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Changes in crystalline structure of microspheres of corn starch and amylose under isothermal and temperature cycling treatments
Industrial Crops and Products 51 (2013) 220–223
Contents lists available at ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Short communication
Changes in crystalline structure of microspheres of corn starch and amylose under isothermal and temperature cycling treatments Xia Bai a , Zhao Dong a , Xiuli Wu a,b , Jin Tong a , Jiang Zhou a,∗ a Key Laboratory of Bionic Engineering (Ministry of Education), College of Biological and Agricultural Engineering, Jilin University, 5988 Renmin Street, Changchun 130022, China b College of Biological Science and Technology, Changchun University, 6543 Weixing Road, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 14 May 2013 Received in revised form 4 September 2013 Accepted 5 September 2013 Keywords: Starch microsphere Crystalline structure Temperature treatments
a b s t r a c t Microspheres of corn starch and amylose were prepared by precipitating starch paste solution with absolute ethanol. The V-type crystalline structure was observed in the obtained microspheres. By storing the microspheres at three temperature conditions under 95% RH, constant temperatures at 8 ◦ C and 30 ◦ C as well as temperature cycles at 8 ◦ C for 1 day and at 30 ◦ C for 1 day, for different periods of time, changes in crystalline structure of the microspheres were investigated by X-ray diffraction (XRD) analysis. The results showed that the diffraction peaks of V-type crystalline structure vanished after the temperature treatments. Retrogradation yielding A-type crystalline structure took place in all the corn starch microspheres and only the amylose microspheres with 8 ◦ C/30 ◦ C treatment. Comparing with the isothermal treatments, the temperature cycling accelerated retrogradation of the microspheres. © 2013 Elsevier B.V. All rights reserved.
1. Introduction As one of the most abundant biopolymers in nature, starch is found in the tissues of many plants and has been considered as an alternative material for industrial applications because of its availability from renewable resources, biodegradability, derivability, non-toxicity and low cost. Starch microspheres, as one of starch products, have found applications in pharmaceutical industry (Fang et al., 2008), food industry (Glenn et al., 2010), material industry (Ma et al., 2008) and environmental engineering (Miao et al., 2010; Yang et al., 2010). The microstructure of starch microspheres has deep effects on these applications because it will determine performance of the microspheres, such as loading and release characteristics, mechanical properties and adsorption performance. Although there are several approaches to prepare starch microspheres, such as spray drying (Glenn et al., 2010), emulsioncrosslinking technique (Atyabi et al., 2006; Miao et al., 2010) and emulsion-gelation process followed by supercritical drying (García-González et al., 2012), the precipitation is a commonly used method (Ma et al., 2008; Tan et al., 2009; Chin et al., 2011). The precipitation process involves a successive addition of a dilute starch solution which is prepared by using solvent or gelatinizing starch
∗ Corresponding author. Tel.: +86 431 85095760x414; fax: +86 431 85095760x888. E-mail address: [email protected] (J. Zhou). 0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2013.09.007
in water to a nonsolvent or inversely. Therefore, the microstructure of the starch microspheres is dependant upon the preparation conditions and compositions. It is known that retrogradation will take place in gelatinized starch. Starch retrogradation is a non-equilibrium thermoreversible recrystallization process, its rate and extent depend on ratio and structures of amylose and amylopectin, storage temperature and time as well as water content. The retrogradation can be accelerated by treating gelatinized starch with temperature cycles between a temperature near the glass transition temperature and a higher temperature up to the melting temperature (Jacobson and BeMiller, 1998). Retrogradation of starch has been the subject of considerable investigation, but most research was carried out with high-moisture and intermediate-moisture starch materials such as pastes and gels (Jacobson and BeMiller, 1998; Park et al., 2009; Zhou et al., 2010). No detailed information is available about the retrogradation of starch microspheres with low moisture content. Understanding the changes of crystalline structure and crystallinity in starch microspheres is necessary for applications of starch microspheres. In this paper, starch microspheres were prepared by precipitation using corn starch and amylose. Then, the microspheres were stored under isothermal (8 ◦ C and 30 ◦ C) as well as temperature cycles of 8 ◦ C and 30 ◦ C for different periods to investigate changes in crystalline structures aiming at getting deeper insights of the influence of storage conditions on microstructure of starch microspheres.
