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The Brewing Process - Wort & Oxygen |
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Page 7 of 9
The Wort and Oxygen
The use of oxygen in brewing is a double-edged sword. The yeast utilize oxygen in their growth processes, although they don't exactly breath. Boiling the wort drives out the dissolved oxygen, which is why aeration of some sort is needed prior to fermentation. Once the yeast use up all of the oxygen in the wort for growth and reproduction, they get down to the anaerobic business of turning sugar into alcohol and CO2 that we call fermentation. Prior aeration of the wort is the best way to ensure that there are enough yeast cells for a good fermentation.
Aeration of the wort can be accomplished several ways: shaking the container, pouring the wort into the fermenter so it splashes, or even hooking up an airstone to an aquarium air pump and letting that bubble for an hour. For the latter method, (which is popular) everything must be sanitized! Otherwise, welcome to Infection City. These instructions recommend shaking the starter and pouring/shaking the wort. More on this later.
On the other hand, if oxygen is introduced while the wort is still hot, the oxygen will oxidize the wort and this is a problem. This will cause oxidation of the beer later which can manifest as a wet cardboard taste after several weeks. The key to preventing oxidation is to not aerate when the wort is above 80°F. In addition, if oxygen is introduced after the fermentation has started, it may not be completely utilized by the yeast and will later cause off-flavors. This is why it is important to cool the wort rapidly to below 80F, to prevent oxidation, and then aerate it to provide the dissolved oxygen that the yeast need to help growth and reproduction. Cooling rapidly between 90 and 130°F is important because this region is ideal for bacterial growth to be established in the wort. See the Cooling The Wort section for suggested methods.
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