General properties of fermentation vessels

Several fermentation systems are used in brewing. These reflect the type of beer which is being made, the traditions of the country of production and possibly the volume throughput and modernity of the brewery. Within these systems many different types of fermenting vessel may be used. These differ in terms of capacity, materials of construction, geometry and mode of operation.

The plethora of types of fermenting vessel mirrors the diversity of brewing operations that are encountered throughout the world. Thus, fermenter design must cater for the requirements of the small unsophisticated cottage industry, brewing beer for domestic consumption, through to the very large ultramodern brewing factory producing brands for the international market place. In between these two extremes is the traditional brewery using vessels with a design pedigree that is centuries old. Other breweries use vessels made to a similar traditional design but constructed from new materials, which are better suited to the modern industry. In addition, the efficiency or ease of use of traditional vessels may be improved by the introduction of new methods for monitoring and controlling the processes occurring within them.

A recent phenomenon is the micro brewery, producing small volumes of often specialist beer types for one or a few retail outlets. The fermenting vessels used are normally of traditional design, but the relatively small volumes involved allow the use of materials such as plastics that have hitherto not been utilised for this purpose in the brewing industry. A further rather niche area requiring fermentation vessels of a special design is that of research and development. Academic and commercial institutions involved in brewing research use laboratory and pilot scale fermentation facilities, which are designed to act as model versions of their production-scale counterparts. Several different small-scale fermentation systems are in common use.

Vessel design, the method of operation, the variety of yeast used and the type of beer produced are all intimately interrelated. In the case of many traditional fermentation vessels, these relationships have been derived empirically using the technology, scientific knowledge and engineering expertise available at some time in the past. Brewing is an innately conservative industry and naturally there is little will to change any part of the process that could result in a reduction in product quality. This applies particularly to fermentation since this stage of brewing is critical to the development of beer flavour. Consequently, there can be resistance to replacing vessels of traditional design with modern substitutes, designed purely on functional grounds to take full advantage of current knowledge. Thus, traditional vessels continue to be used and replaced, some shortcomings being accepted as the price to be paid to maintain a perceived product quality.

On the other hand, the conservatism of parts of the brewing industry has always coexisted with a more radical element, which has sought improvement by innovation. For example, the use of aluminium cylindroconical fermenting vessels together with a method for producing clear worts was first applied to lager brewing at the end of the nineteenth century (Nathan, 1930a, b). This author demonstrated that his fermentation system allowed the production of lager in approximately 12 days compared to the then conventional open fermenter and cellaring system, which took several months to complete. However, whether or not the beers produced by the rapid process were a precise organoleptic match for their slower brewed counterparts was the subject of some controversy. This debate continues; thus, German wheat beers are traditionally produced in large open top-cropping fermenters at relatively high temperature (25 to 30°C). In many breweries this practice continues, whereas in others closed, bottom-cropping cylindroconical fermenters are employed. It has been claimed that the change in vessel type has resulted in wheat beer which has a blander character compared to the traditionally produced product (Hook, 1994).

In many countries in recent years, there has been a trend towards fewer and larger breweries. To meet commercial needs these breweries have tended to use ever-larger individual fermentation batch sizes. These very large fermentations represent a considerable business risk should the resultant beer be in any way atypical. In almost all cases, these very large vessels are of a closed cylindroconical or a related type because of their superior hygiene, ease of monitoring, ability to control and excellent utilisation of available ground area. The trend towards the increasing use of cylindroconical vessels seems set to continue (Maule, 1986).

It may be readily appreciated from the preceding discussion that there is no single ideal fermenting vessel, merely one that is fit for the purpose to which it is put. However, there are some common process and performance criteria. These may be considered in the following categories:

(1) Materials of construction

(2) Hygiene

(3) Capacity

(4) Vessel geometry

(5) Facilities for monitoring and control. 5.1.1 Materials used for vessel construction

The materials from which fermentation vessels are fabricated must have sufficient mechanical strength and rigidity to withstand the forces exerted upon them by the volume of fermenting wort. Conversely, they may require to be sufficiently light or malleable, or have other properties which make them easily formed into the required shape and facilitate installation into the brewery. The internal surfaces of the vessel must be capable of being cleaned with ease and be durable enough to withstand sterilisation regimes and the rigours of long-term use. In addition, the internal surfaces should be smooth so as not to present potential sites for microbial colonisation. The material in contact with the fermenting liquid must be inert; first, to ensure that no beer or wort components are adsorbed onto its surface, and, second, such that no potentially toxic and/or flavour tainting materials are leached out into the product.

