Overview of malting and brewing

Brewer's yeast Saccharomyces can grow on sugar anaerobically by fermenting it to ethanol:

C6H12O6 ^ 2C2H5OH + 2CO2

While malt and yeast contribute substantially to the character of beers, the quality of beer is at least as much a function of the water and, especially, of the hops used in its production.

Barley starch supplies most of the sugars from which the alcohol is derived in the majority of the world's beers. Historically, this is because, unlike other cereals, barley retains its husk on threshing and this husk traditionally forms the filter bed through which the liquid extract of sugars is separated in the brewery. Even so, some beers are made largely from wheat while others are from sorghum.

The starch in barley is enclosed in a cell wall and proteins and these wrappings are stripped away in the malting process (essentially a limited germination of the barley grains), leaving the starch largely preserved. Removal of the wall framework softens the grain and makes it more readily milled.

Water

Water

Adjuncts Hops

Saccharomyces

Barley

Malt

Wort t

Beer

Steep 14 -18°C, 48 h Germinate 16-20°C, 4-6 days Kiln 50-110° C, 24 h

Store 20 °C, 4 weeks Mill, mash 40-72°C 1-2 h Wort separation 1.5-4 h Boil 100 °C 45 min-2 h Clarify

Malting

Brewing

Fermentation and conditioning

Fermentation 6-25°C, 3-14 days Maturation (varies)

Filter, stabilise, package

Downstream processing and packaging

Fig. 2.1 Overview of malting and brewing.

Not only that, unpleasant grainy and astringent characters are removed during malting.

In the brewery, the malted grain must first be milled to produce relatively fine particles, which are for the most part starch. The particles are then intimately mixed with hot water in a process called mashing. The water must possess the right mix of salts. For example, fine ales are produced from waters with high levels of calcium while famous pilsners are from waters with low levels of calcium. Typically mashes have a thickness of three parts water to one part malt and contain a stand at around 65°C, at which temperature the granules of starch are converted by gelatinisation from an indigestible granular state into a 'melted' form that is much more susceptible to enzymatic digestion. The enzymes that break down the starch are called the amylases. They are developed during the malting process, but only start to act once the gelatinisation of the starch has occurred in the mash tun. Some brewers will have added starch from other sources, such as maize (corn) or rice, to supplement that from malt. These other sources are called adjuncts. After perhaps an hour of mashing, the liquid portion of the mash, known as wort, is recovered, either by straining through the residual spent grains or by filtering through plates. The wort is run to the kettle (sometimes known as the copper, even though they are nowadays fabricated from stainless steel) where it is boiled, usually for around 1 h. Boiling serves various functions, including sterilisation of wort, precipitation of proteins (which would otherwise come out of solution in the finished beer and cause cloudiness), and the driving away of unpleasant grainy characters originating in the barley. Many brewers also add some adjunct sugars at this stage, at which most brewers introduce at least a proportion of their hops.

The hops have two principal components: resins and essential oils. The resins (so-called a-acids) are changed ('isomerised') during boiling to yield iso-a-acids, which provide the bitterness to beer. This process is rather inefficient. Nowadays, hops are often extracted with liquefied carbon dioxide and the extract is either added to the kettle or extensively isomerised outside the brewery for addition to the finished beer (thereby avoiding losses due to the tendency of the bitter substances to stick on to yeast). The oils are responsible for the 'hoppy nose' on beer. They are very volatile and if the hops are all added at the start of the boil, then all of the aroma will be blown up the chimney (stack). In traditional lager brewing, a proportion of the hops is held back and only added towards the end of boiling, which allows the oils to remain in the wort. For obvious reasons, this process is called late hopping. In traditional ale production, a handful of hops is added to the cask at the end of the process, enabling a complex mixture of oils to give a distinctive character to such products. This is called dry hopping. Liquid carbon dioxide can be used to extract oils as well as resins and these extracts can also be added late in the process to make modifications to beer flavour.

After the removal of the precipitate produced during boiling ('hot break', 'trub'), the hopped wort is cooled and pitched with yeast. There are many strains of brewing yeast and brewers carefully look after their own strains because of their importance in determining brand identity. Fundamentally brewing yeast can be divided into ale and lager strains, the former type collecting at the surface of the fermenting wort and the latter settling at the bottom of a fermentation (although this differentiation is becoming blurred with modern fermenters). Both types need a little oxygen to trigger off their metabolism, but otherwise the alcoholic fermentation is anaerobic. Ale fermentations are usually complete within a few days at temperatures as high as 20° C, whereas lager fermentations at temperatures as low as 6°C can take several weeks. Fermentation is complete when the desired alcohol content has been reached and when an unpleasant butterscotch flavour, which develops during all fermentations, has been mopped up by yeast. The yeast is harvested for use in the next fermentation.