X. Bai et al. / Industrial Crops and Products 51 (2013) 220–223
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2. Experimental 2.1. Materials The corn starch (with about 25% amylose) was purchased from Changchun Jincheng Corn Development Co. Ltd. (Changchun, China). The amylose was purchased from Shaanxi Tianwei Biological Production Co. Ltd. (Xi’an, China). Absolute ethanol was purchased from Beijing Beihua Fine Chemicals Co. Ltd. (Beijing, China). All the materials were used as received. 2.2. Preparation of corn starch microspheres and amylose microspheres Ten grams of starch (corn starch or amylose) was mixed with 200 mL of deionized water, and the mixture was heated to 95 ◦ C and kept at this temperature for 1 h in water bath with constant stirring. After the temperature of the gelatinized starch solution was reduced to 50 ◦ C, 200 mL of absolute ethanol was dropwise added at the speed of 3 mL/min under constant stirring of 400 rpm. When the obtained microsphere suspension was cooled to room temperature, another 200 mL of absolute ethanol was dropwise added with constant stirring. The precipitate was collected by centrifugation with 6000 rpm, washed two times by using absolute ethanol to remove water, then air-dried at room temperature overnight (12 h). 2.3. Morphology observation Morphologies of corn starch microspheres and amylose microspheres were observed using a Philips XL30 FEG-ESEM (FEI Philips Electroscan, Mass., USA). Samples were mounted on specimen stubs with carbon black tape and then sputter-coated with gold before observation. 2.4. Temperature treating conditions
Fig. 1. SEM images of corn starch microspheres (a) and amylose microspheres (b).
The obtained starch microspheres were weighed into glass petri-dishes and stored under 95% RH at three different temperature conditions: constant temperatures at 8 ◦ C and 30 ◦ C, and temperature cycles at 8 ◦ C for 1 day and at 30 ◦ C for 1 day (8/30 ◦ C). 2.5. X-ray diffraction (XRD) analysis The crystalline structures of the starch microspheres were characterized using a Rigaku D/max-2500 X-ray diffractometer (Rigaku ˚ at Corporation, Tokyo, Japan) with Cu-K␣ radiation ( = 1.542 A) 40 kV and 250 mA. The XRD patterns were recorded over the 2 range of 4–40◦ at a speed of 2◦ /min. Relative crystallinity was evaluated from the ratio of the areas of the diffraction peaks to the area of the whole diffraction pattern subtracted amorphous background patterns (Nara and Komiya, 1983). 2.6. Moisture content Samples were dried in a vacuum oven at 105 ◦ C until constant weight. The moisture content (MC) of the starch microspheres was calculated using the measured wet weight, WW , and the dry weight, WD , by MC =
WW − WD × 100% WD
(1)
3. Results and discussion 3.1. Corn starch microspheres (CSM) The obtained CSM consisted of particles ranging in size from approximately 15 m to 100 m with a mean size of 37.40 m
