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Freeze-thaw stability of rice starch modified by Improved Extrusion Cooking Technology
Accepted Manuscript Title: Freeze-thaw stability of rice starch modified by Improved Extrusion Cooking Technology Author: Jiangping Ye Xiuting Hu Fang Zhang Chong Fang Chengmei Liu Shunjing Luoab PII: DOI: Reference:
S0144-8617(16)30548-3 http://dx.doi.org/doi:10.1016/j.carbpol.2016.05.026 CARP 11095
To appear in: Received date: Revised date: Accepted date:
17-3-2016 7-5-2016 11-5-2016
Please cite this article as: Ye, Jiangping., Hu, Xiuting., Zhang, Fang., Fang, Chong., Liu, Chengmei., & Luoab, Shunjing., Freeze-thaw stability of rice starch modified by Improved Extrusion Cooking Technology.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2016.05.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Freeze-thaw stability of rice starch modified by Improved Extrusion Cooking Technology Jiangping Yea,b, Xiuting Hua,b, Fang Zhanga,b, Chong Fanga,b, Chengmei Liu
a,b
,*
Shunjing Luoa,b**
a
State Key Laboratory of Food Science and Technology, Nanchang University,
Nanchang 330047, China b
School of Food Science and Technology, Nanchang University, Nanchang 330047,
China
*
Corresponding author
Chengmei Liu Tel. /Fax: +86-791-88305871 E-mail address: [email protected] **
Corresponding author
Shunjing Luo Tel. /Fax: +86-791-88304983 E-mail address: [email protected]
1
Highlights
IECT-modified rice starch had better FT stability than native starch;
IECT inhibited retrogradation of rice starch;
IECT may be suitable for producing frozen foods.
Abstract This study aimed to explore freeze-thaw (FT) stability of rice starch modified by Improved Extrusion Cooking Technology (IECT). FT stability of IECT-modified rice starch was investigated and compared with native one. Syneresis and SEM analysis showed that IECT-modified rice starch had better FT stability than native starch. Furthermore, IECT-modified rice starch had less significant changes in the rheological parameters during the FT cycles than the native starch. XRD and iodine binding analysis demonstrated that IECT treatment inhibited the association of rice starch, especially amylose retrogradation. Additionally, the peak at around 20o was detected in XRD patterns of IECT-modified rice starch, which confirmed the formation of amylose-lipid complex during the IECT treatment. These results suggested that the IECT treatment could improve FT stability of rice starch, which was ascribed to inhibition of starch retrogradation by IECT.
Keywords: Improved extrusion cooking technology; rice starch; freeze-thaw stability
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1. Introduction As the pace of life continues to increase, the demand of frozen ready-to-eat products dramatically increased. Starch was incorporated in many of them as one of the main raw material or additives (Sae-kang & Suphantharika, 2006). Generally, starch was repeatedly suffered from freezing and thawing, when starch-based products go through factory to table. In the freezing process, water in those frozen foods turned into ice, resulting in physical stress to the food matrix and phase separation within the food matrix (Charoenrein & Preechathammawong, 2012). Upon thawing, the moisture can be readily expressed from the network of starch gels (Karim, Norziah & Seow, 2000). In addition, freezing and thawing caused changes of texture, drip loss, and deterioration in the overall quality. These changes were attributed to starch retrogradation (Lee, Baek, Cha, Park & Lim, 2002; Ferrero, Martino, & Zaritzky, 1994; Muadklay & Charoenrein, 2008), inducing that such products were unacceptable to consumers. For food professional researchers, it is a difficult and required task to develop foods in such a way as to minimize the negative effects induced by freezing and thawing. Previous studies showed that there were three main ways to prevent or retard the changes during freezing and thawing of starch-based foods. The first way was shortening the time required for the freezing (Muadklay & Charoenrein, 2008) and thawing (Teng, Chin & Yusof, 2013). But it was usually involved in high initial costs. The second method was to use food additives (Arunyanart & Charoenrein, 2008; Charoenrein & Preechathammawong, 2012; Charoenrein, Tatirat, Rengsutthi
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& Thongngam, 2011; Katekhong & Charoenrein, 2012; Muadklay & Charoenrein, 2008). It was found that FT stability could be effectively improved by these additives. The positive effect of additives was due to the reduction of retrogradation induced by retarding interaction of starch chains. However, an increasing number of consumers demand few additives or even no additives. The third one was emerged as the requirement. It was modified starch with less retrogradation to improve the FT stability. Among them, there was a growing interest in the physical modification of starch, especially in food applications (Zavareze & Dias, 2011), as it was considered to be a clean-label starches or highly safe ingredients. It was reported that high hydrostatic pressure (Chotipratoom, Choi, Bae, Kim & Baik, 2015), ultrasound (Sit, Misra & Deka, 2014), gamma irradiation (Zhu, 2016), and heat-moisture treatment (Yadav, Guleria & Yadav, 2013) could improve the freeze-thaw stability of starch. However, these studies evaluated the FT stability only by syneresis, and these techniques were not easy to achieve industrialization. Therefore, application of industrial methods to improve the FT stability and investigation on morphology and other properties during FT cycles were still required. Extrusion cooking, as a multi-step, multi-functional and thermal/mechanical process, has been widely used in food industry (Zhang, Bai & Zhang, 2011). It provides low cost, high productivity, and versatility to conventional methods, especially for processing starch and starch-based products (Guha & Ali, 2011). Our lab designed an extruder with longer screw, lower screw speed, lower temperature and higher pressure named Improved Extrusion Cooking Technology (IECT). The
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expansion of extrudate was hardly changed, and the textured cereal rice made by IECT showed good texture properties (Liu et al., 2011). Furthermore, IECT-modified rice starch had lower retrogradation degree than native rice starch (Zhang et al., 2014). So it was speculated that IECT-modified rice starch might have good FT stability. Therefore, the objective of this study was to explore the FT stability of IECT-modified rice starch. Specifically, syneresis, microstructure, crystallinity and other properties of IECT-modified rice starch were investigated and compared with those of native starch.
