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In vitro starch digestibility of rice flour is not affected by method of cooking
Accepted Manuscript In vitro starch digestibility of rice flour is not affected by method of cooking Shujun Wang, Peiyan Li, Teng Zhang, Jinglin Yu, Shuo Wang, Les Copeland PII:
S0023-6438(17)30427-9
DOI:
10.1016/j.lwt.2017.06.023
Reference:
YFSTL 6318
To appear in:
LWT - Food Science and Technology
Received Date: 26 March 2017 Revised Date:
9 June 2017
Accepted Date: 12 June 2017
Please cite this article as: Wang, S., Li, P., Zhang, T., Yu, J., Wang, S., Copeland, L., In vitro starch digestibility of rice flour is not affected by method of cooking, LWT - Food Science and Technology (2017), doi: 10.1016/j.lwt.2017.06.023. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
In vitro starch digestibility of rice flour is not affected by method of
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cooking
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Shujun Wanga*, Peiyan Lia, Teng Zhanga, Jinglin Yub, Shuo Wangac*, Les Copelandd
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a
Key Laboratory of Food Nutrition and Safety, Ministry of Education, Tianjin
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University of Science & Technology, Tianjin 300457, China
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Research Centre of Modern Analytical Technique, Tianjin University of Science
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& Technology, Tianjin 300457,China
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Beijing Advanced Innovation Center for Food Nutrition and Human Health,
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Beijing Technology & Business University, Beijing 100048, China
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School of Life and Environmental Sciences, University of Sydney, NSW
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Australia 2006
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* Corresponding authors: Dr. Shujun Wang or Dr. Shuo Wang
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Mailing address: No 29, 13th Avenue, Tianjin Economic and Developmental Area
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(TEDA), Tianjin 300457, China
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Phone: 86-22-60912486
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E-mail address: [email protected] or [email protected] 1
ACCEPTED MANUSCRIPT Abstract: This study evaluated the effects of different cooking methods on in vitro
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starch digestibility of three rice varieties. Results from differential scanning
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calorimetry and X-ray diffraction of cooked rice flours showed that rice starch was
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completely gelatinized after cooking. The overall paste viscosities of cooked indica
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hybrid flour (IHF) and japonica flour (JF) were lower than those of their
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corresponding raw flours. However, cooked waxy flour (WF) showed much higher
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paste viscosities than raw flours, especially the viscosity at the start of RVA testing.
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Cooking increased greatly the rate and extent of starch digestion of IHF and JF, but
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had a small effect on WF. There were small differences in rate and extent of starch
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digestion of rice after cooking under various conditions. Cooking methods were found
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to have little effect on in vitro starch digestibility of rice. Viscosity of the gelatinized
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waxy starch matrix affected in vitro starch digestibility. This study enables us to gain a
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better understanding of cooking effects on in vitro digestibility of starch in rice flour.
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Keywords: cooked rice, cooking method, starch amylolysis, starch gelatinization,
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starch paste viscosity.
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ACCEPTED MANUSCRIPT 1. Introduction
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Rice is the predominant staple food in many countries in the world, accounting for
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20% of the global dietary energy intake (Darandakumbura, wijesinghe, & Prasantha,
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2013). Rice is consumed mainly as intact kernels after cooking with water. However,
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there has been a significant recent increase in rice flour production for utilization in
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baby formulae and gluten-free foods (Hera, Gomez, & Rosell, 2013; Ngamnikom &
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Songsermpong, 2011). Rice flour offers health benefits and many rice-based food
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products, particularly rice bread, are produced for celiac disorder patients with wheat
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allergy (Qian & Zhang, 2013). Rice and rice-based foods are often classified as
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having a high glycemic index (GI), which may constitute a public health problem for
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populations who rely heavily on these foods (Hsu, Lu, Chang, & Chiang, 2015). With
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concerns over the increasing incidence of diabetes worldwide, there is a pressing need
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to find rice-based foods with low GI by screening rice varieties and optimizing
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processing protocols.
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On heating in excess of water, starch is gelatinized, leading to the disruption of starch
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structures and enhanced susceptibility of starch to amylolysis. Starchy foods differ
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greatly in the rates at which they are digested and absorbed into the bloodstream. The
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rate and extent of native starch digestion are determined by many factors including
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botanical source, granule size, surface characters (such as pores or channels, and
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surface proteins and lipids), amylose content and fine structure of starch polymers; 3
ACCEPTED MANUSCRIPT starch crystallinity, modification and starch-lipid interactions (Singh, Dartois, & Kaur,
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2010; Wang & Copeland, 2013). In contrast, the digestibility of cooked starch is
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affected by the extent of starch gelatinization and retrogradation, interactions between
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starch and other components, and the properties of the food matrix (Chung, Liu,
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Huang, Yin, & Li, 2010; Dhital, Dabit, Zhang, Flanagan, & Shrestha, 2015; Lee, Lee,
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Han, Lee, & Rhee, 2005; Sagum & Arcot, 2000; Srikaeo & Sopade, 2010; Zhang,
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Dhital, & Gidley, 2015). Rice is usually cooked with water in an electric cooker or a
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microwave oven, or by boiling or steaming (Jung et al., 2009). Some studies have
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shown that different cooking methods can have a significant effect on in vitro and in
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vivo digestibility of rice starch and on blood glucose response in rats (Lee et al., 2005;
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Li, Han, Xu, Xiong, & Zhao, 2014; Rashmi & Urooj, 2003; Reed, Ai, Leutcher, &
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Jane, 2013). For example, Rashmi & Urooj (2003) have reported that pressure
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cooking, boiling, straining and steaming methods influence the nutritionally important
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starch fractions in rice varieties. Among these methods, steaming resulted in lower
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rapidly digestible starch and higher slowly digestible starch in all rice varieties. Lee et
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al. (2005) found that the extent of in vitro starch hydrolysis in cooked rice samples
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followed the order: autoclave > stone pot > electric cooker > microwave, consistent
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with the order of degree of starch gelatinization. Reed et al. (2013) showed that
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stir-fried rice displayed the least starch hydrolysis rates compared with steamed and
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pilaf rice. However, a recent study has shown that only a low degree of gelatinization
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is required to greatly increase the susceptibility of starch to amylolysis ( Wang, Wang,
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vitro digestibility of rice, wheat and lotus starches ( Wang, Sun, Wang, Wang, &
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Copeland, 2016; Wang et al., 2017). Therefore, there is a need to re-evaluate the
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effects of different cooking methods on in vitro starch digestibility of cooked rice.
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A good understanding of the effect of various cooking methods on starch digestibility
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of foods can help consumer to select appropriate cooking practices to improve the
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nutritional quality of starchy food products. This study aimed to understand the effects
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of different cooking protocols, including cooking at atmospheric pressure (AP),
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pressure cooking (P), pre-soaking (S) and varying the ratio of rice to water, on starch
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in vitro enzymatic hydrolysis of rice starch. Three rice varieties, a non-waxy indica
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hybrid rice, a non-waxy japonica rice and waxy rice, were used in the study. These
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varieties represent a range of popular rice grain types.
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2. Materials and methods
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2.1 Materials
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Grains of three rice (Oryza sativa) varieties differing in amylose content were used in
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the present study. Indica hybrid rice (IHR, Hualiangyou) was kindly provided by
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Huazhong Agricultural University (Wuhan, China). Japonica rice (JR) and waxy rice
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(WR) were purchased from a local supermarket in Tianjin. Total Starch Assay Kit,
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glucose oxidase/peroxidase (GOPOD) kit and amyloglucosidase (AMG, 3260 U/ml)
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Ireland). Amylose (A0512) and amylopectin (A8515) from potato starch, porcine
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pancreatic α-amylase (PPA, A3176, EC 3.2.1.1, typeVI-B from porcine pancreas, 15
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units/mg), pepsin (P7012, pepsin from gastric mucosa, ≥2500 units/mg), trypsin
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(T0303, trypsin from porcine pancreas Type IX-S, lyophilized powder, 13000-20000
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BAEE units/mg protein) and α-chymotrypsin (C4129, α-chymotrypsin from bovine
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pancreas Type Ⅱ, lyophilized power, ≥40 units/mg protein) were purchased from
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Sigma Chemical Co. (St. Louis, Mo., U.S.A.). Other chemical reagents were all of
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analytical grade.
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2.2 Preparation of raw rice flours and chemical composition analysis
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Rice grains were ground using a household high-speed multi-function grinder (BJ-350,
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DeqingBaijie Electrical Appliance Co. Ltd, China). The resulting flours were passed
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through a 150 µm sieve and used for chemical composition analysis. Total starch
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content of rice flour was determined using the Total Starch Assay Kit following the
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procedure provided by the manufacturer. The nitrogen content of rice flour was
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determined by standard Kjeldahl methodology. Protein content was estimated by
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multiplying the nitrogen content by a conversion factor of 6.25. Total amylose content
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and lipid content of rice flour were measured according to Chrastil (1987) and the
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AACC International Approved Method 44-15.02, respectively. All analyses were
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performed in triplicate.
