Hordein is the storage protein of barley. Cereal proteins have been classified into four major categories based upon their solubility in various solvents. The alcohol-soluble fraction in cereals is composed of storage proteins called prolamines. The prolamines of barley are called hordeins. Hordeins account for approximately 35%–50% of the total seed protein. They are important in brewing for several reasons.
Hordein proteins are rich in the amino acids glutamate and proline, both of which are important to yeast nutrition. Glutamate (glutamic acid) is the amino acid most preferred by yeast and proline is not absorbed at all. As a result, finished beer has higher levels of proline than of any other amino acid.
Another reason is that hordeins such as wheat prolamines (gliadins) are primarily responsible for triggering the autoimmune response that characterizes celiac disease. The amino acid sequences responsible for celiac disease are present in hordeins as well. Wheat beers have been shown to have significantly higher prolamine levels than beers made from only barley or adjunct beers made using corn or rice.
Finally, hordeins are also thought to play a role in beer foaming and haze formation. The proteins have been shown to complex with polyphenolic compounds present in barley and hops, precipitating and resulting in haze. Hordeins, along with other proteins, have been revealed to be important in the favorable formation of beer foam.
Hordein proteins play both positive and negative roles in brewing.