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amylopectin ratios
Journal of Cereal Science 51 (2010) 388e391
Contents lists available at ScienceDirect
Journal of Cereal Science journal homepage: www.elsevier.com/locate/jcs
Glass transition temperature of starches with different amylose/amylopectin ratios Peng Liu a, Long Yu a, b, *, Xueyu Wang a, Dan Li a, Ling Chen a, Xiaoxi Li a a b
CPFRR, School of Light Industry and Food, ERCPSP, South China University of Technology, PR China Commonwealth Scientific and Industrial Research Organisation Materials Science and Engineering, Melbourne, Vic. 3169, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2009 Received in revised form 26 January 2010 Accepted 10 February 2010
The glass transition temperatures (Tg) of starch with different amylose/amylopectin ratios were systematically studied by a high-speed DSC. The cornstarches with different amylose contents (waxy 0; maize 23, G50 50 and G80 80) were used as model materials. The high heating speed (up to 300 C/min) allows the weak Tg of starch to be visible and the true Tg was calculated by applying linear regression to the results from different heating rates. It is confirmed for the first time, that the higher the amylose content is, the higher the Tg is for the same kind of starch. The sequence of true Tg of cornstarch is G80 > G50 > maize > waxy when samples contain the same moisture content, which corresponds to their amylose/amylopectin ratio. It was found that Tg was increased from about 52 to 60 C with increasing amylose content from 0 to 80 for the samples containing about 13% moisture. The microstructure and phase transition were used to explain this phenomenon, in particular the multiphase transitions that occur in high-amylose starches at higher temperatures, and the gel-ball structure of gelatinized amylopectin. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Starch Glass transition temperature Amylose/amylopectin High-speed DSC
1. Introduction It is well known that the glass transition temperature (Tg) represents the narrow temperature range of the transition of polymers from a hard and brittle glass into a softer, rubbery state. The origin of the Tg is the onset of large-scale cooperative motion of polymer chain segments (of the order of 20e50 consecutive carbon atoms) (Ebewele, 2000). Polymer Tg generally depends on chain flexibility, stiffness, geometric configuration, copolymerization, molecular weight, branching, cross-linking, crystallinity, plasticization, pressure, and rate of testing (Stevens, 1999). In practice, the most popular and conventional techniques to detect the Tg are differential scanning calorimeter (DSC) and dynamic mechanical analysis (DMA). However, the measurement of Tg for starch and starch-based materials by DSC is difficult since the change of heat capacity or the signal from heat flow is usually much weaker than that of conventional polymers (Liu et al., 2006; Yu and Christie, 2001). Furthermore, the multiple phase transitions that starch undergoes during heating and the instability of water (such as evaporation) contained in starch make it more difficult to study * Corresponding author at: Commonwealth Scientific and Industrial Research Organisation Materials Science and Engineering, Melbourne, Vic. 3169, Australia. Tel.: þ61 3 9545 2777; fax: þ61 3 9544 1128. E-mail address: [email protected] (L. Yu). 0733-5210/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcs.2010.02.007
the thermal behavior of starch materials using DSC (Liu et al., 2009b; Yu and Christie, 2001; Zeleznak and Hoseney, 1987). The moisture evaporation during heating is also the reason that other techniques such as DMA cannot be simply used to measure the Tg of starch. Recently a high-speed differential scanning calorimetry (Hyper-DSC), has attracted much attention for observing glass transitions using its extremely high heating rate to enlarge the weak signal and minimize the effect of losing moisture (Gramaglia et al., 2005; Katayama et al., 2008; Liu P. et al., 2009; McGregor and Bines, 2008; McGregor et al., 2004; Saunders et al., 2004). It is well known that most of the granular native starches are a mixture of amylose, a linear structure of alpha-1,4 linked glucose units, and amylopectin, a highly branched structure of short alpha1,4 chains linked by alpha-1,6 bonds. Most native starches are semicrystalline with crystallinity about 20e45% (Whistler et al., 1984; Zobel, 1988). Amylose and the branching points of amylopectin form the amorphous regions. The short branching chains in amylopectin are the main crystalline component in granular starch. The molecular weight of amylopectin is about 100 times larger than that of amylose. Study of the effect of the amylose/amylopectin ratio on Tg has both scientific and commercial importance. Arvanitoyannis et al. (1994) and Psomiadou et al. (1996) reported that the Tg of potato starch was slightly higher than that of rice starch, and the Tg was decreased by various plasticizers. Based on the results of extracted, then acetated amylose and amylopectin
P. Liu et al. / Journal of Cereal Science 51 (2010) 388e391