In traditional ale brewing, the beer is now mixed with hops, some priming sugars and with isinglass finings from the swim bladders of certain fish, which settle out the solids in the cask.

In traditional lager brewing, the 'green beer' is matured by several weeks of cold storage, prior to filtering.

Nowadays, the majority of beers, both ales and lagers, receive a relatively short conditioning period after fermentation and before filtration. This conditioning is ideally performed at -1 °C or lower (but not so low as to freeze the beer) for a minimum of 3 days, under which conditions more proteins drop out of the solution, making the beer less likely to cloud in the package or glass.

The filtered beer is adjusted to the required carbonation before packaging into cans, kegs, or glass or plastic bottles.

Although it is possible to make beer using raw barley and added enzymes (so-called barley brewing), this is extremely unusual. Unmalted barley alone is unsuitable for brewing beer because (1) it is hard and difficult to mill; (2) it lacks most of the enzymes needed to produce fermentable components in wort; (3) it contains complex viscous materials that slow down solid-liquid separation processes in the brewery, which may cause clarity problems in beer and (4) it contains unpleasant raw and grainy characters and is devoid of pleasant flavours associated with malt.

Barley belongs to the grass family. Its Latin name is Hordeum vulgare, though this term tends to be retained for six-row barley (discussed later), with Hordeum distichon being used for two-row barley. The part of the plant of interest to the brewer is the grain on the ear. Sometimes this is referred to as the seed, but individual grains are generally called kernels or corns. A schematic diagram of a single barley corn is shown in Fig. 2.2. Four components of the kernel are particularly significant:

(1) the embryo, which is the baby plant;

(2) the starchy endosperm, which is the food reserve for the embryo;

Barley

Embryo

.Micropyle

Barleystarch Brewing Diagram

Embryo

.Micropyle

(3) the aleurone layer, which generates the enzymes that degrade the starchy endosperm;

(4) the husk (hull), which is the protective layer around the corn. Barley is unusual amongst cereals in retaining a husk after threshing and this tissue is traditionally important for its role as a filter medium in the brewhouse when the wort is separated from spent grains.

The first stage in malting is to expose the grain to water, which enters an undamaged grain solely through the micropyle and progressively hydrates the embryo and the endosperm. This switches on the metabolism of the embryo, which sends hormonal signals to the aleurone layer, triggers that switch on the synthesis of enzymes responsible for digesting the components of the starchy endosperm. The digestion products migrate to the embryo and sustain its growth.

The aim is controlled germination, to soften the grain, remove troublesome materials and expose starch without promoting excessive growth of the embryo that would be wasteful (malting loss). The three stages of commercial malting are

(1) steeping, which brings the moisture content of the grain to a level sufficient to allow metabolism to be triggered in the grain;

(2) germination, during which the contents of the starchy endosperm are substantially degraded ('modification') resulting in a softening of the grain;

(3) kilning, in which the moisture is reduced to a level low enough to arrest modification.

The embryo and aleurone are both living tissues, but the starchy endosperm is dead. It is a mass of cells, each of which comprises a relatively thin cell wall (approximately 2 ^m) inside which are packed many starch granules amidst a matrix of protein (see Fig. 2.3). This starch and protein (and also the cell-wall

Cell wall

Large starch granule

Protein matrix

Cell wall

Large starch granule

Starch Granule Cell Wall Protein

Small starch granule

Protein matrix

Fig. 2.3 A single cell within the starchy endosperm of barley. Only a very small number of the multitude of small and large starch granules are depicted.

Small starch granule

Fig. 2.3 A single cell within the starchy endosperm of barley. Only a very small number of the multitude of small and large starch granules are depicted.

materials) are the food reserves for the embryo. However, the brewer's interest in them is as the source of fermentable sugars and assimilable amino acids that the yeast will use during alcoholic fermentation.

