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Effect of milling methods on the properties of rice flour and gluten-free rice bread
LWT - Food Science and Technology 108 (2019) 137–144
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Effect of milling methods on the properties of rice flour and gluten-free rice bread
T
Tong Wua,b,c,1, Lili Wangb,1, Yan Lia,c, Haifeng Qiana,c, Liya Liub, Litao Tongb, Xianrong Zhoub, Li Wanga,c,∗, Sumei Zhoub,∗∗ a
State Key Laboratory of Food Science and Technology, Jiangnan University, 1800 Lihu Avenue, Wuxi, 214122, China Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences, Beijing, 100193, China c School of Food Science and Technology, Jiangnan University, 1800 Lihu Avenue, Wuxi, 214122, China b
ARTICLE INFO
ABSTRACT
Keywords: Rice flour Rice milling Damaged starch Gluten-free Rice bread
The properties of rice flour are very important for making gluten-free rice bread. In this study, rice flour was prepared using different milling methods (wet-milling, cyclone-milling and ultrafine-milling) to investigate their effect on the properties of gluten-free rice bread. The milling method was found to significantly affect the damaged starch content, state of the starch granule, gelatinization temperature and absorption enthalpy of rice flour (p < 0.05). The starch granules of flour prepared by wet milling had a high integrity, low damaged starch content (2.80 g/100 g), high gelatinization temperature, and high values of absorption enthalpy and gel strength compared to ultrafine-milling and cyclone-milling methods. As the deformation of the starch in the rice grains decreased, the amount of completely gelatinized starch granules in the bread decreased, with its specific volume also decreasing from 3.1 to 1.2 mL/g. Increase in starch damage content was observed to decrease the specific volume of bread, consequently increasing the hardness of bread. Also, uneven pores were observed, and the number and size of internal pores decreased. This indicated that wet-milled rice flour with a low content of damaged starch and higher starch granules integrity produced better gluten-free rice bread.
1. Introduction Celiac disease (CD) is an autoimmune enteropathy induced in individuals with susceptible genes when ingesting gluten-containing grains such as wheat, barley, and buckwheat, and their products (Di Sabatino & Corazza, 2009). CD has been recognized by the medical community for almost 100 years. The incidence of CD in Caucasians has been found to be 1% (Dubé et al., 2005) with its incidence reported to be rising in Asia (Makharia et al., 2014) and also in China (Kang, Kang, Green, Gwee, & Ho, 2013). Because of its diverse and complex causes, the only feasible and effective way to relieve its symptoms is to give patients a gluten-free diet. In general, the raw grain materials used for producing gluten-free foods are rice, corn, sorghum, buckwheat, millet, and potato. Of these materials, rice has the properties of being easily digested and absorbed, and it is also hypoallergenic, mild and colorless, with a high yield (Gujral & Rosell, 2004; Torbica, Hadnađev, & Dapčević, 2010).
Therefore, many studies on using rice to prepare bread, cakes, biscuits, and noodles have been conducted. Rice (Oryza sativa L) is a crop grown worldwide and is an important staple food for about 50% of the world's population, mainly in Asia (FAO, 2013, pp. 1–289). China has cultivated rice for more than 5,000 years (You, 1994), with its current production ranking first in the world (Li, Su, & Liu, 2016). China has many traditional products that use rice as the raw material, such as rice cakes, rice noodles, rice wine, rice sponge cake, and rice dumplings. The consumption habits of Chinese people have also been changing from traditional foods to non-traditional foods such as bread, cakes and pasta, usually made from wheat flour. Rice bread made from rice flour may provide a new direction for rice processing, improve rice usage, and meet the demand for bread for people who are allergic to gluten. Different milling methods have been reported to affect the properties of rice flour significantly (Kadan, Bryant, & Miller, 2010), especially the particle size of the rice flour, the content of damaged starch, and the state of the starch granules. Kim & Shin (Kim & Shin, 2014) studied the
∗ Corresponding author. State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, 1800 Lihu Avenue, Wuxi, 214122, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (L. Wang), [email protected] (S. Zhou). 1 These authors contributed equally to this work.
https://doi.org/10.1016/j.lwt.2019.03.050 Received 11 December 2018; Received in revised form 15 March 2019; Accepted 16 March 2019 Available online 20 March 2019 0023-6438/ © 2019 Elsevier Ltd. All rights reserved.
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effect of the particle size of rice flour on gluten-free rice cupcakes, finding that those prepared with moderately-sized rice flour particles had a better quality. Previous studies in our laboratory have found that the damaged starch content in rice flour had a great influence on the quality of rice noodles (Tong et al., 2015). Wet milling is a traditional method used in China with many studies showing that it is more suitable for producing rice flour than dry milling (Kumar, Malleshi, & SilaBhattacharya, 2008). However, wet milling generates a large quantity of wastewater which pollutes the environment (Tong et al., 2017). In contrast, dry milling produces less wastewater, but its particle size of the powder is too big. It is possible to obtain rice powder with a fine particle size without generating a large amount of wastewater by using the ultrafine milling method. The present study therefore aims to compare the rheological properties of rice flour doughs using rice flour prepared by different milling methods and their effects on rice bread regarding its volume, texture and quality characteristics to improve the quality of rice bread.
Germany). 12 g sample (14% moisture content) was dispersed in 100 mL of distilled water. The suspension was warmed up according to the following procedure: ramping from 30 °C to 95 °C at a rate of 7.5 °C/ min, holding at 95 °C for 5 min, then cooling to 50 °C at a rate of 7.5 °C/ min, holding at 50 °C for 5 min. The mixer speed was 250 r/min and the viscosity was expressed as BU. 2.6. Gel strength of rice flour A certain amount of rice flour was added with water to form a suspension with a mass fraction of 15% (on a dry basis). The mixture was heated and stirred for 1 min on a magnetic heating stirrer, and the pre-gelatinized starch paste was equally distributed into three 10 mL beakers, and the gelatinization was continued for 30 min in a 95 °C water bath after sealing. After being removed from water bath, it was stabilized at room temperature and cooled at 4 °C overnight. The Texture Analyzer (TA, TA-XT2i, Stable Micrio System, England) was used to measure the gel strength using the TPA (texture profile analysis) mode. The test probe P/0.5R was selected. The pre-test speed and test speed were 0.5 mm/s, the measured speed was 5.0 mm/s, the compression distance was 10.0 mm, the triggering force was 5.0 g, the compression interval was 2 s. The experiment was repeated three times, and the average was taken.
2. Materials and methods 2.1. Materials Non-waxy white Wuchang rice was purchased from Wuchang Wanfu Rice Industry Co., Ltd.(Harbin, Heilongjiang province of China) in 2017. Psyllium gum and Sodium caseinate were purchased from Zhengzhou, Henan province. Extruded rice flour was obtained from Nanchang, Jiangxi province. White sugar, butter and the instant dry yeast were obtained from a local market in Beijing China.
2.7. DSC analysis Thermal behaviour measurements were performed on a DSC-Q200 differential scanning calorimeter (TA, USA) using the aluminum pans. 4 mg (dry weight) of rice flour was added to the crucible, distilled water in a ratio of 1:2 (w/w) was added, and the mixture was allowed to stand overnight after sealing. The sealed blank was used as a control, and the thermal characteristic curve was measured by heating from 20 °C to 110 °C at a rate of 5 °C/min. The starting temperature (To), the peak temperature (Tp), the termination temperature (Tc), the gelatinization temperature range (Tr), and the gelatinization absorption heat enthalpy (ΔH) were obtained by analyzing the spectrum in the DSC software.
