Fermentation
Fundamentally brewing yeast can be divided into ale and lager strains, the former type collecting at the surface of the fermenting wort and the latter settling to the bottom of a fermentation (although this differentiation is becoming blurred with modern fermenters). Both types need a little oxygen to trigger off their metabolism, but otherwise the alcoholic fermentation is anaerobic. Ale fermentations are usually complete within a few days at temperatures as high as 20°C, whereas lager fermentations at as low as 6°C can take several weeks. Fermentation is complete when the desired alcohol content has been reached and when an unpleasant butterscotch flavour which develops during all fermentations has been mopped up by yeast. The yeast is harvested for use in the next fermentation.
In traditional ale brewing the beer is now mixed with hops, some priming sugars and with isinglass finings from the swim bladders of certain fish, which settle out the solids in the cask.
In traditional lager brewing the 'green beer' is matured by several weeks of cold storage, prior to filtering.
Nowadays,the majority of beers, both ales and lagers, receive a relatively short conditioning period after fermentation and before filtration.This conditioning is ideally performed at -1°C for a minimum of three days, under whichconditions more proteins drop out of solution, making the beer less likely to go cloudy in the package or glass.
The filtered beer is adjusted to the required carbonation before packaging into cans, kegs or glass or plastic bottles.