J Food Sci Technol (Nov–Dec 2010) 47(6):644–649
DOI 10.1007/s13197-010-0106-1
ORIGINAL ARTICLE
A comparative study on starch digestibility, glycemic index
and resistant starch of pigmented (‘Njavara’ and ‘Jyothi’)
and a non-pigmented (‘IR 64’) rice varieties
G. Deepa & Vasudeva Singh & K. Akhilender Naidu
Revised: 16 June 2010 / Accepted: 21 June 2010 / Published online: 9 October 2010
# Association of Food Scientists & Technologists (India) 2010
Abstract In vitro starch digestibility and glycemic indices
of three rice varieties- ‘Njavara’, ‘Jyothi’ (pigmented rice
verities) and ‘IR 64’ (non-pigmented rice) with similar
amylose content were studied. Starch digestibility studies
showed differences in glycemic response in three types of
rice. The rate of starch hydrolysis was maximum (67.3%) in
‘Njavara’ rice compared to other two rice varieties.
‘Njavara’ exhibited the lowest kinetic constant (k) indicating
inherent resistance to enzymatic hydrolysis. The glycemic
load (GL) and glycemic index (GI) of ‘Njavara’ were similar
to ‘Jyothi’ and ‘IR 64’. Resistant starch content was high in
pigmented rice varieties compared to ‘IR 64’. The resistant
starch content of dehusked and cooked rice increased with
the storage time at refrigeration temperature (4°C). ‘Njavara’
is an easily digestible rice and can be used for baby and
geriatric foods.
Keywords Rice . Njavara . Jyothi . Resistant starch . In vitro
starch digestibility . Glycemic index . Glycemic load
Introduction
Carbohydrates are a major source of energy in human diet.
In recent years, obesity and diabetes are the foremost
K. A. Naidu (*)
Department of Biochemistry and Nutrition,
Central Food Technological and Research Institute
(Council of Scientific and Industrial Research),
Mysore 570020, India
e-mail: kanaidu@mailcity.com
G. Deepa : V. Singh
Department of Grain Science and Technology,
Central Food Technological and Research Institute
(Council of Scientific and Industrial Research),
Mysore 570020, India
problems of mankind. Carbohydrates are not just a source
of calories but specific types of carbohydrates are included
in the diet depending on the physiological disorder. People
are aware of the alterations in blood glucose levels or
glycemic index after consuming carbohydrate rich food.
Currently, consumption of whole grain diet has received
considerable attention due to its health benefits in attenuating
chronic diseases including cardiovascular disease, type II
diabetes, cancer and gastrointestinal disorders (Marquart et
al. 2002). Whole grain cereals are a unique source of
dietary fibre containing several bioactive compounds
(Englyst et al. 1995) and nutrients (Marquart et al. 2002).
Resistant starch is the residual fractions of starch, resistant
to enzyme hydrolysis, entering the large intestine along
with dietary fibre. Though resistant starch accounts only a
small proportion of the total calorie intake, its effect is
similar to those of other fibre components (Bjorck 1996).
The glycemic index (GI) and resistant starch (RS) content
have been established as important indicators of starch
digestibility.
Rice is the most important cereal crop and one of the
staple foods of the world’s population. It is an easily
digestable fine cereal, producing high glycemic index
(Wolever et al. 1990) and low colonic fermentation (Kerlin
et al. 1984). The GI of brown rice has been reported to be
96, white rice is 83, while the freshly cooked rice is 64–93
(Miller et al. 1992). However, discrepancies in GI of rice
have been reported due to differences in varieties, amylose
content (Sagum and Arcot 2000; Frei et al. 2003),
processing and cooking (Frei et al. 2003), particle size
(Snow and O’Dea 1981), physicochemical characters like
gelatinization (Chung et al. 2006; Panlasigui et al. 1991),
amylose/ amylopectin ratio (Juliano and Goddard 1986),
lipid-amylose complex (Guraya et al. 1997) and differential
susceptibility to amylolytic enzymes (Jenkins et al. 1982).
Rice is considered as a good source of insoluble fibre.
