Once you've chosen the right yeast for your brew, you aren't finished. You will need to make sure you pitch the right amount of yeast into well-oxygenated wort at the right temperatures, etc., in order to get the best beer. The conditions under which your yeast do their job will have a major impact on the outcome. Understanding these variables and the yeast life cycle is critical to getting optimal performance from your yeast.
The quantity of yeast initially pitched will affect the production of aromatic and flavor compounds and will also impact the overall health of the yeast. Proper pitching rates (discussed later) are required for the best balance.
There are many ways you can impair the health of your yeast. Over-pitching and under-pitching are two ways. Low oxygen rates in the wort the yeast is pitched into, poor temperature control, yeast autolysis, and other adverse environmental conditions are equally bad, if not worse.
According to Yeast Supplemental Material - From the American Brewer's Guild, the following are the major stresses that impair the vitality of the yeast (the following is a summary of the original list):
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Note that excessive pitching rates will also impair your yeast by not allowing adequate yeast growth (new yeast grows to replace old yeast, keeping healthy younger yeast in the culture; excessive pitching keeps more of the older yeast involved, and the viability is reduced).
Per (Wyeast's Recommended Pitch Rates page):
A low pitch rate can lead to:
High pitch rates can lead to:
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Another quote from the above Wyeast page, regarding re-using yeast:
The following are guidelines that should be followed when re-using yeast:
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Hell, if you look around, you'll find that everyone has the "Right Amount" for batch size 'x'. More often than not, someone will spout off about using a gallon of starter for a 10 gallon batch of beer. This is over-pitching in the extreme. Here's what they're really telling you: We want you to over-pitch by a huge quantity to minimize lag times and thus make up for improper cleaning and sanitizing and/or to get a very fast fermentation. Forget about re-using your yeast afterwards...they want you coming back for more, anyways. Do you LIKE diacetyl? Sure, maybe in tiny quantities in a Scottish Ale, but in that Kölschbier? If you over-pitch that severely, you'll be learning to love it sooner or later.
So when someone goes off about these high pitching rates, you'll know what they're trying to do.
What you REALLY want is more along the lines of 250mL per 5 gallons. This and the above on why this discrepancy exists comes from a professional brewer friend answering my questions to him about this. I might over-pitch a bit, around 450mL for about a 6 gallon batch, but that's mostly out of habit....
Unlike fermentation temperatures, when you are in the middle of starting and stepping up your yeast, the correct temperature is 75 deg.F. Depending on how much control you have over internal temperatures (in the tin can I'm currently stuck living in, it's nearly impossible), you may need some extra help. Using an insulating material based on bubble-wrap and aluminum foil (Reflectix) and a USPS Priority Mail box I just happened to have laying around, I came up with the following:
The IncuBox - Front View
The IncuBox - Side View
The lid, when closed, covers the remaining side with insulation as well.