Fig. 2.5 Structures of hop alpha acids (kindly provided by Richard Webster, Bass Brewers).
between 2 and 15% of the weight of the hop cone. During the copper boil the alpha acids undergo isomerisation to form the cis and trans forms of the humulones. It is these iso-alpha acids that impart the bitter character to beer. In addition, iso-alpha acids have antiseptic properties and bittered beers have much better keeping properties than the earlier unbittered ales.
The hop oil fraction accounts for 0.05-2.0% of the weight of the cone. They comprise a complex mixture of more than 250 components, which have subtle floral, spicy, citrus and estery aromas and tastes. Varieties of hops which are rich in hop oils are often referred to as aroma hops. These varieties cannot be used throughout the copper boil since the oils are volatile. Thus, they are added towards the end of the boil or possibly post-fermentation, known as dry hopping.
In addition to the dried cones, several other hop-derived products are available for use in commercial brewing. The use of whole hops is now comparatively rare. The simplest processed hop products are powdered and pelleted dried cones. The hop cones are milled and then pressed into pellets and sealed into foil packets from which air has been removed. Removal of oxygen produces a more stable product. Stabilisation may be further promoted by the addition of magnesium hydroxide to form the magnesium salts of the alpha acids. Further heat processing is possible by heat treating the pellet to produce a hop preparation in which the alpha acids are pre-isomerised.
Extracted hop resins are also available. Early commercial processes used organic solvents to extract hop resins, now liquid carbon dioxide is more often employed. The latter avoids the possibility of organic solvent residues being introduced to beers with the hop extracts and by manipulation of the conditions of extraction it is possible to obtain a fraction rich in alpha acids but without unwanted impurities. The same technique can be applied to aroma varieties to obtain hop oil extracts. These extracts may be blended to produce bespoke flavours and aromas for individual customers.
Use of pre-isomerised resins provides a reproducible means of bittering beers with a product which generates a minimum of waste material and is stable. These purified products have the further advantage that they may be modified chemically to introduce desirable properties and remove undesirable characters. Thus, hop derived products are now in use which have enhanced bittering qualities, promote good beer head formation and a reduced potential to generate adverse flavours due to interactions with light (Kember, 1990; Bradley, 1997).
The first stage of the brewery process proper is the preparation of'sweet wort'. The steps involved are (1) milling, in which the malt and any other solid source of extract are converted into a coarse flour known as the grist, (2) 'mashing', in which the grist is suspended in water and heated to prepare an aqueous extract from the grist and finally (3) a separation stage, in which the spent grains are removed to leave partially clarified sweet wort.
The charge of material delivered to the mill (Fig. 2.6) is calculated to produce an appropriately sized batch of wort with a desired sugar concentration. In addition to malt, other solid adjuncts may be included; alternatively, solid adjuncts not requiring
Fig. 2.6 Malt mill (kindly provided by Harry White. Bass Brewers).
milling may be added to the grist after milling. Milling is designed to release the contents of the malt endosperm from the husk and to reduce the mixture to a particle size which provides optimum operation in the mashing stage. If the particle size is too large enzymic degradation is inefficient. If the particle size is too small wort separation is impeded. Milling may be wet or dry depending on the composition of the grist and the preferences of the particular brewery.
During mashing the grist is suspended in water and the mixture heated. Many of the components of wort arise at this time by simple solution. Others are formed as a result of reactions between malt-derived enzymes and their substrates, which become possible because of the provision of an aqueous environment and the presence of an appropriate combination of pH, temperature and ions. The elevated temperature gelatinises starch granules, in other words disrupts their crystalline structure, so that they are susceptible to attack by amylases. Both a- and (3-amylases from malt are active during mashing, although the latter is more heat labile and its activity does not persist for long in high temperature mashes. Similarly, limit dextrinase is heat labile and is rapidly denatured during mashing.
a-amylase requires calcium ions for activity and if necessary it is usual to add calcium sulphate to the mash liquor to provide this ion. In addition, calcium takes part in reactions with other ions in water and helps maintain a low pH. The optimum pH for amylase activity is approximately pH 5.3. The concerted action of the amylases produces mainly maltose and but leaves significant concentrations of unde-graded dextrins. The action of amylases reduces wort viscosity due to starch granules. Elevated viscosity can be caused by malt (3-glucans and these carbohydrates may persist since the malt (3-glucanase is heat labile. To avoid this problem heat stable commercial enzyme preparations may be added.