J Food Sci Technol (Nov–Dec 2010) 47(6):644–649
There are limited studies on brown rice or dehusked rice
compared to milled rice.
‘Njavara’ var. rice (Oryza sativa L.) is a medicinal red
rice variety endemic to Kerala, in India. Morphologically,
‘Njavara’ is similar to ordinary rice with husk colour
varying from golden yellow to brownish black, depending
upon the edaphic and climatic conditions (Menon 2004).
The medicinal quality of ‘Njavara’ is preserved by using
only dehusked rice. It is the main component of ‘Njavara’
kizhi, an Ayurvedic treatment where a bolus is prepared by
cooking rice with milk and certain herbs like Sida rectusa
and Alpinia galangal and massaged over entire body for
treatment of paralysis, arthritis and neurological problems.
The local uses of ‘Njavara’ include consumption of rice
cooked in copper vessel, to prevent rheumatic complaints.
The rice is consumed to give high energy and gain weight.
‘Njavara’ is also recommended for lactating mothers and
growing babies. ‘Njavara’ rice cooked along with medicinal
herbs (Monsoon porridge) is consumed during monsoon
season to increase immunity. ‘Jyothi’ (‘PTB 39’) is a hybrid,
red rice variety consumed as a staple food in Kerala. ‘IR 64’
is a non-pigmented hybrid variety, known for its palatability
and high yield. Our previous study reports the nutrient
composition and physicochemical properties of ‘Njavara’
rice (Deepa et al. 2008). This study reports the starch
digestibility, GI and RS of brown rice flour of a medicinal
rice –‘Njavara’ and two non-medicinal rice varieties
‘Jyothi’ and ‘IR 64’.
Materials and methods
‘Njavara’ paddy was brought from Padma Ayurveda, Mannar
(Kerala) while ‘Jyothi’ and ‘IR 64’ paddy were procured
from Agriculture Products Marketing Cooperative market in
Bandipalya, Mysore. Paddy harvested in December 2003 was
obtained and stored at room temperature (27±2°C) for one
year and five months and then shifted to cold (4–6°C) until
use. Amylases (Bacillus amyloliquefaciens), glucosidase
(Rhizopus sp.) and pepsin were purchased from Sigma
Chemical (USA). Glucose oxidase peroxidase kit was
purchased from Monozyme India Pvt. Ltd. (Secundrabad,
India). All other chemicals used were of analytical grade.
The paddy samples were dehusked using rubber roll
dehusker (Satake Corporation, Tokyo, Japan) and ground
into flour (−60 mesh) using a rice mill (Surabhi, India). The
dehusked rice and brown rice flour were stored at 4°C until
use.
Sample preparation Brown rice flour (10%), dry basis, was
made into slurry in glass distilled water and made up to
500 ml. The slurry was poured into Brabender viscograph
bowl and heated from 30 to 95°C, maintained at 95°C for
645
20 min and then cooled to 30°C, at a rate of 1.5o C per min
with constant stirring. The cooked rice flour paste was then
cooled to room temperature and then processed as.
(a) Freeze dried (0 h), (b) Kept at 4°C for 24 h and then
freeze dried (24 h) and (c) Kept at 4°C for 48 h and
then freeze dried (48 h).
The samples were stored at 4°C until used for the
experiments.
Total starch Brown rice flour (−60 mesh) and processed
rice flour (100 mg db), were dispersed in 50 ml of water
and treated with Termamyl (100 μl) and incubated in
boiling water bath for 10 min, cooled and equilibrated at
60°C. Solubilized starch was then hydrolyzed by adding
glucosidase (6 mg in 0.6 ml acetate buffer pH 4.6) and
incubated in shaking water bath at 60°C for 2 h. The
samples were centrifuged and filtered. The supernatant was
made up to a known volume. The glucose concentration in
the supernatant was determined using glucose oxidase
peroxidase kit at 505 nm. Starch was calculated as glucose
x 0.9.
