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Thermal and rheological properties of brown flour from Indica rice
Journal of Cereal Science 70 (2016) 270e274
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Thermal and rheological properties of brown flour from Indica rice Lingxu Ye a, Changsheng Wang b, Shujun Wang c, Sumei Zhou a, **, Xingxun Liu a, * a
Institute of Food Science and Technology (IFST), Chinese Academy of Agricultural Science (CAAS), Beijing 100193, China Department of Chemical Engineering, CREPEC, Polytechnique Montr eal, Station Centre-Ville, Montr eal, QC, Canada c Key Laboratory of Food Nutrition and Safety, Ministry of Education, Tianjin University of Science & Technology, Tianjin 300457, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 April 2016 Received in revised form 6 July 2016 Accepted 8 July 2016 Available online 11 July 2016
The thermal, paste and rheological properties of brown flours from four Indica rice subspecies with different amylose content were examined using Differential scanning calorimetry (DSC), Brabender Viscometer and rheometer. Peak, final and setback viscosities (p < 0.05) increased with increasing amylose content from Brabender micro Visco-Amylo-Graph (MVA), but the phase transition temperatures of brown rice flour from DSC (p < 0.05) decreased with increasing amylose content. Rheological properties were determined by steady shear, small amplitude oscillatory shear (SAOS) and thixotropic experiments. SAOS results showed a gel-like viscoelastic behavior with G0 higher than G00 . Steady-shear results showed that the brown rice flour exhibited a non-Newtonian shear-thinning behavior and the flow curves can be well described by the Herschel-Bulkley model. The upward-downward rheograms showed that brown rice flour gel, except IR-1, had a hysteresis loop, indicating a strong thixotropic behavior. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Brown rice flour Rheology properties Pasting properties Thermal properties
1. Introduction Rice is an excellent source of energy for about half world’s human and it is particularly important in Asia’s food consumption. Brown rice, produced by dehulling the grain, is considered as a whole grain food and its color may be light brown, green, reddish or black (Gunaratne et al., 2013). White rice, also known as milled rice or polished rice, is currently the most common form consumed. Compared with white rice, brown rice flour has an amount of fiber (non-starch polysaccharides, NPS) and bioactive molecules such as polyphenol, which can reduce the risk of chronic diseases including hypercholesterolemia, cardiovascular disease, and type II diabetes (Frolich et al., 2013). Recently, brown rice has drawn increasing attention due to its additional health benefits. The thermal and rheological properties are important to understand brown rice processing and digestion in human. For example, flow behavior, thixotropy and dynamical viscoelastic properties are most important properties in rheology, which are related to production processing and application. In detail, small oscillatory frequency sweep is extremely sensitive to the structure
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Zhou), [email protected] (X. Liu). http://dx.doi.org/10.1016/j.jcs.2016.07.007 0733-5210/© 2016 Elsevier Ltd. All rights reserved.
information of samples, which makes it useful to evaluate gelation behavior and kinetics (Liu et al., 2015), and it can provide some additional information such as structure and energy consumption. The rheological properties are also important to the analysis of flow conditions in product processes and prediction of product stability (Li et al., 2014). Rheological properties also correlate to the textural attributes, which in turn determine the sensory characteristics and consumer acceptability of the products (Li et al., 2014). The thermal, pasting and rheological properties of brown rice n ~ ez et al., 2007; Varavinit et al., 2003; flour have been reported (Iba Wu et al., 2013; Zhu et al., 2010). Wu et al. (2013) showed that the gelatinization temperature exhibited no significant differences between the brown rice flour and isolated starch. However, the pasting viscosity of starch is several times higher than that of n ~ ez et al. (2007) have studied the brown flour (Zhu et al., 2010). Iba viscoelastic properties of waxy and non-waxy rice flours. After removal of protein and lipids with aqueous alkaline or detergent solutions, pasting temperatures decreased, but the viscosities of both starches increased. The rheological properties of rice flour are also shown to be affected by damaged starch content and particles size (Asmeda et al., 2016) as well as apparent amylose content (Kong et al., 2015a). Rheological properties of rice flour are of high importance to design new products with desired sensory and textural attributes. However, the comprehensive and systematic study on the
L. Ye et al. / Journal of Cereal Science 70 (2016) 270e274
rheological properties of brown rice flour is still limited. In this work, Indica, a main rice subspecies cultivated in South China, was used to investigate the rheological properties of gels produced from brown rice flour using steady and oscillatory shear experiments. This work provides an important reference to the brown rice application. 2. Material and methods 2.1. Raw materials Four Indica rice (IR) cultivars from China main rice production area (such as Guangdong, Hunan province) were used in this study (Table 1). All the brown rice was harvested in the year of 2014 and stored at 4 C. Beihan1# was kindly provided by Shandong Academy of Agricultural Science, while Jinnongsimiao and Yuxiangyouzhan were kindly provided by Guangdong Academy of Agricultural Science, China. Xing2# was supplied by Hunan Jinjian Seed Industry Science & Technology Co., Ltd. 2.2. Sample preparation Before all tests, the raw rice grains were dehulled to produce brown rice (with bran layers and germ still intact). In this study, rice grains were de-husked (BLH-3250, Zhejiang Bethlehem, China) and grounded to pass through a 100-mesh sieve on a Cyclone Sample Mill (CT410, Foss Scino, China) for further use. 2.3. Chemical analyses Protein content (N content x5.95) was determined by combustion (AACC method 46-11A) using Automatic Kjeldahl Nitrogen Analyser (Kjeltes2300, FOSS, Inc, USA). Total starch content was measured by Megazyme assay kit (K-TSTA 09/14) from Megazyme International Ireland Ltd. (Co. Wicklow, Ireland) following the manufacturer’s instructions. Amylose content of starches was analyzed in triplicate with an amylose/amylopectin assay kit (Megazyme International Ireland Ltd., Bray, Co. Wicklow, Ireland). Total fat was measured using a Soxhelt extractor, with petroleum ether as the extraction solvent (AACC method 08-01). 2.4. Pasting properties of brown rice flour The pasting properties were monitored using a Brabender Micro Visco-Amylo-Graph (MVA) (Brabender OHG, Duisburg, Germany) according to the classic method (Mariotti et al., 2005, 2009). 12 g of the sample was dispersed in 100 mL of distilled water, scaling both flour and water weight on 14% flour moisture basis. The suspensions were subjected (stirring at 250 RPM/min and using a 300 cm$gf cartridge) to the following standard temperature profile: heating from 30 C up to 95 C at a rate of 7.5 C/min, holding at 95 C for 5 min, cooling from 95 C to 50 C at a rate of 7.5 C/min and holding at 50 C. The pasting temperature range was obtained as the range between the temperature at the start of increase of viscosity and that at which it remains constant. The peak, breakdown, and setback viscosity were also recorded.