Fermentation is an exothermic process and it is necessary to apply cooling to maintain a desired temperature. In addition to the need for providing a method of attemperation, the materials used for fermenter construction must have appropriate properties of thermal conductivity. Thus, where attemperation is achieved via external cooling jackets the inner surfaces of the fermenter should be made from materials with high thermal conductivity to ensure rapid and efficient heat transfer from wort to coolant. The exterior walls of the vessel, which enclose the cooling jackets, should then be provided with low-thermal-conductivity lagging material to prevent heat pick-up from the environment. Conversely, where attemperation is achieved via devices submerged in the fermenting liquid, the walls of the vessel should be largely constructed from materials with low thermal conductivity to minimise heat pick-up from the fermenting room.

Many materials have been used for the construction of fermenting vessels and these are summarised in Table 5.1. Frequently the chosen material had much to do with what could be obtained in a particular locality, the availability of a large work force skilled in the use of such materials and, by inference, cost. The tendency to use locally

Table 5.1 Materials used for the construction of fermenting vessels.

Material Comments

Natural materials

Hollowed gourds, animal skins Stone, slate

Wood

Fabricated materials

Plastics

Ceramics Glass

Concrete Aluminium

Iron, steel Copper Stainless steel

Used for domestic brewing, particularly 'native' beers.

Durable, non-porous, inert, low thermal conductivity, very heavy, only suitable for comparatively small square or rectangular vessels

Durable, relatively light, easily worked and shaped, must be well-seasoned, some timbers require inert lining, low thermal conductivity, now expensive, needs manual cleaning.

Light, can be formed into any shape, inexpensive, easily deformed, may not be resistant to heat and chemicals, low thermal conductivity, must be inert, not easily cleaned.

Inert and non-porous (particularly if glazed), easily worked, inexpensive, low thermal conductivity, fragile and only suitable for small scale domestic brewing.

Light, can be formed into any shape, inert, non-porous, relatively inexpensive, easily cleaned, medium thermal conductivity, very fragile and only suitable for laboratory or domestic vessels. Used in form of vitreous lining with some production scale vessels.

Durable, strong, relatively inexpensive, can be formed into any shape, relatively heavy but can be used for large vessels, low thermal conductivity, requires inert lining material.

Light, durable, relatively inexpensive, easily worked, can be used as light lining material or as main fabric of vessels, inert, easily cleaned, some alloys susceptible to attack by alkalis, high thermal conductivity, subject to electrochemical corrosion.

Strong, relatively inexpensive, easily worked, durable, high thermal conductivity, subject to rusting so must have inert lining fitted.

Strong, easily worked, durable, very expensive, easily cleaned, very high thermal conductivity.

Strong, very durable, expensive, difficult to machine, excellent cleaning properties, austenitic types very corrosion resistant, can be formed into any shape, high thermal conductivity.

available materials inevitably resulted in vessels which were peculiar to a particular geographical region. In more recent times, and in particular with vessels of non-traditional design, it has been more usual to choose construction materials purely on the basis of cost and functionality.

Historically, fermenting vessels were most commonly constructed from wood, frequently using a circular design and employing the same skills required to cooper casks. In many ways wood is an ideal material for the task. It is, or perhaps was, universally available at reasonable cost, it is durable and strong yet easy to work with. It is not toxic to yeast, it does not impart flavour to the beer and it permits low rates of heat transfer. It does suffer the disadvantage that it is always porous to a certain extent thereby providing potential sites for microbial infection.

The problem of porosity can be minimised by choosing an appropriate wood. Ideally the timber should be hard, close grained, as far as possible free from knots and well seasoned. Favoured woods were oak, kauri pine, red deal and cypress (Lloyd Hind, 1940). The first two of these can be used in vessels where the wood is in direct contact with the fermenting wort. Some of the disadvantages of using wood for fermenter construction can be eliminated by providing an impermeable and inert coating layer between the timber carcase and the product. Materials which have been used include waxes, enamel, pitch and varnish. Lloyd Hind (1940) recommended that lining materials such as metal sheeting should be avoided since this arrangement may promote rotting of the underlying timber. However, there are reports of wooden vessels lined with copper which had shown no deterioration after 70 years of service (Lowe, 1991).

