Physicochemical and rheological properties of flour and starch from Thai pigmented rice cultivars

International Journal of Biological Macromolecules 137 (2019) 666–675

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Physicochemical and rheological properties of flour and starch from Thai pigmented rice cultivars Nuttinee Tangsrianugul a, Rungtiwa Wongsagonsup b, Manop Suphantharika a,⁎ a b

Department of Biotechnology, Faculty of Science, Mahidol University, Rama 6 Road, Bangkok 10400, Thailand Food Technology Division, School of Interdisciplinary Studies, Mahidol University, Kanchanaburi Campus, Kanchanaburi 71150, Thailand

a r t i c l e

i n f o

Article history: Received 10 April 2019 Received in revised form 14 June 2019 Accepted 24 June 2019 Available online 26 June 2019 Keywords: Pigmented rice Amylose content Physicochemical properties

a b s t r a c t Flour and starch from four Thai pigmented rice cultivars, i.e., Riceberry (RB), Hom Nil (HN), Niaw Dang (ND), and Kum Pleuak Khao (KP) were compared for their physicochemical and rheological properties. Amylose content of all rice starches decreased in the following order: RB (12.09%) N HN (8.14%) N KP (2.87%) ~ ND (2.77%). The HN starch had the lowest proportion of amylopectin short A chains, while the KP starch showed the highest. Pasting temperature, setback, and final viscosity increased, while breakdown and swelling power decreased with increasing amylose content for both flour and starch samples. The flours and starches from the RB and HN showed greater onset and peak temperatures and enthalpy change (ΔH) of gelatinization than those from the ND and KP. Moreover, the gelatinization temperatures of all starches were significantly lower, but ΔH was higher than their flour counterparts. Dynamic viscoelastic tests revealed weak-gel like behavior of all flour and starch gels as evidenced by their G′ N G″ and tan δ values were smaller than unity. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Rice (Oryza sativa L.) is a major cereal crop and a staple food of over half of the world's population. Although rice is consumed mostly as white rice, there are special rice cultivars that contain pigments in the pericarp or in the bran part of the rice kernel such as black rice, purple rice, red rice, and brown rice. The pigments composed of a mixture of anthocyanins which its grain color range from various shade of red and purple to black [1]. Recently, consumption of pigmented rice is increasing due to its benefit on human health that is attributed to the presence of phenolic compounds such as phenolic acids and anthocyanins [2]. Many studies have demonstrated antioxidant activity, anticancer, anti-inflammatory, antiallergic, antimutagenic and hypoglycemic activities of pigmented rice [1,3]. In addition, pigmented rice is a good source of minerals, fiber, vitamins, and other phytochemicals which has a good effect on health [4,5]. Anthocyanins, a group of natural pigments, are water soluble polyphenolic compounds that are sensitive to many environmental factors such as temperature, oxygen, light, and pH [6]. The color and benefit of anthocyanins are vulnerable to be destroyed by these factors, thus their applications in foods may be restricted. Therefore, dry-milling process was used in this study to obtain rice flour from dehulled rice and maintain anthocyanins. Normally, the pigmented rice is consumed ⁎ Corresponding author. E-mail address: [email protected] (M. Suphantharika).

https://doi.org/10.1016/j.ijbiomac.2019.06.196 0141-8130/© 2019 Elsevier B.V. All rights reserved.

both as whole grain as well as processed products for example; as a food colorant in bread, ice cream and liquor [7], traditional colored rice cakes in Korea, brewing colored red wines [8] and noodles [9]. Starch is the major component of rice and consists of two polysaccharides, amylose and amylopectin. Amylose is a predominantly linear molecule of α (1 → 4) linked D glucopyranosyl units with a few branches, while amylopectin has a larger molecular weight and highly branched molecule with the branch points being α (1 → 6) linked D glucopyranosyl units. The amylose/amylopectin ratio of starch mostly influences the starch functional properties. The swelling behavior of cereal starch is primarily a property of its amylopectin content, whereas amylose acts as both a diluent and an inhibitor to swelling [10]. Variations in the structural features including amylose/amylopectin ratio, granular size, degree of polymerization and chain length distribution of amylopectin, amylose-lipid complexes, and presence of noncarbohydrate content (such as lipids and proteins) of starch, result in differences in physicochemical properties of starch, such as swelling power, solubility, pasting and thermal properties and rheological properties of starch gels [11–13]. The physicochemical characteristics of flour and starch are of great importance because of their extensive utilization in the food and non-food industries. An understanding of the relationship between structural characteristics and physicochemical properties of rice flour and starch is very important for optimizing in starch-based food products and their applications. Several studies have done about antioxidant activity or anthocyanin profiles of pigmented rice. However, data about physicochemical and

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rheological properties of pigmented rice are rather scanty [14–16]. Thus, the main objective of this study was to determine the chemical compositions and physicochemical properties as well as rheological properties of flours and starches from various Thai pigmented rice cultivars having different amylose contents. 2. Materials and methods

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2.5. Scanning electron microscopy (SEM) Granular morphology of flour and starch samples from four Thai pigmented rice cultivars were obtained using a scanning electron microscopy (model JSM-6610LV, JEOL, Tokyo, Japan). The samples were mounted on aluminum stubs using double sided adhesive tape and afterwards were coated with a thin layer of gold at an accelerating voltage of 15 kV. The images were captured at a magnification of 2500.

2.1. Materials 2.6. Color measurement Four different pigmented rice cultivars were used in this study. Riceberry (RB) and Hom Nil (HN) were donated by Kasetsart University, Kamphaeng Saen Campus, Thailand. Niaw Dang (ND) and Kum Pleuak Khao (KP) were purchased from local market in Khon Kaen, Thailand. Dehulled rice grains were milled using a pin mill machine (Hosokawa Alpine, Augsburg, Germany) and passed through a 100-mesh sieve to obtain rice flour. The samples were kept at −18 °C before analyses. All reagents were of analytical grade and distilled water was used for preparation of all solution.

