Beer Fermentation
Fermentation Systems
There are many different fermentation systems that are used worldwide that have evolved based on available technology, brewing materials, and perceived product quality. The following is a brief description of some of the more common fermentation systems in use today.
Cylindroconical
Cylindroconical fermenters are the most commonly used fermentation systems used today to produce both lagers and ales. As the name implies, the enclosed vessels are vertical cylinders with a conical base and, normally, a dished top, as shown in Figure 13.1. This design allows for easy yeast collection and CIP cleaning. They range in size between 100 and 7,000 hl, have from a 1:5 to a 3:1 ratio of height to diameter, and work under pressures of from 1 to 1.3 bars above atmospheric pressure. In fermentation vessels with a ratio greater than 3:1, there is a tendency for increased production of higher alcohols at the expense of esters (5). Vessel geometry plays an important role in fermentation. As the height-to-diameter ratio increases, so does the mixing of yeast and wort, as well as the fermentation rate.
Traditional Ale Top-Skimming System
This is the traditional ale skimming system used in the United Kingdom. It utilizes top-fermenting yeast cropped for re-pitching by skimming from the surface. The vessels are normally shallow and flat-bottomed and may be round, square, or rectangular. Traditionally, these vessels are open-topped to facilitate skimming and are located in a well-ventilated room to disperse the carbon dioxide evolved during fermentation.
Traditional Ale Dropping System
This is a variation of the ale top-skimming system, whereby the wort is pitched, collected, and partially fermented in one vessel (the settling tanks) and then dropped or transferred to a second vessel after 24 to 36 hours to complete the fermentation (53). Trub and cold break are left behind in the first vessel, and the transfer also rouses the fermentation.
Yorkshire Square System
This system, as shown in Figure 13.2, is used in the north of England to produce ales with a clean, round palate. It is a variation of the ale fermentation system adapted to enable the use of a flocculent yeast strain or a two-strain system with one of the yeasts being flocculent. The vessels are normally square or rectangular in shape, and constructed of stainless steel. The Yorkshire square system consists of a lower compartment with a gently sloping upper deck located just above the fill level of the vessel.
Burton Union System
This system was commonly used in Burton-on-Trent and produces pale ales with a fruity character by using "powdery" non-flocculent yeast. The wort is collected and pitched in a separate vessel and then transferred to the union set after 24 to 48 hours. The union set consists typically of 2 rows of wooden casks, 24 to 50 in number and each of 7 hl capacity, positioned under a cooled trough. Each cask is fitted with a swan neck that overhangs a slightly inclined trough mounted above the casks.
Open Square Fermenters
Traditional lager systems in Continental Europe and still common in Eastern Europe utilize open square fermenters (see Figure 13.3) similar to those used in the traditional ale top system.
Dual Purpose Vessel System
The vessel characteristics that make cylindroconicals so suitable for fermenting vessels also make them ideal for conditioning tanks. This has led to the installation of dual-purpose vessels where primary fermentation and conditioning are carried out in the same vessel. Dual-purpose vessels have cooling jackets located high in the vessel for fermentation and low in the vessel for conditioning. Other differences are that conditioning tanks are normally required to be top-pressured to maintain carbonation levels, which is not required in fermenters.
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