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Short Communication
Dietary fibre and fermentability characteristics of root crops and legumes
Aida C. Mallillin
1
, Trinidad P. Trinidad
1
*, Ruby Raterta
2
, Kevin Dagbay
3
and Anacleta S. Loyola
1
1
Department of Science and Technology (FNRI-DOST), Food and Nutrition Research Institute, Gen. Santos Ave., Bicutan,
Taguig, Metro Manila 1631, Philippines
2
Biological Sciences, University of Santo Tomas, Espana, Manila, Philippines
3
Biochemistry, University of the Philippines, P. Gil St., Ermita, Manila, Philippines
(Received 16 August 2007 – Revised 16 November 2007 – Accepted 23 November 2007 – First published online 11 March 2008)
The dietary fibre and fermentability characteristics of local root crops and legumes were determined. Total, soluble and insoluble fibre were
determined in six root crops (kamote, gabi, potato, tugi, ube, cassava) and ten legumes (mungbean, soyabean, peanut, pole sitao, cowpea, chickpea,
green pea, lima bean, kidney bean and pigeon pea) using Association of Official Analytical Chemists methods. The dietary fibre from test foods was
isolated and fermented in vitro using human faecal inoculum simulating conditions in the human colon. The SCFA, e.g. acetate, propionate, butyrate,
produced after fibre fermentation was measured using HPLC. The dietary fibre content of root crops ranged from 4·6 to 13·5 g/100 g while legumes
ranged from 20·9 to 46·9 g/100 g, suggesting that root crops and legumes are good sources of dietary fibre. Significant amounts of SCFA were pro-
duced after in vitro fermentation of the fibre isolate of both root crops and legumes. The best sources (as mmol/g fibre isolate) of acetate among the
legumes were pole sitao (5·6 (
SEM 0·5)) and mungbean (5·3 (SEM 0·1)) and among the root crops, tugi (2·5 (SEM 0·4)) and cassava (2·4 (SE M 0·1));
of propionate, kidney bean (7·2 (
SEM 1·5)) and pigeon pea (3·3 (SEM 0·2)) for legumes, and tugi (1·8 (SEM 0·2)) for root crops; and of butyrate, peanut
(6·0 (
SEM 0·2)) and cowpea (5·4 (SEM 0·2)) for legumes, and tugi (0·8 (SEM 0·0)) and cassava (0·8 (SEM 0·0)) for root crops. In conclusion, root crops
and legumes are good sources of dietary fibre and produced SCFA after fibre fermentation, such as acetate, propionate and butyrate. SCFA pro-
duction after in vitro fermentation can be estimated using human faecal inoculum and can be used to model the human colon.
Dietary fibre: Legumes: Root crops: Fermentation
Increased intake of dietary fibre is recommended in the proper
control of chronic diseases, e.g. diabetes mellitus, CVD and
cancer
(1,2)
. It is now well established that dietary fibre is not
metabolized in the small intestine and reaches the colon
where it is fermented to produce SCFA such as acetate, pro-
pionate and butyrate, and gases such as carbon dioxide,
methane and hydrogen
(3,4)
. SCFA contributes 6·3–8·4 kJ/g
dietary fibre (1·5– 2·0 kcal/g dietary fibre)
(5)
. Propionate has
been shown to inhibit the activity of the enzyme 3-hydroxy-
3-methylglutaryl-CoA reductase, the limiting enzyme for
cholesterol synthesis
(6)
. Butyrate enhances cell differentiation
thus preventing tumour formation in the colon
(7)
. Dietary fibre
can be soluble or insoluble depending upon solubility in water.
Water-soluble fibre can form viscous solutions. Increased
viscosity in the intestine slows intestinal transit time, delays
gastric emptying
(8,9)
, and slows glucose and sterol absorption
by the intestine
(10,11)
. Viscous soluble fibre can control the
release of glucose with time thus lowering postprandial
blood glucose and insulin levels, and serum cholesterol
(9)
.
Insoluble fibre consists of lignin, cellulose and hemicellulose.
It has usually high water-holding capacity and contributes
to increased faecal bulk and frequency of defecation
(12,13)
.
Moreover, all fibres, both soluble and insoluble, can entrap
bile acid and prevents its re-absorption in the liver thus
inhibiting cholesterol synthesis
(14)
. The consumption of
root crops and tubers in the Philippines is 19 g/d while con-
sumption of beans, nuts and seeds is 10 g/d
(15)
. The present
study will determine the dietary fibre and fermentability
characteristics of root crops and legumes.
