Malting Technologies

Barley or other grain may be delivered to a malting factory, or maltings, by road, rail, or barge (1-4,6-10). Before a load is accepted, each batch of grain will be checked, usually for viability, varietal purity, freedom from insect pests, moldy grains (which may lead to poorly flavored malt and carry mycotoxins and agents that cause beers to gush, or overfoam) (1,5), freedom from physical damage and premature sprouting, the appropriate protein (N X 6.25) content, moisture content, and screenings (thin grains). If the load is substandard it may be rejected or accepted at a reduced price. The grain may be moved pneumatically, by drag-flight or helical screw conveyors, or by bucket elevators. The grain will quickly be precleaned, by sieving and aspiration, to remove most of the coarse and fine impurities and will be passed over magnets to remove fragments of iron and steel. If necessary the grain will later be dried to less than 12% moisture for prolonged storage. It will also be thoroughly cleaned to remove grains of other cereals, dust, weed seeds, small stones, and broken grains. The grain may also be graded, by sieving, to separate it into width classes, which may be malted separately. Usually the thinnest grains, or screenings, will be rejected and used for animal feed, but occasionally they are made into low-extract, enzyme-rich malt. The grain will be stored, at least until any dormancy has declined, under conditions that prevent its deterioration and the development of pest infestations.

The malting process itself may be carried out in many different sorts of plants. Old-fashioned floor maltings are still in limited use for making small quantities of specialist malts. Most malt is made in pneumatic maltings, which may operate in batch, continuous, or semicontinuous fashions. Most malting is carried out in batches, and steeping, germination, and kilning operations are carried out in different vessels. However, vessels are in use that combine steeping and germination (SGV), germination, kilning, and steeping (SGKV), and germination and kilning (GKV). Modern steeps are either cylindrical with conical bottoms or circular with flat bases. The latter have perforated false floors on which the grain rests, and they have mechanical leveling and unloading devices. Each is equipped to add and remove water, to blow compressed air into the base of the steep when the grain is underwater, and to suck air down through the vessel for C02-extraction between immersions. Sometimes provision is made to wash the grain as it is conveyed to the steep. When the grain is sufficiently moist (e.g. 44-46% fresh weight for many barley brewing malts and up to 50% for some distilling malts) it is drained and transferred to a germination compartment. Where their use is allowed, additives such as gibberellic acid and (though rarely) a bromate salt may be applied to the grain during the transfer by spraying on a solution (1,4). Gibberellic acid supplements the grains' own gibberellins and accelerates the formation of many enzymes, thus speeding the rate of modification. Bromates reduce rootlet growth, respiration, and therefore total malting losses (e.g., to 6% dry basis) and the levels of soluble protein released when the treated malt is mashed. Treated malts contain traces of bromide ions.

Traditionally, barley is germinated cool (10-14°C; 50-57°F) on smooth tiled or concrete floors. It used to be raked or turned by workers with shovels to control the temperature and prevent the rootlets from matting together. These processes are now partly mechanized (1,4). If the batch of barley (piece) becomes too warm, it is spread more thinly, for example, to 3 in., to allow it to cool. Conversely, if it is too cool, it is piled thicker, to 6 in. or more, to retain the heat. In modern plants grain temperatures and ventilation are achieved pneumatically, that is, by passing a variable flow of temperature-controlled and water-saturated air through a comparatively thick bed of grain (2 ft in the older plants, up to 8 ft in modern plants; about 4 ft is the present average). Maximum airflow rates may be 0.14 to 0.2 m3/ ton of barley steeped/s (11). Mechanical devices are used, about three times per day, to lighten the bed, to turn it, and to prevent the rootlets from matting. Various temperature regimes are used depending on the grain and the type of malt being produced, usually in the range of 13 to 18°C (54.4-64.0°F). In drums, horizontal metal cylinders, turning is achieved by intermittent rotation about the long axis. For most purposes drum batch sizes (average about 50 tons) are uneconomically small. Most malt is germinated in compartment vessels, usually of up to 300 tons capacity (of original barley). However, one Scottish combination steeping, germination, and kilning unit has a batch size of 520 tons of barley. Most compartments, like the early Saladin boxes, are rectangular, but newer plants are usually circular. In compartments grain is supported on a perforated deck through which the fan-driven stream of cooled and humidified air passes. Turning is achieved by a row of vertical, rotating helical screws, or augers, that move slowly through the grain mass, lifting it and separating the roots. In rectangular boxes the turners are supported from a carriage that moves slowly over the bed of grain. With circular compartments the set of turners, mounted below their support, may rotate around the center of the static bed of grain, or the bed of grain may be rotated and thus moved through a static row of turners.

