Guest Post: DIY Stir Plate

Underpitching yeast to your wort on brew day can lead to off flavours in your final product and may impede the yeast’s ability to fully ferment your beer (called under-attenuation). One of the easiest ways to improve your beer is to pitch an adequate amount of yeast in your wort. In order to achieve a proper pitch rate, you’re left with only a few options. In most cases, this means purchasing several packages of yeast or creating a yeast starter to achieve your desired cell count. Since yeast is quite expensive, most people choose to create starters. The general idea of a yeast starter is that you brew a mini beer–usually a litre or two–and pitch your yeast in it, keeping it oxygenated for maximal yeast cell reproduction. Oxygenation can be done by hand, but serious brewers use magnetic stir plates.

A magnetic stir plate is simply a platform containing a small motor that spins a magnet under a vessel holding liquid. A small, teflon-coated, stir bar is then added to the liquid; it spins by the force of the magnet attached to the motor, causing the liquid to swirl and effectively oxygenating a yeast starter.

Stir plates can cost between $80 and $150 CAD (plus tax and shipping). For a home brewer, this is just one of the many expenses that stand between you and awesome beer, and it’s usually cost prohibitive for many who are just starting out in this rewarding hobby. This is exactly why I recently decided to build my own magnetic stir plate using materials that I already had lying around my house. 

There are plenty of instructions for creating your own stir plate on the web. A simple YouTube search for “diy stir plate” turns up more than a few results. Before starting my shopping list, I took inventory of things I had lying around the house. Without looking too hard, I found:

  • An old wooden box in my basement. Not sure what it was used for, but it looked neat
  • A computer that I hadn’t used in about 6 years, containing fans, perfect for spinning my magnets
  • A few wires salvaged from an ethernet cable
  • A magnetic name tag containing powerful rare earth magnets
  • A 9V DC power adapter

With these items in hand, I only had to spend about $8 on other items:

  • A small switch to turn the stir plate on/off
  • A potentiometer to control power reaching the fan
  • A stir bar, which I purchased locally from Shortfinger Brewing Co.

The wiring was relatively easy, and didn’t take much time. I started by making Stir Plate Wiringsure that the fan would work when directly connected to the power adapter. Then I wired in the switch to ensure I could switch it on and off. Finally I connected the potentiometer in the circuit.

I cut a hole in the top of the box because the lid was a bit thick and I wanted to get my magnets as close to the surface as possible. For this reason, I also raised the fan up using a little section of wood.

In the end, the magnets sit just below the surface of the box when it’s closed. The stir bar is held strongly in place and spins perfectly in any vessel that I have tested. A lot of the “pros” use huge Erlenmeyer flasks for their starters, Close-up of Magnetsbut a growler or 1-gallon glass jug is effectively the same and much cheaper than a 5L flask. 

Making this stir plate was a quick and easy task that I accomplished in an afternoon. I saved myself $100 or more by making it myself (money that I will now obviously spend on other homebrew equipment and ingredients). This was a very easy way to create better yeast starters, achieve better pitch rates and brew better beer.

ABOUT THE AUTHOR:   

Chris Inch is a local software developer, beekeeper, and homebrewer. Follow him on Twitter and Instagram, and check out both his personal and beekeeping blogs. Cheers! 


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