I had thought that the point of using a siphon when moving wort/beer is to prevent agitation and aeration and a ruined beer. So we use the siphon when moving cooled wort to the carboy for fermentation, when moving the fermented beer to the secondary fermenter and then lastly when moving the beer to the bottling bucket. However, I recently talked to another homebrewer who would strive for maximum agitation when pitching their yeast so the yeast is really integrated. That really confused me. So my question is, When is aeration good and when is aeration bad? (I assume this will also answer the question, When is the siphon best used?)

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Aeration post boil and post cooling, but pre fermentation, is a good thing. Aeration post boil, but pre cooling, can result in hot side aeration, which can oxidize your beer. You can also aerate up to about 12 hours into fermentation, but usually not after that point. The point is not to "integrate" the yeast. Yeast use oxygen to synthesize sterols, which are used to build new cell walls as the yeast grows. A lack of oxygen limits yeast growth. Aerating/splashing pre boil has been found to not be as much of an issue as was once thought. The boil will drive off any oxygen picked up pre boil. Aeration/spalshing post fermentation (like at bottling or kegging)will not be driven off and will reuslt in oxidation and premature staling.

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