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Fig. 2.12 A lauter tun. Drawing courtesy of Briggs of Burton.
Fig. 2.13 A mash filter. Photograph courtesy of Briggs of Burton.

sizes (the sand versus clay analogy used earlier). Furthermore, the grains bed depth is particularly shallow (2-3 in.), being nothing more than the distance between the adjacent plates.

Water

Since water represents at least 90% of the composition of most beers, it will clearly have a major direct impact on the product, particularly in terms of flavour and clarity. The nature of the water, however, exerts its influence much earlier in the process, through the impact of the salts it contains on enzymic and chemical processes, through the determination of pH, etc.

Water in breweries comes either from wells owned by the brewer (cf. the famous water of Burton-on-Trent in England or Pilsen in the Czech Republic) or from municipal supplies; especially in the latter instance, the water will be subjected to clean-up procedures, such as charcoal filtration, to eliminate undesirable taints and colours.

The ionic composition of the water in four brewing centres is given in Table 2.3. The water in Burton is clearly very hard, both permanent and temporary. By contrast, the water in Pilsen is extremely soft. It is clear that the nature of the water has had some impact on the quality of the different beer styles traditionally produced in these two centres; however, the rationale for the differences is less than fully satisfactorily explained.

The water composition can be adjusted, either by adding or by removing ions. Thus, calcium levels may be increased in order to promote the precipitation of oxalic acid as oxalate, to lower the pH by reaction with phosphate ions (3Ca2+ + 2HPO4- ^ Ca3(PO4)2 + 2H+) and to promote amylase action. (The optimum pH for mashing is between 5.2 and 5.4.) The alkalinity of water used for sparging (alkalinity is largely determined by the content of carbonate and bicarbonate) may be reduced to less than 50 ppm in order to limit the extraction of tannins. Ions such as iron and copper must be as low as possible to preclude oxidation. Furthermore, water may need to be of different standards for different purposes. The microbiological status of water used for slurrying yeast or for use downstream generally is important. Water used for diluting high-gravity streams must be of low oxygen content, and its ionic composition will be critical. When ions need to be removed, the likeliest approach is ion-exchange resin technology.

Table 2.3 Ionic composition (mg L 1) of water.

Component

Burton

Pilsen

Dublin

Munich

Calcium

352

7

119

80

Magnesium

24

8

4

19

Sulphate

820

6

54

6

Chloride

16

5

19

1

Bicarbonate

320

37

319

333

Name Side chain (R)

Humulone —CO-CH2-CH(CH3)2 isovaleryl

Cohumulone —CO-CH(CH3)2 isobutyryl

Adhumulone —CO-CH(CH3)-CH2-CH3 2-methylbutyryl

Name Side chain (R)

Humulone —CO-CH2-CH(CH3)2 isovaleryl

Cohumulone —CO-CH(CH3)2 isobutyryl

Adhumulone —CO-CH(CH3)-CH2-CH3 2-methylbutyryl

Fig. 2.14 Hop resins.

Hop oils "

Sulphur-containing compounds (e.g. CH3SSSCH3 dimethyl trisulphide)

Oxygen-containing compounds (mostly oxidised mono-, di-and sesquiterpenes)

Humulene epoxide

Humulene epoxide

Examples

Oxygen-containing compounds (mostly oxidised mono-, di-and sesquiterpenes)

Examples

Linalool Karahenone

Monoterpenes Sesquiterpenes Hydrocarbons and diterpenes (e.g. ^-farnesene) (e.g. myrcene)

Humulene

Humulene

Hops

The hop, Humulus lupulus, is rich in resins (Fig. 2.14) and oils (Fig. 2.15), the former being the source of bitterness, the latter the source of aroma. The hop is remarkable amongst agricultural crops in that essentially its sole outlet is for brewing. Hops are grown in all temperate regions of the world, with approximately one-third coming from Germany.

