particle size affects structural and in vitro digestion
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Accepted Manuscript
Particle size affects structural and in vitro digestion properties ofcooked rice flours
Adil Muhammad Farooq, Chao Li, Siqian Chen, Xiong Fu, BinZhang, Qiang Huang
PII: S0141-8130(18)31137-1DOI: doi:10.1016/j.ijbiomac.2018.06.071Reference: BIOMAC 9911
To appear in: International Journal of Biological Macromolecules
Received date: 9 March 2018Revised date: 11 June 2018Accepted date: 13 June 2018
Please cite this article as: Adil Muhammad Farooq, Chao Li, Siqian Chen, Xiong Fu, BinZhang, Qiang Huang , Particle size affects structural and in vitro digestion properties ofcooked rice flours. Biomac (2017), doi:10.1016/j.ijbiomac.2018.06.071
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Particle size affects structural and in vitro digestion properties of cooked rice flours
Adil Muhammad Farooq1, Chao Li
1,2, Siqian Chen
3, Xiong Fu
1,2, Bin Zhang
1,2*, Qiang
Huang1,2
*
1 School of Food Science and Engineering, Guangdong Province Key Laboratory for Green
Processing of Natural Products and Product Safety, South China University of Technology,
Guangzhou 510640, China
2 Overseas Expertise Introduction Center for Discipline Innovation of Food Nutrition and
Human Health (111 Center), Guangzhou, China
3 Centre for Nutrition and Food Sciences, The University of Queensland, St Lucia, QLD 4072,
Australia
Corresponding authors
Tel.: +86 20 8711 3845; fax: +86 20 8711 3848.
E-mails: [email protected] (B. Zhang); [email protected] (Q. Huang).
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Abstract
The aim of this study was to identify the contributions made by size fractions of four milled
rice (i.e., waxy, white, black and brown rice) to structural and in vitro starch digestion
properties after cooking. Rice grains were hammer-milled in a controlled manner, and the
coarse (300 - 450 µm), medium (150 - 300 µm) and fine size (< 150 µm) fractions were
segregated through vertical sieving. All samples displayed monophasic digestograms, and
starch digestion rate and extent for size fractionated rice flours were predicted through the
Logarithm of Slope model. It was found that digestion rate and extent were markedly reduced
with increasing particle size within each rice variety. Of the four rice varieties, non-waxy rice
flour fractions showed lower digestion rate and extent compared to the waxy counterpart,
possibly due to the formation starch-lipid complexes as judged by XRD with ca. 4%-8% V-
type crystalline structure remained after cooking. We suggested that the less rigid
morphological structure and almost amorphous conformation lead to the high digestion rate
and extent during simulated intestinal starch digestion. These findings will help develop
functional rice ingredients with desirable digestion behaviour and attenuated postprandial
glycemic responses.
Keywords: particle size, structural properties, rice flours, digestion kinetics
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1. Introduction
Rice, rich in glycemic carbohydrates, is consumed after cooking as a traditional staple food
particularly in Asia. Significant upsurge of rice flour is observed in recent decades, due to its
wide use in food products such as baby foods, breakfast cereals, crackers, rice noodles and
snack foods [1]. Starch with slow digestion rate and extent is preferred due to their potential
health and nutritional benefits through modulating insulin secretion and postprandial
glycemic response [2, 3].
As a result of grain milling, damage to starch granules and whole grain structure where starch
granules interact with the cell wall and protein matrices is commonly observed in flour,
which further influences its properties related to processing (e.g. water absorption and
solubility) and nutritional value (e.g. starch digestion rate) of finished foods [4-7]. Al-Rabadi
et al. (2012) found that the hydration properties response to hydrothermal treatment and
extent of starch digestion could be manipulated by the particular size fractions of uncooked
milled grains [8]. As for cooked milled grains, the plant tissue matrix significantly limited the
water and heat transfer on starch gelatinization, thus decreased the digestion rate with
increase of particle size (>80 µm) [9]. Edwards et al. (2015) reported that the coarse particles
(2 mm particles) were digested in a slower rate than that of fine particles (0.2 mm particles),
thereby influencing the glycemic response of coarse or smooth wheat porridges [10]. In
general, the effect of particle size on starch digestion has typically been associated with the
available surface area per unit volume for the enzyme binding/diffusion process [8, 11].