measured by using a laser particle size analyzer (BT-9300H, Bettersize Instruments Ltd., Dandong, China). Fig. 1(a) showed the SEM image of the CSM, which revealed that the CSM were basically spherical shape with some concavities and small irregular fragments on surface, though some particles were irregularly shaped. X-ray diffraction patterns of the CSM stored at three temperature conditions for 0, 2, 6, 10 and 20 days were given in Fig. 2(a). The fresh CSM possess a typical V-type crystalline structure with diffraction peaks at 7.7◦ , 13.3◦ and 20.4◦ of 2. This result is different from that in the starch microspheres prepared using an aqueous two-phase system consisting of two structurally different polymers (poly(ethylene glycol) and starch), which gave A-type diffraction patterns (Elfstrand et al., 2006). However, after 2 days storage under the selected conditions, the diffraction peaks of V-type crystalline structure disappeared and the A-type crystalline structure with diffraction peaks at 15.3◦ , 17.2◦ , 19.8◦ and 22.8◦ of 2 was formed in the sample stored at 8 ◦ C for 20 days, and the ones at 30 ◦ C for 10 and 20 days, as well as the ones at 8/30 ◦ C for 6, 10 and 20 days. These observations suggest that, for the CSM, the temperature cycling treatment at 8/30 ◦ C is more helpful to formation of the A-type structure than the isothermal treatments at 8 ◦ C or 30 ◦ C. The relative crystallinity and moisture content of the CSM stored under the three conditions were presented in Table 1. Compared to the crystallinity of 6.41% in the crosslinked starch microspheres prepared from soluble starch and N,N -methylenebisacrylamide (Miao et al., 2010), the crystallinity (16.58%) of the fresh CSM in this study was much higher. The data in Table 1 indicated that storing the CSM at 8/30 ◦ C gave rise to higher rate and extent of retrogradation compared with storing isothermally at 8 ◦ C and 30 ◦ C. This may be because the temperature cycling induces a stepwise nucleation
222
X. Bai et al. / Industrial Crops and Products 51 (2013) 220–223
Table 1 Crystallinity and moisture content of corn starch microspheres (CSM) and amylose microspheres (AM) stored at different conditions. 8 ◦C
30 ◦ C
8/30 ◦ C
Sample
Time (day)
MC (%)
Crystallinity (%)
MC (%)
Crystallinity (%)
CSM
0 2 6 10 20
16.58 1.26 1.97 3.92 5.94
± ± ± ± ±
0.63 0.10 0.05 0.13 0.47
5.27 23.16 27.26 28.45 21.25
16.58 4.53 2.19 4.61 11.68
± ± ± ± ±
0.63 0.29 0.38 0.27 0.25
5.27 16.88 20.35 21.21 16.50
16.58 1.95 5.59 7.43 14.10
± ± ± ± ±
0.63 0.08 0.05 0.22 0.28
5.27 20.14 20.39 19.71 15.32
AM
0 2 6 10
10.39 9.10 9.47 3.76
± ± ± ±
0.37 0.66 0.82 0.25
8.07 22.80 20.14 22.12
10.39 4.85 5.48 4.71
± ± ± ±
0.37 0.51 0.69 0.15
8.07 21.89 19.51 20.27
10.39 9.32 11.40 10.73
± ± ± ±
0.37 0.24 0.65 0.21
8.07 20.89 14.89 16.67
Crystallinity (%)
MC (%)
hydrogen bonding were fewer, this would reduce moisture content in the microspheres. 3.2. Amylose microspheres (AM)
Fig. 2. X-ray diffraction patterns of corn starch microspheres (a) and amylose microspheres (b) stored at different conditions.
