Overview of Malting Technology
Many malting plants are used, but only the most common types will be described. Malting technology is comparatively safe, provided that certain precautions are observed. Some sometimes unfamiliar, risks are due to carbon dioxide, grain, and malt dust. As grain is steeped and germinated, it liberates carbon dioxide. This heavy gas can pool, so checking that vessels and confined spaces are ventilated before they are entered is essential.
Dust must be confined and cleaned away not only because it becomes damp and a breeding ground for insects and microbes but also because, when breathed, it can cause allergies and fungal lung infections, and it can form explosive mixtures when mixed with air. All handling equipment must be earthed (grounded) to prevent sparks, which might trigger an explosion, and all conveyors, ducts, etc., should have explosion vents.
Modern malt factories process large batches of grain (often 200t/300t batches), and the process stages are highly automated so that processing conditions are reproducible and the workforce needed to produce each tonne of malt is minimized. In large maltings, grain is delivered in bulk by ship, barge, train, or lorry. Before unloading begins, the bulk should be inspected and sampled for analysis. When the quality has been agreed upon, unloading begins. Grain is usually sucked from the holds of vessels, and this pneumatic system may be used to empty rail wagons or lorries, or there may be emptied under gravity. Lorries usually unload into an intake pit by tipping or from a hopper. The grain runs into the pit, is ventilated to remove dust, and is equipped with a coarse moving screen (sieve) to catch and remove coarse impurities such as straw and large stones. Each lorry is weighed onto the site and off after unloading. The difference in weights gives the amount of grain unloaded.
During malting, the grain will be moved several times. The equipment used varies but usually includes bucket elevators, helical screw conveyors (worms), belt conveyors, and chain and flight conveyors. Less usually, the grain will be moved using pneumatic conveyors. It is highly desirable that to avoid cross-contamination, the equipment used to move grain is entirely separate from that used to move malt. The freshly delivered barley is conveyed to a “green grain” bin for temporary storage. Here it will remain, usually ventilated with fresh air, until it can be precleaned and dried if necessary. Precleaning involves rapid screening to remove gross impurities, such as sand, straw, stones, and string, which are either appreciably larger or smaller than the grains, and aspiration with air to remove dust. The dust from this and other locations is trapped in cyclones and textile-sleeve filters. The grain also passes over magnetic separators, which retain iron and steel impurities.
In Northern Europe, the grain must be dried (to 12% moisture or less) before it can be safely stored. Drying and pre-cleaning may be carried out before the grain is delivered, but because of the risk of heat damage caused by inexpert drying, some maltsters do not allow this. The drying temperatures are lower for more moist grain because wetter grain is more easily damaged by heat. Batch drying can occur in malt kilns or steeping, germination, kilning units, or dedicated batch driers.
In these, the grain rests on a perforated floor or deck, and warm air is passed through it (e.g., for eight hours) until the grain has been dried sufficiently. The grain then may or may not be cooled, depending on whether it is to be committed to long-term storage or it is to be stored warm for a short period to overcome dormancy (i.e., to hasten post-harvest maturation).
In flow-through dryers, the grain passes downwards under gravity in a stream regulated by valves. The grain passes through a series of zones where it meets air at different temperatures and is successfully warmed, dried, and cooled. If there is to be a period of warm storage, the cooling may be limited or omitted so that the grain reaching the store is at 30-40°C (86-104°F) rather than 15°C (59°F) or less, which is desirable for long-term storage.
The dried grain may be thoroughly cleaned immediately or after warm storage. This process is less rushed than pre-cleaning, and so is more thorough. The grain is screened to remove thin corns, and sometimes it is graded into size classes (e.g., above and below 2.5 mm width), which are malted separately. The screens used may be flat and oscillate horizontally or be rotating cylinders. Currently, the quality of the grain on delivery in the UK is so good that apart from aspiration, screening, and passage over magnetic separators, this is all the required cleaning. The clean barley may be stored in flat-bed stores, bins, or silos.
If storage is to be for an extended period, then the grain can be treated with an approved insecticide. The grain must be ventilated if it is held relatively moist (>12%). At a 12% moisture content, grain can be stored for some months at or below 15°C, but for periods over about six months, a moisture content of 10% is safer. Stores must be regularly inspected for signs of insect infestation and fungal attack, and depredations by birds or rodents. The temperature of the grain, determined by probes positioned at various sites and depths, should be recorded weekly, and any undue increase should be acted on as a sign of deterioration.
Grain is weighed on its way to the steep(s). Suppose abrasion (limited physical battering or rubbing the grains together) is to be employed. In that case, this is carried out before steeping, as grain can be treated at rates of only 10-12 t/h, and malting batch sizes are often as large as 300 t, so this amount of treated grain must be accumulated before steeping can begin. Historically, steeps were barrels or shallow troughs where grain rested underwater at depths of 1-2 ft. (0.31-0.62 m). Numerous patterns of steeping vessels have been used. Those preferred now are either flatbed or conical-bottomed steeps. Flat-bed steeps are circular in plan view, and the grain is supported on a perforated deck above the true base, so there is a plenum beneath the deck. For a 200 t batch size, the steep might have a diameter of 15 m (49.2 ft.; Gibson, 1989). Initial depths (before the grain swells) maybe 1.5-1.8 m (approx. 4.9-5.9 ft.).
Grain is loaded in from above, dropping through sprays of water that quench the dust falling into the water. The bed is leveled with a rotating spreader called a miracle. Air may be blown in beneath the deck while the grain is immersed, and in dry periods, air may be sucked down through the grain. The steeped grain is discharged through ports impelled by the miracle. Such steps allow relatively even grain treatment since the bed depth is comparatively shallow, but the water used to fill the plenum is “waste,” and so effluent volumes are large. In addition, it is difficult to keep the plenum chamber clean.