Structural and functional characteristics of Japonica rice starches with different amylose contents

ABSTRACT

Fully understanding the structures and properties of starches with different amylose contents is important to agriculture and food industries. Structural and functional characteristics of three Japonica rice starches differing in amylose content (<0.5%, 18.8%, and 33.3% for waxy, normal, and high-amylose varieties, respectively) were investigated using a range of characterization methods. As amylose content increased, the weight-average molecular weight, short-range order, relative crystallinity, and lamellar peak intensity of starch decreased. High-amylose starch contained a lower proportion of A chains than waxy and normal starches. High-amylose starch displayed a C-type crystalline structure, different from the A-type crystalline structure of waxy and normal starches. Compared with waxy and normal starches, high-amylose starch exhibited higher granule size (volume-average diameter), pasting temperature and higher resistant starch content, but lower peak viscosity, breakdown viscosity, swelling power, and rapidly digestible starch content. These results can provide reference for the exploitation of Japonica rice starches.

RESUMEN

Para la agricultura y la industria alimentaria es importante comprender plenamente las estructuras y las propiedades de los almidones con diferentes contenidos de amilosa. Por ello, el presente estudio se propuso investigar las características estructurales y funcionales de tres almidones de arroz Japonica que difieren en su contenido de amilosa (<0.5%, 18.8% y 33.3% para las variedades cerosa, normal y con alto contenido de amilosa, respectivamente), utilizando varios métodos de caracterización. Así, pudo constatarse que, a medida que el contenido de amilosa aumenta, disminuyen el peso molecular medio, el orden de corto alcance, la cristalinidad relativa y la intensidad del pico laminar del almidón. Por otra parte, se comprobó que el almidón de alta amilosa contiene menor proporción de cadenas A que los almidones cerosos y normales, y que presenta una estructura cristalina de tipo C, diferente de la estructura cristalina de tipo A de los almidones cerosos y normales. En comparación con los almidones cerosos y normales, en el almidón de alta amilosa se constató un mayor tamaño de gránulo (diámetro medio del volumen), mayor temperatura de pegado y mayor contenido de almidón resistente, aunque posee menor viscosidad máxima, menor viscosidad de ruptura, menor poder de hinchamiento y menor contenido de almidón rápidamente digerible. En conclusión, estos resultados pueden servir de referencia para el aprovechamiento de los almidones de arroz Japonica.

KEYWORDS:

• Rice
• starch
• structure
• properties

PALABRAS CLAVE:

• arroz
• almidón
• estructura
• propiedades

1. Introduction

Rice (Oryza sativa L.) is one of the most important cereal crops providing more than half of the world’s staple food (Z. H. Zhang et al., 2020). Varieties of rice cultivars are utilized in diverse food products because of their different quality properties (Kim et al., 2019; Tang et al., 2021). Asian cultivated rice is classified into two major subspecies, Indica and Japonica (Cheng et al., 2019). Japonica rice grains are mainly characterized by round oval shape and have higher yield than Indica rice.

Starch is the most abundant energy storage carbohydrate of rice. Starch polymers are generally synthesized in the form of granules that have a semi-crystalline structure (Singh & Sogi, 2018). Starch is mainly composed of two glucans, the essentially linear amylose with a few branches and large amounts of highly branched short-chain amylopectin. These semi-crystalline starch granules form a layered organization with alternating semi-crystalline and amorphous growth rings (Khatun et al., 2019; L. Lin et al., 2019).

Amylose content is one of the major factors influencing the structure and properties of starch, which further affects the quality of starch-enriched foods. Oyeyinka et al. (2018) found that the relative crystallinity (RC) of white bitter yam starch (amylose content 15.09%) was significantly higher than that of the yellow bitter yam (amylose content 16.95%). Naidoo et al. (2015) reported that the wild and amadumbe starches (amylose content 20%) contained higher resistant starch (RS) content than the cultivated amadumbe starch (amylose content 12%). X. Zhou et al. (2018) reported that the amylose content had a positive correlation with the RS content of Indica rice starch. Z. H. Li et al. (2016) reported that the amylopectin-rich Indica starch has lower final and setback viscosities than the amylose-rich starch. Y. Y. Zhang et al. (2019) found that the amylose content showed a positive correlation with the setback, final viscosity, and pasting temperature of chestnut starch, but a negative correlation with its breakdown value.

