A Comparison of Barley and Malt Polypeptides that Inhibit Green Malt Endoproteinases

The brewing quality of a wort is strongly affected by its amino acid/peptide/protein ratio, which in turn is determined by the endoproteinases active during malting and mashing. In addition to the multiple endoproteinases present in germinating barley and malt, we previously showed that inhibitors of these enzymes, which will affect the rate and extent of protein hydrolysis, occur in ungerminated barley. We are characterizing these inhibitors to determine how important they are to the malting and brewing processes. Contrary to reports by other researchers, this study demonstrates that similar inhibitors are also present in kilned malt, at levels about 2.5 times higher than those of barley. Both barley and malt contain two classes of inhibitors, Ia and Ib. We have now separated the la fractions (containing low-molecular-weight, proteinaceous compounds) from barley and malt into multiple forms by ion exchange chromatography. The separated la fractions specifically inhibited the activities of particular electrophoretically separated green malt endoproteinases. All of the barley Ia fractions restricted the activities of the same green malt endoproteinases, as did all of those from malt. This probably indicates that all of the barley inhibitors are similar, and that the same is also true for the malt inhibitors. Ia fractions from barley suppressed only cysteine class proteinases, while those from malt inhibited a serine proteinase, in addition to the cysteine proteinases.