Most beer can be bottled or kegged straight from the fermenter, whether it has been racked or not. However, there are some beers that benefit from further conditioning before being put into bottles or a keg.
Regardless of whether you intend to do extended conditioning, leaving the beer in the fermenter or racking vessel for up to a week after fermentation finishes gives the yeast a chance to clean up after itself and consume certain compounds it produces during fermentation. This results in a better, more rounded beer.
However, some beers benefit from extended cold or cool conditioning.
"Lager" means "to store". It has its origins in the days when brewers would store their beer in caves during winter. They realised that this improved the beer and allowed it to develop its characteristic crisp, clean taste. Today, the best lager beers are lagered for up to several months, cheaper lagers for a couple of days. Some ales are also lagered, or stored at cold temperatures, to produce a more rounded flavour. Lagering is most effective when done to a large volume, so before the beer is bottled.
Step by step: Lagering
During lagering, the fermented beer is held as close to freezing as possible for an extended period. Unless you live in an extremely cold climate, this probably means you will need a temperature-controlled fridge or, ideally, a freezer because most refrigerators will struggle to maintain a low enough temperature.
- It's likely the beer you will be lagering is, in fact, a lager so make sure you do a diacetyl rest to remove any butterscotch flavours.
- Rack the beer. The beer can be transferred to a fermenter or to any plastic or glass container. Because the beer has finished fermentation, a container that's just big enough to fit all the beer is preferable and no headspace is required.
- Place the beer into a fridge or freezer.
- Set the temperature at 0C. If the temperature is controlled accurately the beer will not freeze because the sugar and alcohol in it lowers its freezing point to below 0C.
- Lager the beer for a few weeks to three months. Your patience will be rewarded with a well-rounded, crisp beer.
- Bottle or keg.
High-gravity, malty beers that are high in alcohol, such as strong ales, barley wines and imperial stouts, can benefit from being racked and left in a cool place such as a cellar for several months to a year before bottling or kegging. For example, the recipe for Oliver and Geoff's Millennium Ale was based on Thomas Hardy's Ale and recommends conditioning for three months after fermentation has finished and before bottling.
During the conditioning period the beer will undergo further changes and become smoother and mellower.