Food Chemistry 172 (2015) 757–765
Contents lists available at ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
In vitro digestibility and physicochemical properties of milled rice Sushil Dhital a, Laura Dabit b, Bin Zhang a, Bernadine Flanagan a, Ashok K. Shrestha b,⇑ a ARC Centre of Excellence in Plant Cell Walls, Centre for Nutrition and Food Sciences, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, St. Lucia, Brisbane, Queensland 4072, Australia b Nutrition and Food Science Program, School of Science and Health, University of Western Sydney, Richmond, Hawkesbury Campus, Sydney, NSW 2753, Australia
a r t i c l e
i n f o
Article history: Received 23 April 2014 Received in revised form 28 August 2014 Accepted 24 September 2014 Available online 2 October 2014 Keywords: Rice Amylose Gelatinization Supra-molecular structure Digestibility
a b s t r a c t Rice is a staple diet as well as a major ingredient in many processed foods. The physicochemical and supra-molecular structure of eight rice varieties with amylose content from 9% to 19% were studied to elucidate the factors responsible for variation in enzymatic digestibility of raw and cooked rice. Parboiled rice had a digestion rate coefficient almost 4.5 times higher than the least digestible Low GI rice. The rate coefficient was found to be independent of helical structure and long range molecular order, possibly attributed to the effect of rice flour architecture. Strong swelling and pasting behaviour and lower gelatinisation temperature were linked with apparently higher in vitro digestibility but the relationship was statistically insignificant. It is concluded that the enzymatic susceptibility of rice flours are independent of supra-molecular structure and are most likely controlled by external factors not limited to particle size, presence of intact cell wall and other non-starch polymers. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Rice (Oryzae sativa, L.) is the staple food for over half of the world’s population. Though it is mostly consumed in cooked form, the inclusion of rice flour in a number of processed foods, such as in baby formula and gluten free foods, is commonplace. Rice starch is highly digestible; therefore can increase the post-prandial blood glucose level associated with diet-related health complications (Mohan, Radhika, Vijayalakshmi, & Sudha, 2010). The huge surge in metabolic syndrome in the Asian population is often linked with excessive consumption of rapidly digestible food sources, such as cooked rice (Hu, 2011). In order to combat this problem, consumption of minimally or un-milled rice, parboiled rice, high-amylose, high fibre and mixed grain including rice is gaining popularity. On the other hand, rice varieties with high amylose content, that are claimed to have low glycaemic response, are commercially available. The enzymatic susceptibility of rice, rice flour or isolated rice starch has been studied extensively (Chung, Liu, Lee, & Wei, 2011; Okuda, Aramaki, Koseki, Satoh, & Hashizume, 2005; Wani, Singh, & Shah, 2012) with inconclusive findings that long range crystalline and short range molecular structures in part controls the enzymic digestibility (Chung, Lim, & Lim, 2006; Chung, Wang, Yin, & Li, 2010; Chung et al., 2011; Reed, Ai, Leutcher, & ⇑ Corresponding author. Tel.: +61 2 4570 1296; fax: +61 2 4570 1383. E-mail address: [email protected] (A.K. Shrestha). http://dx.doi.org/10.1016/j.foodchem.2014.09.138 0308-8146/Ó 2014 Elsevier Ltd. All rights reserved.
Jane, 2013; Wani et al., 2012). There is enough evidence that high amylose cereals, such as mutant maize and barley, are less susceptible to amylolytic enzymes compared to wild varieties primarily due to the interaction of amylose with longer branch chains of amylopectin strengthening the granular structure (Bird, Shrestha, Lopez-Rubio, & Gidley, 2009; Sasaki, Kohyama, Suzuki, Noel, & Ring, 2009; Shrestha et al., 2012). However, the majority of rice starches are rich in amylopectin, and are disrupted with complete or partial loss of supra-molecular and granular structure during high temperature processing, such as cooking, enhancing the enzyme susceptibility (Chung et al., 2011). Some other parameters, such as higher peak and breakdown viscosities during shear cooking, are also associated positively with both amylopectin content and enzymic susceptibility (Benmoussa, Moldenhauer, & Hamaker, 2007; Chung et al., 2011). Apart from amylose content and molecular/supra-molecular structure; cultivars, chemical/physical modification and processing; architectural factors, such as granule/particle size and porosity; intactness of cell walls; and presence of anti-nutrients, also control the rate and extent of enzymatic hydrolysis of starches (Bird et al., 2009; Wani et al., 2012). Choi, Kim, Kang, Nam, and Friedman (2010) have demonstrated the potential value of black rice bran as an anti-inflammatory and anti-allergic food ingredient and possibly also as a therapeutic agent for the treatment and prevention of diseases associated with chronic inflammation. Though studied extensively, most of the studies regarding the enzyme susceptibility of rice starch/flour are limited to either
758
S. Dhital et al. / Food Chemistry 172 (2015) 757–765
non-commercial varieties (mutants or genetically modified crops) or with few rice varieties which may not be sufficient to draw the meaningful relationship between the molecular, supramolecular structure and enzymic susceptibility. Furthermore, very limited reports are available regarding the causal relationship between the raw and cooked rice flours. In order to address these limitations, this study demonstrated the relationship between the supra-molecular structure (short and long range molecular order), thermal and rheological properties, non-starch polysaccharides and enzymic susceptibility of both raw and cooked rice flours using a range of techniques covering wider range of length scales.
