4 x 400 hi vessels

1 x 1600 hi vessel

Volume output

1600 hi (4 x 400 hi batches).

1600 hi (1 x 1600 hi batch).


Good - system has ability to ferment four simultaneous product lines.

Poor - only one product line can be fermented at a time.

Vessel space utilisation



Capital costs

Cheaper individual vessels but four required.

More complex pipe-work, valving etc.

Costs of monitoring/control equipment for four vessels

More expensive vessel but only one required. Simpler pipework but may need heavier duty pumps. May need higher capacity brewhouse.

Revenue costs

Large number of unit operations -filling, cropping, racking, CiP.

Coolant jacket surface area of four small vessels approximately 1.6 x jacket surface area of one large vessel.

Fewer unit operations for same volume throughput but longer time required for each process step.

Greater individual coolant load for larger diameter vessel.

In modern breweries, the trend towards larger batch sizes has provided the impetus for the almost universal adoption of cylindroconical fermenters. Typically, vessels have a capacity in the range 1500^000 hi although vessels of 12000 hi are in use. Probably this is close to, or greater than, the practical maximum for all but the largest breweries. Very large vessels are frequently used for both fermentation and conditioning ('dual-purpose vessels' - see Section 5.4.3). To accommodate these dual functions, modifications are required, particularly to the provision for cooling. The influence of capacity on fermenter performance is intimately related to vessel aspect ratio and geometry. These topics are discussed together in Section 5.1.4.

Very-large-capacity fermenters are employed where high-volume throughputs are required. An alternative approach, particularly where a single beer quality is produced, is to consider the use of continuous fermentation (Masschelein, 1994). Continuous beer fermentation systems are described in Sections 5.6 and 5.7.

5.1.4 Fermenter geometry

The geometry of the fermenting vessel must facilitate the operations that have to be performed within it, be suitable for the fermentation characteristics of the chosen yeast strain and be appropriate for the quality of beer produced.

The method of addition of wort should be arranged such that the risk of contamination is minimised and foam generation is not favoured. Conversely, filling of vessels should produce sufficient turbulence to ensure that the yeast is evenly distributed throughout the wort. Brewery fermentation vessels are not usually provided with mechanical stirrers and carbon dioxide bubble formation provides a natural method of agitation. Vessel geometry should be such that gas evolution maintains homogeneous conditions during active fermentation so as to ensure consistent yeast count throughout the body of the wort and to assist with attemperation. However, uncontrolled foam generation should be avoided, which would result in loss of product. Similarly, gas evolution rates should be controlled to minimise stripping of volatile flavour components and evaporation of ethanol. On economic, environmental and safety grounds, it is desirable to provide a means for collecting the evolved carbon dioxide.

At the end of fermentation and possibly during its course, it is necessary to separate yeast from the green beer. The geometry of the vessel should facilitate this operation and be appropriate to the type of yeast that is being used. Vessel emptying should be rapid but not cause turbulent flow and the vessel should drain completely to minimise losses. Before the vessel can be re-used it must be cleaned and disinfected as discussed in Section 5.1.2. These processes should be facilitated by the design of the vessel.

Vessels may be open or closed. Originally, the majority of shallow types were of open construction. However, modern variants tend to be of the closed construction. Deep vessels are invariably closed. The requirement for improved hygiene provided the impetus for the introduction of closed fermenters. This has on occasion produced unexpected results. For example, it has been reported that closed fermenters have increased susceptibility to infection by lactic acid bacteria, possibly a result of the very low oxygen tensions associated with these vessels (Ulenberg et al., 1972; Hoggan, 1978). Cleaning and sterilisation of closed vessels is facilitated by their design; however, anti-vacuum devices must be fitted to eliminate the possibility of collapse. Shuttlewood (1984) identifies a number of circumstances under which a partial vacuum may arise. These are:

(1) emptying an unvented vessel under gravity;

(2) cooling down after hot cleaning or sterilisation;

(3) chemical reactions occurring in the head space (particularly conversion of gaseous carbon dioxide to sodium carbonate when using sodium hydroxide as a cleaning agent); and

(4) siphoning of vessel contents from a top-mounted down pipe.

