Presto Chango: Turn A Slow Cooker Into A Stacking Dehydrator (w/ Apple Chips Recipe)

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

To Assemble:



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:

1 medium to large apple

1 cup water

2 teaspoons lemon or lime juice

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.

Low Temperature Baking - A Journey Of Three Paths

This blog documents my journey into low temperature baking (LTB), which is gentle baking at temperatures below 300°F/150°C (mostly below 250°F/121°C), cooking times up to several hours and occasionally days. Low temperature baking includes slow baking, steaming, and dehydration or "warm air baking", the main use of heat in raw foods cuisine. Technically, steaming and dehydration aren't baking, but if they are producing bakery-style goods, then on VaporBaker, they are forms of baking.

LTB can offer better flavor, eveness of cooking, different textures and a healthier composition compared to the furnace heat of conventional ovens. LTB benefits the environment because of lower energy consumption. LTB appliances themselves are very energy efficient. According to Planet Green, slow cookers, for example, are 3 times more energy efficient than toaster ovens (never mind the huge wall ovens in homes), AFTER taking into account the longer operating times of a cooker.

This article only broadly describes LTB methods and techniques. The blog itself is a compendium of my LTB recipes, baking tips and techniques and equipment. Because the recipes are culinary experiments, the ingredient quantities may sometimes barely serve one, though if desired, they could be doubled or tripled to serve more.

There's been renewed interest in low temperature cooking techniques, such as steaming and its haute cuisine cousin sous vide for entree and main course dishes, but not as much for bakery and dessert type goods. Steam baking has been in existence since the discovery of boiling water. Today, it is widespread, if not the favored form of baking. The English have their steamed sweet and savory puddings (which, to Americans, are really steamed cakes and pies), the Asian cuisines have their steamed buns and other snacks, the Americans have their steamed quickbreads like Boston brown bread.

Slow baking is less common. In old times, bread could be baked in a dutch oven buried in the ground and surrounded by lumps of hot coal. That setup can be replicated by a slow cooker on the kitchen countertop or by an oven set at low temperatures. Standard slow cooker baking, however, is not necessarily LTB, because a slow cooker can get much hotter than 300°F over time. Slow cookers also bake inconsistently, because heat power varies from model to model. If its internal temperature can be monitored and held down, a slow cooker can be an excellent LTB appliance.

While raw foodies every now and then do steam and boil foods briefly, warm air baking in a dehydrator is the preferred form of "cooking" to preserve nutrients. Raw foods cakes and breads are mixtures of ground nuts, grains and fruits, shaped into loaves and disks or poured onto trays, and dried at temperatures around 105-115°F/40-46°C. This very low temperature range preserves natural enzymes in food. Raw foods chefs are brilliantly inventive, on the one hand trying to mimic the taste and texture of cooked foods, on the other hand innovating with creations unique to their cuisine.

The dehydration recipes on VaporBaker will not necessarily contain 100% raw ingredients and dehydrator temperatures may go higher than recommended by raw guidelines (food can dry at temperatures up to 140°F/60°C) . Where possible, I will suggest a raw equivalent. Warm air baking is a worthwhile technique outside the raw foods and food preservation context. There's more to it than beef jerky and fruit roll-ups.

My cardinal rule for VaporBaker LTB is temperatures should not exceed 300°F and preferably not higher than 250°F, the point at which toxic compounds (like trans-fatty acids, acrylamides and advanced glycation end products or AGEs) begin forming.¹ Controversy rages as to whether these compounds pose a significant threat to human health, but the goal of avoiding them helps to set the upper range of LTB temperatures.

It's possible to bake with exacting heat these days. All dehydrator models (except entry level) have a thermostat. Steam baking (dry and wet) is inherently precision, because water always boils (at sea level) at 212°F. LTB ovens (such as an electronic countertop oven or a slow cooker with temperature control) can output precise heat if they have a thermostat. However, it's always a good idea to monitor oven and food temperatures with cooking thermometers.

Despite all this talk of precision, I try not to obsess over baking temperatures except when testing recipes. For general baking, so long as the numbers remain within guidelines, I let the food cook until it's done, raising or lowering the heat whenever I happen to pass the baking station. Only in a restaurant where customers expect consistency is high precision important. I reach for my trusty instant read thermometer less often than I'd like to admit. The eyes, nose, fingers, a thin wood skewer can be as good a gauge of doneness.

THE EQUIPMENT

The primary LTB appliances are ovens and slow cookers, steamers and dehydrators. For slow baking, conventional home ovens are less than ideal, because their construction and thermostats aren't designed for LTB. New and affordable products do address the LTB market. There are countertop ovens claiming high temperature accuracy priced at less than $200 US. At the other end, passive solar ovens, heated only by the rays of the sun shining down on them, have no trouble making 250°F, even in the dead of winter. Somewhere in the middle are slow cookers, which are more affordable, more versatile, more reliable (compared to a solar oven), but they aren't "set and forget" appliances Thus, the greater part of this article is about how to bake in a slow cooker.

