Phase four: the MATURATION. Most types of beer require a period of maturation. The purpose of this is to let the beer clarify, gain sparkle through the further development of carbon dioxide (in a secondary fermentation) and mellow in palate. Brewers some times refer to this as "conditioning". It may take anything from a week or two to several months. The longer type of maturation, classically carried out at cold temperature around 32° F, is sometimes known as "lagering", the German word for "storage".
Phase five: the FILTERING. When the brewer considers that the beer is mature and ready for kegging or bottling, he may first filter it to remove any remaining protein solids and yeast cells, so that the end product will be crystal clear. Some top fermentation beers are not filtered, though they may be centrifuged.

Phase six: the BOTTLING. Some specialty bottles and kegged beers, especially from Belgium, have yet a further fermentation. This type of beer is given fresh yeast and/or sugar in the bottle. It is then placed for about two to three weeks in a warm room, at about 75° F, to promote this third fermentation. These "bottle/keg-conditioned" beers will develop their palate over months and even years. Beers that have this type of further conditioning are not pasteurized. They are LIVING beers. The bottles of such beers can be stored for years, just like good wine.

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