2. Materials and methods 2.1 Materials Rice starch was kindly gifted by Golden Agriculture Biotech Co., Ltd, Jiangxi, China. It consisted of 22.9% amylose, 0.34% protein, 0.20% fat (Fu, Luo, BeMiller, Liu & Liu, 2015). All other chemicals and reagents were of analytical reagent grade. 2.2 Modification of rice starch by IECT treatment The modification of rice starch by IECT treatment was executed as described previously. The IECT experiment was performed on a single-screw extruder (designed in our laboratory, and made in Jinan Saixin Machinery Ltd., China). The screw was 100 mm in diameter, with the length at 1950 cm. Additionally, the compression ratio was 2.84:1, when a forward element was used (Liu et al., 2011). The independent processing variables for the extrusion were addressed as follows: the mass ratio of rice starch at 1:1.5, feed screw rate at 30 rpm, screw speed at 37.5 rpm. Moreover, the
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temperature profiles in the feed, mix, screw conveyor, shearing compression metering, die head zones were kept constant at 50, 65, 85, 100, 95oC, respectively. IECT-modified rice starch was then immediately dried. Afterwards, these samples were ground and passed through a 100-mesh sieve. 2.3 Freezing and thawing treatment Freezing and thawing treatment was carried out according to the previous study with minor modification (Pongsawatmanit & Srijunthongsiri, 2008). Rice starch suspension and IECT-modified rice starch suspension (8%, w/w rice starch on a dry basis) were prepared by blending starch in distilled water and continuously stirring at 250 rpm for 1 h. Then, these suspensions were gelatinized by placing in a water bath at 95oC for 25 min with continuously stirring. Finally, all of the samples were placed in an incubator at 25oC for 4 h. Afterwards, the starch gel samples were frozen in a chest freezer (Siemens refrigerator, model BCD-610W) at -20oC for 20 h and then thawed in an incubator at 25oC for 4 h. This was one FT cycle. The FT cycle was repeated for seven cycles. Each thawed sample was immediately dried. 2.4 Syneresis measurement Syneresis measurement was executed by the method of Charoenrein, Tatirat and Muadklay (2008) with modification. The top of syringe was cut off to put into a 50 mL centrifuge tube and about 15 holes were drilled at the bottom. Then, a part of needle was cut down for fitting in the centrifuge tube. The equipment as illustrated in Fig.1, was used to measure syneresis. Afterwards, the thawed gel samples were removed from syringes to those drilled syringes with filter paper at the bottom. Next,
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the drilled syringes were placed in centrifuge tubes. The tubes were then centrifuged at 4000 g for 15 min. The percentage of syneresis was then calculated as the ratio of the amount of liquid separated (g) to the total weight (g) of the gel before centrifugation. 2.5 Scanning electron microscopy analysis Freeze-dried samples were cut with a razor blade and cut surface samples were prepared by sticking onto double-sided adhesive tape attached to a circular specimen stub. The samples were viewed using an environmental scanning electron microscope (ESEM) (Quanta200F, FEI Deutschland GmbH, Kassel, Germany) at 30 kV and 3.0 spot size (Chen et al., 2012). 2.6 Dynamic rheological measurement Native rice starch and IECT-modified starch slurries (8%, w/w rice starch on a dry basis) were loaded between parallel plates (1 mm gap) with a Peltier temperature control system in a rheometer (DCR-302, Anton Paar, Austria). Methyl silicone oil was then applied on the geometry’s periphery to prevent water evaporation. Then, a strain of 2% and frequency of 1Hz was applied, and the sample was equilibrated at 25°C for 15 min. Then, a frequency sweep was conducted from 0.01 to 100 S-1. Afterwards, the samples were immediately cooled to −20oC for 8 min and held for 1 min, and then heated to 25oC at 20oC/min and held for 1 min. Small amplitude oscillatory measurements (frequency sweep) were performed again. The FT cycle was repeated for up to 7 cycles. The storage modulus (G’), loss modulus (G”), and loss tangent (tanδ= G”/G’) were obtained (Yamazaki et al., 2013).