The color of rice grains, flour and starch samples was measured using a chroma meter (Minolta Chroma Meter, model CR 300 Series, Japan) as L*, a*, and b* values which was calibrated using a white standard porcelain plate (L* = 97.10, a* = −0.07, b* = +1.97). The L* value represented lightness, +a* value represented redness, −a* value represented greenness, +b* value represented yellowness, and –b* value represented blueness. Each sample was placed into a glass Petri-dish (6-cm diameter, 1-cm thickness). All analyses were run in triplicate.

2.2. Rice starch preparation 2.7. Amylopectin branch chain length distribution Rice starch was isolated using alkaline steeping method adapted from Yu et al. [17]. Rice flour sample (100 g) was soaked in sodium hydroxide (NaOH) solution (0.3% w/v, 400 mL). The slurry was stirred at room temperature (25 °C) for 18 h. Subsequently, the slurry was blended using a blender (Model MX-J210GN, Panasonic, Tokyo, Japan) for 2 min, filtrated through a 200-mesh sieve and centrifuged at 2465 ×g for 5 min. The supernatant was drained off and any yellow surface layer was manually scraped off from the starch. The underlying starch layer was reslurried with 0.3% NaOH and centrifuged at 2465 ×g for 5 min. This process was repeated until there was no yellow layer. The underlying starch layer was reslurried with distilled water, neutralized to pH 6.5 by adding 0.5 M hydrochloric acid (HCl) solution and centrifuged again. Afterward, the neutralized starch was washed using distilled water three times and the last time washed with 100% ethanol solution. The starch was dried at 45 °C overnight, ground and sieved using a 100-mesh sieve. The samples were kept at −18 °C before analyses. 2.3. Chemical analysis

The sample preparation before measurement of amylopectin branch chain length distribution was performed following the procedure of Kittisuban et al. [19]. Each rice starch samples (1% w/v, 900 μL) was mixed with 100 mM sodium acetate buffer (pH 5.0, 100 μL) and then heated for 20 min at 100 °C. The mixtures were incubated with pullulanase (0.72 U, Megazyme International Ireland Ltd., Wicklow, Ireland) and isoamylase (0.1 U, Megazyme International Ireland Ltd., Wicklow, Ireland) at 40 °C for 48 h to hydrolyze the α 1,6 linkages. The debranched samples were boiled for 10 min at 100 °C to inactivate the enzymes. Samples were filtered through a 0.45 μm nylon filter. The amylopectin debranched linear chain length distributions were determined by using a high-performance anion-exchange chromatography system equipped with a pulsed amperometric detector (HPAEC-PAD) (Dionex, Sunnyvale, CA, USA.) according to the method of Bertoft [20]. Chain length distribution with degree of polymerization (DP) was characterized as percentage of the total peak area. The weight-average degree of polymerization (DPw) of the sample was calculated using the equipment software.

Chemical analyses were performed for rice flour and starch samples. Moisture content, fat, protein, ash, and total dietary fiber were determined according to methods of the Association of Official Analytical Chemists [18]. Moisture content was determined by using a direct heating method at 105 °C to a constant weight. Crude fat was analyzed by the Soxhlet extraction method, using petroleum ether as the organic solvent. Protein content was measured as total nitrogen content by Kjeldahl method which a factor of 5.95 was used for conversion of nitrogen to crude protein. Ash content was determined by high temperature incineration in a furnace (600 °C). Total dietary fiber content was measured with the enzymatic-gravimetric method. Amylose content and total starch were determined using the “Amylose/Amylopectin Assay Kit” and “Total Starch Assay Kit”, respectively by Megazyme International Ireland Ltd. (Bray Business Park, Bray, Co. Wicklow, Ireland). The results were measured in triplicate and reported on a dry weight basis, except for the moisture content.

2.8. Swelling power and solubility

2.4. Particle size determination

Thermal properties of rice flour and starch samples from four Thai pigmented rice cultivars were analyzed by a differential scanning calorimeter (DSC 1, Mettler Toledo, Schwerzenbach, Switzerland). Rice flour and starch samples were suspended in distilled water (30% w/w, dry basis) with mild stirring for 15 min. The well stirred sample suspensions were exactly weighed (10–15 mg) into 40 μL aluminum pans and immediately hermetically sealed to prevent moisture loss. Scans were

The granule sizes of rice flour and starch were analyzed by a laser diffraction particle size analyzer (Mastersizer 2000, Malvern Instruments Ltd., Worcestershire, UK) using distilled water as a solvent. Particle size was reported as the median diameter (d(0.5)) which is the granule size at which 50% of all the granules by volume are smaller.

Swelling power and solubility of rice flour and starch samples from four Thai pigmented rice cultivars were determined by the modified method of Yu et al. [17]. Flour and starch samples were mixed with distilled water (1%, w/v) and heated in a water bath at 90 °C for 30 min with minimum shear condition. After heating, the samples were cooled to room temperature and centrifuged at 1615 ×g for 15 min. The supernatant was decanted into an aluminum cup and dried at 105 °C to constant weight and weighed. The wet sediment fraction was weighed for swelling power determination. The swelling power is the ratio of the wet weight of sediment to its dry weight, and the solubility is the percentage of dry mass of soluble molecules in supernatant to the dry mass of whole sample. 2.9. Differential scanning calorimetry (DSC)