271
2.5. Differential scanning calorimetry Thermal transition measurements of brown rice flour were determined using a Modulated Differential Scanning Calorimeter MDSC 2920 instrument (TA Instruments Inc., Delaware, USA) equipped with a thermal analysis data station and data recording software. The sample was prepared according to the procedure of Wang et al. (Wang et al., 2014) with a slight modification. Briefly, about 3 mg of samples was weighed into 40 mL aluminum pans. Distilled water was added with a pipette to obtain a water/sample ratio of 3:1 in the DSC pans. The starch-water mixtures were blended gently and left overnight at room temperature before DSC analysis. The pans were heated from 30 to 120 C at a scanning rate of 10 C/min and an empty pan was used as a reference. The Universal Analysis 2000 software was used to analyze the main endotherm of the DSC traces for start (To), peak (Tp) and conclusion (Tc) temperatures and enthalpy change (△H). 2.6. Rheological properties Starch dispersions (5% w/w) were moderately stirred for 15 min, then heated at 95 C for 30 min by a magnetic stirrer. The hot paste obtained was immediately transferred to the rheometer plate and cooled down to 25 C for rheological tests. All the tests were performed in an MCR 301 rheometer (Anton-paar Austria, DE) using a parallel plate geometry (d ¼ 50 mm). 2.6.1. Small amplitude oscillatory shear (SAOS) Frequency sweep was carried out at 25 C in a frequency range of 1e100 rad/s with a strain of 0.1, which was within the identified linear viscoelastic regime. The storage modulus (G0 ), loss modulus (G00 ), loss tangent (tan q ¼ G00 /G0 ) and complex viscosity (h*) were recorded as a function of frequency (u). 2.6.2. Steady-shear Steady shear flow measurements were carried out at 25 C, and the shear rate increased from 0.01 to 1001. Shear stress (t) and steady shear viscosity (h) were recorded as a function of shear rate. Tests were carried out in automatic mode to ensure that steady state at each shear rate was reached. 2.6.3. Thixotropic properties The thixotropic properties of brown rice flour gel were characterized by using hysteresis experiments which consisted of a threestep operation (upward curve, plateau curve and downward curve). An increasing shear rate ramp at a constant shear rate of 1 s1 to 100 s1, followed by a plateau at the maximum shear rate for 50 s1, and thereafter, the ramp was reversed to measure downward flow curve from 100 to 1 s1, the upward and downward flow curves should be the same for a time-independent liquid and should not superpose in the case of a time-dependent liquid. The area enclosed between up curves and down curves obtained by increasing and decreasing shear rate measurements was calculated by the software in Rheometer of MCR 301.
Table 1 Sources and chemical composition of brown rice power samples. Code
Cultivar
Protein (%)
Total fat (%)
IR-1 IR-2 IR-3 IR-4
Beihan1 Xing2 Zhongguangxiang Yuxiangyouzhan
11.94 ± 0.05a 9.28 ± 0.01b 8.38 ± 0.11c 7.77 ± 0.15d
3.86 2.18 2.77 2.65
± ± ± ±
0.16a 0.20c 0.21b 0.02b
Total starch (%) 75.52 75.75 78.21 76.95
± ± ± ±
1.16a 0.89a 2.22a 1.22a
Apparent amylose (%) 10.40 15.90 20.56 26.50
± ± ± ±
0.19d 0.01c 1.33b 0.26a
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2.7. Statistical analysis
Table 3 Gelatinization parameters of different brown rice flour by DSC.