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2.7 Iodine binding analysis Iodine binding analysis was performed using a UV/visible Spectrophotometer (U-2900 Spectrophotometer, Hitachi, Ltd., Japan) according to the method described previously (Miao et al., 2015) with minor modification. One hundred milligrams of dried starch sample was dissolved in 10.0 mL of 90% DMSO and then diluted to 50.0 mL with deionized water. An aliquot of 5.0 mL was diluted with deionized water to 50.0 mL, and then 1.0 mL of iodine reagent (0.20% I2 + 2.0% KI) was added. After vortexing and standing for 15 min, the absorbance spectra were analyzed from 450 to 800 nm. 2.8 X-ray diffraction analysis Wide-angle X-ray scattering measurements of dried samples (moisture content around 8%) were using a XRD DI SYSTEM (BEDE Group, UK) with Cu Ka radiation at 40 kV and 30 mA. X-ray diffraction data were collected for 2θ from 5 to 35° at a scanning rate of 0.5 °/min at room temperature. 2.9 Statistical analysis Statistical analysis was carried out using data analysis functions in Origin 8.5 (OriginLab Corporation, Northamptaon, MA) and significant differences between the results were calculated by analysis of variance (ANOVA). Differences at p<0.05 were considered to be significant.
3. Results and discussion 3.1 Syneresis Freeze-thaw stability represents the ability of starch to withstand the
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undesirable physical changes that may occur during freezing and thawing. The freeze-thaw stability during FT cycles (0, 1st, 3rd, 5th and 7th) was evaluated with the percentage of syneresis as an index. Syneresis of native rice starch and IECT-modified rice starch during FT cycles were presented in Fig.2. Under the experimental conditions, syneresis of native rice starch in the 1st FT cycle reached 31.4%, and decreased to 26.5% after 3rd FT cycles. Then, the syneresis gradually decreased from 24.3% to 23.2% through the subsequent FT cycles. The reduction of syneresis after 1st FT cycle might be ascribed to the formation of rough-texture gel with a honeycomb-like structure that allowed it to reabsorb part of the separation water (Deetae, et al 2008; Yuan & Thompson, 1998). Syneresis of IECT-modified rice starch was significantly decreased (p<0.05) during FT cycles. In detail, the syneresis of IECT-modified rice starch was 27.5% in the 1st FT cycle, and the syneresis value changed slightly from 23.1% to 19.3% through 3rd to 7th cycles. Syneresis in a freeze-thawed gel was caused by an increase in molecular associations between starch chains, in particular the retrogradation of amylose (Ferrero, Martino & Zaritzky, 1994; Morris, 1990). It was speculated that IECT suppressed the associations of starch chains, which improved FT stability of rice starch. 3.2 Microstructure of FT rice starch gels The morphology change of rice starch and IECT-modified rice starch during FT cycles was analyzed by SEM. The native rice starch gel before freezing was compact with some honeycomb network (Fig. 3a). After 1st FT cycle, pores of honeycomb structure became larger and clearer (Fig. 3b), which could be attributed to the fact
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that starch molecules rearranged in order and ice crystal formation (Charoenrein & Preechathammawong, 2012). In the following 3rd to 7th FT cycles, the pore size of native rice starch kept unchanged, and the matrix surrounding the pores were thicker (Fig. 3c-3e). This result was well correlated with the results of Section 3.1. The separation water was much more than the reabsorption water, as the honeycomb structure was formed in the 1st FT cycle. However, the honeycomb structure reabsorbed part of the separation water, leading to the decreasing of syneresis in the following FT cycles. Unfrozen IECT-modified starch appeared unconfined and less organized than native rice starch (Fig. 3A), suggesting that the structure of rice starch was changed by IECT. The honeycomb structure of IECT-modified starch gels gradually shaped with the increasing of FT cycles (Fig. 3B-3E), However, the pores of IECT-modified starch gels was smaller than those of the corresponding native starch. Consequently, these results suggested that IECT treatment effectively stabilized the microstructure of the rice starch gel during FT cycles. 3.3 Rheological properties Fig.4 showed changes of G’ and tanδ as a function of frequency (f) for rice starches after repeated FT cycles. G’ and G” of the native rice starch gel were frequency independent over a large time scale. Additionally G’ was much larger than G” throughout the frequency sweep range (G” data not shown), suggesting that these systems behaved as a solid-like material (Achayuthakan & Suphantharika, 2008). The G’ values of rice starch dramatically increased after 1st FT treatment, and then slightly increased with increasing the FT cycles. But change of tanδ was opposite. It
