Qualitative and quantitative changes in barley seed protein patterns during the malting process analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis with respect to malting quality.

Abstract

Seeds of two barley cultivars, similar in total protein content and malt extract yield but different in their final attenuation values, were malted. Samples taken at daily intervals during the malting process were extracted sequentially with Tris-HCl buffer, aqueous 2-propanol, aqueous 2-propanol containing 0.5% dithiothreitol, and 4 M urea, containing 0.5% dithiothreitol and 1% Nonidet P-40. The protein composition of these extracts was analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and computer densitometry to determine whether differences observed in the rate or extent of protein modification are related to the malting quality character final attenuation. It was found that, common to both cultivars, the albumin and globulin proteins were relatively resistant to proteolysis, whereas the hordeins suffered a dramatic breakdown during malting, with the D hordein being degraded most rapidly, followed by the B and C hordeins. Besides these similarities, differences between both cultivars were observed in the relative rates of D hordein degradation, as this rate was considerably higher in the cultivar with high malting quality. Similar, but much less distinct kinetics were seen with certain B hordeins. Since a possible relationship might exist between the rate of proteolysis of the D hordeins and the character final attenuation, we analyzed a larger number of barley cultivars with different final attenuation values with a simplified technique. For the ten cultivars examined, differences during germination were again seen in the rates of modification of the D hordeins. However, significant correlations between the D hordein breakdown and final attenuation values were not obtained, so that we propose that there exists at best a loose correlation between the relative rate of proteolysis of these proteins and the malting quality character final attenuation.