Slate or stone are natural materials which have been used for fermenter construction. They share many of the advantages of wood, for example, inertness, durability and low thermal conductivity and in addition, they are totally non-porous. On the other hand, they are heavy, not easily worked and suitable only for use in vessels with square or rectangular cross-section. They have tended to be used where local supplies were available and in vessels of particular traditional design as typified by the Yorkshire stone square. Although slate is eminently suitable for these fermenters and many remain in use, where they have been replaced the new vessels have been built to a similar design but using more modern materials such as stainless steel (Griffin, 1996). Wood and slate are suitable only for vessels of limited capacity, typically less than 80 hectolitres. Wooden vessels with metal linings may be used for larger vessels with volumes of 150-300 hectolitres. For even larger vessels, it is necessary to use fabricated construction materials such as concrete or various metals.

Reinforced concrete is durable, relatively inexpensive and can be formed into any desired configuration. It has been used for the construction of fermenting vessels, usually of cylindrical design and of any desired capacity. It has high mechanical strength but is relatively heavy, and therefore has tended to be used in new installations where the vessels form part of the fabric of the fermentation room. As in the case of some woods, concrete vessels need to have an inert lining material in contact with the fermenting wort. Lloyd Hind (1940) describes two patented linings, Aus-tralite and Ebon, specifically developed for bonding to the inner surface of concrete fermenting vessels. In addition to these materials, aluminium liners have been used.

Metals are the most advantageous of all materials. They are strong, durable, can be fabricated into any shape, inert, non-toxic and easily cleanable. Metals have high coefficients of thermal conductivity and consequently where internal or jacket attemperation is used the external walls of vessels must be lagged. Several metals are used for fermenter construction, notably aluminium, copper, iron, steel and stainless steel. The metal may be used as comparatively thin sheets forming a liner, with other materials used to give structural strength. Alternatively, the entire vessel may be constructed from thicker gauge metal. As in the case of the other materials already described, inner coatings may be used to form an inert barrier between the metal and the fermenting liquid. The coating may be added purposely or it may take the form of a naturally occurring layer of metal oxide.

Aluminium is light, relatively cheap and easily welded and formed, and in the past has been much favoured for vessel construction, for example, the original cylin-droconical vessels introduced by Nathan (1930b). It may be used as a lining material with wooden or concrete vessels, as already described, or as the main fabric of the vessel. Aluminium alloys have greater strength than the pure metal but are more susceptible to corrosion. To overcome this it is common to use alloys that have a thin coating of pure aluminium. The major disadvantage of aluminium is that it is attacked by concentrated alkalis and consequently caution is required in the choice of cleaning materials. More significantly, it can be subject to electrochemical corrosion. This can be a particular problem where aluminium vessels are fitted with internal copper attemperators (Schoffel, 1970; Junger & Kruse, 1972). Thus, the charged species in the fermenting wort allow an electrochemical potential to be set up between the two metal components. Severe corrosion to aluminium may occur, especially if any microscopic fissures are present. The effect must be prevented by not using different metals in the same vessel or by insulating the aluminium with an inert layer of lacquer.

Copper has been used widely for small open square ale fermenters in many traditional UK breweries. Many of these vessels have been in constant use for a hundred or more years. Copper has excellent properties with respect to inertness, strength, durability, ease of working and cleaning. It is interesting to consider that relatively low concentrations of copper ions are toxic to many brewing yeast strains (see Section 8.3.3.2) and yet copper fermenting vessels have been used to no apparent detriment. It must be assumed that the layer of copper oxide, which is always present, exerts a protective effect. Use of copper in new vessels is now precluded on economic grounds. Where it has been replaced with the now more usual stainless steel, deleterious changes in beer flavour have been observed, for example, increase in the concentrations of sulphidic components (R. Wharton, personal communication). The explanation is that the presence of copper ions removes some sulphur-containing beer constituents in the form of insoluble copper sulphide.

Iron and steel have both been used for fermenter construction. They possess most of the advantages of metals in general and are relatively inexpensive. Both suffer the obvious drawback of rusting and they must be lined. As in the case of concrete, several proprietary inert resins have been used. In addition, glass lined vessels are common. The vitreous lining is fused to the metal vessel body by means of a heat treatment. The lining is fragile and great care must be taken to ensure that it is not damaged. Epoxy lined steel vessels have a very smooth finish which is claimed to be superior to polished stainless steel for cleaning and efficient removal of yeast crops from conical fermenters (Hollis et al., 1989).

