Commercial brewers go to extreme lengths to reduce the amount of oxygen in their finished beers, and as a home brewer you should be concerned about oxygen as well. Oxygen can spoil the long term stability of your beer flavor and clarity and lead to a variety of off flavors.
Oxygen in Fermentation versus Finished Beer
Let me start by saying that before fermentation, oxygen is generally a good thing. In fact oxygen is needed to allow healthy yeast growth during fermentation, which is why many brewers aerate their beer before they pitch the yeast. Unless you use pure oxygen it is difficult to over-oxygenate your wort before fermentation.
After the yeast has fermented, however, oxygen is generally considered a contaminant. Many of us who rented party kegs or beer balls with a hand pump on them in our college days saw first-hand how oxygen spoils beer. These keg hand pumps put oxygen directly in the keg which meant that the beer would spoil within a very short period, often giving it a stale flavor within 24 hours. So the kegs needed to be consumed rapidly to compensate.
Oxygen, even in very small quantities is bad for finished beer. Not only does it rapidly spoil your beer if present in large quantities, it can also damage the long term flavor stability of your beer even in small quantities.
Clarity also suffers in beer with free oxygen as the oxygen will interact with polyphenols and tannins in the beer to create chill haze and eventually a permanent haze in the beer.
Once fermentation is complete, a layer of carbon dioxide, which is heavier than air, sits above the beer and provides a protective layer in your fermenter. If you don’t disturb the beer, that layer does a good job of protecting your beer as it ages.
For home brewers, oxygen is most often introduced when transferring beer from one vessel to another and also during the bottling or kegging process. Excessive splashing during transfer, small leaks in your siphon or kegging system or a poor seal on your bottle caps can lead to excessive oxygen.
Avoiding Oxygen in Beer
One strategy is simply to avoid transferring your beer as much as possible. Many brewers now skip the secondary fermentation entirely and bottle directly from their primary fermenter. Commercial brewers make use of conical fermenters, which let them remove excess yeast and sediment without transferring their beer.
Another important point is to use proper oxygen barrier containers if you are fermenting or storing your beer for any significant length of time. Glass and metal are good oxygen barriers, so use a glass or stainless fermenter if you plan to store your beer for an extended period.
Splashing during transfer and bottling is a large source of oxygen. Auto-siphon devices with a poor seal also often will pull oxygen in when siphoning. This can be seen as bubbles near the seal when siphoning. If you have a leaky auto siphon you should discard it or add sterile water above the seal so it pulls water and not air if needed.
When kegging you need to minimize splashing and do a good purge with CO2 after filling the keg. You can do this by standing the keg upright and simply releasing the pressure relief valve several times while the gas is turned on. The heavier CO2 will displace the oxygen in the keg, protecting your beer.
For bottling, you should try to keep your headspace to a minimum (generally an inch or so is sufficient), avoid splashing your beer when filling and use oxygen absorbing bottle caps if you have access to them. Also be sure to carefully adjust and check your bottle capper on some bottles as even a small leak will leave your beer undercarbonated and also stale.
Hopefully these simple tips will help you to be more aware of oxygen and some of the ways you can minimize oxygen in your finished beer.
from Brad Smith at Beersmith