Barley grain, or seeds, are allowed to sprout for a short amount of time and then dried to prevent further growth. This process is called malting and allows the grain to convert part of its stored starches to starches that can easily be converted to fermentable sugars, and to produce enzymes and nutrients that are important later in the brewing process. Malts are often described as under modified or fully modified and this simply refers to how long the grain was allowed germinate, and thus how well the starches have been converted and most importantly how well those nutrients have developed. Sometimes, this malted barley will be further processed by roasting, toasting, or even smoking, to allow the brewmaster to add special characteristics or complement other flavors expected in the beer.
There are two main types of barley used to make malt; two row and six row. These terms refer to the number of barley corns in the sheaf. Six row barley tends to yield a larger amount of grain per measured area of farmland, but two row generally produces larger barley corns and thus a better yield of convertible starch per measured amount of grain. However, it is generally held that six row barley will have a higher enzymatic power, which is important when using adjuncts without (or with very little of) the needed enzymes to convert their own starches during the brewing process (such as rice or corn).
Malt also varies in colour and its capacity to convert starch to sugar (diastatic power). This variation is determined by the modification level of the malt (described earlier) and by the length of time that the malt is heated after it has been germinated. Lagers and Pilseners tend to use undermodified malt, that is malt which the starch grains are relatively intact such as pale malt, while ales use more modified malts, such as crystal malts. Stouts use heavily modified malts such as chocolate, or roast barley (which technically is not a malt as it hasn’t been germinated).