2. Materials and methods 2.1. Materials The following enzymes and chemicals were obtained from local distributors: a-amylase (Sigma A-3176 Type VI-B from porcine pancreas), pepsin (Sigma P-6887, from gastric porcine mucosa), pancreatin (Sigma P-1750 from porcine pancreas), amyloglucosidase, (AMG 300, Novozyme), enzyme glucose reagent (TR15103, Thermoelectron), pure potato amylose (A0512, Sigma) and amylopectin (S9679, Sigma). Eight commercial brand rice varieties were purchased from the local supermarket. Low GI brown long grain, Sushi rice Japanese style, Vita-Rice parboiled long grain and Black rice were of Sunrice brand (SunRice, Leeton, NSW, Australia), whereas Arborio and Jasmine rice were of Home Brand (Woolworths Homebrand, Bella Vista, NSW, Australia). Basmati rice was an organic product from Macro wholefoods (Bella Vista, NSW, Australia) whereas RicePlus™ was from Plus Nutrition Pty Ltd, Mordialloc, VIC, Australia. As a product, RicePlus™ is unique as it has a mixture of ingredients: Brown Rice Long Grain (25%), White Basmati (24%), BARLEYmaxÒ, Pearl Barley, Quinoa, Red Basmati (5%), Black Rice (5%), Black Sesame Seeds. All rice samples were ground using a B-2B Mini blender (Homemaker, Australia) into moderately fine particles. The ground rice that passed through 600 lM but was retained in 400 lM (rice grits) was used for all analytical purpose. The particle size range was selected to minimise the deviation arising from difference in particle size as reported for starch granules (Dhital, Shrestha, & Gidley, 2010) or grain fragments (Al-Rabadi, Gilbert, & Gidley, 2009) and also to mimic the size of chewed cooked rice during mastication. The sieved flours were stored in air tight glass bottles at room temperature until further use. 2.2. Chemical composition The moisture content of the ground rice samples was measured by the standard air oven drying at 105 °C overnight (AOAC, 2005). The level of starch present in the ground rice was measured by acid digestion method followed by Lane and Enyon method (ASEAN Manual of Food Analysis, 2011). The apparent amylose content of the ground samples were analysed by iodine colorimetric method (Hoover & Ratnayake, 2005). All samples were analysed in triplicates. 2.3. Crystallinity (X-ray diffraction) XRD measurements of samples were made with a Panalytical X’Pert Pro diffractometer. The instrument was equipped with a copper X-ray generator (k = 1.54 A°), programmable incident beam divergence slit and diffracted beam scatter slit, and an X’celerator high speed detector. X-ray diffraction patterns were acquired at
room temperature over the 2H range of 2–40° with a step size of 0.0330° 2H and a count time of 400 s per step. Crystallinity was calculated using the modified curve-fitting procedure as described by Lopez-Rubio, Flanagan, Gilbert, and Gidley (2008). Repeat analyses indicated an accuracy of ±1% for crystallinity values. 2.4. Fourier transform infrared spectroscopy (FTIR) Fourier transformed infrared (FT-IR) spectra of starch in rice flour were recorded on a FT-IR spectrometer (Nicolet 5700, Thermo Electron Corporation, Madison, WI, USA) using an attenuated total reflectance (ATR) single reflectance cell with a diamond crystal. For each spectrum, 32 scans were recorded over the range of 1200– 800 cm1 at room temperature (about 22 °C) at a resolution of 4 cm1, co-added and Fourier transformed. The background spectrum was recorded on air and subtracted from the sample spectrum. The ratio of absorbance at wave numbers 1047/1022 was calculated to represent ordered short range starch structures. 