Although much of the suspended yeast will settle to the bottom of the storage tank by gravity sedimentation, it can be very time consuming in preparing beers for filtration. Consequently, the brewer can add fining agents at the onset or during storage to speedup the sedimentation process. Alternatively, the brewer can use centrifugation to remove yeast and other solids after fermentation. Each of these processes is described in the following sections.
Although good clarity can be obtained from simple sedimentation, better results can be obtained in less time by using fining agents-isinglass and gelatin. The use of finings is not universal. They find their widest employment in the United Kingdom with some ale brewers but there has been renewed interest in North America.
Isinglass is a traditional "real ale" clarifier used in the United Kingdom, where the style of beer benefits from a 48 hour clarification before or after casking. It is also used for fining chilled and filtered beers. Isinglass is a gelatinous substance derived from the internal membranes of fish bladders and comes in many different forms. The currently accepted mechanism involves a direct interaction of positively charged isinglass with negatively charged yeast to form flocs, which precipitate. Its effectiveness in settling ale yeast varies with the strain of yeast, and it is generally not recommended for precipitating lager yeast.
Centrifuging is a popular method of reducing the yeast content of beer and is often used where fining agents are not used or are used in conjunction with fining agents. Many brewers who practice accelerated cold conditioning use centrifuges, as they offer a greater degree of control over yeast count and eliminate the time needed for maturation and fining. The yeast count in beer ex-centrifuge can be controlled to a level of 0.05 x 106 cells/ml (29). The flocculation characteristics of the yeast are of less importance as long as the yeast remains in suspension for most of the fermentation. Optimal yeast separation is achieved at temperatures between 3 and 5°C (29). There are two principal ways to use centrifuges for yeast removal after fermentation.
Click on the following topics for more information on beer conditioning.