| Popis: |
Flours from dehusked barley and malt were fractionated to obtain water-insoluble cell wall material (WIS). A mass balance of these fractionations was determined. Arabinoxylans were extracted from this WIS in high purity and yield with Ba(OH) 2 , and subfractionated with graded ethanol precipitation.The structural elements present in these arabinoxylans and in arabinoxylans isolated from wort were determined. These arabinoxylans all consisted of a backbone of (1-->4)-linkedβ-D-xylopyranose units (Xyl p ), a proportion of which were substituted with alfa-L- arabinofuranose (Ara f ) at O-2 and/or O-3 of the Xyl p units. A new feature was the presence in barley and malt arabinoxylans of a large amount of Xyl p units carrying a single Ara f substituent at O-2. The amounts of Xyl p substituted at O-2 or at both O-2 and O-3 increased with increasing substitution of the xylan backbone. The wort arabinoxylans were found to be exceptionally rich in O-2,3-disubstituted Xyl p .A number of fragments could be isolated after degradation of barley and malt arabinoxylans with endoxylanase 1 from Aspergillus awamori . The structures of the isolated fragments were determined. From the structures found, it could be shown that the position of Ara f substituents on the xylose residues of the arabinoxylan influenced the extent of enzymic degradation of the xylan backbone, substituents at O-2 being more efficient than substituents at O-3 with the enzyme used. From these data and the linkage composition of undegradable arabinoxylan fractions, it was concluded that the distribution of Ara f substituents over the xylan chain was not random, but fairly regular.The arabinoxylans extracted from barley and malt cell wall material appeared to be very similar in composition and structural features. This implies that changes during malting are small or extremely localized. |