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For rice grains that were not pre-soaked (U), 100 g were added to 100 mL, 150 mL,
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and 200 mL distilled water and cooked using either a rice cooker (WZA-0512, Shunde
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Electrical Appliance Co. Ltd. Guangdong, China) or an electric pressure cooker
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(YG-D420, Beijing Liven Sci.-Tech. Co. Ltd. Beijing, China), respectively. The rice
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was cooked in the rice cooker for 10 min, followed by a 10 min-holding period at the
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warming setting of the cooker. Pressure cooking was conducted using a fixed cooking
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setting of the cooker (70 kPa) and cooking was considered complete when the lip of
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the cooker could be opened. In the case of pre-soaking before cooking (S), 100 g of
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rice grains were kept in 100 mL, 150 mL, and 200 mL distilled water for 1 h at room
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temperature before cooking as described above. The cooked rice grains were frozen at
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-80 oC overnight and then freeze-dried for 24 h in a freeze dryer (Ningbo Scientz
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Biotechnology Co. Ltd., China). The dried, cooked rice grains were ground using a
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ball mill and passed through a 150 µm sieve. The multiple samples described in the
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Tables and Figures are referred to by a shorthand nomenclature, as illustrated in the
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following examples: IHF-AP-U-n:m and WF-P-S-n:m represent, respectively,
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un-soaked indica rice cooked at atmospheric pressure at a rice/water ratio of n:m and
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pre-soaked waxy rice cooked in a pressure cooker at a rice/water ratio of n:m.
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2.4 Crystallinity determined by X-ray diffraction (XRD) 7
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Shimadzu, Tokyo, Japan) with a Cu-Kα source (λ = 0.154 nm) operating at 40 kV and
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30 mA. Raw and cooked rice flours were equilibrated over a saturated NaCl solution
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at room temperature for one week before analysis. The samples were examined over
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an angular range of 5 - 35o (2θ) at a scanning speed of 2 o/min and a step size of 0.02o.
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The relative crystallinity (%) was quantified as the ratio of the crystalline area to the
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total area under the diffractograms using the Origin software (Version 8.0, Microcal
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Inc., Northampton, MA, USA).
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2.5 Thermal properties
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Thermal properties of rice flours were determined using a Differential Scanning
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Calorimeter (DSC, 200F3, Netzsch, Germany). Raw and cooked rice flours (3 mg, dry
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weight basis) were weighed accurately into aluminum pans. Distilled water was added
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to obtain a rice flour/water ratio of 1:5 (w/v) in the DSC pans. The pans were sealed
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and allowed to stand overnight at room temperature before heating from 20 to 120 oC
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at a rate of 10 oC/min. An empty pan was used as the reference. The onset (To), peak
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(Tp), conclusion (Tc) temperatures, gelatinization temperature range (Tc-To), and
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gelatinization enthalpy change (△H) were obtained through data recording software.
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All measurements were performed in triplicate.
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2.6 Pasting properties 8
ACCEPTED MANUSCRIPT The pasting profiles of rice flours were monitored using a Rapid Visco Analyser-4
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(RVA-4) (Perten Instruments, Warriewood, Australia) according to a method described
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elsewhere(Lee et al., 2005). Raw and cooked rice flours (2.6 g, dry weight basis) were
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weighed exactly into the RVA canisters, and distilled water was added to make a total
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weight of 28 g. The rice flour suspension was held in the RVA at 50 oC for 1 min,
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heated from 50 to 95 oC at a rate of 6 oC/min, held at 95 oC for 5 min, cooled from 95
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to 50 oC at a rate of 6 oC/min, and held at 50 oC for 2 min. The heating process was
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accompanied by a constant shear at 960 rpm for the first 10 s followed by a constant
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shear at 160 rpm until the end of the analysis. Peak viscosity (PV), trough viscosity
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(TV), final viscosity (FV), and pasting temperature (PT) were obtained from the RVA
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profiles. The breakdown (BD) and setback (SB) viscosities were calculated using the
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Thermocline software provided with the instrument. All measurements were done in
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triplicate.
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2.7 In vitro starch digestion
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In vitro starch digestibility was determined according to a modified two-stage
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gastric-intestinal protocol as follows (Wang, Li, Zhang, Wang, & Copeland, 2017).
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For the gastric digestion stage, raw and cooked rice flours (both containing 100 mg
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starch, dry weight basis) were incubated in 5 mL of pepsin solution (1 mg/mL, ≥
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2500 U/mg) at 37 oC with continuous agitation (260 rpm) for 30 min. The solution
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was neutralized with 5 mL of 0.01 M NaOH and mixed with 25 mL of sodium acetate
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intestinal fluid (SIF) containing porcine pancreatic 8000 U α-amylase and 200 U
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amyloglucosidase, 1 mL of trypsin (3000 U) and 1mL of α-chymotrypsin (20 U) was
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added to the above neutralized solution, and incubated at 37 oC with continuous
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stirring (260 rpm) for 2 h. Aliquots (0.2 mL) were taken and mixed with 0.8 mL
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anhydrous ethanol to inactivate the enzymes. After centrifugation, the glucose content
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in the supernatant was determined by the Megazyme GOPOD kit.
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As in vitro starch hydrolysis may be represented as a first order kinetic process, the
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digestograms of starch hydrolysis were fitted to the first-order rate equation, Ct=C∞
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(1-e-kt) (Goni, Garcia-Alonso, & Saura-Calixto, 1997), which can be considered as a
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single exponential decay equation, with the rate of reaction slowing due to substrate
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depletion (Dhital, Warren, Butterworth, Ellis, & Gidley, 2015). In the equation, Ct is
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the amount of starch digested at time t, C∞ is the evaluated amount of starch digested
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at the reaction end point, k is the first order rate constant and t is the digestion time
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(min). Both C∞ and k parameters were calculated for each starch sample on the basis
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of the obtained curves during simulated intestinal digestion.
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2.9 Statistical analysis
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All analyses were performed at least in triplicate, and the results are reported as the 10
ACCEPTED MANUSCRIPT mean values and standard deviations. In the case of XRD, only one measurement was
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performed. One-way analysis of variance (ANOVA) followed by Duncan’s multiple
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range test (p<0.05) was conducted to determine the significant differences between
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mean values using the SPSS 19.0 Statistical Software Program (SPSS, Inc. Chicago,
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IL, U.S.A.).
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3.1 Composition of raw rice flours
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Total starch content of IHF, JF, and WF was 72.8, 76.3, and 89.9%, respectively. The
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apparent amylose content of IHF, JF and WF was 8.1, 14.3 and 0.4%, respectively.
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Lipid content of IHF, JF and WF was 0.8, 0.6 and 0.6%, respectively, consistent with
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previous reports (Chung et al., 2010; Reed et al., 2013), as were crude protein content
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of IHF, JF and WF values of 8.9, 7.9 and 7.2%, respectively (Chung et al., 2010; Mir
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& Bosco, 2014; Reed et al., 2013; Zhu, Liu, Wilson, Gu, & Shi, 2011).
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3.2 Crystallinity of rice flours
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The X-ray diffraction patterns of raw and cooked rice flours are presented in Figure 1.
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As the diffraction patterns of non-waxy rice flours were similar, only JF and WF are
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shown. Raw JF (Figure 1A) and WF (Figure 1B) displayed the typical A-type
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diffraction patterns with strong diffraction peaks at about 15° and 23° (2θ), and an
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unresolved doublet at around 17° and 18° (2θ). The relative crystallinity of raw rice 11
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the typical A-type diffraction patterns of JF and WF disappeared, indicating that
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starches in these rice flours were completely gelatinized. Cooked JF and IHF showed
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two weak diffraction peaks at 13° and 20° (2θ), which were attributed to the Vh-type
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amylose–lipid complexes (Chanvrier et al., 2007; Rewthong, Soponronnarit,
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Taechapairoj, Tungtrakul, & Prachayawarakorn, 2011). However, these diffraction
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peaks were not observed for cooked WF. The diffraction peaks of amylose-lipid
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complex were more obvious for JF-P-U-1:1 and JF-P-S-1:1 than for other cooked rice
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flours, consistent with pressure cooking facilitating the formation of more
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amylose-lipid complexes compared to cooking at atmospheric pressure (Le Bail et al.,
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2013; Meng, Ma, Sun, Wang, & Liu, 2014). On the other hand, high moisture content
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in the outer part of the kernels may limit the formation of amylose–lipid complex
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(Derycke et al., 2005), which could explain the weaker diffraction peaks in other
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samples (Figure 1A).
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3.3 Thermal properties of rice flours
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As the thermograms of the non-waxy flours were similar, only those of JF and WF are
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shown (Figures 1C and 1D). Raw IHF had higher gelatinization temperatures (~ 82 oC)
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compared with raw JF and WF (~ 67 oC), in agreement with the findings reported by
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(Chung et al., 2010). The gelatinization enthalpies were 6.2, 8.2 and 10.5 J/g for raw
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IHF, JF and WF, respectively. After cooking, the three rice flours did not show any 12
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endothermic transitions over a temperature range of 20-120 oC (Figure 1C and 1D), a
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further indication of complete gelatinization of the cooked rice starch, consistent with
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the XRD results.
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The RVA pasting curves of raw and cooked rice flours are depicted in Figure 2, and
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the corresponding viscosity parameters are listed in Table 1. There were significant
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differences in the peak, trough, breakdown, final and setback viscosities between rice
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flours. After cooking at atmospheric pressure, IHF and JF showed similarly-shaped
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RVA pasting profiles to the raw counterparts, although the viscosities were greatly
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reduced (Figures 2A and 2C). This indicated that there were some ordered structural
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remnants, undetected by XRD and DSC in these cooked rice flours, which swelled
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and developed paste viscosity during RVA heating. In addition to decreases in paste
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viscosities, the RVA pasting profiles of IHF and JF after pressure cooking differed
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considerably (Figures 2B and 2D) in comparison with those of rice cooked at
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atmospheric pressure. For example, pressure-cooked JF had an initial pasting
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viscosity that increased very slowly during the RVA heating stage. These results
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indicated that little structural organization remained in pressure-cooked rice flours,
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and that interaction of other components (like protein, cell wall polysaccharides) and
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gelatinized starch during RVA tests may influence the overall pasting profiles of the
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pressure-cooked flours. Most of the cooked IHF and JF showed increased pasting
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indicating disruption of starch granules during cooking, in agreement with a previous
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report (Lee et al., 2005). The peak, trough, final, and setback viscosities of IHF and JF
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obtained after atmosphere pressure cooking decreased significantly with increasing
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water/rice ratio, regardless of whether rice grains were pre-soaked or not (Table 1),
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indicating there was greater disruption of starch granules cooked under higher water
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content. However, under pressure cooking conditions, the peak, trough and
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breakdown viscosities of cooked JF, with or without pre-soaking, increased
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significantly with increasing water/rice ratio. This observation is consistent with the
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formation of more amylose-lipid complex under pressure cooking at lower water/rice
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ratio, which inhibited the swelling of amylopectin and in turn the starch paste
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viscosity.