and propagation and therefore promotes the growth of crystalline regions (Silverio et al., 2000). It was noted that the moisture content in the samples increased with storing time first but decreased after 10 days storage at 8 ◦ C and 30 ◦ C, and 6 days storage at 8/30 ◦ C. Moisture content in starch films was also observed decreasing with increasing of crystallinity (Fama et al., 2007). The microspheres absorbed water when stored at 95% RH, and water could enhance mobility of starch molecules and help starch molecules reassociate into crystalline segments of the A-type. With propagation of crystallization, the available hydroxyl groups to absorb water through
The obtained AM consisted of particles ranging in size from approximately 8 m to over 150 m with a mean size of 42.05 m measured by using a laser particle size analyzer (BT-9300H, Bettersize Instruments Ltd., Dandong, China). Fig. 1(b) presented the SEM image of the AM, which showed considerable difference in appearance compared with the CSM, i.e., the shape was quite irregular and the surface of most particles was rough and porous. X-ray diffraction patterns of the AM stored at three temperature conditions for 0, 2, 6 and 10 days were given in Fig. 2(b). The relative crystallinity and moisture content of the AM stored at the three temperature conditions were also presented in Table 1. The fresh AM also possess the V-type crystalline structure, but the diffraction peaks become broader comparing with that of fresh CSM, indicating the crystallite size in AM is smaller since the width at half height of a peak is inversely proportional to the crystallite size. Differing from the CSM, the V-type structure in AM still existed after 6 days storage at 8 ◦ C although the intensity of diffraction peaks decreased. It was also noted from Fig. 2(b) that no A-type structure was detectable in the AM stored at 8 ◦ C and 30 ◦ C, but A-type structure was formed after 2 days storage at 8/30 ◦ C, more rapid than in CSM. It is generally believed that there are three phases to polymer crystallization, i.e., nucleation, propagation and maturation (Jacobson and BeMiller, 1998). The observed results indicated that the V-type crystalline structure in the AM was more stable during the storage at 8 ◦ C, these small crystals would act as physical crosslinking points and hinder rearrangement of amylose molecules so that propagation of A-type crystalline structure can not take place even nucleation may already occurs. On the other hand, although the V-type crystalline structure was disrupted during the storage at 30 ◦ C, this temperature (30 ◦ C) may be too high for nucleation of the A-type crystalline structure. Therefore, no A-type crystalline structure was formed in the AM under these isothermal treatments. For the rapid formation of the A-type crystalline structure in the AS stored at 8/30 ◦ C, the reason probably is that the lower temperature storage allows more nuclei of A-type crystalline structure be formed and the higher temperature storage promotes the propagation of these nuclei. Compared with the CSM, there is no amylopectin in the AM, which may facilitate rescrystallization of amylose. 4. Conclusion The fresh corn starch microspheres and amylose microspheres prepared by precipitating starch paste solution with absolute ethanol possess the V-type crystalline structure with crystallinity of 16.58% and 10.39% respectively. The V-type crystalline structure
X. Bai et al. / Industrial Crops and Products 51 (2013) 220–223
vanished after the temperature treatments under 95% RH, isothermally at 8 ◦ C and 30 ◦ C, as well as temperature cycle of 8 ◦ C/30 ◦ C. Retrogradation yielding A-type crystalline structure took place in the treated microspheres. The recrystallization rate was higher in 8 ◦ C/30 ◦ C cycling and the amylose microspheres recystallized more rapidly than the corn starch microspheres. The crystallinity of the corn starch microspheres and amylose microspheres reached 7.43% and 10.73%, respectively, after five 8 ◦ C/30 ◦ C cycles. Acknowledgements The authors are grateful to the National Natural Science Foundation of China (No. 51273083), Plan of Science and Technology Development of Jilin Province of China (No. 20110722) and “985 Project” of Jilin University for financial support. References Atyabi, F., Manoochehri, S., Moghadam, S.H., Dinarvand, R., 2006. Cross-linked starch microspheres: effect of cross-linking condition on the microsphere characteristics. Arch. Pharm. Res. 29, 1179–1786. Chin, S.F., Pang, S.C., Tay, S.H., 2011. Size controlled synthesis of starch nanoparticles by a simple nanoprecipitation method. Carbohydr. Polym. 86, 1817–1819. Elfstrand, L., Eliassion, A.C., Jonsson, M., Reslow, M., Wahlgren, M., 2006. From starch to starch microspheres: factors controlling the microspheres quality. Starch/Starke 58, 381–390. Fama, L., Goyanes, S., Gerschenson, L., 2007. Influence of storage time at room temperature on the physicochemical properties of cassava starch films. Carbohydr. Polym. 70, 265–273.
