Build Your Own Yogurt Maker, Sous-vide Cooker, and All-purpose Fermenter for $40

I make a half gallon of yogurt, twice a month.  I like making my own yogurt for many reasons, but it's a bit of a pain.  Since I make large batches, I can't use a standard yogurt maker.  I often get distracted and over-heat the milk, and the method I use to incubate the yogurt is wildly inefficient (my beloved Excalibur dehydrator).  I also need a constant warm temperature for various other fermentation projects, and that's often difficult to achieve with the tools I have.

I finally found a better solution: a temperature controller that accurately regulates the temperature of a slow cooker by turning an outlet on or off.  I simply set the temperature of the controller, place the temperature probe into the slow cooker, and plug the slow cooker into the temperature controller outlet.  The slow cooker then stays at whatever temperature I want.  Here's what the temperature controller looks like:

Once built, the temperature controller with or without the slow cooker can be used for a variety of other tasks (including regulating cooling devices).  Here are some ideas that come to mind:

• Sous-vide cooker
• High-capacity yogurt maker
• Bread dough riser
• All-purpose thermophilic fermenter (e.g., for tempeh, natto, koji)
• Beer/cider/wine fermentation temperature controller
• Kegerator controller
• Freezer-to-fridge conversion
• Egg incubator
• Soil temperature controller for seed starting

Don't worry, I'm not turning into a food blogger.  But this sous-vide-cooked
chicken I made with my DIY temperature controller was pretty tasty.
I used this recipe from NomNom Paleo.

You can build the whole thing for about $40, including the slow cooker.

A few caveats before we get started:

• If you're a sous-vide aficionado, this method may not satisfy you, because it doesn't circulate the water (requires occasional stirring) and it's slow to heat.
• If you want to use it as a sous vide cooker, you'll need a vacuum sealer.  I like my FoodSaver, but other brands will presumably work fine.  These are often available on Craig's List.
• Did I mention it takes a long time to heat up?  Slow cookers cook slowly and efficiently.  You can speed things up by starting with hot water.
• The temperature controller is rated to 10 amperes (A) of current, which on a typical 120 volt circuit (United States) translates to 1,200 watts.  Everything except the temperature probe should be rated to 10 A or greater (you'll be fine if you follow my materials recommendations).  Don't use this setup to control appliances that draw more than 1,200 watts.  Slow cookers draw up to 250 watts.  
• This project involves wiring, and I'm not an electrician.  If you have ANY concerns about the wiring, consult a licensed electrician.

Materials

For your convenience, I've provided links to Amazon.com product pages.  If you purchase using these links, you'll be supporting my work at no additional cost to yourself.  If you scrounge them up from your garage/kitchen or purchase them used, even better.

Here's what you'll need to purchase or scrounge:

• Temperature controller with sensor.
• Plastic project box (7" x 5" x 3").  Radio Shack sells them for $7.49.  You can also use this one from Amazon, but it's a bit more expensive and you'll have to put the temp controller in the front face rather than the top because it's not deep enough.
• 8+ foot long three-prong extension cord (rated to 10 A or more).
• Wall outlet with two plugs (standard 15 A duplex receptacle).  This is best purchased at a hardware store and should cost less than a dollar.  Here's the Home Depot page for the receptacle I used.
• Wall plate for the outlet.  This is also best purchased at a hardware store and should cost less than 50 cents.  Here's the Home Depot page for the cover I used.
• 3 twist-on wire connectors.
• Electrical tape.
• Rubber wiring grommets (one 1/4" and one 3/8").  I purchased these from Home Depot.
• Slow cooker.  It must have an actual manual on-off switch, not digital controls!  The $40 price tag assumes you buy one used for under $10 like I did.  I got mine at Goodwill.  

These are the tools you'll need:

• Screwdriver with small Phillips-head bits
• Drill with various sizes of bits
• Wire cutter
• Wire stripper
• Rotary tool (e.g., Dremel) OR some other way to cut rectangular windows into the plastic project box.
• Adhesive.  Preferably a glue gun, but any adhesive rated for rubber will do.

Procedure

1.  Cut the extension cord and retrieve interior wires.  You want to leave the male end intact because you'll be using it to plug in your box.  Leave as much intact cord attached to the male end as you think you'll need for the final product (I left 5 feet).

Next, remove the three interior wires from a 5-foot section of the female end of the extension cord.  Typically, they will be white (neutral), black (hot), and green (ground).  The wires are all functionally equivalent but it's best to use standard color coding in your wiring so it's more intuitive if you have to repair it later.  They should look similar to the photo on the left.