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performed from 25 to 100 °C at a controlled constant heating rate of 10 °C/min. A sealed empty aluminum pan was used as a reference. The enthalpy change of gelatinization (ΔH) and transition temperatures, namely the onset temperature (To), peak temperature (Tp), and conclusion temperature (Tc) were determined. The ΔH was calculated, based on the area of the main endothermic peak, and expressed in terms of J/g of dry sample. 2.10. Determination of pasting properties The pasting properties of rice flour and starch samples were determined using a Rapid Visco-Analyzer (model RVA-4C, Newport Scientific Pty. Ltd., Warriewood, Australia). Rice flour and starch samples were suspended in distilled water (8% w/w, dry weight basis; a total weight of 28 g) with mild stirring for 15 min in an RVA canister. The slurries were stirred manually using a plastic paddle for 20–30 s before insertion into the RVA machine. A programmed heating and cooling cycle was used: held at 50 °C for 1 min, heated to 95 °C within 3 min 42 s, held at 95 °C for 2 min 30 s, cooled to 50 °C within 3 min 48 s and held at 50 °C for 2 min, while maintaining a rotation speed of 160 rpm. The viscosity was expressed in Rapid visco units (RVU). 2.11. Rheology measurement The freshly prepared gels of rice flour and starch samples obtained from the RVA machine were determined for dynamic viscoelastic properties by using a rheometer (Physica MCR 150, Anton Paar GmbH, Stuttgard, Germany) with a cone and plate geometry sensor (1° cone angle, 50 mm diameter, and 0.05 mm gap) which was equilibrated at 25 °C. For dynamic viscoelastic measurement, the linear viscoelastic range was determined with strain sweep (0.01–100%) at a fixed frequency of 10 rad/s. After that, a dynamic frequency sweep was conducted by applying a constant strain of 0.5% which was within the linear region, over a frequency range of 0.1–100 rad/s. The mechanical spectra were obtained recording the dynamic moduli G′, G″ and tan δ as a function of frequency. The G′ is the dynamic elastic or storage modulus, related to the material response as a solid. The G″ is the dynamic viscous or loss modulus, related to the material response as a fluid. The tan δ is the loss tangent defined as the ratio of G″ to G′. 2.12. Statistical analysis The data reported were expressed as mean ± standard deviation of triplicate measurement. A one-way analysis of variance (ANOVA) and Duncan's multiple range test were used to establish the significant differences (p ≤ 0.05) among the mean values. The correlation analysis among different parameters were determined with Pearson's correlation test. Statistical analyses were performed using SPSS Version 14.0 for Windows (SPSS Inc., Chicago, IL, USA.).

3. Results and discussion 3.1. Chemical compositions The chemical compositions of four pigmented rice flour samples and their isolated starches are presented in Table 1. Protein, fat, ash, and total dietary fiber contents of rice flours from four pigmented rice cultivars varied in the ranges of 8.52–9.68%, 3.61–4.35%, 1.34–1.68%, and 3.68–4.76% (w/w), respectively while their starch counterparts consisted of 0.67–0.79%, 0–0.1%, 0.03–0.14%, and 1.46–1.60%, respectively. Rice flours contained higher amount of protein, fat, ash, and total dietary fiber contents than their starch counterparts. These results indicated that the protein, fat, ash, and fiber were almost completely removed during alkaline steeping method. The total dietary fiber contents of rice flours are consistent with Sompong et al. [5] who reported that total dietary fiber contents of nine pigmented rice varieties from Thailand, China, and Sri Lanka ranged from 2.5 to 4.5% (w/w). The amylose content of rice flour samples was found to be lower than that of the respective rice starch samples, which is in good agreement with the previous studies [17,21,22]. It could be attributed to the removal of proteins and fats during starch extraction, which results in the elevation of the proportion of starch, and therefore amylose content in the samples [22]. However, the large difference between the amylose contents of the flour and starch of the RB cultivar (6.3% vs 12.1%) suggested that particle size of the samples might affect the determination of amylose content using the Amylose/Amylopectin Assay Kit in this study. The RB grains exhibited a more rigid structure as evidenced by a significantly (p ≤ 0.05) larger particle size (d(0.5)) than the other flour samples (Table 1). The large particle size of the RB flour might limit an extraction of amylose, which will be enzymatically hydrolyzed to D-glucose, leading to an underestimate of amylose content of the RB flour sample. Moreover, the assay kit procedure suggests relative standard deviations of b5% for starches and 10% for cereal flours. Therefore, the amylose content of flour samples exhibited more error than that of the starch samples. This could be due to the present of the other components such as proteins and lipids which could interfere the amylose assay. According to the classification of IRRI [23], rice starch can be classified based on its amylose content into waxy (0–2% amylose) and non-waxy: very low (2–9%), low (10–20%), intermediate (20–25%), and high (25–33%) amylose starches. In this study, RB (12.09%) was classified as low amylose starch whereas HN (8.14%), ND (2.77%), and KP (2.87%) were classified as very low amylose starch. Differences in amylose content of rice starches have been reported to be influenced by rice cultivars, growing zone, and environment [21,24]. The total starch analysis also showed that N95% total starch content was found for all rice starch samples and the remaining (~5%) might be impurity in the starch samples. These values (N95%) indicated

Table 1 Chemical compositions (%)a and median particle size [d(0.5)] of four Thai pigmented rice flour and starch samples.b Sample

Moisture

Crude protein

Fat

Ash

Total dietary fiber

Total starch

Amylose content

d(0.5)c (μm)

Flour

RB HN ND KP

9.98 ± 0.01a 8.50 ± 0.00c 9.08 ± 0.18b 10.18 ± 0.04a

8.65 ± 0.01b 9.68 ± 0.00a 8.52 ± 0.02c 8.62 ± 0.03b

4.35 ± 0.03a 3.71 ± 0.02c 4.02 ± 0.09b 3.61 ± 0.05d

1.68 ± 0.01a 1.34 ± 0.01d 1.45 ± 0.01c 1.64 ± 0.00b

4.49 ± 0.12b 4.58 ± 0.13ab 3.68 ± 0.07c 4.76 ± 0.04a

77.06 ± 1.58a 78.09 ± 1.23a 76.71 ± 1.95a 75.30 ± 0.40a

6.31 ± 0.35a 6.11 ± 0.37a 2.17 ± 0.13b 2.45 ± 0.18b

71.48 ± 0.60a 54.13 ± 0.86d 57.71 ± 0.44c 64.33 ± 1.87b

Starch

RB HN ND KP

9.54 ± 0.08d 12.39 ± 0.25a 11.82 ± 0.04b 11.30 ± 0.18c

0.67 ± 0.01b 0.79 ± 0.01a 0.67 ± 0.01b 0.69 ± 0.02b

n.d. 0.10 ± 0.00 n.d. n.d.

