Insulated stainless steel shell
Fig. 5.4 (a) Traditional wooden open fermenter with bottom drain, (b) Lined wooden open square fermenter. (c) Stainless steel closed square fermenter.
standards, typically less than 30 hi. Although used mainly for top fermentation, similar vessels are used with bottom-cropping yeasts, for example, traditional Pilsner lager production in the Czech Republic (Hlavacek, 1977). Belgian Iambic beers (Section 18.104.22.168) are fermented in closed casks (with an aperture for the egress of carbon dioxide) made from oak or chestnut and originally used to hold wine (de Keersmaecker, 1996).
A slightly more sophisticated vessel, an open square fermenter, is shown in Fig. 5.4(b). These may also be constructed from wood, but in this case there is an inner metal liner, typically aluminium or copper. This is more easily cleaned than a wooden surface. Control of fermentation rate is facilitated by an internal attemperator. To ensure good heat transfer this is usually constructed from copper. Open square fermenters have modest capacities, most often within the range 30-50 hi. Like the primitive cask fermenter, the low aspect ratio of the open square favours the formation of a yeast head. Removal of this is via a wide mouthed drain ('parachute'), attached to the side of the vessel. The height of the parachute is adjustable to allow for different wort depths and thickness of yeast head. The skimming board consists of a length of wood or polypropylene, which allows the yeast head to be pushed towards the parachute inlet. Other methods for collecting the yeast head are used, for example, suction pumps. With this arrangement it is possible to pass the yeast through a filter press and return the beer filtrate to the vessel, thereby reducing losses associated with top cropping.
In operation, both the open square and the cask type vessels require a high degree of manual input. In particular, such vessels are normally cleaned by hand. It is
possible to automate the cleaning of open squares by providing a temporary cover, usually made of polypropylene. Apart from containing the process, the cover incorporates spray balls that distribute the cleaning agent over the surface of the vessel. The residues are collected via the vessel bottom drain. Even with this automatic system, it is still advisable to remove the attemperator for separate cleaning.
The modern incarnation of the square fermenter is shown in Fig. 5.4 (c). These vessels are now usually constructed entirely from stainless steel and typically have capacities of up to 500 hi. Larger vessels have been used, for example, the Guinness brewery in Dublin, in the 1960s installed stainless steel rectangular fermenters with capacities of 5200 and 11500 hi (Lindsay & Larson, 1975). Closed square or rectangular fermenters are provided with a dedicated CiP system. Attemperation is achieved by wall mounted cooling jackets. Apart from being more efficient, these lead to much improved hygiene since the interior of the vessel is relatively uncluttered and therefore much more easily cleaned. Temperature is measured with a thermometer mounted in the wall of the vessel. If desired, output from this may be used to regulate the flow of coolant and thereby provide automatic attemperation. Since the vessels are of a closed design, it is possible to collect carbon dioxide. Modern closed square fermenters may be used for both top and bottom fermentation. If the former is used there are facilities for automatic skimming of the yeast head via a suction pump. The yeast head may be pushed towards the pump inlet using a gas jet. The base of the vessel slopes towards the outlet, which assists with removal of the yeast when used for bottom fermentation.
Traditional open square or cask type fermenters are not fitted with mechanical stirrers and compared to large cylindrical vessels natural agitation is limited. This can be a problem where particularly flocculent strains of yeast are used since sedimentation of yeast cells may cause premature slowing or even cessation of fermentation. Furthermore, several traditional ale fermentations use mixed yeast strains. If one strain is particularly flocculent it may sediment differentially and eventually disappears from the subsequent top crop. In the United Kingdom, several fermentation systems have been developed specifically to overcome these problems. In the dropping system, the fermentation is started in one open square fermenter and then after 24-36 hours, the partially fermented wort is transferred to another similar vessel. This resuspends the yeast, ensures thorough mixing and high fermentation rates. In addition, the transfer has a cleansing action. Thus, if carefully managed it is possible to transfer the partially fermented wort and yeast but leave much of the non-yeast solid material behind. However, there are some disadvantages. The process uses two vessels for each fermentation, and therefore is expensive in terms of labour and doubles the number of cleans required. In addition, the approach is inherently inefficient since the wetting losses associated with emptying vessels are also doubled.
The Yorkshire stone or slate square is a traditional open vessel designed specifically for use with flocculent ale strains (Fig. 5.6). It is a one-vessel system, and therefore overcomes the doubled cleaning disadvantage of the dropping system. As the name suggests, such vessels were originally constructed from slabs of stone or slate and were small, in the region of 30-50 hi. Modern versions are constructed from stainless steel or aluminium and have capacities of 300-400 hi.
Several variations have evolved; however, the characteristic of the vessel is that it is divided into two compartments by a partition or 'deck', positioned towards the top of the vertical walls. The deck slopes slightly, either from the centre or from one side to the other and is perforated by a central circular manhole. The latter has a raised rim about 15 cm high. In addition, the upper and lower compartments are connected by a number of tubes, known as 'organ pipes', which extend downwards almost to the bottom of the vessel. At the periphery of the deck, there is a drain for removal of yeast. A further aperture in the deck allows access for dipping. Vessels were originally cooled by circulating cold water through an external jacket; later these were superseded by submerged attemperators with the inlet and outlet fed through the manhole.