Resistant Starch (RS) Native and processed rice flour
(100 mg, db) were suspended in water (50 ml) and treated
with Termamyl (100 μl) at 95°C for 45 min, cooled,
centrifuged and supernatant was discarded. The residue was
hydrolyzed with protease (10 mg in phosphate buffer
pH 7.5) and amyloglucosidase (10 mg 0.1 M acetate buffer
pH 4.75) to remove proteins and hydrolyze starch,
respectively. The residues were dissolved in 2 M KOH,
incubated with amyloglucosidase for 35 min at 60°C to
hydrolyze RS. Glucose content in the above samples was
determined using glucose oxidase peroxidase kit. Digestible
starch was calculated as the difference between total starch
and RS.
Starch kinetics Brown rice flour (50 mg, db) was cooked in
5.0 ml of water for 30 min and incubated with 10 ml of
pepsin solution prepared in HCl-KCl buffer pH 1.5 at 40° C
for 1 h in a shaking water bath. The volume of the samples
was made to 25 ml using Tris-Maleate buffer (pH 6.9).
Reaction was started by adding α-amylase (2.6 units in
5 ml of buffer pH 6.9) and the samples were incubated at
37o C in a shaking water bath. One ml of the sample aliquot
was collected at intervals of 30 min for 3 h. The enzyme
activity in these aliquots was inactivated by heating at 100°C
for 5 min and refrigerated until the end of the incubation
period. To these aliquots, 3 ml of 0.4 M sodium acetate
buffer (pH 4.75) and 60 μl amyloglucosidase were added to
hydrolyze the digested starch to glucose. The samples were
incubated at 60 °C for 45 min. The glucose content in each
aliquot was estimated using glucose oxidase peroxidase kit.
646
Glucose was converted into starch by multiplying with 0.9.
All the experiments were conducted thrice and with
triplicates in each analysis. The kinetics of starch digestion
was estimated by non-linear first order equation established
by Goni et al. (1997).
C ¼ Ca 1 e kt ;
where, C corresponds to the concentration of starch hydrolyzed at time t. Cα represents the equilibrium concentration
i.e. the percentage of starch hydrolyzed after 180 min. k is
the kinetic constant. The parameters Cα and k were estimated
for each cultivar based on the data obtained from the in vitro
starch hydrolysis procedure. Parameters were estimated
using SYSTAT (Sigma Plot 10) software, MS Office version.
The hydrolysis index (HI) was calculated as the
percentage of total glucose released from the samples as
compared to that released from standard glucose (0–
180 min). The glycemic indices of the samples were
estimated according to the equation of Goni et al. (1997),
with the use of glucose as the reference food:
GI ¼ 39:71Ca þ 0:549 HI
Glycemic load (GL) was estimated indirectly by multiplying the amount of carbohydrate contained in a nominal
serving size (150 g) of rice with GI value of specific rice
variety, divided by 100 (Salmeron et al. 1997).The
available carbohydrate per serving (33 g carbohydrate/
serving) of boiled brown rice in India was obtained from
literature (Kurup and Krishnamurthy 1992; Foster-Powell et
al. 2002).
GL ¼ GI carbohydrate net content per portion in g=100
Statistical analysis Analysis of variance (ANOVA) was
performed by using SPSS system for windows version 7.5.
Duncan’s multiple range tests were conducted for comparison of means at p<0.05. Simple correlation coefficients
were calculated for the relationships between nutrient
composition (carbohydrate, protein, lipids and dietary fibre)
and food indexes like total starch content, resistant starch
and digestible starch and among the food indices.
Results and discussion
The total starch content was 79–89% in three rice varieties
(Table 1) ‘IR 64’ had more starch content than the two red
rice varieties, ‘Njavara’ and ‘Jyothi’. The resistant starch
content was 0.6–1% in three rice varieties. The pigmented
rice varieties showed more resistant starch content than the
un-pigmented variety, ‘IR 64’. Resistant starch of all the
rice varieties increased with storage at refrigeration (4°C).
