Filtered beer
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Filtered beer is beer which has been cleaned of significant contact with yeast.
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[edit] Filtering methods
There are several methods used for clearing yeast from beer.
Some methods involve filtration media such as sheets or candles, while others use a fine powder made of, for example, diatomaceous earth, also called kieselguhr, which is introduced into the beer and recirculated past screens to form a filtration bed.
Filters range from rough filters that remove much of the yeast and any solids (e.g. hops, grain particles) left in the beer, to filters tight enough to strain color and body from the beer. Normally used filtration ratings are divided into rough, fine and sterile. Rough filtration leaves some cloudiness in the beer, but it is noticeably clearer than unfiltered beer. Fine filtration gives a glass of beer that you could read a newspaper through, with no noticeable cloudiness. Finally, as its name implies, sterile filtration is fine enough that almost all microorganisms in the beer are removed during the filtration process.
[edit] Sheet (pad) filters
These filters use pre-made media and are relatively straightforward. The sheets are manufactured to allow only particles smaller than a given size through, and the brewer is free to choose how finely to filter the beer. The sheets are placed into the filtering frame, sterilized (with hot water, for example) and then used to filter the beer. The sheets can be flushed if the filter becomes blocked, and usually the sheets are disposable and are replaced between filtration sessions. Often the sheets contain powdered filtration media to aid in filtration.
It should be kept in mind that pre-made filters have two sides. One with loose holes, and the other with tight holes. Flow goes from the side with loose holes to the side with the tight holes, with the intent that large particles get stuck in the large holes while leaving enough room around the particles and filter medium for smaller particles to go through and get stuck in tighter holes.
Sheets are sold in nominal ratings, and typically 90% of particles larger than the nominal rating are caught by the sheet.
[edit] Kieselguhr filters
Filters that use a powder medium are considerably more complicated to operate, but can filter much more beer before needing to be regenerated. Common media include diatomaceous earth, or kieselguhr, and perlite.
[edit] Cold filtering
A method that is gaining popularity is cold filtering in which the beer is chilled before or during a sheet filtering process. When beer is chilled the protein molecules tend to clump together and so are easier to filter out.
[edit] Lagering
Beer can also be filtered by allowing the yeast to settle to the bottom of the container and the clear or bright beer is then decanted off the sediment. The period spent allowing the beer to settle is known either as lagering or as conditioning. With ale conditioning finings are used to assist in clearing the beer.
[edit] Bright beer
The term used by brewers for beer which has been cleaned of yeast is bright beer; such beer is usually held in bright tanks before bottling or additional treatment.
[edit] Pasteurisation
An additional process that is sometimes done to filtered beer is pasteurisation in which the beer is flash heated to kill off any bacteria. Because this process is known to have an adverse effect on the flavour of the beer the practise is declining in most breweries, though some British regional breweries still pasteurise.
[edit] Brewery conditioned
In the UK a beer which has been filtered in the brewery is known as brewery conditioned beer. A brewery conditioned beer may just be filtered, or it may also be pasteurised. The term is used to differentiate such beers from those which are still in contact with the yeast which are known as bottle conditioned, cask conditioned or real ale.
[edit] Drop bright
When the yeast in a beer has settled it is said to have dropped bright or fallen bright. This means the beer is clear rather than cloudy.
