By mel
In addition to judging the eight entries into the Golden Growler Home brew beer competition at the March meeting of the DUFF club, club members also got a look at Jim Kinsman's homemade stir plate. "What is a stir Plate?" you ask.
A stir plate consists of spinning permanent magnets placed beneath a flask. The flask contains liquid and a stir magnet. The spinning magnets beneath the flask cause the stir magnet within the flask to rotate and keep the solution continually rich in oxygen. One can buy commercial stir plates for the Home Brewer such as Northern Brewer's units costing from $96.99 to $201.99 or separate stir plate replacement magnets for $6.50 - $6.99 but much of the fun in home brewing is building our own equipment.
Why use a stir plate? Because yeast that is grown in an oxygen rich environment will have a 10 to 15 fold increase in the number of yeast cells due to the continuous agitation/aeration. Yeast can grow in an aerobic or anaerobic (with or without oxygen) environment. When it has lots of oxygen, it reproduces and converts sugar to carbon dioxide and water, when it does not have oxygen it converts the sugar incompletely and much more slowly to ethanol and carbon dioxide. Optimal yeast growth and fermentation are two completely different processes and is the reason why you want high oxygen when making a yeast starter but no oxygen when fermenting your wort.
Now back to Jim Kinsman's home made stir plate. The components include a DC fan motor from an old computer, a DC plug in transformer, a couple little circle magnets glued onto the fan blade, a rheostat, most of these components are mounted in a rubber maid type bowl.
The lid is placed on the bowl and the flask sets on top. If you look closely at the photo on the right, you will see the vortex created in the water as the magnet spins in the flask. (one magnet, though spinning, seems caught in still motion at the bottom of the flask while a second is lying on the counter to the left of the flask.)
Thanks Jim for the education and demonstration of your stir plate and to the article, "Yeast Propagation and Maintenance, Principles and Practices" for helping me understand this subject.
Works consulted:
Friday, March 14, 2008
Building a Stir Plate
Posted by DUFF Man Site Admin at 8:25 AM
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1 Comment:
Nice article Mel! Very good research done on the subject. I do disagree slightly with one statement. At least it is not always true.
You stated, "Optimal yeast growth and fermentation are two completely different processes and is the reason why you want high oxygen when making a yeast starter but no oxygen when fermenting your wort." The first part is true. To produce healthy yeast, oxygen is necessary.
If you do not pitch enough yeast to completely ferment out your wort, you do need to have oxygen present in that wort so the yeast can reproduce. It will reproduce until it uses up all the oxygen, or until it has reached its population limit. Any remaining oxygen will be scrubbed out by the fermentation process, and will not affect your beer.
Aerating at the end of fermentation is what should be avoided at all costs. As this will cause oxidation and off flavors.
This is one reason I like to use some of the Fermentis dry yeasts. One packet, according to their literature, is enough to ferment 5 gallons of beer with no need for aeration. I used their US-05 in the IPA I brought to the meeting the other day.
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