Some 35^10% of malt proteins are solubilised during mashing, depending on the temperature and pH. However, malt proteases are heat labile and thus more protein solubilisation occurs during malting than mashing. Lower mash temperatures favour greater concentrations of wort total soluble nitrogen. The lower the mash tem perature, the greater the proportion of total soluble nitrogen arising as free a-amino nitrogen. Thus, mashing temperatures of 64-68°C favour rapid starch conversion and high extract formation, whereas, 50-55°C favour high a-amino nitrogen.
Different mashing systems are used depending on the type of beer being brewed. For ale fermentations a single temperature mash, usually 65°C, is employed in a process referred to as infusion mashing. Such a method suits the more highly modified worts, which tend to be used for production of ale worts. In this case the relatively high and constant temperature is appropriate for the formation of both adequate fermentable sugar and total soluble nitrogen.
Lager beers are traditionally brewed using a less well-modified wort. In this case it is necessary to use a relatively low initial temperature to promote proteolysis followed by a higher temperature for starch gelatinisation and amylolytic attack. In a traditional process this is achieved by a process termed decoction mashing in which successive proportions of the mash are removed and boiled then returned so as to increase gradually the temperature of the mash. A disadvantage of this method is that the boiling step denatures a significant proportion of malt enzymes. An alternative and now widely adopted method is programmed temperature mashing in which the temperature is gradually increased in a series of steps so as to allow progressive enzymic degradation of proteins and carbohydrates.
Infusion and decoction mashing require different brewery plant. Infusion mashing systems employ a mash tun for the conversion step (mash conversion stand), often with a Steel's mash mixer to take the milled grist and ensure it is mixed with hot liquor. Care must be taken to ensure that the mash reaches the desired temperature. This is achieved by calculating what temperature of mash liquor is required, termed the striking heat, which corrects for the reduction in temperature due to mixing hot water with the colder grist. This may be assisted by heating the grist with steam prior to mixing with hot liquor. Attemperation in the mash tun may be accomplished by direct injection of steam or by introducing hot liquor into the base of the vessel, termed 'underletting'. In modern installations heating jackets are provided.
The mash tun (Fig. 2.7) is a circular closed stainless steel vessel of hygienic design. Raised in the base of the vessel is a false bottom plate consisting of a mesh of slotted holes. In the headspace of the vessel there are two rotating arms, which spray liquor
onto the surface of the mash, known as 'sparging'. Usually there is a facility for moving the mash using a series of rotating rakes. These may be used during the mashing process itself but mainly serve to remove spent grains by moving them towards a discharge point in the base of the vessel.
The mash is allowed to stand in the vessel for a desired period after which time the grains settle to form a bed through which the wort is filtered. The filtration process is known as the run-off and must be managed carefully so as to avoid compressing the bed. The mash tun is usually fitted with some device to ensure that the wort is removed in an even manner across the whole bed. In the first phase of run-off, the wort is recycled back into the mash tun to allow a stable bed to form and ensure that bright wort is obtained. Once the bulk of the wort has been removed the bed is sparged with hot liquor to remove liquid entrained in the bed. This is allowed to proceed until the concentration of the wort in the run-off falls to a value at which further treatment is uneconomic. The sweet wort may be stored in a holding vessel prior to delivery to the copper.