Material and methods
Test foods
In the present study, six root crops (ube (Dioscorea alata), gabi
(Colocasia esculenta), tugi (Dioscorea esculenta), potato (Sola-
num tuberosum), camote ( Ipomoea batatas) and cassava (Man-
ihot esculanta)) and ten legumes (cowpea (Vigna uriguiculata
(L.) Walp.), mungbean (Vigna radiate (L.) Wilczek), pole
sitao (Vigna uriguiculata subsp. Sesuipedalos L. Verde), chick-
peas (Cicer arietinum), green peas (Pisum sativum L.), peanut
(Arachis hypogaea L.), pigeon pea (Cajanus cajan), kidney
beans (Phaseolus vilgaris L.), lima beans (Phaseolus lunatus)
and soyabeans (Glycine soja)) were used as test foods. Food
* Corresponding author: Dr Trinidad P. Trinidad, fax þ 632 837 2934, email tptrinidad@yahoo.com
British Journal of Nutrition (2008), 100, 485–488 doi:10.1017/S000711450891151X
q The Authors 2008
British Journal of Nutrition
samples were bought in local markets. Root crops were cooked
in water while legumes were soaked in water overnight, and
boiled to cook the next day. Both root crops and legumes
were freeze-dried before analysis.
Analytical methods
The proximate analysis of all test food was determined using
Association of Official Analytical Chemists methods, and the
test foods were also analysed for total, soluble and insoluble
fibre using Association of Official Analytical Chemists
methods
(16,17)
against a standard wheat flour (NBS Standard
Reference Material 1567 A; Gaithersburg, MD, USA).
In vitro fermentation
Dietary fibre isolates from each test food were fermented
in vitro using human faecal inoculum
(18)
. Dietary fibre was
isolated from test samples using Association of Official Ana-
lytical Chemists methods. Fibre isolate (0·5 g) was weighed in
a serum bottle. Fermentation media (40 ml of a mixture of
2 litres deionized water, 1 litre 0·5
M-sodium bicarbonate
buffer solution, 1 litre macromineral solution, 5 ml 0·1 % resa-
zurin) and reducing solution (2 ml of a mixture of 1·25 g
cysteine-HCl þ fifty pellets of potassium hydroxide in
100 ml deionized water and 1·25 g sodium sulphide in
100 ml deionized water) were added to the serum bottle and
flushed with carbon dioxide until colourless. The bottles
were sealed with rubber stoppers and an aluminium seal and
stored at 48C. The next day the bottles were placed in a water-
bath for 1 –2 h at 378C. A 10 ml faecal inoculum (1:15 dilution
of fresh faeces from a healthy female human volunteer eating
an unspecified diet with no intake of antibiotics for a year) was
added into each bottle and the mixture was incubated for 24 h
at 378C without mixing. The fermented digest was filtered
and read in a high-pressure liquid chromatograph (LC10
Shimadzu, Shimadzu, Tokyo, Japan) to measure SCFA against
a volatile acid standard mixture of acetate, propionate and
butyrate (Supelco, Philadelphia, PA, USA). No internal stan-
dards were used.
Statistical analysis
Differences between test foods and food analyses were deter-
mined by ANOVA and Duncan’s multiple range test using the
Statistical Analysis System program (SAS Institute Inc., Cary,
NC, USA).
Results
Table 1 shows the nutritional composition of root crops and
legumes. Legumes are the best sources of protein and are sig-
nificantly greater than root crops (Table 1; P, 0·05). Root
crops have significantly lower fat content than legumes
(Table 1; P, 0·05). The dietary fibre content of root crops
ranged from 4·6 to 13·5 g/100 g while legumes ranged from
20·9 to 46·9 g/100 g, suggesting that both root crops and
legumes are good sources of dietary fibre (Table 1). The
dietary fibre content of legumes was significantly greater
than that of root crops. Soyabean has significantly greater diet-
ary fibre content (46·9 (
SEM 3·4)g/100 g) among the legumes
while gabi has significantly greater dietary fibre content
(13·5 (
SEM 0·1)g/100 g) among the root crops (Table 1;
P, 0·05). The ratio of insoluble to soluble fibre in root
crops ranged from 1:1 to 2:1 while for legumes 5:1 to 5:19
excluding kidney beans, which was approximately 99 %
insoluble fibre. However, the ratio of insoluble to soluble
fibre in all test foods was not significantly associated with pro-
duction of SCFA (Table 1; P, 0·05). Significant amounts of
SCFA were produced after fermentation of the fibre isolate
of both root crops and legumes, with cowpea, kidney bean
and peanut having the greatest production of SCFA for
legumes, and tugi and cassava for root crops (Table 1). The
best sources (as mmol/g fibre isolate) of acetate among the
legumes were pole sitao (5·6 (
SEM 0·5)) and mungbean (5·3
(
SEM 0·1)) while among the root crops, tugi (2·5 (SEM 0·4))
and cassava (2·4 (
SEM 0·1)); for propionate, kidney bean (7·2
(
SEM 1·5)) and pigeon pea (3·3 (SEM 0·2); Table 1); and for
butyrate, peanut (6·0 (
SEM 0·2)) and cowpea (5·4 (SEM 0·2))
among the legumes, and tugi (0·8 (
SEM 0·0)) and cassava
(0·8 (
SEM 0·0); Table 1) among the root crops.