All the measurements were conducted at least in duplicate. Analysis of variance and least significant difference tests were carried out using the SPSS statistical package version 20.0 (SPSS Inc., Chicago, IL, USA). Tukey’s test was applied to detect differences of the means, and p < 0.05 was considered to be statistically significant. 3. Results and discussion 3.1. Chemical composition analyses Protein, fat, and total starch content of the brown rice samples are listed in Table 1. Protein, fat and starch are important substantial basic to evaluate the processing and nutritious quality of rice. From Table 1, it can be seen that the protein content varied from 7.77% for IR-4 to 11.94% for IR-1 while the fat content ranged from 2.18 to 3.86%. The apparent true amylose content was in the range 10.4e26.5%, whereas the total starch content showed no significance difference. Generally, amylose is essentially a linear structure of a-1,4-linked glucose units, which always make the thermal and rheological properties differ of highly branched polymers. 3.2. Pasting properties Pasting behavior is reported to influence the cooking and eating quality and functionality of rice (Tian et al., 2009). The MVA profiles of the brown flour slurries can be seen in Fig. S1. The corresponding pasting parameters of four rice flours are shown in Table 2. Pasting temperature varied from 69.2 to 72.0 C. IR-4 showed the highest pasting temperature, followed by IR-3 and IR-2, and IR-1 showed the lowest pasting temperature. The order of pasting temperature is consistent with that of amylose content and inconsistent with protein content, indicating that amylose and protein contents play an important role in determining the pasting temperature of rice flour. Two experimental phenomena have been observed before, which may be used to explain it: (1) the protein removal imparts decreased initial gelatinization temperature to starch has been reported by Lim et al. (Lim et al., 1999); (2) The higher pasting temperature of amylose rich rice starch may be due to its lower swelling ability (Kong et al., 2015a). Peak viscosity reflects water-binding capacity or the extent of granule swelling, and it is the major determinant for the quality of final products since the swollen and collapsed granules affect the texture of products (Wani et al., 2012); Peak viscosity of brown rice flour ranges from 378.0 for IR-1 to 581.0 BU for IR-4. It should be noted that the peak viscosity of brown rice increases with increasing amylose content and decreasing protein content. More protein hydration during heating may cause the network linked by disulfide bonds (Xie et al., 2008), resulting in phase separation and lower peak viscosity. Another possible reason to explain this phenomenon is higher a-amylases in amylopecin-rich rice may cause the starch hydrolysis (Zhu et al., 2010). Final viscosity in the brown rice fours ranged from 387 to 850 BU, with the highest for IR-4 and
Code
TO ( C)
TP ( C)
IR-1 IR-2 IR-3 IR-4
68.6 ± 0.36a 65.11 ± 0.36b 62.90 ± 0.21c 60.20 ± 0.10d
74.57 72.80 68.81 67.72
± ± ± ±
DH (J/g)
TC ( C) 0.32a 0.1b 0.0c 0.26d
79.80 77.80 74.37 73.90
± ± ± ±
0.31a 0.40b 0.21c 0.35d
6.14 6.67 6.61 6.71
± ± ± ±
0.16b 0.24a 0.05a 0.44a
Values in each column with the same superscript are not different (p < 0.05).
lowest for IR-1. The increase in final viscosity during the cooling processing may be due to the aggregation or rearrangement of the amylose molecules (Arocas et al., 2009; Kong et al., 2015b). Breakdown viscosity reflects the starch paste resistance against heat and shear, while setback viscosity exhibits the tendency of starch pastes to retrogradation. Here IR-1 shows the lowest breakdown viscosity, indicating that the network structure of IR-1 is easy to be destroyed. The setback viscosity is usually related with the amylose content of rice flour, and the increase in setback viscosity with amylose content supports this conclusion. 3.3. Thermal properties Fig. S2 shows the gelatinization endotherms of various brown rice flours measured by DSC. A large gelatinization endotherm at about 70 C was observed for all the samples. This endotherm may be associated with the gelatinization of starch (Liu et al., 2013; Wang and Copeland, 2013) and denaturation of protein. The gelatinization parameters of various rice flours are listed in Table 3. In combination with Table 1, it can be seen that the onset (To), peak (Tp) and conclusion (Tc) of gelatinization temperatures decreased with increasing amylose content. This phenomenon is consistent with rice starch (Kong et al., 2015b), but inconsistent with the work of Liu et al. (Liu et al., 2006), who reported that the corn starch with higher amylose content had higher gelatinization temperature. Actually, the gelatinization endotherm (always called endotherm “G”) reflects the changes from limited swelling to maximum swelling of starch granules (mainly amylopectin molecules) and partial dissolution of starch polymers (mainly amylose molecules) (Wang and Copeland, 2013; Bao et al., 2009). It should be noted that the order of brown rice flour gelatinization temperature by DSC is totally different from the result of MVA. The interaction between starch and protein/fat may induce different change in heat fluxed for DSC or viscosity for MVA. For example, there is not any change of gelatinization temperature for waxy starch-fat complex from DSC, but the pasting temperature (by MVA) increased for amylose rich starch-fat complex (Wang et al., 2016). This new phenomenon will be studied in future. 3.4. Rheological properties 3.4.1. Shear experiment Typical mechanical spectrum (frequency dependence of G0 , G00 and h*) of brown rice flour (5% w/w) at 25 C is shown in Fig. 1 and Fig. S3. All the samples showed a linear decrease in the complex
Table 2 Pasting parameters of different brown rice flour. Code
Pasting temp ( C)
IR-1 IR-2 IR-3 IR-4
69.2 71.3 71.1 72.0
± ± ± ±
0.07c 0.11b 0.17b 0.28a
Peak viscosity (BU) 378.0 450.0 469.0 581.0
± ± ± ±
1.41d 4.14c 3.35b 12.73a
Trough viscosity (BU) 265.0 274.0 286.0 402.0
± ± ± ±
Values in each column with the same superscript are not different (p < 0.05).
0.71d 1.32c 4.13b 2.83a
Final viscosity (BU) 387.0 561.0 611.0 850.0
± ± ± ±
4.24d 3.41c 2.13b 1.41a
Breakdown (BU) 112.0 175.0 182.0 178.0
± ± ± ±
2.83b 1.23a 3.14a 9.19a
Setback (BU) 162.0 361.0 421.0 652.0
± ± ± ±
4.95d 5.62c 1.01b 18.38a
L. Ye et al. / Journal of Cereal Science 70 (2016) 270e274
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4 showed higher G0 and G00 than IR-1 and IR-2 (Fig. 1), indicating that higher amylose content favors the formation of good gel network. G0 showed little frequency dependency except for IR-1 if 0 applying the power-law model of G0 ¼ K 0 un , and the relaxation 0 exponent n decreases with increasing amylose content (Table 4), indicating that higher amylose content favors the formation of elastic gels. Interestingly, G00 is more frequency-dependent, and the exponent varies from 0.26 to 0.43.