Cracks and micro-porous structures also facilitated the rate of enzymatic adsorption and
binding by increasing the effective surface area. Processing conditions such as thermal
processing, extrusion cooking, and post-cooking refrigerated storage significantly alter the
physical state of starch granules and food microstructure that could further affect the
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digestibility of starch matrix. Such changes cause cell rupturing and cell separation due to
destabilization of pectic materials [12].
The aim of the present work was to elucidate the effects of particle size and rice varieties
(white, black, brown and waxy) on structural properties and digestibility of cooked flours. A
hammer milling was used to disrupt plant tissue into flour particles in a controlled manner,
and the coarse (300 - 450 µm), medium (150 - 300 µm) and fine size (< 150 µm) fractions
were segregated through vertical sieving. We further investigated the structural and
physiochemical properties of size fractionated rice flours, to better understand their in vitro
digestion kinetics. The results will be helpful in developing functional rice ingredients with
potential applications in the management and prevention of hyperglycaemic related diseases.
2. Materials and methods
2.1. Materials
Four types of rice grain, i.e., white (WH), brown (BR), black (BL), waxy (WX) rice, were
purchased from a local market (Guangzhou, China), and ground by a hammer mill with the
screen size of 1 mm (Miller 6850, Metuchen, NJ, USA). The milled flours were segregated
by size using three screen sieves (pore opening, 150, 300, and 450 µm respectively) under
gravity with mechanical agitation in a sieve shaker (Labtech, China), to obtain fine (<150
µm), medium (150-300 µm) and coarse (300-450 µm) fractions. Moisture, protein and fat
content of all rice flour particle size were determined according to AOAC methods [13].
Total starch assay kits were purchased from Megazyme International Ltd. (Co. Wicklow,
Ireland). The other chemicals used in this study were all analytical grade.
2.2. Sample Preparation
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The flour sample was cooked in a 95 oC water bath for 30 min, then centrifuged at 1500 g for
10 min, and the fresh pellet obtained for light microscopy, particle size analysis. Cooked
flour samples subjected to freeze-drying were used for SEM and X-ray diffraction analysis.
2.3. Morphological structure
2.3.1. Scanning electron microscopy (SEM)
The raw and cooked flour samples were mounted on aluminium stubs with carbon tape, and
sputter-coated with a thin film of gold (10 nm). Morphology of raw and freeze-dried flour
samples were observed by using a scanning electron microscope (Zeiss EVO18, Germany)
operated at 10 kV accelerating voltage. All the images were taken at 100× magnification.
2.3.2. Light microscopy
Light microscopy observation was performed by using an BX-51 light microscope (Olympus,
Tokyo, Japan). One drop of fresh cooked rice flour suspension was placed on a glass slide
and stained with 2% (w/v) iodine solution. The images were recorded at 200× magnification.
2.4. Particle size distribution
Particle size analysis was carried out using a Mastersizer Hydro 2000MU (Malvern
Instruments, Worcestershire, UK) by following the previous report [14]. A refractive index of
1.33 was used to calculate the particle size of rice flour. Samples were added to circulating
distilled water until an obscuration of >10% was recorded.
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2.5. X-ray diffraction (XRD)
X-ray diffractometer (Rigaku, Tokyo, Japan) was operated at 40 kV and 40 mA with Cu Kα
radiation (λ=0.154 nm). The samples were packed tightly in a rectangular glass cell and
scanned over the range of 4-35° 2θ angles at a rate of 2°/min [15]. The relative crystallinity
was calculated as the ratio of the crystalline peak area to the total diffraction using PeakFit
software (Version 4.0, Systat Software Inc., San Jose, CA, USA).