Discussion
Colonic fermentation studies ideally should be done in vivo.
However, the difficulty of experimental variables often
makes in vivo studies difficult to conduct. Moreover, the
rate and extent of fermentation determined from SCFA pro-
duction cannot be estimated with much accuracy in vivo
(18)
.
The in vitro fermentation method using human faecal inocu-
lum has been used to study rate and extent of fermentation
(18)
.
The method provided SCFA molar ratios which are similar to
those seen in vivo and therefore can be used to model the
human colon. It demonstrated that faecal materials collected
from man at different times of the year provide a sufficiently
uniform source of micro-organisms so that fermentation
values such as fibre digestibility and gas or SCFA production
can be compared between studies
(18)
. It also showed the differ-
ences in fermentability of different fibre sources. For example,
pectin was found to be most rapidly fermented followed by
psyllium gum, tragacanth gum and cellulose
(18)
. However, in
the present study we did not determine the rate and extent
of fermentation of the different test foods since it was not
one of the objectives of the study. We only determined the
SCFA produced after 24 h of fermentation which mimics
human fermentation in the colon.
The study showed that root crops and legumes are good
sources of dietary fibre. Although both root crops and legumes
have significant amounts of dietary fibre, there must be some
differences in their physiological effects. Root crops contained
low fat and high fibre and can be a good substitute for bread
and rice. Flaxseed (28·0 g/100 g dietary fibre) may be considered
a legume and has been shown to have some protective and pre-
ventive effects on CVD
(19)
and colon and breast cancer
(20 – 22)
.
The physiologic effect of legumes may be similar to flaxseed.
Legumes maybe a better laxative than root crops because of
the high percentage of insoluble fibre. Intakes of legumes may
increase faecal bulk and decrease transit time due to excellent
water-holding capacity. Legumes may also serve to dilute sec-
ondary bile acids in the colon, thereby reducing colonic
exposure to these substances. The higher percentage of
soluble fibre in root crops compared to legumes may have phys-
A. C. Mallillin et al.486
British Journal of Nutrition
Table 1. Nutritional composition of root crops and legumes (g/100 g sample)
(Mean values with their standard errors)
Fat Protein CHO* Dietary fibre Soluble fibre Insoluble fibre Acetate† Propionate† Butyrate†
Test foods Mean
SEM Mean SEM Mean SEM Mean SEM Mean SEM Mean SEM Mean SEM Mean SEM Mean SEM
Root crops
Kamote 0·6
k
0·2 3·4
h
0·0 89·6
c
0·3 8·1
j
0·1 4·8
c
0·4 3·2
i
0·2 1·1
gx
0·2 0·4
hz
0·0 0·5
jy
0·0
Gabi 0·4
k
0·0 1·1
j
0·0 90·7
b
0·2 13·5
g
0·1 3·6
e
0·1 9·5
f
0·2 1·1
gx
0·1 0·2
iy
0·0 0·2
ky
0·0
Potato 1·0
j
0·1 10·0
e
0·1 77·1
e
0·4 7·6
k
0·2 3·6
e
0·1 4·0
h
0·2 0·2
hy
0·0 0·1
jy
0·1 0·6
hijx
0·1
Tugi 0·2
l
0·1 4·8
g
0·1 75·6
f
0·8 10·3
i
0·1 3·6
e
0·1 7·0
g
0·2 2·5
dex
0·4 1·8
dey
0·2 0·8
gz
0·0
Ube 0·4
k
0·0 5·2
f
0·1 87·0
d
0·3 11·8
h
0·2 4·4
d
0·1 7·4
g
0·2 0·2
hx
0·0 0·1
jy
0·0 0·1
kx
0·1
Cassava 0·3
l
0·0 2·4
i
0·0 91·1
a
0·1 4·6
l
0·2 1·4
g
0·4 3·2