0.14 ± 0.01a 0.11 ± 0.01b 0.06 ± 0.01c 0.03 ± 0.00d

1.46 ± 0.04b 1.48 ± 0.01b 1.54 ± 0.06ab 1.60 ± 0.03a

96.59 ± 0.25a 95.21 ± 1.25a 95.94 ± 0.10a 96.09 ± 1.15a

12.09 ± 0.51a 8.14 ± 0.03b 2.77 ± 0.06c 2.87 ± 0.22c

4.33 ± 0.04d 4.94 ± 0.05b 4.85 ± 0.03c 5.57 ± 0.46a

RB: Riceberry; HN: Hom Nil; ND: Niaw Dang; KP: Kum Pleuak Khao; n.d: not detected. a Dry basis except for moisture content. b Mean ± standard deviation values of flour and starch samples in each column with the different letters are significantly different (p ≤ 0.05). c d(0.5) is the granule size at which 50% of all the granules by volume are smaller.

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the successful isolation of pure starch by this alkaline steeping method [25]. 3.2. Morphological characteristics and particle size The scanning electron micrographs of rice flour and starch samples from four Thai pigmented rice varieties are shown in Fig. 1. For the rice flour samples, the starch granules were varied in size and shape and formed a clump in the cell wall matrices with scatter of small spherical aleurone grains as shown in Fig. 1A–D. These results are in good agreement with those reported by Reddy et al. [26] who observed that the starch granules of the pigmented rice flours were surrounded by non-starch components, such as protein and fat bodies. It could be

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seen that after starch extraction process, the starch granules were released from the cell wall matrices and the coated proteins and lipids leading to the morphological change of the granules from closely spherical to polygonal [26] and some starch granules were clustered as shown in Fig. 1E–H. These results suggested that more than one granules are produced simultaneously in a single amyloplast and the compound starch granules are tightly packed together and difficult to separate [27]. The median diameters (d(0.5)) of all rice flour samples were in the range of 54.13–71.48 μm (Table 1). The d(0.5) values of all rice starch samples (4.33–5.57 μm) were much smaller than those of rice flour samples because the rice starch granules were individually separated from other components during rice starch extraction process. The granular size and shape of rice starch samples from four Thai

Fig. 1. SEM micrographs of flour (A–D) and starch (E–H) samples from four Thai pigmented rice cultivars. RB: Riceberry; HN: Hom Nil; ND: Niaw Dang; KP: Kum Pleuak Khao; AG: aleurone grain; SG: starch granule.

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pigmented rice varieties observed in this study are in agreement with previous studies [22,24] reporting that rice starch granules sizes were in the range of 2–7 μm. The size of starch granules varies depended on rice cultivars, growing conditions as well as plant physiology [24].

3.3. Color measurement Photographs and color values of rice grains, flours and starch samples of four Thai pigmented rice cultivars are shown in Fig. 2 and Table 2, respectively. The RB rice grains and flour had the significantly (p ≤ 0.05) lowest L* values, indicating the darkest samples among the rice varieties studied. It might be due to the presence of anthocyanin pigments on the outer layer of the kernels that existed even after grinding. On the other hand, the ND rice grains and flour had the significantly (p ≤ 0.05) highest L*, a*, and b* values. The +a* values of ND rice grains and flour were the highest due to reddish brown color of external layers of the kernels. The obvious differences in rice grain and flour colors are due to the accumulation of anthocyanins and proanthocyanidins which produce black, purple, and red colors [4]. There are many studies reported that the pigmented rice as a potent source of antioxidants and have the ability to reduce and prevent the risk of chronic diseases [1,3]. The color values of all rice flours showed the similar trend as their corresponding rice grains. However, the L* value of rice flour samples was greater than that of the respective rice grains because the endosperm in rice grains, which is mostly white in color, was exposed by dry-milling process. These rice flour samples had more lightness than rice grains. The L* values of rice starch samples from four Thai pigmented rice cultivars ranged from 97.2 to 98.4. The compositions of rice grains mostly consisted of starch (over 90%) which is naturally white in color. The L* value which is higher than 90 gives a satisfactory whiteness for purity of starch [28]. Moreover, the rice starch samples had less a* and b* values than the corresponding rice flour samples. These results suggested that anthocyanins, the major pigments of pigmented rice and water soluble compounds, can be dissolved in water and removed during rice starch isolation process. The color of rice starch samples from four Thai pigmented rice cultivars could not be visibly differentiated even if the statistical analysis results presented a significant difference.

3.4. Amylopectin branch chain length distribution Normalized chromatograms of branch chain length distribution of amylopectin isolated from rice starch samples of four Thai pigmented rice cultivars are represented in Fig. 3 and the computed results of peak area ratio of each chain type are shown in Table 3. The average amylopectin branch chain length of the RB, HN, ND, and KP starches were 17.1, 17.2, 17.3, and 17.7 anhydroglucose unit (AGU), respectively. Amylopectin branch chain length distributions were classified into different ranges of degree of polymerization (DP) from short to long chain as follows: short A chains (DP 6–12), B1 chains (DP 13–24), B2 chains (DP 25–36), and B3+ chains (DP ≥ 37). Fig. 3 shows that the chromatograms of RB, HN, ND, and KP starches were A-type starches. The A-type starches had higher proportions (34.4–44.6%) of short A chains (DP 6–12) and lower proportions (5.6–10.5%) of B3+ long chains (DP ≥ 37) [12]. The KP had the highest proportion of short A chains but lower in B1 chains, whereas the HN had a lowest proportion of short A chains. These results are in agreement with previous data that rice starch containing low amylose content had a higher proportion of short A chains [11,13,19]. 3.5. Swelling power Swelling power, the ability of starch associate with water via hydrogen bonding under specific conditions such as temperature and water availability, of rice flour and starch samples from four Thai pigmented rice cultivars are shown in Table 4. The ND and KP had higher swelling power than the RB and HN for both flour and starch samples. Tester and Morrison [10] proposed that swelling is a property of amylopectin because the weakness of the intra- and inter- molecular coherence in starch, whereas amylose act as an inhibitor of swelling. This coincides with our results since the ND and KP with a low amylose content swelled greater than the RB and HN with a high amylose content. When compare among the starch samples the results showed that the RB starch had the lowest swelling power. This could be due to the RB starch had a small proportion of short A chains but high proportion of long B3+ chains (Table 3). The short branch chains of amylopectin would contributed to a more disordered packing of double helices, resulting in an easier swelling. On the other hand, the presence of the

Fig. 2. The rice grains, flour and starch samples of four Thai pigmented rice cultivars. RB: Riceberry; HN: Hom Nil; ND: Niaw Dang; KP: Kum Pleuak Khao.

