Structural Characterisation of Water-extractable and Water-unextractable Arabinoxylans in Wheat Bran
2002
Maes, C. | Delcour, J.A.
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 α-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 °C) 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.
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