Kilning is the heating of germinated barley to dry it and develop malty, biscuit-like flavors. The largest portion of malt in most beers today is pale malt that is only gently dried at relatively low heat to preserve the integrity of its enzymes.
Kilning is invariably done in two or three stages. Initially, most of the surface moisture of the germinated grain is driven off. At the final stage, the malt is “cured.” The goal is to reduce the grain’s moisture content from about 40% to 50% down to at least 4% to 6%. Different maltsters use different temperatures and time intervals for the different kilning phases. A typical sequence is step one at 50°C to 60°C (122°F to 140°F), step two at 65°C to 75°C (149°F to 167°F), and a curing step at 80°C to 105°C (176°F to 221°F). The sequencing of the temperature levels is important. If the grain is heated too moist at too high a temperature, its enzymes would be denatured and thus it would be rendered useless for mashing.
Traditional kiln designs are simple vessels with a heat source at the bottom, one or two floors—perforated for air flow—to hold the grain, and vents at the top for air evacuation. Because kilning is a highly energy-intensive process, most malting plants use various heat exchangers to reclaim heat from the hot air that leaves the kiln.