Nutritional and physicochemical characterisation of dietary fibre in wheat fractions

Abstract: Dietary fibre is fermented in the colon to short-chain fatty acids (SCFAs), of which especially butyric and propionic acid may have health-promoting effects. One source of dietary fibre may be by-products from wheat, which are currently used as animal feed. The aim of the work presented in this thesis was to perform physicochemical and nutritional characterisation of dietary fibre in waste fractions from wheat, and to examine the potential for using carbohydrate-rich fractions commercially for the design of products with specific health benefits. The dietary fibre content, including arabinoxylan (AX) and fructan, was determined in six different milling fractions, distillers’ grains (DG) from ethanol production and streams from the wet fractionation of wheat flour into starch and gluten. The highest fructan content was found in one stream from wet fractionation, while the shorts and bran fractions were the milling fractions with the highest fructan content. Using xylanase in wet fractionation decreased the molecular weight of the AX and increased the proportion of soluble AX, but did not produce significant amounts of arabinoxylan oligosaccharides or affect the fructans. Mannoproteins and (1→3) and (1→6) β-glucans from the yeast and unidentified low-molecular-weight carbohydrates mainly composed of arabinose were only found in DG. Rats were fed diets containing shorts and fractions thereof (an insoluble, a water-soluble and an ethanol-soluble fraction) and wet process fractions (DG and two fractions from wet fractionation, produced with and without xylanase), separated into supernatant and pellet fractions. The fractions with a high content of soluble AX and fructan (the water-soluble fraction from shorts and the supernatant fractions from wet fractionation), and the supernatant from DG, with a high content of dietary fibre composed of xylose and arabinose, led to the formation of higher amounts of propionic acid in the hindgut, and lower ratios of acetic to propionic acid in the portal blood of the rats, than the other diets. Particularly high proportions of butyric acid were observed with the reference diet containing a mixture of oligofructose and raffinose. The results further indicated that wheat fructans of the molecular weight studied resulted in a higher proportion of propionic acid and a lower proportion of butyric acid than oligofructose. The main differences between the wet process fractions were found between the supernatant and pellet fractions, rather than between the original fractions. Rats fed the supernatant diets generally had higher caecal pools of most SCFAs and lower ratios of acetic to propionic acid in the portal blood than rats fed the corresponding pellet diets. One important difference was the bifidogenic effect of the DG supernatant, although all supernatants led to higher caecal concentrations of Enterobacteriaceae. No effect was seen following xylanase treatment, indicating that the degree of degradation was not sufficient and/or the AX content too low to produce a measurable effect.