In operation, wort is added to a depth of a few centimetres above or below the
height of the deck. The optimum depth is judged by experience and depends on the properties of the yeast that is used. As the fermentation proceeds, yeast rises through the manhole where it is retained on the deck. Beer separates from the yeast head and drains back into the lower compartment via the organ pipes. If very flocculent yeast strains are used the contents of the vessel may be continuously roused using a pumped recirculation system. At the appropriate time, the rousing is stopped and the yeast settles onto the deck from where it may be collected by pushing it into the yeast drain. After the yeast has been skimmed the beer is removed via the bottom drain.
Yorkshire squares, in common with other traditional fermenters, are inefficient since they require considerable manual input. Beer losses are high due to the method of yeast cropping. However, regardless of these shortcomings they continue to be used since it is claimed that the beer is superior to the same product produced in more modern closed vessels. In one Yorkshire brewery, new squares have recently been fitted (Griffin, 1996). These were chosen in preference to the ubiquitous cylindro-conical vessels. The fermentation uses a mixture of two yeast strains and it is claimed that both are necessary to achieve beer with the desired qualities. The strains have differing flocculation characters and it was found that when used in cylindroconical fermenters this resulted in loss of one yeast type.
The new Yorkshire squares have a capacity of 880 hi and have been redesigned to improve efficiency and vessel hygiene. They are enclosed so that collection of carbon dioxide is possible. Automatic CiP systems are fitted and attemperation is automatic via glycol-containing wall cooling jackets. The yeast skimming procedure is automated with each vessel having two suction points set into the deck. These are connected to vacuum yeast collection tanks. After the bulk of the yeast has been removed, the remainder is pushed automatically towards the suction points by rows of water jets located above the deck. Flow through each row of jet heads is controlled sequentially as the yeast advances towards the exit drain. A similar arrangement in the lower compartment controls the removal of tank bottoms.
The Burton Union system of fermentation derives from central England where it was commonly used for the production of ales that required a non-flocculent, or 'powdery' yeast strain. It includes elements of both the dropping system and the Yorkshire square. Thus, the fermentation is started in a conventional open or closed square fermenter. After 24-36 hours, when the fermentation is proceeding vigorously, the contents of the square are transferred into the Burton union fermenter or 'set'.
The essential features of a Burton union set are shown in Fig. 5.7(a), (b) and (c). The actively fermenting wort is transferred to a number of pairs of attemperated wooden casks, termed unions. The casks are suspended below a central 'top trough' to which they are linked via stainless steel tubes known as swan necks. As the fermentation proceeds, yeast and entrained liquid moves upwards from the casks into the top trough. This is cooled using an internal attemperator and this causes the yeast to sediment. The top trough is slightly inclined and this allows beer to run into the feeder trough from whence it returns to the casks via the side rods. When the fermentation is complete most of the yeast is in the top trough and from here it is collected via a drain in the feeder trough. Beneath each row of casks is the bottom trough. This is used to collect the beer from the casks via bottom drain cocks and direct it towards the racking tank. The height of the drain cocks is adjustable so that there is some control over the quantity of yeast transferred with the beer.
The capacity of each union cask is approximately 150 imperial gallons (c. 1 hi). The number of casks varies from 24, up to between 50 and 60. Thus, a large set with say 56 casks would have a total capacity in the region of 326 hi. When this system was in common usage in the United Kingdom, a brewery such as Bass, at Burton-upon-Trent, had a hundred sets of this capacity in regular use (see Fig. 5.8). Now, only one brewery, Marstons, also of Burton-upon-Trent, continues to use the union system.
It has many disadvantages, in fact so many it is perhaps difficult to see how it ever arose, let alone survived! For instance, it requires both a conventional fermenter as the well as the union set and this is expensive in terms of plant utilisation. The union set is complex and difficult to clean. Much of the cleaning and routine operation needs a high degree of manual input. The large number of casks and the open trough arrangement presents a significant microbiological risk. The casks require the skills of a cooper to fabricate and maintain. The efficiency of the system is very poor since beer losses are high due to the multiplicity of vessels.
On the other hand, the beer produced from Burton union sets is of the highest quality, as is the yeast collected from the top trough. In the past, efforts were made to simplify the design of the system and improve the hygiene but retain the essential features. For example, replacement of the oak casks with a single long shallow stainless steel tank, fitted with CiP facilities. Such plant was installed at the Bass Brewery in Burton-upon-Trent but it was not considered a success. There were undesirable changes in both beer flavour and the quality of the yeast crop (Steve
Price, personal communication). The now sole user of the Burton union system, Marstons, has recently invested in new plant (Harvey, 1992). The design was totally traditional and the investment was justified since it was claimed that when using conventional fermenters it was not possible to match the existing beer flavour or collect sufficient good quality yeast for re-pitching purposes.
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