My Stir Plate Works Better as a Blender

My DIY, computer fan and hard drive magnet stir plate spins a vortex than would make a typhoon jealous, but it's performance when it comes to propagating yeast is less than stellar. In a side by side test an un-agitated starter outperformed this stir plate.

The first stir bar I used was a cut off screw. In retrospect I imagine that on a microscopic level the threads of the screw were like a course knife plowing through the yeast. When examining the cells produced with this bar the viability was very low, and the cells that were alive were ragged and thin. They weren't anything like the plumb smooth and defined cells that I am used to seeing from a fresh starter.

Next a hard drive magnet was used. This has produced similar results.

Is the problem my stir bar? Is the velocity of a computer fan just too high? Any one else look at their cells made with a DIY stir plate?

What kind of side-by-side test? Are you looking at your yeast through a microscope? I have a hard time imagining that something as relatively big as screw blade could mechanically damage yeast cells.

your power supply will impact the speed more than the fan, what voltage are you using?

many people here (myself included) use computer fans with no issue

MalFet said:

What kind of side-by-side test? Are you looking at your yeast through a microscope? I have a hard time imagining that something as relatively big as screw blade could mechanically damage yeast cells.

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Yes, with a microscope. Using a hemocytometer and cell counts with MB viability staining.

Your question made me go over the numbers a little closer. Looks like I over reacted. When I saw the low viability I panicked, but this was from an old slurry that started with a pretty low viable cell count. in big round numbers, it started as 40 billion total cells at a 50% viability. It ended at 90 billion total cells at 75% viability.

When I look at it as the number of viable cells instead of viability it all adds up.

terrapinj said:

your power supply will impact the speed more than the fan, what voltage are you using?

many people here (myself included) use computer fans with no issue

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I'm using a bench top supply that I built out of a computer power supply. There is an adjustable linear regulator on one of the +12v legs so that I can change the voltage. The speed doesn't seem to change much until it gets near the stall voltage.

WoodlandBrew said:

Yes, with a microscope. Using a hemocytometer and cell counts with MB viability staining.

Your question made me go over the numbers a little closer. Looks like I over reacted. When I saw the low viability I panicked, but this was from an old slurry that started with a pretty low viable cell count. in big round numbers, it started as 40 billion total cells at a 50% viability. It ended at 90 billion total cells at 75% viability.

When I look at it as the number of viable cells instead of viability it all adds up.

Click to expand...

Ah, definitely you'll need to make sure you're comparing apples to apples. Old slurry will have particularly poor growth if you don't add nutrient, for example.

Anyway, glad it checks out.

WoodlandBrew said:

I'm using a bench top supply that I built out of a computer power supply. There is an adjustable linear regulator on one of the +12v legs so that I can change the voltage. The speed doesn't seem to change much until it gets near the stall voltage.

Click to expand...

I have built several stir plates using this controller and old 92mm computer fans. Hooked up to a 12V DC output adapter it provides beautiful linear control of the speed from stall to flat out. I prefer to avoid creating too much of a vortex. I like the vortex to not extend more than 50% of the liquid depth. I always use proper stir bars and have very strong magnets (8 in total - 2 stacks of 4) on the fan so it never throws the bar.