Fully understanding the structures and properties of Japonica rice starches with different amylose contents can provide references for agricultural researchers to breed structure-targeted varieties and can also help food manufacturers to choose proper raw materials and optimize processing technologies. Generally, the amylose content of Japonica rice starch ranges from 0% to 20% (Z. H. Ma et al., 2017). To date, there have been few systematic studies reporting the structure and properties of Japonica rice starch with amylose content higher than 20%. In recent years, considerable efforts have been paid to breeding techniques to modify the amylose content of Japonica rice (Cui et al., 2020). In this research, a Japonica rice starch with amylose content of 33.3% was compared with waxy and normal Japonica rice starches in terms of structural and functional properties. This research work can provide an important basis for the exploitation of Japonica rice starches.

2. Materials and methods

2.1. Materials

High-amylose Japonica rice (variety Jiangtangdao1) was kindly supplied by Shanghai Academy of Agricultural Sciences (Shanghai, China). Waxy and normal Japonica rice (varieties of taihunuo and huai5) were obtained from a local grain trader (Hubei, China). Porcine pancreatic-amylase (P7545), amyloglucosidase (A7095), and isoamyloase (A3176) were purchased from Sigma Chemical Co. (St. Louis, MO, USA). Glucose oxidase/peroxidase (GOPOD) kit was purchased from Megazyme International Ireland Ltd. (Bray Co., Wicklow, Ireland). Other chemical reagents were of analytical grade without being otherwise stated.

2.2. Starch isolation

Starch was isolated from white rice according to the method of Wang et al. (2017). According to the method of L. S. Lin et al. (2017), the amylose contents of waxy, normal, and high-amylose starches were determined as <0.5%, 18.8%, and 33.3%, respectively.

2.3. Determination of molecular weight distribution

Molecular weight distribution of rice starch was determined by a gel-permeation chromatography (GPC) in conjunction with DAWN-HELEOS-II multi-angle laser-light scattering detector (MALLS) (Wyatt Technology, CA, USA) and refractive index (RI) detector (Optilab Wyatt, USA). Starch sample (5 mg) was completely dissolved in 2 mL of DMSO containing 5 mM LiBr by heating in a boiling-water bath. The produced starch solution was then cooled to room temperature and filtered through a 0.22 μm PTFE membrane. The solution sample (100 μL) was subjected into the three serially connected columns of Ohpak SB-805 HQ, Ohpak SB-804 HQ, and Ohpak SB-803 HQ (300 × 8.0 mm, Shodex, Tokyo, Japan) and eluted with DMSO containing 5 mM LiBr solution at a flow rate of 0.4 mL/min. Data were collected and processed using Wyatt Astra software (Version 5.3.4.14, Wyatt Technology, CA, USA).

2.4. Amylopectin chain length distribution measurement

Amylopectin chain length distribution of the rice starches was determined according to the method of Shi et al. (2018). Starch (5 mg/mL) in 50-mM sodium acetate was debranched with isoamylase. The amylopectin chain length distribution of starch was analyzed using a high-performance anion chromatography with a pulsed amperometric detector (HPAEC-PAD, ICS-5000+; Dionex Corporation, Sunnyvale, CA, USA).

2.5. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) analysis

The ATR-FTIR spectra of starches were measured using a Nicolet iS50 FTIR spectroscopy (Thermo Scientific, Waltham, MA, USA) with a built-in diamond attenuated total reflection (ATR). The spectral region of 1200–800 cm⁻¹ was deconvoluted with a half-band width and enhancement factor of 19 cm⁻¹ and 1.9, respectively (Xiao et al., 2019).

2.6. X-ray diffraction (XRD) analysis

The crystalline structures of starch samples were evaluated using an Xpert PRO diffractometer (PANalytical B.V., Almelo, The Netherlands), operating at 40 kW and 40 mA with Cu Kα radiation source (λ = 0.154 nm). XRD patterns were acquired for a diffraction angle (2θ) range from 3° to 50° at a scanning speed of 10° min⁻¹ and a scanning step of 0.033°. The relative crystallinity (%) was quantitatively estimated by using the Origin software (Version 8.0, Microcal Inc., Northampton, MA, USA) according to the method of Wang et al. (2014).

