Wheat bran AX properties and choice of xylanase affect enzymic production of wheat bran-derived arabinoxylan-oligosaccharides

Wheat bran-derived arabinoxylan-oligosaccharides (AXOS) have recently been shown to potentially exert prebiotic effects. In this study, fifteen bran samples obtained by milling different wheat cultivars were treated with xylanases from Hypocrea jecorina (XHJ), Aspergillus aculeatus (XAA) and Pseudoalteromonas haloplanktis (XPH) to assess the impact of bran source and xylanase properties on the AXOS yield and structure. The total arabinoxylan (AX) extraction yield was higher with XHJ (8.2-10.7%) and XAA (8.2-10.8%) than with XPH (6.9-9.5%). Irrespective of the enzyme, a significant negative correlation was observed between extraction yield and arabinose to xylose (A/X) ratio of the bran AX (r = -0.7), but not between yield and bran AX level. The A/X ratio of the extracted material was in a 0.27-0.34 range for all bran samples and all enzymes, which, combined with yield data and microscopic analysis, indicated primary hydrolysis of aleurone and nucellar epidermis AX. The average degree of polymerization (avDP) of the extracted AX was very low for all enzymes (2-3), owing to the release of high levels of monomeric arabinose and xylose. The release of these monosaccharides could be ascribed to (1) the activity of wheat bran-associated enzymes (arabinofuranosidases and xylosidases), (2) the hydrolytic properties of the xylanases themselves, and (3) the presence of xylosidases as contaminations in the enzyme preparation, in that order of importance. Heat treatment of bran before xylanase treatment significantly decreased the levels of monomeric arabinose and xylose in the extract, without affecting the extraction yield, resulting in a higher avDP (3-7), thus yielding true AXOS. Overall, for AXOS production, wheat cultivars with a low bran A/X ratio of their AX are preferable as starting materials, and inactivation of bran-associated enzymes before incubation is desirable. The XHJ xylanase was the best enzyme for wheat bran-derived AXOS production.