J Food Sci Technol (Nov–Dec 2010) 47(6):644–649
Table 1 Total starch, resistant starch and digestible starch of brown rice
flour (% w/w dry weight) after cooking and stored at refrigerated temp
Parameters
Njavara
Jyothi
Total starch
Native
79.56±0.28 a
0h
85.58±3.84 b
24 h
85.50±3.29 a,b
48 h
85.24±2.67 b
Resistant starch
Native
0.80±0.06 b
0h
0.80±0.02 a
24 h
0.94±0.15 a,b
48 h
1.05±0.03 a
Digestible starch
Native
79.16±0.42 a
0h
84.78±2.70 b
24 h
48 h
84.05±2.48
84.19±1.86
b
b
IR 64
80.67±2.52
84.69±0.60
86.62±0.40
86.33±0.27
0.83±0.01
0.83±0.01
0.98±0.17
1.10±0.27
a
b
b
b
a
0.68±0.08
0.64±0.04
0.68±0.02
0.71±0.05
a
a
a
80. 67±1.78 a
83.86±0.42 b
85.64±0.11
85.23±0.08
84.30±1.29
88.89±0.43
89.34±0.65
89.30±0.64
b
b
a
a
a
a
c
b
b
b
83.96±0.83
88.24±0.35
a
88.66±0.45
88.60±0.50
a
a
a
Values within the same row with different superscripts are significantly different (p<0.05) (n=3)
Various physical factors like stirring, water-starch ratio,
cooking and cooling regimes affect resistant starch formation (Garcia-Alonso et al. 1999). In order to avoid these
discrepancies all the rice varieties were cooked in Brabender
Viscograph Type 801202 (Duisburg, FRG). Resistant starch
formation is also influenced by amylose-amylopectin ratio
in rice (Frei et al. 2003). Starch when cooked and cooled,
rearrangement of amylose and amylopectin chains occur
(retrogradation), which leads to increase in crystalline nature
(B-type) of starch granules (Jane and Robyt 1984) and
decreased starch digestibility. During retrogradation the
amylose chains form double helix structure (Jane and Robyt
1984) while amylopectin crystallization occurs by reassociation of the outermost short chains (Ring et al.
1987). Retrogradation of amylose is a more rapid process,
occurring immediately, while cooling, but amylopectin
requires longer time and hence storage conditions are
important factors affecting retrogradation (Garcia-Alonso
et al. 1999).
The digestible starch among the three varieties varied
from 79 to 84% with ‘IR 64’ having more digestible
fraction of starch while ‘Njavara’ showing the least
(Table 1). In processed dehusked rice, the digestibility
increased by 5–6%. Native starches are mostly indigestible.
Cooking in excess of water leads to swelling of starch
granules followed by disintegration, exposing the starch
chains and making them more accessible to the action of
digestive enzymes. In this study also the observed total
starch and digestible starch after gelatinization (0, 24 and
48 h) is due to exposure of amylose and amylopectin chains
J Food Sci Technol (Nov–Dec 2010) 47(6):644–649
647
Table 2 Correlation coefficients among total starch, resistant starch and digestible starch in vitro
Total starch
Native
Total starch
Native
0h
24 h
48 h
1
0.483
0.579
0.615
Resistant starch
Native
−0.566
0h
−0.735*
24 h
−0.540
48 h
−0.690*
Digestive starch
Native
0.996#
0h
0.497
24 h
0.586
48 h
0.646
Resistant starch
0h
24 h
48 h
Native
0h
Digestive starch
24 h
1
0.884#
0.912#
1
0.992#
−0.811#
−0.731*
−0.820#
−0.565
–0.713*
–0.681*
–0.755*
−0.538
–0.793*
–0.762*
–0.795*
−0.612
1
0.966#
0.770*
0.801#
1
0.704*
0.822#
1
0.866#
0.478
1.00#
0.894#
0.910#
0.593
0.885#
0.999#
0.982#
0.620
0.915#
0.995#
0.997#
−0.518
–0.825#
–0.730*
–0.823#
–0.690*
–0.748*
–0.695*
–0.797*
−0.524
–0.823#
–0.787*
–0.832#
48 h
Native
0h
24 h
1
0.491
0.598
0.645
1
0.895#
0.915#
1
0.987#
48 h
1
1
−0.648
−0.580
−0.572
–0.675*
1
*Correlation is significant at 0.05 level
#Correlation is significant at 0.01 level
to the action of enzyme (amylase and glucosidase) leading
to its breakdown to glucose.