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was observed that tanδ decreased after 1st and 3rd FT cycles. In addition, the magnitudes of tanδ were within the range of 0.032–0.119. IECT-modified rice starch had much less G’ than native rice starch during FT cycles, implying that rheological behavior of IECT-modified rice starch became weaker and less solid-like during the process of repeated FT cycles (Yamazaki et al., 2013). The tanδ of IECT-modified rice starch only significantly decreased after the 1st FT cycle, and then slightly varied with increasing FT cycles. It suggested that IECT modification could maintain a stable rheological behavior despite undergoing all the FT cycles. Besides, the tanδ of IECT-modified rice starch was higher than that of native rice starch. When repeating FT treatments were performed on the rheometer plate, the decrease of tanδ was observed. The result implied that repeating FT treatments promoted starch molecule association. It might be due to retrogradation of leached components and interaction between molecules remaining inside the granule, reinforcing the gel structure during cooling (Kaur, Singh, Singh & McCarthy, 2008). However, the association of starch molecules was inhibited by IECT. 3.4. Iodine binding analysis Starch can form inclusion complex with polyiodide ions, which gives rise to a characteristic deep blue color. The amylose content of starch was often colorimetrically determined from the iodine complexation. However, once dispersed amylose chains formed double-helical associations upon retrogradation, it would gradually lose its ability to form blue complex with iodine (Wang, Li, Copeland, Niu & Wang, 2015). As a result, the iodine ability and blue value (BV) could reflect
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retrogradation of amylose. Native and IECT-modified rice starch during FT cycles showed different absorptivity of iodine binding spectra (Fig. 5). The iodine binding values of IECT-modified rice starch was higher than those of native rice starch. The BV at 635 nm of native rice starch substantially decreased from 0.613 to 0.518, while the BV of IECT-modified rice starch slightly decreased from 0.636 to 0.622 during FT cycles. The results suggested that the amylose retrogradation was hinted by IECT treatment. 3.5. X-ray diffraction analysis The XRD patterns of native and IECT-modified rice starch from FT cycles were shown in Fig.6. Gelatinized native rice starch was amorphous and showed an essentially diffused pattern. After the 1st FT cycle, it began to recrystallize, and the crystalline peak 16.9° appeared. At 3rd FT cycles, its intensity slightly increased. In the subsequent FT cycles, no changes occurred, except the slight increasing of the intensity. However, the X-ray diffractogram of IECT-modified starch during FT treatment was different from that of native rice starch. The diffraction peak at 2θ of 16.9°appeared until 7th FT cycles, and the intensity of the crystalline peaks at 16.9° was much lower than that of native rice starch. These results suggested that IECT inhibited the retrogradation of rice starch during FT cycles. Peculiarly, IECT-modified rice starch had an obvious crystallinity peak at 20°. The peak at 20° indicated a well-formed V-type crystallinity (Osella et al., 2005). V-type crystallinity was usually the amylose complexed with fatty acids or phospholipids (Koksel, Sahbaz & Ozboy, 1993), which could hinder the amylose rearrangements (Wu, Chen,
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Li & Li, 2009). These results demonstrated that better FT stability of IECT-modified rice starch was ascribed to that IECT modification inhibited the amylose retrogradation.
4. Conclusions This work demonstrated that IECT treatment could effectively improve the stability of rice starch gels subjected to repeatedly FT cycles. Better FT stability of IECT-modified rice starch was confirmed by reducing syneresis and retarding changes of honeycomb structure. The mechanism of good FT stability of IECT-modified rice starch was that IECT modification inhibited the starch re-associations, particularly amylose retrogradation. In conclusion, IECT is an applicable and promising technique for producing frozen starch products.
Acknowledgement This study was financially supported by National Natural Science Foundation of China (31271953) and State Key Laboratory of Food Science and Technology, Nanchang University (SKLF-ZZA-201608).
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Fig.1. The apparatus for syneresis. 35 Native rice starch IECT-modified rice starch
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Syneresis (%)
25 20 15 10 5 0 0
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Freeze-thaw cycles
Fig.2. Syneresis of native rice starch and IECT-modified rice starch gels during FT cycles.
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Fig.3. SEM images of native rice starch and IECT-modified rice starch gels during FT cycles 1000
Native rice starch
Native rice starch
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-1 Frequency (s ) IECT-modified rice starch
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Fig.4. Changes in dynamic rheological properties of native rice starch and IECT-modified rice starch gels during FT cycles.