When it is ready the green malt is usually transferred to a kiln, where it will be dried and slightly cooked in a stream of warm air. The temperatures used depend on the type of malt being made (1-4,6-11). For pale malts the initial air-on temperature may be 50 to 60°C (122-140°F), rising to 80 to 105°C (176-221°F). The temperatures of the grain and the air leaving the grain bed are less as the air is cooled by evaporating moisture from the malt. The grain is supported on a perforated deck, and the air moves through it from below. Kilns may be rectangular or circular in cross-section and may be of single-, double-, or, rarely, triple-deck construction. In modern kilns the grain beds may be 2 to 6 ft deep, and outside North America they are not turned. In double kilns the wetter batch, on the top deck, receives some air from the earlier, drier batch of malt below. This arrangement saves heat, and a similar effect is achieved by linking kilns with air ducts (1,4). Usually air-to-air heat exchangers or other devices are used to abstract heat from the warm and wet outgoing kiln air and convey it to the dry-air inlet, saving heat and reducing fuel usage. Sometimes, toward the end of the curing period when the malt is nearly dry, a large proportion of the hot air is recirculated, also to save heat. Kilns may be directly fired, that is, furnace gasses pass directly through the bed of malt. Alternatively, the furnace heats a heat exchanger that in turn heats the air. With such indirect heating the furnace gasses do not come into contact with the malt. Sometimes sulphur dioxide is added to the kiln airstream to reduce the color of the malt, to make the malt more acid so that the pH of the mashed malt is reduced, to increase the levels of soluble nitrogenous substances in the wort, and to prevent the formation of carcinogenic nitrosamines on the surfaces of the malt grains. The quantities of oxides of nitrogen that reach the malt during kilning are also limited, to minimize the formation of nitrosamines. Malt for making Scotch whiskey is flavored with peat smoke, applied during kilning, and rauchmalz, used in the making of some special German beers, is flavored with hardwood smoke (1-3,8,9). At the end of kilning the malt is cooled and dressed—that is, the sprouts or rootlets, which are now brittle, are broken by agitation, and they and dust are removed by sieving and aspiration. These, together with thin grains and grain dust, are used for animal feed, often after being pelletized. Some special malts are finished in roasting drums, commonly of 0.5-5 tons capacity (1,8). Brown, chocolate, amber, and black malts are made from plump, lightly kilned pale malts. In the United Kingdom roasted barley is regarded as a special malt. It imparts a sharper, drier flavor than true roasted malts. The malt is loaded into a horizontal metal cylinder that rotates around its long axis and, for this purpose, is heated from the outside. The temperature is carefully raised until the correct coloration is achieved. For making black malts, when the highest temperatures are used, heating begins at about 75°C (167°F) and the temperature is raised to about 225°C (437°F). The products are quickly cooled, to fix their composition and reduce the risk of fire. They should not be charred. Any substantial amount of charcoal in these products makes them worthless. They deteriorate on storage and therefore are used as soon as possible. During roasting objectionable fumes are formed, and these must be destroyed, for example, by being consumed in an afterburner.

Crystal and caramel malts are prepared differently. Wet malt, either green malt or a pale malt that has been re-wetted, is warmed in a closed cylinder, with no drying allowed, to 65 to 70°C (149-158°F). At this temperature the endosperms of the grains are mashed and the contents liquefy to a sugary mass. When all the grains have liquid contents, hot air is passed through the drums to dry the grains and to develop the desired color. At lower temperatures pale, luscious-flavored products are made, whereas at higher temperatures progressively darker and more strongly flavored materials are produced. When the malts are cooled the endosperm contents set solid and become hard, with a brittle, barley sugar-like texture. These are the only dry malts that should not be friable.

When malts are stored, care is taken to ensure that they are not mixed with unmalted grain; do not become damp; and do not become infested with insects, rodents, or other pests. Normally malt is transported in bulk, but for some purposes it is stored and moved in laminated, moisture-proof sacks of up to a 1-ton capacity.

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