The wall around each cell of the starchy endosperm comprises 75% j-glucan, 20% pentosan, 5% protein and some acids, notably acetic acid and the phenolic acid, ferulic acid. The j-glucan comprises long linear chains of glucose units joined through j-linkages. Approximately 70% of these linkages are between C-1 and C-4 of adjacent glucosyl units (so-called j 1-4 links, just as in cellulose) and the remainder are between C-1 and C-3 of adjacent glucoses (j 1-3 links, which are not found in cellulose) (Fig. 2.4). These 1-3 links disrupt the structure of the j-glucan molecule and make it less ordered, more soluble and digestible than cellulose. Much less is known about the pentosan (arabinoxylan, Fig. 2.5) component of the wall, and it is generally believed that it is less easily solubilised and difficult to breakdown when compared with the j-glucan, and that it largely remains in the spent grains after mashing. The cell-wall polysaccharides are problematic because they restrict the yield of extract. They do this either when they are insoluble (by wrapping around the starch components) or when they are solubilised (by restricting the flow of wort from spent grains during wort separation). Dissolved but undegraded j-glucans also increase the viscosity of beer and slow down filtration. They

Fig. 2.4 Mixed linkage f -glucan in the starchy endosperm cell wall of barley. The 1-3 linkages occur every third or fourth glucosyl, although there are 'cellulosic' regions wherein there are longer sequences of 1-4 linked glucosyls. ~ indicates that the chain continues in either direction - molecular weights of these glucans can be many millions.

Fig. 2.4 Mixed linkage f -glucan in the starchy endosperm cell wall of barley. The 1-3 linkages occur every third or fourth glucosyl, although there are 'cellulosic' regions wherein there are longer sequences of 1-4 linked glucosyls. ~ indicates that the chain continues in either direction - molecular weights of these glucans can be many millions.

Ferulic Acid Linkage
OH

Fig. 2.5 Pentosans in the walls of barley comprise a linear backbone of j 1 ^ 4 linked xylo-syl residues with arabinose attached through either a I ^ 2 or a1 ^ 3 bonding. Although not depicted here, the arabinose residues are variously esterified with either ferulic acid or acetic acid.

Fig. 2.5 Pentosans in the walls of barley comprise a linear backbone of j 1 ^ 4 linked xylo-syl residues with arabinose attached through either a I ^ 2 or a1 ^ 3 bonding. Although not depicted here, the arabinose residues are variously esterified with either ferulic acid or acetic acid.

Foams Proteins Lamella

■ Arabinoxylan E® p-Glucan ■ Protein-rich middle lamella

Fig. 2.6 Current understanding of the structure of the cell walls of barley endosperm. Walls surrounding adjacent cells are cemented by a protein-rich middle lamella. To this is attached arabinoxylan, within which is the j-glucan.

■ Arabinoxylan E® p-Glucan ■ Protein-rich middle lamella

Fig. 2.6 Current understanding of the structure of the cell walls of barley endosperm. Walls surrounding adjacent cells are cemented by a protein-rich middle lamella. To this is attached arabinoxylan, within which is the j-glucan.

are prone to drop out of solution as hazes, precipitates or gels. Conversely it has been claimed that j-glucans have positive health attributes for the human, by lowering cholesterol levels and contributing to dietary fibre.

The enzymic breakdown of j-glucan during the germination of barley and later in mashing is in two stages: solubilisation and hydrolysis. Several enzymes (collectively the activity is referred to by the trivial name 'solubilase') may be involved in releasing j -glucan from the cell wall, including esterases that hydrolyse ester bonds believed to cement polysaccharides, perhaps to the protein-rich middle lamella. The most recent evidence, however, is that the pentosan component encloses much of the glucan (Fig. 2.6), and accordingly pentosanases are efficient solubilases. This is despite the observations that pentosans are less digestible than glucans. j-Glucans are hydrolysed by endo-j-glucanases (endo enzymes hydrolyse bonds inside a polymeric molecule, releasing smaller units, which are subsequently broken down by exo enzymes that chop off one unit at a time, commencing at one end of the molecule). These enzymes convert viscous j-glucan molecules to non-viscous oligosaccharides comprising three or four glucose units. Less well-understood enzymes are responsible for converting these oligosaccharides to glucose. There is little if any j-glucanase in raw barley, it being developed during the germination phase of malting in response to gibberellins. Endo-j-glucanase is extremely sensitive to heat, meaning that it is essential that malt is kilned very carefully to conserve this enzyme if it is necessary that it should complete the task of glucan degradation in the brewhouse. This is especially important if the brewer is using j-glucan-rich adjuncts such as unmalted barley, flaked barley and roasted barley. It is also the reason why brewers often employ a low temperature start to their mashing processes. Alternatively, some brewers add exogenous heat-stable j-glucanases of microbial origin.

