Bioprocessing, Structure and Material Properties of Arabinoxylans

Doctoral thesis, 2008

This work has focused on using agriculture residues as raw material for new polymeric materials. Arabinoxylans have been isolated from barley husks and endosperm fiber using alkali extraction, and the chemical composition and detailed structure have been determined. Different isolation conditions, pre-treatments, and purification steps were investigated to find an optimized and environmentally sustainable process. It was shown that a key operation for successful extraction of arabinoxylan from barley husk is the degradation of the lignin network prior to extraction. Delignification was not needed when using endosperm fiber as raw material. The extracts were mixtures of arabinoxylan, β-glucan, starch, and proteins. Sequential purification showed that residual starch makes arabinoxylan films more brittle; β-glucan makes them more flexible, and proteins have little or no effect at all. If only mechanical properties of films are taken into account, enzymatic removal of starch is the only purification needed. This is very useful information when designing an isolation process, since the number of operations should be kept at a minimum for economic and environmental reasons. Differences in the molecular structure between arabinoxylans from husk and fiber were determined. Barley husk-derived arabinoxylan was found to have disaccharide branches with a terminal xylose unit on the arabinose, and also glucuronic acid substituents, as shown by NMR. Barley fiber arabinoxylan, on the other hand, had xylose residues disubstituted with only arabinose. The Ara/Xyl ratio was much higher in the fiber sample, 0.63 compared to 0.22 for the husk arabinoxylans. Films have been made by casting from water solution and material and barrier properties were evaluated. The films were transparent, strong, rather stiff and moisture sensitive. They had good gas barrier properties, and thus have a potential application as biodegradable oxygen barrier films in multilayer food packaging. The isolated arabinoxylan formed films without addition of external plasticizer. This special property was explained by internal plasticizing by water, caused by the ability of arabinose to attract water. Controlled debranching of arabinose substituents showed that the degree of arabinose substitution had a large effect on water holding capacity, plasticization, and crystallinity of the material. Optimal mechanical properties were obtained for arabinoxylan with an Ara/Xyl ratio of 0.35-0.40. Enzymatic treatment was demonstrated to be a very attractive tool for tailor-making xylans with specific properties. This work shows that bioprocessing of agricultural residues through an optimized, environmentally sustainable process, together with an understanding of the relationship between structure and properties of biopolymers will lead to the development of a biorefinery for the production of future materials, chemicals, and fuels.