The Protein Reserves of the Barley Grain and Their Degradation During Malting and Brewing

Abstract

The hordeins of barley are the main components of the grain protein, comprising B (sulphur rich), C (sulphur poor) and D (high relative molecular mass) species. A gel fraction can be isolated that consists of the D and part of the B hordein. Other important components of the reserve protein are B-amylase, protein Z and enzyme inhibitors as well as endosperm cell wall protein and protein associated with the starch granules.Degradation of protein is mediated by peptidase enzymes, most important of which are the endopeptidases and the exopeptidases; the latter having specificity for the carboxy-or amino-terminal region of a polypeptide chain. A very high activity of carboxypeptidase develops In the germinating barley grain. Five carboxypeptidases have been identified in malt and three fully characterized. Several endopeptidases have been identified, which have optimum activity close to pH 5.0 (that of the starchy endosperm) and studies with active site Inhibitors suggest that sulphydryl dependent species are the most important However, only limited use has been made of hordein as a substrate in the measurement of peptidase activity in malt Lack of a complete understanding on the peptidases and their regulation in the germinating barley is a major limiting factor in the knowledge on the mobilization of the protein reserves during malting and mashing. Both de novo synthesis and activation of the peptidases have been demonstrated and the hormones gibberellic acid and abscisic acid appear to have a regulatory role. A controlled modification of the protein reserve is required so that there is an appropriate supply of amino acids for the brewers yeast but retention of proteins important in the desirable foaming component of beer. It is also important that proteins which can interfere in the final stages of brewing, e.g. those with chill haze forming potential, are broken down. An early hydrolysis is required of cell wall proteins and those proteins intimately associated with the starch granules.

Molecular basis of barley quality

“…The influence of storage protein within the grain, its subsequent degradation during malting and brewing, the function of proteinases, exopeptidases, and carboxypeptidases, and the influence of these factors on malt and beer quality has been recently reviewed (Wallace and Lance 1988). The mechanisms and control of the breakdown of storage proteins have only been partially unravelled.…”

Enzymes from Cereal and Fusarium Metabolism Involved in the Malting Process – A Review

“…Depending on their protease class, proteases break down peptide bonds by hydrolysis at specifics regions inside the protein structure. Similar to the amylolytic enzymes, in the class of proteolytic enzymes there are two protease groups: exo- [101,102] and endo-enzymes [103] to degrade proteins from outside or inside, respectively. Furthermore, barley proteases can be categorized into four protease classes, depending on their mechanism of catabolism: [103,104] cysteine proteases, metalloproteases, aspartic proteases, and serine class proteases.…”

Brewing with up to 40% unmalted oats (Avena sativa) and sorghum (Sorghum bicolor): a review

“…Strelec et al . reported at least six aminopeptidases (EC 3.4.11.—) with optimum activities at a neutral/alkaline pH in germinated barley, which therefore have limited relevance in malting/mashing (Table ). Furthermore, a proline‐specific dipeptidyl‐peptidase IV (EC 3.4.14.5) and dipeptidases (EC 3.4.13.—) are also present in germinating barley grains (Table ).…”

“…reported at least six aminopeptidases (EC 3.4.11.—) with optimum activities at a neutral/alkaline pH in germinated barley, which therefore have limited relevance in malting/mashing (Table ). Furthermore, a proline‐specific dipeptidyl‐peptidase IV (EC 3.4.14.5) and dipeptidases (EC 3.4.13.—) are also present in germinating barley grains (Table ). The use of unmalted cereal adjuncts in brewing can result in an inadequate breakdown of the endosperm storage proteins (soluble nitrogen/free amino nitrogen deficiency), adversely affecting the fermentation and the filtration processes as well as the beer quality (haze formation, poor foam stability, off‐flavours) .…”

“…Furthermore, a proline‐specific dipeptidyl‐peptidase IV (EC 3.4.14.5) and dipeptidases (EC 3.4.13.—) are also present in germinating barley grains (Table ). The use of unmalted cereal adjuncts in brewing can result in an inadequate breakdown of the endosperm storage proteins (soluble nitrogen/free amino nitrogen deficiency), adversely affecting the fermentation and the filtration processes as well as the beer quality (haze formation, poor foam stability, off‐flavours) . In order to prevent problems such as these, exogenous metalloendopeptidases (EC 3.4.24.—), derived from Bacillus species, are commonly added to brewery mashes .…”