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Cooked WF exhibited quite different RVA pasting behavior to that of the non-waxy
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rice starches. The RVA profile had an initial high viscosity, indicative of the presence
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of cold water-swelling starch, which forms when gelatinized starch is dried and then
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rehydrated (Chung et al., 2010; Jane, 1992). Cooked and cooled low-amylose
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glutinous rice exhibited a sharp cold water-swelling peak (Chung et al., 2010). In the
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absence of amylose, retrograded waxy starches lack the extended molecular networks
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gels characteristic of non-waxy starches (Tang & Copeland, 2007). Interestingly, and
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in contrast to the two non-waxy varieties, all the paste viscosities of cooked WF were
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higher than those of the raw WF, whereas the pasting temperatures were lower than
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those of raw WF. Similar results were also reported in previous studies (Chung et al.,
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2010; Lai, 2001).
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The in vitro enzymatic digestograms of raw and cooked JF and WF are shown in
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Figure 3. The digestograms of cooked IHF resembled those of cooked JF (not shown).
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No starch hydrolysis occurred with any of the rice flours during the simulated gastric
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digestion in the absence of amylolytic enzymes. Subsequently, the digestograms of the
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cooked rice flours were characterized by an initial rapid rate followed by a more
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gradual rate, reaching a plateau after about 80 min during the small intestinal
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digestion process. Cooking increased significantly the extent and rate of starch
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digestion in JF (Figures 3A and 3B), but there were essentially no differences between
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the variously cooked rice flours. The final hydrolysis percentage of cooked JF was
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above 95%, much higher than 80% of raw JF, and the values of C∞ and k for starch
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digestion in cooked rice flours were higher than those of raw flours (Table 2). The
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higher hydrolysis percentage and faster digestion rate of cooked JF is attributed to the
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disruption of starch granule structures during cooking, which facilitates the
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access/binding of enzyme to the substrate (Wang & Copeland, 2013; Wang, Sun,
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Wang, Wang, & Copeland, 2016). Although there were differences in pasting profiles
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of the cooked non-waxy rice flours and in amounts of amylose-lipid complexes, the
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method of cooking had no significant effect on in vitro enzymatic digestion of starch
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in rice flours.
319 Cooking had only a small effect on in vitro starch digestograms of WF, especially for
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samples cooked at atmosphere (Figures 3C and 3D). The hydrolysis percentages of
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cooked WF were slightly higher than those of raw WF in the early stage of hydrolysis,
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accompanied by small increases in k values (Table 2). While cooking also disrupted
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the granular structure of waxy rice starch, it did not greatly increase the in vitro starch
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digestibility of WF, in contrast to the results observed for IHF and JF. The fact that
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there was only a small increase in the rate and extent of in vitro starch digestibility of
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waxy rice after cooking could be related to the cold-water swelling properties
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observed in the RVA profiles of the cooked WF suspensions, it is possible the higher
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viscosity inhibited diffusion and access of enzymes to starch substrates, which is a
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key rate-limiting step for starch digestion (Wang et al., 2017). Viscosity of the
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medium can affect starch digestion, with a high viscosity of the food matrix
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considered to reduce the rate of digestion and absorption of starch (Bordoloi, Singh, &
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Kaur, 2012). As for JF and IHF, cooking method had little effect on in vitro enzymatic
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digestion of starch in cooked WF.
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4. Conclusions
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The present study has shown that cooking methods had little effect on in vitro 16
ACCEPTED MANUSCRIPT enzymatic starch digestion percentages or the first order rate constants of non-waxy
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and waxy rice flours. Gelatinization of starch in rice flours increased the in vitro
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starch digestibility of cooked non-waxy IHF and JF. However, cooking did not
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increase greatly in vitro starch digestibility of WF. The viscosity of the gelatinized
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starch in cooked rice appeared to play an important role in determining starch
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digestibility. Increased viscosity could account for there being little change in final
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starch digestibility of waxy rice flour after cooking, and also for the lower final
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digestion percentage of the starch in cooked waxy rice compared to the non-waxy
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varieties.
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Acknowledgements
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This work was supported by the State key research and development plan "modern
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food processing and food storage and transportation technology and equipment" (No.
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2017YFD0400200) and by the National Natural Science Foundation of China
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(31522043, 31401651).
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AC C
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356 357 358 17
ACCEPTED MANUSCRIPT 359
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459
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462
physicochemical properties of rice (Oryza sativa L.) flours and starches
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ACCEPTED MANUSCRIPT Figure captions Figure 1. X-ray diffraction patterns of japonica flour (A) and waxy rice flour (B); and DSC thermograms of japonica flour (C) and waxy rice flour (D) before and after cooking. JF/WF-AP-U-1:1.5;
JF/WF-AP-U-1:1;
JF/WF-AP-U-1:2;
JF/WF-AP-S-1:1;
JF/WF-AP-S-1:2;
JF/WF-P-U-1:1;
JF/WF-P-U-1:2;
JF/WF-P-S-1:1;
JF/WF-AP-S-1:1.5; JF/WF-P-U-1:1.5;
JF/WF-P-S-1:1.5;
