Extensive Dry Ball Milling of Wheat and Rye Bran Leads to in Situ Production of Arabinoxylan Oligosaccharides through Nanoscale Fragmentation

This study investigated the potential of ball milling as a dry process for in situ production of arabinoxylan oligosaccharides (AXOS) in arabinoxylan (AX)-rich wheat and rye bran. An extensive lab-scale ball mill treatment (120 h, 50% jar volume capacity) increased the wheat bran water-extractable arabinoxylan (WE-AX) level from 4% (untreated bran) to 61% of the wheat bran AX. Extractable AX fragments had an arabinose/xylose ratio (A/X ratio) of 0.72 and a molecular mass (MM) of 15 kDa. Ball milling of rye bran gave rise to similar results, with the A/X ratio of the extractable AX being considerably lower (0.51). Optimization of the ball mill treatment by varying the degree of filling of the milling jar permitted us to obtain equally high WE-AX levels (>70%) in wheat and rye bran within a 24 h ball-milling period. Ball milling at optimal conditions (24 h, 16% jar volume capacity) yielded wheat bran AXOS, with an A/X ratio of 0.65 and a MM of 6 kDa. Ball milling (24 h, 50% jar volume capacity) of pericarp-enriched wheat bran increased the WE-AX level from 1% (untreated pericarp) to 63%. The extractable material had a high A/X ratio (0.97) and a low MM (5 kDa). Fluorescence microscopy revealed that the extensive ball mill treatment led to the almost complete disappearance of discernible tissue structures in the ball-milled material, indicating bran particle size reductions down to the nanoscale level. It further visualized the aggregation of the treated material. These results show that AXOS can be produced in situ from wheat or rye bran in a single-stage dry milling process, rendering a wet extraction step redundant. The higher A/X ratio of the obtained AXOS than of enzymically produced wheat bran-derived AXOS offers perspectives for the production of a wide range of AXOS structures. Moreover, ball milling makes upgrading of the low-value pericarp layer feasible.