2.5. Rice starch pasting properties (RVA) Rapid Visco Analyser (RVA) (Newport Scientific, RVA model 4, New South Wales, Thermocline Software Version 2.2) was used to measure the rheological properties of all rice starches. Starch suspension (8%, w/w) was prepared by weighing starch (2 ± 0.1 g) into the RVA canister and making up the total weight to 25 ± 1 g with distilled-deionised water. After equilibration at 50 °C for 1 min, the starch suspension was heated at a rate of 6 °C/min to 95 °C, maintained at 95 °C for 5 min, cooled to 50 °C at a rate of 6 °C/min, and then maintained at 50 °C for 2 min. A paddle speed of 160 rpm was used throughout, except for the first 10 s when a speed of 960 rpm was used to disperse the sample. Pasting temperature (PT), peak viscosity (PV), hot paste (trough) viscosity (HPV), and cool paste (end) viscosity (CPV) were measured from the RVA software. 2.6. Differential scanning calorimeter (DSC) DSC 1 (Mettler Toledo, Schwerzenbach, Switzerland) with internal coolant and nitrogen/air purge gas was used to determine gelatinisation/melting temperatures of starches in uncooked flour. DSC was calibrated for the heat flow and melting temperature using indium and zinc as standards. An empty crucible was used as a reference. Rice flour (4 ± 0.1 mg) was mixed with deionised water (12 ± 0.3 mg) in a DSC pan (low pressure 40 ll aluminium pan). The crucible was left for about 120 min for equilibrium hydration of the sample before analysis. All analyses were carried out in duplicate. The pans were held at 10 °C for 5 min and then heated to 120 °C at 5 °C/min. The onset (To), peak (Tp), conclusion temperatures (Tc) and the enthalpy of gelatinisation (DH, J/g) were determined using the built-in software (STARe System, Mettler Toledo, Schwerzenbach, Switzerland). 2.7. In vitro digestion of raw starch In vitro starch digestion of ground rice was carried following method described by Shrestha et al. (2010) that mimics the biochemical conditions of mouth, stomach and small intestine using porcine salivary amylase, pepsin and pancreatin and amyloglucosidase as sources of hydrolysing enzymes, respectively. The resulting oligomers from the action of the amylases were converted to glucose using fungal amyloglucosidase and quantified with GOPOD enzymic assay kit as described by Shrestha et al. (2010). The results were presented as percentage starch hydrolysis, using the conversion factor of 0.9 for anhydrous glucose to starch.
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S. Dhital et al. / Food Chemistry 172 (2015) 757–765
2.8. In vitro digestion of cooked starch Considering the digestibility of cooked rice is likely to be different from raw rice, rice grains were cooked in excess water as suggested by Reed et al. (2013). The milled rice fragments (500 mg per 10 ml water) were heated to boiling temperature in shakingheating water bath for 10 min operating at 30 strokes per minutes. The temperature of the water bath and content of the bottle was quickly lowered down to 37 °C using an ice water bath. Once the temperature was set, in vitro digestion of cooked starch was carried out immediately (as to minimise the effect of retrogradation) following the protocol mentioned previously. 2.9. Data fitting and statistical analysis The standard first-order rate equation (Eq. (1)) was used to investigate the kinetics of starch digestion under simulated small intestine conditions by a-amylase.
