a lot of the oxygen that was in the water has come out. All those bubbles during boiling are water vapor – H2O – a third of which is O, or oxygen. This oxygen is a vital part of the fermentation process, as yeast are aerobic creatures, meaning they need oxygen to thrive and procreate, just like we do.
The problem is how to get oxygen back into the wort. The method shown here is to let the cooled wort splash around as it flows down to the fermentation bucket. That foam in the bucket is an indicator that air is getting back into the liquid. Another method is to stir the wort vigorously once it is in the bucket. I do not like this method because I invariably become a little too vigorous and splash some on the floor. I hate cleaning up. The lazy solution is to just avoid having to do clean up by developing elegant solutions that help you avoid work.
As you can see in the photo, the valve on my brew pot is open only partway. This tends to make the flow of the liquid less fluid, for lack of a better word. The flow splashes a little more because it is not smooth. All this causes mixing with the air and brings enough oxygen back into the liquid.
Another method is to use an oxygen injection system. This is similar to something you might have in an aquarium tank to keep your fishies from suffocating. This is an effective way to accomplish the task but the thing that I do not like about this method is that it requires extra equipment, like a tank of oxygen and an oxygen stone doodad or some other device. It is a lot of overhead for a relatively simple process, in my book, although some people love this method.
One tool I have that makes aerating wort very easy is a drill-mounted stir rod. It fits on my drill and does a great job of whipping the liquid around and getting some good air all up in there. Normally I use this device to drive off gas when I make wine, but it does just as well in oxygenating my wort. In addition, the paddles fold up and allow the device to fit into the neck of a carboy.
Whatever method you use, be sure not to skip this important step. Your yeast will thank you by ensuring that your brew is nice and potent.