2.7. Small angle X-ray scattering (SAXS) analysis

The SAXS analysis of starches was performed on a Bruker NanoStar SAXS instrument (Bruker AXS GmbH, Karlsruhe, Germany) equipped with Vantec 2000 detector and pinhole collimation for point focus geometry as previously descried by Z. Ma et al. (2018). The SAXS dataset was analyzed using DIFFRAC plus NanoFit software. Parameters of the SAXS spectrum were determined according to the simple graphical method (Ren et al., 2020).

2.8. Determination of particle size distribution

The particle size distributions of starch samples were measured with the laser light scattering method using the Malvern Masterizer 3000 instrument (Malvern Instruments Ltd., Worcestershire, UK). Distilled water was used as the suspension media.

2.9. Scanning electron microscopy (SEM)

Starch samples were coated with gold in a vacuum evaporator, and then the coated samples were viewed for their morphologies by a scanning electron microscope (S-3000 N, Hitachi High-Technologies Corp.; Tokyo, Japan) at an accelerating voltage of 15 kV.

2.10. Pasting properties determination

A Rapid Visco-analyzer (RVA, Newport Scientific, Warriewood, New South Wales, Australia) was used to analyze the pasting properties of starch samples. Starch (3.0 g, dry weight) was weighed exactly into an RVA canister, and distilled water was added to make a total weight of 28 g. The suspensions were equilibrated at 50°C for 1 min, heated to 95°C at a rate of 12°C/min, held at 95°C for 2.5 min, and cooled to 50°C at a rate of 12°C/min and held at 50°C for 1.5 min. The speed of the mixing paddle was 960 rpm for the first 10 s and then 160 rpm for the remainder of the experiment.

2.11. Swelling power and solubility determinations

Swelling power and solubility of starches were determined by heating starch–water slurries in a water bath at temperatures ranging from 50°C to 90°C in 10°C intervals according to the procedures of Wang and Copeland (2012) with some minor modifications. Briefly stated, the 4% starch slurry in deionized water was heated for 30 min at a specific temperature, the samples were swirled by hand after 10 and 20 min to resuspend precipitated starch, cooled at room temperature for 10 min. The slurries were then centrifuged at 2615 ×g for 20 min, and the supernatant was removed and dried by evaporation at 105°C. The dried soluble fraction and the sedimented swollen granules were weighed to determine solubility (%) and swelling power (g/g) using the following formulae:

$\begin{array}{rl}& \mathrm{S}\mathrm{w}\mathrm{e}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{p}\mathrm{o}\mathrm{w}\mathrm{e}\mathrm{r}\left(\mathrm{g}/\mathrm{g}\right)=\left(\begin{array}{c}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{m}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{d}\mathrm{s}\mathrm{w}\mathrm{o}\mathrm{l}\mathrm{l}\mathrm{e}\mathrm{n}\\ \mathrm{g}\mathrm{r}\mathrm{a}\mathrm{n}\mathrm{u}\mathrm{l}\mathrm{e}\mathrm{s}\end{array}\right)\\ & /\left(\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{g}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{h}\right)\end{array}$

$\begin{array}{rl}& \mathrm{S}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{b}\mathrm{i}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{y}\left(\mathrm{\%}\right)=\left(\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{s}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{b}\mathrm{l}\mathrm{e}\mathrm{s}\right)/\\ & \left(\mathrm{d}\mathrm{r}\mathrm{y}\mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{o}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{g}\mathrm{i}\mathrm{n}\mathrm{a}\mathrm{l}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{r}\mathrm{c}\mathrm{h}\right)\end{array}$

2.12. In vitro starch digestibility assay

In vitro digestibility of the rice starches was tested according to the method of X. H. Ma et al. (2020). Briefly, the starch sample was suspended in a sodium acetate buffer and the mixture was heated at 95°C for 30 min. The gelatinized samples were treated with an enzyme mixture (pancreas α-amylase and amyloglucosidase) at 37°C. The glucose content of the enzymatic hydrolysates at 0, 20 and 120 min of hydrolysis was determined to calculate the rapidly digestible starch (RDS, digested within 20 min), slowly digestible starch (SDS, digested between 20 and 120 min) and resistant starch (RS, undigestible starch after 120 min).

2.13. Statistical analysis

Data are averages of duplicate determinations. An analysis of variance with a significance level of 5% was conducted and Duncan’s multiple range test was applied to determine differences between means using the commercial statistical package (SPSS, Inc, Chicago, IL, USA).