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Table 2 The color (as L*, a*, and b*) of rice grains, flour and starch samples of four Thai pigmented rice cultivars.a Sample

RB HN ND KP

Grain

Flour

Starch

L*

a*

b*

L*

a*

b*

L*

a*

b*

27.1 ± 1.1c 35.9 ± 1.3b 43.6 ± 1.4a 33.5 ± 2.3b

3.4 ± 0.4c 7.9 ± 0.5b 17.2 ± 0.8a 8.1 ± 0.7b

1.1 ± 0.4d 8.9 ± 0.8b 23.1 ± 0.5a 6.7 ± 0.7c

69.7 ± 0.3c 79.2 ± 0.4a 79.6 ± 0.1a 74.7 ± 0.3b

2.7 ± 0.0b 2.3 ± 0.1c 5.9 ± 0.1a 2.7 ± 0.0b

3.3 ± 0.1d 5.9 ± 0.0b 10.5 ± 0.0a 3.4 ± 0.1c

97.9 ± 0.0c 97.3 ± 0.1b 97.2 ± 0.0b 98.4 ± 0.1a

0.4 ± 0.0b 0.4 ± 0.0b 0.5 ± 0.0a 0.2 ± 0.0c

1.3 ± 0.0c 1.5 ± 0.0b 2.1 ± 0.0a 1.5 ± 0.0b

RB: Riceberry; HN: Hom Nil; ND: Niaw Dang; KP: Kum Pleuak Khao. a Mean ± standard deviation values in each column with the different letters are significantly different (p ≤ 0.05).

long branch chains of amylopectin increases the stability of crystalline structure then suppresses the swelling of starch granule [11]. Thus, a larger amount of amylose and longer amylopectin branch chain length of the RB could be attributed to a reduction of swelling power. A significant correlation between swelling power and amylose content is shown in Table 5. The results showed that the amylose content is negatively correlated to swelling power for both flour and starch samples (r = −0.949, and –0.971, p ≤ 0.01, respectively). However, no significant correlation was found between swelling power and amylopectin branch chain length distribution. It could be due to a less of structural variations among rice samples. A similar result was also reported in the previous studies investigating rice starch with different amylose contents as well as rice cultivars [11,17,21,29]. In addition, the swelling power of rice flour samples was lower than that of their derived starch samples. The swelling power of rice flours varied from 13.5 to 19.2 g/g whereas rice starches was in the ranged of 31.0–55.6 g/g. The previous studies reported that the swelling power of flours depends on proteins and lipids or channel in rice flour granules [17]. The packing of starch granules with proteins will form a stiff matrix then tend to encourage a strong hydrogen bonding from charged side chain which retard accessibility of water into the starch granules [30]. Moreover, lipids can form complex with amylose which reduce charged molecules and then inhibit swelling [31].

3.6. Solubility Solubility is the ability of solids to dissolve or disperse in an aqueous solution (mostly water) and related to the presence of soluble molecules like amylose [10]. The solubility of rice flours and starches are presented in Table 4. The results showed that the solubility of flour samples from the RB and HN was significantly (p ≤ 0.05) lower than those from the ND and KP. This could be due to a significantly (p ≤ 0.05) higher amylose content of the RB and HN which can form complex with lipid in the flour samples. The amylose-lipid complexes are insoluble and require high temperature to dissociate (N 95 °C) [11]. Thus, the formation of amylose-lipid complexes within starch granules might retard the amylose leaching [10]. In contrast, the solubility of starch samples from the RB and HN was significantly (p ≤ 0.05) greater than those from the ND and KP. These results are in agreement with those previously reported on the solubility of starches isolated from 14 rice cultivars by Kong et al. [29] who found that high-amylose rice starches presented higher water solubility than waxy or low-amylose starches. It could be attributed to the higher amylose content of the RB and HN which resulted in amylose being leached to a larger extent during heating. In addition, the removal of lipids during the production of starch would contribute to the high amylose leaching. Therefore, the solubility of rice starch is influenced by amylose leaching. These results indicated that solubility of

Fig. 3. Amylopectin branch chain length distributions of four Thai pigmented rice starch samples by HPAEC-PAD. RB: Riceberry; HN: Hom Nil; ND: Niaw Dang; KP: Kum Pleuak Khao.

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Table 3 Average chain length and amylopectin branch chain length distributions of four Thai pigmented rice starch samples.a Sample

Amylopectin branch chain length distribution (%)

RB HN ND KP

Average chain length (AGU)

DP 6-12

DP 13-24

DP 25-36

DP ≥ 37

39.0 ± 2.6b 34.4 ± 0.5d 37.9 ± 2.6 cd 44.6 ± 0.0a

46.6 ± 3.5a 50.9 ± 0.2a 46.7 ± 3.5a 35.5 ± 1.2b

6.6 ± 0.3b 9.1 ± 1.2a 8.2 ± 0.6a 9.4 ± 0.1a

7.8 ± 0.7b 5.6 ± 0.7c 7.3 ± 1.3bc 10.5 ± 1.1a

17.1 ± 0.0b 17.2 ± 0.0b 17.3 ± 0.3b 17.7 ± 0.4a

RB: Riceberry; HN: Hom Nil; ND: Niaw Dang; KP: Kum Pleuak Khao; DP: degree of polymerization; AGU: anhydroglucose unit. a Mean ± standard deviation values in each column with the different letters are significantly different (p ≤ 0.05).