2.  Cut the following wires:

White: two 10" lengths
Black: five 10" lengths
Green: one 10" length

Strip about 3/8" off each end of all wires.

3.  Cut windows into the lid of the project box for the temperature controller and receptacle.  You want them both to be able to fit into their slots snugly.  Before you cut, make sure your placement allows enough space for both the wall plate and the controller (it's a tight fit).


The temperature controller has two orange sliders that you should remove before fitting it into its slot.  Disregard these for your measurements of window size.  Re-attach them after the controller is fitted and slide them up to fix the controller into its slot.

Place the receptacle in its slot.  Mark the lid where screws should go to fix the receptacle to the lid, and drill appropriately-sized holes.  If you used a 7" x 5" x 3" project box, it should look like the photo to the left when you're done.

Remove the protective plate on the back of the controller, exposing the wiring hookups.

4.  Drill holes into the front and back of the project box for the electrical wire and temperature probe.  These holes should be just large enough for you to squeeze in your 1/4" and 3/8" rubber gaskets (which will protect your wires from abrasion and give the box a finished look).

Insert your gaskets, verifying that they are wide enough to accommodate the thin end of the electrical cord and the probe.

5.  Wire it.  My electrical skills are approximately nil, so I relied on others to help me with the wiring.  There are many online tutorials on how to wire this temperature controller and similar units.  Here's the video I used.  His wiring works, although unlike him I would stick to standard color coding for the hot, neutral, and ground wires.

Here's how the wiring should look once it's complete.










6.  Fix everything in place.  Ideally, after everything is wired you should fix the power cord and temperature probe wires to the grommets as they exit the box on the inside.  This prevents them from tugging on your interior wiring.  A glue gun would work perfectly for this, although any rubber-rated adhesive should work.

Use the orange sliders to fix the temperature controller into its slot.  Screw the receptacle into its slot and screw the wall plate onto the receptacle.  Screw the lid onto the project box.  You're done!

7.  Use it!  The instruction manual explains how to switch between Celsius and Fahrenheit, how to set the temperature, and how to set the temperature range.

You should now have one plug that turns on when the temperature drops below the desired range (heating), and one that turns on when the temperature exceeds the desired range (cooling).  I recommend labeling each outlet accordingly.

You can use this temperature controller for a wide variety of projects.  Let me know in the comments what you use it for!

A note about sous-vide cooking

For sous-vide cooking, I recommend starting with hot water and using the system to regulate temperature from there.  You can also pre-heat the ceramic of the crock pot by putting a bit of hot water into it and dumping it before filling it with more hot water.  Crock pots heat slowly, even on high, so if you start with cold water you might have to wait around for two hours before your water bath gets up to temperature.

The larger your crock pot, the better it will work for sous-vide cooking, because the larger thermal mass of the water will buffer the temperature so that it doesn't drop as much when you add your food.

Also, since the crock pot doesn't circulate water to maintain even heat throughout the water bath, give it a stir and check the temperature before adding your food, and stir it periodically as it's cooking.

A note about yogurt making

You can both scald and incubate your yogurt using this setup.  When scalding, I recommend filling the crock halfway with warm water and placing your jars into it (not too hot or the temperature contrast may crack your glass) to speed up the process.  After scalding at 180 F, simply set the controller to 110 F and let it cool down.  Once it's reached 110 F, add your starter*.

I'm still experimenting with probe placement.  Last time, I put the probe in the upper layer of the milk.  It worked all right, but the temperature tended to overshoot by a few degrees sometimes, so I had to set it to 108 F.  It did end up making excellent yogurt.  Next time, I'll try putting the probe into the water.


* I have a little yogurt starter trick to share.  Those starter packets (e.g., Yogourmet) are so expensive that they essentially eliminate the financial incentive to make your own yogurt.  There is a better way.  I buy a quart of yogurt that I like (e.g., Nancy's), and I freeze cubes of it using a standard ice cube tray.  Each cube (~2 tbsp) is enough to start 1/2 gallon of yogurt.  The key to maximizing the viability of the culture is to freeze the cubes as quickly as possible, but thaw them slowly.  Once you've loaded your ice cube tray, place it into the fridge for 15 minutes to chill it, then place it into the coldest freezer you have.  Once frozen, pop out the cubes and store them in a ziplock bag in the freezer.  When you want to use one, let it thaw in a little bowl on the counter at room temperature.