Abstract/Summary

The relationships between wheat dietary fibre (DF) and prebiotic effects were investigated by testing how various DF types in different combinations and how the structure, of the major DF component of wheat, arabinoxylan (AX) modulate prebiotic activity. It was hypothesised that DFs could act synergistically resulting in different prebiotic activities when fermented in vitro at different ratios. The DF, arabinogalactan-peptide (AGP) was isolated from wheat flour and characterised, AX and mixed-linked β-glucan were commercially bought. The three fractions were tested singly and in combination in in vitro fermentation studies to assess their prebiotic activity when used individually and in different combinations. SCFAs produced from in vitro fermentation were measured using HPLC; flow-FISH was used for bacterial enumeration. Increases in bifidobacteria and acetate were observed with combinations of AGP and AX fractions. There were no additive effects on prebiotic activity when AGP and AX were combined; the greatest increases in total SCFAs, bifidobacteria and total bacteria were observed when AX and β-glucan were combined at a 3:1 ratio. The importance of AX structure was investigated using AX isolated from transgenic wheat with an altered arabinosylation pattern and pre-incubation of AX fractions in a simulated gastric digest prior to in vitro colonic fermentation. AX isolated from flour of homozygous TaXAT1 RNAi transgenic wheat showed lower prebiotic activity compared to AX isolated from flour of the corresponding azygous control wheat. Pre-incubation of AX in an in vitro gastric digest resulted in smaller AXOS, a lower A:X ratio, improved bacterial composition and slower fermentation compared to the control. Based on these results, three soluble arabinoxylan (AX) fractions from wheat flour with different average molecular mass (MM) and one insoluble AX fraction were subjected to in vitro gastric and duodenal digestion prior to in vitro fermentation, resulting in an increase in smaller AXOS, decreases in MM, viscosity and A:X ratio and a reduction in bound ferulic acid. Differences in prebiotic activity were observed between AX samples of different MM after changes in structure caused by in vitro digestion. The AX fraction of 323kDa gave the greatest increases in SCFA production, while the insoluble AX fraction showed the smallest increases in SCFA production.