The brewing process

The major stages that together comprise the brewing process for a European-style beer made by a traditional batch process are shown in Fig. 2.2. The process consists of three phases: wort manufacture, fermentation and post-fermentation processing. Each of these phases contains several distinct steps. The precise detail of each step depends, to some extent, on the nature of the beer being made and the plant used. However, there are common themes and these are described in this section.

The raw materials for wort production are water, malt, hops and, possibly, adjuncts. In addition, other materials may be used which assist in the process of wort preparation. The essential features of these ingredients are discussed in following sections. These are termed process aids and are distinguished from raw materials in that they may not persist into the finished beverage.

Barley

Malting JH,

Malt

Milling

Grist

Mashing JH^

Mash

Wort separation

Sweet wort Boiling JH,

Bitter wort

Fermentation

Green beer Conditioning ^

Mature beer Packaging ^

Packaged beer

Fig. 2.2 Major steps in the brewing process.

2.3.1 Malting

Several cereals may be malted; however, the description given here is the process applied to barley. In the malting process the barley grain is encouraged to undergo controlled germination (Briggs, 1987). This is initiated by wetting the grains, known as steeping. In the subsequent germination phase enzyme systems are activated as the barley grain begins to mobilise starch reserves to provide carbon and energy for development of the embryo. At an appropriate point the germination process is arrested by application of heat, termed kilning. This stabilises the grain such that in malt the relevant enzymes and reserve materials are available for subsequent extraction and further degradation to release fermentable sugars during wort production.

Particular varieties of barley (Hordeum distichon and H. vulgare) are cultivated specifically for malting. Key properties are an ability to undergo even germination within a given time period, possession of disease resistance and, most importantly, formation of a large grain containing an appropriate balance of starch and nitrogen. In general, a low nitrogen content is preferred.

A cross-sectional diagram of a barley grain is shown in Fig. 2.3. The endosperm consists of a protein mesh in which starch grains, both large and small, are embedded. Starch accounts for 55-65% of the total grain weight. Some 75-80% of the starch is in the form of the branched polymer, amylopectin (D-glucose, a-(l->4) and a-(l->6) linkages) and 20-25% amylose (D-glucose, a-(l->4) linkages only). Important protein components of the barley grain include globulins, albumins, hordein and glutelin. The first two of these are mainly enzyme proteins, the others are structural and degraded during malting. The relative proportions of starch and protein influence the appearance of the endosperm. Thus, it is described as 'steely' or 'floury' where protein

Endosperm

Endosperm

Obesumbacterium Proteus

Testa

Fig. 2.3 Diagrammatic representation of a cross-section through a barley grain.

Testa

Fig. 2.3 Diagrammatic representation of a cross-section through a barley grain.

or starch levels are high, respectively. Utilisation of the starchy and nitrogenous components of the endosperm is catalysed by amylases and proteases, which are located in the aleurone layer of the grain.

The barley grain contains many other components that contribute to wort composition including sucrose, vitamins, minerals, polyphenols, nucleotides and lipids. Some components have the potential to produce problems during wort production. For example, malting barley endosperm contains approximately 4% (3-glucans. These must be degraded during malting, since barley (3-glucanases are heat labile and are inactivated during wort mashing. Failure to degrade (3-glucans results in high viscosity worts, which create problems in run-off. Barley varieties containing large quantities of (3-glucans are unsuitable for brewing.

Steeping is managed in a variety of ways depending on the sophistication of the maltings. It is common practice to soak the barley grains in water with periods of exposure to air. The steep water may contain a biocide (see Section 8.2.1.1) to minimise surface microbial growth. During steeping the water content of the barley grains increases to 42^15% and this initiates germination, termed chitting. Steep water, which is removed from the barley grains, contains some tannins, which can impart bitter taints in beers. Exposure of the embryo to moisture stimulates the formation of the plant hormone gibberellic acid. This is transported to the aleurone layer, where it stimulates activation of endosperm-degrading enzymes. Occasionally exogenous gibberellic acid may be added to the grains to encourage more rapid germination, although this is now relatively uncommon.