2.6. Thermal properties
Gelatinization properties were analysed using differential scanning calorimetry (DSC-8000,
Perkin Elmer, Norwalk, CT, USA). Raw flour samples (3 mg, dry starch basis, dsb) with
deionized water (70%, w/w) were placed in an aluminium pan. The pan was sealed, and the
samples was allowed to equilibrate overnight at room temperature. The sample was scanned
at 10 oC/min heating rate from 30 to 120
oC. Gelatinization enthalpy (∆H, J/g dry starch),
onset (To), peak (Tp) and conclusion (Tc) temperatures were obtained using the software
supplied with the DSC instrument (Perkin Elmer, Norwalk, CT, USA).
2.7. Swelling power and solubility
Swelling power (SP) and solubility (S) of the raw rice flour and starch samples were
determined according to the previous method [16,17] with slight modification. About 500 mg
(dsb) of sample was cooked in about 20 mL of water at temperatures 90 °C for 30 min. Then
cooled to room temperature and centrifuged at 2,600×g for 15 min. Supernatant was decanted
carefully and kept, and the residue was weighed for SP determination. The supernatant was
poured out from the tube to a glass dish (of known weight) and kept on a boiling water bath
for evaporation. Afterwards, the dish was dried at 105 °C to constant and weighed. Solubility
and swelling power was estimated with the following:
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(eq. 1)
(eq. 2)
where Wt is the weight of the wet sediment, Wr is the weight of the dried supernatant, W is the
weight of sample (mg, dsb).
2.8. In vitro starch digestion
In vitro starch digestion procedure was adapted from the method described elsewhere with
slight modifications [18]. Flour sample (∼50 mg, db) was cooked at 95 oC for 30 min in 15
mL phosphate buffered saline (PBS, pH 7.4) (P3813, Sigma) buffer with constant mixing,
and then cooled down to 37 oC before adding the 2.5 units of porcine pancreatic amylase. At
each time intervals up to 120 min, 300 µL of aliquot was mixed with 1200 µL of ice-cold
sodium carbonate solution (0.5 M) to stop the reaction and centrifuged at 4000 g for 5 min to
remove the undigested starch. The concentration of maltose equivalent (reducing sugar) in
the supernatant was determined by the para-hydroxybenzoic acid hydrazide (PAHBAH)
assay (H9882, Sigma) [19] and the maltose equivalent released (%) was calculated as follows.
(eq. 3)
The starch digestion kinetic profiles were then fitted to the first-order equation using
Logarithm of Slope (LOS) analysis as described previously [18].
ln (dC/dt) = ln (C∞k) − kt (eq. 4)
where t is the digestion time (min), C is digested starch at incubation time t, C∞ is digestion at
infinite time, and k is rate constant (min−1
). The plot of ln (dC/dt) against digestion time t is
linear with a slope of −k, and the C∞ can be calculated from the intercept of the equation and
slope k.
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2.9. Statistical analysis
Results were expressed as means of triplicate measurements unless otherwise specified.
Analysis of variance (ANOVA) was used to determine significance at p < 0.05 using SPSS
18.0 statistical software for Windows (SPSS, Inc., Chicago, IL, USA).
3. Results and discussion
3.1 Morphological structure
Table 1 shows different parameters of the size distribution, namely the 10th (d0.1), median
(d0.5) and 90th (d0.9) percentiles. The average particle size (d0.5) of fine fraction ranged from
85 to 113µm, and d0.5 value of medium and coarse fractions is in the range of 279-297µm and
509-541µm respectively. These particle size parameters from different fractions of rice flour
confirmed that coarse, medium and fine fractions were successfully segregated through
vertical sieving. Figure 1 shows the morphological structure of size fractionated flours. It was
observed that the all rice flour particles are irregular polyhedral in shape, and their surface
exhibited smooth or plain superficial appearance without obvious pores. Coarse fractions may
include more multicellular structures that contain encapsulated cell walls, compared to the
fine fractions (Figure 1). The total starch content of rice flour samples with various sizes are
presented in Table 1. It was noteworthy that all fine fractions showed lower starch content
values, probably due to the loss of starch granules during milling and sieving (<150 µm).