C ¼ 1 eKt
ð1Þ
where C = starch digested (expressed as % db) at incubation time t (min), 1 C = undigested starch remaining after incubation time t, K = digestion rate coefficient (min1). For the purpose of data fitting, the value of K was obtained by a linear-least-squares fit of the solution of Eq. (1), viz., as the slope of a plot of ln(1 C) against t for each granule size as described by Dhital et al. (2010). Results are expressed as means with standard deviations of at least duplicate measurements. In the case of XRD and FTIR, only one measurement was performed as these techniques give exactly reproducible results. Analysis of variance (ANOVA) was used to determine the least significance at p < 0.05 using Minitab 16 (Minitab Inc., State College, PA), and correlation coefficients were determined using Microsoft Office Excel 2011. 3. Results and discussion 3.1. Nutritional composition of polished rice Table 1 shows the percentage distribution of nutrients in rice kernels. Except for moisture, starch and amylose content, other parameters were extracted from the nutritional information panel in the primary original packet. In general, energy contribution from rice varies considerably e.g., 1460 kJ/100 g in Sushi to 1548 kJ/ 100 g in Arborio. Sushi rice had the lowest level of protein, 6.0%, whereas RicePlus™ had the highest, 10.1%. The protein content of milled rice varies significantly with cultivars and is also affected by environmental factors and processing parameters (Juliano, 1990). Composition data from the label also showed the wider difference in fibre and lipid content of rice, between 0.5–11.9% and 0.5–4.6%, respectively (Table 1). It is understood that the high level of lipid, dietary fibre and protein in RicePlus™ are due to
incorporation of other grains in order to increase the level essential nutrients, such as resistant starch and non-starch polysaccharides. The level of starch in rice varieties differs considerably. A review by Wani et al. (2012) showed the starch content from rice cultivars varies from 59% to 71.6%. Whereas, current starch analysis of 8 rice varieties showed a relatively larger range, 60.9% in RicePlus™ to 83.1% Arborio (Table 1). RicePlus™ had the lowest starch content which can be explained by the nature of this product that contained mixture of multi-grains, exceptionally rich in dietary fibre (11.9%). Starch content from the current rice sample showed the majority of the samples had higher than 71.6% starch e.g. Sushi, Vita parboiled, Arborio, Basmati and Jasmine. These values are not very different from Japonica (84%), Indica (83%) and Waxy (80%) as reported by Reed et al. (2013). The lipid content of the rice samples varied from 0.5% to 4.6%, which is higher than previously reviewed by Wani et al. (2012). Rice starch is bland tasting, creamy, spreadable, and has a smooth texture that makes it an ideal material for various food formulations. Higher digestibility, hypoallergenicity, smaller granules size, greater acid resistance and a wide range of amylose:amylopectin ratios are other functionalities of rice starch (Lawal et al., 2011; Mitchell, 2009; Wani et al., 2012). 3.2. Apparent amylose content Amylose is probably the most important component affecting the physico-chemical, rheological, thermal and nutritional properties of starch based materials. The starch iodine complex method was used to analyse the apparent amylose content and gave values ranging from 9.1% to 18.8% (Table 1). It has been well publicized that the measurement of amylose in starch based materials is method dependent. However, a number of factors (as reviewed by Fitzerald et al., 2011) can affect the accuracy of analysis resulting in variation among published results. For example, Arborio, a Japonica variety, reported to contain 18.8% amylose (Chung et al., 2011), is significantly higher than the currently analysed value, 14.2%. On the basis of apparent amylose content, rice is generally classified as: waxy (0–2% amylose); very low (5–12% amylose); low (12–20% amylose); intermediate (20–25%); or high (25–33% amylose) (Wani et al., 2012). Based on above classification, rice starches were classified as Very low amylose: Low GI, Sushi, Black, and Jasmine; and Low amylose: Vita rice parboiled, RicePlus™, Arborio, and Basmati. 3.3. Crystallinity of rice starch flour Starch molecules in a crystalline alignment give rise to the peaks in X-ray diffractograms, whereas starch molecules in amorphous regions contribute to the diffuse regions of the XRD patterns. The X-ray diffraction pattern of the individual rice flours are displayed in Fig. 1. Native rice starches clearly displayed A-type
Table 1 Chemical composition of rice samples (g/100 g).a
a
Rice
Energy, kJ
Watera
Proteins
Starcha
Amylosea
Fibre
Lipid
Low GI Sushi Vita Par Black RicePlus™ Arborio Basmati Jasmine
1490 1460 1500 1490 1470 1548 1530 1500
12.5 ± 0.5 12.8 ± 1.1 12.7 ± 0.9 11.1 ± 0.8 12.1 ± 1.4 14.4 ± 1.2 12.2 ± 1.0 13.3 ± 0.9
8.1 6.0 8.3 8.1 10.1 6.8 8.9 8.0
71.6 ± 2.5 78.4 ± 1.8 79.8 ± 2.1 71.0 ± 1.5 60.9 ± 1.9 83.1 ± 2.9 77.5 ± 3.0 82.2 ± 3.1
9.9 ± 0.8 10.2 ± 0.4 13.1 ± 0.8 9.1 ± 0.5 18.8 ± 1.5 14.2 ± 1.1 15.0 ± 1.8 10.0 ± 1.0
3.4
Contents lists available at ScienceDirect
Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
In vitro digestibility and physicochemical properties of milled rice Sushil Dhital a, Laura Dabit b, Bin Zhang a, Bernadine Flanagan a, Ashok K. Shrestha b,⇑ a ARC Centre of Excellence in Plant Cell Walls, Centre for Nutrition and Food Sciences, Queensland Alliance for Agriculture and Food Innovation, The University of Queensland, St. Lucia, Brisbane, Queensland 4072, Australia b Nutrition and Food Science Program, School of Science and Health, University of Western Sydney, Richmond, Hawkesbury Campus, Sydney, NSW 2753, Australia