3. Results and discussion

3.1. Molecular weight distribution and chain length distribution of amylopectin

The curves of molecular weight and refractive index versus eluent time for GPC-MALLS-RI analysis of three Japonica rice starches are shown in Figure 1. For the three starches, the molecular weight decreased as the amylose content increased (p < .5). Weight-average molecular weights (M_w) of the waxy, normal, and high-amylose starches were 1.93 × 10⁸, 1.61 × 10⁸, and 0.98 × 10⁸ g/mol, respectively. As the amylose content increased, polydispersity indexes of the starch increased (p < .05), suggesting that the molecular weight distribution of the starch became wider with the increase of amylose content. The polydispersity indexes (PI) of the three starches were 3.03, 5.51, and 7.15, respectively.

Figure 1. The curves of molecular weight and refractive index versus eluent time for waxy (a), normal (b), and high-amylose (c) Japonica rice starches determined by gel permeation chromatography coupled with multi-angle light scattering and refractive index (GPC-MALLS-RI) detection. Mw = weight-average molecular weight; PI = polydispersity index.

Figura 1. Curvas del peso molecular y del índice de refracción versus el tiempo de eluyente para los almidones de arroz Japonica ceroso (a), normal (b) y de alta amilosa (c), determinadas mediante cromatografía de permeación en gel acoplada a la dispersión de luz multiángulo y a la detección del índice de refracción (GPC-MALLS-RI). Mw = peso molecular medio; PI = índice de polidispersidad

Amylopectin chain length distribution of three starches is shown in Figure 2 and four fractions were grouped in Table 1. All three starches showed similar bimodal profiles in the chain length distribution of amylopectin. The first peak appeared at around DP 12, and the second peak was at around DP 42. Similar results were reported in the study of Indica rice starch (Cao et al., 2020; Khatun et al., 2019). As compared to the waxy and normal starches, the high-amylose starch had a lower amount of short A chains and higher amount of long chains (B2 and B3). Z. H. Li et al. (2016) reported similar results for the Indica rice starch.

Table 1. Amylopectin chain length distribution of waxy, normal, and high-amylose Japonica rice starches.^†

Tabla 1. Distribución de la longitud de la cadena de amilopectina en los almidones de arroz Japonica ceroso, normal y de alta amilosa.^†

	Fraction (%)
	A (DP 6–12)	B1 (DP 13–24)	B2 (DP 25–36)	B3 (DP ≥ 37)
Waxy	28.4 ± 0.5^c	48.3 ± 0.5^b	11.3 ± 0.3^b	11.9 ± 0.2^b
Normal	31.2 ± 0.4^a	47.7 ± 0.3^b	10.7 ± 0.2^c	10.3 ± 0.3^c
High-amylose	19.4 ± 0.2^b	49.9 ± 0.1^a	13.7 ± 0.2^a	17.1 ± 0.3^a

^†Data were expressed as means ± SD. Means within a column that had the same letter were not significantly different (α = 0.05). DP = degree of depolymerization.

^†Los datos figuran como media ± DE. Dentro de una columna, las medias que presentan una misma letra no son significativamente diferentes (α = 0.05). DP = grado de despolimerización.

Figure 2. Amylopectin chain distributions of waxy (a), normal (b), and high-amylose (c) Japonica rice starches.

Figura 2. Distribución de las cadenas de amilopectina de los almidones de arroz Japonica ceroso (a), normal (b) y de alta amilosa (c)

3.2. Short-range ordered structure

ATR-FTIR spectra can be used to characterize the amount of short-range ordered structure in starch. The ratio of absorbance 1047/1022 cm⁻¹ in deconvoluted FT-IR spectra is used to quantify the amount of ordered starch to amorphous starch (Q. Li et al., 2019). The deconvoluted ATR-FTIR spectra of three Japonica rice starches in the wavenumber range of 1200–800 cm⁻¹ are shown in Figure 3. The three starches had a significant difference in absorbance ratio of 1047/1022 cm⁻¹ (p < .05). Absorbance ratios of 1047/1022 cm⁻¹ for the waxy, normal, and high-amylose starches were 0.631, 0.590, and 0.533, respectively, suggesting that the amount of short-range ordered structure decreased as the amylose content increased. This result was consistent with corn and wheat starches as previously reported by Chen et al. (2019) and Karwasra et al. (2017). Amylose molecules can entangle within the crystalline regions of amylopectin and interfere with the formation of double helices between branch chains of amylopectin molecules (Chen et al., 2019).