SC
JF/WF-P-S-1:2
RI PT
Raw JF/WF;
M AN U
Figure 2. RVA pasting profiles of indica hybrid flour (A and B), japonica flour (C and D) and waxy flour (E and F) before and after cooking. A, C and E represent rice cooked at atmospheric pressure, B, D and F represent pressure-cooked rice. Raw IHF/JF/WF;
IHF/JF/WF-AP-U-1:1; IHF/JF/WF-AP-U-1:2;
IHF/JF/WF-AP-S-1:1;
IHF/JF/WF-AP-S-1:1.5;
IHF/JF/WF-AP-S-1:2;
IHF/JF/WF-P-U-1:1;
IHF/JF/WF-P-U-1:1.5;
IHF/JF/WF-P-U-1:2;
IHF/JF/WF-P-S-1:1;
IHF/JF/WF-P-S-1:1.5;
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IHF/JF/WF-AP-U-1:1.5;
IHF/JF/WF-P-S-1:2
AC C
Figure 3. In vitro starch hydrolysis curves of japonica flour (A and B) and waxy flour (C and D) before and after cooking. A and C represent rice cooked at atmospheric pressure, B and D represent pressure-cooked rice. Raw JF/WF;
JF/WF-AP-U-1:1;
JF/WF-AP-U-1:2;
JF/WF-AP-S-1:1;
JF/WF-AP-S-1:2;
JF/WF-P-U-1:1;
JF/WF-P-U-1:2;
JF/WF-P-S-1:1;
JF/WF-P-S-1:2 23
JF/WF-AP-U-1:1.5; JF/WF-AP-S-1:1.5; JF/WF-P-U-1:1.5; JF/WF-P-S-1:1.5;
ACCEPTED MANUSCRIPT Figure 1 12000
12000
A
B 10000
8000
6000
4000
2000
8000
RI PT
D iffraction in te nsity
6000
4000
2000
0
0
5
10
15
20
25
30
35
5
Diffraction angle (2θ)
20
25
30
35
D
M AN U
8.0 7.5
7.5
7.0
H eat flow (m w /m g)
7.0
H eat flow (m w /m g)
15
Diffraction angle (2θ)
C
8.0
10
SC
D iffra ctio n inte nsity
10000
6.5 6.0 5.5
6.5 6.0 5.5 5.0
4.5 4.0 30
40
50
60
TE D
5.0
70
80
90
100
110
o
AC C
EP
Temperature ( C)
24
4.5 4.0 30
40
50
60
70
80 o
Temperature( C)
90
100
110
ACCEPTED MANUSCRIPT Figure 2
A 3000
90 85
2500
100
B
95
3000
90 85
2500
80
80 2000
75
2000
75
RI PT 70
70 1500
1500
65
65 60
1000
60
1000
55
55 500
50
500
50 45
0
5
10
15
20
0
25
5
3500
100
C
3500
D
95 3000
15
20
25
100 95
M AN U
3000
90
90 85
2500
V isco sity(cp )
10
Time (min)
Time (min)
85
2500
80
80
2000
75
2000
75 70
70
1500
1500
65
65 60
1000
60
1000
55
55
500
50
0
5
10
TE D
45
0
15
20
500
45 0
25
100
E
5
90 85
EP
2000
1500
AC C
V isco sity(cp )
2500
25
3500
100
F
95
3000
90 85
2500
80 75
70
70 1500
65
65
60
60
1000
55
55 500
50
50
45
45
0
10
20
2000
75
500
5
15
80
1000
0
10
Time (min)
95
3000
50
0
Time (min) 3500
0
15
20
25
0
Time (min)
5
10
15
Time (min)
25
T em perature ( ℃ )
0
SC
45 0
20
25
T em perature ( ℃ )
V iscosity(cP )
3500
95
Tem perature( ℃ )
100
3500
ACCEPTED MANUSCRIPT Figure 3
90
80
60 40
80
RI PT
Starch hydrolysis(%)
60 40
20
20
0
0 0
20 SGF
40
60
80
100
Time (min)
120 SIF
140
0
160
C
D
100
40
60
80
100
Time (min)
120
140
160
120 SIF
140
160
SIF
M AN U
100
20 SGF
SC
Starch hydrolysis(%)
90
90
Starch hydrolysis (%)
90
80
60 40 20
80
60
40 20
0 40
60
80
100
Time (min)
120 SIF
140
160
EP
20 SGF
TE D
0 0
AC C
Starch hydrolysis(%)
B
100
A
100
26
0
20 SGF
40
60
80
100
Time (min)
ACCEPTED MANUSCRIPT Table 1 Pasting properties of IHF, JF and WF before and after cooking at different cooking methods. Samples
PV (cP)
TV (cP)
BD (cP)
2981.5±20.5i
1452.5±3.5j
1529.0±17.0j
FV (cP)
SB (cP)
PT (oC)
IHF Raw IHF
2661.0±0.0j
1208.5±3.5j
82.7±0.1b
1522.0±80.6h
1314.5±54.4i
207.5±26.2g
2281.0±86.3i
966.5±31.8i
88.7±0.0c
IHF-AP-U-1:1.5
913.0±1.4ef
863.0±2.8h
50.0±1.4cd
1442.0±2.8g
579.0±0.0e
91.1±0.0d
IHF-AP-U-1:2
529.5±2.1b
516.0±1.4d
13.5±3.5ab
870.0±1.4c
354.0±2.8c
ND
IHF-AP-S-1:1
862.5±16.3e
767.0±9.9g
95.5±6.4f
1422.5±24.8g
655.5±14.9g
91.3±0.3d
IHF-AP-S-1:1.5
720.5±2.1d
685.0±5.7e
35.5±3.5bc
1270.5±6.4e
IHF-AP-S-1:2
607.0±1.4c
541.5±3.5d
65.5±2.1de
984.0±14.1d
IHF-P-U-1:1
457.0±18.4a
457.0±18.4bc
0.0±0.0a
735.0±19.8b
IHF-P-U-1:1.5
431.0±0.0a
354.5±4.9a
76.5±5.0ef
647.0±8.5a
1174.5±9.2g
841.0±4.2h
333.5±13.4h
1565.0±9.9h
IHF-P-S-1:1
446.0±1.4a
425.0±1.4b
21.0±0.0ab
753.0±7.1b
585.5±0.7e
93.5±0.0e
442.5±10.6d
ND
278.0±1.4a
ND
292.5±3.5a
ND
724.0±14.1h
ND
328.0±5.7b
ND
SC
IHF-P-U-1:2
RI PT
IHF-AP-U-1:1
IHF-P-S-1:1.5
926.0±4.2f
728.5±3.5f
197.5±7.8g
1355.5±6.4f
627.0±2.8f
ND
IHF-P-S-1:2
935.0±18.4f
469.0±1.4c
466.0±19.8i
843.0±4.2c
374.0±2.8c
ND
Raw JF
2631.0±21.2j
1056.5±2.1j
2215.0±0.0j
1158.5±2.1k
85.1±0.0b
JF-AP-U-1:1
960.5±7.8h
914.5±0.7i
JF-AP-U-1:1.5
619.5±12.0d
591.5±13.4f
JF-AP-U-1:2
522.0±7.1c
520.0±7.1d
JF-AP-S-1:1
1010.5±2.1i
911.5±0.7i
JF-AP-S-1:1.5
778.5±0.7f
727.5±3.5h
JF-AP-S-1:2
706.0±14.1e
640.5±20.5g
354.5±0.7a
346.5±0.7a
JF-P-U-1:1.5
480.5±2.1b
381.0±2.8b
JF-P-U-1:2
606.0±17.0d
JF-P-S-1:1
494.0±9.9b
46.0±7.1cd
1614.0±5.7h
699.5±5.0i
92.3±0.0c
28.0±1.4bc
1017.0±21.2e
425.5±7.8e
94.7±0.1d
2.0±0.0a
868.5±6.4d
348.5±13.4c
ND
99.0±2.8e
1762.0±21.2i
850.5±21.9j
92.9±0.3c
51.0±2.8d
1380.5±0.7g
653.0±4.2h
94.3±0.0d
65.5±6.4d
1187.0±41.1f
546.5±20.5g
ND
8.0±1.4ab
586.0±1.4a
239.5±0.7a
ND
99.5±0.7e
702.5±5.0b
321.5±2.1b
ND
TE D
JF-P-U-1:1
M AN U
JF 1574.5±19.1h
459.0±25.5c
147.0±8.5f
845.5±38.9d
386.5±13.4d
ND
449.0±4.2c
45.0±5.7cd
793.0±9.9c
344.0±5.7bc
ND
JF-P-S-1:1.5
717.0±5.7e
JF-P-S-1:2
844.5±26.2g
556.0±8.5e
161.0±2.8f
1025.5±9.2e
469.5±0.7f
ND
559.0±2.8e
285.5±23.3g
1005.0±7.1e
446.0±4.2e
50.8±1.0a
Raw WF
1226.5±3.5a
333.0±1.4a
492.5±3.5a
159.5±2.1a
65.3±0.2b
WF-AP-U-1:1
1726.5±105.4b
1113.5±12.0i
613.0±117.4a
WF-AP-U-1:1.5
1742.5±55.9b
978.0±50.9h
764.5±106.8ab
1794.5±23.3k
681.0±11.3i
50.2±0.1a
1496.5±68.6j
518.5±17.7h
49.7±0.6a
WF-AP-U-1:2
1835.0±128.7bc
875.5±41.7g
959.5±170.4abc
1307.5±68.6h
432.0±8.5g
50.1±0.0a
WF-AP-S-1:1
2614.0±41.0e
872.0±18.4g
1742.0±22.6e
1336.5±30.4hi
464.5±12.0g
49.9±0.2a
WF-AP-S-1:1.5
2141.0±49.5cd
842.0±5.7fg
1299.0±55.2cd
1226.0±2.8gh
384.0±2.8f
49.6±0.1a
WF-AP-S-1:2
2408.0±287.1de
806.0±11.3ef
1602.0±275.8de
1183.0±32.5fg
377.0±21.2f
49.9±0.3a
WF-P-U-1:1
1897.0±138.6bc
784.5±4.9ef
1112.5±143.5bc
1150.5±0.7efg
366.0±4.2ef
50.1±0.0a
WF-P-U-1:1.5
1821.0±67.9bc
772.0±4.2de
1049.0±72.1bc
1132.0±15.6ef
360.0±11.3ef
50.0±0.0a
AC C
EP
WF
893.5±2.1abc
WF-P-U-1:2
1687.5±251.0b
748.5±47.4de
939.0±298.4abc
1087.5±85.6de
339.0±38.2de
49.9±0.2a
WF-P-S-1:1
1513.5±202.9ab
718.0±22.6cd
795.5±225.6ab
1037.0±24.0cd
319.0±1.4cd
49.7±0.0a
WF-P-S-1:1.5
1581.0±15.6ab
687.0±21.2bc
894.0±36.8abc
990.0±26.9bc
303.0±5.7bc
49.8±0.0a
WF-P-S-1:2
1566.5±314.7ab
647.5±26.2b
919.0±288.5abc
923.5±48.8b
276.0±22.6b
50.0±0.0a
Data are means ± standard deviations. Within the same raw rice flour, values followed by different letters in the same column are significantly different (P <0.05). IHF: indica hybrid rice flour; JF: japonica flour; WF: waxy flour; AP: cooked at atmospheric pressure; P: pressure cooked; U: un-soaked, S: pre-soaked; 1:1, 1:1.5 and 1:2 represent the ratio of rice to water, respectively. PV, peak viscosity; TV, trough viscosity; BD: breakdown viscosity; FV: final viscosity; SB: setback viscosity; PT: pasting temperature. ND, Not detected .Not detected means there is no defined pasting temperature during the heating process.
27
ACCEPTED MANUSCRIPT Table 2 kinetics parameters of cooked and their corresponding raw rice flour C∞ (%)
K (min-1)
Raw JF
83.1±0.3a
0.030±0.000a
JF-AP-U-1:1
98.0±0.8d
0.045±0.002b
JF-AP-U-1:1.5
95.8±0.4bc
0.051±0.001cd
JF-AP-U-1:2
97.7±0.6d
0.049±0.000c
JF-AP-S-1:1
97.0±0.0bcd
0.049±0.000c
JF-AP-S-1:1.5
97.4±1.2cd
JF-AP-S-1:2
95.6±0.5b
JF-P-U-1:1
97.6±1.5d
JF-P-U-1:1.5
98.2±0.6d
JF-P-U-1:2
97.7±0.4d
JF-P-S-1:1
100.0±0.6e
JF-P-S-1:1.5
98.4±0.1d
JF-P-S-1:2
97.3±0.6bcd
0.057±0.002ef
Raw WF
85.5±1.1d
0.041±0.001a
WF-AP-U-1:1
84.1±0.5bc
0.057±0.003b
WF-AP-U-1:1.5
81.9±0.3a
0.063±0.001c
WF-AP-U-1:2
81.8±0.5a
0.063±0.001c
WF-AP-S-1:1
83.3±0.2bc
0.065±0.002cd
WF-AP-S-1:1.5
83.4±0.6bc
0.056±0.001b
83.2±0.9bc
0.058±0.002b
84.2±0.5bc
0.066±0.002cde
83.8±0.7bc
0.065±0.004cd
83.2±0.4bc
0.071±0.003f
84.3±0.5c
0.069±0.002def
83.8±0.6bc
0.070±0.000ef
82.9±0.2ab
0.071±0.001f
WF-P-U-1:1.5 WF-P-U-1:2 WF-P-S-1:1 WF-P-S-1:1.5
0.050±0.003c
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0.057±0.000ef
0.056±0.001ef 0.058±0.003f
0.060±0.001f
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WF-P-S-1:2
0.054±0.000de
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WF-P-U-1:1
0.057±0.002ef
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WF-AP-S-1:2
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Samples
Data are means ± standard deviations. Within the same raw rice flour, values followed by different letters in the same column are significantly different (P <0.05).IHF: indica hybrid rice flour; JF: japonica flour; WF: waxy flour; AP: cooked at atmospheric pressure; P: pressure cooked; U: un-soaked, S: pre-soaked; 1:1, 1:1.5 and 1:2 represent the ratio of rice to water, respectively. C∞: equilibrium concentration, K: kinetic constant.