Size fractionated rice flours were cooked at 95 oC for 30 min, and the morphology of the
freshly cooked samples was observed under light microscopy. The light micrographs
exhibited in Figure 2 showed that flour particles lost their densely packed structure after
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heating, and some deformed particles aggregated with each other and formed small or large
lumps. Starch swelling, and gelatinization exerted pressure on the cell walls of rice flour
during cooking that ruptured flour particles [20], and the physical entrapment and
retrogradation of leached amylose molecules might result in aggregation phenomenon of
cooked flours [21]. It was noteworthy that the coarse and medium fractions of non-waxy
varieties swelled extensively but remained mostly intact, whereas all fine fractions showed
more homogeneous matrix (Figure 2). In addition, cooking process disrupted most of particle
structure for the all WX flour factions, with a dispersion of smaller particles and dissolved
polymer molecules observed (Figure 2). The SEM images of cooked rice flour samples
subjected to freeze-drying showed spongier and more open structure containing organized
pores and thin walls (supplementary data Fig. S1), which usually is the evidence of soft gel
[22].
3.2 XRD
The XRD profiles of size fractionated rice flours before and after cooking are presented in
Figure 3. Raw rice flours displayed typical A-type diffraction pattern with strong diffraction
pattern peaks at 15.0o and 23.0
o 2θ, and an unresolved doublet at ca. 17.0
o and 18.2
o 2θ
(Figure 3A). The relative crystallinity of all raw rice flour samples ranged from 20% to 26%,
consistent with a report elsewhere [23]. Apparent difference was observed in the relative
crystallinity of rice flour among four varieties, with waxy rice samples showing the highest
crystallinity value. Moreover, coarse samples exhibited the similar crystallinity values
compared to the fine and medium fractions.
Figure 3B shows the XRD diffractograms and relative crystallinity of size fractionated
samples subjected to cooking and freeze-drying procedures. The typical A-type diffraction
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patterns of all cooked samples disappeared after cooking, indicating that starch granules were
completely gelatinized, and their semi-crystalline structure was destroyed. All X-ray
diffractograms of cooked non-waxy rice flour samples showed two peaks at ca. 13o and 20
o
2θ compared to raw flours, attributing to the formation of V-type amylose-lipid complexes
[24, 25]. However, the diffractograms of cooked WX flours displayed only a board
amorphous peak without characteristic V-type polymorph peaks, due to the reason that highly
branched and large number of short branch chains of amylopectin molecules cannot form
starch-lipid complex. The crystallinity values ranged from 4% to 8% for the fractions of
cooked non-waxy varieties but lowered than 1% for the waxy rice fractions (Figure 3B). Fine
fractions of non-waxy varieties showed slight higher crystallinity value than that of medium
and coarse fractions, suggesting the formation of starch-lipid complex is favourable between
dispersed starch and lipid molecules rather than in the encapsulated cells of cooked medium
and coarse fractions.
3.3 Thermal properties
The gelatinization temperatures and enthalpy change of size fractionated rice flour samples
are summarized in Table 2. Starch gelatinization is an endothermic process, involving
dissociation of amylopectin double helices from a semi-crystalline structure to an amorphous
conformation. The gelatinization temperatures (To, Tp and Tc) ranged from 67.2 to 73.7°C,
72.7 to 79.0°C, and 81.5 to 88.4°C, respectively. Among the four rice varieties, WX rice
samples showed lower To values (67.2-70.4 oC) than non-waxy rice samples (68.3-73.7
oC).
Coarse fractions of all rice flours exhibited significantly lower To values than that of the
medium and fine counterparts, suggesting that retarded gelatinization happened for the cell
wall encapsulated starch granules.
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Enthalpy change (∆H) is a combination of the endothermic disruption process of ordered
structure and the exothermic process of granular swelling, and mainly influenced by the
melting of overall amylopectin crystals rather than separation of double helices within starch
crystallites [26]. Significant differences in ∆H were observed between different particle size
fractions of all four varieties. Coarse fractions always showed lower ∆H value as compared to
medium and fine fractions, possibly due to the reason that the cell wall encapsulation limited
starch swelling and gelatinization [27]. In additional, size fractionated waxy rice flour
samples showed significantly higher ∆H values (Table 2) than that of the non-waxy rice
counterparts, consistent with the highest crystallinity value (Figure 3A) formed by the double
helix stacking of amylopectin molecules.