Other advantages accrue with closed vessels; for example, collection of carbon dioxide is possible. Vessels of closed design may be pressurised. This can be used to regulate yeast growth and thereby provide a practical measure for controlling the development of yeast-derived organoleptic volatile components of beer (see Section

There are two general geometries used for fermentation vessels. Low-aspect-ratio types, as exemplified by open and closed squares or horizontal cylindrical types, and the high-aspect-ratio variety, as in the cylindroconical or related vessel. Historically, shallow vessels pre-date high-aspect-ratio designs. In the authoritative text of de Clerck, first published in 1948 and then in translation in the UK in 1954, it was recommended that fermenters should be at least 1 metre in depth but not greater than 2 metres. This author advised that settling of yeast and solids would be unacceptably slow and cleaning too difficult in vessels of greater depth.

The move towards deeper and enclosed vessels was pioneered by Nathan (1930a, b) by his introduction at the end of the nineteenth century of the aluminium cylindroconical design. Advances in hygienic design and ease of construction fostered by the use of construction materials such as mild steel and latterly stainless steels promoted the use of larger vessels. Nevertheless progress was slow and the widespread use of large fermenters did not begin until the late 1950s. Initially there was much resistance to changing vessel geometry because of the fear of adversely affecting beer quality. Consequently, capacities were increased simply by building larger vessels to a traditional design but using modern construction materials. For example, the St James Gate Brewery of Guinness in Dublin, Ireland, installed, in 1957, enlarged stainless steel rectangular vessels with capacities of up to 5200 hi and depths approaching 7 m.

During the following three decades, a body of experience was gained which demonstrated that deep vessels of novel design were not a bar to successful brewing. The benefits of economies of scale overrode the misgivings of the conservative and many breweries installed tall high-volume vessels. For example, the Whitbread Brewery in the United Kingdom installed cylindroconical fermenters with capacities of 1200 hi and wort depths of 6 m. Using these vessels, ales were produced that were considered analytical and organoleptic matches for similar beer fermented in traditional shallow open vessels (Shardlow & Thompson, 1971; Shardlow, 1972). Mackie (1985) reported similar findings when cylindroconical vessels where used for the production of British cask-conditioned ales. Ulenberg et al. (1972) described the construction and application for lager production at the Heineken Brewery at Zouterwoude in the Netherlands of a giant cylindroconical vessel of 4800 hi capacity and overall height of 25 m. Many other tall vessel designs were introduced during the 1960s and '70s. These include dished-ended cylinders, cylinders with cones of varying included angles, cylinders with sloping bases and even spheres with conical bases. In many cases, these new vessels were designed for the dual purposes of fermentation and conditioning. The construction and operation of these is described later in this chapter.

Although tall deep vessels are now the norm it is still of benefit to compare their performance with shallower types. Low-aspect-ratio vessels are generally associated with the use of top-cropping yeast strains and high-aspect-ratio vessels for use with bottom-cropping yeast strains. The physiological basis of top- and bottom-fermenting yeast strains is discussed elsewhere (Sections 4.2.2 and The aspect ratio of the fermenter, coupled with the method of yeast cropping, has several ramifications. In top-fermenting systems, there is an opportunity to purify the yeast used in subsequent fermentations. The first yeast head, which contains much entrained wort solids, is discarded. The second yeast head, which forms as the result of vigorous growth during active fermentation, is cropped and retained for subsequent re-pitching. Other non-yeast solids and mainly dead yeast cells sediment. Thus, top-cropped yeast tends to be very clean and of high viability. Many traditional breweries have used the same culture of top-cropped yeast repeatedly pitched and cropped for many tens of years with no apparent decline in performance. However, there are several disadvantages. The method of cropping may be labour intensive or require elaborate and difficult-to-clean plant. Hop utilisation is poor in top fermenters since the bittering components, iso-homulones, selectively bind to yeast cell walls (Section 4.4.2) and hydrophobic species present in foams and these are selectively removed during cropping. Overall efficiencies in terms of product yields are low because of beer losses associated with the cropping regime. In addition, compared to bottom-cropping systems, there is more opportunity for the yeast to be exposed to air. Apart from the increased risk of microbiological contamination, this also can result in reduced fermentation efficiency. Thus, excessive yeast growth in subsequent fermentations may be promoted due to sterol synthesis in response to oxygen exposure during cropping from the previous fermentation.

