is the external protective envelope of cereal grains. It makes up about 10% of the seed’s mass. It is composed of a number of layers of dead cells with high levels of inert cellulose, lignin, arabinoxylan, and other carbohydrate polymers. It provides a strong barrier to penetration of water and acts as a deterrent to insects that could damage the seed. Silica and antioxidant phenols are also present and provide protection against microbial attacks.

In barley, the husk is composed of three layers and is tightly attached to the underlying waxy pericarp layer, which is also inert and impervious to gas transfer. Below the pericarp is the testa with high levels of lipid material. Together these layers act to keep the barley grain inert for all practical purposes until germination. Abrasion of the husk allows penetration of water and exogenous growth hormones and hastens germination.

The husk is important in brewing because it forms the filter basis for separating solid particles from liquids as the wort is run off at the end of mashing. Cracking of the barley grain during milling should produce large husk particles that can perform the filtration function efficiently. Excessive tearing or crushing of the husk would reduce particle size and produce cloudy worts. The husk imparts flavors to malts, accounting for the tannic “crispness” of many six-row malts. In general, six-row barleys will have a greater husk-to-seed ratio, particularly when there is a notable size difference between the central and lateral kernels. Brewers generally wish to avoid the extraction of astringent tannins and haze-causing polyphenols from grain husk. As a result, they will generally keep sparging temperatures below 77.8°C (172°F) and stop the runoff from the mash when the runoff gravity drops much below 2°P. See sparging.

Barley husks are easily colonized by microorganisms, particularly molds such as Fusarium, which can produce mycotoxins and compounds that can induce gushing in beer. Barley grains protect themselves by producing antimicrobial compounds to act against such fungi. One of the modern barley breeding objectives is the development of more resistant varieties that minimize the potential for microbial growth while delivering improved malt quality.

Naked (hull-less) barleys have a free threshing husk, similar to that of wheat, which, if used as the only malt in the mash, will cause a highly compact grain bed that is difficult to lauter. However, advances in mash filtration technology are under development that will make greater use of hull-less malted barleys in brewing possible.