Cask ale is served without externally supplied carbon dioxide or nitrogen. The idea that it is served "flat" with no bubbles at all, though, is a misconception; properly conditioned cask ale contains bubbles of carbon dioxide generated by the natural action of the yeast living in the beer while it is transported and stored1[›]. In fact, the yeast will actually produce more carbon dioxide than is required, and it is for this reason that the cask is vented through the spile hole in the shive.
Two kinds of spile are available. Initially, "soft" or porous spiles are used, made of open-grained softwood, bamboo, or harder wood with cuts in. These allow the excess gas to escape. Once the cellarman judges this process to be complete, a hard spile is inserted which does not allow any more gas out of the cask. These are made of harder wood (usually still technically "softwood", but denser and more solid than soft spiles). Plastic pegs intended for sealing shives are occasionally used as hard spiles.
During service, the spile must be removed to allow air in to replace the beer drawn off. Once the spile is reinserted enough gas will come out of solution to replace what was lost, but the amount of carbon dioxide available in the system - free or dissolved - is limited, so it is important that the spile is kept in as much as possible. This is one of several reasons for real ale's short shelf-life once a cask is opened - if too much carbon dioxide is lost, the beer will be flat. Typically, the beer will be good only for two to four days2[›]; this short life is why it is important that a pub serving real ale have sufficient turnover for casks to be emptied while still at their best.
Like many such older terms, the word spile has other local meanings. For example:
- At Indian Harbour in Nova Scotia, the shores on which fish processing "stores" were located were called spiles.
- Spile is also a term used in boatbuilding to describe the means/actions which a builder uses to determine the shape of any curved piece needing to be fitted to a curved surface.
^ 1: Properly served, there should be no significant quantities of this yeast present in the beer as it is drunk. During the conditioning process, finings help to gather the yeast into clumps ("flocculation") which sink into the belly of the cask below the tap. Only if the cask is shaken, tilted too far, or served too early will the yeast find its way into the glass.
^ 2: Stronger beers will last longer, mostly because the increased alcohol content serves as a preservative. Nevertheless, they may be a little flat by the end of their life. This is not catastrophic, as most strong beers are somewhat dark, heavy and sweet and thus rely less on carbonation compared to light, hoppy "golden ales".
- ""Flap Jack Do It Again Transcript"". Good Eats Fan Page. May 21, 2006.
- Cellarmanship - Caring for Real Ale, 2nd Edition, 1992. Published by CAMRA. ISBN 1-85249-063-2