Wheat Starch Granules Cross Section
Fig. 2.7 The cross-sectional structure of a starch granule.

The starch in the cells of the starchy endosperm is in two forms: large granules (approximately 25 ^m) and small granules (5 ^m). The structure of granules is quite complex, having crystalline and amorphous regions (Fig. 2.7). I address starch later, in the context of mashing.

The proteins in the starchy endosperm may be classified according to their solubility characteristics. The two most relevant classes are the albumins (water-soluble, some 10-15% of the total) and the hordeins (alcohol-soluble, some 85-90% of the total). In the starchy endosperm of barley, the latter are quantitatively the most significant: they are the storage proteins. They need to be substantially degraded in order that the starch can be accessed and amino acids (which will be used by the yeast) generated. Their partial degradation products can also contribute to haze formation via cross-linking with polyphenols. Excessive proteolysis should not occur, however, as some partially degraded protein is required to afford stable foam to beer. Most of the proteolysis occurs during germination rather than subsequent mashing, probably because endogenous molecules that can inhibit the endo-proteinases are kept apart from these enzymes by compartmentalisation in the grain, but when the malt is milled, this disrupts the separation and the inhibitors can now exert their effect. There may be some ongoing protein extraction and precipitation during mashing, and peptides are converted into amino acids at this stage through the action of carboxypeptidases. The endo-peptidases are synthesised during germination in response to gibberellin and they are relatively heat-labile (like the endo-^-glucanases). Substantial carboxypeptidase is present in raw barley and it further increases to abundant levels during germination. It is a heat-resistant enzyme and is unlikely to be limiting. Thus, the extent of protein degradation is largely a function of the extent of proteinase activity during germination.

Much effort is devoted to breeding malting barleys that give high yields of 'extract' (i.e. fermentable material dissolved as wort). The hygiene status of the barley is also very important, and pesticide usage may be important to avoid the risk of infection from organisms such as Fusarium. Barleys may be

Three Row Six Row Barley Structure

Two-row Six-row

Fig. 2.8 Two-row and six-row ears of barley. Photograph courtesy of Dr Paul Schwarz.

Two-row Six-row

Fig. 2.8 Two-row and six-row ears of barley. Photograph courtesy of Dr Paul Schwarz.

two-row, in which only one kernel develops at each node on the ear and it appears as if there is one kernel on either side of the axis of the ear, or six-row in which there are three corns per node (Fig. 2.8). Obviously there is less room for the individual kernels in the latter case and they tend to be somewhat twisted and smaller and therefore less desirable. Farmers are restricted in how much nitrogenous fertiliser they can use because the grain will accumulate protein at the expense of starch in the endosperm, and it is the starch (ergo fermentable sugar) that is especially desirable. Maltsters pay a 'malting premium' for the right variety, grown to have the desired level of protein. There must be some protein present, as this is the fraction of the grain which includes the enzymes and which is the origin of amino acids (for yeast metabolism) and foam polypeptide. The amount of protein needed in malt will depend on whether the brewer intends to use some adjunct material as a substitute for malt. For example, corn syrup is a rich source of sugar but not of amino acids, which will need to come from the malt.

Dead grain will not germinate, so batches of barley must pass viability tests.

Most barley in the Northern Hemisphere is sown between January and April and is referred to as Spring Barley. The earlier the sowing, the better the yield and lower the protein levels because starch accumulates throughout the growing season. In locales with mild winters, some varieties (Winter Barleys) are sown in September and October. Best yields of grain are in locales where there is a cool, damp growing season allowing steady growth, and then fine, dry weather at harvest to ripen and dry the grain. Grain grown through very

Table 2.1 World production of barley (3-year average, 1998-2000).

Production

Percentage of world

Countries

(thousand tons)

production

World

132 393

_

Canada

13124

9.9

Germany

12 671

9.5

Russian Fed.

11222

8.5

France

10036

7.5

Spain

9 871

7.4

Turkey

7 533

5.6

USA

6908

5.2

UK

6 566

5.0

Ukraine

6389

4.8

Australia

5 372

4.1

hot, dry summers is thin, poorly filled and has high nitrogen. Malting barley is grown in many countries (Table 2.1).