Removal of yeast crops from bottom-fermenting systems such as cylindroconicals is efficient, can be automated and beer losses are minimal. Since the vessel is closed there is little possibility of exposure to atmospheric oxygen, and therefore maintaining high growth efficiencies is facilitated. On the other hand, unless very bright worts are used, the crop will contain relatively high levels of entrained non-yeast solids and there may be further enrichment with each subsequent repitching. In top-cropping systems, the yeast is removed relatively early in the fermentation when it is in good condition. With bottom-cropping vessels the yeast is not usually removed until the fermentation is completed. Consequently, the yeast may remain in the cone of a deep vessel for quite long periods of time, subjected to the physiological stresses of high hydrostatic pressure, temperature hotspots and high concentrations of carbon dioxide and ethanol. It is possible to crop at intervals during the fermentation (see Section 6.7.2); however, this introduces the possibility of selecting for yeast variants with increased flocculence characteristics. In addition, if multiple cropping is practised, the advantages of ease of yeast handling associated with closed vessels are obviously diminished. The combination of solids enrichment and deterioration due to additive stresses or selection of non-standard variants mitigates against continued cropping and repitching. For this reason where deep enclosed vessels are employed it is usual to limit the number of serial fermentations and periodically introduce newly propagated yeast (see Section 7.2).

The flocculence characteristics of the yeast are important with regard to the aspect ratio of the vessel. Very flocculent yeast, if no remedial action is taken, can settle out before the fermentation is complete with the result that the desired degree of wort attemperation is not achieved. Some traditional shallow vessels were designed specifically to be used with highly flocculent yeast strains, for example, Yorkshire stone squares and the Burton Union system. With these systems the design and mode of operation ensures that yeast remains in contact with the fermenting wort (see Section 5.3). Where flocculent yeast strains are used in other vessel types it may be necessary to provide some means of agitation other than carbon dioxide evolution, to prevent premature sedimentation. It is easier to keep yeast in suspension in shallow vessels of comparatively small volume, compared to their larger, high-aspect-ratio counterparts. In the latter case, it may be necessary to provide a mechanical agitator or to pump the fermenting wort from the base of the vessel back up to the top using a recirculation loop. Obviously, this represents additional capital and revenue costs. Alternatively, when fermentations are scaled up and there is a change from shallow to deep vessels it may be necessary to select a less flocculent yeast strain.

The mixing characteristics of shallow and deep vessels are significantly different. Contrary to expectation, agitation produced by carbon dioxide evolution is much more vigorous in tall thin vessels compared to shallow types. Bishop (1938) reported a detailed study of his observations on carbon dioxide evolution in open square fermenters. He noted that gas bubbles form only at the bottom of fermenters. Sedi-mented solid particulates but not yeast cells acted as nucleating sites. As the result of a tour de force of meticulous observation, Bishop concluded that gas bubbles arose only from particles which had rough or creviced surfaces. Presumably, this explained why the relatively smooth yeast cells did not act as nucleation sites. Once formed, the stream of bubbles rose through the fermenting wort carrying yeast cells within the bubble film. At the surface of the wort the small bubbles collapsed and coalesced into a loose mesh of larger bubbles with entrapped yeast cells held within.

In deep vessels using bottom fermenting yeast, gas bubbles have also been reported to form on sedimented particulates but in this case no yeast cells were observed within the bubble film. Instead yeast cells were simply re-suspended in the wake of rising bubbles (Delente et al., 1968; Ladenburg, 1968). These authors confirmed the earlier observations of Bishop (1938) that no gas bubbles were formed directly from yeast cells. They further noted that the gas bubbles increased in diameter as they rose through the wort and that all bubbles were roughly the same size at any given height within the fermenter. The following interpretation was advanced. Gas bubbles break away from the nucleation site and rise due to buoyancy and in so doing re-suspend yeast cells. Carbon dioxide is evolved at the same rate throughout the entire body of fermenting wort. However, within the body of the vessel the evolving carbon dioxide diffuses directly into the ascending bubbles thereby causing them to grow, an effect further magnified by the decreasing hydrostatic pressure. In growing the bubbles become more buoyant, and their rate of ascent and power to drag both increase. The power of agitation due to the gas bubbles varies in proportion to the rate of fermentation and the logarithm of the height of the fermenter (Fig. 5.1). As may be seen,