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ACCEPTED MANUSCRIPT Highlights
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Cooking method had little effect on in vitro starch digestion in rice flours Pressure cooking at low water content favored formation of amylose-lipid complex Cooked waxy flour displayed less starch hydrolysis than cooked non-waxy flours Viscosity of the gelatinized starch influenced in vitro digestion of cooked rice
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S0023-6438(17)30427-9
DOI:
10.1016/j.lwt.2017.06.023
Reference:
YFSTL 6318
To appear in:
LWT - Food Science and Technology
Received Date: 26 March 2017 Revised Date:
9 June 2017
Accepted Date: 12 June 2017
Please cite this article as: Wang, S., Li, P., Zhang, T., Yu, J., Wang, S., Copeland, L., In vitro starch digestibility of rice flour is not affected by method of cooking, LWT - Food Science and Technology (2017), doi: 10.1016/j.lwt.2017.06.023. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT 1
In vitro starch digestibility of rice flour is not affected by method of
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cooking
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Shujun Wanga*, Peiyan Lia, Teng Zhanga, Jinglin Yub, Shuo Wangac*, Les Copelandd
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a
Key Laboratory of Food Nutrition and Safety, Ministry of Education, Tianjin
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University of Science & Technology, Tianjin 300457, China
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Research Centre of Modern Analytical Technique, Tianjin University of Science
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& Technology, Tianjin 300457,China
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Beijing Advanced Innovation Center for Food Nutrition and Human Health,
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Beijing Technology & Business University, Beijing 100048, China
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School of Life and Environmental Sciences, University of Sydney, NSW
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Australia 2006
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* Corresponding authors: Dr. Shujun Wang or Dr. Shuo Wang
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Mailing address: No 29, 13th Avenue, Tianjin Economic and Developmental Area
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(TEDA), Tianjin 300457, China
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Phone: 86-22-60912486
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E-mail address: [email protected] or [email protected] 1
ACCEPTED MANUSCRIPT Abstract: This study evaluated the effects of different cooking methods on in vitro
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starch digestibility of three rice varieties. Results from differential scanning
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calorimetry and X-ray diffraction of cooked rice flours showed that rice starch was
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completely gelatinized after cooking. The overall paste viscosities of cooked indica
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hybrid flour (IHF) and japonica flour (JF) were lower than those of their
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corresponding raw flours. However, cooked waxy flour (WF) showed much higher
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paste viscosities than raw flours, especially the viscosity at the start of RVA testing.
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Cooking increased greatly the rate and extent of starch digestion of IHF and JF, but
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had a small effect on WF. There were small differences in rate and extent of starch
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digestion of rice after cooking under various conditions. Cooking methods were found
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to have little effect on in vitro starch digestibility of rice. Viscosity of the gelatinized
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waxy starch matrix affected in vitro starch digestibility. This study enables us to gain a
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better understanding of cooking effects on in vitro digestibility of starch in rice flour.
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Keywords: cooked rice, cooking method, starch amylolysis, starch gelatinization,
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starch paste viscosity.
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ACCEPTED MANUSCRIPT 1. Introduction
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Rice is the predominant staple food in many countries in the world, accounting for
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20% of the global dietary energy intake (Darandakumbura, wijesinghe, & Prasantha,
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2013). Rice is consumed mainly as intact kernels after cooking with water. However,
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there has been a significant recent increase in rice flour production for utilization in
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baby formulae and gluten-free foods (Hera, Gomez, & Rosell, 2013; Ngamnikom &
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Songsermpong, 2011). Rice flour offers health benefits and many rice-based food
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products, particularly rice bread, are produced for celiac disorder patients with wheat
52
allergy (Qian & Zhang, 2013). Rice and rice-based foods are often classified as
53
having a high glycemic index (GI), which may constitute a public health problem for
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populations who rely heavily on these foods (Hsu, Lu, Chang, & Chiang, 2015). With
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concerns over the increasing incidence of diabetes worldwide, there is a pressing need
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to find rice-based foods with low GI by screening rice varieties and optimizing
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processing protocols.
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On heating in excess of water, starch is gelatinized, leading to the disruption of starch
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structures and enhanced susceptibility of starch to amylolysis. Starchy foods differ
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greatly in the rates at which they are digested and absorbed into the bloodstream. The
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rate and extent of native starch digestion are determined by many factors including
63
botanical source, granule size, surface characters (such as pores or channels, and
64
surface proteins and lipids), amylose content and fine structure of starch polymers; 3
ACCEPTED MANUSCRIPT starch crystallinity, modification and starch-lipid interactions (Singh, Dartois, & Kaur,
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2010; Wang & Copeland, 2013). In contrast, the digestibility of cooked starch is
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affected by the extent of starch gelatinization and retrogradation, interactions between
68
starch and other components, and the properties of the food matrix (Chung, Liu,
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Huang, Yin, & Li, 2010; Dhital, Dabit, Zhang, Flanagan, & Shrestha, 2015; Lee, Lee,
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Han, Lee, & Rhee, 2005; Sagum & Arcot, 2000; Srikaeo & Sopade, 2010; Zhang,
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Dhital, & Gidley, 2015). Rice is usually cooked with water in an electric cooker or a
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microwave oven, or by boiling or steaming (Jung et al., 2009). Some studies have
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shown that different cooking methods can have a significant effect on in vitro and in
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vivo digestibility of rice starch and on blood glucose response in rats (Lee et al., 2005;
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Li, Han, Xu, Xiong, & Zhao, 2014; Rashmi & Urooj, 2003; Reed, Ai, Leutcher, &
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Jane, 2013). For example, Rashmi & Urooj (2003) have reported that pressure
77
cooking, boiling, straining and steaming methods influence the nutritionally important
78
starch fractions in rice varieties. Among these methods, steaming resulted in lower
79
rapidly digestible starch and higher slowly digestible starch in all rice varieties. Lee et
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al. (2005) found that the extent of in vitro starch hydrolysis in cooked rice samples
81
followed the order: autoclave > stone pot > electric cooker > microwave, consistent
82
with the order of degree of starch gelatinization. Reed et al. (2013) showed that
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stir-fried rice displayed the least starch hydrolysis rates compared with steamed and
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pilaf rice. However, a recent study has shown that only a low degree of gelatinization
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is required to greatly increase the susceptibility of starch to amylolysis ( Wang, Wang,
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ACCEPTED MANUSCRIPT Liu, Wang, & Copeland, 2017) and that degree of gelatinization has little effect on in
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vitro digestibility of rice, wheat and lotus starches ( Wang, Sun, Wang, Wang, &
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Copeland, 2016; Wang et al., 2017). Therefore, there is a need to re-evaluate the
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effects of different cooking methods on in vitro starch digestibility of cooked rice.
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A good understanding of the effect of various cooking methods on starch digestibility
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of foods can help consumer to select appropriate cooking practices to improve the
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nutritional quality of starchy food products. This study aimed to understand the effects
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of different cooking protocols, including cooking at atmospheric pressure (AP),
95
pressure cooking (P), pre-soaking (S) and varying the ratio of rice to water, on starch
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in vitro enzymatic hydrolysis of rice starch. Three rice varieties, a non-waxy indica
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hybrid rice, a non-waxy japonica rice and waxy rice, were used in the study. These
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varieties represent a range of popular rice grain types.
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2. Materials and methods
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2.1 Materials
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Grains of three rice (Oryza sativa) varieties differing in amylose content were used in
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the present study. Indica hybrid rice (IHR, Hualiangyou) was kindly provided by
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Huazhong Agricultural University (Wuhan, China). Japonica rice (JR) and waxy rice
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(WR) were purchased from a local supermarket in Tianjin. Total Starch Assay Kit,
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glucose oxidase/peroxidase (GOPOD) kit and amyloglucosidase (AMG, 3260 U/ml)
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Ireland). Amylose (A0512) and amylopectin (A8515) from potato starch, porcine
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pancreatic α-amylase (PPA, A3176, EC 3.2.1.1, typeVI-B from porcine pancreas, 15
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units/mg), pepsin (P7012, pepsin from gastric mucosa, ≥2500 units/mg), trypsin
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(T0303, trypsin from porcine pancreas Type IX-S, lyophilized powder, 13000-20000
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BAEE units/mg protein) and α-chymotrypsin (C4129, α-chymotrypsin from bovine
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pancreas Type Ⅱ, lyophilized power, ≥40 units/mg protein) were purchased from
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Sigma Chemical Co. (St. Louis, Mo., U.S.A.). Other chemical reagents were all of
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analytical grade.