3.4 Swelling power and solubility
Heating in excess water disrupted crystalline structure of starch and exposed more hydroxyl
groups with water molecules, which enhanced the swelling and solubility of rice flour [28].
As shown in Table 2, the swelling power of size fractionated WH, BR, BL and WX rice
flours varied from 5.3 to 12.7 g/g, in line with previously reported results [29]. Fine particles
of all rice varieties showed significantly higher swelling power values than other size
fractions, whereas coarse particles demonstrated the lowest swelling power possibly due to
the higher rigidity and lower surface area. This is in agreement with previous results showing
that swelling power is negatively related to particle size [30]. In addition, swelling power of
rice flour also affected by amylose and protein content, which inhibit the granular swelling
due to the formation of extensive and strong network [23]. Apparent differences were also
observed in the swelling power among rice varieties. It was noteworthy that the WX fractions
showed remarkably higher swelling power, ranging from 9.3 to 12.7 g/g than that of WH, BR
and BL fractions, probably attributed to their less rigid particle structure.
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The solubility of rice flour samples was ranged from 6.7% to 14.1% (Table 2), which was in
agreement with a previous report [23]. Significant differences were observed in the solubility
of rice among different size fractions. Coarse particles also had lower solubility compared
with fine and medium particles. The solubility of rice flour samples was significantly
decreased with the increase in particle size due to lower surface area. Solubility of WX flour
samples was higher than that of non-waxy flour samples regardless of particle size. Solubility
of WX flour sample fraction decreased with the increase in particle size and ranged from
11.2-14.1%. Moreover, the solubility of WX rice flour was found to be higher than that of
non-waxy rice flours, due to the less compact granular structure that disintegrated easily and
increased the solubility of WX flour sample.
3.5 In vitro starch digestion
The in vitro starch digestion kinetic profiles of size fractionated WH, BR, BL and WX rice
flours subjected to the cooking process were monitored by a reducing sugar assay, as shown
in Figure 4. In order to obtain logarithmic digestion curves and fit first-order kinetics for all
flour samples, a fixed porcine pancreatic α-amylase condition (2.5 units per 50 mg starch)
was used to convert sufficient starch substrate to oligosaccharide products over the time
course. LOS plots, as shown in Supplementary Data Figure S2, were well applied to the
starch digestion kinetic profiles to obtain first-order coefficients (k), showing that all
digestion profiles can be described by a single linear (one rate constant) pseudo-first order
process (R2> 0.90). Single rate coefficients of cooked flour samples and digestion extents
after 120 min of digestion are summarized in Table 3.
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It was observed that both particle size and botanical origin of rice flours played a role on
starch digestion kinetic profiles. Within each rice variety, rate coefficients and extents after
120 min of digestion were markedly retarded with increasing particle size (Table 3), and all
the four rice varieties showed the same trend. This is indicative of reduced starch
bioaccessibility where the cell wall integrity imposes a physical barrier to enzyme access [3].
This is also probably related to the surface area of rice flour particles. Coarse particles have
less surface area compared with medium and fine particles, leading to lower extent of water
diffusion and enzyme susceptibility. Moreover, other factors such as particle architecture,
crystalline pattern, degree of crystallinity, surface pores or channels, degree of
polymerization, non-starch components and their interactions with starch, and amylose to
amylopectin ratio could also influence digestion rate [27]. Among four varieties, waxy rice
samples showed higher k values and reducing sugar released values compared to the
counterparts for WH, BR and BL varieties, indicating that waxy rice flour is generally more
easily digested than non-waxy rice varieties. The fine and medium fractions of waxy rice
samples showed higher both digestion rate (0.089 and 0.049 min-1
) and extent (58.2% and
54.6%), probably related to the less rigid morphological structure (shown in Figure 2) and
almost amorphous conformation (crystallinity <1%, Figure 3B) after cooking. These data also
could be supported by their higher swelling power and solubility (Table 2) of waxy rice flour
samples, resulting in high susceptibility to enzymatic hydrolysis. For the coarse fraction (300-
450 µm), it was noteworthy that narrow range of digestion extent results (30.9%-35.7%) was
observed for the non-waxy varieties, whereas waxy variety showed ca. 10% higher starch
digested after 120 min hydrolysis. Speculatively, the lower digestion extents of non-waxy
flours may form starch-lipid complexes which remains indigestible, which was confirmed by
our XRD profile with ca. 4%-8% V-type crystalline structure after cooking (Figure 3B).