Grain arrives at the maltings by road or rail and, as the transport waits, the barley will be weighed and a sample tested for viability, nitrogen content and moisture. Expert evaluation will also provide a view on how clean the sample is in terms of weed content and whether the grain 'smells sweet'. Once accepted, the barley will be cleaned and screened to remove small grain and dust, before passing into a silo, perhaps via a drying operation in areas with damp climates. Grain should be dry to counter infection and outgrowth.

It is essential that the barley store is protected from the elements, yet it must also be ventilated, because barley, like other cereals, is susceptible to various infections, for example, Fusarium, storage fungi such as Penicillium and Aspergillus, Mildew, and pests, for example, aphids and weevils.

Steeping is probably the most critical stage in malting. If homogeneous malt is to be obtained (which will go on to 'behave' predictably in the brewery), then the aim must be to hydrate the kernels in a batch of barley evenly. Steeping regimes are determined on a barley-by-barley basis by small-scale trials but most varieties need to be taken to 42-46%. Apart from water, barley needs oxygen in order to support respiration in the embryo and aleurone. Oxygen access is inhibited if grain is submerged for excessive periods in water, a phenomenon which directly led to the use of interrupted steeping operations. Rather than submerge barley in water and leave it, grain is steeped for a period of time, before removing the water for a so-called 'air-rest' period. Then a further steep is performed and so on. Air rests serve the additional purpose of removing carbon dioxide and ethanol, either of which will suppress respiration. A typical steeping regime may involve an initial steep to 32-38% moisture (lower for more water-sensitive barleys). The start of germination is prompted by an air rest of 10-20 h, followed by a second steep to raise the water content to 40-42%. Emergence of the root tip ('chitting') is encouraged by a second air rest of 10-15 h, before the final steep to the target moisture. The entire steeping operation may take 48-52 h.

Gibberellic acid (GA, itself produced in a commercial fermentation reaction from the fungus Gibberella) is added in some parts of the world to supplement the native gibberellins of the grain. Although some users of malt prohibit its use, GA can successfully accelerate the malting process. It is sprayed on to grain at levels between 0.1 and 0.5 ppm as it passes from the last steep to the germination vessel.

The hormones migrate to the aleurone to regulate enzyme synthesis, for the most part to promote the synthesis of enzymes that break down successively j -glucan, protein and starch. The gibberellin first reaches the aleurone nearest to the embryo and therefore, enzyme release is initially into the proximal endosperm. Breakdown of the endosperm ('modification'), therefore, passes in a band from proximal to distal regions of the grain.

Traditionally, steeped barley was spread out to a depth of up to 10 cm on the floors of long, low buildings and germinated for periods up to 10 days. Men would use rakes either to thin out the grain ('the piece') or pile it up depending on whether the batch needed its temperature lowered or raised: the aim was to maintain it at 13-16°C. Very few such floor maltings survive because of their labour intensity, and a diversity of pneumatic (mechanical) germination equipment is now used. Newer germination vessels are circular, made of steel or concrete, with capacities of as much as 500 tons and with turning machinery that is microprocessor-controlled. A modern malting plant is arranged in a tower format, with vessels vertically stacked, steeping tanks uppermost.

Germination in a pneumatic plant is generally at 16-20° C. Once the whole endosperm is readily squeezed out and if the shoot initials (the acrospire) are about three-quarters the length of the grain (the acrospire grows the length of the kernel between the testa and the aleurone and emerges from the husk at the distal end of the corn), then the 'green malt' is ready for kilning.

Through the controlled drying (kilning) of green malt, the maltster is able to

(1) arrest modification and render malt stable for storage;

(2) ensure survival of enzymes for mashing;

(3) introduce desirable flavour and colour characteristics and eliminate undesirable flavours.

Drying should commence at a relatively low temperature to ensure survival of the most heat-sensitive enzymes (enzymes are more resistant to heat when the moisture content is low). This is followed by a progressive increase of temperature to effect the flavour and colour changes (Maillard reaction) and complete drying within the limited turnaround time available (typically under 24 h). There is a great variety of kiln designs, but most modern ones feature deep beds of malt. They have a source of heat for warming incoming air, a fan to drive or pull the air through the bed, together with the necessary loading and stripping systems. The grain is supported on a wedge-wire floor that permits air to pass through the bed, which is likely to be up to 1.2m deep.