During the germination stage the growing barley is held as a bed which is slowly turned over to keep each grain separate and facilitate even germination. In traditional floor maltings this is a manual process, whereas in modern facilities it is automated. A high humidity is maintained and a temperature between 13 and 16°C. Cooling is required since germination is exothermic. As germination proceeds endo-glucanases, pentosanases, endoproteases and amylases are released from the aleurone layer and slowly perfuse into and degrade the cell structure of the endosperm, thereby releasing starch grains. This process is termed modification and a well-modified malt would be one in which degradation was extensive. In germination 75% of the (3-glucan and 40% of the protein within the endosperm is solubilised. Surprisingly, only 10% of the starch is degraded, leaving the bulk entire for extraction during wort production.

Germination is arrested by drying the green malt in a process termed 'kilning'. Drying is gentle in order to prevent inactivation of malt enzymes required during wort formation. Typically, temperatures are increased slowly from between 25 and 30°C up to between 60 and 70°C. During this period air is blown through the malt bed to facilitate removal of moisture and the water content of the malt is gradually reduced to approximately 4%. Kilning serves several functions. Notably it renders the malt into a stable form in which it may be stored for long periods and also reduces the surface microbial loading. Some flavour and colour reactions occur which contribute to beer quality. The latter reactions are characteristic of certain malts, particularly those used for ales, in which there may be a final high temperature heat treatment known as curing. A disadvantage of such malts is that considerable enzyme inactivation occurs and there is now a trend towards using paler malts supplemented with other sources of colour and flavour added later in the wort manufacturing process.

Malt is stored in 'silos' (Fig. 2.4), both in the makings and at the brewery. On malt intake, malt dust is distributed into the air and may well be a prime source of brewery specific micro-organisms (see Section 8.1.4.2).

Fig. 2.4 Malt silo (kindly provided by Richard Webster. Bass Brewers).

2.3.2 Adjuncts

Adjuncts, or secondary brewing agents, are defined as any source of fermentable sugar that is used in the brewing process and is not derived directly from malted barley. They are widely used in all countries, although the German beer purity law, the Reinheitsgebot, prohibits their use in beers for domestic consumption. Indeed in Germany, adjuncts are taken to mean any addition to the brewing process other than water, malt, hops and yeast (Stewart, 1995).

Two groups of adjunct are used, solid and liquid. Solid adjuncts require some processing and are usually incorporated into the mash. Liquid adjuncts can be used with no treatment and are added to the copper. In addition, liquid adjuncts are used post-primary fermentation in the form of'primings' to provide fermentable sugar for secondary fermentations and as a means of adjusting beer sweetness.

Adjuncts may be used simply as sources of fermentable sugars, which are less expensive than malt, although this is not usually the sole motive. Thus, they are used for their contribution to beer colour and flavour and they may be added to improve beer foam performance. Liquid adjuncts added to the copper serve as a valuable means of improving the productivity of the brewhouse. This is of particular importance in high-gravity brewing (see Section 2.5) where the wort sugar concentration may be conveniently increased by addition of liquid syrups. Perhaps most importantly, they provide the brewer with a positive method of controlling the gross composition of wort. For example, influencing the ratio of carbon to nitrogen as a means of producing beers with particular flavours.

Although individual adjuncts may be less expensive alternatives to malt, other costs are associated with their handling. For example, liquid syrup adjuncts require storage tanks, which may need to be heated to maintain a manageable viscosity. In addition, storage must be hygienic to prevent microbial spoilage. Although many solid adjuncts by-pass the malting stage of brewing, they may require specific plant for extracting fermentable sugars.