Amylose content seems the main determinant, as waxy varieties with large amount of small
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branch chains in amylopectin is prone to rapid amylolysis compared to amylose. In addition,
interaction between phenolic compounds and starch or enzyme could be another factor
affecting the digestibility of rice flour. An et al. (2016) reported that phenolic compounds
present in black rice decreased the digestibility by effectively inhibiting the digestive
enzymes activities [31].
4. Conclusions
Particle size of rice flour significantly affected its structural and in vitro starch digestion
properties. The swelling power and solubility of all rice flours was significantly reduced with
an increase in particle size. Furthermore, the gelatinization temperature and enthalpy value of
all rice flours were significantly lowered with the increase in particle size. After cooking, the
digestion rate and extent of non-waxy rice fractions were lower than those of waxy rice ones,
due to the formation of amylose-lipid complexes. It was also observed that the digestion rate
and extent of fine particles were higher compared to the medium or coarse fractions, possibly
due to the higher particle rigidity and lower surface area of the bigger sizes. These findings
have important implications for how rice flours with different varieties are best processed and
utilized to achieve slow starch digestibility and attenuated postprandial glycaemic responses
in human body.
Abbreviations
BL, black rice flour; BR, brown rice flour; C, coarse size fraction; CK, cooked; DSC,
differential scanning calorimeter; F, fine size fraction; LOS, log of slop; M, medium size
fraction; PBS, phosphate buffered saline; PAHBAH, para-hydroxybenzoic acid hydrazide; R,
raw; SEM, scanning electron microscopy; To, onset temperature; Tp, peak temperature; Tc,
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conclusion temperature; WH, white rice flour; WX, waxy rice flour; XRD, X-ray diffraction ;
∆H, enthalpy change.
Acknowledgements
We thank the financial support received from the National Natural Science Foundation of
China (31701546), Natural Science Foundation of Guangdong Province (2017A030313207),
National Key Research and Development Program of China (2017YFD0400502), Science
and Technology Planning Project of Nansha, Guangzhou (2016GJ001), the Fundamental
Research Funds for the Central Universities of China (2017MS100), and the 111 Project
(B17018). Farooq Muhammad Adil would like to thank South China University of
Technology for providing opportunity and support for his study in China.
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List of Figures
Fig. 1 Scanning electron microscopy (SEM) micrographs of rice flours; R, raw; WH, white
rice flour; BR, brown rice flour; BL, black rice flour; WX, waxy rice flour; F, fine; M,
medium; C, coarse.
Fig. 2 Light microscopy images of cooked rice flour samples (200×); WH, white rice flour;
BR, brown rice flour; BL, black rice flour; WX, waxy rice flour; F, fine; M, medium; C,
coarse.
Fig. 3 XRD patterns and relative crystallinity of raw and cooked rice flour samples. (A) R,
raw (B) CK, cooked, WH; white rice flour, BR; brown rice flour, BL; black rice flour WX,
waxy rice flour F, fine; M, medium, C, coarse.
Fig. 4 Digestion kinetic profiles of cooked rice flours (A) WH; white rice flour, (B) BR;
brown rice flour, (C) BL; black rice flour (D) WX, waxy rice flour; F, fine; M, medium; C,
coarse.
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Table 1 Chemical composition and particle size of fractionated rice flours.
1WH, white rice flour; BR, brown rice flour; BL, black rice flour; WX, waxy rice flour; F, fine; M, medium; C, coarse. All data are averages of
three measurements with standard deviation. Means within a column with different letters are significantly different (p < 0.05) by LSD least
significant test.
2 d0.1, d0.5, and d0.9 indicate 10th, 50th (median) and 90th percentiles respectively.
