During the COVID-19 pandemic and run-up to the 2020 U.S. Election, I started watching the Great British Baking Show (a.k.a. Great British Bake Off) on Netflix as a way to reduce stress. When one of the contestants mentioned a “proofing drawer” my ears perked up. Soon I understood that the proofing drawer (a.k.a. warming drawer) as a tool that provided a warm, temperature-controlled environment that would keep a batch of dough happy and growing.

My personal interest in this appliance is driven by the room temperature in my house Berkeley, California, which is usually too cool for most bread fermentation (typically 65 F/18 C)¹, but I don’t have space or energy to research and install a commercial proofing drawer.

Evolution of the DIY Proofing Drawer

A few years ago, while living in another house in Berkeley, I also faced the room temperature problem. Full of creative inspiration from the Maker Movement of the time, I built a DIY proofing drawer, using a kitchen cabinet as the container and a 60 Watt incandescent light bulb as the heat source. An Arduino microcontroller with various accessories controlled the temperature in the cabinet. It worked OK, but I didn’t use it much because it was a pain to set up — empty the cabinet, dig out my Arduino stuff, set it up, etc.

In my current house, I don’t have a cabinet that could serve as a proofing box. But one day I noticed an unused IKEA plastic plant pot in my garage, and realized that it could hold my favorite dough containers and most of my favorite bread pans. So I filed that idea away in my memory.

Another day I noticed my “Christmas Stuff” box on a shelf and realized that a string of old-style incandescent Christmas lights would be an excellent heat source. They would be a safer than a single a single 60 W (or more) light bulb because the heat is spread among many small bulbs instead of being concentrated in one bulb. A string of small lamps is also a more diffuse heat source, enveloping the rising dough with gentle heat, instead of blasting one side of the dough with all of the energy.

Putting the Pieces Together

I dug out my Arduino stuff, reworked the design, updated the programming, and put it all together. Unlike my previous design, this setup is self-contained, so I can leave it on a kitchen chair in an out-of-the-way place, ready to help me make better bread.

Here is a high-level overview of the set-up (a full listing of components is in my first Arduino meets bread baking post):

• Microcontroller: Arduino Uno with extensions. This reads the temperature, controls the lights, logs the temperature
• Power Switcher: PowerSwitch Tail. This enables the microcontroller to safely switch the lights on and off. Unfortunately, the PowerSwitch Tail is no longer available, but Adafruit (my favorite on-line store for this kind of equipment) has a replacement called Four Outlet Power Relay that has the same or better functionality
• Temperature Sensor: Thermocouple. This reads the temperature. I am partial to thermocouples but admit that they can be a headache — to make this work with Arduino I needed an extension module. Simpler temperature sensing alternatives include the TMP36 and DS18B20
• Heat Source: A string of Christmas lights with incandescent bulbs, 120 W in this case. To get 120 W from LED bulbs you will need a very long string, as the power requirements for LEDs are far lower². To be sure, a string of LEDs would work, but the heat up time would be longer.
• Container: the IKEA PS FEJÖ plant pot. This pot is 15″ diameter at top, 10″ diameter at bottom, and 13″ height (38 x 25 x 33 cm). To keep the dough container away from the lights, I put an overturned bowl at the bottom of the pot. For bread pans, I built a structure that fits into the mid-section of the pot (see photo below).

The next illustration shows how the components fit together.

The DIY Proofing Box In Action

I have been using my DIY proofing box for a few months and I am quite pleased with its performance. My bread dough is rising nicely in the controlled temperature, the finished loaves have great flavor and structure. The chart below shows the temperatures inside the box and in the room during a recent test: the box is a cozy 75 F, while the room was around 63 F.

The next photo shows a loaf in its final proofing stage, with the cover removed to reveal the contents of the box. At the bottom you can see the bundle of lights. The white cable near the top is the thermocouple (the sensing element is a small bead at the end). The bread pan support is a criss-cross of wood that I put together in the garage from scraps. It’s rustic, but effective and inexpensive.

How Hot Can It Get?

I wondered about the maximum possible temperature inside the box, so I connected the lights to the main power-strip, put on the cover, and started datalogging. The figure below shows what I found: the peak temperature for this setup is about 97 F (~36 C). I can’t think of a recipe where I will need 97 F, but someday I might run into one.

What’s Next For The DIY Proofing Box?

There are some improvements to consider:

• Changing the target temperature is a pain in the neck: remove Arduino from the rig, bring to computer, find the right Sketch, update the target, upload to Arduino. I could add an LCD display with buttons that would let me modify the target temperature (as I used in earlier Arduino kitchen experiments). But a far simpler solution would be a potentiometer with discrete set points (e.g., one that has 10 clicks available) in a voltage divider: for example, Click One is 65 F, Click Two is 67 F, and so on.
• Handling large flat loaves, like Priya’s Smoky Jalapeño Bread (in the Great British Bake Off recipe library) is not possible. This one is pretty simple to solve: find a different container, like a plastic storage bin or clean cardboard box.
• Replacing the thermocouple with something like a TMP36 analog temperature sensor or 1-Wire DS18B20 digital thermometer, which would directly connect to the Arduino’s inputs instead of requiring a special breakout board. So far I haven’t been having good luck with the TMP36 — their readings don’t match measurements by type-K thermocouples.

Another Place for Dough Proofing From The GBBS

Update, 7/3/21: Netflix’s Great British Baking Show (Collection 7, episode 1) reveals Rosie’s not very common method of ensuring a good rise for her bread dough: she puts the dough bowl in her pet python Minnie’s vivarium.

Important note: Attempt the projects described in this blog at your own risk and observe common sense safety precautions.

1. To be sure, there are certain bread recipes where the baker wants fermentation at temperatures around 65 F (or even lower — some call for overnight fermentation in the refrigerator), as there’s a rule of thumb in bread baking that low temperature leads to high flavor.
2. A small fraction of the energy input to an incandescent bulb is released as visible light, the rest is heat (at invisible infrared wavelengths). LEDs convert far more energy to visible light. For example, I found a 300-bulb string of LEDs that consumes only 21 W.