Fig. 5.1 Relationship between power output due to carbon dioxide evolution and fermenter height (adapted from Delente et al., 1968).

since the power output progressively flattens out, the advantages of good mixing in large-aspect-ratio vessels are eventually lost. The superior mixing characteristics of tall cylindrical vessels are further enhanced by the provision of wall cooling jackets and a conical base. With this configuration, the yeast cells are dragged up through the central core of the vessel with the ascending gas bubbles. At the top, the liquid flow is directed towards the walls. Here the cooling effect increases the density of the fermenting wort and the yeast cells are carried back down towards the base of the vessel there to be returned into the path of the central rising gas stream. Eventually when the rate of gas evolution slows due to depletion of fermentable sugars, the yeast sediments in the base of the cone, thereby facilitating cropping. The patterns of agitation are illustrated in Fig. 5.2. Further discussion of patterns of agitation as a function of vessel geometry may be found in Section 5.4.2.

The use of high-aspect-ratio vessels has been linked with perturbations in beer flavour. Vrieling (1978) reported that the concentrations of the important flavour compounds, ethyl acetate and iso-amyl acetate, formed during fermentation were inversely related to vessel height. Other authors have observed that changing fermenting vessels from low- to high-aspect-ratio types resulted in faster fermentation rates, greater attenuation of worts, increased concentrations of higher alcohols, increased bitterness, increased yeast growth, reduced beer nitrogen, increased dissolved carbon dioxide and lower beer pH (Shardlow, 1972; Ulenberg et al., 1972; Hoggan, 1978; Masschelein, 1986a, 1989).

The alteration in dissolved carbon dioxide is obviously a consequence of the increased hydrostatic head. The improved hop utilisation is a result of the reduced loss rate of bittering compounds associated with top cropping of yeast. Masschelein (1994) identified the high agitation rate as being the principal cause of the other changes. High rates of carbon dioxide evolution promote loss of'volatiles' due to gas stripping. More significantly, good mixing increases the suspended yeast count and this results in high fermentation rates and enhanced yeast growth. Increases in biomass yields are associated with greater utilisation of wort nitrogen, elevated levels of

Solubility Factor Agitation

higher alcohols and reduced pH. In addition, a greater proportion of the intracellular pool of cytosolic acetyl-CoA is used to fuel growth at the expense of ester formation. Apart from increased agitation, tall vessels can influence yeast growth because the hydrostatic head increases oxygen solubility. The dissolved oxygen concentration provided in wort at the start of fermentation is the primary mechanism by which the extent of subsequent yeast growth is controlled.

Other factors may be implicated in vessel-size-associated differences in fermentation performance. Large vessels can take a long time to fill and this may require several batches of wort. It is common practice to pitch all the yeast with the first portion of wort. If the filling stage is very prolonged there is an opportunity for the yeast to start budding before wort addition is completed. Should oxygen be supplied to all the wort, a higher biomass yield per unit volume will be achieved in a large vessel as compared to that obtained with a smaller vessel capable of being filled with a single batch of wort. Other flavour anomalies may arise where wort collection is prolonged. Masschelein (1981) observed that vicinal diketone concentrations were three-times higher where a 1200 hi fermenter was filled over a 24 hour period, compared to a similar vessel filled in 4 hours.

