In England the methods of applying the top fermentation system may be classified as follows: (A) The Cleansing System: (a) Skimming System, (b) Dropping System (pontos or ordinary dropping system), (c) Burton Union System. (B) The Yorkshire Stone Square System.Fig. 4. - Fermenting Round.
(A) In (a) the Skimming System the fermentation from start to finish takes place in wooden vessels (termed "squares" or "rounds"), fitted with an attemperator and a parachute or other similar skimming device for removing or "skimming" the yeast at the end of the fermentation (fig. 4). The principle of (b) the Dropping System is that the beer undergoes only the main fermentation in the "round" or "square," and is then dropped down into a second vessel or vessels, in which fermentation and cleansing are completed. The ponto system of dropping, which is now somewhat old-fashioned, consists in discharging the beer into a series of vat-like vessels, fitted with a peculiarly-shaped overflow lip. The yeast works its way out of the vessel over the lip, and then flows into a gutter and is collected. The pontos are kept filled with beer by means of a vessel placed at a higher level. In the ordinary dropping system the partly fermented beer is let down from the "squares" and "rounds" into large vessels, termed dropping or skimming "backs." These are fitted with attemperators, and parachutes for the removal of yeast, in much the same way as in the skimming system. As a rule the parachute covers the whole width of the back. (c) The Burton Union System is really an improved ponto system.
A series of casks, supplied with beer at the cleansing stage from a feed vessel, are mounted so that they may rotate axially. Each cask is fitted with an attemperator, a pipe and cock at the base for the removal of the finished beer and "bottoms," and lastly with a swan neck fitting through a bung-hole and commanding a common gutter. This system yields excellent results for certain classes of beers, and many Burton brewers think it is essential for obtaining the Burton character. Fig. 6 (Plate II.) shows the process in operation in Messrs Allsopp's brewery.
(B) The Stone Square System, which is only used to a certain extent (exclusively in the north of England), practically consists in pumping the fermenting wort from one to the other of two superimposed square vessels, connected with one another by means of a man-hole and a valve. These squares are built of stone and kept very cool. At the end of the fermentation the yeast (after closing the man-hole) is removed from the top square.
After the fermentation and cleansing operations are completed, the beer is racked off (sometimes after passing a few hours in a settling tank) into storage vessels or trade casks. The finest "stock" and "pale" ales are stored from six weeks to three months prior to going out, but "running" beers (mild ales, etc.) are frequently sent out of the brewery within a week or ten days of mashing. It is usual to add some hops in cask (this is called dry hopping) in the case of many of the better beers. Running beers, which must be put into condition rapidly, or beers that have become flat, are generally primed. Priming consists in adding a small quantity of sugar solution to the beer in cask. This rapidly ferments and so produces "condition."