Figure 3. Deconvoluted attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) spectra of waxy, normal, and high-amylose Japonica rice starches.

Figura 3. Espectros deconvolucionados y atenuados de reflectancia total -espectroscopia de infrarrojos por transformada de Fourier (ATR-FTIR) de almidones de arroz Japonica ceroso, normal y con alto contenido en amilosa

3.3. Crystalline structure

The XRD patterns of three Japonica rice starches are shown in Figure 4. Waxy and normal starches exhibited peaks at 2θ angles of 15.0°, 17.0°, 17.9°, and 23.0°, indicating a typical A-type diffraction pattern (Cai et al., 2019). Different from waxy and normal starches, high-amylose starch exhibited a C-type pattern, with peaks at 2θ angles of 5.6°, 15.0°, 17.0°, 19.6° and 23° (X. X. Wang et al., 2018). The different crystalline pattern of high-amylose starch could be ascribed to their higher proportions of long chains (B2 and B3) in amylopectin. According to Cheetham and Tao (1998), long chains favor the formation of B-type crystallinity and short chains benefit the A-type crystallinity. The C-type crystalline structure is a combination of both A- and B-type structures. As the chain length increases, the crystalline type of starch can change from A- to B-type via C-type. The relative crystallinity of the starch decreased with increasing amylose content (p < .05). The relative crystallinities of waxy, normal, and high-amylose starches were 38.2%, 31.5%, and 28.3%, respectively. Similar result was previously reported for maize starch (Wang et al., 2014). Amylopectin is thought to be chiefly responsible for the semi-crystalline organization in starch granules and amylose is generally considered as an amorphous polymer (Zhong et al., 2020).

Figure 4. X-ray diffraction (XRD) patterns of waxy, normal, and high-amylose Japonica rice starches.

Figura 4. Patrones de difracción de rayos X (XRD) de los almidones de arroz Japonica ceroso, normal y de alta amilosa

3.4. Lamellar structure

The SAXS patterns of three Japonica rice starches are shown in Figure 5. All three starches presented a well-resolved scattering peak at q of 0.06–0.08 Å⁻¹. The peak position was consistent with previous research in rice starch and other starches (N. Li et al., 2019; G. T. Li et al., 2019; Yang et al., 2016). The peak position and intensity reflect the size of lamellae and the electron density difference between the crystalline and amorphous regions of the lamellae, respectively (L. Zhang et al., 2018). The three starches did not show a significant difference in lamellar thickness (d = 2π/q), which was around 9.2 nm. Waxy starch presented the highest lamellar peak intensity (503.4 a.u.), followed by normal starch (255.3 a.u.), while high-amylose starch showed the lowest value (174.0 a.u.). Our observation is consistent with the previous research (Koroteeva et al., 2007), which suggested that amylose can disturb the order of lamellar structure, leading to the defectiveness of lamellae.

Figure 5. Small angle X-ray scattering (SAXS) patterns of waxy, normal, and high-amylose Japonica rice starches.

Figura 5. Patrones de dispersión de rayos X de ángulo pequeño (SAXS) de los almidones de arroz Japonica ceroso, normal y de alta amilosa

3.5. Particle size distribution and morphology

SEM images of starches are shown in Figure 6 and particle size distributions of three Japonica rice starches are shown in Figure 7. For all three starches, granules mainly exhibited polyhedral and irregular shape (Figure 6). All three starches displayed a unimodal distribution pattern in particle size (Figure 7). Among the three starches, waxy starch presented the narrowest distribution of particle size and high-amylose starch showed the widest distribution. Volume-average diameter (D[4,3]) of the starch increased with increasing amylose content (p < .05). D[4,3] values of the waxy, normal, and high-amylose starches were 5.33, 7.26, and 8.67 μm, respectively.

Figure 6. Particle size distributions of waxy, normal, and high-amylose Japonica rice starches.

Figura 6. Distribución del tamaño de las partículas de los almidones de arroz Japonica ceroso, normal y de alta amilosa

Figure 7. Scanning electric microscopy (SEM) images of waxy (a), normal (b), and high-amylose (c) Japonica rice starches.