rice starch samples was positively correlated with amylose content (r = 0.958, p ≤ 0.01) but negatively correlated in the case of flour samples (r = −0.907, p ≤ 0.01) (Table 5). However, there is no significant correlation between solubility of the starch samples and amylopectin branch chain length distribution except the B2 chains (DP 13-24) (r = 0.605, p ≤ 0.05). Furthermore, the solubility of rice starch samples was lower than that of rice flour samples. These results are in agreement with those reported in previous studies for rice cultivars [17,21]. It might be due to the compositions of rice flour which are composed of more soluble molecules such as protein, soluble fiber, and also anthocyanins (water soluble pigments), thus the solubility values of rice flour were increased. 3.7. Differential scanning calorimetry The thermal properties of rice flour and starch samples from four Thai pigmented rice cultivars are showed in Table 4. The gelatinization temperatures (onset, To; peak, Tp; and conclusion, Tc) and enthalpy change of gelatinization (ΔH) were significantly different among rice cultivars. The result showed that the HN had the highest gelatinization temperatures for both flour and starch samples. Since the HN contained high amylose content and also the smallest proportion of amylopectin short A chains among the rice starch samples. On the other hand, the KP which contains the largest proportion of amylopectin short A chains, displayed the lowest gelatinization temperatures. These results are consistent with Chung et al. [11] who reported that rice starch containing the highest amylose content and the smallest proportion of DP 6-12 amylopectin showed higher gelatinization temperatures. Gidley and Bulpin [32] postulated that the presence of short chains (DP b 10) in amylopectin decrease the stability of double helix, which could reduce the gelatinization temperatures. The shorter chains require a lower temperature to dissociate completely than that required for the longer double helices. It was found in this study (Table 5) that To and Tp of the flour samples correlated positively (r = 0.980 and 0.993, p ≤ 0.01, respectively) with the amylose content, while the starch samples showed a negative correlation (r = −0.815 and –0.737, p ≤ 0.01, respectively) with short branch-chains (DP 6-12) amylopectin.

Moreover, the ΔH of ND and KP were higher than those of the RB and HN for both flour and starch samples. It may have been because the ND and KP contained lower amylose content than the RB and HN. These results are in agreement with those observed for maize, potato, barley [33], and rice starch varieties [11] in which the ΔH of waxy starches were higher than those of non-waxy starches. Jane et al. [12] suggested that the lower amylose or waxy starches are known to have greater gelatinization enthalpy since these starches mainly consist of amylopectin which constitutes more crystalline (double helix) and less amorphous regions. Thus, they have a more compact physical structure and require more energy to breakdown intermolecular bonds in the starch granules. The negative correlation was found between ΔH and amylose content for both flour and starch samples (r = −0.974 and –0.842, p ≤ 0.01, respectively) (Table 5). On the other hand, ΔH did not have a significant correlation with the amylopectin branch chain length distribution in this study. The differences in these parameters may have been due to the differences in amylose/amylopectin content, granular architecture, and molecular structure of the crystalline regions which related with amylopectin branch chain lengths and distribution [32]. Furthermore, the thermal properties of rice flour were different from its isolated starch counterpart. The gelatinization temperatures of starches were significantly (p ≤ 0.05) lower, while ΔH was higher than those of the flour counterparts. This could be attributed to the presence of non-starch components in the rice flours such as protein and lipid affected on the heat transfer hindrance for starch gelatinization [34]. In addition, cell wall material in the rice flours could also act as a barrier to retard water movement to the starch granules [35]. 3.8. Pasting properties All pasting parameters of the rice flours and starches are summarized in Table 4. The lowest (p ≤ 0.05) peak viscosity (PV) was observed for the RB for both flour and starch samples. It meant that the starch granules of RB cultivar have a low ability to bind with water via hydrogen bonding since the RB contained the highest amylose content. For all starch and flour samples tested, it was found that final viscosity (FV), setback (SB), and pasting temperature (PT) increased, whereas

Table 4 Swelling power, solubility, thermal and pasting properties, of four Thai pigmented rice flour and starch samples.a Sample

SP (g/g)

S (%)

Thermal properties

Pasting properties

To (°C)

Tp (°C)

Tc (°C)

ΔH (J/g)

PT (°C)

PV (RVU)

BD (RVU)

FV (RVU)

SB (RVU)

