There are various types of malting plants in use, but we will only discuss the most common ones. Malting technology is generally safe as long as certain precautions are taken. However, there are some risks associated with carbon dioxide, grain dust, and malt dust, which may be unfamiliar to some. Carbon dioxide is released as the grain is soaked and germinated. This dense gas can accumulate, so it is important to ensure that vessels and confined spaces are well ventilated before entering them.
Dust should be contained and cleaned up not only because it becomes damp and a breeding ground for insects and microbes, but also because inhaling it can cause allergies and fungal lung infections. Additionally, when dust mixes with air, it can form explosive mixtures. All handling equipment needs to be grounded to prevent sparks that could trigger explosions, and conveyors and ducts should have explosion vents.
Modern malt factories handle large batches of grain (often 200t/300t batches) and have highly automated processes to ensure reproducible conditions and minimize manpower required for each tonne of malt produced. In large maltings, grain is delivered in bulk via ship, barge, train, or truck. Before unloading, the bulk should be inspected and sampled for analysis to determine its quality. Once the quality is confirmed, unloading can proceed. Grain from vessels is usually suctioned out using pneumatic systems, while rail wagons or trucks can be emptied by gravity or with the help of pneumatic systems. Trucks typically unload into an intake pit through tipping or from a hopper. The pit is ventilated to remove dust and equipped with a coarse moving screen (sieve) to remove impurities like straw and large stones. Each truck is weighed upon arrival and after unloading to calculate the amount of grain unloaded.
During malting, the grain is moved multiple times using various equipment, including bucket elevators, helical screw conveyors, belt conveyors, chain and flight conveyors, and occasionally pneumatic conveyors. To prevent cross-contamination, it is important that the equipment used for moving grain is completely separate from that used for moving malt. The freshly delivered barley is conveyed to a temporary storage called the “green grain” bin. It stays there, usually ventilated with fresh air, until it can undergo pre-cleaning and drying if necessary. Pre-cleaning involves rapid screening to remove large impurities like sand, straw, stones, and string, as well as aspiration to remove dust. Dust from various locations is collected in cyclones and textile-sleeve filters. The grain also goes through magnetic separators to remove iron and steel impurities.
In Northern Europe, the grain usually needs to be dried to a moisture content of 12% or less before safe storage. Some maltsters don’t allow drying before delivery due to the risk of heat damage caused by inexperienced drying. Drying temperatures are lower for wetter grain to avoid damage. Batch drying can be done in malt kilns, steeping, germination, and kilning units, or dedicated batch dryers. In these systems, warm air is passed through the grain for several hours until it is sufficiently dried. Cooling may or may not be done depending on whether the grain will be stored long-term or kept warm for a short period to hasten post-harvest maturation.
Flow-through dryers work by allowing the grain to pass downward under gravity through different temperature zones, where it is successively warmed, dried, and cooled. If warm storage is required, the cooling process may be limited or omitted, resulting in grain that is stored at temperatures around 30-40°C (86-104°F) instead of 15°C (59°F) or lower, which is suitable for long-term storage.
The dried grain can now undergo thorough cleaning, either immediately or after warm storage. This process is more comprehensive than pre-cleaning. The grain is screened to remove small kernels and sometimes graded into size classes (e.g., above and below 2.5 mm width) for separate malting. Screens can be flat and oscillate horizontally or rotating cylinders. Currently, the quality of grain upon delivery in the UK is generally excellent, so apart from aspiration, screening, and magnetic separation, no additional cleaning is required. Clean barley can be stored in flat-bed stores, bins, or silos.
If the grain will be stored for an extended period, it can be treated with an approved insecticide. If the grain has a relatively high moisture content (>12%), ventilation is necessary. Grain with a moisture content of 12% can be stored for several months at or below 15°C, but for periods over six months, a moisture content of 10% is safer. Regular inspections of the storage facilities should be conducted to detect signs of insect infestation, fungal attack, or damage caused by birds or rodents. The temperature of the grain should be recorded weekly using probes placed at various sites and depths, and any significant increase should be addressed as it may indicate deterioration.
Grain is weighed before entering the steeping process. If limited physical agitation or rubbing of grains together (abrasion) is to be employed, this is done prior to steeping. Since the treatment rate for grain is only 10-12 t/h and malting batch sizes are often as high as 300 t, enough treated grain must be accumulated before steeping can begin. Historically, steeps were barrels or shallow troughs filled with water that submerged the grain at depths of 1-2 ft (0.31-0.62 m). Various patterns of steeping vessels have been used, but the preferred ones today are flat-bed or conical-bottomed steeps. Flat-bed steeps are circular in shape with a perforated deck supporting the grain above the actual base, creating a plenum beneath the deck. For a 200 t batch size, a steep might have a diameter of 15 m (49.2 ft). Initial depths before grain swelling can range from 1.5 to 1.8 m (approx. 4.9-5.9 ft).
Grain is loaded from above, passing through water sprays that quench the dust and into the water. A rotating spreader called a giracleur is used to level the bed. Air can be blown in under the deck while the grain is submerged, and during dry periods, air may be sucked down through the grain. The steeped grain is discharged through ports driven by the giracleur. Such steeping systems allow for relatively even treatment of the grain due to the shallow bed depth. However, the water used to fill the plenum becomes waste, resulting in large effluent volumes. Additionally, keeping the plenum chamber clean can be challenging.