Magnetic Stirrer

This paper says that magnetic stirrers can give rise to 10-15 times more yeast compared with not stirring. So, I of course wanted one, given that I’ve got a fridge full of smack packs and vials and the glycerin for freezing yeast is on it’s way from http://vinogbar.no.

Magnetic stirrers are often available on ebay, particularly in the states, but I then remembered the different voltage requirements and it all seemed a bit too much hassle and cost. The BrewWiki StirPlate page has many links to instructions for building a stirrer, which clearly describs how it is built.After reading, I figured it didn’t seem that hard and decide to give it a go.

I had a cheap 25w soldering iron, a 12 volt PC fan, an old hard drive and a small green lunchbox, but not much else. I bought resistors, breadboard, cables and other electric components from Futurlec. Component prices were lower than anything else I could find in Europe, and shipping to Norway was very reasonable ($5), although it did take several weeks for the order to arrive. It worked out most economical to place two orders, each under $30. The biggest saving was that the order was tax free when imported, as each order was for less than 200 NOK ($30). In fact, in addition to tax at 25% the post office also adds 85kr (ca. $15) for the privillege of charging you the VAT, so paying tax on small orders can make the order very expensive. The components needed for the stirrer could easily have been sent as one package, but I bought some project boxes, prototype board and components needed to build a temperature controller.

The key components in my stirrer are:

• A 12v PC fan
• A beatiful flourescent green lunchbox
• 1k potentiometer – 1K was the lowest resistance available, so I bought a dual pot in case I needed to halv the resistance by connecting the two sides together.
• Knob for potentiometer.
• DC Connector, so the stirrer can be pluged directly into a battery saver.
• I also needed some lead-free solder and a helping hand.

As most of the articles suggest, I used magnets from an old hard drive. It was difficult to get the magnets off the backing plates, as the magnets break easily if too much force is used. When mounting the magnets on the fan, I had to also use the backing plate or else the magnetic field would stop the fan from spinning freely. (If I had realized this to begin with, I probably wouldn’t have tried getting them off the backing plate first.)

To balance the backing plates and magnets on the fan, I cut out a cardboard disc slightly smaller than the fan and used sticky tape to fix it all together. The tape was secure enough to hold the magnet and backing plates, but flexible enough that I could reposition it to find the balance point.

Some articles recommend balancing the magnets on a pencil to find the center of gravity, but that didn’t work – the magnets and plates were heavy so that even the slightest movement caused it to fall off.Instead I just connected the fan to the power and listened to how much vibration there was. When the assembly close to center and there was no audible vibration, I picked up the fan and held it in my hand to feel how much vibration there was. After a few minutes adjusting, the fan was pretty much center, and it spins silently at the speeds I will be using.

The fan is about 1cm narrower than the bredth of the lunchbox, so it’s wedged in firmly with some folded thin cardboard. This was quick and simple, and acts as a shock absorber for any vibration that might occur. A hole for the DC connector and pot, a few solder joints and we were ready to roll.

I had 3 magnetic PTFE stirbars in different sizes from www.onlinesciencemall.com. The medium (7mm) and the large (11mm) work best.

When there is airlock activity, there is a clear correlation between the speed and the frequency of bubbles in the airlock. A quick spin, lots of bubbles – a slow spin, fewer bubbles. Well that was my amusement for the weekend. I wonder if the yeast really are metabolizing faster or if it’s just more CO2 being released from the solution due to the stirring.

I’ve made a couple of lagers which got off to a good start, so it seems 1L is fine for medium gravity beers, but I for the higher gravity beers a 2L starter will be needed, and also for larger batches when I scale up later this year. I will buy some larger Erlenmeyer flasks from http://kptnaturfag.no/search?query=erlenmeyer