Build Your Own Stir Plate

This page describes a method for making a simple stir plate, suitable for stirring starters.

The reason for making such a stir plate is that some research has shown that a stirred starter produces considerably more yeast than one that is just left to sit. The following plot is take from Yeast Propagation and Maintenance: Principles and Practices by MB Raines-Casselman, Ph.D.

Other handy links can be found on the Yeast page.

See also the Brew board stir plate FAQ.

What you will need

Please note that there is plenty of scope for improvisation, the approach taken here may not be the simplest method depending on what fan(s) and other parts you have available.

1. A 12V DC fan such as found in PC power supplies. Other voltages will work if you have the right power supply.
2. Two rare-earth (strong!) magnets around 12x5mm. I bought mine from Alpha Magnetics in Boronia for 75c each ( GST).
3. Plastic plate (must be non magnetic) for the flask to sit on. Furthermore with the aluminium plate I first tried, although it didn't interfere with the magnetic coupling between the motor and stir bar, it really gave the motor a hernia so I resorted to wood. I will replace with perspex when I get a chance.
4. Misc machine screws and spacers. I ended up using 40mm M3 screws which allowed me to fit two 9mm spacers (threaded) to the motor and some M3 short machine screws to hold the platform down.
5. One or more stir bars. I made the mistake of buying the variety that has a ring in the middle to act as a pivot. Well unfortunately it pivots every revolution and the rattling forced me to leave the starter out in the garage overnight. I will be getting the "no ring" version very soon. I sourced these from Science Supply Australia in Mitcham. They also have conical flasks.
6. You will also need a suitable vessel for stirring in, such as a conical flask, ideal because it has a flat base. Common glass vessels typically have a convex bottom (looking at the inside) making it difficult to centre the stir bar.

Construction

You may notice that the steel ring (washer) that the magnets are glued to are not mentioned in the "what you will need" section. This is optional and other designs such as shown in the_stain's homemade stirplate project have the magnets glued directly to the fan.

After some frigging around, I ended up using heat glue to fix the washer to the fan and the magnets are pretty much holding their own - a dob of heat glue would make sure of it but I don't plan spinning it too fast.

Note that the further apart you have the magnets, the greater the risk of unbalancing the fan which will result in unwanted vibration and shortened fan life. If you look closely, other articles simply glue the magnets onto the fan hub - for all I know this may be superior to my more indirect method.

I then made a simple plywood platform and used some plastic standoffs to keep the fan a safe distance from the top of the stir plate (see comments below re conductive platform).

I found that the 9mm spacers actually made it quite difficult to get the stir bar central and reasonably quiet. Adding another 9mm (ie further away) made for significantly greater usability. This put the magnets around 25mm from the surface of the stir plate. Further away you lose drive, closer and things tend to be a bit twitchy. Note that the actual distance you use will depend on a number of factors (including type of magnets, spacing), I recommend trying starting at a smaller gap then experiment with different thickness packing until you find a nice compromise.

I found out that a conductive stir plate base almost completely stuffs the motor (speed dramatically lowered). I am an electrical engineer and supposed to know about this stuff and I deliberately used an aluminium base, expecting that a non-ferrous material would have no impact. When I realised the motor was close to stalling I first though it might have been something to do with induced currents in the mounting screws that were electrically connected via the aluminium plate. Not so. I guess it is a conductor in a moving magnetic field so it is fair to expect that current would be generated.

Updated waterproof platform

Since the plywood is susceptible to moisture, I made a new platform from perspex after the initial test run.

Then I got sick of lugging around my lab power supply so I built this with an LM317. Don't forget that the tab of the LM317 is live and unless you insulate the heatsink, it is also. This heatsink I just hacked from an old CPU cooler.

No guarantees on the circuit, I drew this from memory, if in doubt, google for LM317 application or similar. You could possibly get away with no heatsink at all, certainly a smaller one should be sufficient for most combinations of power and fan.

Test drive

The stirrer in action with a 1L conical.

Stirring up the acid wash. This is a very thick mixture, about 300ml of very compacted yeast diluted to 1L. Just for reference, this is a 1L flask.

First crack at a starter, now stepped up to 2L flask. There is a lot working against it, yeast is a month old, lots of acid from wash in starter and tonight looks like its going to be cold out in the garage. The yeast is 3rd generation US Ale II (1272) yeast.

Maybe a 2L flask aint big enough. This is after 18 hours, bearing in mind that it was down to 14°C overnight, it really only kicked in about 5 hours ago.

I could attach a blowoff but the krausen is thick, rich, presumably healthy yeast and I would want to collect it somehow. In this case I just resanitised the aluminium foil, skimmed the exposed yeast off the top and swirled the flask to disperse the foam.

36 hours after firing it up and that is quite a bit of yeast. Keep in mind that the first 12 hours were quite cool this could perhaps have turned around in 24 hours. To be fair, I have no comparison with a non stirred starter under the same conditions.

The starter ended up at SG of 1.010 and the pH 4. This is in-line with a "normal" fermentation (see yeast FAQ although I have never measured the pH of my beer before carbonation. The lesson learned here is not to be too paranoid about tossing acid washed yeast into a starter (such paranoia fueled my yeast centrifuge).

But does it make good beer?

Well the only real test is how the beer turns out. I pitched this yeast into a stout and it was bubbling away in under 2 hours. So far so good.

I also used the "cold pitching" method - ie the yeast comes straight from the fridge, into the fermenter. There are claims around that this produces a healthier start to fermentation, contrary to the often quoted "temperature shock" susceptibility of yeast. In reality, I recall only ever seeing this quoted in the context of quickly dropping temperatures, not the other way round. There are some theories going around relating to "cold pitching" which involve yeast consuming resources while it is warming up and cold pitching utilises these resources during fermentation.

This beer is a similar recipe and fermented under similar conditions to the previous stout I brewed and when I it is complete I will post the plots of SG vs fermentation time.

The gravity vs time for two similar brews, the second (green) I cold pitched, the stirred yeast described above. The recipes are Sweet Stout #1 my "regular" yeast routine and Sweet Stout #2 with the cold pitched, stirred protocol. Two things stick out to me (well three if you include "this isn't a very well controlled experiment"), the cold pitched, stirred yeast:

• Was 10-20 hours ahead of the other yeast.
• Didn't attenuate as well.

When the second yeast bottomed out, I opened the fermenter and gave it a thorough (sanitary) stir, dropping it down a couple more points. I didn't have the patience to keep repeating this to see how low it would go.

I was speculating that a stirred starter favours more flocculent yeast cells than a regular starter. In a non stirred starter, the most flocculant yeast drop out sooner and then get buried by more yeast, leaving the last part of fermentation to what is still in suspension. With a stirplate, yeast don't have an opportunity to flocculate. Of course there are a number of other variables in this experiment. FWIW, I noticed that the starter yeast settled really quick after turning off the stirplate and chilling it. Poor little yeast buggers must have been tired out after all that stirring.

A Real Life Example

This is CL400 yeast that I built up from a pin head size to this in less than 48 hours. There is some hot break in there from the wort but that is a significant amount of yeast. I was ready to build it up a second time (sterlized wort in jars ready to go) but I don't think I will bother. The method I use to make this is described here.

Big Brother

And here is the mother of all stirrers - this could probably even stir the mash but certainly could be used in a fermenter.

Continue reading here: Beer Flavours

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