2.2. Preparation of rice flour Three kinds of rice flour were prepared by wet-milled flour, cyclonemilled flour and Ultrafine-milled flour, respectively. For wet-milled flour, 1 kg of rice (14% moisture content, wet basis) were soaked in 2 L water at room temperature for 24 h, followed by ground with a grinder (YU8022, Wet miller, Hebei, China). For the cyclone-milled flour, the rice was pulverized into flour using a cyclone mill (CT410, FOSS Scino (Suzhou) Co., Ltd., Suzhou, China) and passed through a 100 mesh sieve. For Ultrafine-milled flour, rice was grinded with an Ultrafine grinder and passed through a 100 mesh sieve. The three rice flours were freeze-dried till to its moisture content 5% (wet basis) and then stored at 4 °C for further analysis.
2.8. Bread making procedure The bread formula for bread consisted of 30 g rice flour with different milling treatments, 7.5 g white sugar, 5 g butter, 5 g sodium caseinate, 2.5 extrusion rice flour, 0.5 g psyllium gum, 0.5 g instant dry yeast. The psyllium gum was pre-dissolved in 60 °C hot water, and cooled to room temperature and finally mixed with white sugar and instant dry yeast. The rice flour and extrusion rice flour were poured into the mixture and mixed for 4 min with a pin mixer, then the butter was added and stirred for 4 min. The dough was then divided into 70 g of dough and placed in a baking tray and fermented for 60 min (temperature 38 °C, humidity 80%). The oven was preheated for 30 min and bread was baked at 150 °C, for 15 min and taken out. The bread was cooled for one hour at room temperature before any further analysis.
2.3. Determination of rice flour properties Moisture, protein and lipid contents of rice flour were measured according to AACC methods 44-15A, 46-11A, and 30-10, respectively (2000). The total starch content and the degree of starch damage of the different rice flours were measured using a Total Starch Kit and a Starch Damage Assay Kit (K-SDAM, Megazyme International Ltd., Wicklow, Ireland). Experiment repeated three times, and the average was taken. 2.4. Measurement of particle size distributions
2.9. Evaluation of bread quality
Particle size distribution of three kinds of rice flour was determined by Malvern MS3000 laser particle size analyzer (MS3000, Malvern instruments, UK). Air was chosen as the optical model of light scattering and the refractive index (1.000) was used. After determining the background light scattering, 5 g of the sample powder was poured into the apparatus to start measurement, and then the results of particle size distribution was shown after 2 min.
Bread volume was measured by the seed displacement method (AACC 72-10), and specific volume was calculated from the volume divided by weight. The textural properties of the bread crumbs were measured using a compression test with a Texture Analyzer (TA, TAXT2i, Stable Micrio System, England). Rice flour bread was prepared one day in advance, and then cut into small pieces of 2 × 2 × 1.5 cm. The bread slice was subjected to a double compression cycle with a 50% original height deformation using a 36 mm flat aluminum compression disk (probe P/36). The measuring conditions are as follows: pre-test/ post-measurement speed of 2.0 mm/s, test speed of 1 mm/s, relaxation time of 5 s; trigger force of 10 g; trigger mode - automatic.
2.5. Gelatinization characteristics of rice flour The gelatinization properties of rice flour were determined using a Brabender viscometer (MVAG803202, Brabender Gmbh & Co.KG, 138
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severity of processing. More mechanical and thermal energy is produced during dry milling than wet milling, resulting in higher levels of damaged rice starch (Kumar et al., 2008). Table 1 also shows that the different milling methods had a significant effect on the particle size of the rice flour. The average particle sizes of WF and UF were 37.8 μm and 32.4 μm, respectively. However, the UF particle size distribution was the narrowest, from only 20.4–86.5 μm. CF exhibited the largest average particle size of 74.2 μm and the widest particle size distribution, from 43.6 to 191.5 μm. Producing UF involves a high mechanical impact on grains during the highspeed operation and a considerable impact between the grinding media and the grains during honing, leading to a small particle size and a narrow distribution range. For WF, the rice grains were easily pulverized because of the soaking and consequent softening so that the rice flour obtained had a particle size slightly larger than that of UF.
2.10. Rheological properties of rice flour dough The following ingredients (g/100 g flour) were used to make dough: sugar (25 g/100 g), butter (16.7 g/100 g), Sodium caseinate (16.7 g/ 100 g), extruded rice flour (8.3 g/100 g), Psyllium gum (1.67 g/100 g). The rheological properties of the fresh dough were measured by a rheometer (Physica MCR 301, Anton Paar GmbH, Stutgart, Germany). The freshly prepared dough sample was placed on a test platform, the mold (2.5 cm in diameter, flat plate) was pressed down, the control plate spacing was 1.0 mm, the excess sample was scraped off, and the silicone oil was sealed to prevent water loss during the test. After standing for 5 min and releasing the strain, the storage modulus (G') and the loss modulus (G") were tested with the scanning frequency (ω) at 25 °C, strain 0.1%, and frequency range 0.1–100 Hz). 2.11. Fermentation rheological properties of rice flour dough
3.2. Rice flour microstructure
The rheological characteristics of rice flour dough during fermetation obtained by different milling methods were determined by Rheofermentometer F3 (Tripette et Renaud, France). The rice flour dough was prepared using the dough preparation method in the bread making process, and 250 g of the dough was weighed and placed in a fermentation basket for measurement. The test was carried out at 38 °C with a weight of 0 kg and a test time of 3 h.
The morphology of the rice flour particles from the different milling methods are shown in the scanning electron microscope images (Fig. 1). As shown in Fig. 1A, A', the starch granules from WF were intact, small and round, with a smooth surface. CF involves milling the rice granules using the high-speed rotary impact of a pulverizing blade, so the shape of the flour particles was irregular, the starch granules bare, with an increased number of small particles (Fig. 1B, B'). The particles of UF had sharp edges and a sheet-like shape, and their starch structure was destroyed, exposing a large amount of the starch granule (Fig. 1C, C').
2.12. Scanning electron microscopy (SEM) analysis For SEM analysis, the chopped bread crumbs were frozen in liquid nitrogen and lyophilized, and the rice flour samples were also lyophilized to remove moisture. The freeze-dried samples were sputter coated with gold-palladium to make them electrically conductive. The sample was then examined and an image was recorded with a scanning electron microscope (SEM Hitachi S-570, Hitachi, Co., Ltd.,Tokyo, Japan) at an acceleration voltage of 10 kV. The rice flour samples were observed at magnifications of ×1,000 and ×2,500 and the rice bread samples were observed at magnifications of ×50 and ×1,000. In the case of ×1,000 magnification, the outside and inside of the breadcrumbs were examined.
3.3. Gelatinization characteristics of rice flour
Data were analyzed using statistical software Origin 9.0 and SPSS 19.0. Analysis of variance (ANOVA) followed by multiple comparisons of Duncan and LSD tests were used to assess statistical differences between the mean values (p < 0.05).
The gelatinization characteristics of rice flour were shown in Table 2. The gelatinization temperature of WF was 67.6 °C, with that of UF was only 48.4 °C, which may be related to the integrity and size of starch granules (Marshall, 1992). The gelatinization temperature and damaged starch content were negatively correlated. The WF particles exhibited the highest peak viscosity, while the UF was less than half of the WF peak viscosity, only 326, probably because of the high starch content of UF, consistent with the results reported by Heo et al. (Heo, Lee, Shim, Yoo, & Lee, 2013). The WF particles exhibited a higher peak viscosity than the dry-milled flours. The UF particles exhibited the lowest disintegration value, indicating their high stability. The retrogradation value usually reflects the degree of aging or regeneration of the starch paste, i.e. the strength of the cooled gel formed. The regenerated values for CF and WF were significantly higher than those for UF, indicating their greater gel strength.