Correlation among total starch (TS), resistant starch (RS)
and digestible starch (DS) is presented in Table 2. Starch
indices (TS, RS and DS) were correlated with carbohydrate,
protein, lipid and dietary fibre content of the three rice
varieties reported elsewhere (Deepa et al. 2008). As shown
in Table 3, TS and DS released were observed to be
Table 3 Correlation coefficients between nutrient composition and total starch release, resistant starch and digestive starch during in vitro starch
digestion
Dietary fibre
Parameters
Total starch
Native
0h
24 h
48 h
Resistant starch
Native
0h
24 h
48 h
Digestive starch
Native
0h
24 h
48 h
Carbohydrate
Protein
Lipid
0.592
0.623
0.541
0.569
−0.518
−0.316
−0.710*
−0.660*
−0.526
−0.657
−0.629
−0.642
-0.505
−0.516
−0.732
−0.638
0.316
0.369
0.452
0.433
0.519
0.625
0.565
0.598
−0.543
−0.321
−0.702
−0.666
*Correlation is significant at 0.05 level
#Correlation is significant at 0.01 level
0.504
0.475
0.851#
0.697*
−0.530
−0.656
−0.657
−0.672
Soluble
Insoluble
Total
0.063
0.377
0.037
0.148
−0.526
−0.463
−0.780*
−0.773*
−0.522
−0.448
−0.776*
−0.764*
−0.673
−0.579
−0.491
−0.603
0.590
0.624
0.491
0.523
0.568
0.600
0.471
0.499
−0.011
0.389
0.072
0.202
−0.525
−0.473
−0.772
−0.775
−0.524
−0.457
−0.767
−0.765
648
J Food Sci Technol (Nov–Dec 2010) 47(6):644–649
inversely related to protein, insoluble and total dietary fibre
content. However, it was positively correlated to the
carbohydrate and soluble dietary fibre content (Table 3).
The release of TS and DS is dependent on starch, lipid and
protein complexes which make them less susceptible to the
action of amylolytic enzymes (Holm et al. 1986). Similar
results have been reported in other rice varieties by Urooj
and Puttraj (1999). RS was inversely proportional to the
amount of starch digested. Further, RS was directly
proportional to protein, insoluble and total dietary fibre
content while negatively correlated to carbohydrate content
and soluble dietary fibre content. Yao et al. (2002) reported
that lipid-amylose complex decreases the amount of
amylose available to interact with the external chains of
amylopectin to form resistant starch.
In vitro method for measuring the rate of hydrolysis of
starch has been suggested as an inexpensive and less time
consuming method compared to measuring in vivo starch
digestion (Jenkins et al. 1987). O’Dea et al. (1980) reported
that in rice postprandial glucose and insulin responses
correlate closely to the in vitro rates of hydrolysis. The
present study on in vitro starch digestion showed that
‘Njavara’ is easily digestible (starch released 67% in
180 min) than ‘Jyothi’ and ‘IR 64’ (Table 4). The starch
hydrolyzed in first 30 min was similar (58–59%) in all the
three varieties of rice. However, after 30 min a gradual
increase in starch digestion was observed in ‘Njavara’.
‘Njavara’, ‘Jyothi’ and ‘IR 64’ exhibited a plateau at
approximately 60 min of hydrolysis. ‘Njavara’ and ‘Jyothi’
have been reported to have similar amylose content of 23%
but they differed in their rate of digestion (Deepa et al.
2008). This is in agreement with the earlier reports that rice
varieties with similar amylose content differ in their
digestibility due to differences in their physiochemical
properties like gelatinization temperature (Panlasigui et al.