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2.2 Preparation of raw rice flours and chemical composition analysis
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Rice grains were ground using a household high-speed multi-function grinder (BJ-350,
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DeqingBaijie Electrical Appliance Co. Ltd, China). The resulting flours were passed
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through a 150 µm sieve and used for chemical composition analysis. Total starch
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content of rice flour was determined using the Total Starch Assay Kit following the
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procedure provided by the manufacturer. The nitrogen content of rice flour was
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determined by standard Kjeldahl methodology. Protein content was estimated by
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multiplying the nitrogen content by a conversion factor of 6.25. Total amylose content
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and lipid content of rice flour were measured according to Chrastil (1987) and the
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AACC International Approved Method 44-15.02, respectively. All analyses were
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performed in triplicate.
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For rice grains that were not pre-soaked (U), 100 g were added to 100 mL, 150 mL,
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and 200 mL distilled water and cooked using either a rice cooker (WZA-0512, Shunde
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Electrical Appliance Co. Ltd. Guangdong, China) or an electric pressure cooker
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(YG-D420, Beijing Liven Sci.-Tech. Co. Ltd. Beijing, China), respectively. The rice
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was cooked in the rice cooker for 10 min, followed by a 10 min-holding period at the
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warming setting of the cooker. Pressure cooking was conducted using a fixed cooking
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setting of the cooker (70 kPa) and cooking was considered complete when the lip of
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the cooker could be opened. In the case of pre-soaking before cooking (S), 100 g of
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rice grains were kept in 100 mL, 150 mL, and 200 mL distilled water for 1 h at room
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temperature before cooking as described above. The cooked rice grains were frozen at
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-80 oC overnight and then freeze-dried for 24 h in a freeze dryer (Ningbo Scientz
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Biotechnology Co. Ltd., China). The dried, cooked rice grains were ground using a
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ball mill and passed through a 150 µm sieve. The multiple samples described in the
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Tables and Figures are referred to by a shorthand nomenclature, as illustrated in the
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following examples: IHF-AP-U-n:m and WF-P-S-n:m represent, respectively,
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un-soaked indica rice cooked at atmospheric pressure at a rice/water ratio of n:m and
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pre-soaked waxy rice cooked in a pressure cooker at a rice/water ratio of n:m.
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2.4 Crystallinity determined by X-ray diffraction (XRD) 7
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Shimadzu, Tokyo, Japan) with a Cu-Kα source (λ = 0.154 nm) operating at 40 kV and
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30 mA. Raw and cooked rice flours were equilibrated over a saturated NaCl solution
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at room temperature for one week before analysis. The samples were examined over
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an angular range of 5 - 35o (2θ) at a scanning speed of 2 o/min and a step size of 0.02o.
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The relative crystallinity (%) was quantified as the ratio of the crystalline area to the
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total area under the diffractograms using the Origin software (Version 8.0, Microcal
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Inc., Northampton, MA, USA).
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2.5 Thermal properties
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Thermal properties of rice flours were determined using a Differential Scanning
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Calorimeter (DSC, 200F3, Netzsch, Germany). Raw and cooked rice flours (3 mg, dry
161
weight basis) were weighed accurately into aluminum pans. Distilled water was added
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to obtain a rice flour/water ratio of 1:5 (w/v) in the DSC pans. The pans were sealed
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and allowed to stand overnight at room temperature before heating from 20 to 120 oC
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at a rate of 10 oC/min. An empty pan was used as the reference. The onset (To), peak
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(Tp), conclusion (Tc) temperatures, gelatinization temperature range (Tc-To), and
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gelatinization enthalpy change (△H) were obtained through data recording software.
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All measurements were performed in triplicate.
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2.6 Pasting properties 8
ACCEPTED MANUSCRIPT The pasting profiles of rice flours were monitored using a Rapid Visco Analyser-4
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(RVA-4) (Perten Instruments, Warriewood, Australia) according to a method described
172
elsewhere(Lee et al., 2005). Raw and cooked rice flours (2.6 g, dry weight basis) were
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weighed exactly into the RVA canisters, and distilled water was added to make a total
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weight of 28 g. The rice flour suspension was held in the RVA at 50 oC for 1 min,
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heated from 50 to 95 oC at a rate of 6 oC/min, held at 95 oC for 5 min, cooled from 95
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to 50 oC at a rate of 6 oC/min, and held at 50 oC for 2 min. The heating process was
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accompanied by a constant shear at 960 rpm for the first 10 s followed by a constant
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shear at 160 rpm until the end of the analysis. Peak viscosity (PV), trough viscosity
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(TV), final viscosity (FV), and pasting temperature (PT) were obtained from the RVA
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profiles. The breakdown (BD) and setback (SB) viscosities were calculated using the
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Thermocline software provided with the instrument. All measurements were done in
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triplicate.
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2.7 In vitro starch digestion
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In vitro starch digestibility was determined according to a modified two-stage
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gastric-intestinal protocol as follows (Wang, Li, Zhang, Wang, & Copeland, 2017).
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For the gastric digestion stage, raw and cooked rice flours (both containing 100 mg
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starch, dry weight basis) were incubated in 5 mL of pepsin solution (1 mg/mL, ≥
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2500 U/mg) at 37 oC with continuous agitation (260 rpm) for 30 min. The solution
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was neutralized with 5 mL of 0.01 M NaOH and mixed with 25 mL of sodium acetate
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ACCEPTED MANUSCRIPT buffer (pH 6, 0.2 M) to stop enzyme reactions. Subsequently, 5 mL of simulated
192
intestinal fluid (SIF) containing porcine pancreatic 8000 U α-amylase and 200 U
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amyloglucosidase, 1 mL of trypsin (3000 U) and 1mL of α-chymotrypsin (20 U) was
194
added to the above neutralized solution, and incubated at 37 oC with continuous
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stirring (260 rpm) for 2 h. Aliquots (0.2 mL) were taken and mixed with 0.8 mL
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anhydrous ethanol to inactivate the enzymes. After centrifugation, the glucose content
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in the supernatant was determined by the Megazyme GOPOD kit.
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As in vitro starch hydrolysis may be represented as a first order kinetic process, the
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digestograms of starch hydrolysis were fitted to the first-order rate equation, Ct=C∞
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(1-e-kt) (Goni, Garcia-Alonso, & Saura-Calixto, 1997), which can be considered as a
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single exponential decay equation, with the rate of reaction slowing due to substrate
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depletion (Dhital, Warren, Butterworth, Ellis, & Gidley, 2015). In the equation, Ct is
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the amount of starch digested at time t, C∞ is the evaluated amount of starch digested
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at the reaction end point, k is the first order rate constant and t is the digestion time
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(min). Both C∞ and k parameters were calculated for each starch sample on the basis
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of the obtained curves during simulated intestinal digestion.
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2.9 Statistical analysis
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All analyses were performed at least in triplicate, and the results are reported as the 10
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performed. One-way analysis of variance (ANOVA) followed by Duncan’s multiple
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range test (p<0.05) was conducted to determine the significant differences between
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mean values using the SPSS 19.0 Statistical Software Program (SPSS, Inc. Chicago,
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IL, U.S.A.).
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3.1 Composition of raw rice flours
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Total starch content of IHF, JF, and WF was 72.8, 76.3, and 89.9%, respectively. The
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apparent amylose content of IHF, JF and WF was 8.1, 14.3 and 0.4%, respectively.
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Lipid content of IHF, JF and WF was 0.8, 0.6 and 0.6%, respectively, consistent with
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previous reports (Chung et al., 2010; Reed et al., 2013), as were crude protein content
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of IHF, JF and WF values of 8.9, 7.9 and 7.2%, respectively (Chung et al., 2010; Mir
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& Bosco, 2014; Reed et al., 2013; Zhu, Liu, Wilson, Gu, & Shi, 2011).
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3.2 Crystallinity of rice flours
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The X-ray diffraction patterns of raw and cooked rice flours are presented in Figure 1.
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As the diffraction patterns of non-waxy rice flours were similar, only JF and WF are
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shown. Raw JF (Figure 1A) and WF (Figure 1B) displayed the typical A-type
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diffraction patterns with strong diffraction peaks at about 15° and 23° (2θ), and an
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unresolved doublet at around 17° and 18° (2θ). The relative crystallinity of raw rice 11
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the typical A-type diffraction patterns of JF and WF disappeared, indicating that
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starches in these rice flours were completely gelatinized. Cooked JF and IHF showed
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two weak diffraction peaks at 13° and 20° (2θ), which were attributed to the Vh-type
237
amylose–lipid complexes (Chanvrier et al., 2007; Rewthong, Soponronnarit,
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Taechapairoj, Tungtrakul, & Prachayawarakorn, 2011). However, these diffraction
239
peaks were not observed for cooked WF. The diffraction peaks of amylose-lipid
240
complex were more obvious for JF-P-U-1:1 and JF-P-S-1:1 than for other cooked rice
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flours, consistent with pressure cooking facilitating the formation of more
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amylose-lipid complexes compared to cooking at atmospheric pressure (Le Bail et al.,
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2013; Meng, Ma, Sun, Wang, & Liu, 2014). On the other hand, high moisture content
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in the outer part of the kernels may limit the formation of amylose–lipid complex
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(Derycke et al., 2005), which could explain the weaker diffraction peaks in other
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samples (Figure 1A).
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3.3 Thermal properties of rice flours
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As the thermograms of the non-waxy flours were similar, only those of JF and WF are
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shown (Figures 1C and 1D). Raw IHF had higher gelatinization temperatures (~ 82 oC)
251
compared with raw JF and WF (~ 67 oC), in agreement with the findings reported by
252
(Chung et al., 2010). The gelatinization enthalpies were 6.2, 8.2 and 10.5 J/g for raw
253
IHF, JF and WF, respectively. After cooking, the three rice flours did not show any 12
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endothermic transitions over a temperature range of 20-120 oC (Figure 1C and 1D), a
255
further indication of complete gelatinization of the cooked rice starch, consistent with
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the XRD results.
