There are two stages of fermentation. Using a staggering amount of imagination and creativity, they have been dubbed 'primary fermentation' and 'secondary fermentation'...
So you’ve introduced your yeast into wort that has been cooled and aerated. The oxygen in the wort is used by the yeast cells in multiplying, and when the oxygen is all gone the yeast then goes to work to produce alcohol and CO2 from most of the sugars that are present in the wort. Every time you open the lid of your fermentation bin at this stage you run the risk of introducing an infection, so it’s good practice to leave well alone. If you do sneak a peek however, you might see a quite thick foamy
crust sitting on top of the beer. This is called the krausen. There is no need to be alarmed, this is a natural by product of the yeast cells doing their job. Some brewers take a sterilised spoon and skim off the krausen; I used to do this myself, but after some experimenting I don’t bother these days because I’ve found it makes no difference to the beer.
The krausen will sink back into solution after a short while. Very occasionally due to an overactive fermentation you might be unlucky enough to be faced with a situation as shown in the above photo; here the krausen is produced with such force that it has blown the lid off the fermentation bin, is running down the sides of the bin and is threatening to go on the rampage throughout the surrounding countryside.
If you like messing with a hydrometer, primary fermentation has ended when the reading bottoms out and remains constant, say at 1.010 or 1.012. On really 'big' beers such as Twelfth Night At Toad Hall you could find the reading bottoming out at quite a high figure such as 1.022. If, like me, hydrometers don't float your boat, simply leave the wort alone. When brewing normal strength beers you can safely assume primary fermentation has ended one week after you first noticed the yeast had kicked in; if you're in any doubt, no harm will come to the beer if you leave it for 14 days. In the case of very strong beers I'd recommend you leave it for at least 14 days and a week longer than that if you're unsure.
Very little fermentation takes place during secondary fermentation; it’s really just a phase where the beer undergoes some conditioning, when yet more yeast cells drop out of solution and flavours and aromas mellow to some extent. All you need to do is to siphon off the beer from the fermentation bin in which primary fermentation took place, into a sterilised fermentation bin, taking great care not to transfer any of the trub. Stretch a large fine straining bag over the neck of the sterile bin to catch any large pieces of trub hanging in suspension. When siphoning is complete, close the lid and leave it to stand at room temperature for a couple of weeks and you’re done. If you wish, just in case some further minor fermentation has taken place, you can take a further hydrometer reading at the end of the two weeks. Your reading will be identical, or extremely close to, the reading you took at the end of primary fermentation.
Many brewers these days do not bother with the secondary fermentation stage, believing it to be out of date and redundant - and of course it’s up to you; feel free to experiment. My own view for what it's worth is that secondary fermentation still has a place. It takes almost no work at all to accomplish, and when the beer is siphoned off at the end of secondary fermentation, for bottling or kegging, there is always a small amount of trub left behind – debris that will find its way into the finished beer if secondary fermentation isn’t done. It’s very difficult to siphon off beer in primary for bottling or kegging without disturbing the bed and getting at least a little of the trub in your beer, which makes it cloudy. And of course if you are brewing a beer that requires additions such as fruit and dry hopping, then secondary fermentation is a good place to do it because you don’t have to worry about stirring up much trub back into suspension when you stir in any additions.
So you've now had your beer sitting in secondary fermentation for two weeks and now we need to think about getting it into either bottles or a keg; the choice is up to you. Let's start out by looking at Bottling...