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

In the beer industry beta-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 watersoluble beta-glucan contents of barley during malting. Total and water-soluble beta-glucans were analyzed from two different malts that originated from the same barley but varied in germination time from 36 h (malt A) to 72 h (malt B). Water-soluble beta-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 beta-glucan content decreased from barley to malt due to the action of pglucanase and was greatest in malt B (where 92% of beta-glucans were degraded) which highlights the influence of germination time. beta-Glucan solubility increased during malting, again particularly in malt B, where most of beta-glucans became soluble. The beta-glucanase activity also affected the molecular weight of the polymers which ranged from 298 103 g/mol in barley to 293 " 103 and 218 " 103 g/mol in malts A and B respectively. The molar mass of the most abundant fraction decreased from barley (256 " 103 g/mol) to malt A (112 " 103 g/mol) and malt B (89 " 103 g/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 " 103 g/mol increased, while the high molecular weight fraction (between 200 " 103 g/mol and 400 " 103 g/mol) decreased. Moreover, the presence of a higher molecular weight fraction (14-16%) beyond 400 " 103 g/mol which does not change during malting was observed. The Mark-Houwink constants a and log k confirmed the random coil conformation of soluble beta-glucans and showed an increase in the compactness of the macromolecules from barley to malts. (C) 2014 Elsevier Ltd. All rights reserved.