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The RVA pasting curves of raw and cooked rice flours are depicted in Figure 2, and
260
the corresponding viscosity parameters are listed in Table 1. There were significant
261
differences in the peak, trough, breakdown, final and setback viscosities between rice
262
flours. After cooking at atmospheric pressure, IHF and JF showed similarly-shaped
263
RVA pasting profiles to the raw counterparts, although the viscosities were greatly
264
reduced (Figures 2A and 2C). This indicated that there were some ordered structural
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remnants, undetected by XRD and DSC in these cooked rice flours, which swelled
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and developed paste viscosity during RVA heating. In addition to decreases in paste
267
viscosities, the RVA pasting profiles of IHF and JF after pressure cooking differed
268
considerably (Figures 2B and 2D) in comparison with those of rice cooked at
269
atmospheric pressure. For example, pressure-cooked JF had an initial pasting
270
viscosity that increased very slowly during the RVA heating stage. These results
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indicated that little structural organization remained in pressure-cooked rice flours,
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and that interaction of other components (like protein, cell wall polysaccharides) and
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gelatinized starch during RVA tests may influence the overall pasting profiles of the
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pressure-cooked flours. Most of the cooked IHF and JF showed increased pasting
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indicating disruption of starch granules during cooking, in agreement with a previous
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report (Lee et al., 2005). The peak, trough, final, and setback viscosities of IHF and JF
278
obtained after atmosphere pressure cooking decreased significantly with increasing
279
water/rice ratio, regardless of whether rice grains were pre-soaked or not (Table 1),
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indicating there was greater disruption of starch granules cooked under higher water
281
content. However, under pressure cooking conditions, the peak, trough and
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breakdown viscosities of cooked JF, with or without pre-soaking, increased
283
significantly with increasing water/rice ratio. This observation is consistent with the
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formation of more amylose-lipid complex under pressure cooking at lower water/rice
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ratio, which inhibited the swelling of amylopectin and in turn the starch paste
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viscosity.
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Cooked WF exhibited quite different RVA pasting behavior to that of the non-waxy
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rice starches. The RVA profile had an initial high viscosity, indicative of the presence
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of cold water-swelling starch, which forms when gelatinized starch is dried and then
291
rehydrated (Chung et al., 2010; Jane, 1992). Cooked and cooled low-amylose
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glutinous rice exhibited a sharp cold water-swelling peak (Chung et al., 2010). In the
293
absence of amylose, retrograded waxy starches lack the extended molecular networks
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gels characteristic of non-waxy starches (Tang & Copeland, 2007). Interestingly, and
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in contrast to the two non-waxy varieties, all the paste viscosities of cooked WF were
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higher than those of the raw WF, whereas the pasting temperatures were lower than
297
those of raw WF. Similar results were also reported in previous studies (Chung et al.,
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2010; Lai, 2001).
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The in vitro enzymatic digestograms of raw and cooked JF and WF are shown in
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Figure 3. The digestograms of cooked IHF resembled those of cooked JF (not shown).
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No starch hydrolysis occurred with any of the rice flours during the simulated gastric
304
digestion in the absence of amylolytic enzymes. Subsequently, the digestograms of the
305
cooked rice flours were characterized by an initial rapid rate followed by a more
306
gradual rate, reaching a plateau after about 80 min during the small intestinal
307
digestion process. Cooking increased significantly the extent and rate of starch
308
digestion in JF (Figures 3A and 3B), but there were essentially no differences between
309
the variously cooked rice flours. The final hydrolysis percentage of cooked JF was
310
above 95%, much higher than 80% of raw JF, and the values of C∞ and k for starch
311
digestion in cooked rice flours were higher than those of raw flours (Table 2). The
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higher hydrolysis percentage and faster digestion rate of cooked JF is attributed to the
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disruption of starch granule structures during cooking, which facilitates the
314
access/binding of enzyme to the substrate (Wang & Copeland, 2013; Wang, Sun,
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Wang, Wang, & Copeland, 2016). Although there were differences in pasting profiles
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of the cooked non-waxy rice flours and in amounts of amylose-lipid complexes, the
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method of cooking had no significant effect on in vitro enzymatic digestion of starch
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in rice flours.
319 Cooking had only a small effect on in vitro starch digestograms of WF, especially for
321
samples cooked at atmosphere (Figures 3C and 3D). The hydrolysis percentages of
322
cooked WF were slightly higher than those of raw WF in the early stage of hydrolysis,
323
accompanied by small increases in k values (Table 2). While cooking also disrupted
324
the granular structure of waxy rice starch, it did not greatly increase the in vitro starch
325
digestibility of WF, in contrast to the results observed for IHF and JF. The fact that
326
there was only a small increase in the rate and extent of in vitro starch digestibility of
327
waxy rice after cooking could be related to the cold-water swelling properties
328
observed in the RVA profiles of the cooked WF suspensions, it is possible the higher
329
viscosity inhibited diffusion and access of enzymes to starch substrates, which is a
330
key rate-limiting step for starch digestion (Wang et al., 2017). Viscosity of the
331
medium can affect starch digestion, with a high viscosity of the food matrix
332
considered to reduce the rate of digestion and absorption of starch (Bordoloi, Singh, &
333
Kaur, 2012). As for JF and IHF, cooking method had little effect on in vitro enzymatic
334
digestion of starch in cooked WF.
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335 336
4. Conclusions
337
The present study has shown that cooking methods had little effect on in vitro 16
ACCEPTED MANUSCRIPT enzymatic starch digestion percentages or the first order rate constants of non-waxy
339
and waxy rice flours. Gelatinization of starch in rice flours increased the in vitro
340
starch digestibility of cooked non-waxy IHF and JF. However, cooking did not
341
increase greatly in vitro starch digestibility of WF. The viscosity of the gelatinized
342
starch in cooked rice appeared to play an important role in determining starch
343
digestibility. Increased viscosity could account for there being little change in final
344
starch digestibility of waxy rice flour after cooking, and also for the lower final
345
digestion percentage of the starch in cooked waxy rice compared to the non-waxy
346
varieties.