a r t i c l e
i n f o
Article history: Received 23 April 2014 Received in revised form 28 August 2014 Accepted 24 September 2014 Available online 2 October 2014 Keywords: Rice Amylose Gelatinization Supra-molecular structure Digestibility
a b s t r a c t Rice is a staple diet as well as a major ingredient in many processed foods. The physicochemical and supra-molecular structure of eight rice varieties with amylose content from 9% to 19% were studied to elucidate the factors responsible for variation in enzymatic digestibility of raw and cooked rice. Parboiled rice had a digestion rate coefficient almost 4.5 times higher than the least digestible Low GI rice. The rate coefficient was found to be independent of helical structure and long range molecular order, possibly attributed to the effect of rice flour architecture. Strong swelling and pasting behaviour and lower gelatinisation temperature were linked with apparently higher in vitro digestibility but the relationship was statistically insignificant. It is concluded that the enzymatic susceptibility of rice flours are independent of supra-molecular structure and are most likely controlled by external factors not limited to particle size, presence of intact cell wall and other non-starch polymers. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Rice (Oryzae sativa, L.) is the staple food for over half of the world’s population. Though it is mostly consumed in cooked form, the inclusion of rice flour in a number of processed foods, such as in baby formula and gluten free foods, is commonplace. Rice starch is highly digestible; therefore can increase the post-prandial blood glucose level associated with diet-related health complications (Mohan, Radhika, Vijayalakshmi, & Sudha, 2010). The huge surge in metabolic syndrome in the Asian population is often linked with excessive consumption of rapidly digestible food sources, such as cooked rice (Hu, 2011). In order to combat this problem, consumption of minimally or un-milled rice, parboiled rice, high-amylose, high fibre and mixed grain including rice is gaining popularity. On the other hand, rice varieties with high amylose content, that are claimed to have low glycaemic response, are commercially available. The enzymatic susceptibility of rice, rice flour or isolated rice starch has been studied extensively (Chung, Liu, Lee, & Wei, 2011; Okuda, Aramaki, Koseki, Satoh, & Hashizume, 2005; Wani, Singh, & Shah, 2012) with inconclusive findings that long range crystalline and short range molecular structures in part controls the enzymic digestibility (Chung, Lim, & Lim, 2006; Chung, Wang, Yin, & Li, 2010; Chung et al., 2011; Reed, Ai, Leutcher, & ⇑ Corresponding author. Tel.: +61 2 4570 1296; fax: +61 2 4570 1383. E-mail address: [email protected] (A.K. Shrestha). http://dx.doi.org/10.1016/j.foodchem.2014.09.138 0308-8146/Ó 2014 Elsevier Ltd. All rights reserved.
Jane, 2013; Wani et al., 2012). There is enough evidence that high amylose cereals, such as mutant maize and barley, are less susceptible to amylolytic enzymes compared to wild varieties primarily due to the interaction of amylose with longer branch chains of amylopectin strengthening the granular structure (Bird, Shrestha, Lopez-Rubio, & Gidley, 2009; Sasaki, Kohyama, Suzuki, Noel, & Ring, 2009; Shrestha et al., 2012). However, the majority of rice starches are rich in amylopectin, and are disrupted with complete or partial loss of supra-molecular and granular structure during high temperature processing, such as cooking, enhancing the enzyme susceptibility (Chung et al., 2011). Some other parameters, such as higher peak and breakdown viscosities during shear cooking, are also associated positively with both amylopectin content and enzymic susceptibility (Benmoussa, Moldenhauer, & Hamaker, 2007; Chung et al., 2011). Apart from amylose content and molecular/supra-molecular structure; cultivars, chemical/physical modification and processing; architectural factors, such as granule/particle size and porosity; intactness of cell walls; and presence of anti-nutrients, also control the rate and extent of enzymatic hydrolysis of starches (Bird et al., 2009; Wani et al., 2012). Choi, Kim, Kang, Nam, and Friedman (2010) have demonstrated the potential value of black rice bran as an anti-inflammatory and anti-allergic food ingredient and possibly also as a therapeutic agent for the treatment and prevention of diseases associated with chronic inflammation. Though studied extensively, most of the studies regarding the enzyme susceptibility of rice starch/flour are limited to either
758
S. Dhital et al. / Food Chemistry 172 (2015) 757–765
non-commercial varieties (mutants or genetically modified crops) or with few rice varieties which may not be sufficient to draw the meaningful relationship between the molecular, supramolecular structure and enzymic susceptibility. Furthermore, very limited reports are available regarding the causal relationship between the raw and cooked rice flours. In order to address these limitations, this study demonstrated the relationship between the supra-molecular structure (short and long range molecular order), thermal and rheological properties, non-starch polysaccharides and enzymic susceptibility of both raw and cooked rice flours using a range of techniques covering wider range of length scales.