Fig. 1. Complex viscosities as a function of angular frequency of brown rice flour.
viscosity h* with frequency on a log-log scale (see Fig. 1), and the slope is about 0.79, showing a typical shear thinning behavior. The slope is a little steeper than that (¼0.76) described by Morris (Liu et al., 2015; Morris, 1990), which was used to describe the “weak gel” characteristics of a polysaccharide gel formed by overlapping and entangled flexible random coil chains. Inconsistent with the data from MVA (Table 2), IR-4 shows lower viscosity than IR-3, which may be due to the faster retrogradation. The same trend was also found in the steady shear experiments. The Herschel-Bulkley model of t ¼ t0 þ kgm was used to evaluate the flow properties of brown rice flour gel from the steady shear experiment data (Wang et al., 2011). From Table 4, it can be seen that in all cases, t0 was greater than zero and m was less than 1 (t0 > 0 and m < 1), which means that it is a yield-pseudo plastic fluid. High amylose content could result in a low m value, reflecting a strong shear-thinning behavior. On the other hand, G0 is over G00 in the tested frequency range (Fig. 1) for all the samples, showing a gel-like behavior. IR-3 and IR-
3.4.2. Thixotropic properties Thixotropy is an important non-Newtonian fluid characteristics, which is characterized by the change of the apparent viscosity decrease under constant shearing condition, or hysteresis loops appearing with the shear rate changing circularly (Li et al., 2014). The area of hysteresis loops is considered to reflect the degree of thixotropy. Generally, the greater the hysteresis area, the stronger the thixotropic properties are. A negative value of hysteresis loops area represents the damage of brown rice flour gel structure upon increasing shear rate, whereas a positive value represents its structure recovery (Benchabane and Bekkour, 2008). Fig. S4 shows the hysteresis loops of brown rice flour gel, and the corresponding data of the hysteresis loops area obtained from the software of Rheometer are presented in Table 4. It can be seen that IR-2 and IR-3 exhibited stronger thixotropic properties in the given test range. In contrast, IR-1 showed a negative value of hysteresis loops area, suggesting the gel structure has been destroyed at higher shearing rate, the result is consistent with breakdown viscosity of IR-1 in Table 2. Based on all the information above, Fig. 2 represents the structure change of brown rice flour during gelatinization and gel formation. During heating with enough water, starch-rich rice flours undergo starch gelatinization which involves granular swelling, crystalline zone of starch melting, molecular components from the granular exudation and eventually total distribution of the granules. DSC and MVA analysis were also used to monitor this processing. Then, on cooling, amylose molecules begin to re-associate, forming a precipitate or gel and becoming opaque in the process,
Table 4 Rheological characteristics of brown starch based on Herschel-bulkley model for flown curve and power law for dynamic viscoelasticity. Code
IR-1 IR-2 IR-3 IR-4
Flow parameters
Ring area
K
m
t0
R2
1.41 4.62 5.33 3.97
0.58 0.41 0.34 0.30
0.52 1.65 0.77 0.21
0.99 0.99 0.99 0.99
80.02 ± 10.08 283.07 ± 30.04 358.21 ± 35.64 121.79 ± 28.59
Fig. 2. Schematic representation of changes of brown rice flour during gelatinization and gel formation.
Viscoelasticity n0
n00
0.42 0.14 0.06 0.05
0.43 0.32 0.26 0.37
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L. Ye et al. / Journal of Cereal Science 70 (2016) 270e274
called retrogradation or setback. Leached amylose and swollen granules arrange themselves in a special three-dimensional conformation by entanglement of molecule chains, formation of junction zones, and embedding of swollen granules Because they form complexes with both amylose and amylopectin, lipids or protein would alter the three dimensional conformation by assembling with amylose or amylopectin and thus the thermal and n ~ ez et al., 2007). mechanical properties of the composite gel (Iba However, the retrogradation time may affect the gel quality. The detailed information about the fine structure of amylopectin (such as chain length and distribution) to affect the viscoelastic of brown rice flour will be studied in future. 4. Conclusion Peak, final and setback viscosity increased with increasing amylose content from MVA, but the phase transition temperature of brown rice flour decreased with increased amylose content. SAOS results showed that brown rice flour gels presented a somehow solid-like viscoelastic behavior. The brown rice flour formed a weak gel. Steady-shear results showed that the brown rice flour exhibited non-Newtonian shear-thinning behavior and the flow curves could be well described by the Herschel-Bulkley model. The upward-downward rheograms showed that except IR-1, other three samples exhibited a strong thixotropic behavior. Conflict of interest The authors declare that there is no conflict of interests regarding the publication of this paper. Acknowledgements The authors from China would like to acknowledge the research funds NFSC (31301554), Fundamental Research Funds for Chinese Academy of Agricultural Sciences (2015ZL048) and Special Fund for Agro-scientific research in the Public Interest (Grant No. 201403063-03). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jcs.2016.07.007. References Arocas, A., Sanz, T., Fiszman, S.M., 2009. Clean label starches as thickeners in white sauces. Shearing, heating and freeze/thaw stability. Food Hydrocoll. 23, 2031e2037. Asmeda, R., Noorlaila, A., Norziah, M.H., 2016. Relationships of damaged starch granules and particle size distribution with pasting and thermal profiles of milled MR263 rice flour. Food Chem. 191, 45e51. Bao, J., Xiao, P., Hiratsuka, M., Sun, M., Umemoto, T., 2009. Granule-bound SSIIa protein content and its relationship with amylopectin structure and
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Thermal and rheological properties of brown flour from Indica rice Lingxu Ye a, Changsheng Wang b, Shujun Wang c, Sumei Zhou a, **, Xingxun Liu a, * a
Institute of Food Science and Technology (IFST), Chinese Academy of Agricultural Science (CAAS), Beijing 100193, China Department of Chemical Engineering, CREPEC, Polytechnique Montr eal, Station Centre-Ville, Montr eal, QC, Canada c Key Laboratory of Food Nutrition and Safety, Ministry of Education, Tianjin University of Science & Technology, Tianjin 300457, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 April 2016 Received in revised form 6 July 2016 Accepted 8 July 2016 Available online 11 July 2016