Commonly used solid adjuncts are barley, wheat, maize, rice and triticale. These come in many forms to facilitate extraction of sugars. For example, in the form of dried rolled flakes, as a milled grain, in micronised or torrefied form (product of rapid heating of cereal grains), as a flour (brewer's wheat flour) and as 'grits'. The latter category is the commonly used adjunct forms of rice and maize, particularly popular in the United States. Grits are by-products of processing of the cereals and consist mainly of endosperm. As discussed subsequently, a key part of the mashing process is to subject malt and other cereal sources of extract to a heat-step during which starch grains are gelatinised and therefore made accessible to enzymic degradation. In a typical mashing regime this is achieved within a temperature range of 63 to 65°C. However, the starch from maize and rice grits requires a higher temperature to gelatinise starch (65 to 75°C) and where these adjuncts are used it is necessary to use a separate cooker to provide a high temperature pre-treatment prior to addition to the main mash.

Liquid adjuncts come in the form of various syrups, which are characterised by their concentration, purity, colour and sugar spectrum. Thus, they may range from an almost pure glucose syrup, which is entirely fermentable, through to a crude hydro-

lysate of a cereal starch, which has been subject to treatment with various amylases. In between are a whole range of syrups, which may contain a mixture of fermentable sugars and dextrins. Less pure cereal extracts, particularly those derived from barley and wheat, may contain significant concentrations of nitrogen. Clearly the use of these will influence yeast nutrition and by implication the formation of ethanol and flavour metabolites during fermentation, in a totally different way to pure sugar adjuncts. The choice depends upon cost and the desired application. Caramels are a specific group of liquid adjuncts that are prepared by subjecting sugar solutions to a high temperature heating process. Caramélisation is promoted by aluminium ions which act as catalysts. These are used mainly for colour and flavour adjustment and now frequently in preference to dark roasted malts. The latter group of speciality malts, which are used principally as a source of colour and flavour, as opposed to extract, may also be considered as being adjuncts.

2.3.3 Brewing water

The ionic composition of water used for wort preparation exerts a crucial influence on the successful outcome of the brewing process. For optimum process performance and in order to maintain the highest product quality, brewing of different beer styles requires water (liquor) with particular ionic spectra. This fact has long been recognised by empirical observation even though the underlying science was not understood. Thus, availability of appropriate local water supplies accounts for the rise to prominence of the more famous centres of brewing excellence. For example, the reputations of towns and cities such as Pilsen, Dublin, Burton-upon-Trent, Dortmund and Munich are largely based on the qualities of the local water. Pilsen has very soft water with less than 10 ppm each of calcium, magnesium and sulphate and 10-20 ppm of bicarbonate (note: ppm or parts per million is equivalent to mg 1 *). Burton-upon-Trent well water is high in permanent hardness. It contains 250-300 ppm calcium and bicarbonate, more than 600 ppm sulphate and 60-70 ppm magnesium. Munich water is intermediate with 70-80 ppm calcium, 10-20 ppm sulphate and magnesium together with 150 ppm bicarbonate.

For many breweries, particularly those using a traditional process the composition of local water supplies continues to be of importance as it is used without modification to ionic composition. However, in many modern plants this has become an irrelevance since it is usual to de- mineralise all brewing liquor and then add back salts to achieve a desired ionic composition. This allows a single brewery to produce water appropriate for any beer style. It follows that it is advantageous if the local supply of water is soft since de-ionisation requirements are small. This is especially so since in any case most water is used for non-brewing purposes and has to be softened for most applications to avoid corrosion of plant.

The ions in brewing water have vital roles in the various stages involved in wort formation; they contribute to yeast nutrition, they influence yeast technological properties and have an impact on beer flavour. Key ions must be present in sufficient concentrations to exert positive effects but not be too high to cause inhibition or impart undesirable flavours. More importantly key ions must be present in balanced quantities. The sum of the effects on the brewing process and beer quality of the ionic composition of brewing liquor are complex since many synergistic and antagonistic interactions are possible. Some of the more notable effects are summarised in Table 2.2. With regard to wort production the most significant requirement is regulation of pH, especially during mashing and to a lesser extent during the copper boil. It is essential to maintain a low pH for efficient starch breakdown and proteolysis during mashing. Wort buffering capacity is provided by phosphates derived from malt. The pH is reduced by interactions between calcium ions and phosphates and with proteins and polypeptides which contain glutamate and aspartate in reactions which liberate protons. Taylor (1990) argued that interactions between calcium and amino groups was likely to be of greatest importance to control of wort pH since phosphate is a relatively poor buffer at wort pH values (pH 5.0-5.3).