2. Materials and methods 2.1. Materials The following enzymes and chemicals were obtained from local distributors: a-amylase (Sigma A-3176 Type VI-B from porcine pancreas), pepsin (Sigma P-6887, from gastric porcine mucosa), pancreatin (Sigma P-1750 from porcine pancreas), amyloglucosidase, (AMG 300, Novozyme), enzyme glucose reagent (TR15103, Thermoelectron), pure potato amylose (A0512, Sigma) and amylopectin (S9679, Sigma). Eight commercial brand rice varieties were purchased from the local supermarket. Low GI brown long grain, Sushi rice Japanese style, Vita-Rice parboiled long grain and Black rice were of Sunrice brand (SunRice, Leeton, NSW, Australia), whereas Arborio and Jasmine rice were of Home Brand (Woolworths Homebrand, Bella Vista, NSW, Australia). Basmati rice was an organic product from Macro wholefoods (Bella Vista, NSW, Australia) whereas RicePlus™ was from Plus Nutrition Pty Ltd, Mordialloc, VIC, Australia. As a product, RicePlus™ is unique as it has a mixture of ingredients: Brown Rice Long Grain (25%), White Basmati (24%), BARLEYmaxÒ, Pearl Barley, Quinoa, Red Basmati (5%), Black Rice (5%), Black Sesame Seeds. All rice samples were ground using a B-2B Mini blender (Homemaker, Australia) into moderately fine particles. The ground rice that passed through 600 lM but was retained in 400 lM (rice grits) was used for all analytical purpose. The particle size range was selected to minimise the deviation arising from difference in particle size as reported for starch granules (Dhital, Shrestha, & Gidley, 2010) or grain fragments (Al-Rabadi, Gilbert, & Gidley, 2009) and also to mimic the size of chewed cooked rice during mastication. The sieved flours were stored in air tight glass bottles at room temperature until further use. 2.2. Chemical composition The moisture content of the ground rice samples was measured by the standard air oven drying at 105 °C overnight (AOAC, 2005). The level of starch present in the ground rice was measured by acid digestion method followed by Lane and Enyon method (ASEAN Manual of Food Analysis, 2011). The apparent amylose content of the ground samples were analysed by iodine colorimetric method (Hoover & Ratnayake, 2005). All samples were analysed in triplicates. 2.3. Crystallinity (X-ray diffraction) XRD measurements of samples were made with a Panalytical X’Pert Pro diffractometer. The instrument was equipped with a copper X-ray generator (k = 1.54 A°), programmable incident beam divergence slit and diffracted beam scatter slit, and an X’celerator high speed detector. X-ray diffraction patterns were acquired at
room temperature over the 2H range of 2–40° with a step size of 0.0330° 2H and a count time of 400 s per step. Crystallinity was calculated using the modified curve-fitting procedure as described by Lopez-Rubio, Flanagan, Gilbert, and Gidley (2008). Repeat analyses indicated an accuracy of ±1% for crystallinity values. 2.4. Fourier transform infrared spectroscopy (FTIR) Fourier transformed infrared (FT-IR) spectra of starch in rice flour were recorded on a FT-IR spectrometer (Nicolet 5700, Thermo Electron Corporation, Madison, WI, USA) using an attenuated total reflectance (ATR) single reflectance cell with a diamond crystal. For each spectrum, 32 scans were recorded over the range of 1200– 800 cm1 at room temperature (about 22 °C) at a resolution of 4 cm1, co-added and Fourier transformed. The background spectrum was recorded on air and subtracted from the sample spectrum. The ratio of absorbance at wave numbers 1047/1022 was calculated to represent ordered short range starch structures. 