Abstract
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.
Original language | English |
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Qualification | Doctor Degree |
Awarding Institution |
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Supervisors/Advisors |
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Award date | 15 Aug 2003 |
Place of Publication | Espoo |
Publisher | |
Print ISBNs | 951-38-6229-1 |
Electronic ISBNs | 951-38-6230-5 |
Publication status | Published - 2003 |
MoE publication type | G5 Doctoral dissertation (article) |
Keywords
- dietary fibres
- rye bran
- fermentation
- in vitro fermentation
- carbohydrates
- metabolism
- prebiotics
- lignans
- extraction
- chemical analysis