I have read reviews of cookers that heat up so fast, they boil water within an hour. They are fine for baking if tempered with an external temperature control. For manually adjusted heat, these cookers may run too hot for LTB, because one will be constantly scrambling to turn the heat up or down to maintain a steady temperature. My cookers half full of water and set on HIGH for several hours don't boil water (they may simmer after many hours, but don't boil). When buying cookers, check the display models for power consumption. This number has a "W" at the end (for "Watts") and is found near the AC cord exit on the appliance next to the voltage spec. If the number is too high, it could indicate an fast-heating cooker. On the bottom of my 1.5-quart cooker, it reads 120V @ 120W. On my 5.5-quart cooker, it reads 120V @280W. By comparison, toaster ovens and countertop ovens are rated between 1200W and 2000W.

Slow cookers are cheap. I recommend getting two: a small one and a big one. The small one should be round and is ideal for baking small quantities of food, for turning into a dehydrator and as a heat source for rising bread dough. My 1.5-quart cooker cost me $6 US. The large one could be either round or oval. I like the oval crocks for full-size loaves of bread and small cakes. My 5-quart round will hold an 8-inch cake pan with room to spare for a thermometer. A large cooker can bake larger quantities of food, not only because the crock is larger, but also because the heating element is more powerful. The large ones aren't expensive either. A basic 6-quart model can cost as little as $25 US (my 5.5-quart cooker with digital timer was $26 on sale and my standard 5-quart cost just $20). Don't be swayed by fancy controls and features.



Speaking of fancy controls, basic slow cookers (the kind with single-knob heat selector) can be upgraded with an add-on temperature control, the same kind of accessory sold for home sous-vide setups. For LTB, a temperature control is optional, but very helpful. One of my large cookers cannot accept a temperature control, and baking with it is an exercise in patience and vigilance to keep the temperature steady.

Slow cooker temperature controls range in price from as little as $10 for a DIY solution to as much as $200. The simplest controls are repurposed lamp dimmers. The gray box in the picture above is an example of a homemade control. See the link above for more information about how temperature controls work and how to choose one. A temperature control is not a requirement for LTB, but it will make life easier. Again, to anyone who plans to bake a lot in a slow cooker, I STRONGLY recommend adding a temperature control.

High-tech steam ovens (which can cost several times more than a convection oven) cook food the same way as a stovetop steamer - with boiling water. Stainless steel steamers with the width to accommodate full-size cake pans are higher-end or specialty items these days, but my hefty, 9.5-inch inside diameter, 3-piece stovetop steamer set was just $25 on closeout. Two-tier bamboo steamers up to 14" in diameter sell for less than $20 (put these over a saucepan of boiling water - no wok needed). My 7.5-inch stainless steel steamer insert cost me $4 US at a close out store, and fit perfectly in a 2.5-quart saucepot I already owned (the lid fit the steamer too). This smaller insert has the height to hold my 4.5-inch tall bundt pan with room to spare.

Dehydrators (or warm air ovens) are still considered esoteric. General home goods stores stock one or two basic models for around $20-$40 US, but the cheaper ones don't have thermostats. I've seen plans for homemade dehydrators powered by the sun, a room heater, terrarium heater or hair dryer. A slow cooker with a temperature control dries food better than most entry-level dehydrators and can be converted into a multi-tiered dehydrator (see Presto Chango: Turn A Slow Cooker Into A Dehydrator). A regular oven with the door propped open a crack is a no-cost solution, if the oven will operate so far down the thermostat.

Most home kitchens already have most of the equipment for LTB. There certainly is expensive LTB gear for those who can afford it, such as precision countertop ovens, standalone steam ovens, add-on slow cooker thermostats and cookers with temperature dials (an electric roaster is like a slow cooker with an oven thermostat), but a modest investment is all it takes to get started.



Thermometers: Thermometers are essential for checking food and oven temperatures. Slow cookers don't even have thermostats, so I have to monitor cooker temperatures with an analog (dial) oven thermometer or a digital remote probe thermometer. Don't allow them to touch the bottom and walls of the crock, which are hotter than the air inside the crock and will give misleading readings. Put an analog oven thermometer on a trivet (see picture below) or make a stand for it from craft wire. Similarly, a temperature probe should be protected by a wire coil sheath that dissipates heat buildup.



For other situations, I use analog and digital instant-read thermometers. Discount home goods stores sell analog (dial) ones for as little as $6, but the costlier digital thermometers have the advantage of faster and more accurate read times. One of my digitals has a large display, which is easier to read but more cumbersome to hold. The smaller one stabilizes faster.

