Beta-Glucans are polymers in which residues of the monomer glucose are linked through glycosidic linkages in the β-configuration. These linkages can be between different carbon atoms on the glucose molecule. In a brewing context there are two significant β-glucans, found in barley and yeast respectively.
In the cell walls of the starchy endosperm of barley is a highly polymerized β-glucan in which β linkages are either between the number one and four carbons of successive glucoses or between the one and three carbons. In most of the molecule there is a β1→3 linkage occurring after every third or fourth β1→4 linkage. The occurrence of the β1→3 linkages tends to break up regularity that makes solely β1→4 linked glucoses (viz. cellulose) highly crystalline and resilient to attack.
Barley β-glucans are highly viscous and can cause a number of problems in brewing, notably reduced rates of wort separation and beer filtration and also the formation of hazes, gels, and precipitates. On the other hand, they do represent soluble fiber and barley β-glucan on that basis is touted for its healthful properties.
A different β-glucan is found in the cell walls of yeast, where it accounts for 30%–60% of the total material and affords rigidity. Here, the linkages are β1→3 and β1→6. Whereas the barley β-glucan is linear, that from yeast comprises a backbone of β1→3 linked glucosyls, with branches linked to the backbone by β1→6 bonds.