Pentosan

Pentosans are polymers composed of pentoses. In contrast to cellulose, which is composed of hexose monomers, pentosans are derived from five-carbon sugars such as xylose. Pentosan-rich biomass is the precursor to furfural.
The pentosan content has been determined for many natural materials:

29-25%: oat hulls, cottonseed hulls, barley, sugarcane bagasse, sunflower husks
24-20% wheat straw, flax shives, hazelnut shells, birchwood, eucalyptus wood
8% pinewood
3% peanut shells

Pentosans can act as heparinoids, glycosaminoglycans which are derivatives of heparin.
They can have an influence on bread quality.