from native potato starch, previous studies (Bizot et al., 1997; Gowie and Henshall, 1976; Oford et al., 1989) have deduced that the Tg of amylose is higher than that of amylopectin, with the explanation that linear chains appear to favor chainechain interactions and induce-partial crystallinity. Branched chains, on the other hand, have chain end effects and flexibility of branch points. However, starches used for previous work with different amylose content came from different sources, or the amylose used in these studies was extracted from starch granules by n-butanol. There is no systematic report about the Tg of the same kind of starch with different amylose/amylopectin ratios. Furthermore, because of the problems from conventional DSC mentioned before, the results of the measured Tg from previous reports are significantly different and sometimes contradictory (Liu P. et al., 2009). In a previous work, we have successfully detected the Tg of cornstarch film using a high-speed DSC (Liu P. et al., 2009). In this work, cornstarch with different amylose/amylopectin ratios (waxy 0; maize 23, G50 50 and G80 80) are used as model materials to study their Tg by high-speed DSC with heating rate up to 300 C/ min. The true Tg of samples with different moisture content will be calculated by the method of linear regression equation through plotting the measured results from different heating rates. The effect of amylose content on Tg will be discussed based on their microstructures and phase transitions. 2. Experimental 2.1. Materials and sample preparation Cornstarches with different amylose/amylopectin ratios were used in the experimental work. All the starches are commercially available and were kindly supplied by Penford (Australia). Table 1 shows details of the starches studied. Starch film was prepared by casting. For waxy and maize starch, a 3% starch suspension was first stirred and then heated to 98 C, and kept for 2 h to fully gelatinize the starch (Chen et al., 2007). For G50 and G80, the suspension was heated to 125 C for 1 h (Liu et al., 2006). Then the solution was cast on glass and dried by airflow at 50 C for 24 h. The thickness of the films is about 0.10e0.12 mm. The film was stored in different relative humidity environments (24%, 33%, and 54%), which were maintained by saturated CH3COOK, MgCl, and Mg(NO3)2 solutions respectively for 48 h to equilibrate the moisture content. The actual moisture contents of the film samples were measured by drying samples in a vacuum at 130 C for 24 h.
389
The specimen with about 2 mg was sealed in an aluminium pan (PE No. 0219-0041). Tg was taken as the half variation in heat capacity occurring at the transition. All results were the average of triplicate parallel experiments. All of the measured results, such as sample temperature, time, Tg, and heating rate, were imported into Excel software to obtain the linear regression equation and linear dependent coefficient (R2). The R2 was used to evaluate the linearity of variable X vs. Y, with the value of R2 decreasing from 1 as the linear fit decreases. 3. Results and discussions Figs. 1 and 2 show the typical DSC thermograms of gelatinized cornstarch of pure amylopectin (waxy) and high amylose (G80) starches containing similar moisture content at different heating rates. It can be seen that there are no gelatinization endotherms being detected in the curves, which indicates that the starches have been fully gelatinized. Furthermore, there is no clear step change being detected when the heating rate is lower for all starches, and the size of the step change increased with increasing heating rate for both samples. The medial point of the step change (Tg) increased with increasing heating rate for all samples. Due to moisture evaporation, there is an upward excursion on each curve when the temperature exceeds 95 C. Similar curves for other starch samples with different moisture content have also been observed. Fig. 3 shows the plots of measured Tg vs. heating rate for different starch films with different moisture content. It can be seen that the Tg increases linearly with increasing heating rate for all starches (with R2 > 0.90). Measured Tg increased with decreasing moisture content for all starches, which is expected. In this work, the extrapolation method was used to calculate the extrapolated Tg. Theoretically, the heating rate can only change the kinetics of a thermal event when all other heating conditions are constant, such as film thickness, DSC pans, and the headspace between sample film and cover of the pan, which results in a linear relationship between Tg and heating rate (see Fig. 3). The extrapolated Tg can be obtained through a linear regression equation (Liu et al., 2009a,b), which represents the theoretical Tg under an extremely slow heating rate (approaching 0 C/min). Based on the results from Fig. 3, the extrapolated Tg was calculated for various starches. The slope and extrapolated Tg calculated from the regression equation are listed in Table 2. As expected, it is observed that the extrapolated Tg increased with decreasing moisture content. It has been noted that the slopes are not parallel for a starch containing different moisture content. This could be the
2.2. High-speed DSC A PerkineElmer DSC Diamond-1 with an internal coolant (Intercooler 1P) and nitrogen purge gas were used in the experimental work. The instrument was calibrated for temperature and heat flow using indium and zinc as the standards. A baseline using an empty pan was established for each corresponding heating rate.