3. Results and discussion
3.4. Gel strength of rice flour
3.1. Basic ingredients of rice flour
It can be seen from Table 3 that the gel properties of different rice flours are quite different. The mean hardness of the WF gel was 3.2 N, which was significantly higher than that of the UF gel, but not significantly different from that of the CF gel. Regarding adhesiveness, the WF exhibited a significantly higher viscosity (p < 0.05) than the CF and UF which did not differ significantly (p > 0.05). In terms of
2.13. Statistical analysis
Table 1shows that the different milling methods had no significant effect on the total starch, crude fat and crude protein contents of the rice flour. The different milling methods had a great effect on the content of damaged starch of the rice flour which increased with the
Table 1 Effect of different milling methods on the characteristics and particle size distribution of rice flour. Sample
UF CF WF
Properties
Particle size distributions
Crude protein (g/100 g)
Crude fat (g/100 g)
Total starch (g/100 g)
Damaged starch (g/100 g)
D30 (μm)
D50 (μm)
D90 (μm)
4.0 ± 0.5a 4.4 ± 0.5a 4.3 ± 0.1a
0.31 ± 0.05a 0.33 ± 0.08a 0.28 ± 0.02a
88.6 ± 0.7a 87.7 ± 0.8a 89.2 ± 1.2a
54.41 ± 0.65a 12.02 ± 0.31b 2.80 ± 0.12c
20.4 ± 0.0c 43.6 ± 0.3a 24.2 ± 0.1b
32.4 ± 0.1c 74.2 ± 0.5a 37.8 ± 0.0b
86.5 ± 1.0c 191.5 ± 2.1a 108.5 ± 0.7b
UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively. Different letters within a column indicate significant differences between mean values (n = 3) (p < 0.05). 139
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Fig. 1. Scanning electron microscope images of the microstructure of rice flour (left: × 1000, right: × 2500). Note: A, B, and C show rice flour from wet-milling (WF), cyclone-milling (CF), and ultrafine-milling (UF), respectively.
springiness, the values of CF and UF were higher than that of WF. The chewiness values of WF and CF were higher than that of UF, and the recovery performance was also good. The gel hardness value of rice flour gels can reflect the gel properties of starch. The hardness of starch gels is affected by many factors, such as the content of damaged starch, the structural properties of rice granules and the composition and structure of starch (Wang et al., 2010). Compared with WF and CF, UF has the lowest gel hardness, this may be related to the damaged starch contents caused by the different milling methods. The content of damaged starch of UF was greatest, so the gel formed after gelatinization was weak, making it difficult to form
a network structure strong enough to support rice flour bread. This also caused the rice flour bread prepared with UF to collapse after baking and cooling. 3.5. DSC analysis Table 4 shows that different milling methods had a significant effect on the thermodynamic properties of the rice flour. The damaged starch content of the UF was the highest (Table 1), resulting in the smallest To value (Table 4), while the WF with the lowest damaged starch content and intact starch granules exhibited the highest To value. To has been
Table 2 Effect of different milling methods on the gelatinization characteristics of rice flours measured using a Brabender viscometer. Rice flour
Initial pasting temperature (°C)
Viscosity (BU) Peak
Trough
(P) UF CF WF
c
48.4 ± 0.4 62.3 ± 0.9b 67.6 ± 0.0a
Final
(T) c
326.0 ± 1.4 611.5 ± 7.8b 694.0 ± 2.8a
Breakdown
(F) c
128.0 ± 4.2 352.0 ± 0.0b 416.5 ± 2.1a
Setback
(P-T) c
217.5 ± 3.5 625.5 ± 2.1b 699.5 ± 4.9a
(F-T) c
199.0 ± 2.8 257.5 ± 6.4b 276.5 ± 0.7a
96.5 ± 3.5b 404.0 ± 5.7a 379.0 ± 4.2a
UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively. Different letters within a column indicate significant differences between mean values (n = 3) (p < 0.05). 140
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Table 3 Effect of different milling methods on the gel strength of rice flours measured by a Texture Analyzer. Rice flour UF CF WF
Hardness/N
Adhesiveness/N·s b
1.03 ± 0.022 3.14 ± 0.209a 3.20 ± 0.125a
Springiness b
Cohesion ab
−6.583 ± 0.060 −5.798 ± 0.510b −9.966 ± 0.144a
93.2 ± 1.64 96.8 ± 0.41a 89.4 ± 1.18b
Chewiness/N b
0.53 ± 0.03 0.56 ± 0.01ab 0.60 ± 0.01a
b
52.4 ± 0.2 178.1 ± 8.7a 173.8 ± 7.4a
Resilience 0.02 ± 0.000c 0.11 ± 0.009a 0.09 ± 0.001b
UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively. Different letters within a column indicate significant differences between mean values (n = 3) (p < 0.05). Table 4 Effect of different milling methods on results of DSC analysis of rice flours. Rice flour
To/°C
Tp/°C
UF CF WF
45.79 ± 0.04d 50.72 ± 0.38c 53.55 ± 0.39a
55.85 ± 0.49b 62.09 ± 0.33a 61.70 ± 0.27a
Tc/°C
Tr/°C 63.25 ± 0.08b 69.98 ± 0.42a 68.82 ± 0.60a
ΔH/J.g-1 17.46 ± 0.12b 19.26 ± 0.80a 15.27 ± 0.22c
1.87 ± 0.07c 7.92 ± 0.08b 9.99 ± 0.02a
UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively. Different letters within a column indicate significant differences between mean values (n = 3) (p < 0.05). Fig. 2. Effect of different milling methods on (a) changes in the storage (G′, black solid) and loss (G″, white open) moduli of rice flour dough (▲: CF, ■: WF, ●: UF, △: CF, □: WF, ○: UF) with frequency and (b) changes in tan δ of rice flour dough (▲: CF, ■: WF, ●: UF) with frequency. (UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wetmilled flour, respectively).
Fig. 3. Effect of different milling methods on (a) changes in dough fermentation curve (Black, blue, red and lines are WF, CF, UF,respectively) and (b) changes in gas release curve (A1, retention volume) (A2, volume of CO2 lost) (The picture is WF, CF, UF from top to bottom). (UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
reported to be related to the milling method, the damaged starch content, and the particle size distribution (Hasjim, Li, & Dhital, 2013). The peak temperature, Tp, and termination temperature, Tc, were negatively correlated with the volume median diameter (Hasjim et al., 2013). There was no significant difference between the termination temperature, Tc, of the WF and CF with that of the UF being lower because of its fine particle size. The gelatinization temperature range, Tr, is related to the size and uniformity of the starch granules. The three milling methods had a significant effect on the gelatinization temperature range of the rice flour: that of WF was the smallest and that of CF the largest. It was possible that the particle size of WF, being less than that of CF, required less heat to cause the starch to gelatinize. There was a significant difference in the gelatinization absorption heat enthalpy, ΔH, during the transition from a crystalline to an amorphous structure. WF had a low content of damaged starch so the starch granules were almost completely intact, so required more heat to gelatinize the starch. The UF starch had a high content of damaged and incomplete particles,
Fig. 4. Effect of different milling methods on the whole and cross-sectional images of the crumbs of rice bread. UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively. 141
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Table 5 Effect of different milling methods on the textural properties of rice bread. Rice flour sample
Textural properties Hardness (N)
UF CF WF
Specific volume (mL/g) Adhesiveness (N.sec)
a
30.82 ± 1.24 21.85 ± 2.16b 9.44 ± 0.41c
a
−0.013 ± 0.003 −0.011 ± 0.001b −0.004 ± 0.001c
Resilience (%)
cohesion
b
Springiness (%) b
23.65 ± 3.42 23.53 ± 0.25b 33.32 ± 0.47a
0.57 ± 0.05 0.53 ± 0.02b 0.65 ± 0.01a
b
84.7 ± 8.44 83.9 ± 1.24b 95.4 ± 1.28a
Chewiness 14.74 ± 3.34a 9.72 ± 0.81b 5.81 ± 0.27c
1.2 ± 0.1c 2.3 ± 0.2b 3.1 ± 0.1a
UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively. Measurements of specific volume and weight loss were performed in duplicate. Measurements of textural properties were made on two central slices from two loaves of each dough. Different letters within a column indicate significant differences between mean values (n = 3) (p < 0.05).