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0.8
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Absorbance
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b FT 0 cycles FT 1 cycles FT 3 cycles FT 5 cycles FT 7 cycles
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0.3 Samples a0 a1 a3 a5 a7 BV(635nm) 0.613 0.602 0.603 0.602 0.581
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b0 b1 b3 b5 b7 0.636 0.631 0.628 0.624 0.622
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Fig.5. Iodine absorption spectra of (a) native rice starch and (b) IECT-modified rice starch during FT cycles. Native rice starch
IECT-modified rice starch
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Fig.6. X-ray diffractograms of native rice starch and IECT-modified rice starch during FT cycles.
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S0144-8617(16)30548-3 http://dx.doi.org/doi:10.1016/j.carbpol.2016.05.026 CARP 11095
To appear in: Received date: Revised date: Accepted date:
17-3-2016 7-5-2016 11-5-2016
Please cite this article as: Ye, Jiangping., Hu, Xiuting., Zhang, Fang., Fang, Chong., Liu, Chengmei., & Luoab, Shunjing., Freeze-thaw stability of rice starch modified by Improved Extrusion Cooking Technology.Carbohydrate Polymers http://dx.doi.org/10.1016/j.carbpol.2016.05.026 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Freeze-thaw stability of rice starch modified by Improved Extrusion Cooking Technology Jiangping Yea,b, Xiuting Hua,b, Fang Zhanga,b, Chong Fanga,b, Chengmei Liu
a,b
,*
Shunjing Luoa,b**
a
State Key Laboratory of Food Science and Technology, Nanchang University,
Nanchang 330047, China b
School of Food Science and Technology, Nanchang University, Nanchang 330047,
China
*
Corresponding author
Chengmei Liu Tel. /Fax: +86-791-88305871 E-mail address: [email protected] **
Corresponding author
Shunjing Luo Tel. /Fax: +86-791-88304983 E-mail address: [email protected]
1
Highlights
IECT-modified rice starch had better FT stability than native starch;
IECT inhibited retrogradation of rice starch;
IECT may be suitable for producing frozen foods.
Abstract This study aimed to explore freeze-thaw (FT) stability of rice starch modified by Improved Extrusion Cooking Technology (IECT). FT stability of IECT-modified rice starch was investigated and compared with native one. Syneresis and SEM analysis showed that IECT-modified rice starch had better FT stability than native starch. Furthermore, IECT-modified rice starch had less significant changes in the rheological parameters during the FT cycles than the native starch. XRD and iodine binding analysis demonstrated that IECT treatment inhibited the association of rice starch, especially amylose retrogradation. Additionally, the peak at around 20o was detected in XRD patterns of IECT-modified rice starch, which confirmed the formation of amylose-lipid complex during the IECT treatment. These results suggested that the IECT treatment could improve FT stability of rice starch, which was ascribed to inhibition of starch retrogradation by IECT.
Keywords: Improved extrusion cooking technology; rice starch; freeze-thaw stability
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1. Introduction As the pace of life continues to increase, the demand of frozen ready-to-eat products dramatically increased. Starch was incorporated in many of them as one of the main raw material or additives (Sae-kang & Suphantharika, 2006). Generally, starch was repeatedly suffered from freezing and thawing, when starch-based products go through factory to table. In the freezing process, water in those frozen foods turned into ice, resulting in physical stress to the food matrix and phase separation within the food matrix (Charoenrein & Preechathammawong, 2012). Upon thawing, the moisture can be readily expressed from the network of starch gels (Karim, Norziah & Seow, 2000). In addition, freezing and thawing caused changes of texture, drip loss, and deterioration in the overall quality. These changes were attributed to starch retrogradation (Lee, Baek, Cha, Park & Lim, 2002; Ferrero, Martino, & Zaritzky, 1994; Muadklay & Charoenrein, 2008), inducing that such products were unacceptable to consumers. For food professional researchers, it is a difficult and required task to develop foods in such a way as to minimize the negative effects induced by freezing and thawing. Previous studies showed that there were three main ways to prevent or retard the changes during freezing and thawing of starch-based foods. The first way was shortening the time required for the freezing (Muadklay & Charoenrein, 2008) and thawing (Teng, Chin & Yusof, 2013). But it was usually involved in high initial costs. The second method was to use food additives (Arunyanart & Charoenrein, 2008; Charoenrein & Preechathammawong, 2012; Charoenrein, Tatirat, Rengsutthi
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& Thongngam, 2011; Katekhong & Charoenrein, 2012; Muadklay & Charoenrein, 2008). It was found that FT stability could be effectively improved by these additives. The positive effect of additives was due to the reduction of retrogradation induced by retarding interaction of starch chains. However, an increasing number of consumers demand few additives or even no additives. The third one was emerged as the requirement. It was modified starch with less retrogradation to improve the FT stability. Among them, there was a growing interest in the physical modification of starch, especially in food applications (Zavareze & Dias, 2011), as it was considered to be a clean-label starches or highly safe ingredients. It was reported that high hydrostatic pressure (Chotipratoom, Choi, Bae, Kim & Baik, 2015), ultrasound (Sit, Misra & Deka, 2014), gamma irradiation (Zhu, 2016), and heat-moisture treatment (Yadav, Guleria & Yadav, 2013) could improve the freeze-thaw stability of starch. However, these studies evaluated the FT stability only by syneresis, and these techniques were not easy to achieve industrialization. Therefore, application of industrial methods to improve the FT stability and investigation on morphology and other properties during FT cycles were still required. Extrusion cooking, as a multi-step, multi-functional and thermal/mechanical process, has been widely used in food industry (Zhang, Bai & Zhang, 2011). It provides low cost, high productivity, and versatility to conventional methods, especially for processing starch and starch-based products (Guha & Ali, 2011). Our lab designed an extruder with longer screw, lower screw speed, lower temperature and higher pressure named Improved Extrusion Cooking Technology (IECT). The
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expansion of extrudate was hardly changed, and the textured cereal rice made by IECT showed good texture properties (Liu et al., 2011). Furthermore, IECT-modified rice starch had lower retrogradation degree than native rice starch (Zhang et al., 2014). So it was speculated that IECT-modified rice starch might have good FT stability. Therefore, the objective of this study was to explore the FT stability of IECT-modified rice starch. Specifically, syneresis, microstructure, crystallinity and other properties of IECT-modified rice starch were investigated and compared with those of native starch.
