Xylose is a five-carbon sugar (or pentose) that is the principal monosaccharide of hemicellulose and one of the most abundant sugars in nature. Most of the xylose in wheat or barley is joined into larger molecules with arabinose, another pentose; together they are referred to as arabinoxylan. This polymer usually consists of roughly equal parts xylose and arabinose and can constitute up to 10% of brewing grains. Most of this arabinoxylan is nonsoluble and will remain in the spent grain following lautering. However, 1–2 g/l arabinoxylan is found in a typical hopped wort and has been shown to affect the wort’s viscosity. Some of the simpler arabinoxylo-oligosaccharides created by breakdown of the arabinoxylans are found in beer, anywhere from 0.8 to 2 g/l. These polysaccharides can affect the mouthfeel of a beer, giving roundness, and are generally considered taste-neutral. Yeast strains used in brewing do not ferment xylose, so if any were present in the wort, it would not be fermented.