Test the accuracy of a new thermometer and account for any discrepancies in use. The first way to test it is by dipping it in boiling water, IF the boiling point of water at that location is known. At sea level, water boils at 212°F/100°C, but at higher elevations, water boils at lower temperatures. If the boiling point of water at a specific location is not known, research it online.

The other way to test a thermometer is to dip it in ice water. It should read 32°F/0°C. Not all thermometers read down the the freezing point of water. My candy thermometer doesn't go that low.

Note 1: instant-read thermometers have a probe for inserting into food. The heat sensor in the probe must contact the food to get an accurate reading. In an analog thermometer, the sensor is about 1.5 inches up from the tip of the probe. In a digital thermometer, the sensor is at or very near the tip of the probe, and is more appropriate for testing low-rise cakes and cookies.

Note 2: if the thermometer has a slow response, pre-heat the probe by inserting it in the food for a few seconds before turning it on.

BAKING TIPS

Browning: Of the three types of low temperature baking on VaporBaker, only slow-baking in a dry oven can brown food in a reasonable amount of time. From a chemical perspective, browning is the result of Maillard reactions, enzymatic reactions and the caramelization of sugars. Of these, caramelization is the primary source of browning in baking below 250°F/121°C. Fructose, for example, caramelizes at 230°F/120°C, but glucose and maltose do so at over 320°F/160°C. In general, bread browns at 221°F to 239°F (105°C to 115°C)², and meat browns at around 302°F/150°C. Browning (especially browning from high temperature Maillard reactions), while imparting delicious flavor, can create toxic compounds. Therefore, when food must be browned, it should be very lightly browned and exposed to high temperatures briefly.





Pre-Heating A Slow-Cooker Oven: In many recipes, the slow cooker must be preheated to a target temperature. For a large cooker (like my oval 5.5 quart), monitor the temperature with an oven thermometer. The thermometer should be on a stand or trivet and not touch the crock, which will be hotter than the air inside. Position the oven thermometer so it won't interfere with the placement of the food, but the dial is still visible.

For a small cooker, if a mini thermometer that will fit unobtrusively in the crock isn't available, a digital remote probe thermometer can monitor the cooker temperature. The probe should be protected with a metal coil to dissipate any hot spots and to stop the probe from touching the crock. The lid can hold the probe in place (re-position to balance if necessary). The method creates a gap, because the lid isn't seated. Put a towel around the lid to block the gap and prevent the hot air from escaping.

Once the thermometer is positioned, put the lid on and cover it with some kitchen towels or a slow cooker draft stop to block heat loss. If the cooker has an external temperature control, follow the control's instructions for pre-heating. Otherwise, turn the cooker to HIGH and heat until it reaches the target temperature. Then turn down to WARM or LOW to steady the temperature.

An empty slow cooker can take over an hour to reach 250°F/121°C (faster if it is insulated with a draft stop). If the batter or dough must be baked as soon as it's ready, wait until the cooker is near or at pre-heat temperature before preparing the dough or batter.



Adjusting Slow Cooker Temperature: If the cooker has an external temperature control, follow the control's instructions for maintaining temperature.

Slow cookers heat up slowly, so check the temperature periodically by reading an in-place oven thermometer or a digital thermometer with a remote probe installed inside the cooker. (Be sure the keep the thermometer or probe away from the bottom and sides of the crock to get the most accurate readings.) To read an analog oven thermometer when there are moisture-absorbing paper towels under the cooker's lid, cut or tear a small hole in the towels so the thermometer will be visible through the glass lid (see picture above). If the hole extends under the lid itself and allows heat to escape, put a kitchen towel around the lid to block the gap.

The average slow cooker (without a fine temperature control) only has 3 settings: WARM, LOW and HIGH. Once the cooker attains a target temperature, shift back and forth among these settings to steady the temperature. If the temperature is too high and there is no lower setting, either turn off the cooker or position the lid askew to vent heat or both. When the temperature has fallen below the target temperature, turn the cooker back on and/or re-seat the lid.

Whatever the oven temperature is maintained manually or automatically, VaporBaker's choice baking temperature for LTB is 250°F/121°C and a maximum of 300°F/150°C. If the cooker should go above 250°F, the food isn't necessarily damaged, because chances are good that it took a while to go that high and the food hasn't been exposed to the highest temperatures for all that time. Simply reduce the temperature and continue baking.

No-Thermometer Baking: If there is no thermometer available or it is too inconvenient to track the temperature, then watch the browning on the bread or pastries. Most breads and pastries begin browning below 250°F/121°C, so if the surface is just starting to brown, then the surface temperature may not have exceeded 250°F/121°C yet. If the surface gets too brown, food temperature may be past 250°F and the inside may be overbaked. Slow-baking without a thermometer is risky, but feasible.