2. Materials and methods 2.1 Materials Rice starch was kindly gifted by Golden Agriculture Biotech Co., Ltd, Jiangxi, China. It consisted of 22.9% amylose, 0.34% protein, 0.20% fat (Fu, Luo, BeMiller, Liu & Liu, 2015). All other chemicals and reagents were of analytical reagent grade. 2.2 Modification of rice starch by IECT treatment The modification of rice starch by IECT treatment was executed as described previously. The IECT experiment was performed on a single-screw extruder (designed in our laboratory, and made in Jinan Saixin Machinery Ltd., China). The screw was 100 mm in diameter, with the length at 1950 cm. Additionally, the compression ratio was 2.84:1, when a forward element was used (Liu et al., 2011). The independent processing variables for the extrusion were addressed as follows: the mass ratio of rice starch at 1:1.5, feed screw rate at 30 rpm, screw speed at 37.5 rpm. Moreover, the
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temperature profiles in the feed, mix, screw conveyor, shearing compression metering, die head zones were kept constant at 50, 65, 85, 100, 95oC, respectively. IECT-modified rice starch was then immediately dried. Afterwards, these samples were ground and passed through a 100-mesh sieve. 2.3 Freezing and thawing treatment Freezing and thawing treatment was carried out according to the previous study with minor modification (Pongsawatmanit & Srijunthongsiri, 2008). Rice starch suspension and IECT-modified rice starch suspension (8%, w/w rice starch on a dry basis) were prepared by blending starch in distilled water and continuously stirring at 250 rpm for 1 h. Then, these suspensions were gelatinized by placing in a water bath at 95oC for 25 min with continuously stirring. Finally, all of the samples were placed in an incubator at 25oC for 4 h. Afterwards, the starch gel samples were frozen in a chest freezer (Siemens refrigerator, model BCD-610W) at -20oC for 20 h and then thawed in an incubator at 25oC for 4 h. This was one FT cycle. The FT cycle was repeated for seven cycles. Each thawed sample was immediately dried. 2.4 Syneresis measurement Syneresis measurement was executed by the method of Charoenrein, Tatirat and Muadklay (2008) with modification. The top of syringe was cut off to put into a 50 mL centrifuge tube and about 15 holes were drilled at the bottom. Then, a part of needle was cut down for fitting in the centrifuge tube. The equipment as illustrated in Fig.1, was used to measure syneresis. Afterwards, the thawed gel samples were removed from syringes to those drilled syringes with filter paper at the bottom. Next,
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the drilled syringes were placed in centrifuge tubes. The tubes were then centrifuged at 4000 g for 15 min. The percentage of syneresis was then calculated as the ratio of the amount of liquid separated (g) to the total weight (g) of the gel before centrifugation. 2.5 Scanning electron microscopy analysis Freeze-dried samples were cut with a razor blade and cut surface samples were prepared by sticking onto double-sided adhesive tape attached to a circular specimen stub. The samples were viewed using an environmental scanning electron microscope (ESEM) (Quanta200F, FEI Deutschland GmbH, Kassel, Germany) at 30 kV and 3.0 spot size (Chen et al., 2012). 2.6 Dynamic rheological measurement Native rice starch and IECT-modified starch slurries (8%, w/w rice starch on a dry basis) were loaded between parallel plates (1 mm gap) with a Peltier temperature control system in a rheometer (DCR-302, Anton Paar, Austria). Methyl silicone oil was then applied on the geometry’s periphery to prevent water evaporation. Then, a strain of 2% and frequency of 1Hz was applied, and the sample was equilibrated at 25°C for 15 min. Then, a frequency sweep was conducted from 0.01 to 100 S-1. Afterwards, the samples were immediately cooled to −20oC for 8 min and held for 1 min, and then heated to 25oC at 20oC/min and held for 1 min. Small amplitude oscillatory measurements (frequency sweep) were performed again. The FT cycle was repeated for up to 7 cycles. The storage modulus (G’), loss modulus (G”), and loss tangent (tanδ= G”/G’) were obtained (Yamazaki et al., 2013).