The thermal, paste and rheological properties of brown flours from four Indica rice subspecies with different amylose content were examined using Differential scanning calorimetry (DSC), Brabender Viscometer and rheometer. Peak, final and setback viscosities (p < 0.05) increased with increasing amylose content from Brabender micro Visco-Amylo-Graph (MVA), but the phase transition temperatures of brown rice flour from DSC (p < 0.05) decreased with increasing amylose content. Rheological properties were determined by steady shear, small amplitude oscillatory shear (SAOS) and thixotropic experiments. SAOS results showed a gel-like viscoelastic behavior with G0 higher than G00 . Steady-shear results showed that the brown rice flour exhibited a non-Newtonian shear-thinning behavior and the flow curves can be well described by the Herschel-Bulkley model. The upward-downward rheograms showed that brown rice flour gel, except IR-1, had a hysteresis loop, indicating a strong thixotropic behavior. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Brown rice flour Rheology properties Pasting properties Thermal properties
1. Introduction Rice is an excellent source of energy for about half world’s human and it is particularly important in Asia’s food consumption. Brown rice, produced by dehulling the grain, is considered as a whole grain food and its color may be light brown, green, reddish or black (Gunaratne et al., 2013). White rice, also known as milled rice or polished rice, is currently the most common form consumed. Compared with white rice, brown rice flour has an amount of fiber (non-starch polysaccharides, NPS) and bioactive molecules such as polyphenol, which can reduce the risk of chronic diseases including hypercholesterolemia, cardiovascular disease, and type II diabetes (Frolich et al., 2013). Recently, brown rice has drawn increasing attention due to its additional health benefits. The thermal and rheological properties are important to understand brown rice processing and digestion in human. For example, flow behavior, thixotropy and dynamical viscoelastic properties are most important properties in rheology, which are related to production processing and application. In detail, small oscillatory frequency sweep is extremely sensitive to the structure
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Zhou), [email protected] (X. Liu). http://dx.doi.org/10.1016/j.jcs.2016.07.007 0733-5210/© 2016 Elsevier Ltd. All rights reserved.
information of samples, which makes it useful to evaluate gelation behavior and kinetics (Liu et al., 2015), and it can provide some additional information such as structure and energy consumption. The rheological properties are also important to the analysis of flow conditions in product processes and prediction of product stability (Li et al., 2014). Rheological properties also correlate to the textural attributes, which in turn determine the sensory characteristics and consumer acceptability of the products (Li et al., 2014). The thermal, pasting and rheological properties of brown rice n ~ ez et al., 2007; Varavinit et al., 2003; flour have been reported (Iba Wu et al., 2013; Zhu et al., 2010). Wu et al. (2013) showed that the gelatinization temperature exhibited no significant differences between the brown rice flour and isolated starch. However, the pasting viscosity of starch is several times higher than that of n ~ ez et al. (2007) have studied the brown flour (Zhu et al., 2010). Iba viscoelastic properties of waxy and non-waxy rice flours. After removal of protein and lipids with aqueous alkaline or detergent solutions, pasting temperatures decreased, but the viscosities of both starches increased. The rheological properties of rice flour are also shown to be affected by damaged starch content and particles size (Asmeda et al., 2016) as well as apparent amylose content (Kong et al., 2015a). Rheological properties of rice flour are of high importance to design new products with desired sensory and textural attributes. However, the comprehensive and systematic study on the
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rheological properties of brown rice flour is still limited. In this work, Indica, a main rice subspecies cultivated in South China, was used to investigate the rheological properties of gels produced from brown rice flour using steady and oscillatory shear experiments. This work provides an important reference to the brown rice application. 2. Material and methods 2.1. Raw materials Four Indica rice (IR) cultivars from China main rice production area (such as Guangdong, Hunan province) were used in this study (Table 1). All the brown rice was harvested in the year of 2014 and stored at 4 C. Beihan1# was kindly provided by Shandong Academy of Agricultural Science, while Jinnongsimiao and Yuxiangyouzhan were kindly provided by Guangdong Academy of Agricultural Science, China. Xing2# was supplied by Hunan Jinjian Seed Industry Science & Technology Co., Ltd. 2.2. Sample preparation Before all tests, the raw rice grains were dehulled to produce brown rice (with bran layers and germ still intact). In this study, rice grains were de-husked (BLH-3250, Zhejiang Bethlehem, China) and grounded to pass through a 100-mesh sieve on a Cyclone Sample Mill (CT410, Foss Scino, China) for further use. 2.3. Chemical analyses Protein content (N content x5.95) was determined by combustion (AACC method 46-11A) using Automatic Kjeldahl Nitrogen Analyser (Kjeltes2300, FOSS, Inc, USA). Total starch content was measured by Megazyme assay kit (K-TSTA 09/14) from Megazyme International Ireland Ltd. (Co. Wicklow, Ireland) following the manufacturer’s instructions. Amylose content of starches was analyzed in triplicate with an amylose/amylopectin assay kit (Megazyme International Ireland Ltd., Bray, Co. Wicklow, Ireland). Total fat was measured using a Soxhelt extractor, with petroleum ether as the extraction solvent (AACC method 08-01). 2.4. Pasting properties of brown rice flour The pasting properties were monitored using a Brabender Micro Visco-Amylo-Graph (MVA) (Brabender OHG, Duisburg, Germany) according to the classic method (Mariotti et al., 2005, 2009). 12 g of the sample was dispersed in 100 mL of distilled water, scaling both flour and water weight on 14% flour moisture basis. The suspensions were subjected (stirring at 250 RPM/min and using a 300 cm$gf cartridge) to the following standard temperature profile: heating from 30 C up to 95 C at a rate of 7.5 C/min, holding at 95 C for 5 min, cooling from 95 C to 50 C at a rate of 7.5 C/min and holding at 50 C. The pasting temperature range was obtained as the range between the temperature at the start of increase of viscosity and that at which it remains constant. The peak, breakdown, and setback viscosity were also recorded.
