be·ta-glu·can (b??t?-glo?o?k?n? -k?n)n. Any of various polysaccharides, such as cellulose, that are composed of glucose monomers and are found chiefly in the cell walls of fungi and plants, especially oats and barley. Beta-glucans are added to foods as a source of dietary fiber and are used as dietary supplements.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.