2.5. Rice starch pasting properties (RVA) Rapid Visco Analyser (RVA) (Newport Scientific, RVA model 4, New South Wales, Thermocline Software Version 2.2) was used to measure the rheological properties of all rice starches. Starch suspension (8%, w/w) was prepared by weighing starch (2 ± 0.1 g) into the RVA canister and making up the total weight to 25 ± 1 g with distilled-deionised water. After equilibration at 50 °C for 1 min, the starch suspension was heated at a rate of 6 °C/min to 95 °C, maintained at 95 °C for 5 min, cooled to 50 °C at a rate of 6 °C/min, and then maintained at 50 °C for 2 min. A paddle speed of 160 rpm was used throughout, except for the first 10 s when a speed of 960 rpm was used to disperse the sample. Pasting temperature (PT), peak viscosity (PV), hot paste (trough) viscosity (HPV), and cool paste (end) viscosity (CPV) were measured from the RVA software. 2.6. Differential scanning calorimeter (DSC) DSC 1 (Mettler Toledo, Schwerzenbach, Switzerland) with internal coolant and nitrogen/air purge gas was used to determine gelatinisation/melting temperatures of starches in uncooked flour. DSC was calibrated for the heat flow and melting temperature using indium and zinc as standards. An empty crucible was used as a reference. Rice flour (4 ± 0.1 mg) was mixed with deionised water (12 ± 0.3 mg) in a DSC pan (low pressure 40 ll aluminium pan). The crucible was left for about 120 min for equilibrium hydration of the sample before analysis. All analyses were carried out in duplicate. The pans were held at 10 °C for 5 min and then heated to 120 °C at 5 °C/min. The onset (To), peak (Tp), conclusion temperatures (Tc) and the enthalpy of gelatinisation (DH, J/g) were determined using the built-in software (STARe System, Mettler Toledo, Schwerzenbach, Switzerland). 2.7. In vitro digestion of raw starch In vitro starch digestion of ground rice was carried following method described by Shrestha et al. (2010) that mimics the biochemical conditions of mouth, stomach and small intestine using porcine salivary amylase, pepsin and pancreatin and amyloglucosidase as sources of hydrolysing enzymes, respectively. The resulting oligomers from the action of the amylases were converted to glucose using fungal amyloglucosidase and quantified with GOPOD enzymic assay kit as described by Shrestha et al. (2010). The results were presented as percentage starch hydrolysis, using the conversion factor of 0.9 for anhydrous glucose to starch.
759
S. Dhital et al. / Food Chemistry 172 (2015) 757–765
2.8. In vitro digestion of cooked starch Considering the digestibility of cooked rice is likely to be different from raw rice, rice grains were cooked in excess water as suggested by Reed et al. (2013). The milled rice fragments (500 mg per 10 ml water) were heated to boiling temperature in shakingheating water bath for 10 min operating at 30 strokes per minutes. The temperature of the water bath and content of the bottle was quickly lowered down to 37 °C using an ice water bath. Once the temperature was set, in vitro digestion of cooked starch was carried out immediately (as to minimise the effect of retrogradation) following the protocol mentioned previously. 2.9. Data fitting and statistical analysis The standard first-order rate equation (Eq. (1)) was used to investigate the kinetics of starch digestion under simulated small intestine conditions by a-amylase.