271
2.5. Differential scanning calorimetry Thermal transition measurements of brown rice flour were determined using a Modulated Differential Scanning Calorimeter MDSC 2920 instrument (TA Instruments Inc., Delaware, USA) equipped with a thermal analysis data station and data recording software. The sample was prepared according to the procedure of Wang et al. (Wang et al., 2014) with a slight modification. Briefly, about 3 mg of samples was weighed into 40 mL aluminum pans. Distilled water was added with a pipette to obtain a water/sample ratio of 3:1 in the DSC pans. The starch-water mixtures were blended gently and left overnight at room temperature before DSC analysis. The pans were heated from 30 to 120 C at a scanning rate of 10 C/min and an empty pan was used as a reference. The Universal Analysis 2000 software was used to analyze the main endotherm of the DSC traces for start (To), peak (Tp) and conclusion (Tc) temperatures and enthalpy change (△H). 2.6. Rheological properties Starch dispersions (5% w/w) were moderately stirred for 15 min, then heated at 95 C for 30 min by a magnetic stirrer. The hot paste obtained was immediately transferred to the rheometer plate and cooled down to 25 C for rheological tests. All the tests were performed in an MCR 301 rheometer (Anton-paar Austria, DE) using a parallel plate geometry (d ¼ 50 mm). 2.6.1. Small amplitude oscillatory shear (SAOS) Frequency sweep was carried out at 25 C in a frequency range of 1e100 rad/s with a strain of 0.1, which was within the identified linear viscoelastic regime. The storage modulus (G0 ), loss modulus (G00 ), loss tangent (tan q ¼ G00 /G0 ) and complex viscosity (h*) were recorded as a function of frequency (u). 2.6.2. Steady-shear Steady shear flow measurements were carried out at 25 C, and the shear rate increased from 0.01 to 1001. Shear stress (t) and steady shear viscosity (h) were recorded as a function of shear rate. Tests were carried out in automatic mode to ensure that steady state at each shear rate was reached. 2.6.3. Thixotropic properties The thixotropic properties of brown rice flour gel were characterized by using hysteresis experiments which consisted of a threestep operation (upward curve, plateau curve and downward curve). An increasing shear rate ramp at a constant shear rate of 1 s1 to 100 s1, followed by a plateau at the maximum shear rate for 50 s1, and thereafter, the ramp was reversed to measure downward flow curve from 100 to 1 s1, the upward and downward flow curves should be the same for a time-independent liquid and should not superpose in the case of a time-dependent liquid. The area enclosed between up curves and down curves obtained by increasing and decreasing shear rate measurements was calculated by the software in Rheometer of MCR 301.
Table 1 Sources and chemical composition of brown rice power samples. Code
Cultivar
Protein (%)
Total fat (%)
IR-1 IR-2 IR-3 IR-4
Beihan1 Xing2 Zhongguangxiang Yuxiangyouzhan
11.94 ± 0.05a 9.28 ± 0.01b 8.38 ± 0.11c 7.77 ± 0.15d
3.86 2.18 2.77 2.65
± ± ± ±
0.16a 0.20c 0.21b 0.02b
Total starch (%) 75.52 75.75 78.21 76.95
± ± ± ±
1.16a 0.89a 2.22a 1.22a
Apparent amylose (%) 10.40 15.90 20.56 26.50
± ± ± ±
0.19d 0.01c 1.33b 0.26a
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2.7. Statistical analysis
Table 3 Gelatinization parameters of different brown rice flour by DSC.
