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Sunday 25 September 2011

Brewing stir plate

As an avid homebrewer, I'm constantly trying ways to improve the quality of my brew. Unless you brew Westvleteren, there is always room for improvement. Tweaks to the recipes, better equipment and better technique are all ways I try to better my beer. And as a maker, I find joy out of building better brew gear.

Whilst reading Brew Like A Monk, I learned of the importance of yeast in beer, in particular how the Belgians don't just chuck in whatever yeast they get their hands on, but rather consider it one of the most important aspects to get right. Careful yeast propagation, concentrations and temperatures all affect beer quite dramatically.

So after been enlightened on the culture of yeast [bad pun, sorry], I decided that my brews were going to get a better yeasting.

Having upgraded some time back from dry yeast sachets to liquid yeast, I learned that homebrew vials are still not optimal for pitching. The virility of the yeast decreases over time, and during the period between packaging in America and me chucking it into the brew in Australia, a significant percentage of the cells die, with the reduced cell count suboptimal for a brew.

Further, after primary fermentation, carbonation from bottling often relies on residual yeast cells floating in the beer. This is also not ideal, and using fresh yeast (about 1/10 original pitch rate) will allow final fermentation to occur with much more robust yeast.

In order to increase the cell count to the ideal numbers, a starter is generally recommended. Create a wort of about 1.040 gravity, and then add yeast to it, let it grow and get to maximum viability, ready to tackle even a very high gravity brew.

Many people just make it in a jar, shake for a bit, then let it sit for a few hours. But this can still leave the yeast under oxygenated, and their growth is slowed somewhat.

So to keep things moving and oxygen going in, a stir plate is often used, which is what I have built. A stir plate just uses a magnetic bar inside a wort flask, and a rotating magnet outside of it. The rotating magnet makes the bar spin, stirring the mixture constantly.

My stir plate is very simple in principle. It uses a computer fan with magnets mounted on it, and sits under an Erlenmeyer flask containing the wort/yeast. The rest of it is really just chassis and speed control. Simply running the motor from raw 12volts won't work, as the fan spins way too fast to hold the bar.

I have used a 555 based speed control, with a pot on the front to dial in the speed. This gives me from 0 to 100% speed. It's built on a small perf board and based off this circuit.

The magnets are rare earth magnets bought off eBay, though I suppose I could have pilfered some from old hard disks. Some tweaking of the spacing had to be done in order to get optimal pull on the stir bar. To high and they lift and scrape on the lid. Too low and they don't have the strength to turn the bar.

The chassis is built from small aluminium stock. Hardware stores here sell 1-3m lengths of 1.5 or 3mm bars, angle and other shapes, which I absolutely love for projects, as it is really easy to work with, looks pretty good, and very versatile. My milling machine and drill press were perfect for getting each piece just the right size. M3 screws and a hand tap hold it all together - and a sheet of perspex (Plexiglass) serves as a top. Using a metal lid is not ideal, as eddy currents from the moving magnets could slow the speed.

Some bling was also added - a few green LEDs around the sides shining into the works give a nice techy look to the unit, as well as a little illumination.

I've used it a few times so far, and have been very happy with it. A Duvel clone has reached 90% attenuation, a stout has excellent flavour, and a Scotch Ale in the fermenter has a firm krausen on top.

Stir plate yeast starters are a permanent part of my brews now, and should be for you too.


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