C ¼ 1 eKt
ð1Þ
where C = starch digested (expressed as % db) at incubation time t (min), 1 C = undigested starch remaining after incubation time t, K = digestion rate coefficient (min1). For the purpose of data fitting, the value of K was obtained by a linear-least-squares fit of the solution of Eq. (1), viz., as the slope of a plot of ln(1 C) against t for each granule size as described by Dhital et al. (2010). Results are expressed as means with standard deviations of at least duplicate measurements. In the case of XRD and FTIR, only one measurement was performed as these techniques give exactly reproducible results. Analysis of variance (ANOVA) was used to determine the least significance at p < 0.05 using Minitab 16 (Minitab Inc., State College, PA), and correlation coefficients were determined using Microsoft Office Excel 2011. 3. Results and discussion 3.1. Nutritional composition of polished rice Table 1 shows the percentage distribution of nutrients in rice kernels. Except for moisture, starch and amylose content, other parameters were extracted from the nutritional information panel in the primary original packet. In general, energy contribution from rice varies considerably e.g., 1460 kJ/100 g in Sushi to 1548 kJ/ 100 g in Arborio. Sushi rice had the lowest level of protein, 6.0%, whereas RicePlus™ had the highest, 10.1%. The protein content of milled rice varies significantly with cultivars and is also affected by environmental factors and processing parameters (Juliano, 1990). Composition data from the label also showed the wider difference in fibre and lipid content of rice, between 0.5–11.9% and 0.5–4.6%, respectively (Table 1). It is understood that the high level of lipid, dietary fibre and protein in RicePlus™ are due to
incorporation of other grains in order to increase the level essential nutrients, such as resistant starch and non-starch polysaccharides. The level of starch in rice varieties differs considerably. A review by Wani et al. (2012) showed the starch content from rice cultivars varies from 59% to 71.6%. Whereas, current starch analysis of 8 rice varieties showed a relatively larger range, 60.9% in RicePlus™ to 83.1% Arborio (Table 1). RicePlus™ had the lowest starch content which can be explained by the nature of this product that contained mixture of multi-grains, exceptionally rich in dietary fibre (11.9%). Starch content from the current rice sample showed the majority of the samples had higher than 71.6% starch e.g. Sushi, Vita parboiled, Arborio, Basmati and Jasmine. These values are not very different from Japonica (84%), Indica (83%) and Waxy (80%) as reported by Reed et al. (2013). The lipid content of the rice samples varied from 0.5% to 4.6%, which is higher than previously reviewed by Wani et al. (2012). Rice starch is bland tasting, creamy, spreadable, and has a smooth texture that makes it an ideal material for various food formulations. Higher digestibility, hypoallergenicity, smaller granules size, greater acid resistance and a wide range of amylose:amylopectin ratios are other functionalities of rice starch (Lawal et al., 2011; Mitchell, 2009; Wani et al., 2012). 3.2. Apparent amylose content Amylose is probably the most important component affecting the physico-chemical, rheological, thermal and nutritional properties of starch based materials. The starch iodine complex method was used to analyse the apparent amylose content and gave values ranging from 9.1% to 18.8% (Table 1). It has been well publicized that the measurement of amylose in starch based materials is method dependent. However, a number of factors (as reviewed by Fitzerald et al., 2011) can affect the accuracy of analysis resulting in variation among published results. For example, Arborio, a Japonica variety, reported to contain 18.8% amylose (Chung et al., 2011), is significantly higher than the currently analysed value, 14.2%. On the basis of apparent amylose content, rice is generally classified as: waxy (0–2% amylose); very low (5–12% amylose); low (12–20% amylose); intermediate (20–25%); or high (25–33% amylose) (Wani et al., 2012). Based on above classification, rice starches were classified as Very low amylose: Low GI, Sushi, Black, and Jasmine; and Low amylose: Vita rice parboiled, RicePlus™, Arborio, and Basmati. 3.3. Crystallinity of rice starch flour Starch molecules in a crystalline alignment give rise to the peaks in X-ray diffractograms, whereas starch molecules in amorphous regions contribute to the diffuse regions of the XRD patterns. The X-ray diffraction pattern of the individual rice flours are displayed in Fig. 1. Native rice starches clearly displayed A-type
Table 1 Chemical composition of rice samples (g/100 g).a
a
Rice
Energy, kJ
Watera
Proteins
Starcha
Amylosea
Fibre
Lipid
Low GI Sushi Vita Par Black RicePlus™ Arborio Basmati Jasmine
1490 1460 1500 1490 1470 1548 1530 1500
12.5 ± 0.5 12.8 ± 1.1 12.7 ± 0.9 11.1 ± 0.8 12.1 ± 1.4 14.4 ± 1.2 12.2 ± 1.0 13.3 ± 0.9
8.1 6.0 8.3 8.1 10.1 6.8 8.9 8.0
71.6 ± 2.5 78.4 ± 1.8 79.8 ± 2.1 71.0 ± 1.5 60.9 ± 1.9 83.1 ± 2.9 77.5 ± 3.0 82.2 ± 3.1
9.9 ± 0.8 10.2 ± 0.4 13.1 ± 0.8 9.1 ± 0.5 18.8 ± 1.5 14.2 ± 1.1 15.0 ± 1.8 10.0 ± 1.0
3.4
