Homebrewing: How To Build Your Own Stir Plate

Peter Reed

Brewers make wort and yeast make beer, right? And lots of healthy yeast can make low- or high-alcohol beers with fast, strong, and reliable fermentations. So what can the brewer do to get lots of healthy yeast? Make a starter. And to make a better starter, use a stir plate.

Joe Postma introduced yeast starters on Serious Eats a few months back, but I'll walk you through it a bit more now, and then we'll get building.

One of the most important parts of making good beer is a strong fermentation with plenty of healthy yeast. You can buy yeast from Wyeast or White Labs who (I like to think of them as sentient beings) are healthy and ready to pitch. But if your starting gravity is higher than 1.028 (according to Mr. Malty), you will need more than one packet or vial of yeast to have enough cells to ferment a 5 gallon batch of beer. You can buy more than one package of yeast, but that starts to get very expensive. Instead, you should use a starter to expand your one hundred billion yeast cells in the package to the number you need.

"the stir plate will significantly improve any starter by continuously driving off carbon dioxide and stirring in oxygen"

If that weren't reason enough, there is a second great reason to make a starter: with a starter, you can rev up your yeast so that they are excited, active, even exuberant when they hit your wort. Exuberant yeast make for a fast and healthy fermentation and better, cleaner beer. While this may seem like a nerdy throwback to high school chemistry, the stir plate will significantly improve any starter by continuously driving off carbon dioxide and stirring in oxygen, which help the yeast to grow and to replicate to their full potential.

Making a starter is simple. Mix 100 grams light dried malt extract (DME) and 1/8 teaspoon yeast nutrient into 1 liter or quart of water. Bring to a boil, let boil for five minutes, covering for the last minute to sterilize the lid. Chill to the pitching temperature for your beer, then pour into a sanitized jar or flask, add your sanitized stir bar, cover loosely with (sanitized) aluminum foil and put it on your stir plate. This low-gravity starter provides enough nutrients to the yeast to get them revved up, but won't overburden them with a high-alcohol, stressful environment. And its flavor is pretty neutral. If you want to use dark DME for a dark beer, that's fine, but not necessary.

When you're ready to pitch the yeast, slowly chill the starter, decant off the clear, spent wort on top, and pour your revved up yeast into your wort.

You can buy a stir plate from a lab supply store or some homebrew supply stores. But building a stir plate is easy, cheap, and fun: in short, the perfect DIY project.

When I built my first stir plate, I started with the DIY instructions at stirstarters.com. These instructions are largely informed by that site. But I used a dead external hard drive for many of the parts and I improvised much of the construction.

What you'll need:

The computer cooling fan and other electronic parts can be purchased online or from a store like RadioShack. Stir bars are available from lab supplies or online.

Step 1

Gut the hard drive. Open the hard drive case and start taking apart the innards. Harvest the rare earth magnets from the reader arms and reserve the female plug for the power supply and the power supply cord. The case should be empty before you move on to the next step.

Step 2

Glue the piece of pipe to the fan so that it is centered, then glue the magnets to the pipe, making sure the fan stays balanced. Later, when you mount the fan into the hard drive case, you want the magnets to be as close as possible to the top surface of the case.

Step 3

Assemble the small electronic components on the circuit board, using excess wire harvested from the fan and the soldering iron. Follow the circuitry guide from stirstarters.com. Be absolutely certain about the wiring, and make sure no wires are crossed! (I wasn't careful with my second stir plate and started two small electronics fires as result...)

Step 4

Stir plate guts--it doesn't have to be pretty.

Mount the fan, circuit board, and potentiometer into the hard drive case. You may need to be creative about how to make all of the parts fit. (I used aluminum foil and epoxy.) Drill a hole in the case for the potentiometer arm to fit through, then attach the knob to the outside. Align the circuit board so that the power supply cord can be plugged in.

Step 5

Close the hard drive case and plug in the power supply to the wall. Put some water and the stir bar in a jar and test your new stir plate!

To use your stir plate properly, you only need a small dimple in the surface of the starter solution—no massive whirlpool—adjust the knob on the potentiometer accordingly. A convenient calculator is available at Mr. Malty to help you figure out how many yeast cells you need and how much starter to make to use with your new stir plate.