Fig. 5. Scanning electron microscope images of the microstructure of rice bread (left: × 50, right: × 1000). Note: D, E, and F are internal images of breadcrumbs prepared from wet-milled, cyclone-milled and ultrafine-milled flour, respectively (black arrows indicate starch residues, and white arrows deformed starch granules).
resulting in less heat absorption.
This indicated that the dough was harder at low frequencies then as the frequency increased, the dough showed a transition from being solidlike to liquid-like. These results are related to the content of damaged starch of the sample and the granular structural integrity of the starch (Han, Campanella, Mix, & Hamaker, 2002). UF had the highest content of damaged starch, resulting in the lowest values of G' and G'', indicating that the elasticity and viscosity of its dough were weak, and therefore not conducive to the production and proofing processes required for breadmaking. In contrast, the WF and CF, with their lower contents of damaged starch, exhibited higher values of G' and G''.
3.6. Dynamic rheological properties of rice flour dough Fig. 2 shows that all frequencies tested, the storage modulus (G') was higher than the loss modulus (G'') for all dough samples, indicating that the elastic characteristics of the dough were higher than its viscous characteristics. As the frequency increases, the G' and G'' of all samples increases, indicating that the viscoelasticity is increased. The G' and G'' of UF dough are the lowest, indicating that their viscoelasticity is poor. At lower scan frequencies, the dough prepared from the UF exhibited the lowest tan δ, and as the frequency increased, tan δ also increased. 142
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3.7. Fermentation rheological properties of rice flour dough
4. Conclusions
Fig. 3 (a) shows that the maximum height of the dough was in the order: WF > CF > UF. The dough of WF was highest at 32.1 mm with dough prepared from UF being lowest at only 4.8 mm. The time for the dough to reach its maximum height was also in the order: WF > CF > UF, with respective times of 20, 25, and 35 min. Fig. 3 (b) shows that the gas release rate for UF was the most rapid. This was probably caused by the high content of damaged starch in UF, which the yeast was more likely to use and so produce gas (Purna, Miller, Seib, Graybosch, & Shi, 2011). The UF dough lost the largest volume of CO2 (A2), while WF dough lost the smallest. The WF dough had the largest retention volume (A1) and UF dough the smallest, which led to a much higher volume for WF dough during fermentation.
Compared to ultrafine and cyclone powders, rice flour prepared by wet milling has lower impaired starch content, more intact starch granules, higher gelatinization temperature and absorption enthalpy, and higher gel strength. The texture and appearance of bread prepared with wet ground rice flour is best suited to meet consumer expectations compared to ultrafine and cyclonic. Therefore, for making gluten-free rice bread, it is essential to choose the appropriate milling method for the rice. Rice flour with a low damaged starch content, intact starch granules and moderate particle size is a good raw material for preparing gluten-free rice bread. In future studies, the appropriate rice flour will be selected to prepare desirable gluten-free rice bread using sensory evaluation. Acknowledgments
3.8. Rice bread characteristics
This work was supported by the National Natural Science Foundation of China (No. 31471617), the National Key Research and Development Program (2016YFD0400205-101) and the Fundamental Research Funds for the Central Universities (No. JUSRP51708A). This research was also partly funded by the Innovation Project of Chinese Academy of Agricultural Sciences (CAAS) and national first-class discipline program of Food Science and Technology (JUFSTR20180103). We thank Philip Creed, PhD, from Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.
The whole and cross-sectional images of rice flour bread are shown in the Fig. 4. The bread prepared with WF exhibited the highest specific volume, the best pore structure. The bread made with CF was of a small volume, with small, unevenly-dispersed internal pores. The UF bread had the smallest volume and almost no internal porosity. After cooling at room temperature, the top surface of the UF bread caved in, even though the expansion had remained constant during the baking process in the oven. This was related to the high content of damaged starch and the state of the starch in UF. The specific volume and textural characteristics of the rice breads are listed in Table 5. The bread made with UF had the highest hardness, while WF bread had the lowest hardness. Compared to the bread made with UF and CF, the WF bread had the highest specific volume, springiness and resilience was also the best. The main reason for this were the differences in the content of damaged starch in the different rice flours. The specific volume of rice bread was found to decrease as the content of damaged starch in the rice flour increased (Masakatsu, Chie, Nao, & Kumiko, 2012). Different milling methods have been shown to affect the specific volume of the bread, the size of the bubbles and the uniformity of the breadcrumb (Lee & Lee, 2006). Bread prepared from CF formed irregular and uneven bubbles, whose size in the crumbs was smaller than in bread prepared using WF. Therefore, it would be expected that bread prepared from rice flour with a low content of damaged starch would form more bubbles, and increase the volume of the baked rice bread.
References Alvarezjubete, L., Auty, M., Arendt, E. K., & Gallagher, E. (2010). Baking properties and microstructure of pseudocereal flours in gluten-free bread formulations. European Food Research & Technology, 230(3), 437. Demirkesen, I., Sumnu, G., & Sahin, S. (2013). Image analysis of gluten-free breads prepared with chestnut and rice flour and baked in different ovens. Food and Bioprocess Technology, 6(7), 1749–1758. Di Sabatino, A., & Corazza, G. R. (2009). Coeliac disease. The Lancet, 373(9673), 1480–1493. Dubé, C., Rostom, A., Sy, R., Cranney, A., Saloojee, N., Garritty, C.,., et al. (2005). The prevalence of celiac disease in average-risk and at-risk Western European populations: A systematic review. Gastroenterology, 128(4), S57–S67. FAO (2013). FAO (2013) FAO statistical yearbook world food and agriculture. Food and Agriculture Organization of the United Nations). Gujral, H. S., & Rosell, C. M. (2004). Functionality of rice flour modified with a microbial transglutaminase. Journal of Cereal Science, 39(2), 225–230. Han, X. H., Campanella, O. H., Mix, N. C., & Hamaker, B. R. (2002). Consequence of starch damage on rheological properties of maize starch pastes. Cereal Chemistry, 79(6), 897–901. Hasjim, J., Li, E., & Dhital, S. (2013). Milling of rice grains: Effects of starch/flour structures on gelatinization and pasting properties. Carbohydrate Polymers, 92(1), 682–690. Heo, S., Lee, S. M., Shim, J. H., Yoo, S. H., & Lee, S. (2013). Effect of dry- and wet-milled rice flours on the quality attributes of gluten-free dough and noodles. Journal of Food Engineering, 116(1), 213–217. Kadan, R. S., Bryant, R. J., & Miller, J. A. (2010). Effects of milling on functional properties of rice flour. Journal of Food Science, 73(4), E151–E154. Kang, J. Y., Kang, A. H., Green, A., Gwee, K. A., & Ho, K. Y. (2013). Systematic review: Worldwide variation in the frequency of coeliac disease and changes over time. Alimentary Pharmacology and Therapeutics, 38(3), 226–245. Kim, J. M., & Shin, M. (2014). Effects of particle size distributions of rice flour on the quality of gluten-free rice cupcakes. LWT - Food Science and Technology, 59(1), 526–532. Kumar, C., Malleshi, N. G., & Bhattacharya, Sila (2008). A comparison of selected quality attributes of flours: Effects of dry and wet grinding methods. International Journal of Food Properties, 11(4), 845–857. Lee, M. H., & Lee, Y. T. (2006). Bread-making properties of rice flours produced by dry, wet and semi-wet milling. Journal of the Korean Society of Food Science & Nutrition, 35(7). Li, T. F., Su, Y. Q., & Liu, D. N. (2016). Research on status Quo,Difficult position and development strategy of rice processing industry. Food Industry, 37(10), 224–229. Makharia, G. K., Mulder, C. J. J., Goh, K. L., Ahuja, V., Bai, J. C., Catassi, C., et al. (2014). Issues associated with the emergence of coeliac disease in the asia-pacific region: A working party report of the world gastroenterology organization and the asian pacific association of gastroenterology. Journal of Gastroenterology and Hepatology, 29(4), 666–677. Marshall, W. E. (1992). Effect of degree of milling of brown rice and particle size of milled
