Monitoring of malting process by characterization of glycation of barley protein Z

Glycation is a common form of non-enzymatic modification that influences the properties of proteins. Although occurrence of protein glycation in food is known for several years, qualitative and quantitative analysis of its reaction products is still a challenging task. Our attention was focused on monitoring of changes in the glycation degree of protein Z, one of the most important protein components of barley malt that influences the properties of beer. The modification during the malting period was studied by a combination of gel electrophoresis, liquid chromatography and mass spectrometry. Water soluble proteins were extracted from barley grains at different times of malting and separated by gel electrophoresis. Protein Z was clearly identified in the band at a position of about 43 kDa and further studied by MS with matrix-assisted laser desorption/ionization technique in detail. From the analysis, we found that protein Z glycation is detectable from the second day of malting. The glycated peptide was observed as a weak signal of m/z 2,619 in the MS spectrum. To confirm the presence of glycation, the MS/MS experiments of both unmodified (m/z 2,457) and modified peptides (m/z 2,619) were carried out. We successfully established a procedure based on the monitoring of glycated peptides in positive reflectron mode to monitor the progress of malting using protein Z as a model protein that might be a potential marker reflecting this technological process. We also demonstrated the high suitability of MS/MS analyses to provide more structural information on peptide modifications.