Table 1 List of starches studied and their characterization. Starch
Amylose content (%)
Molecular weight (MW)a
Crystal patternb
Crystallinity (%)b
Waxy Maize G50 G80
0 26 50 80
20,787,000 13,000,000 5,115,000 673,000
A A B B
42.3 38.3 31.3 28.3
a b
Molecular weight measured by GPC (supplied by Penford (Australia)). Measured by wide-angle X-ray diffraction (see Liu et al., 2009a).
Fig. 1. DSC thermograms of waxy film with 13.2% moisture content under different heating rates ( C/min): 270, 250, 230, 200, 180, 160, 120 and 20 (from top to bottom).
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P. Liu et al. / Journal of Cereal Science 51 (2010) 388e391 Table 2 The linear regression equation and extrapolated Tg of films from G80, G50, maize starch and waxy cornstarches. Moisture Regression equation content (%) G80
9.3 11.5 13.3 G50 9.1 11.8 13.4 Maize 8.7 11.6 13.3 Waxy 8.7 11.2 13.2
y y y y y y y y y y y y
¼ 0.0723x ¼ 0.0643x ¼ 0.0416x ¼ 0.0591x ¼ 0.0559x ¼ 0.0476x ¼ 0.0749x ¼ 0.0594x ¼ 0.0515x ¼ 0.0488x ¼ 0.0534x ¼ 0.0612x
þ 73.518 þ 69.133 þ 60.368 þ 70.163 þ 62.957 þ 60.747 þ 67.328 þ 61.363 þ 57.424 þ 59.565 þ 57.191 þ 53.894
Slope
Extrapolate Dependent coefficient (R2) Tg ( C)
0.0723 0.0643 0.0416 0.0591 0.0559 0.0476 0.0749 0.0594 0.0515 0.0488 0.0534 0.0612
0.9847 0.9576 0.9117 0.9711 0.9555 0.9780 0.9596 0.9912 0.9571 0.9534 0.9933 0.9229
73.5 69.1 60.4 70.2 63.0 60.7 67.3 61.4 57.4 59.6 57.2 53.9
Fig. 2. DSC thermograms of G80 film with 13.8% moisture content under different heating rates ( C/min): 300, 250, 200, 180, 160, 140 and 20 (from top to bottom).
nature of the thermal behaviors for the samples measured by DSC. Different thermal conductivity or heat capacity of different samples containing different moisture content could cause this. This phenomenon will not affect the extrapolated results. Fig. 4 shows the plot of extrapolated Tg of various starches vs. different moisture content. The extrapolated Tg increased with decreasing moisture content for all starches. Furthermore, the linear regression equation and the true Tg can also be calculated from Fig. 4. The sequence of true Tg of the cornstarch is G80 > G50 > maize > waxy when samples contain the same moisture content, which corresponds to their amylose/amylopectin ratio. This is the first confirmation that the higher the amylose content is, the higher the Tg is for the same kind of starch. Previous studies (Bizot et al., 1997; Gowie and Henshall, 1976; Oford et al., 1989) considered that linear chains appear to favor chainechain
interactions and induce-partial crystallinity. Branched chains, in contrast, have chain end effects and flexibility of branch points. We would like to use the crystal structure and the microstructure of gelatinized starch to further explain this phenomenon. The starches with B type crystal structure (G50 and G80) show higher Tg than that with A type crystal structure (waxy and maize) (see Table 1). It has also been noted that high amylopectin starches have higher molecular weight and higher crystallinity but lower Tg. Our previous study (Yu and Christie, 2005) has shown that the double helical, crystalline structures formed by the short branched chains in amylopectin are torn apart during gelatinization. However, these short-branched chains remain in a regular pattern by retaining a certain “memory”. These short branched chains form gel-balls that are comprised mainly of chains from the same sub-main chain. In addition, one amylopectin molecule may form a relatively separate super-globe. The molecular entanglements between gel-balls and
90
75
Maize
Waxy 80
Tg (
Tg (
)
)
70
70
8.70% 11.2% 13.20%
65
60 100
150
200
Heating rate (
250
60
300
Series1 8.7% Series2 11.6% Series3 13.3% 90
140
/min)
190
Heating rate (
240
/min)
100
90
G80
G50 90
Tg (
Tg (
)
)
80
70
9.30% ♦ 11.80% ■ 13.40% 150
200
Heating rate (
250
/min)
9.30% 11.50% ♦ 13.30%
70
▲
60 100
80
▲ ■
300
60
90
140
190
Heating rate (
240
290
/min)
Fig. 3. Plots of measured Tg vs. heating rate for different starch films containing different moisture content.
