Effects of malting on molecular weight distribution and content of water-extractable β-glucans in barley

In the beer industry β-glucans are extensively studied non-starch polysaccharides due to their ability to increase the viscosity of solutions and to form gels. The current study was designed to determine the total and water-soluble β-glucan contents of barley during malting. Total and water-soluble β-glucans were analyzed from two different malts that originated from the same barley but varied in germination time from 36h (malt A) to 72h (malt B). Water-soluble β-glucans were also characterized using high-performance size-exclusion chromatography with triple-detector analysis (HPSEC-TDA) to evaluate the variation in molecular weight distributions, intrinsic viscosity, radius of gyration, Mark-Houwink parameters and polydispersity and thus the overall structural changes during malting. Total β-glucan content decreased from barley to malt due to the action of β-glucanase and was greatest in malt B (where 92% of β-glucans were degraded) which highlights the influence of germination time. β-Glucan solubility increased during malting, again particularly in malt B, where most of β-glucans became soluble. The β-glucanase activity also affected the molecular weight of the polymers which ranged from 298·10³g/mol in barley to 293·10³ and 218·10³g/mol in malts A and B respectively. The molar mass of the most abundant fraction decreased from barley (256·10³g/mol) to malt A (112·10³g/mol) and malt B (89·10³g/mol), again highlighting the effect of the longer germination time. Proceeding from barley to malt, the cumulative molar mass distribution function confirmed that the weight fraction of polymers below 200·10³g/mol increased, while the high molecular weight fraction (between 200·10³g/mol and 400·10³g/mol) decreased. Moreover, the presence of a higher molecular weight fraction (14-16%) beyond 400·10³g/mol which does not change during malting was observed. The Mark-Houwink constants α and log k confirmed the random coil conformation of soluble β-glucans and showed an increase in the compactness of the macromolecules from barley to malts.