Hops are hardy, climbing herbaceous perennial plants grown in gardens using characteristic string frameworks to support them. It is only the female plant that is cultivated, as it is the one that develops the hop cone (Fig. 2.16). Their rootstock remains in the ground year on year and is spaced in an appropriate fashion for effective horticultural procedures (e.g. spraying by tractors passing between rows). In recent years, so-called dwarf varieties have been

Fig. 2.16 Hop cones. Photograph courtesy of Yakima Chief.

bred, which retain the bittering and aroma potential of 'traditional' hops but which grow to a shorter height (6-8 ft as opposed to twice as big). As a result, they are much easier to harvest and there is less wastage of pesticide during spraying. Dwarf hop gardens are also much cheaper to establish.

Hops are susceptible to a wide range of diseases and pests. The most serious problems come from Verticillium wilt, downy mildew, mould and the damson-hop aphid. Varieties differ in their susceptibility to infestation and have been progressively selected on this basis. Nonetheless, it is frequently necessary to apply pesticides, which are always stringently evaluated for their influence on hop quality, for any effect they may have on the brewing process and, of course, for their safety.

Hops are generally classified into two categories: aroma hops and bittering hops. All hops are capable of providing both bitterness and aroma. Some hops, however, such as the Czech variety Saaz, have a relatively high ratio of oil to resin and the character of the oil component is particularly prized. Such varieties command higher prices and are known as aroma varieties. They are seldom used as the sole source of bitterness and aroma in a beer: a cheaper, higher a-acid hop (a bittering variety) is used to provide the bulk of the bitterness, with the prized aroma variety added late in the boil for the contribution of its own unique blend of oils. Those brewers requiring hops solely as a source of bitterness may well opt for a cheaper variety, ensuring its use early in the kettle boil so that the provision of bitterness is maximised and unwanted aroma is driven off.

The use of whole cone hops is comparatively uncommon nowadays. Many brewers use hops that have been hammer-milled and then compressed into pellets. In this form they are more stable, more efficiently utilised and do not present the brewer with the problem of separating out the vegetative parts of the hop plant. Some use hop extracts that are derived by dissolving the resins in liquid carbon dioxide, followed by a chemical isomerisation if the bitterness is to be added to the finished beer rather than in the boiling stage. Recent years have been marked by an enormous increase in the use of such pre-isomerised extracts after they have been modified by reduction. One of the side chains on the iso-a-acids is susceptible to cleavage by light; it then reacts with traces of sulphidic materials in beer to produce 2-methyl-3-butene-1-thiol (MBT), a substance that imparts an intensely unpleasant skunky character to beer. If the side chain is reduced, it no longer produces MBT. For this reason, beers that are destined for packaging in green or clear glass bottles are often produced using these modified bitterness preparations, which have the added advantage of possessing increased foam-stabilising and antimicrobial properties.

Wort boiling and clarification

The boiling of wort serves various functions, primary amongst which are the isomerisation of the hop resins (a-acids) to the more soluble and bitter iso-a-acids, sterilisation, the driving off of unwanted volatile materials, the precipitation of protein/polyphenol complexes (as 'hot break' or 'trub') and concentration of the wort. The extent of wort boiling is normally described in terms of percentage evaporation. Water is usually boiled off at a rate of about 4% h-1 and the duration of boiling is likely to be 1-2h. Brew kettles are sometimes referred to as 'coppers', reflecting the original metal from which they were fabricated (Fig. 2.17). These days they are usually made from stainless steel. Certain fining materials (e.g. a charged polysaccharide from Irish Moss) may be added to promote protein precipitation. This is the stage at which liquid sugar adjunct can be added (Table 2.4). Sugars added in the kettle are called 'wort extenders': they present the opportunity to increase the extract from a brewhouse without investment in extra mashing vessels and wort separation devices. Most sugars are derived from either corn or sugar cane. In the latter case, the principal sugar is either sucrose or fructose plus glucose if the product has been 'inverted'. There are many different corn sugar products, differing in their degree of hydrolysis and therefore fermentability. Through the controlled use of acid but increasingly of starch-degrading enzymes, the supplier can produce preparations with a full range of fermentabilities depending on the needs of the brewer: from 100% glucose through to high dextrin.

Fig. 2.17 Kettle. Wort in this design is siphoned through the external heating device (calandria), thus ensuring an efficient and highly turbulent boil.

Table 2.4 Brewing sugars.