In traditional decoction mashing systems a number of vessels are used. The simplest arrangement is a three-vessel system using a mash mixing vessel, in which grist and liquor are mixed and where conversion takes place, a copper for heating the proportion of mash removed from the mash mixer, and a lauter tun for separating wort and spent grains. More complex systems double up some of the vessels. In some modern systems a double mashing system is used. This consists of a main mash mixing vessel in which the bulk conversion of the malt grist takes place and a secondary cereal cooker for processing solid adjuncts. The mash mixer is a hygienic stainless steel vessel fitted with a large bottom-located agitator to ensure good mixing. Bottom and side jackets provide controlled heating. Mashing involves a ramped temperature profile in which the first low-temperature stand is provided for maximum activity of proteases and (3-glucanases. This is followed by a second higher-temperature stand for starch gelatinisation and amylolysis. A final even higher-temperature short stand may be incorporated, designed to denature enzymes which may cause problems further down stream.
The cereal cooker is particularly popular in the United States where a large proportion of adjuncts such as rice or corn grits are used to produce pale lager beers with blander flavours than European style lagers. In this vessel the adjunct and a small charge of high amylase malt is mixed and held at the high temperature required to gelatinise adjunct starch grains. Operation of mash mixer and cereal adjunct cooker are arranged so that both processes are completed at a similar time which allows mixing of each prior to wort separation in the 'lauter tun'.
The lauter tun (Fig. 2.8) (German for refine or clarify) uses a shallower bed than the mash tun and therefore has a considerably larger diameter. Like the mash tun it has a false bottom with slots for retaining the grains and allowing wort to drain away. Sparge arms are provided and a series of rakes. The blades on the rakes can usually be rotated to either agitate the bed in one orientation or push the grains to the outlet main in the other. The provision of raking allows for the use of a finer grind grist which is characteristic of lauter brewing. In addition, pumps may be used to induce a pressure across the bed and improve the efficiency of run-off. As with the mash tun, the bed functions as a filter through which the wort percolates. Initial recycling allows
the bed to form and sparge liquor is used to extract the maximum volume of wort. Modern lauter tuns are designed to minimise oxygen pick-up which affects beer keeping qualities (Geering, 1996).
Sweet wort and spent grains may be separated using a mash filter (Fig. 2.9). This is a simple plate and frame filter and has the advantage that separation is rapid, efficient and produces a very dry spent grain. However, it has the disadvantage that it is
complex and does not easily produce a bright wort. A modern version of the mash press overcomes these problems (Hermia & Rahier, 1992). This uses polypropylene filter sheets with a fine pore size such that a very fine grind grist can be used. However, this requires the use of a hammer mill as opposed to the more conventional roller types associated with lauter tun breweries. The fine grind allows use of thin beds since the surface area is concomitantly large. Thus, bright worts are formed at relatively high flow rates.
Clarified sweet wort is delivered to the copper (kettle) where it is subjected to a heat treatment. In beers subject to the Reinheitsgebot laws, only hops are added to an all-malt sweet wort and boiled. In other countries additional sources of fermentable sugar in the form of liquid sugar syrup adjuncts may be added with the sweet wort.
The copper boil serves many functions. It sterilises the wort and inactivates all malt enzymes before it is cooled and added to the fermenter. Hop alpha acids are iso-merised during the copper boil. This aspect of copper management is dependent on the type of hops used. Varieties which require extensive heat treatment for iso-merisation are added early on, whereas those which are added for aromatic hop oil content are added towards the end. Inevitably some compromise is necessary between the need for isomerisation and the potential for loss of aromatic components. In addition, prolonged boiling can result in conversion of alpha acids to humulinic acids, which lack bitterness. Removal of most of the essential oil component of hops in the steam exhaust from the copper is essential for balanced flavour in the final beer. Apart from these hop components other volatiles derived from malts, many of which impart undesirable 'vegetable' odours, are also lost. For example, S-methylmethio-nine derived from malt decomposes to dimethyl sulphide during mashing and in the copper boil. The latter is an essential contributor to lager character (see Section 3.7.5).