In large vessels, during active fermentation the circulating yeast is subject to a continually changing hydrostatic pressure. When the turbulent 'active phase' of fermentation is complete, the yeast sediments into the base. This yeast will be exposed to a combination of high hydrostatic pressure, elevated carbon dioxide and possibly increased temperature 'hot spots'. The latter is due to the difficulty of applying cooling to packed yeast which is still generating heat via exothermic metabolism. In addition, the beer surrounding the packed yeast may have a much higher ethanol concentration than that in the upper parts of the fermenter. This is caused by a localised build up of ethanol produced via fermentation of residual sugar and by dissimilation of yeast glycogen reserves (Quain & Tubb 1982). Masschelein and van der Meersche (1976) reported that there may be a degree of stratification in large vessels. In consequence the beer in the bottom may have a different composition to that in the top, due to the presence of yeast excretion products, including amino acids, peptides, nucleotides and organic phosphates. Prolonged residence times lead to the formation of off- flavours and these have been associated with the release of various short-chain fatty acids from the yeast cells (van de Meersche et al., 1979).

Apart from matching vessel size to the needs of the brewery, the optimum capacity and aspect ratio of fermenting vessels is largely governed by the costs of construction and operation. Within certain limits, larger vessels are the most economical since a doubling in volume attracts a cost increase of just 50-60%. Lindsay and Larson (1975) identified three primary contributors to vessel costs: size, geometry and operating pressure. They calculated that for all vessels the logarithm of relative cost was directly proportional to the logarithm of the volume. Vessels of all configurations followed this rule producing a family of parallel lines each with a slope of approximately 0.65. For any vessel type of the same geometry there is a correlation between volume and surface area (surface area oc volume • 0.67). Therefore, the relationship can be further simplified to cost being proportional to surface area raised to the power of 0.97. In other words, there is an almost direct correlation between these two parameters. It follows that in terms of materials costs the most effective geometry for fermenting vessels are those which enclose the maximum volume within the minimum surface area.

The optimum geometry is therefore a sphere and the least effective is one of square or rectangular cross-section. Cylindrical vessels with low aspect ratios are more economical than tall thin varieties. Spherical vessels with conical bases were introduced in the 1970s (Martin et al., 1975). The fermenters were constructed from stainless steel, had a diameter of 10 metres and a capacity of 5000 hi. The geometry favours the easy application of cooling because of the high surface area relative to the volume, and pressurisation is favoured. Costs were reported as being 12% less than all other vessel geometries. However, Lindsay and Larson (1975) considered that spherical vessels were difficult to construct and they were associated with high material wastage.

In practice spherical vessels have found little favour and there has been a trend towards the almost exclusive use of tall cylindroconical vessels, irrespective of the type of beer being produced or the strain of yeast employed. The fears that beer quality would suffer as a result of the change to tall vessels appear to be unfounded. It is interesting to note that when cylindroconical vessels were introduced to the Whit-bread brewery (Shardlow, 1972) it was felt necessary to select a new bottom-cropping yeast strain for production of ales. In later trials a top-cropping variety, hitherto used in shallow fermenters, was tested in a deep vessel. It was noted that in this case the fermentation was normal, as was the beer, but the yeast formed a satisfactory bottom crop.

The effects on fermentation performance and beer quality of higher agitation rates associated with tall vessels were, in fact, recognised by Nathan (1930a, b) and he recommended that this should be counteracted by reducing the fermentation temperature. There are other strategies, apart from temperature, which may be considered when attempting to match performance in large-volume deep vessels with that of shallow types. Thus, wort gravity, dissolved oxygen tension and yeast pitching rate can all be modulated independently to obtain a desired outcome. It is usual to have to reduce hop addition rates to correct for the lower loss rate and clearly this is advantageous. Conversely, because of the hydrostatic head, deep vessels inevitably produce beers with high dissolved carbon dioxide concentrations. This may be acceptable with lagers but many beer qualities will require adjustment down-stream of the fermenter.

Although vessels with high aspect ratios are used with success to produce many beer qualities, a ratio of height to diameter of 3:1 is most commonly encountered. In terms of volume, 1640 hi is the most usual, giving a vessel with dimensions of approximately 4 metres in diameter and 12 metres in height (Shuttlewood, 1984). This represents the optimum compromise between economy of fermenter scale and batch production requirements. Larger vessels tend to be dual-purpose types used for both fermentation and conditioning. Regardless of the pros and cons of traditional versus deep fermenters, the industry has made a decision, and in the vast majority of commercial breweries beer is fermented in large-volume vessels. Thus, it is reported (Derdelinckx & Neven, 1996) that only 5% of total beer production is now produced by top fermentation. This is the almost exclusive preserve of Europe. The distribution is illustrated in Fig. 5.3.