The most favoured material for fermenter construction is undoubtedly stainless steel and the majority of new vessels are now made from this. It has three times the strength of copper and relatively thin gauge sheets (6-10 mm thickness) can be used to fabricate very large vessels. It is totally inert and not subject to corrosion under the conditions encountered in brewery fermentations, provided the correct grade of stainless steel is used and installations are made to appropriate engineering standards. With some caveats, as discussed below, it can be welded with ease and formed into any desired shape. It can be polished to a high degree thereby providing an excellent cleanable and well-draining surface.

Stainless steel is a low-carbon alloy steel containing at least 11.5% chromium. There are three types of stainless steel, termed 'ferritinic', 'martensitic and 'austenitic'. A summary of the alloy composition of these is given in Table 5.2. The brewing industry uses austenitic stainless steels since these are the most resistant to corrosion (Gregory, 1967). The increased resistance is achieved by the addition of nickel and in some grades molybdenum, apart from chromium, which is present in all types of stainless steel. The resistance to corrosion requires oxidising conditions since it is dependent upon the formation of a surface oxide layer. Under reducing conditions and in the presence of chloride ions the oxide layer may be disrupted and not allowed to reform thereby allowing corrosion to occur. To avoid these conditions it is important to ensure that during critical operations such as cleaning oxygen is present.

Table 5.2 Composition of some stainless steel alloys (Perry et al., 1963). Types 304 and 316 are commonly used for brewing fermenter construction.

Alloy

Ferritinic

Martensitic

Austenitic Type 304 Type 316

Composition

13-20% chromium; less than 0.1% carbon

18-20% chromium; more than 7% nickel Cr, 19%; Ni, 10%; C, 0.08% max.

Cr, 16.5-18.5%; Ni, 10% min.; C. 0.08% max.; Mo, 2.25-3.0%; Si, 1.0% max.

Welding can increase the susceptibility of stainless steel to corrosion since heat-induced chromium carbide precipitation causes localised depletion of the protective chromium. In austenitic grades of stainless steel, this problem is ameliorated by stabilising the chromium by the addition of other components such as titanium, tantalum or columbium. Alternatively, or in addition, the carbon content may be reduced to a minimum concentration. No high-carbon steel components must be used for fittings directly welded to fermenting vessels because of the potential for chromium depletion and thereby promotion of corrosion due to weld penetration.

Although austenitic stainless steels are durable, they can suffer from a defect termed stress corrosion cracking. This phenomenon occurs when steel with an applied tensile stress is exposed to a combination of soluble chlorides and oxygen. Under these conditions, the protective surface oxide layer on the stainless steel may be attacked and over a relatively long period pits may develop. This introduces points of weakness such that the tensile stresses cause sudden cracking to occur. Fortunately, this type of corrosion is not appreciable at temperatures below 50° C, and therefore should not be a problem in the case of fermenting vessels. Although fermenters may be exposed to hot chlorine-containing chemicals in the presence of oxygen during the clean cycle, these conditions are transitory and there is time for the protective oxide layer to be reformed. Nevertheless many stainless steel vessels have not been in service for many years and regular inspection to screen for early signs of developing corrosion would seem prudent.

Although stainless steel has been almost universally adopted for the construction of new vessels, the high-quality grades required are expensive. In small installations cheaper alternatives have been occasionally used, for example plastics such as polypropylene (Anonymous, 1991). Polypropylene lacks the strength to be used as the sole material for the construction of large vessels. It has found application as a new lining material where traditional open square fermenters have been refurbished. It is also used to replace wooden headboards which are fitted to square fermenters to contain the rising yeast head and for temporary covers used during cleaning-in-place operations (Haworth, 1983). It is essential that where materials such as plastics are used they are of a type that is totally inert and must not have components that can leach out into the product. This has been reported to be a problem elsewhere. For example, domestic brewing of African beers traditionally used fermenting vessels made from ceramics or gourds. In recent years recycled metal containers have sometimes been substituted and this has resulted in contamination of the beers with metal ions such as zinc, copper and iron (Reilly, 1972).

5.1.2 Vessel hygiene

The hygiene requirements of fermenting vessels are:

(1) All vessels must be capable of being cleaned in between individual fermentations to remove soiling and avoid the possibility of taints being introduced into the product.

(2) It may be necessary to disinfect the vessel, prior to filling with wort, to minimise the risk of subsequent microbial contamination and to ensure that all yeast from the previous fermentation is removed.

(3) In most cases but not all, after the fermentation has commenced the vessel must present a microbiological barrier to the external environment. This is to prevent microbial contaminants gaining entry to the vessel and to confine the yeast within the vessel to minimise the risk of cross-contamination where several yeast strains are used in a single brewery.

