Isolation, Characterization and Material Properties of Arabinoxylan from Barley
The growing amount of household waste and the limited supply of fossil fuel on earth make it important to find renewable resources for the production of biodegradable polymers for a variety of applications, particularly as packaging and other disposable products. Hemicelluloses are important components in all plants and the second most abundant plant biopolymers on earth. Although hemicelluloses are biosynthesized to a large extent they are not yet utilized as polymeric materials.
The aim of the study was to find a suitable method to isolate arabinoxylans from barley in order to produce a film forming product that could be used for preparation of oxygen barrier films or coatings. Arabinoxylan has been extracted from barley husks and endosperm fiber using different methods and the chemical composition and detailed structure has been determined. In the husks, the arabinoxylan is tightly bond to cellulose and lignin, building up a complex composite structure. Relatively high amount of chemicals was needed to render them soluble, and a delignification step was necessary to obtain good yields and high purity. The arabinoxylans in the cell walls of the starchy endosperm were more water soluble and could be extracted using less chemicals. This raw material had a higher starch and protein content. It also contained more β-glucans, which might have an effect on material properties and oxygen barrier of films.
There were also differences in the fine structure of arabinoxylans from husk and fiber. Barley husk arabinoxylan was found to have longer 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 disubstituted xylose residues. The Ara/Xyl ratio was a lot higher in the fiber sample, 0.63 compared to 0.22 for the husk arabinoxylans.
Films have been made by water casting and material and barrier properties of these have been evaluated. The isolated arabinoxylan formed films without addition of external plasticizer. The degree of arabinose substitution had a large effect on water holding capacity and plasticization of the material. 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.