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Adapted from Dufour et al., 2003 and Hough et al., 1971

Adapted from Dufour et al., 2003 and Hough et al., 1971

steps are almost always performed. Clarification can occur via physical separation methods, such as centrifugation, or by the addition of fining agents.

Fining agents act by promoting aggregation or flocculation of yeast cells, and include wood chips, gelatin, and isinglass.The latter, which is derived from fish bladders (really), is more commonly used for ales, especially in the U.K., whereas gelatin is frequently used in the United States for lager-style beers. The suspended cells and other insoluble debris then settle to the bottom of the tank or cask. The clarified, or bright, beer that remains is not cellfree. Therefore, the beer is filtered to remove residual yeast or bacterial cells that still remain.

Several filtration configurations exist, including plate and frame systems, leaf filters, and cartridge filters. The filters or filter sheets do not necessarily exclude cells on their own, that is, the pores within these filters are usually not so small that they directly restrict passage of cells.Yeast and bacterial cells have approximate diameters of about 10 ^m and 2 ^m, respectively, and typical cellulose filters have much larger pore sizes.To remove cells by filtration, then, filtration aids are used.

Filtration aids are inert, insoluble materials, such as cellulose or diatomaceous earth, that act to form a filter bed that traps cells without causing the filter to become plugged or fouled with a layer of cells. Cell counts of less than 10 per ml can be achieved in the filtered beer. Other filter aids, such as silica gel and polyvinylpolypyrrolidone (PVPP), can also be used, but mainly to remove haze-forming proteins and polyphenolic materials, rather than cells (see below).

Despite the effectiveness of these bulk filtration systems, however, there are now many breweries that further filter the beer through membrane filters with pore sizes less than 0.45 ^m. Since bacteria and yeasts cannot pass through these filters, the beer becomes sterile. Many of the large breweries now produce filter-sterilized beer. Because filtration is done at low temperature, this process is also referred to as cold-pasteurized beer (which also has the marketing advantage of distinguishing this process from heat-pasteurized beer).

If a sterile (or polish) filtration step is to be performed, the beer must first be pre-filtered (as described above), to remove cells and other material that would otherwise foul the mem-branes.Although most brewers recognize that filtration is necessary to produce beer with the expected clarity and stability properties, they also realize that filtration may remove flavor, color, body characteristics, and other desirable components. Thus, filtration systems must be carefully designed to achieve the desired appearance without sacrificing flavor and body.

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