Malts, adjuncts and supplementary enzymes

2.1 Grists and other sources of extract

The sources of extract used in brewing are materials used in the mash and materials dissolved during the hop-boil (Chapter 1). In addition, small amounts of sugars may be added to beers as primings or for sweetening. Caramels, coloured malt extracts and Farbebier may also be added to adjust colours. Supplementary enzymes, derived from non-malt sources, may be added to the mash or at later stages of beer production. Malt is the traditional source of enzymes and the extract produced in mashing (Chapters 1 and 4). The contents of this chapter are discussed in more detail elsewhere (Briggs, 1998; Brissart et al., 2000).

2.2 Malting

2.2.1 Malting in outline

Barley (Hordeum vulgare) is the cereal grain most often malted. Wheat (Triticum aestivum) and sorghum (Sorghum vulgare) are also malted in notable quantities (the latter in Africa), but small amounts of rye (Secale cereale), oats (Avena sativum) and millets (various spp.) are also used. The barley grain or corn has a complex structure (Briggs, 1978, 1998, Figs 2.1 and 2.2), and is a single-seeded fruit (a caryopsis). Barley varieties differ in their suitabilities for malting. Barley plants are annual grasses. Some are planted in the autumn (winter barleys) while others are planted in the spring (spring barleys). Grains are arranged in rows, borne on the head, or ear. The number of rows varies, being two in two-rowed varieties and six in six-rowed forms. In mainland Europe winter barleys are usually of poor malting quality, but some of the two-rowed winter varieties grown in the UK (such as Maris Otter, Halcyon and Pearl) are of outstandingly good quality. Good spring malting barleys include Alexis, Chariot, Optic and Prisma. Grains vary in size, shape and chemical composition.

It is important to understand that malts consist of mixtures of grains with differing properties. This heterogeneity, which is reflected in the malt, can give rise to problems in brewing. Barley dimensions vary, usually in the ranges: lengths, 6-12 mm, 0.24- 0.47 in.; widths, 2.7-5.0 mm, 0.11-0.20 in.; thicknesses, 1.8-4.5 mm, 0.07-0.18 in. Two-rowed malting barley grains may have one thousand corn dry weights (TCW) in the range 32-44 g, and some six-rowed barleys have values of about 30 g. Differences between grain sizes must be allowed for when setting brewer's mills. The barley corn is elongated and tapers at the ends (Figs 2.1, 2.2). The dorsal, or rounded side is covered by the lemma, while the ventral, grooved or furrow side is covered by the palea. Together these units constitute the husk. The lemma has five longitudinal ridges, or 'veins' running along it while the palea has two. In threshed grain the apical tip of the lemma is crudely broken off. In the unthreshed grain this is where the extended awn is attached. At the base of the grain, where it was attached to the plant, the rachilla, or basal bristle, lies in the ventral furrow. Rachillae vary greatly in their shapes and sizes, and are of use in helping to identify grain variety. The husk protects the grain from physical damage. In wheat, rye, sorghum and millets (and in some few 'naked' barleys, which are not malted) husks are absent in threshed grain, so the corns are easily damaged.

Within the husk the multi-layered pericarp also has a protective function. Finally, the testa is the layer that 'seals' the interior of the grain from the exterior and limits the inward and outward movements of dissolved substances, such as sugars, amino acids, salts and proteins. This layer invests the entire interior of the grain except at the embryo, where its structure is modified in the micropylar region, and in the furrow, where the two edges are sealed together by the pigment strand. The testa consists of two cuticularized layers between which polyphenolic proanthocyanidins usually occur. At the base of the grain, over the embryo and between the pericarp and the husk, there are two small, hairy structures, the lodicules. During steeping these may distribute water over the embryo, by capillarity. Their varied forms make them valuable aids in identifying a grain's variety.

