The current paper reviews content and variation in fiber and nonstarch polysaccharides
(NSP) of common crops used in broiler diets. The cereal grain is a complex structure,
and its cell walls (CW) differ in their composition and hence properties. Arabinoxylan
(AX), mixed linkage (1→3; 1→4)-β-glucan (β-glucan), cellulose, and the noncarbohydrate
component lignin are the predominant polymers in cereals. They occur in different
proportions depending on the species and tissue type. Rye, triticale, wheat, corn,
and sorghum are all rich in AX, whereas barley and oats contain a high level of β-glucan.
The AX from rye, wheat, and triticale and β-glucan from barley and oats are to a large
extent soluble, whereas the solubility of AX found in corn and sorghum is lower than
the other cereals. The ratio of arabinose to xylose gives a crude indication of the
AX structure, which varies between the endosperm, the aleurone and the outer grain
layers as well as between the same tissues from different grains. Varietal differences
in AX structure of the endosperm are also identified. From the analysis of the released
oligomers after hydrolysis with a specific (1→3,1→4)-β-d-glucan hydrolase, it is found
that the ratio of trisaccharides (degree of polymerization 3) and tetrasaccharides
(degree of polymerization 4) varies depending on the source, being higher in barley
than in oats but lower than in wheat. The molecular weight of β-glucan is higher than
that of AX, and both polymers contribute to the viscosity of the extract. However,
because AX molecules are more resistant to degradation than β-glucan, the use of AX
rich grains in broiler diets is usually more problematic than those containing high
concentrations of β-glucan. The cereal coproducts (brans and hulls) are concentrated
sources of cellulose, lignin, and insoluble AX, but β-glucan can also be present mainly
in rye and wheat brans. The CW composition of seeds and grains of protein crops and
feedstuffs are different from that of cereals. The main CW polymers are pectic substances
(homogalacturonan, rhamnogalacturonan type I and II, xylogalacturonan, and arabinogalactans
type I and II), xyloglucans, and cellulose, but there are significant differences
in the composition of the parenchymatous (cotyledon) tissues and that of the hulls.
In the hulls, cellulose is the predominant polysaccharide, followed by acidic xylans
and pectic substances. The implications of the heterogeneous CW for the action of
exogenous enzymes are discussed.