Flour

RB HN ND KP

13.5 ± 0.5d 14.5 ± 0.2c 18.0 ± 0.2b 19.2 ± 0.3a

12.8 ± 0.5c 11.3 ± 0.2d 18.1 ± 0.2b 22.1 ± 0.3a

62.9 ± 0.1b 68.0 ± 0.0a 62.5 ± 0.2c 61.7 ± 0.1d

70.1 ± 0.2b 74.9 ± 0.1a 69.3 ± 0.2c 69.4 ± 0.3c

76.9 ± 0.4c 81.6 ± 0.1a 76.0 ± 0.1d 78.2 ± 0.7b

8.9 ± 0.1b 9.0 ± 0.1b 10.1 ± 0.1a 9.9 ± 0.2a

88.8 ± 1.4a 85.8 ± 2.0b 69.5 ± 0.9c 69.4 ± 0.0c

88.2 ± 0.7d 110.4 ± 0.8b 123.5 ± 2.5a 101.8 ± 1.5c

26.0 ± 1.5c 39.8 ± 4.7b 49.8 ± 0.8a 47.3 ± 1.6a

142.2 ± 0.6a 138.9 ± 5.4a 92.2 ± 2.6b 68.3 ± 1.7c

80.2 ± 1.3a 68.2 ± 1.4b 18.6 ± 0.2c 13.8 ± 0.2d

Starch

RB HN ND KP

31.0 ± 0.8d 37.9 ± 0.8c 55.6 ± 0.2a 50.1 ± 0.2b

7.8 ± 0.1a 7.4 ± 0.3b 4.7 ± 0.1c 4.7 ± 0.3c

60.0 ± 0.9b 63.8 ± 0.4a 59.5 ± 0.2b 58.0 ± 0.1c

67.6 ± 0.2b 72.2 ± 0.3a 66.3 ± 0.0c 65.8 ± 0.1d

74.8 ± 0.3c 79.5 ± 0.3a 75.3 ± 0.3c 76.6 ± 0.2b

13.2 ± 0.3c 14.4 ± 0.2b 15.4 ± 0.7a 14.6 ± 0.2b

73.6 ± 2.1b 75.7 ± 0.6a 67.2 ± 0.1c 67.2 ± 0.1c

190.2 ± 3.6c 214.5 ± 1.2b 211.5 ± 3.7b 226.8 ± 2.0a

8.4 ± 3.1c 37.7 ± 2.8b 121.3 ± 0.5a 117.8 ± 3.5a

224.8 ± 0.9a 219.6 ± 0.6b 114.3 ± 3.1d 125.8 ± 1.9c

39.6 ± 3.7a 42.8 ± 1.1a 24.0 ± 1.0b 16.8 ± 1.3c

RB: Riceberry; HN: Hom Nil; ND: Niaw Dang; KP: Kum Pleuak Khao; SP: swelling power; S: solubility; To: onset temperature; Tp: peak temperature; Tc: conclusion temperature; ΔH: gelatinization enthalpy; PT: pasting temperature; PV: peak viscosity; BD: breakdown; FV: final viscosity; SB: setback. a Mean ± standard deviation values of flour and starch samples in each column with the different letters are significantly different (p ≤ 0.05).

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673

Table 5 Pearson correlation coefficients between composition and physicochemical properties of rice flour and starch samples. All rice flour samples

SP S To Tp Tc ΔH PT PV BD FV SB G′max G″max tanδG′max

All rice starch samples

Protein

Fat

AC

Size

Protein

Fat

AC

Size

CL

DP6-12

DP13-24

DP25-36

DP≥37

-0.482 -0.658* 0.980** 0.993** 0.939** -0.572 0.534 0.101 -0.134 0.558 0.510 0.227 -0.049 -0.578

-0.573 -0.424 -0.251 -0.319 -0.591* -0.323 0.457 -0.377 -0.656* 0.519 0.510 0.681 0.843** -0.300

-0.949** -0.907** 0.640* 0.642* 0.485 -0.974** 0.989** -0.564 -0.859** 0.944** 0.981** 0.905** 0.721* -0.951**

-0.202 0.101 -0.616* -0.589* -0.513 -0.271 0.233 -0.835** -0.621* 0.071 0.243 0.519 0.584 -0.259

-0.287 0.408 0.834** 0.900** 0.950** -0.010 0.646* 0.279 -0.306 0.469 0.515 0.011 0.081 -0.288

-0.340 0.491 0.922** 0.966** 0.929** -0.017 0.709* 0.139 -0.392 0.546 0.639* 0.130 0.174 -0.441

-0.971** 0.958** 0.461 0.472 -0.014 -0.842** 0.810* -0.801** -0.984** 0.942** 0.854** 0.986** 0.979** -0.776*

0.626* -0.691* -0.308 -0.222 0.355 0.513 -0.535 0.966** 0.727** -0.623* -0.700* -0.831* -0.731* 0.661

0.421 -0.535 -0.390 -0.339 0.049 0.198 -0.473 0.578* 0.519 -0.487 -0.575 -0.887** -0.858** 0.763*

0.319 -0.542 -0.815** -0.737** -0.397 -0.004 -0.593* 0.387 0.460 -0.509 -0.737** -0.267 -0.247 0.676

-0.397 0.605* 0.762** 0.681* 0.273 -0.082 0.619* -0.524 -0.543 0.565 0.776** 0.423 0.386 -0.750**

0.462 -0.420 0.110 0.150 0.580* 0.487 -0.237 0.836** 0.520 -0.382 -0.363 -0.660 -0.556 0.255

0.310 -0.533 -0.801** -0.728** -0.409 -0.035 -0.587* 0.335 0.443 -0.498 -0.706* -0.293 -0.296 0.687

*, ** Correlations are significant at p ≤ 0.05, 0.01, respectively AC: amylose content; CL: average chain length; DP: degree of polymerization; SP: swelling power; S: solubility; To: onset temperature; Tp: peak temperature; Tc: conclusion temperature; ΔH: gelatinization enthalpy; PT: pasting temperature; PV: peak viscosity; BD: breakdown; FV: final viscosity; SB: setback; G′max: the maximum storage modulus; G″max: the maximum loss modulus; tanδG′max: the maximum loss factor

breakdown (BD) values decreased with increasing amylose content. As expected, the amylose content had a significant positive correlation with PT (r = 0.810, p ≤ 0.05), FV (r = 0.942, p ≤ 0.01), and SB (r = 0.854, p ≤ 0.01), while a negative correlation was found with PV (r = −0.801, p ≤ 0.01) and BD (r = −0.984, p ≤ 0.01) for starch samples (Table 5). Similar relationships were also found between amylose content and pasting properties of flour samples except PV which no significant correlation was observed (Table 5). It could be due to the components of rice flour such as lipid or protein which also effect the pasting properties. The pasting behaviors of starch gels have been reported to be affected by amylopectin, amylose, and lipids [10,12]. Amylopectin mainly responsible for swelling of starch granules and pasting, while amylose and lipids inhibit the swelling by maintaining the integrity of the swollen starch granules [10]. The waxy or low amylose rice starch mainly composed of amylopectin with an absence of amyloselipid complexes, thus starch granule can swell easily, providing lower PT and higher PV. During cooling of the starch paste, a more solubilized starch particularly amylose released can reassociate rapidly. The amylose junction zones are formed and viscosity increases again (FV) which reflects gel network formation involving amylose [12,29]. A comparison between the very low amylose ND and KP starches showed that the KP starch exhibited significantly (p ≤ 0.05) higher PV than the ND starch. Zhu [36] stated that starch consisting of amylopectin with more short B-chains displays a lower PV, while those with more long B-chains (B3+) tend to have a higher viscosity. Since the short B-chains tend to give more unparalleled packing of double helices so they tend to have less structural integrity of the swollen granules. In the present study, the KP starch, consisting of a lower proportion of short Bchains and higher proportion of B3+ chains, showed a higher PV. In addition, the starch with high proportion of B3+ chains tend to have a higher FV. The shape of longer amylopectin molecules probably was rather closer to amylose molecules which easily reassociate and contribute a network formation, leading to an increase in FV values [37]. However, the correlation between amylopectin branch chain lengths and pasting parameters was not observed obviously. BD viscosity measured the susceptibility of starch paste to thermal and mechanical shear, while SB viscosity presented the tendency of starch paste to retrograde. The RB and HN exhibited significantly lower BD but higher SB viscosities (p ≤ 0.05) than the ND and KP for both flour and starch samples, indicating that both rice cultivars might be able to withstand more heating and shear stress during cooking and might have a greater tendency to retrograde during cooling than