Within the testa, at the base of the grain, is the small embryo. This is situated towards the dorsal side of the grain. The embryonic axis consists of the coleoptile (the maltster's 'acrospire') pointing towards the apex of the grain and the root sheath (coleorhiza) which surrounds several (typically five) embryonic roots. This appears at the end of the grain, at the onset of germination, as the 'chit'. The axis is the part of the embryo that can grow into a small plant. It is recessed into an expanded part of the embryo called the scutellum (Latin, 'little shield'). Unlike the scutellum in oats, in barley this organ does not grow. Its inner surface, which is faced with a specialized epithelial layer, is pressed against the largest tissue of the grain, the starchy endosperm. With the exception of the embryo all the tissues mentioned so far are dead. All the surface structures, outside the testa, are infested with mixed populations of micro-organisms.

The starchy endosperm is a dead tissue of thin-walled cells packed with starch granules embedded in a protein matrix. The granules occur in two size ranges (usually with diameters 1.7-2.5 µm and 22.5-47.5 µm), which behave differently during malting and brewing. The cell walls are mainly ß-glucans, with some pentosans and a little holocellulose. This tissue contains most of the grain's reserves, although others are present in the embryo and in the aleurone layer. In transverse section the cell walls radiate outwards from a 'crest' of sheaf cells that run along the grain, above the pigment strand. These sheaf cells are devoid of contents and consist of cell walls pressed together, at least in the dry grain. They are not part of the endosperm tissue, the cell walls of which are more readily degraded by enzymes (Briggs, 2002). The outer region of the starchy endosperm, the sub-aleurone layer, is relatively richer in protein (including ß-amylase) and small starch granules but poor in large starch granules. Where the starchy endosperm fits against the scutellum the cells are devoid of contents and the cell walls are pressed together, comprising the crushed-cell or depleted layer. The starchy endosperm, away from the sheaf cells, is surrounded by the aleurone layer (which botanically is also endosperm tissue). On average it is about three cells thick. The cells are alive but do not multiply or grow during germination, have thick cell walls and contain reserves of lipids (fat) and protein, sucrose and possibly fructosans, as well as a full range of functional organelles. They do not contain any starch. A reduced layer of aleurone tissue, a single layer of flattened cells, extends partly over the surface of the embryo. The estimates are approximate, but on a dry weight basis (d.b.) a two-rowed barley corn may consist of husk + pericarp + lodicules, 9-14%; testa, 1-3%; embryo, 2-3.5%; aleurone layer, about 5%; starchy endosperm + sheaf cells, 76-82%. Malting can be understood only by reference to the grain structure and the interactions which occur between the tissues.

Barley is purchased in large amounts. The grain delivered must be of the correct quality, i.e., it must match or exceed in quality a sample seen in advance or an agreed specification. The evaluation of the grain involves both visual and laboratory assessments. Each delivery should be checked before it is unloaded. Delivery may be by railway, barge or (most usually in the UK) by lorry. The grain will be uncovered and inspected for infesting insects, local wetting, admixture of varieties, the presence of ergot sclerotia (poisonous, grain-sized structures produced by the fungus Claviceps purpurea), or any sign of heavy fungal attack. If any of these faults is noted the load is likely to be rejected and, if insects are present, the load will be ordered off the premises. With the exception of varieties with blue-pigmented aleurone layers, (which appear greenish as the blue is viewed through the yellow husk), grain should appear 'bright', with a clean straw-yellow colour. Discoloration is caused by heavy microbial contamination.

Samples of the grain bulk are drawn and sent to the laboratory. The moisture content will be determined. In the UK the grain will be inspected to check that it is predominantly (e.g., > 97%) of one specified variety, that its viability or germinative capacity (GC; checked by tetrazolium staining) is equal to or exceeds the specified limit (at least 98%) and that the total nitrogen content (TN) or crude protein content (6.25 × TN) is within specified limits. Grain moisture and nitrogen contents are usually checked using near-infra-red spectroscopy (NIR), but slower methods may be used. The grain will also be checked for 'pre-germination', since grain that has already...