Structure of anti-nutritional polysaccharides in wheat in relation to feeding value for poultry

Abstract

A typical European poultry diet contains approximately 60-65% wheat. At such a high inclusion level it has been found that the nutritional value of wheat, measured as its apparent metabolisable energy (AME), can vary substantially depending on variety and growing conditions. The importance of this can be judged from the fact that a difference of just 0.5 MJ/kg in the expected AME can have serious financial consequences for a poultry producer. The wheat samples investigated in this study had a similar gross composition but gave AME values ranging from 8.34 to 13.74 MJ/kg dry matter when included as 75% of the poultry diet. Water-soluble, non-starch polysaccharides (NSP), particularly arabinoxylans, derived from the grain cell walls, are considered responsible for the observed depression of AME by increasing digesta viscosity. The increase in the viscosity of digesta is thought in turn to reduce the uptake of all major nutrients in the diet. The aim of this work was to establish whether the amount of soluble NSP alone was responsible for AME depression or whether arabinoxylan structure and size were also determinants.

Arabinoxylans and (1-3)(1-4)-ß-D-glucans exist in all anatomical regions of the grain. Their concentration was greatest in the bran which contained about 4% mixed-link glucan and 30% arabinoxylan and least in the endosperm which contained only 0.5% mixed-link glucan and 1.5% arabinoxylan. Structure and solubility of the arabinoxylan varied depending on the anatomical region and the wheat sample from which they originated. On a whole grain basis, the endosperm contained about 34% of the total mixed-link glucan and 14% of the total arabinoxylan but contributed about 50% of the soluble mixed-linkage glucans and about 60% of the soluble arabinoxylan. The bran contained most of the total NSP and contributed most of the remaining soluble NSP. The aleurone layer contributed little to the whole grain NSP because of the very small amount by weight of aleurone cells present in the grain.

No relationships were found between AME and the concentration of total NSP, soluble NSP, insoluble NSP or starch indicating that amount of starch or NSP was not a primary factor in AME depression. Correlations were found between AME and the ratio of soluble arabinoxylan to (1-3)(1-4)-ß-D-glucan and between AME and the sum of soluble galactose and mannose residues which illustrated the need to consider interactions between polymers. However, no relationship was found between AME and solution viscosity. This is probably because the extraction procedure used did not sufficiently represent the digestive conditions in the upper parts of the chick digestive tract which lead to the solubilisation of the NSP. This result was contrary to that obtained by others who have observed good correlations between intestinal viscosity and AME.

Before differences in the structure of the soluble arabinoxylan could be determined, samples were first partially hydrolysed with a cloned endo-xylanase, which was free from any other detectable activities. The oligosaccharides produced were separated by HPLC and the molecular weight of the oligomers determined using matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF-MS). Detailed structural information was obtained by one-dimensional proton NMR and the two dimensional experiments total correlation spectroscopy (TOCSY) and rotating frame nuclear Overhauser enhancement spectroscopy (ROESY). Six branched oligomers were fully characterised in this way. Six other oligomers were also produced by the enzyme digestion, two of these were identified as being linear xylan chains, the other four were not produced in sufficient quantity to be fully characterised. The chromatograms from the HPLC were used as fingerprints of the arabinoxylan structures from the different wheat samples. It was found that the structure of the arabinoxylans varied between the different wheat samples. AME was found to be affected by the content of very highly branched arabinoxylan which was directly correlated with an increase in AME. Extensive branching would be expected to interfere with polymer interaction reducing the likelihood of association and a concomitant increase in solution viscosity. No other structural features were found which influenced AME. Solution viscosity was also found to be dependant on the molecular weight distribution of the soluble NSP as well as the degree of branching of the arabinoxylan. This was a complex relationship which showed that polysaccharides with low degrees of branching increased viscosity as their molecular weight increased however arabinoxylans with high degrees of branching had high viscosity at lower molecular weights.

It was concluded that AME and viscosity were determined by a number of factors including arabinoxylan concentration and molecular weight, the degree and distribution of branching within the molecule and by its interactions with other polysaccharides. The response of all the wheat samples examined to the action of a single xylanase demonstrated that the viscosity-enhancing ability of released arabinoxylans could be readily disrupted. However, application of enzyme as a practical solution to the depression of AME is of value only to some wheat samples. The work reported here provides information on arabinoxylan structure which may be of use in establishing a routine method of identifying feed wheat which would benefit from enzyme addition.