Newer kilns also use 'indirect firing', in that the products of fuel combustion do not pass through the grain bed, but are sent to exhaust, the air being warmed through a heater battery containing water as the conducting medium. Indirect firing arose because of concerns with the role of oxides of nitrogen present in kiln gases that might have promoted the formation of nitrosamines in malt. Nitrosamine levels are now seldom a problem in malt.

Lower temperatures will give malts of lighter colour and will tend to be employed in the production of malts destined for lager-style beers. Higher temperatures, apart from giving darker malts, also lead to a wholly different flavour spectrum. Lager malts give beers that are relatively rich in sulphur compounds, including DMS. Ale malts have more roast, nutty characters. For both lager and ale malts, kilning is sufficient to eliminate the unpleasant raw, grassy and beany characters associated with green malt.

When kilning is complete, the heat is switched off and the grain is allowed to cool before it is stripped from the kiln in a stream of air at ambient temperatures. On its way to steel or concrete hopper-bottomed storage silos, the malt is 'dressed' to remove dried rootlets, which go to animal feed.

Some malts are produced not for their enzyme content but rather for use by the brewer in relatively small quantities as a source of extra colour and distinct types of flavour. These roast malts may also be useful sources of natural antioxidant materials. There is much interest in these products for the opportunities they present for brewing new styles of beer.

Mashing: the production of sweet wort

Sweet wort is the sugary liquid that is extracted from malt (and other solid adjuncts used at this stage) through the processes of milling, mashing and wort separation. Larger breweries will have raw materials delivered in bulk (rail or road) with increasingly sophisticated unloading and transfer facilities as the size increases. Smaller breweries will have malt, etc. delivered by sack. Railcars may carry up to 80 tons of malt and a truck 20 tons. The conscientious brewer will check the delivery and the vehicle it came in for cleanliness and will representatively sample the bulk. The resultant sample will be inspected visually and smelled before unloading is permitted. Most breweries will spotcheck malt deliveries for key analytical parameters to enable them to monitor the quality of a supplier's material against the agreed contractual specification. Grist materials are stored in silos sized according to brewhouse throughput.

Milling

Before malt or other grains can be extracted, they must be milled. Fundamentally the more extensive the milling, the greater the potential there is to extract materials from the grain. However, in most systems for separating wort from spent grains after mashing, the husk is important as a filter medium.

The more intact the husk, the better the filtration. Therefore, milling must be a compromise between thoroughly grinding the endosperm while leaving the husk as intact as possible.

There are fundamentally two types of milling: dry milling and wet milling. In the former, mills may be either roll, disk or hammer. If wort separation is by a lauter tun (discussed later), then a roll mill is used. If a mash filter is installed, then a hammer (or disk) mill may be employed because the husk is much less important for wort separation by a mash filter. Wet milling, which was adopted from the corn starch process, was introduced into some brewing operations as an opportunity to minimise damage to the husk on milling. By making the husk 'soggy', it is rendered less likely to shatter than would a dry husk.

Mashing

Mashing is the process of mixing milled grist with heated water in order to digest the key components of the malt and generate wort containing all the necessary ingredients for the desired fermentation and aspects of beer quality. Most importantly it is the primary stage for the breakdown of starch.

The starch in the granules is very highly ordered, which tends to make the granules difficult to digest. When granules are heated (in the case of barley starch beyond 55-65°C), the molecular order in the granules is disrupted in a process called gelatinisation. Now that the interactions (even to the point of crystallinity) within the starch have been broken down, the starch molecules become susceptible to enzymic digestion. It is for the purpose of gelatinisation and subsequent enzymic digestion that the mashing process in brewing involves heating.

Although 80-90% of the granules in barley are small, they only account for 10-15% of the total weight of starch. The small granules are substantially degraded during the malting process, whereas degradation of the large granules is restricted to a degree of surface pitting. (This is important, as it is not in the interests of the brewer (or maltster) to have excessive loss of starch, which is needed as the source of sugar for fermentation.)

The starch in barley (as in other plants) is in two molecular forms (Fig. 2.9): amylose, which has very long linear chains of glucose units, and amylopectin, which comprises shorter chains of glucose units that are linked through side chains.

Several enzymes are required for the complete conversion of starch to glucose. «-Amylase, which is an endo enzyme, hydrolyses the a 1-4 bonds within amylose and amylopectin. i-Amylase, an exo enzyme, also hydrolyses a 1-4 bonds, but it approaches the substrate (either intact starch or the lower molecular weight 'dextrins' produced by a-amylase) from the non-reducing end, chopping off units of two glucoses (i.e. molecules of maltose). Limit dextrinase is the third key activity, attacking the a 1-6 side chains in amylopectin.

