Maturation techniques differ among breweries but generally they can be divided into two main methods for finishing beer after primary fermentation: secondary fermentation and cold storage.
Traditionally, maturation involves secondary fermentation of the remaining fermentable extract at a controlled low temperature and with a low yeast count in the green beer. During secondary fermentation, the remaining yeast becomes re-suspended and utilizes the fermentable carbohydrates in the beer. The carbohydrates can come from the residual gravity in the green beer or can be added through priming sugar or kraeusening.
Lagering was developed in Germany for bottom-fermented lagers and involves a long period of cold storage at low temperatures. Although lagering is commonly associated with bottom-fermented beers, some top-fermented beers like Kolsch and Alt beers also require lagering periods.
“Kraeusen” is a German term used to describe the infusion of vigorously fermenting young beer into a larger volume of beer that has undergone primary fermentation. Traditionally, the wort used for kraeusening is obtained from the high ‘kraeusen’ stage of primary fermentation and added in small portions (5-20% by volume) to the green beer to initiate a secondary fermentation. It is suggested to add a volume of kraeusen equal to 10 to 12% of the “green” beer, which contains approximately 2% (w/w) residual extract with a cell count between 10 and 15 million.
Casking originated in the British Isles and is most commonly used for making pale ales (bitters), porters, and stouts. Beer is racked directly from fermenting vessels into casks when fermentation is deemed sufficiently complete (having a residual extract of 0.75 to 2°P) or when the correct amount of yeast is present (0.25-4.00 million cells/ml). If there is too little yeast in the beer, secondary fermentation will be slow and the beer will have insufficient carbon dioxide. However, if there is too much yeast suspended in the beer, secondary fermentation may become violent. While traditionally beer was directly racked to the cask, some brewers pass the beer through rough filtration to improve clarity.
The practice of using priming sugars for bottle-conditioning has been refined by British brewers and is still followed by some craft brewers as well as a few larger British brewers. Belgian brewers are also known to use this method to add unique flavors. Bottle-conditioning usually involves a short time in conditioning tanks to improve overall stability and flavor before adding priming sugars.
With the use of modern refrigeration, carbonation, and filtration equipment, the need for secondary fermentation and long cold storage has been eliminated. The green beer undergoing cold storage is fully attenuated and virtually free from yeast, achieved through higher fermentation temperatures and a diacetyl rest. Cold storage involves relatively short-term storage at temperatures of -2 to 4°C for several weeks or less, compared to the several months required for secondary fermentation and subsequent cold storage.