Type

Carbohydrate distribution (%)

Cane

Invert

Dextrose

High conversion (acid + enzyme)

Glucose chips

Maize syrup

Very-high maltose

High conversion (acid)

High maltose

Low conversion

Maltodextrin

Malt extract

Sucrose predominantly Glucose (50), fructose (50) Glucose (l00)

Glucose (88), maltose (4), maltotriose (2), dextrin (6) Glucose (84), maltose (1), maltotriose (2), dextrin (13) Glucose (45), maltose (38), maltotriose (3), dextrin (14) Glucose (5), maltose (70), maltotriose (10), dextrin (15) Glucose (31), maltose (18), maltotriose (13), dextrin (38) Glucose (10), maltose (60), dextrin (30) Glucose (12), maltose (10), maltotriose (10), dextrin (68) Maltose (1.5), maltotriose (1.5), dextrin (95) Comparable to brewer's wort - also contains nitrogenous components

The products dextrose through maltodextrin are customarily derived by the selective hydrolysis of corn (maize)-derived starch by acid and enzymes to varying extents. Derived from Pauls Malt Brewing Room Book (1998-2000). Bury St Edmunds: Moreton Hall Press.

After boiling, wort is transferred to a clarification device. The system employed for removing insoluble material after boiling depends on the way in which the hopping was carried out. If whole hop cones are used, clarification is through a hop jack (hop back), which is analogous to a lauter tun, but in this case the bed of residual hops constitutes the filter medium. If hop pellets or extracts are used, then the device of choice is the whirlpool, a cylindrical vessel, into which hot wort is transferred tangentially through an opening

Fig. 2.18 A 'whirlpool' (hot wort residence vessel).

Coolant in c^X

Coolant out

Fig. 2.19 A heat exchanger.

0.5-1 m above the base (Fig. 2.18). The wort is set into a rotational flux, which forces trub to a pile in the middle of the vessel.

Wort cooling

Almost all cooling systems these days are of the stainless steel plate heat exchanger type, sometimes called 'paraflows' (Fig. 2.19). Heat is transferred from the wort to a coolant, either water or glycol depending on how low the temperature needs to be taken. At this stage, it is likely that more material will precipitate from solution ('cold break'). Brewers are divided on whether they feel this to be good or bad for fermentation and beer quality. The presence of this break certainly accelerates fermentation and, therefore, it will directly influence yeast metabolism. As in so much of brewing, the aim should be consistency: either consistently 'bright worts' or ones containing a relatively consistent level of trub.

Brewing yeast is Saccharomyces cerevisiae (ale yeast) or Saccharomyces pastorianus (lager yeast). There are many separate strains of brewing yeast, each of which is distinguishable phenotypically [e.g. in the extent to which it will ferment different sugars, or in the amount of oxygen it needs to prompt its growth, or in the amounts of its metabolic products (i.e. flavour spectrum of the resultant beer), or its behaviour in suspension (top versus bottom fermenting, flocculent or non-flocculent)] and genotypically, in terms of its DNA fingerprint.

The fundamental differentiation between ale and lager strains is based on the ability or otherwise to ferment the sugar melibiose (Fig. 2.20): ale strains cannot whereas lager strains can because they produce the enzyme (a-galactosidase) necessary to convert melibiose into glucose and galactose. Ale yeasts also move to the top of open fermentation vessels and are called top-fermenting yeasts. Lager yeasts drop to the bottom of fermenters and are termed bottom-fermenting yeasts. Nowadays it is frequently difficult to make this differentiation, when beers are widely fermented in similar types of vessel (deep cylindro-conical tanks) which tend to equalise the way in which yeast behave in suspension.

We considered yeast structure in Chapter 1. When presented with wort, yeast encounters a selection of carbohydrates which, for a typical all-malt wort, will approximate to maltose (45%), maltotriose (15%), glucose (10%), sucrose (5%), fructose (2%) and dextrin (23%). The dextins (maltotetraose and larger) are unfermentable. The other sugars will ordinarily be utilised in the sequence sucrose, glucose, fructose, maltose, and lastly maltotriose, though there may be some overlap (Fig. 2.21). Sucrose is hydrolysed by an enzyme (invertase) released by the yeast outside the cell, and then the glucose and fructose enter the cell to be metabolised. Maltose and maltotriose also

Yeast

H OH

Melibiose

H OH

H OH

Melibiose

Fig. 2.20 Melibiose.