Limit Dextrinase
(b)
Malted Wheat Rootlets
Fig. 2.9 (Continued).
Fermentation MaltoseFood Fermentation
n= 12-20

Reducing end

B chain

A chain

Non-reducing ends

Fig. 2.9 (a) The basic structure of amylose. Not depicted is the fact that it assumes a helical structure. (b) The basic structure of amylopectin. The individual linear chains adopt a helical conformation. (c) The different types of chain in amylopectin. The different layers in the starch granule result from the ordering of these molecules, interacting with amylose.

«-Amylase develops during the germination phase of malting. It is extremely heat resistant, and also present in very high activity; therefore, it is capable of extensive attack, not only on the starch from malt but also on that from adjuncts added in quantities of 50% or more. j-Amylase is already present in the starchy endosperm of raw barley, in an inactive form through its association with protein Z. It is released during germination by the action of a protease (and perhaps a reducing agent). j -Amylase is considerably more heat-labile than «-amylase, and will be largely destroyed after 30-45 min of mashing at 65°C. Limit dextrinase is similarly heat sensitive. Furthermore, it is developed much later than the other two enzymes, and germination must be prolonged if high levels of this enzyme are to be developed. It is present in several forms (free and bound): the bound form is both synthesised and released during germination. Like the proteinases, there are endogenous inhibitors of limit dextrinase in grain, and this is probably the main factor which determines that some 20% of the starch in most brews is left in the wort as non-fermentable dextrins. Although it is possible to contrive operations that will allow greater conversion of starch to fermentable sugar, in practice, many brewers seeking a fully fermentable wort add a heat-resistant glucoamylase (e.g. from Aspergillus) to the mash (or fermenter). This enzyme has an exo action like j-amylase, but it chops off individual glucose units.

There are several types of mashing which can broadly be classified as infusion mashing, decoction mashing and temperature-programmed mashing.

Micro Brewery Mash Tun
Fig. 2.10 A mash tun.

Whichever type of mashing is employed, the vessels these days are almost exclusively fabricated from stainless steel (once they were copper). What stainless steel loses in heat transfer properties is made up for in its toughness and ability to be cleaned thoroughly by caustic and acidic detergents.

Irrespective of the mashing system, most mashing systems (apart from wet milling operations) incorporate a device for mixing the milled grist with water (which some brewers call 'liquor'). This device, the 'pre-masher', can be of various designs, the classic one being the Steel's masher, which was developed for the traditional infusion mash tun (Fig. 2.10).

Infusion mashing is relatively uncommon, but still championed by traditional brewers of ales. It was designed in England to deal with well-modified ale malts that did not require a low temperature start to mashing in order to deal with residual cell-wall material (fi -glucans). Grist is mixed with water (a typical ratio would be three parts solid to one part water) in a Steel's masher en route to the preheated mash tun, with a single holding temperature, typically 65°C, being employed. This temperature facilitates gelatinisation of starch and subsequent amylolytic action. At the completion of this 'conversion', wort is separated from the spent grains in the same vessel, which incorporates a false bottom and facility to regulate the hydrostatic pressure across the grains bed. The grist is sparged to enable leaching of as much extract as possible from the bed.

Decoction mashing was designed on the mainland continent of Europe to deal with lager malts which were less well-modified than ale malts. Essentially it provides the facility to start mashing at a relatively low temperature, thereby allowing hydrolysis of the fi -glucans present in the malt, followed by raising the temperature to a level sufficient to allow gelatinisation of starch and its subsequent enzymic hydrolysis. The manner by which the temperature increase was achieved was by transferring a portion of the initial mash to a

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Brew Your Own Beer

Brew Your Own Beer

Discover How To Become Your Own Brew Master, With Brew Your Own Beer. It takes more than a recipe to make a great beer. Just using the right ingredients doesn't mean your beer will taste like it was meant to. Most of the time it’s the way a beer is made and served that makes it either an exceptional beer or one that gets dumped into the nearest flower pot.

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Responses

  • CYNTHIA
    What is a barley acrospire?
    6 years ago
  • kimi
    What conditions are used in breweries to ensure that fermentation occurs efficiently?
    5 years ago

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