3.9. SEM image analysis A film-like structure can be formed by the interaction between starch and protein, which envelopes more gas, and thus increase the bread volume (Alvarezjubete, Auty, Arendt, & Gallagher, 2010). Fig. 5 shows that the pores formed in the WF rice bread were large, with those of CF being smaller. The UF-prepared bread, with smaller and fewer internal pores, came from dough with a weak gas holding capacity, resulting in a large amount of gas leakage during fermentation, and so a small bread volume. The SEM images of the breadcrumb at a magnification of × 1000 show that the WF rice bread had more granular residues, because the WF starch granules were the most intact, and its gelatinization temperature was high. This can make it more difficult to achieve complete gelatinization of the starch during baking (Demirkesen, Sumnu, & Sahin, 2013). Therefore, compared with the CF and the UF, bread prepared with WF had more granular residues. The inclusion of a certain quantity of incompletely disintegrated starch granules during the baking process appears to be beneficial for improving bread quality. The reason may be that incompletely gelatinized starch granules, gelatinized starch granules and protein can form a good network structure (Purna et al., 2011), so that the bread is sufficiently rigid to support the structure of the bread without collapsing. 143
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T. Wu, et al. rice on starch gelatinization. Cereal Chemistry, 69(6), 632–636. Masakatsu, Y., Chie, N., Nao, S., & Kumiko, H. (2012). Effect of rice flour milling methods on the quality of rice flour bread. Journal of Nagoya Bunri University, 12, 31–38. Purna, S. K. G., Miller, R. A., Seib, P. A., Graybosch, R. A., & Shi, Y. C. (2011). Volume, texture, and molecular mechanism behind the collapse of bread made with different levels of hard waxy wheat flours. Journal of Cereal Science, 54(1), 37–43. Tong, L. T., Gao, X., Lin, L., Liu, Y., Zhong, K., Liu, L., ... Zhou, S. (2015). Effects of semidry flour milling on the quality attributes of rice flour and rice noodles in China. Journal of Cereal Science, 62, 45–49. Tong, L. T., Zhu, R., Zhou, X., Zhong, K., Wang, L., Liu, L., ... Zhou, S. (2017). Soaking
time of rice in semidry flour milling was shortened by increasing the grains cracks. Journal of Cereal Science, 74, 121–126. Torbica, A., Hadnađev, M., & Dapčević, T. (2010). Rheological, textural and sensory properties of gluten-free bread formulations based on rice and buckwheat flour. Food Hydrocolloids, 24(6–7), 626–632. Wang, Lan, Xie, Bijun, Shi, John, Xue, Sophia, Deng, Qianchun, Yu, Wei, et al. (2010). Physicochemical properties and structure of starches from Chinese rice cultivars. Food Hydrocolloids, 24(2), 208–216. You, Xiuling (1994). Origin and cultivation history of rice in China. Chinese Rice Research Newsletter, 2(1), 9–10.
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Effect of milling methods on the properties of rice flour and gluten-free rice bread
T
Tong Wua,b,c,1, Lili Wangb,1, Yan Lia,c, Haifeng Qiana,c, Liya Liub, Litao Tongb, Xianrong Zhoub, Li Wanga,c,∗, Sumei Zhoub,∗∗ a
State Key Laboratory of Food Science and Technology, Jiangnan University, 1800 Lihu Avenue, Wuxi, 214122, China Institute of Food Science and Technology, Chinese Academy of Agricultural Sciences, Beijing, 100193, China c School of Food Science and Technology, Jiangnan University, 1800 Lihu Avenue, Wuxi, 214122, China b
ARTICLE INFO
ABSTRACT
Keywords: Rice flour Rice milling Damaged starch Gluten-free Rice bread
The properties of rice flour are very important for making gluten-free rice bread. In this study, rice flour was prepared using different milling methods (wet-milling, cyclone-milling and ultrafine-milling) to investigate their effect on the properties of gluten-free rice bread. The milling method was found to significantly affect the damaged starch content, state of the starch granule, gelatinization temperature and absorption enthalpy of rice flour (p < 0.05). The starch granules of flour prepared by wet milling had a high integrity, low damaged starch content (2.80 g/100 g), high gelatinization temperature, and high values of absorption enthalpy and gel strength compared to ultrafine-milling and cyclone-milling methods. As the deformation of the starch in the rice grains decreased, the amount of completely gelatinized starch granules in the bread decreased, with its specific volume also decreasing from 3.1 to 1.2 mL/g. Increase in starch damage content was observed to decrease the specific volume of bread, consequently increasing the hardness of bread. Also, uneven pores were observed, and the number and size of internal pores decreased. This indicated that wet-milled rice flour with a low content of damaged starch and higher starch granules integrity produced better gluten-free rice bread.
1. Introduction Celiac disease (CD) is an autoimmune enteropathy induced in individuals with susceptible genes when ingesting gluten-containing grains such as wheat, barley, and buckwheat, and their products (Di Sabatino & Corazza, 2009). CD has been recognized by the medical community for almost 100 years. The incidence of CD in Caucasians has been found to be 1% (Dubé et al., 2005) with its incidence reported to be rising in Asia (Makharia et al., 2014) and also in China (Kang, Kang, Green, Gwee, & Ho, 2013). Because of its diverse and complex causes, the only feasible and effective way to relieve its symptoms is to give patients a gluten-free diet. In general, the raw grain materials used for producing gluten-free foods are rice, corn, sorghum, buckwheat, millet, and potato. Of these materials, rice has the properties of being easily digested and absorbed, and it is also hypoallergenic, mild and colorless, with a high yield (Gujral & Rosell, 2004; Torbica, Hadnađev, & Dapčević, 2010).
Therefore, many studies on using rice to prepare bread, cakes, biscuits, and noodles have been conducted. Rice (Oryza sativa L) is a crop grown worldwide and is an important staple food for about 50% of the world's population, mainly in Asia (FAO, 2013, pp. 1–289). China has cultivated rice for more than 5,000 years (You, 1994), with its current production ranking first in the world (Li, Su, & Liu, 2016). China has many traditional products that use rice as the raw material, such as rice cakes, rice noodles, rice wine, rice sponge cake, and rice dumplings. The consumption habits of Chinese people have also been changing from traditional foods to non-traditional foods such as bread, cakes and pasta, usually made from wheat flour. Rice bread made from rice flour may provide a new direction for rice processing, improve rice usage, and meet the demand for bread for people who are allergic to gluten. Different milling methods have been reported to affect the properties of rice flour significantly (Kadan, Bryant, & Miller, 2010), especially the particle size of the rice flour, the content of damaged starch, and the state of the starch granules. Kim & Shin (Kim & Shin, 2014) studied the
∗ Corresponding author. State Key Laboratory of Food Science and Technology, School of Food Science and Technology, Jiangnan University, 1800 Lihu Avenue, Wuxi, 214122, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (L. Wang), [email protected] (S. Zhou). 1 These authors contributed equally to this work.
https://doi.org/10.1016/j.lwt.2019.03.050 Received 11 December 2018; Received in revised form 15 March 2019; Accepted 16 March 2019 Available online 20 March 2019 0023-6438/ © 2019 Elsevier Ltd. All rights reserved.
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effect of the particle size of rice flour on gluten-free rice cupcakes, finding that those prepared with moderately-sized rice flour particles had a better quality. Previous studies in our laboratory have found that the damaged starch content in rice flour had a great influence on the quality of rice noodles (Tong et al., 2015). Wet milling is a traditional method used in China with many studies showing that it is more suitable for producing rice flour than dry milling (Kumar, Malleshi, & SilaBhattacharya, 2008). However, wet milling generates a large quantity of wastewater which pollutes the environment (Tong et al., 2017). In contrast, dry milling produces less wastewater, but its particle size of the powder is too big. It is possible to obtain rice powder with a fine particle size without generating a large amount of wastewater by using the ultrafine milling method. The present study therefore aims to compare the rheological properties of rice flour doughs using rice flour prepared by different milling methods and their effects on rice bread regarding its volume, texture and quality characteristics to improve the quality of rice bread.