the ND and KP. These results are in agreement with previous studies [5,11,13,22,33]. A comparison between the pasting properties of flours and starches showed that PV and FV of starch samples were significantly higher, while PT was significantly lower (p ≤ 0.05) than those of the flour samples. These results are similar with the previous researches which investigated pasting properties between flour and starch from different rice cultivars [17,22]. The BD and SB values of the RB and HN starch samples were lower than their rice flour counterparts, whereas the opposite trend was found in the case of the ND and KP cultivars. It may have been because amylose-lipid or amylose-protein complex formations in rice flour samples can increase SB value [38]. Thus, the RB and HN flours, containing higher amylose content and also lipid and protein, displayed higher SB and BD values than their starch counterparts. The pasting properties of rice flour compared to its isolated starch counterpart were different due to the presence of other components in rice flour such as lipid, protein, and fiber as well as the lower amount of starch in flour. 3.9. Rheological measurement Fig. 4 illustrates the dynamic mechanical properties of rice flour and starch gels from four Thai pigmented rice cultivars. These rheograms presented that the storage modulus (G′) was greater than the loss modulus (G″), both moduli showed a slight increase with frequency (ω) (Fig. 4A and B, respectively), and a crossover between these two moduli was not observed throughout the tested frequency range for all flour and starch samples. Therefore, these gels can be classified rheologically as a typical weak gel [39]. Moreover, the dynamic mechanical loss tangent (tan δ), which is the ratio of G″ to G′, of both flour and starch gels tested was much smaller than unity. It meant that both flour and starch gels exhibited as elastic or solid-like behavior. A comparison among rice cultivars showed that G′ and G″ of all gels increased, whereas tan δ decreased with increasing amylose content. The RB gels of both flour and starch samples, which contained the highest amylose content, had higher G′ and G″ and lower tan δ values than the other gels. Thus, we can conclude that the RB gels were more structured and more elastic followed by the HN and ND ≈ KP gels. These results are in agreement with the previous studies, reporting that high amylose rice starch gels showed higher moduli and lower tan δ values [40,41]. Positive correlations were found between G′max and G″max (the maximum values of G′ and G″, respectively) of flour and starch gels and their amylose content, whereas tan δG′max (tan δ at G′max) exhibited a negatively correlation with amylose content

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and G″ values to be greater than those of starch pastes. However, the RB flour gel had lower G′ and higher tan δ value than its starch counterpart gel. The RB had the highest amylose content among the rice varieties studied, resulting in a softer gel of the RB flour sample. Higher amount of amylose-lipid complexes in rice flour formed during gelatinization might lower the G′ value [44]. On the other hand, no or less interaction between lipid or protein with amylose in the RB starch would cause the RB starch gel to become more rigid by the interaction between amylose molecules. The G′, G″, and tan δ values of the HN gels from both flour and starch samples were not that much different. 4. Conclusions The pigmented rice varieties have gained more interest because of their nutritional advantages, i.e. dietary fiber, vitamins, and minerals as well as they are a good source of antioxidant. For the application of pigmented rice in food products, the physicochemical and functional properties are also important. The physicochemical and rheological properties of rice flour and starch from four Thai pigmented rice cultivars with varying amylose contents were different. The ND and KP having low amylose content and high amylopectin showed a significantly higher proportion of short branch chains of amylopectin, swelling power, peak viscosity, breakdown, ΔH, and tan δ values but lower final viscosity, setback, pasting temperature, To, Tp, G′ and G″ than the RB and HN with higher amylose content for both flours and starches. Therefore, not only amylose content but also chain length distribution of amylopectin play an important role in physicochemical and rheological properties of rice starch and rice flour. Furthermore, the physicochemical and rheological properties of rice flours and their isolated rice starch counterparts were significantly different due to the presence of non-starch components in rice flour, e.g. protein, lipid, and cell wall materials. This study provided useful data for the utilization of flour and starch from Thai pigmented rice cultivars, especially for food applications.

Acknowledgements This research was financially supported from the Thailand Research Fund and Mahidol University through the Royal Golden Jubilee Ph.D. Program (Grant no. PHD/0124/2554).The authors thank Metrohm Siam Ltd., Bangkok, Thailand, for providing the rheometer used in rheology experiment. References Fig. 4. Frequency dependence of storage modulus, G′ (A), loss modulus, G″ (B) and loss tangent, tan δ (C) of 8% (w/w) fresh gels prepared from rice flour (closed symbol) and rice starch (open symbol), measured at 0.5% strain and 25 °C. RB: Riceberry; HN: Hom Nil; ND: Niaw Dang; KP: Kum Pleuak Khao.

(Table 5). These results could be attributed to the weaker and more highly swollen granules of low amylose or waxy starch which produced a less rigid paste as compared to normal rice starch granules in which the granule structure was highly strengthened and more rigid. Amylose content was reported to be the important factor which affects the rheological properties of starch [41]. However, there are other factors which influence the rheological properties of starch gels such as branch chain length distribution of amylopectin and starch varieties [42,43]. In addition, different rheological properties were observed between the starch and flour samples of each rice cultivars. The ND and KP flour gels displayed higher G′ and G″ and lower tan δ values than their isolated starch gels, indicating that these flour gels had stronger gel structure than their isolated starch counterpart gels. Kong et al. [40] had reported that the presence of protein in rice flour pastes induced G′

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