Table 2.2 Effects of the ionic composition of water on the brewing process.

Ion Effect

Ammonium Indicative of contamination with decomposed organic material.

Calcium One of the most significant ions with multiple effects. Interacts with phosphate and proteins to reduce pH of mash and promote formation of a bright wort with good runoff. Precipitates oxalate from wort, which can cause hazes and gushing and hazes in beer. Activates a-amylase and proteases in mash. Promotes flocculation of yeast at the end of fermentation. Inhibits extraction of hop resins at high concentrations. Has a bitter astringent taste.

Copper Toxic to brewing yeast at high concentrations. Eliminates H2S from beer as insoluble sulphide.

Iron Toxic to yeast. Can produce hazes and adverse colour changes in beer.

Magnesium Reduces wort pH by interaction with phosphates but less important than calcium.

Important co-factor for many enzymes especially those catalysing dissimilation of pyruvate during fermentation. Essential component of many enzymes involving ATP.

Manganese Co-factor of many yeast and malt enzymes.

Potassium Imparts saline taste to beer.

Sodium In combination with chloride contributes to beer sweetness.

Zinc Inhibitory to yeast growth at high concentration (> 1 ppm) but stimulates fermentation at lower concentrations (0.1-0.3 ppm).

Bicarbonate At high concentrations (> 100 ppm) causes increase of mash pH and concomitant reduction in extract formation.

Chloride Inhibits fermentation at high concentrations (> 600 ppm). Contributes to fullness at low concentration but imparts saline taste at concentrations above 400 ppm.

Nitrate Precursor to nitrosamine formation by Obesumbacterium proteus

Phosphates Interacts with calcium and magnesium to reduce wort pH during mashing. Essential nutrient for yeast growth.

Sulphate Precursor for sulphur-containing amino acid synthesis by yeast in worts with low amino acid content. Precursor for sulphite formation by yeast which improves beer flavour stability. Precursor for sulphide formation by yeast.

The beneficial effects of calcium ions may be defrayed by carbonate. The formation of bicarbonate from carbonate, under acidic conditions in the mash, removes protons and increases wort pH. Two protons are removed for each carbonate ion so a relatively small concentration has the potential to produce a large effect on pH. It is essential, therefore, that temporary hardness is removed from brewing water.

Brewing water must meet standards of purity with respect to contamination, both microbial and chemical. Where municipal water supplies are used, standards are guaranteed by legislation. Since the supplier reserves the right to change the source of water without notice, considerable and abrupt changes in ionic composition are possible. Where brewery borehole water is employed, much less variation in ionic composition would be expected; however, pollution of ground waters can be a problem. To avoid this, where necessary, brewing water should be treated to remove organic contaminants and occasionally treated to reduce bacterial loading (see Section 8.2.1.1).

2.3.4 Hops

The hop plant (Humulus lupulus L.) is a member of the family Cannabinaceae that grows in temperate regions of the world. It is used in brewing to impart both bitterness and floral character. The flavour-active components of hops are resins and essential oils, which are produced in the lupulin glands found at the base seed-bearing bractioles of cones of the female plant. The cones are harvested and it is these that are referred to as hops in the brewing industry. In the traditional process the harvested and separated hop cones were dried in oast houses. Now more usually they are dried in a kiln to a moisture content of no more than 12%. The dried hop cones are packaged in bales, or in the United Kingdom in elongated sacks, termed pockets, for delivery to the brewery.

The bitter character of hops is due to alpha acids, also known as humulones. The three most prevalent alpha acids are humulone, cohumulone and adlupulone (Fig. 2.5). The alpha acid content is a characteristic of particular hop varieties and varies

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