Germany). 12 g sample (14% moisture content) was dispersed in 100 mL of distilled water. The suspension was warmed up according to the following procedure: ramping from 30 °C to 95 °C at a rate of 7.5 °C/ min, holding at 95 °C for 5 min, then cooling to 50 °C at a rate of 7.5 °C/ min, holding at 50 °C for 5 min. The mixer speed was 250 r/min and the viscosity was expressed as BU. 2.6. Gel strength of rice flour A certain amount of rice flour was added with water to form a suspension with a mass fraction of 15% (on a dry basis). The mixture was heated and stirred for 1 min on a magnetic heating stirrer, and the pre-gelatinized starch paste was equally distributed into three 10 mL beakers, and the gelatinization was continued for 30 min in a 95 °C water bath after sealing. After being removed from water bath, it was stabilized at room temperature and cooled at 4 °C overnight. The Texture Analyzer (TA, TA-XT2i, Stable Micrio System, England) was used to measure the gel strength using the TPA (texture profile analysis) mode. The test probe P/0.5R was selected. The pre-test speed and test speed were 0.5 mm/s, the measured speed was 5.0 mm/s, the compression distance was 10.0 mm, the triggering force was 5.0 g, the compression interval was 2 s. The experiment was repeated three times, and the average was taken.
2. Materials and methods 2.1. Materials Non-waxy white Wuchang rice was purchased from Wuchang Wanfu Rice Industry Co., Ltd.(Harbin, Heilongjiang province of China) in 2017. Psyllium gum and Sodium caseinate were purchased from Zhengzhou, Henan province. Extruded rice flour was obtained from Nanchang, Jiangxi province. White sugar, butter and the instant dry yeast were obtained from a local market in Beijing China.
2.7. DSC analysis Thermal behaviour measurements were performed on a DSC-Q200 differential scanning calorimeter (TA, USA) using the aluminum pans. 4 mg (dry weight) of rice flour was added to the crucible, distilled water in a ratio of 1:2 (w/w) was added, and the mixture was allowed to stand overnight after sealing. The sealed blank was used as a control, and the thermal characteristic curve was measured by heating from 20 °C to 110 °C at a rate of 5 °C/min. The starting temperature (To), the peak temperature (Tp), the termination temperature (Tc), the gelatinization temperature range (Tr), and the gelatinization absorption heat enthalpy (ΔH) were obtained by analyzing the spectrum in the DSC software.
2.2. Preparation of rice flour Three kinds of rice flour were prepared by wet-milled flour, cyclonemilled flour and Ultrafine-milled flour, respectively. For wet-milled flour, 1 kg of rice (14% moisture content, wet basis) were soaked in 2 L water at room temperature for 24 h, followed by ground with a grinder (YU8022, Wet miller, Hebei, China). For the cyclone-milled flour, the rice was pulverized into flour using a cyclone mill (CT410, FOSS Scino (Suzhou) Co., Ltd., Suzhou, China) and passed through a 100 mesh sieve. For Ultrafine-milled flour, rice was grinded with an Ultrafine grinder and passed through a 100 mesh sieve. The three rice flours were freeze-dried till to its moisture content 5% (wet basis) and then stored at 4 °C for further analysis.
2.8. Bread making procedure The bread formula for bread consisted of 30 g rice flour with different milling treatments, 7.5 g white sugar, 5 g butter, 5 g sodium caseinate, 2.5 extrusion rice flour, 0.5 g psyllium gum, 0.5 g instant dry yeast. The psyllium gum was pre-dissolved in 60 °C hot water, and cooled to room temperature and finally mixed with white sugar and instant dry yeast. The rice flour and extrusion rice flour were poured into the mixture and mixed for 4 min with a pin mixer, then the butter was added and stirred for 4 min. The dough was then divided into 70 g of dough and placed in a baking tray and fermented for 60 min (temperature 38 °C, humidity 80%). The oven was preheated for 30 min and bread was baked at 150 °C, for 15 min and taken out. The bread was cooled for one hour at room temperature before any further analysis.
2.3. Determination of rice flour properties Moisture, protein and lipid contents of rice flour were measured according to AACC methods 44-15A, 46-11A, and 30-10, respectively (2000). The total starch content and the degree of starch damage of the different rice flours were measured using a Total Starch Kit and a Starch Damage Assay Kit (K-SDAM, Megazyme International Ltd., Wicklow, Ireland). Experiment repeated three times, and the average was taken. 2.4. Measurement of particle size distributions
2.9. Evaluation of bread quality
Particle size distribution of three kinds of rice flour was determined by Malvern MS3000 laser particle size analyzer (MS3000, Malvern instruments, UK). Air was chosen as the optical model of light scattering and the refractive index (1.000) was used. After determining the background light scattering, 5 g of the sample powder was poured into the apparatus to start measurement, and then the results of particle size distribution was shown after 2 min.
Bread volume was measured by the seed displacement method (AACC 72-10), and specific volume was calculated from the volume divided by weight. The textural properties of the bread crumbs were measured using a compression test with a Texture Analyzer (TA, TAXT2i, Stable Micrio System, England). Rice flour bread was prepared one day in advance, and then cut into small pieces of 2 × 2 × 1.5 cm. The bread slice was subjected to a double compression cycle with a 50% original height deformation using a 36 mm flat aluminum compression disk (probe P/36). The measuring conditions are as follows: pre-test/ post-measurement speed of 2.0 mm/s, test speed of 1 mm/s, relaxation time of 5 s; trigger force of 10 g; trigger mode - automatic.
2.5. Gelatinization characteristics of rice flour The gelatinization properties of rice flour were determined using a Brabender viscometer (MVAG803202, Brabender Gmbh & Co.KG, 138
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severity of processing. More mechanical and thermal energy is produced during dry milling than wet milling, resulting in higher levels of damaged rice starch (Kumar et al., 2008). Table 1 also shows that the different milling methods had a significant effect on the particle size of the rice flour. The average particle sizes of WF and UF were 37.8 μm and 32.4 μm, respectively. However, the UF particle size distribution was the narrowest, from only 20.4–86.5 μm. CF exhibited the largest average particle size of 74.2 μm and the widest particle size distribution, from 43.6 to 191.5 μm. Producing UF involves a high mechanical impact on grains during the highspeed operation and a considerable impact between the grinding media and the grains during honing, leading to a small particle size and a narrow distribution range. For WF, the rice grains were easily pulverized because of the soaking and consequent softening so that the rice flour obtained had a particle size slightly larger than that of UF.
2.10. Rheological properties of rice flour dough The following ingredients (g/100 g flour) were used to make dough: sugar (25 g/100 g), butter (16.7 g/100 g), Sodium caseinate (16.7 g/ 100 g), extruded rice flour (8.3 g/100 g), Psyllium gum (1.67 g/100 g). The rheological properties of the fresh dough were measured by a rheometer (Physica MCR 301, Anton Paar GmbH, Stutgart, Germany). The freshly prepared dough sample was placed on a test platform, the mold (2.5 cm in diameter, flat plate) was pressed down, the control plate spacing was 1.0 mm, the excess sample was scraped off, and the silicone oil was sealed to prevent water loss during the test. After standing for 5 min and releasing the strain, the storage modulus (G') and the loss modulus (G") were tested with the scanning frequency (ω) at 25 °C, strain 0.1%, and frequency range 0.1–100 Hz). 2.11. Fermentation rheological properties of rice flour dough
3.2. Rice flour microstructure
The rheological characteristics of rice flour dough during fermetation obtained by different milling methods were determined by Rheofermentometer F3 (Tripette et Renaud, France). The rice flour dough was prepared using the dough preparation method in the bread making process, and 250 g of the dough was weighed and placed in a fermentation basket for measurement. The test was carried out at 38 °C with a weight of 0 kg and a test time of 3 h.
