|No more 1 gallon yeast starters!|
Let's start at the beginning. Yeast are living beings like us, and as such they need oxygen. The more oxygen they have, the healthier the fermentation, and the more the yeast will reproduce additional cells. For a healthy fermentation of your wort you need to make sure that you pitch (add) enough healthy yeast cells. If you don't have enough yeast you may experience: infected finished beer, higher than normal final gravity, excess production of objectionable flavors caused by fusel alcohols, esters, diacetyl and sulfur compounds. I assure you all that stuff is bad.
There are a few of different ways to make sure you have enough healthy yeast. If you use dry yeast, those packets almost always have enough cells. With liquid yeast you can buy additional vials/packages, but that can get expensive in a hurry. The alternative is a yeast starter. All you do is use some dry malt extract to create a low gravity wort, pitch your yeast, and let the yeast ferment and multiply in the yeast starter. After a day or two you can pitch the starter into your finished beer, or put your starter in the refrigerator so the yeast can crash out of suspension. Once that happens you can pour off most of the starter wort, leaving just enough to mix up the yeast cake at the bottom, and pitch that into your wort.
In the past I would ferment my yeast starter in either a half gallon or one gallon growler. To oxygenate the starter wort I would intermittently shake the growler. The shaking helped somewhat compared to just letting the starter sit there, but it still required a large volume of starter wort for a higher alcohol beer, or if the yeast was older and had less viable cells in the package. For the upcoming Learn to Homebrew PArty for PArkinsons we plan on brewing a 10 gallon batch. With just shaking I would have had to made a two gallon starter to have enough cells for the beer we are planning. That clearly would have been unworkable.
The stir plate works when a spinning magnet inside the plate causes a corresponding pill-shaped magnet inside the starter called a stir bar to spin inside the starter and stir the wort.
|That little white pill stirs the liquid in the flask.|
This causes a continuous aeration of the wort, forces CO2 out of the wort, and keeps the yeast cells in suspension as opposed to crashing. All of which enables the starter to generate significantly more yeast cells. This chart demonstrates the impact that a stir plate has:
With winter coming and having bigger, winter beers in the pipeline it was an ideal time to invest in the stir plate. Time is not on my side to have my new winter beer done at the seasonably appropriate time. I will be working on that this weekend. The yeast in the flask will be coming to a porter near you.
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