H U.K. I I Belgium I I Germany XXS1 France + Netherlands

Fig. 5.3 Percentage share of European production of beer by top fermentation (Derdelinckx & Neven. 1996).

5.1.5 Monitoring and control

The design of fermentation vessels must facilitate control of the processes occurring within them. Provision must be made for monitoring the progress of the fermentation, in order that the completion of key stages may be recognised and, if necessary, to identify deviation from normal behaviour and thereby take remedial action. As batch sizes have grown ever bigger, the need to improve the consistency of fermentation performance has provided the impetus for devising improved means for monitoring and control. Whereas the 1960s and '70s were characterised by the widespread adoption of deep closed-fermenting vessels, the intervening period has focused on the development of new and often automatic systems for controlling and monitoring fermentation.

There are three elements to controlling and monitoring brewery fermentations:

(1) establishment of a desired set of initial conditions;

(2) monitoring the progress of the fermentation and applying control to maintain an appropriate rate;

(3) identification of the completion of key stages during fermentation and a point at which the contents of the vessel are deemed to be in a state suitable for transfer to the next stage of processing.

With respect to the initial conditions, the pertinent parameters are wort volume, present gravity, dissolved oxygen concentration, yeast pitching rate and temperature. All of these, with the possible exception of wort volume and present gravity, are established upstream of the fermenter and therefore do not impact directly on fermenter design. However, it may be necessary to make some adjustments to these parameters in the fermenter and appropriate provision must be made to facilitate this. For example, in the event of an unacceptably slow fermentation a possible remedial action would be to add additional oxygen. Vessels should be provided with a means of achieving this aim, either via direct injection of gas into the base of vessels or via an external circulation loop.

In many cases, there may be a requirement to make additions during the course of the fermentation, for example, process aids such as enzymes, fining agents or additional yeast and wort. Vessels must be designed to cater for these needs. There should be a means of measuring the volume of wort within the vessel, either via metered addition or the use of sight-glasses or dipsticks. In some countries, this may be a legal requirement for the purpose of excise duty payments. It may be a practical requirement if the wort collection gravity requires adjustment by dilution with brewing liquor.

During fermentation, it is necessary to monitor progress and usually this is achieved by periodic measurement of the specific gravity of the wort. The achievement of a desired minimum specific gravity marks the end of primary fermentation and signals the commencement of the next stage in the process. In the vast majority of cases, the specific gravity is measured off-line on samples removed from the vessel. These samples may also be used to measure the suspended yeast count and the concentration of flavour metabolites such as vicinal diketones which are also relevant to the progress of some fermentations (Section

In open fermenters, samples of fermenting wort may be obtained with a dropping can or similar implement. Top-opening manway doors in deep vessels allow access to perform the same operation; however, it is more satisfactory to provide a hygienic, cleanable sample point. Where sample cocks are used they must be located in a position which will provide a sample which is representative of the whole of the contents of the vessel. As an alternative to manual sampling and off-line measurement, several commercial systems have been developed which can be fitted into vessels and which provide an automatic measure of specific gravity or other parameters. These are described in Section Apart from providing a continuous measure of the fermentation rate, such systems have the advantage that they provide an output which can be used in automatic feedback control loops.

During fermentation, the primary and usually sole method of controlling rate is via the application of cooling to counterbalance the heat produced by the exo thermic metabolism of the growing yeast. Fermenting vessels must be designed to facilitate the measurement and control of temperature. In unsophisticated traditional vessels, temperature measurement, like gravity, may be performed off-line. Conversely, modern vessels are provided with wall-mounted thermometers. These are usually of a type which provide an output that can be used to provide automatic attemperation. In deep vessels several temperature probes are required and these must be positioned carefully to ensure that readings are representative of the whole vessel.