Wort boiling assists with clarification and removes substances which may cause problems in downstage processes. Thus, polyphenols from malt and hops react with some proteins to form insoluble precipitates. In addition, some of the other proteins coagulate. The resultant mixture is termed 'hot break' or 'trub'. Most of this material is separated from the wort during transfer from copper to fermenter. Oxalate, which can form beer haze, is precipitated as the insoluble calcium salt. The high temperatures in the copper promote many other chemical reactions. Maillard reactions between reducing sugars and amines form melanoidins, which contribute to beer colour. These melanoidins may also displace aldehydes from sulphite adducts in packaged beers, thereby contributing to staling. The boiling process provides an opportunity to concentrate the wort. This allows correction for dilution in the mashing stage due to sparging operations.
Several copper designs are in use (Fig. 2.10). The name suggests the metal originally used for their construction; however, modern vessels are invariably made from stainless steel. Early versions were simply open vessels with a flue for escape of steam and heat provided by an underbuilt fire. Modern coppers are usually heated by steam, either with internal heat exchangers or by circulating the wort through an external
loop fitted with a tube and shell heat exchanger known as a calandria. Such systems use convective forces to drive the boiling wort around the loop.
After the boil is completed, solids in the form of 'trub' (see Section 2.4.4) and any hop material have to be separated from the hot wort before it is cooled and delivered to the fermenting vessel. Various types of plant may be used to accomplish this. Where whole hops are used plant is required to separate the spent cones. Traditionally this is achieved in ale breweries using a hop-back, which is similar in design to a mash tun, in that it has a false-slotted base and sparge arms. Like the mash tun the principle of operation is that the spent hops form a bed through which the hot wort passes and in so doing filters out trub. Sparging provides a means of recovering some of the wort entrained in the hop bed. Traditional lager brewers employ a similar device called a hop-jack or montejus. This is a closed tank fitted with a mechanical agitator. Hot wort is fed in at the top of the vessel and is discharged from the base. Solid materials are retained by an internal mesh, which forms a cage inside the vessel.
Where pelleted hops or hop extracts are used there is insufficient material to make hop-backs or hop-jacks practicable. In this case, which applies to the majority of modern breweries, a whirlpool is used to clarify worts. Several designs are used which are claimed to offer various advantages but all use the same basic principle of operation (Andrews, 1988). The whirlpool consists of a cylindrical insulated vessel into which the hot wort is pumped via a tangentially mounted entry main. This induces a hydrocylone effect in the vessel so that as the liquid circulates, the solid material drops out and forms a compact mound in the centre of the base. The clarified wort can then be run off leaving the solid matter in the whirlpool for subsequent removal.
Clarified wort is cooled using a paraflow or heat exchanger (Fig. 2.11) and pumped to the fermenting vessel where oxygenation and yeast pitching allow the fermentation to
commence. The various processes and types of plant used in the fermentation stage of brewing form the subject of Chapters 5-7 and are not described further here.
When fermentation is complete the beer must be rendered into a form suitable for consumption. Many options are possible depending on the type of beer. Beers subject to a secondary fermentation may simply be removed from the primary fermentation vessel after the bulk of the yeast has been removed and packaged, either in cask or bottle (see Section 6.9). Such types of beers are comparatively rare. Most beer is subject to post-fermentation processing to produce a packaged product, which is stable, both from microbiological, physical and flavour standpoints. The downstream processes involved include 'conditioning' (or 'maturation', 'ageing'), filtration and pasteurisation/sterile filtration.