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Fang, Y.Y., Wang, L.J., Li, D., Li, B.Zh., Bhandari, B., Chen, X.D., 2008. Preparation of crosslinked starch microspheres and their drug loading and releasing properties. Carbohydr. Polym. 74, 379–384. García-González, C.A., Uy, J.J., Alnaief, M., Smirnova, I., 2012. Preparation of tailormade starch-based aerogel microspheres by the emulsion-gelation method. Carbohydr. Polym. 88, 1378–1386. Glenn, G.M., Klamczynski, A.P., Woods, D.F., Chiou, B., Orts, W.J., Iman, S.H., 2010. Encapsulation of plant oils in porous starch microspheres. J. Agric. Food Chem. 58, 4180–4184. Jacobson, M.R., BeMiller, J.N., 1998. Method for determining the rate and extent of accelerated starch retrogradation. Cereal Chem. 75, 22–29. Ma, X.F., Jian, R.J., Chang, P.R., Yu, J.G., 2008. Fabrication and characterization of citric acid-modified starch nanoparticles/plasticized-starch composites. Biomacromolecules 9, 3314–3320. Miao, Z., Ch Li Zh, J., Deng, D., Wang, L., Liu, Y., 2010. Novel crosslinked starch microspheres as adsorbents of Cu2+ . J. Appl. Polym. Sci. 115, 487–490. Nara, S., Komiya, T., 1983. Studies on the relationship between water-saturated state and crystallinity by the diffraction method for moistened potato starch. Starch/Starke 35, 407–410. Park, E.Y., Baik, B.K., Lim, S.T., 2009. Influences of temperature-cycled storage on retrogradation and in vitro digestibility of waxy maize starch gel. J. Cereal Sci. 50, 43–48. Silverio, J., Fredriksson, H., Andersson, R., Eliasson, A.C., Aman, P., 2000. The effect of temperature cycling on the amylopectin retrogradation of starches with different amylopectin unit-chain length distribution. Carbohydr. Polym. 42, 175–184. Tan, Y., Xu, K., Li, L.L., Liu, Ch., Song, Ch. L., Wang, P.X., 2009. Fabrication of sizecontrolled starch-based nanospheres by nanoprecipitation. ACS Appl. Mater. Inter. 1, 956–959. Yang, Y.T., Wei, X., Zh Sun, P., Wan, J.M., 2010. Preparation, characterization and adsorption performance of a novel anionic starch microsphere. Molecules 15, 2872–2885. Zhou, X., Baik, B.K., Wang, R., Lim, S.T., 2010. Retrogradation of waxy and normal corn starch gels by temperature cycling. J. Cereal Sci. 51, 57–65.
Contents lists available at ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Short communication
Changes in crystalline structure of microspheres of corn starch and amylose under isothermal and temperature cycling treatments Xia Bai a , Zhao Dong a , Xiuli Wu a,b , Jin Tong a , Jiang Zhou a,∗ a Key Laboratory of Bionic Engineering (Ministry of Education), College of Biological and Agricultural Engineering, Jilin University, 5988 Renmin Street, Changchun 130022, China b College of Biological Science and Technology, Changchun University, 6543 Weixing Road, Changchun 130012, China
a r t i c l e
i n f o
Article history: Received 14 May 2013 Received in revised form 4 September 2013 Accepted 5 September 2013 Keywords: Starch microsphere Crystalline structure Temperature treatments
a b s t r a c t Microspheres of corn starch and amylose were prepared by precipitating starch paste solution with absolute ethanol. The V-type crystalline structure was observed in the obtained microspheres. By storing the microspheres at three temperature conditions under 95% RH, constant temperatures at 8 ◦ C and 30 ◦ C as well as temperature cycles at 8 ◦ C for 1 day and at 30 ◦ C for 1 day, for different periods of time, changes in crystalline structure of the microspheres were investigated by X-ray diffraction (XRD) analysis. The results showed that the diffraction peaks of V-type crystalline structure vanished after the temperature treatments. Retrogradation yielding A-type crystalline structure took place in all the corn starch microspheres and only the amylose microspheres with 8 ◦ C/30 ◦ C treatment. Comparing with the isothermal treatments, the temperature cycling accelerated retrogradation of the microspheres. © 2013 Elsevier B.V. All rights reserved.
