Oxygen and home brew have an interesting relationship. More specifically, yeast and oxygen have an interesting relationship. It can be confusing to the beginning home brewer, trying to figure out when oxygen is good and when it is bad. I will try to break it down as simply as possible.
When you boil your wort, the boiling processes removes almost all of the oxygen from the solution, however, when you pitch the yeast, they need oxygen to reproduce and adapt to their environment so that they can begin fermenting your beer. But when and how should you oxygenate?
When to Aerate Your Beer
The first place you could oxygenate/aerate the wort is as soon as the boil ends, while you chill it. However, this is actually not a good idea. This is called “hot side aeration,” and can contribute to making your beer taste stale and not store as well. The rule of thumb is that you should not aerate your wort until it is below about 85 degrees. So while you are whirlpooling and/or cooling, make sure not to splash too much and introduce as little oxygen into the hot wort as possible.
When the wort should be aerated/oxygenated is once it is cooled and prior to pitching the yeast. Because they yeast will use oxygen to reproduce, you will need to mix/stir/splash your wort thoroughly for several minutes (if you did a full 5-gallon boil. This is not much of an issue if you are brewing with extract and are adding non-boiled water to the fermentor).
Other ways of oxygenating the cooled wort include using an oxygen stone or using an aquarium pump with an oxygen tank. While using pure oxygen is much quicker and more effective, there are varying opinions on the affects of adding pure oxygen on the taste of the beer. I usually just stick with aerating through a vigorous stir.
Oxygenating at this point allows for the yeast to have a rapid growth and adaptive phase, resulting in more happy yeast to begin fermenting your brew.
When Oxygen is Bad
From the moment you pitch your yeast until your first sip of your brew, you want to avoid any contact with oxygen. As I pointed out earlier, yeast need oxygen to reproduce and adapt at the beginning of the fermentation process, but once they use up that oxygen is when they really begin producing alcohol in the beer. This is an anaerobic process, and any introduction of oxygen could impair this process and cause for a stalled or incomplete fermentation. You want your yeast to concentrate on fermenting your beer at this point, not as much on reproducing.
This is usually not a problem during primary fermentation, because the beer is in a sealed container. However, when the beer is transferred from primary to a secondary fermentor or into a bottling bucket or keg, it is at risk of the introduction of oxygen.
To reduce this, make sure that your siphoning hose is snug to your racking cane/auto siphon. Also, make sure that the end of the transfer tube is all the way to the bottom of the vessel you are transferring into.
When transferring into a keg, you should first prime the keg with carbon dioxide, forcing out the oxygen, as well as making sure that the tube is all the way at the bottom.
The reason you don’t want to add oxygen at this point is because added oxygen causes your beer to stale faster, so it will not age as well.
Obviously, you are always going to end up adding some extra oxygen to your beer (unless you have a fully closed system, but most of us only wish we were that lucky), but as long as you are careful when transferring, bottling and kegging, then you should be just fine and avoid any ill effects of stale, oxygenated beer.