Of course, there is one time when you WANT to oxygenate your beer: when the wort has been cooled, right before you pitch your yeast. While fermentation is anaerobic (meaning oxygen is not required), yeast DO require oxygen for reproduction. So, you want to have some oxygen available in the just-cooled wort for the yeast to use for producing the cell walls for their offspring. The idea is that, at the very beginning of fermentation, the yeast use up the oxygen, reproduce to higher numbers, and then actively ferment the sugars in the wort into alcohol and CO2, making beer.
If we don't add enough oxygen into the wort before fermentation begins, there are a lot of problems that can result. Common ones (and I've had my share of these) include:
- Under-attenuation and stuck fermentations
- Slower fermentation starts, and therefore a higher likelihood of infection
- Longer fermentation times
- Yeast stress and off-flavors
- Lower viability for re-used yeast generations
In the excellent book, "Yeast: The Practical Guide to Beer Fermentation", by Chris White (of White Labs) and Jamil Zainasheff, the authors conclude (from various studies) that 8-10 ppm is the ideal oxygen concentration to have in wort for optimal yeast growth. As homebrewers, there are basically three methods available to use to aerate our wort:
1) Shaking the fermentor - Every homebrewer has used this method at some point. And a lot of veteran homebrewers STILL oxygenate their wort by shaking. In fact, up until my Maine Beer Company Zoe clone a few weeks ago, this is how I've aerated every single past batch. How long you shake, and how much headspace you have in the fermentor will determine how much oxygen you get into the wort. With lots of both, "Yeast" reports that a maximum of 8 ppm of oxygen can be achieved. However, keep in mind that with a 6-6.5 gallon fermentor, and ~5.5 gallons of wort, you don't really have a lot of headspace to work with when you're relying on shaking.
2) Aquarium pump with sintered stone - I haven't used this method, but know some people that do. From what I've heard, it can take quite a while to fully oxygenate wort this way (up to 30 minutes). As with shaking, greater than 8 ppm of oxygen cannot be achieved no matter how long you run the pump.
3) Pure oxygen - The preferred (and most expensive) route, and the only way to get up to (and beyond) 10 ppm of oxygen. This is the method I used for the Zoe clone, through the "Oxygen Aeration System" through Williams Brewing. This involves a stainless steel aeration stone and wand, tubing, and a regulator for a standard small oxygen tank (that you can purchase at most hardware stores). Simply sanitize the stone and wand, drop it in your fermentor, turn on the oxygen till you see bubbling for 30-60 seconds, and the yeast is ready to be pitched.
Of course, there's few homebrewers out there who will spend the large sum of money to buy equipment to measure just how MUCH oxygen they're getting into their wort. Regardless of which aeration method you use, you're guessing at what you're putting in. Even though the Williams system has the regulator to control the oxygen flow, you don't know for sure where to turn the regular knob so that you're getting, say, 1 L of oxygen per minute.
Do you have to worry about getting too MUCH oxygen in your wort? Definitely not if you're shaking or using an aquarium pump. If you're using pure oxygen, it's still probably not a worry. However, "Yeast" does say that really high amounts of pure oxygen can result in "high levels of fusel alcohols, increased acetaldehyde, and other flavor problems". This would be difficult to do accidentally on a homebrew level, however; you'd have to have the oxygen turned on for a significantly longer time to get really high levels that would be detrimental to your beer.
When about aerating really high-gravity beers? It's simply stated in "Yeast" that with beers with OGs higher than 1.092, aeration methods other than pure oxygen simply won't do. In fact, the authors suggest that for all beers with an OG above 1.083, aerate TWICE. That is, before pitching the yeast, and then another dose of oxygen 12 to 18 hours after pitching (this allows the yeast one cell division). This second dose was shown in a study to help speed up fermentation speed and attenuation.
Generally speaking, if you're brewing a low-to-mid-gravity beer, you're probably ok to aerate by shaking, if you put a lot of effort and a good amount of time into it. But when you're brewing bigger beers, or lagers, and you really want to ensure you're getting the right amount of oxygen -> healthier, happier yeast -> better beer, you're better off using pure oxygen. I finally made the move to pure oxygen because I've had many batches (especially "bigger" beers) where I've had trouble hitting my target FG, and now that I've consumed over 60 types of my homebrew, I'm picking up on off-flavors that could be attributed to under-aeration. I felt this was a small investment to make for healthier yeast and (hopefully) better beer.