Dietary fibre components of rye bran and their fermentation in vitro: Dissertation

Dietary fibre (DF) is defined for nutritional purposes as the non-digestible part of plant food. Because DF has many physiological effects along the entire human gastrointestinal tract, it is important for human well-being. Many of the health effects are mediated by the microbial fermentation of DF carbohydrates in the large intestine. Rye is the main source of DF in Finland. Rye bran, in particular, is rich in DF. According to the traditional method of estimating DF the content is about 38 g/100 g on a dry weight basis. Rye bran was also shown to contain a significant fructan concentration (7 g/100 g), which according to the suggested new dietary fibre concept is also a component of DF. Rye grain contained 4.6-6.6 g of fructan/100 g depending on the growth conditions. In traditional soft rye bread, the DF content increases from 11 to 14 g/100 g because of fructan. In the average Finnish rye intake (43 g/day), this means that the total DF intake increases from 7.7 g/day to about 10 g/day. Different rye bran fractions and processed rye bran were prepared in order to study the effect of solubility and processing on the fermentability of rye bran. Before fermentation, substrates were enzymatically digested simulating conditions within the small intestine in order to remove starch and protein. Fermentability was studied in vitro using human faecal inoculum. Soluble rye-bran fractions were fermented rapidly and completely. Rye bran and the rye-bran residue, after water- or alkali-extraction, were fermented at a slower rate, but the fermentation continued throughout the entire fermentation time (24 h). About half of the neutral sugars (arabinose, xylose and glucose) of the total fermentation of rye bran (including the neutral sugars of the inoculum) was consumed over 24 h. Xylanase treatment of rye bran increased the initial rate of fermentation slightly, but after 24 h fermentation the consumption of carbohydrates was the same as in the fermentation of the original rye bran. Short-chain fatty acids (SCFA) were produced, which were related to the consumption of carbohydrates: the higher the rate and quantity of carbohydrate consumption the higher the rate and quantity of SCFA production. However, the ratio of SCFA produced to carbohydrates consumed changed between experiments depending on the faecal inoculum. Butyric and propionic acids were produced in all rye fermentations, but xylanase treatment slightly decreased their production. The most rapid butyric-acid production was shown to be in the fermentation of soluble fractions, but the extent was similar in all rye fermentations. The fermentation rate of rye bran was the same as that of wheat bran, but the extent of fermentation was higher in the case of rye bran. Rye bran was fermented at a slower rate than oat bran. In in vitro experiments with single bacterial strains, all the Bifidobacterium longum strains and one of the Bifidobacterium adolescentis strains examined were able to grow using rye arabinoxylan as the sole carbon source. Arabinoxylan from rye may also have potential as a prebiotic substrate for the proliferation of Bifidobacterium longum, a numerically dominant Bifidobacterium species in the adult human colon. Many Bifidobacterium species were able also to efficiently ferment xylo-oligosaccharides. Rye bran contained plant lignans secoisolariciresinol, matairesinol, syringaresinol, pinoresinol, lariciresinol and isolariciresinol, the sum of which amounted to 5 mg/100 g. The effect of processing of rye bran on the conversion of plant lignans to mammalian lignans (enterodiol, enterolactone) was studied in vitro using a human faecal inoculum. The highest enterodiol formation was found in the fermentation of soluble rye-bran extract. Also, xylanase treatment of rye bran slightly increased enterodiol formation. Enterolactone production was very slow and hardly detectable.