Unravelling the structural heterogeneity and diversity in rheological behavior of alkali-extractable wheat bran arabinoxylans using ammonia pretreatment

The impact of ammonia solution (NH4OH) mild pretreatment (pH 9.5, 25 C-degrees/0.5-2 h), followed by sodium hydroxide extraction (0.1 M NaOH, 60 C-degrees/1-4 h), on the wheat bran (WB) arabinoxylan (AX) extractability, the molecular features of the isolated polysaccharides and their rheological behavior were investigated. AX isolates, obtained from WB treated solely with NH4OH, exhibited lower Mw and a lower degree of xylan backbone substitution (arabinose/xylose ratio), compared to those obtained by combined alkali treatments (NH4OH/ NaOH), as evaluated by HPLC-SEC-RI, FTIR and H-1 NMR analyses. Swelling of bran cell walls, caused by NH4OH, resulted in crevices of the outer bran tissue layers, thus increasing AX extraction yields, as the ammonia pretreatment and NaOH extraction times were extended. 2D DOSY NMR experiments revealed the presence of individual populations exhibiting discrete diffusional characteristics, while in most cases, each population was further subdivided into two sub-populations composed mainly of chains of either monosubstituted or disubstituted anhydro-xylosyl residues, with the latter diffusing more slowly. The steady-shear viscosities of 4% aqueous AX dispersions were positively correlated primarily to the AX Mw, and secondarily to the arabinose/xylose ratio. Dynamic rheometry revealed that extraction only with NH4OH or water provided AX isolates capable of gelification due to oxidative coupling of feruloyl residues covalently attached on AX chains, as evidenced by FTIR, H-1 NMR, and further confirmed by 2D DOSY NMR. Overall, these findings elucidated the structural heterogeneity of AX isolates obtained under varying alkali extraction regimes, along with their diversified rheological behavior.