Yeast starters will jump-start your fermentation, but you can supercharge the process with a stir plate, which uses a spinning magnet to keep the liquid in constant motion. This ensures that the yeast cells don’t settle, it keeps the cells in contact with wort while minimizing the concentration of “poisons” such as alcohol and CO2, and it exposes the starter to a regular influx of oxygen.
You could buy a stir plate for as little as $45 (or easily spend more than $100), but it’s fairly simple to build one. You might even have many of the necessary parts around your house already.
Parts List
12V DC computer fan
9–12V DC power supply
25 ohm potentiometer with a knob to provide a variable resistance
Toggle switch (12V SPST)
Steel washer to serve as a spacer for the magnets
Rare earth magnet(s)
1 short jumper wire
Mounting hardware for the fan:
- Four 2" (5 cm) machine screws
- 8 nuts to fit the screws
- 4 metal washers to fit the screws
- 4 rubber washers to damp vibration
Container
1" (25 mm) plastic-coated magnetic stir bar
Flat bottomed container or flask, 1–2 quarts (1–2 liters)
You can pull a fan from an old computer, and a phone charger or wall adapter transformer can serve as the power supply. The magnets can be scavenged from an old disk drive. There are a number of choices for the housing as well. You could use an electronics project box (8" x 6" x 3"/20 x 15 x 8 cm is a decent size), a wooden cigar box, or anything else with thin walls that can hold the fan and support the weight of the starter.
Tools
Drill
Epoxy or super glue
Soldering iron
Solder
Electrical tape
Build Steps
There are quite a few online guides for building a stir plate. Most are fairly similar, but there are a couple of differences here. In particular, the speed controller here uses a simple potentiometer rather than a slightly more complex voltage regulator setup. In addition, the fan here is mounted to the lid of the container. It may be mounted to the top or the bottom of the container. If it’s mounted to the bottom, you may need a larger spacer to get the magnets close to the top surface.
Prep the box
Mark the screw holes for the fan on the lid of the container, and then drill them out. Drill holes in the front of the box for the potentiometer and the toggle switch. Drill a small hole in the back of the box for the power input.
Set up the fan
Glue the steel washer to the center hub of the fan. Balance is important. Place the washer as close as possible to the center and finger spin the fan to make sure the placement is right. The glue won’t dry immediately, which should give you time to make adjustments. Wait for the glue to dry before continuing.
Center the magnet(s) on the washer (they should stick to the metal). If you’re using arc-shaped magnets from a hard drive, it can be tricky to get the balance right. If you’re using small button magnets, it works better to place two next to each other. Arrange one with the north pole up and the other with the north pole down (opposite polarity). In either case, adjust the placement as needed to keep the fan balanced. Verify that the stir bar will sit on the magnets without hanging off on one side. If you don’t, the stir bar will fly off the magnets once they’re spinning. Put the stir bar away after checking this.
Mount the fan to the under side of the center of the container lid. Run the machine screws through the lid from the top and tighten a nut on each one. Place a rubber washer and metal washer on each screw. Slide the fan onto the screws with the magnets toward the lid. Anchor the fan with a nut on each machine screw. Make sure there’s enough clearance for the fan to spin. If the magnets are touching the lid, loosen the anchoring nuts slightly and try again.
Wire the components
Cut off the jack from the power supply (if necessary) and strip the wire ends. Run the wire into the box through the hole in the back and tie a knot to keep it from slipping back through. Strip the 2 wire ends for the fan. Solder the negative (black) wire from the fan to the negative (black or braid) wire from the power supply. Use electrical tape to tape the soldered joint. Solder the positive (red) wire from the fan to one blade of the toggle switch. Solder the positive (red/non-black) wire from the power supply to the left terminal of the potentiometer (as viewed from the knob side). Solder the jumper wire from the middle terminal of the potentiometer to the other blade of the toggle switch. (If only two terminals on the potentiometer are used, it acts as a voltage regulator.)
Complete the build
Mount the toggle switch to the front of the box. Mount the potentiometer to the front of the box and install the knob.
Using Your Stir Plate
Make a yeast starter just as you would normally: boil the wort and the stir bar to sanitize everything (if you have an Erlenmeyer flask, you can prepare the wort in the flask right on the stove), then chill before adding the yeast. Cover the mouth of your flask or container with a tinfoil cap.
Plug in the stir plate with the power switch in the off position. Place the starter (with the stir bar) on top of the stir plate. Slide the container around a little to make sure the magnet is holding the stir bar. Turn the potentiometer to its lowest setting and turn the power on. Slowly turn up the potentiometer until the stir bar is moving and creating a vortex in the starter.
If the stir bar is thrown off to the side, restart the process. If this continues to happen, try a shorter stir bar. Some people insist that a larger vortex is necessary. I’ve found that movement alone is sufficient.
Don’t forget to remove the stir bar before pitching your starter. It’s easiest to catch the stir bar with another magnet on the outside of the container and then slide it up to the top and out.