All the measurements were conducted at least in duplicate. Analysis of variance and least significant difference tests were carried out using the SPSS statistical package version 20.0 (SPSS Inc., Chicago, IL, USA). Tukey’s test was applied to detect differences of the means, and p < 0.05 was considered to be statistically significant. 3. Results and discussion 3.1. Chemical composition analyses Protein, fat, and total starch content of the brown rice samples are listed in Table 1. Protein, fat and starch are important substantial basic to evaluate the processing and nutritious quality of rice. From Table 1, it can be seen that the protein content varied from 7.77% for IR-4 to 11.94% for IR-1 while the fat content ranged from 2.18 to 3.86%. The apparent true amylose content was in the range 10.4e26.5%, whereas the total starch content showed no significance difference. Generally, amylose is essentially a linear structure of a-1,4-linked glucose units, which always make the thermal and rheological properties differ of highly branched polymers. 3.2. Pasting properties Pasting behavior is reported to influence the cooking and eating quality and functionality of rice (Tian et al., 2009). The MVA profiles of the brown flour slurries can be seen in Fig. S1. The corresponding pasting parameters of four rice flours are shown in Table 2. Pasting temperature varied from 69.2 to 72.0 C. IR-4 showed the highest pasting temperature, followed by IR-3 and IR-2, and IR-1 showed the lowest pasting temperature. The order of pasting temperature is consistent with that of amylose content and inconsistent with protein content, indicating that amylose and protein contents play an important role in determining the pasting temperature of rice flour. Two experimental phenomena have been observed before, which may be used to explain it: (1) the protein removal imparts decreased initial gelatinization temperature to starch has been reported by Lim et al. (Lim et al., 1999); (2) The higher pasting temperature of amylose rich rice starch may be due to its lower swelling ability (Kong et al., 2015a). Peak viscosity reflects water-binding capacity or the extent of granule swelling, and it is the major determinant for the quality of final products since the swollen and collapsed granules affect the texture of products (Wani et al., 2012); Peak viscosity of brown rice flour ranges from 378.0 for IR-1 to 581.0 BU for IR-4. It should be noted that the peak viscosity of brown rice increases with increasing amylose content and decreasing protein content. More protein hydration during heating may cause the network linked by disulfide bonds (Xie et al., 2008), resulting in phase separation and lower peak viscosity. Another possible reason to explain this phenomenon is higher a-amylases in amylopecin-rich rice may cause the starch hydrolysis (Zhu et al., 2010). Final viscosity in the brown rice fours ranged from 387 to 850 BU, with the highest for IR-4 and
Code
TO ( C)
TP ( C)
IR-1 IR-2 IR-3 IR-4
68.6 ± 0.36a 65.11 ± 0.36b 62.90 ± 0.21c 60.20 ± 0.10d
74.57 72.80 68.81 67.72
± ± ± ±
DH (J/g)
TC ( C) 0.32a 0.1b 0.0c 0.26d
79.80 77.80 74.37 73.90
± ± ± ±
0.31a 0.40b 0.21c 0.35d
6.14 6.67 6.61 6.71
± ± ± ±
0.16b 0.24a 0.05a 0.44a
Values in each column with the same superscript are not different (p < 0.05).
lowest for IR-1. The increase in final viscosity during the cooling processing may be due to the aggregation or rearrangement of the amylose molecules (Arocas et al., 2009; Kong et al., 2015b). Breakdown viscosity reflects the starch paste resistance against heat and shear, while setback viscosity exhibits the tendency of starch pastes to retrogradation. Here IR-1 shows the lowest breakdown viscosity, indicating that the network structure of IR-1 is easy to be destroyed. The setback viscosity is usually related with the amylose content of rice flour, and the increase in setback viscosity with amylose content supports this conclusion. 3.3. Thermal properties Fig. S2 shows the gelatinization endotherms of various brown rice flours measured by DSC. A large gelatinization endotherm at about 70 C was observed for all the samples. This endotherm may be associated with the gelatinization of starch (Liu et al., 2013; Wang and Copeland, 2013) and denaturation of protein. The gelatinization parameters of various rice flours are listed in Table 3. In combination with Table 1, it can be seen that the onset (To), peak (Tp) and conclusion (Tc) of gelatinization temperatures decreased with increasing amylose content. This phenomenon is consistent with rice starch (Kong et al., 2015b), but inconsistent with the work of Liu et al. (Liu et al., 2006), who reported that the corn starch with higher amylose content had higher gelatinization temperature. Actually, the gelatinization endotherm (always called endotherm “G”) reflects the changes from limited swelling to maximum swelling of starch granules (mainly amylopectin molecules) and partial dissolution of starch polymers (mainly amylose molecules) (Wang and Copeland, 2013; Bao et al., 2009). It should be noted that the order of brown rice flour gelatinization temperature by DSC is totally different from the result of MVA. The interaction between starch and protein/fat may induce different change in heat fluxed for DSC or viscosity for MVA. For example, there is not any change of gelatinization temperature for waxy starch-fat complex from DSC, but the pasting temperature (by MVA) increased for amylose rich starch-fat complex (Wang et al., 2016). This new phenomenon will be studied in future. 3.4. Rheological properties 3.4.1. Shear experiment Typical mechanical spectrum (frequency dependence of G0 , G00 and h*) of brown rice flour (5% w/w) at 25 C is shown in Fig. 1 and Fig. S3. All the samples showed a linear decrease in the complex
Table 2 Pasting parameters of different brown rice flour. Code
Pasting temp ( C)
IR-1 IR-2 IR-3 IR-4
69.2 71.3 71.1 72.0
± ± ± ±
0.07c 0.11b 0.17b 0.28a
Peak viscosity (BU) 378.0 450.0 469.0 581.0
± ± ± ±
1.41d 4.14c 3.35b 12.73a
Trough viscosity (BU) 265.0 274.0 286.0 402.0
± ± ± ±
Values in each column with the same superscript are not different (p < 0.05).
0.71d 1.32c 4.13b 2.83a
Final viscosity (BU) 387.0 561.0 611.0 850.0
± ± ± ±
4.24d 3.41c 2.13b 1.41a
Breakdown (BU) 112.0 175.0 182.0 178.0
± ± ± ±
2.83b 1.23a 3.14a 9.19a
Setback (BU) 162.0 361.0 421.0 652.0
± ± ± ±
4.95d 5.62c 1.01b 18.38a
L. Ye et al. / Journal of Cereal Science 70 (2016) 270e274
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4 showed higher G0 and G00 than IR-1 and IR-2 (Fig. 1), indicating that higher amylose content favors the formation of good gel network. G0 showed little frequency dependency except for IR-1 if 0 applying the power-law model of G0 ¼ K 0 un , and the relaxation 0 exponent n decreases with increasing amylose content (Table 4), indicating that higher amylose content favors the formation of elastic gels. Interestingly, G00 is more frequency-dependent, and the exponent varies from 0.26 to 0.43.