The morphology of the rice flour particles from the different milling methods are shown in the scanning electron microscope images (Fig. 1). As shown in Fig. 1A, A', the starch granules from WF were intact, small and round, with a smooth surface. CF involves milling the rice granules using the high-speed rotary impact of a pulverizing blade, so the shape of the flour particles was irregular, the starch granules bare, with an increased number of small particles (Fig. 1B, B'). The particles of UF had sharp edges and a sheet-like shape, and their starch structure was destroyed, exposing a large amount of the starch granule (Fig. 1C, C').
2.12. Scanning electron microscopy (SEM) analysis For SEM analysis, the chopped bread crumbs were frozen in liquid nitrogen and lyophilized, and the rice flour samples were also lyophilized to remove moisture. The freeze-dried samples were sputter coated with gold-palladium to make them electrically conductive. The sample was then examined and an image was recorded with a scanning electron microscope (SEM Hitachi S-570, Hitachi, Co., Ltd.,Tokyo, Japan) at an acceleration voltage of 10 kV. The rice flour samples were observed at magnifications of ×1,000 and ×2,500 and the rice bread samples were observed at magnifications of ×50 and ×1,000. In the case of ×1,000 magnification, the outside and inside of the breadcrumbs were examined.
3.3. Gelatinization characteristics of rice flour
Data were analyzed using statistical software Origin 9.0 and SPSS 19.0. Analysis of variance (ANOVA) followed by multiple comparisons of Duncan and LSD tests were used to assess statistical differences between the mean values (p < 0.05).
The gelatinization characteristics of rice flour were shown in Table 2. The gelatinization temperature of WF was 67.6 °C, with that of UF was only 48.4 °C, which may be related to the integrity and size of starch granules (Marshall, 1992). The gelatinization temperature and damaged starch content were negatively correlated. The WF particles exhibited the highest peak viscosity, while the UF was less than half of the WF peak viscosity, only 326, probably because of the high starch content of UF, consistent with the results reported by Heo et al. (Heo, Lee, Shim, Yoo, & Lee, 2013). The WF particles exhibited a higher peak viscosity than the dry-milled flours. The UF particles exhibited the lowest disintegration value, indicating their high stability. The retrogradation value usually reflects the degree of aging or regeneration of the starch paste, i.e. the strength of the cooled gel formed. The regenerated values for CF and WF were significantly higher than those for UF, indicating their greater gel strength.
3. Results and discussion
3.4. Gel strength of rice flour
3.1. Basic ingredients of rice flour
It can be seen from Table 3 that the gel properties of different rice flours are quite different. The mean hardness of the WF gel was 3.2 N, which was significantly higher than that of the UF gel, but not significantly different from that of the CF gel. Regarding adhesiveness, the WF exhibited a significantly higher viscosity (p < 0.05) than the CF and UF which did not differ significantly (p > 0.05). In terms of
2.13. Statistical analysis
Table 1shows that the different milling methods had no significant effect on the total starch, crude fat and crude protein contents of the rice flour. The different milling methods had a great effect on the content of damaged starch of the rice flour which increased with the
Table 1 Effect of different milling methods on the characteristics and particle size distribution of rice flour. Sample
UF CF WF
Properties
Particle size distributions
Crude protein (g/100 g)
Crude fat (g/100 g)
Total starch (g/100 g)
Damaged starch (g/100 g)
D30 (μm)
D50 (μm)
D90 (μm)
4.0 ± 0.5a 4.4 ± 0.5a 4.3 ± 0.1a
0.31 ± 0.05a 0.33 ± 0.08a 0.28 ± 0.02a
88.6 ± 0.7a 87.7 ± 0.8a 89.2 ± 1.2a
54.41 ± 0.65a 12.02 ± 0.31b 2.80 ± 0.12c
20.4 ± 0.0c 43.6 ± 0.3a 24.2 ± 0.1b
32.4 ± 0.1c 74.2 ± 0.5a 37.8 ± 0.0b
86.5 ± 1.0c 191.5 ± 2.1a 108.5 ± 0.7b
UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively. Different letters within a column indicate significant differences between mean values (n = 3) (p < 0.05). 139
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Fig. 1. Scanning electron microscope images of the microstructure of rice flour (left: × 1000, right: × 2500). Note: A, B, and C show rice flour from wet-milling (WF), cyclone-milling (CF), and ultrafine-milling (UF), respectively.
springiness, the values of CF and UF were higher than that of WF. The chewiness values of WF and CF were higher than that of UF, and the recovery performance was also good. The gel hardness value of rice flour gels can reflect the gel properties of starch. The hardness of starch gels is affected by many factors, such as the content of damaged starch, the structural properties of rice granules and the composition and structure of starch (Wang et al., 2010). Compared with WF and CF, UF has the lowest gel hardness, this may be related to the damaged starch contents caused by the different milling methods. The content of damaged starch of UF was greatest, so the gel formed after gelatinization was weak, making it difficult to form
a network structure strong enough to support rice flour bread. This also caused the rice flour bread prepared with UF to collapse after baking and cooling. 3.5. DSC analysis Table 4 shows that different milling methods had a significant effect on the thermodynamic properties of the rice flour. The damaged starch content of the UF was the highest (Table 1), resulting in the smallest To value (Table 4), while the WF with the lowest damaged starch content and intact starch granules exhibited the highest To value. To has been
Table 2 Effect of different milling methods on the gelatinization characteristics of rice flours measured using a Brabender viscometer. Rice flour
Initial pasting temperature (°C)
Viscosity (BU) Peak
Trough
(P) UF CF WF
c
48.4 ± 0.4 62.3 ± 0.9b 67.6 ± 0.0a
Final
(T) c
326.0 ± 1.4 611.5 ± 7.8b 694.0 ± 2.8a
Breakdown
(F) c
128.0 ± 4.2 352.0 ± 0.0b 416.5 ± 2.1a
Setback
(P-T) c
217.5 ± 3.5 625.5 ± 2.1b 699.5 ± 4.9a
(F-T) c
199.0 ± 2.8 257.5 ± 6.4b 276.5 ± 0.7a
96.5 ± 3.5b 404.0 ± 5.7a 379.0 ± 4.2a
UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively. Different letters within a column indicate significant differences between mean values (n = 3) (p < 0.05). 140
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Table 3 Effect of different milling methods on the gel strength of rice flours measured by a Texture Analyzer. Rice flour UF CF WF
Hardness/N
Adhesiveness/N·s b
1.03 ± 0.022 3.14 ± 0.209a 3.20 ± 0.125a
Springiness b
Cohesion ab
−6.583 ± 0.060 −5.798 ± 0.510b −9.966 ± 0.144a
93.2 ± 1.64 96.8 ± 0.41a 89.4 ± 1.18b
Chewiness/N b
0.53 ± 0.03 0.56 ± 0.01ab 0.60 ± 0.01a
b
52.4 ± 0.2 178.1 ± 8.7a 173.8 ± 7.4a
Resilience 0.02 ± 0.000c 0.11 ± 0.009a 0.09 ± 0.001b
UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively. Different letters within a column indicate significant differences between mean values (n = 3) (p < 0.05). Table 4 Effect of different milling methods on results of DSC analysis of rice flours. Rice flour
To/°C
Tp/°C
UF CF WF
45.79 ± 0.04d 50.72 ± 0.38c 53.55 ± 0.39a
55.85 ± 0.49b 62.09 ± 0.33a 61.70 ± 0.27a
Tc/°C
Tr/°C 63.25 ± 0.08b 69.98 ± 0.42a 68.82 ± 0.60a
ΔH/J.g-1 17.46 ± 0.12b 19.26 ± 0.80a 15.27 ± 0.22c
1.87 ± 0.07c 7.92 ± 0.08b 9.99 ± 0.02a
UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively. Different letters within a column indicate significant differences between mean values (n = 3) (p < 0.05). Fig. 2. Effect of different milling methods on (a) changes in the storage (G′, black solid) and loss (G″, white open) moduli of rice flour dough (▲: CF, ■: WF, ●: UF, △: CF, □: WF, ○: UF) with frequency and (b) changes in tan δ of rice flour dough (▲: CF, ■: WF, ●: UF) with frequency. (UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wetmilled flour, respectively).
