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: zhangb@scut.edu.cn (B. Zhang); fechoh@scut.edu.cn (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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[31] J.S. An, I.Y. Bae, S.I. Han, S.J. Lee, H.G. Lee, In vitro potential of phenolic

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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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