1. Introduction As one of the most abundant biopolymers in nature, starch is found in the tissues of many plants and has been considered as an alternative material for industrial applications because of its availability from renewable resources, biodegradability, derivability, non-toxicity and low cost. Starch microspheres, as one of starch products, have found applications in pharmaceutical industry (Fang et al., 2008), food industry (Glenn et al., 2010), material industry (Ma et al., 2008) and environmental engineering (Miao et al., 2010; Yang et al., 2010). The microstructure of starch microspheres has deep effects on these applications because it will determine performance of the microspheres, such as loading and release characteristics, mechanical properties and adsorption performance. Although there are several approaches to prepare starch microspheres, such as spray drying (Glenn et al., 2010), emulsioncrosslinking technique (Atyabi et al., 2006; Miao et al., 2010) and emulsion-gelation process followed by supercritical drying (García-González et al., 2012), the precipitation is a commonly used method (Ma et al., 2008; Tan et al., 2009; Chin et al., 2011). The precipitation process involves a successive addition of a dilute starch solution which is prepared by using solvent or gelatinizing starch
∗ Corresponding author. Tel.: +86 431 85095760x414; fax: +86 431 85095760x888. E-mail address: [email protected] (J. Zhou). 0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2013.09.007
in water to a nonsolvent or inversely. Therefore, the microstructure of the starch microspheres is dependant upon the preparation conditions and compositions. It is known that retrogradation will take place in gelatinized starch. Starch retrogradation is a non-equilibrium thermoreversible recrystallization process, its rate and extent depend on ratio and structures of amylose and amylopectin, storage temperature and time as well as water content. The retrogradation can be accelerated by treating gelatinized starch with temperature cycles between a temperature near the glass transition temperature and a higher temperature up to the melting temperature (Jacobson and BeMiller, 1998). Retrogradation of starch has been the subject of considerable investigation, but most research was carried out with high-moisture and intermediate-moisture starch materials such as pastes and gels (Jacobson and BeMiller, 1998; Park et al., 2009; Zhou et al., 2010). No detailed information is available about the retrogradation of starch microspheres with low moisture content. Understanding the changes of crystalline structure and crystallinity in starch microspheres is necessary for applications of starch microspheres. In this paper, starch microspheres were prepared by precipitation using corn starch and amylose. Then, the microspheres were stored under isothermal (8 ◦ C and 30 ◦ C) as well as temperature cycles of 8 ◦ C and 30 ◦ C for different periods to investigate changes in crystalline structures aiming at getting deeper insights of the influence of storage conditions on microstructure of starch microspheres.
X. Bai et al. / Industrial Crops and Products 51 (2013) 220–223
221
2. Experimental 2.1. Materials The corn starch (with about 25% amylose) was purchased from Changchun Jincheng Corn Development Co. Ltd. (Changchun, China). The amylose was purchased from Shaanxi Tianwei Biological Production Co. Ltd. (Xi’an, China). Absolute ethanol was purchased from Beijing Beihua Fine Chemicals Co. Ltd. (Beijing, China). All the materials were used as received. 2.2. Preparation of corn starch microspheres and amylose microspheres Ten grams of starch (corn starch or amylose) was mixed with 200 mL of deionized water, and the mixture was heated to 95 ◦ C and kept at this temperature for 1 h in water bath with constant stirring. After the temperature of the gelatinized starch solution was reduced to 50 ◦ C, 200 mL of absolute ethanol was dropwise added at the speed of 3 mL/min under constant stirring of 400 rpm. When the obtained microsphere suspension was cooled to room temperature, another 200 mL of absolute ethanol was dropwise added with constant stirring. The precipitate was collected by centrifugation with 6000 rpm, washed two times by using absolute ethanol to remove water, then air-dried at room temperature overnight (12 h). 2.3. Morphology observation Morphologies of corn starch microspheres and amylose microspheres were observed using a Philips XL30 FEG-ESEM (FEI Philips Electroscan, Mass., USA). Samples were mounted on specimen stubs with carbon black tape and then sputter-coated with gold before observation. 2.4. Temperature treating conditions
Fig. 1. SEM images of corn starch microspheres (a) and amylose microspheres (b).