In practice, the rigour with which these requirements are pursued depends upon the sophistication of the fermenter and the type of beer that is being produced. Thus, although all vessels must be capable of being cleaned, it is probable that no fermenting vessel is sterilised in the literal meaning of the word. Instead the somewhat paradoxical concepts of 'near sterility' or 'commercial sterility' are used, as with pasteurisation, to denote a condition in which cleaning and sterilisation procedures reduce microbiological loading to an acceptable level. Treatments which together give physical cleanliness and reduction in microbial counts are sometimes described as 'sanitisation' and the chemical cleaning agents, disinfectants and biocides as 'sani-tisers' (Brennan et al., 1976). See Section 8.2.1.1 for a full discussion of cleaning-in-place (CiP) operations in the brewing industry.

In terms of fermenter hygiene, CiP achieves a hygienic status that is 'fit for purpose'. Additional protection against adventitious microbial contamination is achieved through environmental and product parameters such as closed vessels, anaerobiosis, low pH and the antimicrobial activity of hop components (see Section 8.1.1). Where open fermenters are used it is essential to add the pitched wort as soon as possible after cleaning and disinfection. The blanket of carbon dioxide produced by the yeast presents a natural barrier to contamination. Nevertheless, to ensure good hygiene the design of the fermentation room must prevent materials, microbial or otherwise, inadvertently dropping into open vessels. On the other hand, worts used to produce Belgian Iambic beers are allowed to become contaminated with airborne yeast and other bacteria to start the fermentation (see Section 8.1.4.2). In the case of these products, vessel sterilisation is not necessary and great care is taken to ensure that the microbiological ecology of the fermenting rooms are undisturbed (De Keersmaecker, 1996). At the other end of the spectrum, because of the financial investment associated with very large batch sizes, modern fermenters require a rigorous regime for cleaning and disinfection.

The importance of the materials used for the internal surfaces with regard to ease of cleaning and draining has been discussed in Section 5.1.1. The geometry of the vessel should also facilitate good hygienic practice. In this respect, some traditional fermenters are less than ideal. Those that have square or rectangular cross-section and possibly have internal attemperators are not free draining and have angles that can retain soil. Such vessels may have to be cleaned manually.

Modern vessels tend to be of enclosed design, which assists good hygienic operation. Larger fermenters normally have curved internal geometry that aids both cleaning and draining, and are provided with automatic CiP systems. The latter are permanent plumbed-in systems which automatically clean, rinse and disinfect vessels (see Section 8.2.1.1). They are very effective but require careful setting up. The spray balls must be positioned so that all interior surfaces are cleaned and there must be no shadow areas. All fittings such as sample cocks, valve seals, probes, agitators if fitted, manway door seals etc. must be cleansed properly during the CiP process. The chemicals used must be compatible with all the materials they meet. They must be of an appropriate concentration and the rate at which they are delivered must be sufficient to clean the vessel but not so rapid that draining is impeded.

5.1.3 Fermenter capacity

It is important to size fermenting vessels in relation to the volume of wort that can be produced. In practice, fermenters tend to have a greater capacity than the output of the brewhouse, such that two or three batches of wort are required to fill a vessel. A more disproportionate mismatch between the capacities of fermenter and brewhouse is undesirable since the time taken to produce sufficient wort to fill the vessel would be unacceptable.

The capacity and number of fermenting vessels must be appropriate for the production requirements of the brewery. Where many beer qualities are produced within a single brewery, it is convenient to have a large number of comparatively small fermenters to foster flexibility. Thus, a multiplicity of vessels allows simultaneous brewing of several beer qualities and in addition, it provides a convenient means of fine tuning production in response to seasonal demand in sales. Conversely, if one or a small number of beer qualities are produced, a few large vessels may be more convenient, or alternatively a continuous fermentation process could be considered. However, some degree of compromise is inevitable since few but large vessels and continuous systems are inherently inflexible and cannot respond easily to fluctuating demands for beer volume.

If new vessels are to be installed in a brewery to increase fermentation capacity, the most cost-effective option is fewest and largest, within the constraints already discussed. The economic factors to be considered are capital costs of installation and fabrication versus revenue costs associated with operation. A comparison of a small multi-vessel and single large vessel installation is given in Table 5.3.

Table 5.3 General comparison of installation and running costs of four medium sized (4 x 400 hi) and one large (1600 hi) cylindroconical vessel.
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