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Acknowledgements
349
This work was supported by the State key research and development plan "modern
350
food processing and food storage and transportation technology and equipment" (No.
351
2017YFD0400200) and by the National Natural Science Foundation of China
352
(31522043, 31401651).
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ACCEPTED MANUSCRIPT Figure captions Figure 1. X-ray diffraction patterns of japonica flour (A) and waxy rice flour (B); and DSC thermograms of japonica flour (C) and waxy rice flour (D) before and after cooking. JF/WF-AP-U-1:1.5;
JF/WF-AP-U-1:1;
JF/WF-AP-U-1:2;
JF/WF-AP-S-1:1;
JF/WF-AP-S-1:2;
JF/WF-P-U-1:1;
JF/WF-P-U-1:2;
JF/WF-P-S-1:1;
JF/WF-AP-S-1:1.5; JF/WF-P-U-1:1.5;
JF/WF-P-S-1:1.5;
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Raw JF/WF;
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Figure 2. RVA pasting profiles of indica hybrid flour (A and B), japonica flour (C and D) and waxy flour (E and F) before and after cooking. A, C and E represent rice cooked at atmospheric pressure, B, D and F represent pressure-cooked rice. Raw IHF/JF/WF;
IHF/JF/WF-AP-U-1:1; IHF/JF/WF-AP-U-1:2;
IHF/JF/WF-AP-S-1:1;
IHF/JF/WF-AP-S-1:1.5;
IHF/JF/WF-AP-S-1:2;
IHF/JF/WF-P-U-1:1;
IHF/JF/WF-P-U-1:1.5;
IHF/JF/WF-P-U-1:2;
IHF/JF/WF-P-S-1:1;
IHF/JF/WF-P-S-1:1.5;
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Figure 3. In vitro starch hydrolysis curves of japonica flour (A and B) and waxy flour (C and D) before and after cooking. A and C represent rice cooked at atmospheric pressure, B and D represent pressure-cooked rice. Raw JF/WF;
JF/WF-AP-U-1:1;
JF/WF-AP-U-1:2;
JF/WF-AP-S-1:1;
JF/WF-AP-S-1:2;
JF/WF-P-U-1:1;
JF/WF-P-U-1:2;
JF/WF-P-S-1:1;
JF/WF-P-S-1:2 23
JF/WF-AP-U-1:1.5; JF/WF-AP-S-1:1.5; JF/WF-P-U-1:1.5; JF/WF-P-S-1:1.5;
ACCEPTED MANUSCRIPT Figure 1 12000
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H eat flow (m w /m g)
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H eat flow (m w /m g)
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Diffraction angle (2θ)
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8.0
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ACCEPTED MANUSCRIPT Figure 2
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ACCEPTED MANUSCRIPT Figure 3
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ACCEPTED MANUSCRIPT Table 1 Pasting properties of IHF, JF and WF before and after cooking at different cooking methods. Samples
PV (cP)
TV (cP)
BD (cP)
2981.5±20.5i
1452.5±3.5j
1529.0±17.0j
FV (cP)
SB (cP)
PT (oC)
IHF Raw IHF
2661.0±0.0j
1208.5±3.5j
82.7±0.1b
1522.0±80.6h
1314.5±54.4i
207.5±26.2g
2281.0±86.3i
966.5±31.8i
88.7±0.0c
IHF-AP-U-1:1.5
913.0±1.4ef
863.0±2.8h
50.0±1.4cd
1442.0±2.8g
579.0±0.0e
91.1±0.0d
IHF-AP-U-1:2
529.5±2.1b
516.0±1.4d
13.5±3.5ab
870.0±1.4c
354.0±2.8c
ND
IHF-AP-S-1:1
862.5±16.3e
767.0±9.9g
95.5±6.4f
1422.5±24.8g
655.5±14.9g
91.3±0.3d
IHF-AP-S-1:1.5
720.5±2.1d
685.0±5.7e
35.5±3.5bc
1270.5±6.4e
IHF-AP-S-1:2
607.0±1.4c
541.5±3.5d
65.5±2.1de
984.0±14.1d
IHF-P-U-1:1
457.0±18.4a
457.0±18.4bc
0.0±0.0a
735.0±19.8b
IHF-P-U-1:1.5
431.0±0.0a
354.5±4.9a
76.5±5.0ef
647.0±8.5a
1174.5±9.2g
841.0±4.2h
333.5±13.4h
1565.0±9.9h
IHF-P-S-1:1
446.0±1.4a
425.0±1.4b
21.0±0.0ab
753.0±7.1b
585.5±0.7e
93.5±0.0e
442.5±10.6d
ND
278.0±1.4a
ND
292.5±3.5a
ND
724.0±14.1h
ND
328.0±5.7b
ND
SC
IHF-P-U-1:2
RI PT
IHF-AP-U-1:1
IHF-P-S-1:1.5
926.0±4.2f
728.5±3.5f
197.5±7.8g
1355.5±6.4f
627.0±2.8f
ND
IHF-P-S-1:2
935.0±18.4f
469.0±1.4c
466.0±19.8i
843.0±4.2c
374.0±2.8c
ND
Raw JF
2631.0±21.2j
1056.5±2.1j
2215.0±0.0j
1158.5±2.1k
85.1±0.0b
JF-AP-U-1:1
960.5±7.8h
914.5±0.7i
JF-AP-U-1:1.5
619.5±12.0d
591.5±13.4f
JF-AP-U-1:2
522.0±7.1c
520.0±7.1d
JF-AP-S-1:1
1010.5±2.1i
911.5±0.7i
JF-AP-S-1:1.5
778.5±0.7f
727.5±3.5h
JF-AP-S-1:2
706.0±14.1e
640.5±20.5g
354.5±0.7a
346.5±0.7a
JF-P-U-1:1.5
480.5±2.1b
381.0±2.8b
JF-P-U-1:2
606.0±17.0d
JF-P-S-1:1
494.0±9.9b
46.0±7.1cd
1614.0±5.7h
699.5±5.0i
92.3±0.0c
28.0±1.4bc
1017.0±21.2e
425.5±7.8e
94.7±0.1d
2.0±0.0a
868.5±6.4d
348.5±13.4c
ND
99.0±2.8e
1762.0±21.2i
850.5±21.9j
92.9±0.3c
51.0±2.8d
1380.5±0.7g
653.0±4.2h
94.3±0.0d
65.5±6.4d
1187.0±41.1f
546.5±20.5g
ND
8.0±1.4ab
586.0±1.4a
239.5±0.7a
ND
99.5±0.7e
702.5±5.0b
321.5±2.1b
ND
TE D
JF-P-U-1:1
M AN U
JF 1574.5±19.1h
459.0±25.5c
147.0±8.5f
845.5±38.9d
386.5±13.4d
ND
449.0±4.2c
45.0±5.7cd
793.0±9.9c
344.0±5.7bc
ND
JF-P-S-1:1.5
717.0±5.7e
JF-P-S-1:2
844.5±26.2g
556.0±8.5e
161.0±2.8f
1025.5±9.2e
469.5±0.7f
ND
559.0±2.8e
285.5±23.3g
1005.0±7.1e
446.0±4.2e
50.8±1.0a
Raw WF
1226.5±3.5a
333.0±1.4a
492.5±3.5a
159.5±2.1a
65.3±0.2b
WF-AP-U-1:1
1726.5±105.4b
1113.5±12.0i
613.0±117.4a
WF-AP-U-1:1.5
1742.5±55.9b
978.0±50.9h
764.5±106.8ab
1794.5±23.3k
681.0±11.3i
50.2±0.1a
1496.5±68.6j
518.5±17.7h
49.7±0.6a
WF-AP-U-1:2
1835.0±128.7bc
875.5±41.7g
959.5±170.4abc
1307.5±68.6h
432.0±8.5g
50.1±0.0a
WF-AP-S-1:1
2614.0±41.0e
872.0±18.4g
1742.0±22.6e
1336.5±30.4hi
464.5±12.0g
49.9±0.2a
WF-AP-S-1:1.5
2141.0±49.5cd
842.0±5.7fg
1299.0±55.2cd
1226.0±2.8gh
384.0±2.8f
49.6±0.1a
WF-AP-S-1:2
2408.0±287.1de
806.0±11.3ef
1602.0±275.8de
1183.0±32.5fg
377.0±21.2f
49.9±0.3a
WF-P-U-1:1
1897.0±138.6bc
784.5±4.9ef
1112.5±143.5bc
1150.5±0.7efg
366.0±4.2ef
50.1±0.0a
WF-P-U-1:1.5
1821.0±67.9bc
772.0±4.2de
1049.0±72.1bc
1132.0±15.6ef
360.0±11.3ef
50.0±0.0a
AC C
EP
WF
893.5±2.1abc
WF-P-U-1:2
1687.5±251.0b
748.5±47.4de
939.0±298.4abc
1087.5±85.6de
339.0±38.2de
49.9±0.2a
WF-P-S-1:1
1513.5±202.9ab
718.0±22.6cd
795.5±225.6ab
1037.0±24.0cd
319.0±1.4cd
49.7±0.0a
WF-P-S-1:1.5
1581.0±15.6ab
687.0±21.2bc
894.0±36.8abc
990.0±26.9bc
303.0±5.7bc
49.8±0.0a
WF-P-S-1:2
1566.5±314.7ab
647.5±26.2b
919.0±288.5abc
923.5±48.8b
276.0±22.6b
50.0±0.0a
Data are means ± standard deviations. Within the same raw rice flour, values followed by different letters in the same column are significantly different (P <0.05). IHF: indica hybrid rice flour; JF: japonica flour; WF: waxy flour; AP: cooked at atmospheric pressure; P: pressure cooked; U: un-soaked, S: pre-soaked; 1:1, 1:1.5 and 1:2 represent the ratio of rice to water, respectively. PV, peak viscosity; TV, trough viscosity; BD: breakdown viscosity; FV: final viscosity; SB: setback viscosity; PT: pasting temperature. ND, Not detected .Not detected means there is no defined pasting temperature during the heating process.
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ACCEPTED MANUSCRIPT Table 2 kinetics parameters of cooked and their corresponding raw rice flour C∞ (%)
K (min-1)
Raw JF
83.1±0.3a
0.030±0.000a
JF-AP-U-1:1
98.0±0.8d
0.045±0.002b
JF-AP-U-1:1.5
95.8±0.4bc
0.051±0.001cd
JF-AP-U-1:2
97.7±0.6d
0.049±0.000c
JF-AP-S-1:1
97.0±0.0bcd
0.049±0.000c
JF-AP-S-1:1.5
97.4±1.2cd
JF-AP-S-1:2
95.6±0.5b
JF-P-U-1:1
97.6±1.5d
JF-P-U-1:1.5
98.2±0.6d
JF-P-U-1:2
97.7±0.4d
JF-P-S-1:1
100.0±0.6e
JF-P-S-1:1.5
98.4±0.1d
JF-P-S-1:2
97.3±0.6bcd
0.057±0.002ef
Raw WF
85.5±1.1d
0.041±0.001a
WF-AP-U-1:1
84.1±0.5bc
0.057±0.003b
WF-AP-U-1:1.5
81.9±0.3a
0.063±0.001c
WF-AP-U-1:2
81.8±0.5a
0.063±0.001c
WF-AP-S-1:1
83.3±0.2bc
0.065±0.002cd
WF-AP-S-1:1.5
83.4±0.6bc
0.056±0.001b
83.2±0.9bc
0.058±0.002b
84.2±0.5bc
0.066±0.002cde
83.8±0.7bc
0.065±0.004cd
83.2±0.4bc
0.071±0.003f
84.3±0.5c
0.069±0.002def
83.8±0.6bc
0.070±0.000ef
82.9±0.2ab
0.071±0.001f
WF-P-U-1:1.5 WF-P-U-1:2 WF-P-S-1:1 WF-P-S-1:1.5
0.050±0.003c
SC
0.057±0.000ef
0.056±0.001ef 0.058±0.003f
0.060±0.001f
M AN U
AC C
WF-P-S-1:2
0.054±0.000de
TE D
WF-P-U-1:1
0.057±0.002ef
EP
WF-AP-S-1:2
RI PT
Samples
Data are means ± standard deviations. Within the same raw rice flour, values followed by different letters in the same column are significantly different (P <0.05).IHF: indica hybrid rice flour; JF: japonica flour; WF: waxy flour; AP: cooked at atmospheric pressure; P: pressure cooked; U: un-soaked, S: pre-soaked; 1:1, 1:1.5 and 1:2 represent the ratio of rice to water, respectively. C∞: equilibrium concentration, K: kinetic constant.
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ACCEPTED MANUSCRIPT Highlights
EP
TE D
M AN U
SC
RI PT
Cooking method had little effect on in vitro starch digestion in rice flours Pressure cooking at low water content favored formation of amylose-lipid complex Cooked waxy flour displayed less starch hydrolysis than cooked non-waxy flours Viscosity of the gelatinized starch influenced in vitro digestion of cooked rice
AC C
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