The obtained starch microspheres were weighed into glass petri-dishes and stored under 95% RH at three different temperature conditions: constant temperatures at 8 ◦ C and 30 ◦ C, and temperature cycles at 8 ◦ C for 1 day and at 30 ◦ C for 1 day (8/30 ◦ C). 2.5. X-ray diffraction (XRD) analysis The crystalline structures of the starch microspheres were characterized using a Rigaku D/max-2500 X-ray diffractometer (Rigaku ˚ at Corporation, Tokyo, Japan) with Cu-K␣ radiation ( = 1.542 A) 40 kV and 250 mA. The XRD patterns were recorded over the 2 range of 4–40◦ at a speed of 2◦ /min. Relative crystallinity was evaluated from the ratio of the areas of the diffraction peaks to the area of the whole diffraction pattern subtracted amorphous background patterns (Nara and Komiya, 1983). 2.6. Moisture content Samples were dried in a vacuum oven at 105 ◦ C until constant weight. The moisture content (MC) of the starch microspheres was calculated using the measured wet weight, WW , and the dry weight, WD , by MC =
WW − WD × 100% WD
(1)
3. Results and discussion 3.1. Corn starch microspheres (CSM) The obtained CSM consisted of particles ranging in size from approximately 15 m to 100 m with a mean size of 37.40 m
measured by using a laser particle size analyzer (BT-9300H, Bettersize Instruments Ltd., Dandong, China). Fig. 1(a) showed the SEM image of the CSM, which revealed that the CSM were basically spherical shape with some concavities and small irregular fragments on surface, though some particles were irregularly shaped. X-ray diffraction patterns of the CSM stored at three temperature conditions for 0, 2, 6, 10 and 20 days were given in Fig. 2(a). The fresh CSM possess a typical V-type crystalline structure with diffraction peaks at 7.7◦ , 13.3◦ and 20.4◦ of 2. This result is different from that in the starch microspheres prepared using an aqueous two-phase system consisting of two structurally different polymers (poly(ethylene glycol) and starch), which gave A-type diffraction patterns (Elfstrand et al., 2006). However, after 2 days storage under the selected conditions, the diffraction peaks of V-type crystalline structure disappeared and the A-type crystalline structure with diffraction peaks at 15.3◦ , 17.2◦ , 19.8◦ and 22.8◦ of 2 was formed in the sample stored at 8 ◦ C for 20 days, and the ones at 30 ◦ C for 10 and 20 days, as well as the ones at 8/30 ◦ C for 6, 10 and 20 days. These observations suggest that, for the CSM, the temperature cycling treatment at 8/30 ◦ C is more helpful to formation of the A-type structure than the isothermal treatments at 8 ◦ C or 30 ◦ C. The relative crystallinity and moisture content of the CSM stored under the three conditions were presented in Table 1. Compared to the crystallinity of 6.41% in the crosslinked starch microspheres prepared from soluble starch and N,N -methylenebisacrylamide (Miao et al., 2010), the crystallinity (16.58%) of the fresh CSM in this study was much higher. The data in Table 1 indicated that storing the CSM at 8/30 ◦ C gave rise to higher rate and extent of retrogradation compared with storing isothermally at 8 ◦ C and 30 ◦ C. This may be because the temperature cycling induces a stepwise nucleation
222
X. Bai et al. / Industrial Crops and Products 51 (2013) 220–223
Table 1 Crystallinity and moisture content of corn starch microspheres (CSM) and amylose microspheres (AM) stored at different conditions. 8 ◦C
30 ◦ C
8/30 ◦ C
Sample
Time (day)
MC (%)
Crystallinity (%)
MC (%)
Crystallinity (%)
CSM
0 2 6 10 20
16.58 1.26 1.97 3.92 5.94
± ± ± ± ±
0.63 0.10 0.05 0.13 0.47
5.27 23.16 27.26 28.45 21.25
16.58 4.53 2.19 4.61 11.68
± ± ± ± ±
0.63 0.29 0.38 0.27 0.25
5.27 16.88 20.35 21.21 16.50
16.58 1.95 5.59 7.43 14.10
± ± ± ± ±
0.63 0.08 0.05 0.22 0.28
5.27 20.14 20.39 19.71 15.32
AM
0 2 6 10
10.39 9.10 9.47 3.76
± ± ± ±
0.37 0.66 0.82 0.25
8.07 22.80 20.14 22.12
10.39 4.85 5.48 4.71
± ± ± ±
0.37 0.51 0.69 0.15
8.07 21.89 19.51 20.27
10.39 9.32 11.40 10.73
± ± ± ±
0.37 0.24 0.65 0.21
8.07 20.89 14.89 16.67
Crystallinity (%)
MC (%)
hydrogen bonding were fewer, this would reduce moisture content in the microspheres. 3.2. Amylose microspheres (AM)
Fig. 2. X-ray diffraction patterns of corn starch microspheres (a) and amylose microspheres (b) stored at different conditions.
