A commercial unit which was the insp-iration behind this project. It uses four electromagnets which are sequentially switched to simulate rotation.
So what is a magnetic stirrer? In essence it is a small machine which produces a rotating magnetic field. On it you place a flask or beaker containing the liquid(s) you want to stir.
The stirring action is produced by a short bar magnet encapsulated in an impervious plastic. It is spun by the rotating magnetic field and you can leave it to do its job for as long as you like.
Why stir a mixture yourself when a machine can do it for you?
Magnetic stirrers used in chemistry and biology labs often incorporate a temperature-controlled hotplate. Typically, they use four electromagnets which are alternately switched to provide a pseudo rotating magnetic field. In other words, the magnetic field does not actually rotate but by suitably fiddling with the speed knob you can get the mixture to start spinning and then you increase the speed knob to its desired setting.
We include a photo of the workings of one of these machines.
So now that you have a broad picture of how magnetic stirrers work, you are probably wondering how they are used in home brewing.
OK, we admit it: we're not stirring yeast wort - in fact, it's orange cordial! This photo was taken so you can see the stirring action (the vortex) created by the spinning magnet in the solution, driven by the specially modified fan in the box underneath. The only controls are an on/off switch and a speed controller knob.
In general, beer is made of four things: sugars, hops, water and yeast. When you make a batch of home brew, you mix sugars, hops and water in a big barrel, add your yeast, then soon enough the yeast will “wake up” and start fermenting the sugars to make alcohol.
The trouble is that until the yeast is active you have a barrel of sweet vulnerable wort, just asking for any bacteria to gorge itself and start multiplying.
If unwanted bacteria gets a foothold before the yeast takes over you will get a beer that tastes anywhere from mediocre (if you’re lucky) to yukkk!
Interestingly, every batch of beer is infected to some extent with unwanted bacteria. The job of a good brewer is to keep unwanted bacteria to a minimum.
This is why sanitation and cleanliness is paramount in home brewing. Second to this, we can minimise the chance bacteria will have to get a foothold by pitching yeast that is already activated (awake and active).
This way the yeast will start fermenting the beer right from the start and drastically reduce the opportunity for unwanted bacteria to multiply and spoil your beer.