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2.7 Iodine binding analysis Iodine binding analysis was performed using a UV/visible Spectrophotometer (U-2900 Spectrophotometer, Hitachi, Ltd., Japan) according to the method described previously (Miao et al., 2015) with minor modification. One hundred milligrams of dried starch sample was dissolved in 10.0 mL of 90% DMSO and then diluted to 50.0 mL with deionized water. An aliquot of 5.0 mL was diluted with deionized water to 50.0 mL, and then 1.0 mL of iodine reagent (0.20% I2 + 2.0% KI) was added. After vortexing and standing for 15 min, the absorbance spectra were analyzed from 450 to 800 nm. 2.8 X-ray diffraction analysis Wide-angle X-ray scattering measurements of dried samples (moisture content around 8%) were using a XRD DI SYSTEM (BEDE Group, UK) with Cu Ka radiation at 40 kV and 30 mA. X-ray diffraction data were collected for 2θ from 5 to 35° at a scanning rate of 0.5 °/min at room temperature. 2.9 Statistical analysis Statistical analysis was carried out using data analysis functions in Origin 8.5 (OriginLab Corporation, Northamptaon, MA) and significant differences between the results were calculated by analysis of variance (ANOVA). Differences at p<0.05 were considered to be significant.
3. Results and discussion 3.1 Syneresis Freeze-thaw stability represents the ability of starch to withstand the
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undesirable physical changes that may occur during freezing and thawing. The freeze-thaw stability during FT cycles (0, 1st, 3rd, 5th and 7th) was evaluated with the percentage of syneresis as an index. Syneresis of native rice starch and IECT-modified rice starch during FT cycles were presented in Fig.2. Under the experimental conditions, syneresis of native rice starch in the 1st FT cycle reached 31.4%, and decreased to 26.5% after 3rd FT cycles. Then, the syneresis gradually decreased from 24.3% to 23.2% through the subsequent FT cycles. The reduction of syneresis after 1st FT cycle might be ascribed to the formation of rough-texture gel with a honeycomb-like structure that allowed it to reabsorb part of the separation water (Deetae, et al 2008; Yuan & Thompson, 1998). Syneresis of IECT-modified rice starch was significantly decreased (p<0.05) during FT cycles. In detail, the syneresis of IECT-modified rice starch was 27.5% in the 1st FT cycle, and the syneresis value changed slightly from 23.1% to 19.3% through 3rd to 7th cycles. Syneresis in a freeze-thawed gel was caused by an increase in molecular associations between starch chains, in particular the retrogradation of amylose (Ferrero, Martino & Zaritzky, 1994; Morris, 1990). It was speculated that IECT suppressed the associations of starch chains, which improved FT stability of rice starch. 3.2 Microstructure of FT rice starch gels The morphology change of rice starch and IECT-modified rice starch during FT cycles was analyzed by SEM. The native rice starch gel before freezing was compact with some honeycomb network (Fig. 3a). After 1st FT cycle, pores of honeycomb structure became larger and clearer (Fig. 3b), which could be attributed to the fact
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that starch molecules rearranged in order and ice crystal formation (Charoenrein & Preechathammawong, 2012). In the following 3rd to 7th FT cycles, the pore size of native rice starch kept unchanged, and the matrix surrounding the pores were thicker (Fig. 3c-3e). This result was well correlated with the results of Section 3.1. The separation water was much more than the reabsorption water, as the honeycomb structure was formed in the 1st FT cycle. However, the honeycomb structure reabsorbed part of the separation water, leading to the decreasing of syneresis in the following FT cycles. Unfrozen IECT-modified starch appeared unconfined and less organized than native rice starch (Fig. 3A), suggesting that the structure of rice starch was changed by IECT. The honeycomb structure of IECT-modified starch gels gradually shaped with the increasing of FT cycles (Fig. 3B-3E), However, the pores of IECT-modified starch gels was smaller than those of the corresponding native starch. Consequently, these results suggested that IECT treatment effectively stabilized the microstructure of the rice starch gel during FT cycles. 3.3 Rheological properties Fig.4 showed changes of G’ and tanδ as a function of frequency (f) for rice starches after repeated FT cycles. G’ and G” of the native rice starch gel were frequency independent over a large time scale. Additionally G’ was much larger than G” throughout the frequency sweep range (G” data not shown), suggesting that these systems behaved as a solid-like material (Achayuthakan & Suphantharika, 2008). The G’ values of rice starch dramatically increased after 1st FT treatment, and then slightly increased with increasing the FT cycles. But change of tanδ was opposite. It
