Secondary Fermentation
Cask-Conditioned Beers
Casking has its origins in the British Isles and is most widely used to make pale ales (bitters), porters, and stouts. Cask ale, also known as cask-conditioned beer or “real” ale, is beer that undergoes secondary fermentation in the barrel. Brewers of cask ale don’t interfere with it; they don’t filter it and they don’t pasteurize it. Upon completion of primary fermentation, the beer is usually transferred to a separate racking tank. Here adjustments to color may be made as well as the addition of priming sugars to achieve a desired level of fermentability as well as fining agents. Occasionally, casks are filled directly from fermenter, in which case the additions described must be added to each cask, prior to filling.
Yeast
Beer is racked either directly from fermenting vessels into casks when fermentation is judged sufficiently complete (a residual extract of 0.75–2°P) or when the correct charge of yeast is present (2–4 million cells/mL).
Priming
Once racked onto the cask, various additions are made. These include (1) priming sugars (usually hydrolyzed sucrose, called “invert” sugar) of approximately SG 1.150 at 0.35 to 1.75 L/hL, (2) hops, (3) isinglass finings to promote sedimentation of yeast and clarification of beer, and (4) potassium metabisulfite, which is a bacteriostat at pH values below 4.2 (Hough et al., 1982).
Hops
Hops are added to give beer a fresh, herbal, hop aroma without adding additional bitterness. They are usually in the form of whole hops, Type 100 plugs, Type 90 pellets, or enriched hop powder.
Finings
For cask application, isinglass is added along with any priming sugar, hops, and other ingredients when racking. The isinglass types that form larger flocs are particularly useful to cask ales, where excellent clarity is required in a short period of time (Leather, 1984).
Cellaring
The casks are then sealed and in the brewery for up to 7 days, preferably at a temperature between 13 and 16 degrees C (55 and 61°F), to promote conditioning and fining (Hough et al., 1982).
Venting
After the beer has been cellared for 7 to 10 days it will have built up some carbonation. The casks are usually vented during this period, particularly if storage temperatures are high, either by removing the spile for rapid release of gas or by using a porous spile for a slower escape of gas (Hough et al., 1982).
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