In the case of traditional beers, conditioning involves a period of storage in which flavour maturation occurs and this requires the presence of viable yeast cells. The processes involved are described in Section 6.5. For most beers the primary function of this stage of brewing is to produce beer with a colloidal stability which is appropriate for its projected shelf-life. Some positive flavour adjustments may be made. For example, pre-isomerised hop extract additions may be made to conditioning vessels as opposed to the copper since obviously no heating is required. Recovered beer may also be added back at this stage. Small volumes of recovered beer are generated in many stages of the brewing process, usually where there is a separation stage of liquid from solid and the bulk of a batch of beer has been moved forward to the next stage of processing leaving behind a solid-liquid mixture. A typical example is beer recovered from yeast cropped from a fermenter. The recovered product must be of sufficient quality not to compromise the beer with which it is to be blended. This may involve some intermediate treatment, for example, flash pasteurisation to avoid microbiological contamination.
Colloidal instability of beers may result in the formation of two types of haze, termed chill hazes and permanent hazes. The first of these is a haze that is formed at low temperature but redissolves when the beer is returned to room temperature. Permanent hazes, by definition, once formed persist in beer under all conditions. Hazes are due to interactions between proteins, tannins (polyphenols), polysaccharides and metal ions by mechanisms that are not fully characterised. In the case of chill hazes the interactions are not stabilised, whereas permanent hazes involve the formation of covalent bonds, possibly involving oxygen.
Several chill-proofing treatments are possible. Proteases such as papain reduce the concentration of proteins available for haze formation. However, care must be taken with this approach as it can have a detrimental effect on beer foaming potential. Various adsorbents are used which are themselves insoluble, and therefore are removed with the potential haze-forming material during filtration. Silica gel and bentonite adsorb proteins; Nylon 66 and polyvinylpolypyrrolidone (PVPP) adsorb polyphenols. Another strategy is to add tannic acid, which forms complexes with the proteins that have the potential to produce hazes.
Conditioning is terminated by filtration to produce bright beer. It is advantageous to present beer to filters with as low a solids concentration as possible. To facilitate this beer in conditioning tanks is commonly dosed with isinglass finings to promote sedimentation of suspended solids. Of course, whilst this has the beneficial effect of reducing the solids content in the beer that is to be filtered, there is a concomitant increase in the quantity of solids in the tank bottoms and associated recovered beer. Inevitably, a compromise must be made that suits the individual brewery. Many types of filter are used, depending on the type of operation. Usually two treatments are given, a pre-filtration to remove the bulk of solids followed by a polishing filter to produce brilliantly clear beer. In all cases it is essential to exclude oxygen to prevent adverse staling effects. This is particularly so when all yeast cells have been removed since these have the ability to assimilate low concentrations of oxygen and prevent harmful oxidations.
For beer filtration, plate and frame or leaf filters are most commonly used (Fig. 2.12) although some breweries use a candle filter (Fig. 2.13). In these cases kieselguhr or perlite is used as a filter aid. These filters use a porous membrane on which the filter aid is deposited, termed pre-coating. More filter aid is mixed with the beer as it is fed into the filter, termed body feed. Selection of a suitable grade of filter aid allows operation as rough or polishing filter. The filtered bright beer is held in refrigerated tanks for final adjustments prior to packaging. It is possible at this stage to adjust colour; however, the most important task is to ensure that carbonation is correct.
Beer from bright beer tanks feeds the packaging lines, where the finished product is placed into keg, bottle or can. These processes are clean but not usually sterile and therefore the process culminates with pasteurisation. In the case of keg beers, product is flash pasteurised, in-line, before packaging into steam-sterilised and clean kegs. Cans and bottles are filled, sealed and then tunnel pasteurised.
Severe pasteurisation regimes undoubtedly produce some deterioration in flavour and to overcome this some brewers package canned and bottled beers under sterile conditions. It should be noted that in most countries it is not permitted to use artificial preservatives. Sterile packaging is expensive since it requires that all operations after
the sterilising step have to be performed under aseptic conditions. In the case of a high-speed canning line this is technologically challenging. Sterilisation of beer is achieved by passage through a membrane or candle filter with a pore size of 0.1-0.4 microns, which is sufficiently small to remove micro-organisms. Further discussion on pasteurisation and sterile filtration can be found in Section 220.127.116.11.
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