The two most common defects in home brewed beer are probably oxidation and bad fermentation (incomplete attenuation, too warm, etc.) Most home brewers don't have the temperature controlled fermenters and closed transfer systems that professional breweries have, so it can be hard for us to avoid these defects unless we pay attention and hone our techniques.

Oxidation can be particularly treacherous. A badly fermented beer tastes bad from the beginning,  so you know something is wrong. But an oxidized beer might taste great to begin with, only to slowly become undrinkable. How frustrating is that?

Very frustrating, and I speak from experience. Too many of the batches I made in the 2010 brewing season ended up aging poorly, all thanks to oxidation. This year I decided to focus on improving my techniques in this area, hopefully producing beer with a delicious, long-lasting flavor.

Basically, oxidation can occur during any of these steps:

1. Mashing
2. Fermentation
3. Racking
4. Packaging

It's the last two that are of particular concern, but let's go through all four and see how you can avoid oxidation throughout the brewing process.

Mashing

Oxidation during mashing is normally called Hot Side Aeration (HSA), which is an umbrella term for any sort of oxidation before you pitch yeast. There's some debate over how much of a problem HSA is for home brewers, with Dr. Charlie Bamforth coming down pretty strongly on the "Don't worry about it" side of the debate. On the other side was Dr. George Fix, who details all the nasty effects HSA will have on beer (off flavors, quick staling, etc.) in his book Principles of Brewing Science.

Personally I don't spend too much time worrying about this, but that's largely because my processes don't encourage HSA. I stir my mash once, at the most. I don't splash the wort when running into the kettle or when racking into the fermenter. Instead of aeration for the yeast, I oxygenate my wort which means shorter exposure time.

But if your process includes things like transferring the mash from a mash tun to a lauter tun, or a lot of splashing when moving wort, then you may want to improve those processes in order to reduce HSA.

If you want to learn a lot more about HSA, check out this episode of Brew Strong with Dr. Bamforth. After listening to it you probably won't care about HSA, but you'll know a lot about it.

Fermentation

Once your beer is in the fermenter and yeast has been pitched the yeast should consume available oxygen in order to reproduce. If you over-pitch then the yeast won't need to reproduce as much leaving oxygen in your beer.

Another way to introduce oxygen during fermentation is to frequently open and close the fermenter -- maybe you want to pull samples or just take a peek. Regardless, try to keep the fermenter closed. The yeast will be releasing CO2 which blankets the beer and protects it from oxygen. You're not doing your beer any favors by poking around in there and  disturbing that CO2 blanket.

Once I got patient enough to leave the fermenter closed until fermentation was complete I stopped worrying about introducing oxygen during fermentation.

Racking

Transferring beer from one container to another is the main way to oxidize your beer. It's also a great way to introduce your beer to wild yeast and bacteria that can infect it. But, unless you plan on fermenting in the kettle (which some people do), at some point you'll have to move that beer from Container A to Container B.

Oxidation from racking is one of the main reasons that most brewing pundits recommend against moving your beer to secondary. If you're going to keg, just move the beer from the primary fermenter to the keg. If you plan on bottling, move the beer from primary to bottling bucket.

There are two easy steps to minimize oxidation during racking. First, flush both containers with CO2. Just attach a hose to your CO2 regulator, turn the pressure down low, and fill the containers. Obviously you can't tell when the containers are full  --  just guess. Do not worry about running out of CO2. There's a lot of gas in even the smallest tank.

Second, transfer the beer slowly and gently. Don't splash or agitate the liquid. Doing so increases its surface area, which increases the amount of oxygen it can pick up.

My technique is a bit more complex -- I've moved to closed transfer system in which the beer is never exposed to any oxygen or possible contamination. The fermenter is sealed with a carboy cap that is attached to a racking cane and a CO2 tank. The racking tube connects to a keg that has been sanitized and filled with CO2. I turn on the CO2 tank, which pushes the beer out of the fermenter and into the keg. I in no way invented this setup, which is well documented at The Beer Journals.

Packaging

Packaging is really just another type of racking, so the same techniques apply.

If you're bottling from a bottling bucket, purge the bucket with CO2, add the priming solution and then rack the beer onto the priming solution. Throw some more CO2 on top and then cover the bucket. You shouldn't need to stir in the priming solution. You can also purge the bottles with CO2 before you fill them.

If bottling from a keg, either purge with CO2 or use a filler like the BeerGun, which lets you purge with the push of a button. I fill from a keg with a BeerGun and it works great for me. Remember to purge the headspace before putting the cap on! This is something that I always forget.

Oxygen absorbing caps are also a good choice, although you have to be careful about getting them wet. Once the liner gets wet it activates, meaning the caps are useless if you get them wet too long before bottling. My usual approach is to sanitize as many as I need for a single batch. If any sanitized caps are left over once bottling is done I throw them away.

My understanding is that additional bottle seals -- wax, foil, etc. -- don't help prevent oxidation. They do look nice, though.

I don't know much about corks and their effect on oxidation. From what I can tell this is an ongoing debate, especially amongst wine drinkers.