Effect of wheat bran ball-milling on fragmentation and marker extractability of the aleurone layer

Abstract

Bran (branml) obtained by roller milling of soft (Scipion) and hard (Baroudeur) wheat cultivars was further ball-milled for increasing times and the observed particle size distribution expressed as a dispersion index. Bran (branhi) and aleurone layers were also hand-isolated from the same grains and the pattern of size reduction during ball-milling were compared with branml. Branml and branhi were found to fracture more rapidly than isolated aleurone layers due to the presence of the highly friable pericarp and the possible mechanical constraints due to tissues surrounding the aleurone layer. Previously identified markers of the aleurone layer cell contents (phytates) and cell walls (p-coumaric acid) were used to determine their water extractabilities from ball-milled samples and the state and degree of dissociation of the aleurone layer, either as an isolated tissue or within branml and branhi. The results suggest that ball-milling rapidly induces fractures in walls of cells in the aleurone layer. The partial opening of the cells in the aleurone layer allowed extraction of most (≈70%) of the water-extractable phytates, even though their mean particle size was much larger than the dimensions of the cells. A further increase in extractability of phytates was observed when the particle size was reduced below the aleurone cell dimensions. Although much less soluble, p-coumaric acid followed a similar trend to phytates. The different behaviour of branml and branhi was consistent with a weakening effect of the tissues in the former, probably due to the previous milling process. The bran and aleurone layers from both wheat varieties exhibited a similar behaviour.