Fig. 3. Effect of different milling methods on (a) changes in dough fermentation curve (Black, blue, red and lines are WF, CF, UF,respectively) and (b) changes in gas release curve (A1, retention volume) (A2, volume of CO2 lost) (The picture is WF, CF, UF from top to bottom). (UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
reported to be related to the milling method, the damaged starch content, and the particle size distribution (Hasjim, Li, & Dhital, 2013). The peak temperature, Tp, and termination temperature, Tc, were negatively correlated with the volume median diameter (Hasjim et al., 2013). There was no significant difference between the termination temperature, Tc, of the WF and CF with that of the UF being lower because of its fine particle size. The gelatinization temperature range, Tr, is related to the size and uniformity of the starch granules. The three milling methods had a significant effect on the gelatinization temperature range of the rice flour: that of WF was the smallest and that of CF the largest. It was possible that the particle size of WF, being less than that of CF, required less heat to cause the starch to gelatinize. There was a significant difference in the gelatinization absorption heat enthalpy, ΔH, during the transition from a crystalline to an amorphous structure. WF had a low content of damaged starch so the starch granules were almost completely intact, so required more heat to gelatinize the starch. The UF starch had a high content of damaged and incomplete particles,
Fig. 4. Effect of different milling methods on the whole and cross-sectional images of the crumbs of rice bread. UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively. 141
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Table 5 Effect of different milling methods on the textural properties of rice bread. Rice flour sample
Textural properties Hardness (N)
UF CF WF
Specific volume (mL/g) Adhesiveness (N.sec)
a
30.82 ± 1.24 21.85 ± 2.16b 9.44 ± 0.41c
a
−0.013 ± 0.003 −0.011 ± 0.001b −0.004 ± 0.001c
Resilience (%)
cohesion
b
Springiness (%) b
23.65 ± 3.42 23.53 ± 0.25b 33.32 ± 0.47a
0.57 ± 0.05 0.53 ± 0.02b 0.65 ± 0.01a
b
84.7 ± 8.44 83.9 ± 1.24b 95.4 ± 1.28a
Chewiness 14.74 ± 3.34a 9.72 ± 0.81b 5.81 ± 0.27c
1.2 ± 0.1c 2.3 ± 0.2b 3.1 ± 0.1a
UF, CF, and WF indicate ultrafine-milled, cyclone-milled and wet-milled flour, respectively. Measurements of specific volume and weight loss were performed in duplicate. Measurements of textural properties were made on two central slices from two loaves of each dough. Different letters within a column indicate significant differences between mean values (n = 3) (p < 0.05).
Fig. 5. Scanning electron microscope images of the microstructure of rice bread (left: × 50, right: × 1000). Note: D, E, and F are internal images of breadcrumbs prepared from wet-milled, cyclone-milled and ultrafine-milled flour, respectively (black arrows indicate starch residues, and white arrows deformed starch granules).
resulting in less heat absorption.
This indicated that the dough was harder at low frequencies then as the frequency increased, the dough showed a transition from being solidlike to liquid-like. These results are related to the content of damaged starch of the sample and the granular structural integrity of the starch (Han, Campanella, Mix, & Hamaker, 2002). UF had the highest content of damaged starch, resulting in the lowest values of G' and G'', indicating that the elasticity and viscosity of its dough were weak, and therefore not conducive to the production and proofing processes required for breadmaking. In contrast, the WF and CF, with their lower contents of damaged starch, exhibited higher values of G' and G''.
3.6. Dynamic rheological properties of rice flour dough Fig. 2 shows that all frequencies tested, the storage modulus (G') was higher than the loss modulus (G'') for all dough samples, indicating that the elastic characteristics of the dough were higher than its viscous characteristics. As the frequency increases, the G' and G'' of all samples increases, indicating that the viscoelasticity is increased. The G' and G'' of UF dough are the lowest, indicating that their viscoelasticity is poor. At lower scan frequencies, the dough prepared from the UF exhibited the lowest tan δ, and as the frequency increased, tan δ also increased. 142
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3.7. Fermentation rheological properties of rice flour dough
4. Conclusions
Fig. 3 (a) shows that the maximum height of the dough was in the order: WF > CF > UF. The dough of WF was highest at 32.1 mm with dough prepared from UF being lowest at only 4.8 mm. The time for the dough to reach its maximum height was also in the order: WF > CF > UF, with respective times of 20, 25, and 35 min. Fig. 3 (b) shows that the gas release rate for UF was the most rapid. This was probably caused by the high content of damaged starch in UF, which the yeast was more likely to use and so produce gas (Purna, Miller, Seib, Graybosch, & Shi, 2011). The UF dough lost the largest volume of CO2 (A2), while WF dough lost the smallest. The WF dough had the largest retention volume (A1) and UF dough the smallest, which led to a much higher volume for WF dough during fermentation.
Compared to ultrafine and cyclone powders, rice flour prepared by wet milling has lower impaired starch content, more intact starch granules, higher gelatinization temperature and absorption enthalpy, and higher gel strength. The texture and appearance of bread prepared with wet ground rice flour is best suited to meet consumer expectations compared to ultrafine and cyclonic. Therefore, for making gluten-free rice bread, it is essential to choose the appropriate milling method for the rice. Rice flour with a low damaged starch content, intact starch granules and moderate particle size is a good raw material for preparing gluten-free rice bread. In future studies, the appropriate rice flour will be selected to prepare desirable gluten-free rice bread using sensory evaluation. Acknowledgments
3.8. Rice bread characteristics
This work was supported by the National Natural Science Foundation of China (No. 31471617), the National Key Research and Development Program (2016YFD0400205-101) and the Fundamental Research Funds for the Central Universities (No. JUSRP51708A). This research was also partly funded by the Innovation Project of Chinese Academy of Agricultural Sciences (CAAS) and national first-class discipline program of Food Science and Technology (JUFSTR20180103). We thank Philip Creed, PhD, from Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript.
The whole and cross-sectional images of rice flour bread are shown in the Fig. 4. The bread prepared with WF exhibited the highest specific volume, the best pore structure. The bread made with CF was of a small volume, with small, unevenly-dispersed internal pores. The UF bread had the smallest volume and almost no internal porosity. After cooling at room temperature, the top surface of the UF bread caved in, even though the expansion had remained constant during the baking process in the oven. This was related to the high content of damaged starch and the state of the starch in UF. The specific volume and textural characteristics of the rice breads are listed in Table 5. The bread made with UF had the highest hardness, while WF bread had the lowest hardness. Compared to the bread made with UF and CF, the WF bread had the highest specific volume, springiness and resilience was also the best. The main reason for this were the differences in the content of damaged starch in the different rice flours. The specific volume of rice bread was found to decrease as the content of damaged starch in the rice flour increased (Masakatsu, Chie, Nao, & Kumiko, 2012). Different milling methods have been shown to affect the specific volume of the bread, the size of the bubbles and the uniformity of the breadcrumb (Lee & Lee, 2006). Bread prepared from CF formed irregular and uneven bubbles, whose size in the crumbs was smaller than in bread prepared using WF. Therefore, it would be expected that bread prepared from rice flour with a low content of damaged starch would form more bubbles, and increase the volume of the baked rice bread.
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