It is critical that the beer is fully fermented before you bottle it. If it is not fully fermented then you will have problems with the beer being over-gassed, and bottles possibly exploding.

So, how do you tell if the beer is ready to bottle? Use your hydrometer. Do not rely on the airlock. If your fermenter has a leak then the airlock will stop bubbling before fermentation has finished. Do not rely on just counting the days as per what it says on the tin or what somebody tells you they do. The speed of fermentation can vary with temperature, amount and health of your yeast, and how much there is to ferment.

When the airlock stops bubbling, or close to it, and when the foam has disappeared from the surface of the beer then take a hydrometer reading and record it. Wait two days and take another reading. If the two readings are the same then fermentation has finished. If the second reading is lower, ie a smaller number, then wait another two days and take another reading. Once you have two consecutive readings the same, separated by two days, then you can be fairly sure the beer has finished fermenting. At this point relax: there is no hurry to bottle the beer. Leave the bottling until the weekend when you have enough time. The extra couple of days will benefit the beer by giving it a chance to clear and condition up. However, don’t leave it until the weekend after.

When the yeast are actively fermenting the wort will be very hazy or milky. When the yeast finish fermenting they drop to the bottom of the fermenter. Leave the beer until it is fairly clear before you bottle it. This will help reduce the amount of yeast in the bottle.

There is one last problem that can happen if you rely on the airlock as a fermentation indicator. If you have a very good seal then although fermentation has finished you can still get a very slow bubbling in the airlock as the beer and the gas in the headspace expand as the day warms up. This has fooled many people into thinking that the beer is still fermenting so they have left it for weeks and weeks and the beer has spoiled. What they didn’t see was the beer contracting in the cool of the night and sucking air back in.