1991). The rate of digestion also depends on the granule
size, the amylose/amylopectin ratio, starch protein interaction, amylose/lipid complexes and the level of resistant
Table 4 Rate of starch hydrolysis in brown rice flour
Time, min
‘Njavara’
30
60
90
58.5±0.50
62.4±0.04
63.6±0.83
a
120
150
180
65.6±1.64
66.0±2.06
67.3±0.41
a
a
a
a
58.2±4.86
59.5±2.99
60.5±3.50
a
61.8±4.83
62.3±4.13
63.1±3.63
a
a
a
a
b
59.5±1.79
59.9±1.96
60.3±3.98
a
61.7±1.96
62.2±2.69
62.7±2.93
a
a
a
‘Njavara’
‘Jyothi’
‘IR 64’
Ha
90 (Form)
Hb
90 (Expt)
Cαa
ka
65.2
61.5
61.4
63.6±0.83
60.5±3.50
60.3±3.98
65.2
61.5
61.4
0.073
0.095
0.114
a
Experimental results
b
As per equation C=Cα (1−e −kt )
starch (Sagum and Arcot 2000). In the present study we
observed difference in the gelatinization temperatures
(communicated elsewhere) and resistant starch (Table 1).
According to the studies of Snow and O’Dea (1981)
digestibility of starch is affected by the size of the granule
and surface area to starch ratio for action of hydrolytic
enzymes. The easier digestibility of ‘Njavara’ may be due
to its smaller granular size (Deepa et al. 2008) rendering
more surface area for the action of hydrolytic enzymes.
The hydrolysis kinetics (Table 5) showed that the
equilibrium constant (Cα) of ‘Njavara’ was relatively
higher (65.2) than ‘Jyothi’ and ‘IR 64’ (~ 61). However,
‘Njavara’ exhibited significantly low kinetic constant
(0.073) compared to Jyothi (0.095) and IR 64 (0.114),
indicating that ‘Njavara’ has an inherent resistance to
enzymatic hydrolysis.
Currently, nutritionists recommend that a whole-food
approach rather than a GI approach to measure the
glycemic potency of foods (Monro 2003). HI, GI and GL
obtained for the three varieties are presented in Table 6. All
the three varieties were observed to have similar HI, GI and
GL. Further studies are warranted to understand in vivo
digestibility of ‘Njavara’ rice, as it has high dietary fibre
content compared to ‘Jyothi’ and ‘IR 64’ (Deepa et al.
2008). Dietary fibres are believed to enfold the food, hinder
the action of hydrolytic enzymes in the gut, increase the
viscosity of intestinal contents and thereby reduce the
absorption of carbohydrates, in vivo (Jenkins et al. 1977).
Moreover, the beneficial effects of dietary fibre are nullified
when whole grains are ground. The whole grain flours are
hydrolyzed at the same rate as polished grain flour.
Table 6 Estimated hydrolysis index (HI), glycemic index (GI) and
glycemic load (GL) of brown rice
Sample
HI
GI
GL
‘Njavara’
‘Jyothi’
‘IR 64’
63.9±0.75
60.9±3.99
61.1±2.55
74.8±0.41
73.1±2.19
73.2±1.40
24.7±0.34
24.1±5.24
24.2±6.54
a
b
Values within the same row with different superscripts are significantly different
(p<0.05) (n=3)
Sample
‘IR 64’
‘Jyothi’
a
Table 5 Percentage of starch hyrolysis in 90 min (H 90), equilibrium
constant (Cα) and kinetic constant (k) of brown rice flour
(n=3)
J Food Sci Technol (Nov–Dec 2010) 47(6):644–649
Conclusion
The study suggests that pigmented whole grain rice
(‘Njavara’ and ‘Jyothi’) is a better source of dietary fibre
and resistant starch. The RS content in cooked rice can be
increased by storing at low temperatures. The starch
retrogradation property of these pigmented rice varieties
can be exploited in preparation of healthy food products.
‘Njavara’ rice is observed to be easily digestible than
‘Jyothi’ and ‘IR 64’ based on in vitro starch hydrolysis
study. Thus, Njavara rice could be considered for baby and
geriatric foods.
Acknowledgement Authors thank Prakash V, Director and Salimath
P V, Head, Biochemistry and Nutrition Department, Central Food
Technological Research Institute, Mysore, India for their support and
encouragement in the present study. Deepa Gopinath, CSIR-SRF,
gratefully acknowledges the financial support from CSIR, New Delhi,
in carrying out these investigations. KAN gratefully acknowledges the
financial support in the form of a Project by the Department of
Science and Technology, New Delhi, India.
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