Sample1
Moisture
content (%)
Protein
content (%)
Fat content
(%)
Total starch
content (%)
Particle size parameter (µm)
d0.52 d0.1
2 d0.9
2
WH-F 13.0±0.1b 6.8±0.1
de 2.5±0.1
ef 83.1±1.1
fg 85.7±0.7
a 15.7±0.2
a 204.7±1.5
a
WH-M 12.8±0.0b 7.2±0.2
f 2.2±0.2
de 85.1±1.0
def 294.5±3.7
e 201.6±2.1
ef 431.7±4.9
b
WH-C 11.9±0.2a 7.6±0.0
g 1.8±0.0
bc 86.9±0.3
gh 519.7±3.7
g 327.1±5.4
i 875.9±28.1
b
BR-F 15.2±0.7d 6.1±0.0
ab 3.5±0.2
hi 70.4±1.9
a 110.0±0.2
bc 39.7±0.3
bc 216.6±0.2
a
BR-M 13.6±0.6c 6.4±0.1
bc 3.1±0.0
g 78.7±1.6
cd 282.1±2.3
d 187.8±5.3
de 428.9±20.9
b
BR-C 12.7±0.8b 7.0±0.3
ef 2.4±0.4
def 80.5±2.6
bc 509.1±4.8
f 291.3±30.0
h 938.6±33.0
d
BL-F 16.0±0.0e 7.3±0.1
fg 3.8±0.0
i 68.5±1.9
a 113.5±0.6
c 43.1±0.5
c 222.7±0.9
a
BL-M 15.6±0.4de
7.6±0.0g 3.4±0.2
gh 76.3±0.8
b 279.1±2.4
d 179.6±0.6
d 424.3±17.5
b
BL-C 15.2±0.2d 8.4±0.5
h 2.7±0.4
f 82.5±0.6
de 522.5±2.6
g 272.1±14.3
g 850.7±30.5
b
WX-F 13.0±0.1b 5.9±0.1
a 2.1±0.3
cd 82.7±0.2
de 105.7±0.4
b 25.8±0.4
ab 224.7±0.26
a
WX-M 12.5±0.4b 6.3±0.2
bc 1.6±0.2
b 88.5±1.6
h 297.7±1.3
e 208.1±2.2
f 449.2±5.8
b
WX-C 11.8±0.3a 6.6±0.0
cd 1.0±0.0
a 88.8±2.2
h 541.9±5.9
h 337.4±0.8
i 929.7±65.4
d
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Table 2 Swelling power, solubility and thermal properties of fractionated rice flours
1WH, white rice flour; BR, brown rice flour; BL, black rice flour; WX, waxy rice flour; F,
fine; M, medium; C, coarse. All data are averages of three measurements with standard
deviation. Means within a column with different letters are significantly different (p < 0.05)
by LSD least significant test.
2To, onset temperature; Tp, peak temperature; Tc, conclusion temperature; ∆H, enthalpy
change.
1Sample
Swelling
power
(g.g-1
)
Solubility
(%)
To (oC)
2
Tp (oC)
2
Tc (oC)
2
∆H (J/g)2
WH-F 7.6±0.2f 10.4±0.3
f 71.6±0.2
h 76.8±0.1
de 83.5±0.2
c 5.2±0.5
e
WH-M 6.9±0.0e 9.7±0.1
d 69.6±0.3
d 75.2±0.4
bc 85.0±0.9
de 4.2±0.5
c
WH-C 6.4±0.1d 9.0±0.2
c 68.3±0.1
b 73.0±0.8
a 86.2±0.2
f 1.6±0.2
a
BR-F 7.1±0.1e 8.5±0.1
cd 73.7±0.2
j 78.0±0.2
f 82.4±0.4
b 4.4±0.9
cd
BR-M 6.3±0.0cd
7.8±0.2b 71.4±0.4
h 77.8±0.5
f 83.6±0.1
c 3.2±0.1
b
BR-C 5.7±0.2b 6.9±0.0
a 70.1±0.1
e 74.6±0.2
b 84.8±0.2
d 2.4±0.3
b
BL-F 6.5±0.3d 8.1±0.4
e 72.6±0.1
i 75.0±0.3
bc 81.5±0.1
a 4.1±0.6
c
BL-M 6.0±0.1bc
7.9±0.1d 70.7±0.2
g 74.7±0.4
b 82.5±0.1
b 3.2±0.3
b
BL-C 5.3±0.2a 6.7±0.2
a 69.5±0.1
d 72.7±0.2
a 83.7±0.6
ef 1.6±0.1
a
WX-F 12.7±0.4i 14.1±0.2
i 70.4±0.1
fg 77.4±0.8
ef 85.2±0.2
fg 10.9±0.1
g
WX-M 10.6±0.1h 12.7±0.1
h 68.8±0.2
c 75.9±1.0
cd 87.1±0.6
g 7.4±0.5
f
WX-C 9.3±0.2g 11.2±0.2
g 67.2±0.7
a 79.0±0.5
g 88.4±0.1
h 5.0±0.3
de
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Table 3 Digestion rate coefficient (k, min−1
), maltose equivalent released (C120, %), and C∞
(%) values of different size fracions of rice flours in cooked form.