Slow Cooker Oven Capacity: While some steamer inserts and dehydrator trays can be stacked to expand capacity, standalone ovens and slow cookers have fixed capacity. Ovens come with racks that increase the effective capacity. Slow cookers don't usually come with racks, but they can be made from craft wire.



The picture above shows baking racks made from 16-gauge brass wire (sold in craft stores) to support a second pan over the first or to serve as trivets (if the pans loaded with food aren't too heavy). I shaped the wire with a mini needle-nose pliers. The wire ends are "taped" with a strip of aluminum foil and crimped in place.



My small cooker's crock is only 5 inches in diameter and has room for a single mini pan on the bottom. The picture above shows how a baking rack can expand the capacity. It sits in the small cooker, holding a second cake pan over the first. The triangular rack works well with my 4-inch round tart pans. I crafted rectangular racks with adjustable tines to support a range of odd size pans.

Note: The heat is more concentrated at the bottom of the cooker than the top. I wrapped the bottom pan in a collar of aluminum foil so that the cake would bake evenly with the upper cake.



The wire racks also function as a pan support in my 5.5-quart cooker. The crock in the large cooker above has sloping sides. An 8-inch cake pan (for full-size cakes and flat-breads) will fit only halfway down the crock. The wire stand provides a stable platform for the pan and holds it away from the bottom and sides of the crock. (Very heavy pans though would buckle a wire stand. Then, I recommend a small heatproof baking cup or bowl as a support.)

Caution: stacking too many foods can overload a slow cooker so that it doesn't reach the desired temperature or takes a long time to reach that temperature.





Trivets: When baking in a cooker, a pan should not sit on the bottom of the crock, which can get so hot that it scorches foods. Lay a pan rest or trivet made by rolling aluminum foil into a "Z" or zig-zag or wavy shape on the bottom of the cooker first.

To level the trivet, place a pan on top and gently press down to correct any sloping. I recommend rolling foil trivets that span the bottom of the crock for easier placement of pans.



When stacking pans, the bottom pan is likely to bake slightly faster than the top pan, because it's closer to the heat source. Wrapping the lower pan with an aluminum foil collar can slow the baking. My experience has been that the overbaking is slight, and I usually don't bother a collar.

Dry Steaming vs. Wet Steaming: In dry steaming, the steam never touches the food. Instead, the steam only heats the vessel containing the food. For example, the top part of a double boiler holds the food and is heated by the steam from, or direct contact with, the hot water simmering in the saucepan underneath, but a double boiler makes a poor dry steamer because the steam only heats the bottom of the food pot.



Between dry and wet steaming, wet steaming cooks faster and leavened foods tend to rise higher. Some batters don't form properly under wet steam. In the picture above, both cakes from the same batter were steamed for 10 minutes. The cake on the left was dry steamed, and the one on the right was wet steamed. The wet-steamed cake shriveled up, because it absorbed moisture, which collapsed the cake's structure. If a recipe requires a long steaming time, dry steaming can help prevent sogginess. These are generalities, and recipes may use either method under various conditions.



To dry steam cakes and breads in a steamer or water bath, the cake or bread pan is sealed to block out the cooking steam. Any heatproof pan with a tight lid will work. If the pan doesn't have a lid, cover the top with aluminum foil and secure it with a string before setting it in the steamer. Small pans or rubber molds can be enclosed in a foil pouch formed with room for the food to expand.

In wet steaming, the cooking steam touches the food. A simple steamer can be made by putting a heatproof colander on top of a saucepan of boiling water. It works best if the colander doesn't leak steam from the sides. Bamboo steamers are made for wet steaming, although they can dry steam if the food is covered while steaming. Contrary to some cookbooks, bamboo steamers don't need a wok to hold them; put them over a saucepan of boiling water. Choose a saucepan that closely fits the bottom rim of the steamer.



Water Vapor in a Slow Cooker: Slow cookers are not leak-proof. Steam can escape the crock during baking, but also builds up under the lid and can drip back onto the food. A few layers of paper towels under the cooker's lid will absorb moisture and hold back any condensation that forms on the lid.

¹ Burton, Norah, Concern Regarding Acrylamide In Food, March 20, 2010;
Goldberg, T. (et al.) Advanced Glycoxidation End Products In Commonly Consumed Foods, J Am Diet Assoc. 2004 Aug;104(8):1287-91;
Acrylamide In Foods, MedicineNet.com, 2005.
² Browning, Food-Info.net (Wageningen University, The Netherlands).
Purlis, E., Salvadon, V., Bread Browning Kinetics During Baking, Jrnl of Food Engineering, June 2007.