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Old 05-03-2013, 09:54 AM   #21
BrewInMaVeins
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Correct me if I'm wrong, but from what I know from my Cell Bio classes is that yeast are facultative aerobes (meaning they use Oxygen when they can, but in the lack of oxygen will switch to fermentation) will undergo an exponential growth period due to a surplus of nutrients (in our case malt and oxygen). Once nutrients are used up, the population levels off and starts fermenting (a less energy intensive mode of respiration).

Obviously, we want to pitch at our highest population density, which is right after our exponential growth period. This is probably signified by our krausen when yeast activity is at its highest (around 24 hr period). After krausen, the population levels off due to deficiency in either sugars or oxygen (or both) and the yeast switch into fermentation mode.

All that said, if you want a smooth and active yeast activity off the get go, pitch at high krausen (during the exponential growth phase when the yeast are still actually respiring oxygen, not fermenting.) However, you will have a trade off of adding oxygenated/diluting wort if you are using a 2 Liter starter. I think this is less important if you are making a stout, but say if you are making a clean tasting lager, then this can have a poor effect on flavor. On the other hand, you can refrigerate to precipitate out the yeast, decant off poor tasting beer, and pitch the slurry. However, you compromise the yeast being in a peak respiration state. BUT, this method usually involves warming the yeast up to your wort temp (usually ~68 deg F). This warming up period will awaken you yeast, get them in respiring mode, and they will be somewhat active by the time you pitch to your wort.

My conclusion: chilling and decanting will give you good attenuation, and eliminate the risk of introduce off flavors to your beer. Sorry if that was long-winded, but hopefully it is helpful.

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