Fig. 1. Complex viscosities as a function of angular frequency of brown rice flour.
viscosity h* with frequency on a log-log scale (see Fig. 1), and the slope is about 0.79, showing a typical shear thinning behavior. The slope is a little steeper than that (¼0.76) described by Morris (Liu et al., 2015; Morris, 1990), which was used to describe the “weak gel” characteristics of a polysaccharide gel formed by overlapping and entangled flexible random coil chains. Inconsistent with the data from MVA (Table 2), IR-4 shows lower viscosity than IR-3, which may be due to the faster retrogradation. The same trend was also found in the steady shear experiments. The Herschel-Bulkley model of t ¼ t0 þ kgm was used to evaluate the flow properties of brown rice flour gel from the steady shear experiment data (Wang et al., 2011). From Table 4, it can be seen that in all cases, t0 was greater than zero and m was less than 1 (t0 > 0 and m < 1), which means that it is a yield-pseudo plastic fluid. High amylose content could result in a low m value, reflecting a strong shear-thinning behavior. On the other hand, G0 is over G00 in the tested frequency range (Fig. 1) for all the samples, showing a gel-like behavior. IR-3 and IR-
3.4.2. Thixotropic properties Thixotropy is an important non-Newtonian fluid characteristics, which is characterized by the change of the apparent viscosity decrease under constant shearing condition, or hysteresis loops appearing with the shear rate changing circularly (Li et al., 2014). The area of hysteresis loops is considered to reflect the degree of thixotropy. Generally, the greater the hysteresis area, the stronger the thixotropic properties are. A negative value of hysteresis loops area represents the damage of brown rice flour gel structure upon increasing shear rate, whereas a positive value represents its structure recovery (Benchabane and Bekkour, 2008). Fig. S4 shows the hysteresis loops of brown rice flour gel, and the corresponding data of the hysteresis loops area obtained from the software of Rheometer are presented in Table 4. It can be seen that IR-2 and IR-3 exhibited stronger thixotropic properties in the given test range. In contrast, IR-1 showed a negative value of hysteresis loops area, suggesting the gel structure has been destroyed at higher shearing rate, the result is consistent with breakdown viscosity of IR-1 in Table 2. Based on all the information above, Fig. 2 represents the structure change of brown rice flour during gelatinization and gel formation. During heating with enough water, starch-rich rice flours undergo starch gelatinization which involves granular swelling, crystalline zone of starch melting, molecular components from the granular exudation and eventually total distribution of the granules. DSC and MVA analysis were also used to monitor this processing. Then, on cooling, amylose molecules begin to re-associate, forming a precipitate or gel and becoming opaque in the process,
Table 4 Rheological characteristics of brown starch based on Herschel-bulkley model for flown curve and power law for dynamic viscoelasticity. Code
IR-1 IR-2 IR-3 IR-4
Flow parameters
Ring area
K
m
t0
R2
1.41 4.62 5.33 3.97
0.58 0.41 0.34 0.30
0.52 1.65 0.77 0.21
0.99 0.99 0.99 0.99
80.02 ± 10.08 283.07 ± 30.04 358.21 ± 35.64 121.79 ± 28.59
Fig. 2. Schematic representation of changes of brown rice flour during gelatinization and gel formation.
Viscoelasticity n0
n00
0.42 0.14 0.06 0.05
0.43 0.32 0.26 0.37
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called retrogradation or setback. Leached amylose and swollen granules arrange themselves in a special three-dimensional conformation by entanglement of molecule chains, formation of junction zones, and embedding of swollen granules Because they form complexes with both amylose and amylopectin, lipids or protein would alter the three dimensional conformation by assembling with amylose or amylopectin and thus the thermal and n ~ ez et al., 2007). mechanical properties of the composite gel (Iba However, the retrogradation time may affect the gel quality. The detailed information about the fine structure of amylopectin (such as chain length and distribution) to affect the viscoelastic of brown rice flour will be studied in future. 4. Conclusion Peak, final and setback viscosity increased with increasing amylose content from MVA, but the phase transition temperature of brown rice flour decreased with increased amylose content. SAOS results showed that brown rice flour gels presented a somehow solid-like viscoelastic behavior. The brown rice flour formed a weak gel. Steady-shear results showed that the brown rice flour exhibited non-Newtonian shear-thinning behavior and the flow curves could be well described by the Herschel-Bulkley model. The upward-downward rheograms showed that except IR-1, other three samples exhibited a strong thixotropic behavior. Conflict of interest The authors declare that there is no conflict of interests regarding the publication of this paper. Acknowledgements The authors from China would like to acknowledge the research funds NFSC (31301554), Fundamental Research Funds for Chinese Academy of Agricultural Sciences (2015ZL048) and Special Fund for Agro-scientific research in the Public Interest (Grant No. 201403063-03). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jcs.2016.07.007. References Arocas, A., Sanz, T., Fiszman, S.M., 2009. Clean label starches as thickeners in white sauces. Shearing, heating and freeze/thaw stability. Food Hydrocoll. 23, 2031e2037. Asmeda, R., Noorlaila, A., Norziah, M.H., 2016. Relationships of damaged starch granules and particle size distribution with pasting and thermal profiles of milled MR263 rice flour. Food Chem. 191, 45e51. Bao, J., Xiao, P., Hiratsuka, M., Sun, M., Umemoto, T., 2009. Granule-bound SSIIa protein content and its relationship with amylopectin structure and
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