It is not usual to provide 'external' means of increasing temperature in fermenting vessels, although it is not unknown. Thus, the cylindroconicals described by Shardlow (1972) had facilities for the application of both cooling and heating. However, the latter was found by experience to be superfluous and not used in subsequent installations. Normally the yeast and wort are added at a temperature slightly below that at which the primary fermentation is conducted. When yeast activity commences, the temperature is allowed to rise until a desired set-point is reached and control is then applied.

The degree of cooling which is capable of being generated must be appropriate for the fermentation system and type of beer being produced. Cooling loads may be comparatively slight, for example, in the case of top-cropping ale fermentations carried out in small open squares. These are performed at relatively high temperatures (18 to 22°C) and at the end are subjected to modest chilling (8 to 10°C). Fermentations such as these typically use internal attemperators through which cold water is circulated. Conversely, lager fermentations performed in large deep vessels at lower temperatures (6 to 15°C) and culminating in rapid cooling to between 2 and 4°C to sediment yeast have much greater cooling duties. In these cases, external cooling jackets are the norm and refrigerants such as glycol are required. Dual-purpose vessels require the greatest cooling capacities since in addition to attemperation during primary fermentation they must also cater for cold conditioning at sub-zero temperatures. In the case of these vessels there are four distinct cooling duties: attemperation during primary fermentation, crash cooling at the end of primary fermentation to promote yeast sedimentation, cooling to the conditioning temperature and attemperation at the conditioning temperature.

Occasionally, cooling is achieved by circulating the contents of the vessel through an external heat exchanger. This has some drawbacks since it introduces additional complex and difficult-to-clean components. However, it can be a useful in improving mixing, where a particularly flocculent yeast strain is employed. Nonetheless, wall cooling is an expensive undertaking where large numbers of vessels are employed. An alternative approach is to control the temperature of the fermenting room and use unlagged vessels. Historically, this was standard practice, hence the siting of vessels in cellars or caves, particularly those used for the low-temperature secondary fermentation stage of lager production. Latterly this approach is used as a means of augmenting the cooling applied directly to the vessels.

An interesting case history was reported by de Witt and Hewlett (1974) from a brewery in New Zealand. The authors described a situation in which the brewery required additional capacity for both fermentation and cold conditioning. For the sake of economy of scale, it was decided that large vessels were required and to maximise flexibility they should be dual purpose, suitable for both fermentation and cold conditioning. Three options were considered: outdoor weatherproof insulated tanks fitted with cooling jackets; jacketed, insulated tanks contained in a light construction building; unjacketed and uninsulated tanks contained within an insulated and refrigerated building. It was concluded that the third option was the most economic and this plan was implemented with success. However, this approach has not seen widespread adoption.

In addition to controlling fermentation rate, attemperation is also required to minimise the stresses imposed on the yeast crop. In top-cropping systems the yeast is removed comparatively early in the fermentation and is transferred to the relative security of chilled storage. In the case of deep fermenters, the total residence time of the yeast may be prolonged, and, furthermore, at the end the bulk of the yeast will be sedimented in the base of the vessel. Here local heat generation can occur owing to the combination of exothermy of the packed yeast and lack of convection currents. To avoid deterioration of the crop and improve attemperation, large cylindroconicals are frequently fitted with cone cooling jackets.

The rate of fermentation also can be controlled by the application of pressure. This is generated by restricting the dissipation of evolved C02. Elevated pressure also has been used to modulate levels of beer esters (see Section Obviously this approach is only feasible in closed vessels and, apart from the need to measure and control the pressure, they must be designed to meet appropriate engineering standards. The mandatory requirements relating to pressure vessels cover materials, their nature and thickness, vessel design and method of construction. The regulations apply to the vessels themselves and to associated fittings such as cooling jackets. Vessels classified as pressure types are liable to inspection either by officers of the Government or representatives of the insurer. The most commonly used codes are those applicable in the United States and Canada (American Society of Mechanical Engineers, Boiler and Pressure Vessel Code, ASME Section VIII, Division 1) and their equivalents in the United Kingdom (BS 5500) and Germany (A.D. Merkblatter). The British Standards Institution publication Boilers and Pressure Vessels (1975) details the legislation relevant to 76 political jurisdictions.

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