Wort aeration is a very important step in brewing but is often
overlooked by brewers; it is one of the best ways to prevent fermentation
problems. Even though yeast doesn't need oxygen in the later stages of its life,
it is vital in the early stages when the yeast is growing/multiplying.
Winegrowers Supplies - Wort Aeration
There are basically only 2 rules to aeration:-
1. Do not aerate when the wort is too hot. A cooler wort will accept a lot more air than a warmer one, so it is to your own benefit to wait for it to cool down before aerating. There is also the much-debated issue of hot-side aeration, which could cause off-flavours in your beer. What happens is that when you aerate hot, oxygen binds to certain compounds in the wort so the yeast cannot consume it as it normally would. It can stay bound for several weeks to several months before unbinding again. Once unbound the oxygen does to your beer what it does best - oxydises. So you want the wort to be as close to room temp as possible when you aerate it.
2. Do not aerate after fermentation has begun. It is fine to aerate right after pitching the yeast. Indeed, that's exactly what many people do. However, once fermentation has begun, it is generally considered bad to introduce any oxygen. Though realistically the young beer can benefit from the introduction of fresh oxygen at very least for the first 5 or 6 hours of fermentation, and probably twice that.
Another possible exception to the above rules is that occasionally aerating the wort is the only way to get things going again if your fermentation sticks. Of course, a stuck fermentation is often a sign that the wort wasn't aerated properly in the first place.
Aeration is achieved through a special 'stone', which has thousands of little pores, allowing oxygen to be pushed through under pressure.
Home brewers use a 'stone' a made of sintered particles, a larger version of the type used to oxygenate aquariums. The disadvantage of this type is that it can't be sterilised properly, so needs replacing regularly.
Professional breweries use stainless steel 'stones' with smaller pores that allow finer oxygenation.
The 'stone' is fitted inside a stainless steel 'T' tube, connected 'in line' with the wort flow.
Ideally a medical grade oxygen tank (probably available on Ebay) should be used to feed the 'stone'.
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