Sucrose

Melibiose

Melibiase (lager strains) Fructose Glucose Galactose

Melibiose

Melibiase (lager strains) Fructose Glucose Galactose

Maltose

-► Enzyme-catalysed reaction — ■ ■ — ■ ► Transport

Maltose

-► Enzyme-catalysed reaction — ■ ■ — ■ ► Transport

Fig. 2.21 The uptake of sugars by brewing yeast.

Fig. 2.21 The uptake of sugars by brewing yeast.

Transaminase

Transaminase

Fig. 2.22 The principle of transamination.

Fig. 2.22 The principle of transamination.

enter, through the agency of specific permeases. Inside the cell they are broken down into glucose by an a-glucosidase. Glucose represses the maltose and maltotriose permeases.

The principal route of sugar utilisation in the cell is the EMP pathway of glycolysis (see Chapter 1). Brewing yeast derives most of the nitrogen it needs for synthesis of proteins and nucleic acids from the amino acids in the wort. A series of permeases is responsible for the sequential uptake of the amino acids. It is understood that the amino acids are transaminated to keto acids and held within the yeast until they are required, when they are transaminated back into the corresponding amino acid (Figs 2.22 and 2.23). The amino acid spectrum and level in wort (free amino nitrogen, FAN) is significant as it influences yeast metabolism leading to flavour-active products.

Oxygen is needed by the yeast to synthesise the unsaturated fatty acids and sterols it needs for its membranes. This oxygen is introduced at the wort cooling stage in the quantities that the yeast requires - but no more, because excessive aeration or oxygenation promotes excessive yeast growth, and the more yeast is produced in a fermentation, the less alcohol will be produced. Different yeasts need different amounts of oxygen.

Amino acids

Amino acids

Fig. 2.23 Transamination as part of the metabolism of amino acids by yeast.

Yeast uses its stored reserves of carbohydrate in order to fuel the early stages of metabolism when it is pitched into wort, for example, the synthesis of sterols. There are two principal reserves: glycogen and trehalose. Glycogen is similar in structure to the amylopectin fraction of barley starch. Trehalose is a disaccharide comprising two glucoses linked with an a-1,1 bond between their reducing carbons. The glycogen reserves of yeast build up during fermentation and it is important that they are conserved in the yeast during storage between fermentations. Trehalose may feature as more of a protection against the stress of starvation. It certainly seems to help the survival of yeast under dehydration conditions employed for the storage and shipping of dried yeast.

Pure yeast culture was pioneered by Hansen at Carlsberg in 1883. By a process of dilution, he was able to isolate individual strains and open up the possibility of selecting and growing separate strains for specific purposes. Nowadays brewers maintain their own pure yeast strains. While it is still a fact that some brewers simply use the yeast grown in one fermentation to 'pitch' the following fermentation, and that they have done this for many tens of years, it is much more usual for yeast to be repropagated from a pure culture every 4-6 generations. (When brewers talk of 'generations', they mean successive fermentations; strictly speaking, yeast advances a generation every time it buds, and therefore there are several generations during any individual fermentation.)

Large quantities of yeast are needed to pitch commercial-scale fermentations. They need to be generated by successive scale-up growth from the master culture (Fig. 2.24). Higher yields are possible if fed-batch culture is used. This is the type of procedure used in the production of baker's yeast. It takes advantage of the Crabtree effect, in which high concentrations of sugar drive the yeast to use it fermentatively rather than by respiration. When yeast grows by respiration, it captures much more energy from the sugar and therefore produces much more cell material. In fed-batch culture, the amount of sugar made available to the yeast at any stage is low. Together with the high levels of oxygen in a well-aerated system, the yeast respires and grows substantially.

Master culture +

5 mL sterile hopped wort

| (48 h @ 28° C) 50 mL sterile hopped wort (48 h @ 28° C)

200 mL 200 mL 200 mL 200 mL 200 mL | | (48 hr @ 22° C)| |

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