Aquarium Air Pump
Aquarium Air Pump
Most of the time oxygen is not wanted anywhere near your beer, but there is one exception to this, and that's when you've just pitched your yeast.
At the start of the fermentation, the yeast needs a lot of oxygen to reproduce itself before it gets down to the serious job of producing alcohol - but the long boil will have removed a lot of the dissolved oxygen in the wort. So, when it has cooled down to a suitable temperature for pitching the yeast (which I take to be around 25°C) it is necessary to thoroughly oxygenate the wort to give the yeast a good start.
For years I did this by agitating it with a large wooden spoon, producing a froth nearly two inches thick and (usually) splashing beer on the kitchen floor. But there is an easier way - by using an aquarium air pump to pump air through the wort.
What I use now is a small aquarium air pump made by Interpet. It is called the 'AP Mini' and will pump about a litre a minute of air. Mine cost around £8 a year or so ago and came complete with silicone airline and an airstone, so there's nothing else to buy. I just drop the airstone into the wort and leave it for about 20 minutes to let it get on with aeration while I'm washing up my equipment (keeping an eye on it occasionally, to make sure that it doesn't bubble over).
(It is important to sterilise the stone and tubing when you have finished. I bubble air through plain water, sterilising solution then plain water again to make sure that it is clean.)