In the seed of barley (Hordeum vulgare), proteins specifically expressed therein (seed storage proteins) are present in large quantities, 35–55% of which are comprised of alcohol-soluble hordeins (Shewry 1993, Barley: Chemistry and Technology. pp. 164: American Association of Cereal Chemists).
These hordeins are classified into four types, B, C, D, and γ based on their loci, amino acid compositions, etc; and the content ratio of each to the total hordein is from 70 to 80% for B, from 10 to 20% for C, and 5% or less for D.
It has been reported that these hordeins have a critical role in brewery as amino acid sources for yeast as well as have influence on the color of beer and on the haze formation upon its cooling. These reports did not examine individually classified hordeins, but examined the whole hordeins for their influence on beer quality and beer manufacturing process.
Specifically, Baxter discovered that addition of a reducing agent to mash improved the filtration speed of wort (Baxter, 1980, Brewers Digest 55: 45–47). Furthermore, van den Berg et al. reported that gel protein aggregates, i.e., gel proteins, which are pelleted by centrifugation after the protein extraction with 1.5% SDS from barley exert a detrimental influence on the filterability of wort (van den Berg et al. 1981, Proceeding of the EBC Congress, Copenhagen, 47: 461–469). These reports suggest the correlation between the gel protein resulting from the polymerization of B- and D-hordeins via. disulfide bonds and the wort filterability.
In addition, it has been reported that this gel protein has a highly negative correlation with malt extract (Smith and Lister (1983), Journal of Cereal Science 1: 229–239; Smith and Simpson (1983), Journal of Cereal Science 1: 185–197; Skerritt and Janes (1992), Journal of Cereal Science 16: 219–235; and Howard et al. (1996), Journal of Cereal Science 24: 47–53).
Furthermore, Brennan et al. examined the presence and absence of D-hordein as well as the gel protein content by utilizing isogenic lines based on the barley line which lacked D-hordein, and suggested correlation between the two (Brennan et al. (1998), Journal of Cereal Science 28: 291–299).
From such a standpoint, it has been suggested that if the gel protein can be reduced by some means, there will be the possibility of improving the wort filterability and the wort extract yield.