Figura 7. Imágenes de microscopía eléctrica de barrido (SEM) de los almidones de arroz Japonica ceroso (a), normal (b) y de alta amilosa (c)

3.6. Pasting properties

The RVA curves of the three Japonica rice starches are presented in Figure 8 and RVA parameters of the three rice starches are shown in Table 2. Pasting temperature (PT) of high-amylose starch was higher than those of waxy and normal starches. Similar studies have been reported in high-amylose starches of other crops (Raina et al., 2007; Wang et al., 2014; W. Z. Zhou et al., 2015). High-amylose starch contained more amounts of long chains (B2 and B3), which can delay the gelatinization of starch by strengthening the stability of crystals (W. Z. Zhou et al., 2015). High-amylose starch showed a peak viscosity (PV) of 1275 cP, which was lower than those of waxy starch (2512 cP) and normal starch (3220 cP). Raina et al. (2007) suggested that low-amylose starches attained peak viscosities at lower temperatures than high-amylose starches and presented higher peak viscosities. High-amylose starch exhibited a lower breakdown viscosity (BDV) than the other two starches. Low BDV value of the high-amylose starch might be attributed to the higher amylose content and long side chain amounts of amylopectin, which enhanced the resistance of starch granule to shearing (Park et al., 2007). Setback viscosity (SBV) of high-amylose starch was between those of waxy and normal starches.

Table 2. Pasting parameters of waxy, normal, and high-amylose Japonica rice starches.^†

Tabla 2. Parámetros de pegado de los almidones de arroz Japonica ceroso, normal y de alta amilosa.^†

	PT (°C)	PV (cP)	TRV (cP)	BDV (cP)	FV (cP)	SBV (cP)
Waxy	76.36 ± 0.42^b	2512 ± 36^b	1196 ± 47^a	1316 ± 42^b	1657 ± 9^b	461 ± 49^c
Normal	74.36 ± 0.68^c	3220 ± 45^a	1146 ± 71^a	2074 ± 111^a	2242 ± 126^a	1095 ± 68^a
High-amylose	85.78 ± 0.76^a	1275 ± 21^c	1052 ± 7^b	223 ± 22 ^c	1785 ± 64^b	733 ± 58^b

^†Data were expressed as means ± SD. Means within a column that had the same letter were not significantly different (α = 0.05). PT = pasting temperature; PV = peak viscosity; TRV = trough viscosity; BDV = breakdown viscosity; FV = final viscosity; SBV = setback viscosity.

^†Los datos figuran como medias ± DE. Dentro de una columna, las medias que presentan una misma letra no son significativamente diferentes (α = 0.05). PT = temperatura de pegado; PV = viscosidad de pico; TRV = viscosidad de depresión; BDV = viscosidad de ruptura; FV = viscosidad final; SBV = viscosidad de retroceso.

Figure 8. Rapid Visco-analyzer (RVA) curves of waxy, normal, and high-amylose Japonica rice starches.

Figura 8. Curvas del viscoanalizador rápido (RVA) de los almidones de arroz Japonica ceroso, normal y de alta amilosa

3.7. Swelling power and solubility

Swelling power and solubility of three Japonica rice starches in the temperature range of 50–90°C are shown in Table 3. As temperature increased, all three starches generally exhibited steady increase in swelling power and solubility. However, there were some differences in change rate among the three starches. For the waxy and normal starches, dramatical increase of swelling power and solubility were observed when temperature was increased from 60°C to 70°C, while similar change occurred for the high-amylose starch as temperature increase from 70°C to 80°C. At a temperature of 80°C, heated paste of the waxy starch cannot be centrifuged. At 90°C, this phenomenon happened to both the waxy and normal starches, while swollen granules of the high-amylose starch can still be separated. These results suggested that the high-amylose starch was inferior to waxy and normal starches in swelling property. Swelling of starch is primarily a function of amylopectin while amylose acts as a diluent (Tester & Morrison, 1990). The increase of amylopectin content and fraction of short chains in amylopectin can enhance the swelling property of starch granules (Wang et al., 2017). Compared with high-amylose starch, waxy, and normal starches contained higher amylopectin contents and greater proportions of A chains (DP 6–12) in amylopectin, thus exhibiting better swelling properties.