340
P. Liu et al. / Journal of Cereal Science 51 (2010) 388e391
391
References
Fig. 4. The plots of extrapolated Tg of various starches with different moisture content.
super-globes are much less than those between linear polymeric chains, due to their size and the length of the chains (only 4e6 glucose). These gel-balls require less energy to move than long linear chains, especially when they are lubricated by a plasticizer (water). This explains why high amylopectin materials initially have lower modulus and higher elongation, and lower viscosity during extrusion (Xie et al., 2009; Yu and Christie, 2005). It could also explain why high amylopectin starches exhibit lower Tg. 4. Conclusions The Tg of starch with different amylose/amylopectin ratios was systematically studied using a high-speed DSC with heating rate up to 300 C/min. It was observed that the Tg increases linearly with increasing heating rate for all starches (with R2 > 0.90). Extrapolation was used to calculate the extrapolated Tg. The extrapolated Tg increased with decreasing moisture content for all starches. From the plot of extrapolated Tg vs. moisture content, the true Tg was obtained. It was found that Tg was increased from about 52 to 60 C with increasing amylose content from 0 to 80 for the samples containing about 13% moisture. This confirms for the first time that the higher the amylose content is, the higher the Tg is for the same kind of starch. The sequence of true Tg of cornstarch is G80 > G50 > maize > waxy when samples contain the same moisture content, which corresponds to their amylose/amylopectin ratio. The unique gel-ball structure of gelatinized amylopectin can be used to explain this phenomenon. Acknowledgement The authors from SCUT, China, would like to acknowledge the research funds NRDPHT (863) (2007AA10Z312, 2007AA100407), NKTRDP (2006BAD27B04) and ASTATFP (2009GB23600523).
Arvanitoyannis, I., Kalichevsky, M., Blanshard, J.M.V., Psomiadou, E., 1994. Study of diffusion and permeation of gases in undrawn and uniaxially drawn films made from potato and rice starch conditioned at different relative humidity. Carbohydrate Polymers 24, 1e15. Bizot, H., LeBail, P., Leroux, B., Davy, J., Roger, P., Buleon, A., 1997. Calorimetric evaluation of the glass transition in hydrated, linear and branched polyanhydroglucose compounds. Carbohydrate Polymers 32 (1), 33e50. Chen, P., Yu, L., Kealy, T., Chen, L., Li, L., 2007. Phase transition of starch granules observed by microscope under shearless and shear conditions. Carbohydrate Polymers 68 (3), 495e501. Ebewele, R.O., 2000. Polymer Science and Technology. New York, Washington, D.C.. CRC Press, Baca Raton London. Gowie, J.M.G., Henshall, S.A.E., 1976. The influence of chain length and branching on the glass transition temperature of some polyglucosans. European Polymer Journal 12, 215e218. Gramaglia, D., Conway, B.R., Kett, V.L., Malcolm, R.K., Batchelor, H.K., 2005. High speed DSC (hyper-DSC) as a tool to measure the solubility of a drug within a solid or semi-solid matrix. International Journal of Pharmaceutic 301, 1e5. Katayama, D.S., Carpenter, J.F., Manning, M.C., Randolph, T.W., Setlow, P., Menard, K. P., 2008. Characterization of amorphous solids with weak glass transitions using high ramp rate differential scanning calorimetry. Journal of Pharmaceutical Sciences 97, 1013e1024. Liu, H., Yu, L., Xie, F., Chen, L., 2006. Gelatinization of cornstarch with different amylose/amylopectin content. Carbohydrate Polymers 65, 357e363. Liu, H., Yu, L., Simon, G., Dean, K., Chen, L., 2009a. Effect of annealing on gelatinization and microstructure of corn starches with different amylose/amylopectin ratios. Carbohydrate Polymers 77, 662e669. Liu, H., Yu, L., Xie, F., Chen, L., Li, L., 