Cellulose is usually found in nature as lignocellulose, a composite
of cellulose fibers in a matrix of hemicellulose and lignin. The
lignin, which few microorganisms are able to digest, protects the
carbohydrate against biological attack. Thus, even ruminant animals
that have evolved on diets of lignocellulosic biomass, such
as grasses and forbs, can only extract 50–80% of the energy content
of this plant material because some of the polysaccharides and all
of the lignin pass through the gut undigested [6].