|Jones, Berne - USDA, ARS-RETIRED|
Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 27, 2003
Publication Date: September 18, 2003
Citation: Jones, B., Budde, A.D. 2003. The effect of reducing and oxidizing agents and ph on malt endoproteolytic activities on malt mashes. Journal of Agricultural and Food Chemistry. 51(25):7504-7512. Interpretive Summary: To produce good beer, some but not too many, of the proteins of the barley that is malted and brewed must be degraded into amino acids and peptides. Enzymes that form during malting (germination) carry out this process, but their activities can vary, depending on conditions within the seed and in the 'mash', which is the first step of brewing. To develop malting barleys whose seeds germinate and mash well, and are optimal for beer production, we need to understand exactly how varying the malting and mashing conditions affect the protein-degrading enzymes. This paper describes our studies on how pH and reducing-oxidizing (redox) conditions of a mash affect how its proteins are degraded. When reducing agents (cysteine, beta-mercaptoethanol or dithiothreitol) were added to mashes, more protein was degraded, but this effect was negated when oxidizing agents (diamide, peroxide) were present. Also, the protein degradation was strongly affected by the pH of the mash. This research shows how the activities of the protein-degrading enzymes of a brewing mash can be strongly varied by adjusting its pH or by adding redox agents. This knowledge allows researchers and brewers to alter the protein/amino acid ratios of mashes to study how these changes affect beer quality and various brewing processes. In the end, this will allow brewers to produce better and more cost-effective beers. The information also contributes significantly to scientist's knowledge of the chemistry of seed germination, since it proves that, as has been proposed by other researchers, this chemistry can be strongly altered by redox systems that occur inside plant seeds.
Technical Abstract: The activities of the four endoproteinase classes of malted barley are known to vary with pH, and it seemed likely that the cysteine enzyme activities could be altered by redox agents. This study determined how pH and redox agent concentrations of malt extracts and mashes affected their characteristics. The reducing agents cysteine-HCl, dithiotheritol, and beta-mercaptoethanol increased the proteolysis that occurred in malt extracts and mashes. This increased proteolysis was negated by the addition of the oxidizing agents diamide or hydrogen peroxide. The addition of reducing agents to mashes increased the soluble protein, free amino nitrogen (FAN) and extract values of their resultant worts, and this effect was abolished by the concomitant addition of oxidizing agents. Raising the pH values of mashes strongly reduced their proteolytic activities, soluble protein, FAN and extract values, but not their beta-glucan levels. These results show that several of the major aspects of malting and brewing quality can be adjusted by varying the pH and redox qualities of mashes, which could be helpful to brewers. They also strengthen the previous proposal made by Buchanan et al. that the redox status of plants may play a significant part in controlling their physiology.