Maturation includes all transformations between the end of primary fermentation and the removal of yeast from the beer in preparation for packaging. Although most beer fermentations are technically complete within 3 to 10 days, the vast majority of beers are not yet ready to drink when the yeast finishes its primary work of metabolizing sugars. This is because fermentations tend to produce flavors that are considered undesirable in finished beer. For this reason, beer must undergo some form of maturation to become palatable. Maturation is also referred to variously as conditioning, lagering, and aging.
In traditional brewing practices fermentation and maturation are considered separate steps, but in reality there is significant overlap between the two. Maturation involves many biochemical, chemical, and physical reactions, many of which are not completely understood and elucidated. Vicinal diketones (VDKs, such as buttery-tasting diacetyl and honey-like pentanedione), hydrogen sulfide (rotten eggs), and acetaldehyde (green apples) are primarily responsible for undesirable flavors at the end of primary fermentation. Immature beer is often referred to as “green beer” because it sometimes has the aroma of green apples, the result of elevated levels of acetaldehyde.
In a traditional two-vessel process, lager beers are transferred following primary fermentation and cooling (to approximately 0°C–4°C (32°F– 29.2°F]), into a separate vessel, where residual sugars (maltotriose and sometimes maltose) are slowly fermented. At the same time, off-flavors are reduced, and the beer becomes carbonated as the yeast continues to give off carbon dioxide. Sometimes kräusening, the addition of a small proportion of fermenting beer, is performed before the beer is transferred to a lagering (cold storage) vessel.
Ales are traditionally conditioned by relatively warm storage, usually by holding the beer at 10°C–20°C (50°F–68°F). Because most ale yeasts act quickly at warm temperatures, this storage period can be quite short, and many warm-fermented beers are ready to package within 14 days of brewing. In the UK cask-conditioned ales are traditionally matured (conditioned) unfiltered in the cask in the cellar of the retail outlet (bar or public house). Continuing fermentation in the cask gives the beer a light natural carbonation. Isinglass (collagen) is used to adsorb the yeast and other solid material (protein–polyphenol complexes) and settle them to the bottom of the cask, clarifying the beer before service. Bottle-conditioned beers undergo a secondary fermentation in the bottle. This produces carbonation, but these beers normally require further maturation in the bottle before the beer can be released for sale.
The use of secondary maturation vessels can be expensive and cumbersome, and many breweries now ferment and mature beer in the cylindroconical vessels widely known as Unitanks. These tanks employ their own chilling jackets, obviating the need to send beer into another vessel in a cold cellar. The cone-shaped bottoms of the tanks allow easy removal of the sedimented yeast.
There have been a number of attempts by large breweries to develop continuous systems for the brewing process, including continuous maturation. A continuous maturation process has been developed using immobilized yeast cells for accelerated beer maturation. Yeast cells are immobilized on DEAE cellulose particles or glass beads. To achieve rapid reduction of diacetyl in the immature beer, the original fermentation yeast is removed by centrifugation. This clarified immature beer is heat treated (90°C [194°F] for 7 to 8 min) to convert all of the diacetyl precursor (alpha acetolactate) to diacetyl. Care must be taken to prevent oxygen uptake. After the beer is heat treated, it is rechilled and then slowly flowed through a packed bed column containing immobilized yeast cells.