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THE PROTEIN RESERVES OF THE BARLEY GRAIN AND THEIR DEGRADATION DURING MALTING AND BREWING
Author(s) -
Wallace W.,
Lance R. C. M.
Publication year - 1988
Publication title -
journal of the institute of brewing
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.523
H-Index - 51
eISSN - 2050-0416
pISSN - 0046-9750
DOI - 10.1002/j.2050-0416.1988.tb04598.x
Subject(s) - endosperm , mashing , hordein , carboxypeptidase , biochemistry , exopeptidase , storage protein , aleurone , prolamin , biology , abscisic acid , caryopsis , enzyme , starch , germination , botany , gene
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 β‐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.