Sample1 k (min
-1) Maltose equivalent released (%) C∞ (%)
WH-F 0.061 48.7±0.7i 51.0±0.3
g
WH-M 0.047 42.2±0.2e 47.9±0.5
ef
WH-C 0.031 35.7±0.5b 38.6±0.4
b
BR-F 0.056 45.1±0.3g 46.8±0.5
d
BR-M 0.044 37.1±0.1c 40.9±0.3
c
BR-C 0.032 30.9±0.2a 33.2±0.1
a
BL-F 0.043 47.1±0.1h 48.2±0.4
ef
BL-M 0.039 41.4±0.3d 44.8±0.3
d
BL-C 0.033 34.9±0.3b 40.5±0.9
c
WX-F 0.089 58.2±0.4k 61.2±0.6
i
WX-M 0.049 54.6±0.1j 57.0±0.3
h
WX-C 0.047 43.5±0.6f 47.0±1.9
e
1WH, white rice flour; BR, brown rice flour; BL, black rice flour; WX, waxy rice flour; F,
fine; M, medium; C, coarse. All data are averages of three measurements with standard
deviation. Means within a column with different letters are significantly different (p < 0.05)
by LSD least significant test.
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Fig. 1
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Fig.2
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Fig.3
5 10 15 20 25 30 35
Inte
nsi
ty (
arb.u
nit
s)
2 theta (o)
R-WX-C (26%)
R-WX-M (26%)
R-WX-F (25%)
R-BL-C (22%)
R-BL-F (20%)
R-BR-F (21%)
R-WH-C (23%)
R-WH-M (23%)
R-WH-F (22%)
R-BR-M (21%)
R-BR-C (22%)
R-BL-M (20%)
A
5 10 15 20 25 30 35
Inte
nsi
ty (
arb
.un
its)
2 theta (o)
CK-WX-C (1%)
CK-WX-M (1%)
CK-WX-F (1%)
CK-BL-C (4%)
CK-BL-M (6%)
CK-BL-F (7%)
CK-BR-C (6%)
CK-BR-M (7%)
CK-BR-F (8%)
CK-WH-C (4%)
CK-WH-M (5%)
CK-WH-F (6%)
B
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Fig.4
0 20 40 60 80 100 1200
10
20
30
40
50
60
Model-fit
M
alt
ose e
qu
ivale
nt
rele
ased
(%
)
WH-F
WH-M
WH-C
Digestion time (min)
A
0 20 40 60 80 100 1200
10
20
30
40
50
60
Malt
ose
equ
ivale
nt
rele
ase
d (
%)
Digestion time (min)
BR-F
BR-M
BR-C
B
Model-fit
0 20 40 60 80 100 1200
10
20
30
40
50
60
M
alt
ose e
qu
ivale
nt
rele
ased
(%
)
Digestion time (min)
BL-F
BL-M
BL-C
C
Model-fit
0 20 40 60 80 100 1200
10
20
30
40
50
60
Model-fit
Malt
ose
equ
ivale
nt
rele
ase
d (
%)
Digestion time (min)
WX-F
WX-M
WX-C
D
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