i
0·1 2·4
ex
0·1 0·7
fz
0·1 0·8
gx
0·0
Legumes
Mungbean 5·8
a
0·4 14·6
de
4·9 64·1
gh
5·4 31·7
c
0·1 4·8
c
0·3 26·9
c
0·2 5·3
ax
0·1 1·7
dz
0·0 2·5
dy
0·2
Soyabean 2·8
d
0·1 33·3
a
1·8 55·8
i
1·9 46·9
a
3·4 8·0
a
0·2 38·9
a
0·2 3·8
cx
0·5 1·4
ey
0·2 0·0
Peanut 1·9
h
0·9 22·1
bc
8·5 62·0
gh
9·5 24·1
e
1·7 4·2
cde
0·7 20·0
d
0·7 2·9
dy
0·1 2·2
cz
0·1 6·0
ax
0·2
Pole sitao 4·2
b
0·1 11·5
e
4·4 75·8
efg
4·6 35·0
b
0·2 5·5
b
0·3 29·5
b
0·6 5·6
ax
0·5 1·9
dx
0·1 0·7
hz
0·0
Cowpea 3·2
c
0·1 22·3
c
3·9 67·0
g
4·1 34·0
b
0·6 4·0
de
0·2 29·8
b
0·6 4·8
by
0·1 2·1
cdz
0·2 5·4
bx
0·2
Chickpea 2·3
f
0·1 20·7
c
0·1 69·5
g
0·3 26·2
e
0·7 1·3
g
0·1 24·9
d
0·6 2·8
dx
0·2 0·6
fghz
0·2 0·9
fy
0·0
Green pea 1·0
j
0·1 21·5
cd
0·1 69·4
g
0·3 29·7
d
0·9 2·1
f
0·1 27·6
c
0·6 1·9
fx
0·1 0·7
fy
0·1 0·6
iy
0·0
Lima bean 2·0
g
0·1 23·3
c
1·9 60·1
h
2·5 20·9
f
0·1 3·7
e
0·1 17·7
e
0·2 2·0
fx
0·1 0·6
gy
0·0 0·1
kz
0·1
Kidney bean 2·5
e
0·3 28·3
b
0·0 60·4
h
0·3 29·8
d
0·3 0·4
h
0·1 29·4
b
1·8 0·4
hz
0·2 7·2
ax
1·5 3·6
cy
0·7
Pigeon pea 1·3
i
0·2 24·5
c
0·1 63·2
h
0·4 21·8
f
1·1 2·4
f
0·2 19·4
d
0·6 0·0 3·3
bx
0·2 1·5
ey
0·1
CHO, carbohydrates.
a–l
Mean values within a column with unlike superscript letters were significantly different (P, 0·05).
xyz
For SCFA mean values within a row with unlike superscript letters were significantly different (P, 0·05).
* Calculated by difference (100 – (ash, moisture, fat, protein)).
† SCFA (mmol/g fibre isolate).
Dietary fibre from root crops and legumes 487
British Journal of Nutrition
iological effects related to glucose and cholesterol metab-
olism
(7,8)
. Generally, soluble fibres have a tendency to have
long-term effects associated with better glycaemic control and
lower insulin requirements due to increased insulin sensitivity
or changes in insulin receptors as well as cholesterol-lowering
effects. In terms of SCFA production, a good source of butyrate,
e.g. tugi and cassava (root crops) and peanut and cowpea
(legumes), may have an important role in the prevention for
risk of colon cancer, while a good source of propionate, e.g.
kidney beans and pigeon pea (legumes), may have an important
role in cholesterol-lowering effects.
In conclusion, both root crops and legumes are good sources
of dietary fibre and produced SCFA after fibre fermentation,
such as acetate, propionate and butyrate. SCFA production
after in vitro fermentation can be estimated using human
faecal inoculum and can be used to model the human colon.
Acknowledgements
The project was supported by the Food and Nutrition Research
Institute, Department of Science and Technology, Philippines.
T. P. T. supervised the implementation of the project and
evaluated data and reports; A. C. M. analysed dietary fibre
and SCFA of root crops, conducted data analysis and wrote
the paper; A. S. L. conducted proximate analysis of root
crops, and analysed dietary fibre and SCFA of legumes;
R. R. and K. D. conducted proximate analysis of five legumes
and assisted in the analysis of dietary fibre and SCFA. There
are no conflicts of interest. We would also like to extend our
appreciation to Mr Zoilo B. Villanueva for freeze-drying the
food samples used in the study.
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