kilning is the final part of the malting process, which removes moisture from the germinated grain. It also stops biological activity, preserving the enzymes needed for conversion, and allows the malt to be stored at low temperatures. Drying the malt also generates flavor compounds and allows easier milling. (Kiln-drying is also used to extract moisture from barrel staves to speed up their seasoning, although the process leaves more tannins in the wood than long air-drying outdoors.)
The grain kiln itself is a large chamber with a perforated floor onto which the green malt (at around 35 percent moisture content) is spread. Initially the external moisture is driven off; then as the temperature rises, the water moves from inside to outside of the grain, further lowering moisture. A final period of air-drying drops the moisture down to between 4 and 6 percent.
Today most kilns use hot air to dry the malt, but smoke can be used at the start of the process to impart specific aromas to the grain. The most commonly used fuel for this is peat, but other smokes such as wood can also be used.
As an excessively high kilning temperature will break down the amylolytic enzymes contained in the grain, it is kept at around 72° C. In malt destined for grain whisky this is lower (around 50° C), as higher enzymatic activity is required. During the process the grain will develop color and flavor compounds such as furan, pyrroles, and diketones, which can add smoky chocolate-like aromas. In addition, compounds such as hexanal (which makes a grassy aroma) are produced.
See also barley; malting; and peat.
Buxton, Ian, and Paul S. Hughes. The Science and Commerce of Whisky. Cambridge: Royal Society of Chemistry, 2014.
Lea, Andrew G. H., and John Raymond Piggott, eds. Fermented Beverage Production. New York: Kluwer, 2003.
Russell, Inge, and Graham G. Stewart, eds. Whisky Technology, Production and Marketing, 2nd ed. Amsterdam: Elsevier, 2014.
By: Dave Broom