Structural characterisation of water-extractable and water-unextractable arabinoxylans in wheat bran

Water-extractable arabinoxylan (WE-AX) in wheat bran, of which only a minority originates from adherent endosperm, amounted to 6% of the total arabinoxylan (AX) content in such bran. WE-AX had an arabinose to xylose (A/X)-ratio of 0.45. Graded ethanol precipitation (20-80% range) yielded AX with an A/X-ratio increasing from 0.31 to 0.85. A population of molecular weight (MW) of 50 kDa precipitated between 0 and 40% ethanol and one with much lower apparent MW precipitated at higher ethanol concentrations. Wheat bran unextractable cell wall material (UCM) was obtained as the residue withstanding thermostable alpha-amylase and protease treatments and consisted mainly of AX (ca 45%) and cellulose (30-35%). Two consecutive extractions of UCM with alkaline hydrogen peroxide (AHP; 2.0%, pH 11.5, 4 h, 60 degreesC) resulted in a cellulose rich residue (CRR) containing 33% of the AX originally present in UCM. The average A/X-ratio of AX in CRR was lower than that. in UCM. The extracted AX polymers (ca 45% of the AX originally present in UCM) had a high A/X-ratio (0.82). Their elution profiles showed two polydisperse peaks with apparent MW of respectively 100-120 kDa and 5-10 kDa. Graded ethanol precipitation resulted in a lowly substituted fraction (A/X = 0.40; 0.40% ethanol) of high MW and a fraction of highly substituted AX (A/X>0.95; 40-70% ethanol) containing both low molecular weight (LMW-) as well as high molecular weight (HMW-) AX. At an ethanol concentration of 70% or more, only LMW-AX precipitated. (C) 2002 Elsevier Scieuce Ltd.