Study on hydration properties of wheat bran using bran samples with different particle size

Dietary fiber platforms such as wheat bran exhibit distinctive hydration properties which most probably affect their technological as well as physiological functionality. Whereas hydration properties of fiber impose modifications on food formulations to maintain product quality, they may also affect bulking properties and other beneficial physiological effects in the gastrointestinal tract. To investigate such phenomena, profound understanding of the interaction between dietary fiber platforms and water is a prerequisite. To achieve this, many methods have been used in the past. This has led to different and seemingly contradicting observations on hydration behavior of dietary fiber platforms, while an encompassing theory to explain these findings is not yet available. With this study we aimed to gain insight in the hydration properties of wheat bran as dietary fiber platform model by simultaneously studying and comparing multiple hydration methods performed on a set of six wheat bran samples with average particle sizes ranging between 1687 µm and 77 µm. The sample set was derived from a commercial coarse wheat bran sample by milling with a Cyclotec Sample Mill. Sample hydration properties were evaluated by determining swelling capacity, Enslin-Neff water absorption, Farinograph water absorption and water retention capacity (WRC). The latter was measured according to the prevailing WRC assay but also using a novel WRC assay which allows a better assessment of strongly bound water. Bran with the largest particle size displayed two to three times higher water uptake than the smallest samples based on measurements of swelling capacity, traditional WRC, and Enslin-Neff water absorption. This suggests that destruction of the bran matrix decreases its water uptake potential. However, no significant differences between samples were observed in Farinograph water absorption and WRC values as determined by the novel water retention capacity assay. This was attributed to the mechanical stress imposed on bran in these tests, causing larger particles to release the excess water absorbed in stress free conditions. The differences in outcome between the WRC assays was ascribed to bias caused by particle stacking and reabsorption of water in the conventional WRC method. In conclusion, uptake of water by wheat bran as a dietary platform is rather low and is basically not influenced by particle size when bran is subjected to external forces. Differences in the presence of micropores and stacking efficiency between large and small particles in this case do not affect water uptake since micropore water and stacking water are relatively weakly bound and easily released when subjected to external forces. The water retained by bran when subjected to these forces is most likely held in nanopores of the cell wall matrix and bound to cell wall polysaccharides via hydrogen bonds.