Table 3. Swelling powers and solubilities of waxy, normal, and high-amylose Japonica rice starches.^†

Tabla 3. Poderes de hinchamiento y solubilidad de los almidones de arroz Japonica ceroso, normal y de alta amilosa.^†

	Temperature (°C)
	50	60	70	80	90
Swelling power (g/g)
Waxy	2.02 ± 0.03^b	2.08 ± 0.04 ^c	10.11 ± 0.78^b	–	–
Normal	2.00 ± 0.04^b	2.68 ± 0.01^a	11.54 ± 0.48^a	16.17 ± 0.09^a	–
High-amylose	2.13 ± 0.04^a	2.31 ± 0.01^b	4.32 ± 0.06 ^c	7.47 ± 0.40^b	10.51 ± 0.59
Solubility (%)
Waxy	2.25 ± 0.10^b	2.47 ± 0.06^c	5.73 ± 0.10^b	–	–
Normal	2.41 ± 0.06^b	2.75 ± 0.22^b	8.44 ± 0.69^a	9.66 ± 0.12^b	–
High-amylose	2.69 ± 0.14^a	3.05 ± 0.08^a	5.86 ± 0.01^b	11.89 ± 0.25^a	18.82 ± 0.69

^†Data were expressed as means ± SD. Means within a column that had the same letter were not significantly different (α = 0.05).

^†Los datos figuran como medias ± SD. Dentro de una columna, las medias que tienen una misma letra no son significativamente diferentes (α = 0.05).

3.8. In vitro starch digestibility

The rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) contents of three Japonica starches are presented in Table 4. For the three starches, the contents of RDS, SDS, and RS were in the ranges of 78.74–88.86%, 8.19–11.09%, 2.95–11.70%, respectively. Among the three starches, the waxy starch exhibited the highest RDS content and lowest RS content, while the opposite results were observed for high-amylose starch. This result was in agreement with Noda et al. (2003), which reported that the digestibility of rice starch (amylose content range 3.9–17.2%) was negatively correlated with amylose content. Similar findings were also found in the studies of mung bean and chestnut starches (Hao et al., 2018; Kaur et al., 2011). The differences of three Japonica rice starches in digestibility could be ascribed to the structure of amylose. The double helical structure of amylose is presumably not accessible to the amylase enzyme (Oyeyinka et al., 2017).

Table 4. The RDS, SDS, and RS contents of waxy, normal, and high-amylose Japonica rice starches.^†

Tabla 4. Contenidos de RDS, SDS y RS de los almidones de arroz Japonica ceroso, normal y de alta amilosa.^†

	RDS (%)	SDS (%)	RS (%)
Waxy	88.86 ± 0.51^a	8.19 ± 0.41 ^c	2.95 ± 0.36^c
Normal	84.04 ± 0.42^b	11.09 ± 0.17^a	4.87 ± 0.24^b
High-amylose	78.74 ± 0.17^c	9.56 ± 0.37^b	11.70 ± 0.42^a

^†Data were expressed as means ± SD. Means within a column that had the same letter were not significantly different (α = 0.05). RDS = rapidly digestible starch; SDS = slowly digestible starch; RS = resistant starch.

^†Los datos figuran como medias ± DE. Dentro de una columna, las medias que tienen una misma letra no son significativamente diferentes (α = 0.05). RDS = almidón de digestión rápida; SDS = almidón de digestión lenta; RS = almidón resistente.

4. Conclusion

As the amylose content increased, the weight-average molecular weight of Japonica rice starch decreased. The high-amylose starch contained a lower proportion of A chains and a higher proportion of B2 and B3 chains than the other two starches. The waxy and normal starches showed A-type crystalline structure, but the high-amylose starch exhibited a C-type crystalline structure. Short-range order, relative crystallinity, and lamellar peak intensity of starch decreased with elevated amylose content. As amylose content increased, the volume-average diameter of starch increased and the particle size distribution became wider. High-amylose starch had lower pasting temperature, peak viscosity, breakdown viscosity, and swelling power than waxy and normal rice starches. Among the three starches, the high-amylose starch contained the highest RS content (11.70%) and the lowest RDS content (78.74%).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This research was kindly supported by the Open Fund of Key Laboratory for Deep Processing of Major Grain and Oil (Wuhan Polytechnic University), Ministry of Education [Grant number 2020JYBQGDKFB09]; the Research and Innovation Initiatives of WHPU [grant number 2016y17]; the Excellent Science and Technology Innovation Team of Young and Middle-aged Researchers in Universities of Hubei [grant number LT201911].