and propagation and therefore promotes the growth of crystalline regions (Silverio et al., 2000). It was noted that the moisture content in the samples increased with storing time first but decreased after 10 days storage at 8 ◦ C and 30 ◦ C, and 6 days storage at 8/30 ◦ C. Moisture content in starch films was also observed decreasing with increasing of crystallinity (Fama et al., 2007). The microspheres absorbed water when stored at 95% RH, and water could enhance mobility of starch molecules and help starch molecules reassociate into crystalline segments of the A-type. With propagation of crystallization, the available hydroxyl groups to absorb water through
The obtained AM consisted of particles ranging in size from approximately 8 m to over 150 m with a mean size of 42.05 m measured by using a laser particle size analyzer (BT-9300H, Bettersize Instruments Ltd., Dandong, China). Fig. 1(b) presented the SEM image of the AM, which showed considerable difference in appearance compared with the CSM, i.e., the shape was quite irregular and the surface of most particles was rough and porous. X-ray diffraction patterns of the AM stored at three temperature conditions for 0, 2, 6 and 10 days were given in Fig. 2(b). The relative crystallinity and moisture content of the AM stored at the three temperature conditions were also presented in Table 1. The fresh AM also possess the V-type crystalline structure, but the diffraction peaks become broader comparing with that of fresh CSM, indicating the crystallite size in AM is smaller since the width at half height of a peak is inversely proportional to the crystallite size. Differing from the CSM, the V-type structure in AM still existed after 6 days storage at 8 ◦ C although the intensity of diffraction peaks decreased. It was also noted from Fig. 2(b) that no A-type structure was detectable in the AM stored at 8 ◦ C and 30 ◦ C, but A-type structure was formed after 2 days storage at 8/30 ◦ C, more rapid than in CSM. It is generally believed that there are three phases to polymer crystallization, i.e., nucleation, propagation and maturation (Jacobson and BeMiller, 1998). The observed results indicated that the V-type crystalline structure in the AM was more stable during the storage at 8 ◦ C, these small crystals would act as physical crosslinking points and hinder rearrangement of amylose molecules so that propagation of A-type crystalline structure can not take place even nucleation may already occurs. On the other hand, although the V-type crystalline structure was disrupted during the storage at 30 ◦ C, this temperature (30 ◦ C) may be too high for nucleation of the A-type crystalline structure. Therefore, no A-type crystalline structure was formed in the AM under these isothermal treatments. For the rapid formation of the A-type crystalline structure in the AS stored at 8/30 ◦ C, the reason probably is that the lower temperature storage allows more nuclei of A-type crystalline structure be formed and the higher temperature storage promotes the propagation of these nuclei. Compared with the CSM, there is no amylopectin in the AM, which may facilitate rescrystallization of amylose. 4. Conclusion The fresh corn starch microspheres and amylose microspheres prepared by precipitating starch paste solution with absolute ethanol possess the V-type crystalline structure with crystallinity of 16.58% and 10.39% respectively. The V-type crystalline structure
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