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was observed that tanδ decreased after 1st and 3rd FT cycles. In addition, the magnitudes of tanδ were within the range of 0.032–0.119. IECT-modified rice starch had much less G’ than native rice starch during FT cycles, implying that rheological behavior of IECT-modified rice starch became weaker and less solid-like during the process of repeated FT cycles (Yamazaki et al., 2013). The tanδ of IECT-modified rice starch only significantly decreased after the 1st FT cycle, and then slightly varied with increasing FT cycles. It suggested that IECT modification could maintain a stable rheological behavior despite undergoing all the FT cycles. Besides, the tanδ of IECT-modified rice starch was higher than that of native rice starch. When repeating FT treatments were performed on the rheometer plate, the decrease of tanδ was observed. The result implied that repeating FT treatments promoted starch molecule association. It might be due to retrogradation of leached components and interaction between molecules remaining inside the granule, reinforcing the gel structure during cooling (Kaur, Singh, Singh & McCarthy, 2008). However, the association of starch molecules was inhibited by IECT. 3.4. Iodine binding analysis Starch can form inclusion complex with polyiodide ions, which gives rise to a characteristic deep blue color. The amylose content of starch was often colorimetrically determined from the iodine complexation. However, once dispersed amylose chains formed double-helical associations upon retrogradation, it would gradually lose its ability to form blue complex with iodine (Wang, Li, Copeland, Niu & Wang, 2015). As a result, the iodine ability and blue value (BV) could reflect
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retrogradation of amylose. Native and IECT-modified rice starch during FT cycles showed different absorptivity of iodine binding spectra (Fig. 5). The iodine binding values of IECT-modified rice starch was higher than those of native rice starch. The BV at 635 nm of native rice starch substantially decreased from 0.613 to 0.518, while the BV of IECT-modified rice starch slightly decreased from 0.636 to 0.622 during FT cycles. The results suggested that the amylose retrogradation was hinted by IECT treatment. 3.5. X-ray diffraction analysis The XRD patterns of native and IECT-modified rice starch from FT cycles were shown in Fig.6. Gelatinized native rice starch was amorphous and showed an essentially diffused pattern. After the 1st FT cycle, it began to recrystallize, and the crystalline peak 16.9° appeared. At 3rd FT cycles, its intensity slightly increased. In the subsequent FT cycles, no changes occurred, except the slight increasing of the intensity. However, the X-ray diffractogram of IECT-modified starch during FT treatment was different from that of native rice starch. The diffraction peak at 2θ of 16.9°appeared until 7th FT cycles, and the intensity of the crystalline peaks at 16.9° was much lower than that of native rice starch. These results suggested that IECT inhibited the retrogradation of rice starch during FT cycles. Peculiarly, IECT-modified rice starch had an obvious crystallinity peak at 20°. The peak at 20° indicated a well-formed V-type crystallinity (Osella et al., 2005). V-type crystallinity was usually the amylose complexed with fatty acids or phospholipids (Koksel, Sahbaz & Ozboy, 1993), which could hinder the amylose rearrangements (Wu, Chen,
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Li & Li, 2009). These results demonstrated that better FT stability of IECT-modified rice starch was ascribed to that IECT modification inhibited the amylose retrogradation.
4. Conclusions This work demonstrated that IECT treatment could effectively improve the stability of rice starch gels subjected to repeatedly FT cycles. Better FT stability of IECT-modified rice starch was confirmed by reducing syneresis and retarding changes of honeycomb structure. The mechanism of good FT stability of IECT-modified rice starch was that IECT modification inhibited the starch re-associations, particularly amylose retrogradation. In conclusion, IECT is an applicable and promising technique for producing frozen starch products.
Acknowledgement This study was financially supported by National Natural Science Foundation of China (31271953) and State Key Laboratory of Food Science and Technology, Nanchang University (SKLF-ZZA-201608).
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Fig.1. The apparatus for syneresis. 35 Native rice starch IECT-modified rice starch
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Syneresis (%)
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Fig.2. Syneresis of native rice starch and IECT-modified rice starch gels during FT cycles.
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Fig.3. SEM images of native rice starch and IECT-modified rice starch gels during FT cycles 1000
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Fig.4. Changes in dynamic rheological properties of native rice starch and IECT-modified rice starch gels during FT cycles.
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Fig.5. Iodine absorption spectra of (a) native rice starch and (b) IECT-modified rice starch during FT cycles. Native rice starch
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Fig.6. X-ray diffractograms of native rice starch and IECT-modified rice starch during FT cycles.
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