. Slice each quarter into 1/8" slices and soak them for 10 minutes.
4. Arrange half of the apple slices on the drying tray. Try different arrangements to squeeze more slices onto the tray. If they won't all fit, then the remainder may have to dehydrate in a second batch.
5. Assemble the second drying tray and arrange the remaining slices.
6. Dehydrate at 120°F/49°C, measured at the bottom tray, for 3 hours. On my setup, assuming a 15°F difference between trays, the probe thermometer at the top should read about 105°F. Swap top and bottom trays. If desired, turn chips over to speed drying.
7. Continue dehydrating for another 3 hours or until chips are at desired crispness.
8. Remove chips from dehydrator and store in an airtight container.
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