2009b. Thermal processing of starch-based polymers. Progress in Polymer Science 34, 1348e1368. Liu, P., Yu, L., Liu, H., Chen, L., Li, L., 2009. Glass transition temperature of starch studied by a high-speed DSC. Carbohydrate Polymers 77, 250e253. McGregor, C., Bines, E., 2008. The use of high-speed differential scanning calorimetry (Hyper-DSCTM) in the study of pharmaceutical polymorphs. International Journal of Pharmaceutics 350, 48e52. McGregor, C., Saunders, M.H., Buckton, G., Saklatvala, R.D., 2004. The use of highspeed differential scanning calorimetry (Hyper-DSCTM) to study the thermal properties of carbamazepine polymorphs. Thermochimica Acta 417, 231e237. Oford, P.D., Parker, R., Ring, S.G., Simth, A.C., 1989. Effect of water as a diluent on the glass transition behavior of malto-oligosaccharides, amylose and amylopectin. International Journal of Biological Macromolecules 11, 91e96. Psomiadou, E., Arvanitoyannis, I., Yamamoto, N., 1996. Edible films made from natural resources; microcrystalline cellulose (MCC), methylcellulose (MC) and corn starch and polyols-Part 2. Carbohydrate Polymers 31, 193e204. Saunders, M., Podluii, K., Shergill, S., Buckton, G., Royall, P., 2004. The potential of high speed DSC (Hyper-DSC) for the detection and quantification of small amounts of amorphous content in predominantly crystalline samples. International Journal of Pharmaceutics 274, 35e40. Stevens, M.P., 1999. Polymer Chemistry. Oxford University Press, New York. Whistler, R.L., Bemiller, J.N., Paschall, E.F., 1984. Starch: Chemistry and Technology. Academic Press, London. Xie, F., Yu, L., Su, B., Liu, P., Wang, J., Liu, H., Chen, L., 2009. Rheological properties of starch with different amylose/amylopectin ratios. Journal of Cereal Science 49, 371e377. Yu, L., Christie, G., 2001. Measurement of starch thermal transitions using differential scanning calorimetry. Carbohydrate Polymers 46, 179e184. Yu, L., Christie, G., 2005. Microstructure and mechanical properties of orientated thermoplastic starches. Journal of Materials Science 40, 111e116. Zeleznak, K.L., Hoseney, R.C., 1987. The glass transition in starch. Cereal Chemistry 64, 121e124. Zobel, H.F., 1988. Molecules to granules: a comprehensive starch review. Starch 40, 44e50.
Contents lists available at ScienceDirect
Journal of Cereal Science journal homepage: www.elsevier.com/locate/jcs
Glass transition temperature of starches with different amylose/amylopectin ratios Peng Liu a, Long Yu a, b, *, Xueyu Wang a, Dan Li a, Ling Chen a, Xiaoxi Li a a b
CPFRR, School of Light Industry and Food, ERCPSP, South China University of Technology, PR China Commonwealth Scientific and Industrial Research Organisation Materials Science and Engineering, Melbourne, Vic. 3169, Australia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2009 Received in revised form 26 January 2010 Accepted 10 February 2010
The glass transition temperatures (Tg) of starch with different amylose/amylopectin ratios were systematically studied by a high-speed DSC. The cornstarches with different amylose contents (waxy 0; maize 23, G50 50 and G80 80) were used as model materials. The high heating speed (up to 300 C/min) allows the weak Tg of starch to be visible and the true Tg was calculated by applying linear regression to the results from different heating rates. It is confirmed for the first time, that the higher the amylose content is, the higher the Tg is for the same kind of starch. The sequence of true Tg of cornstarch is G80 > G50 > maize > waxy when samples contain the same moisture content, which corresponds to their amylose/amylopectin ratio. It was found that Tg was increased from about 52 to 60 C with increasing amylose content from 0 to 80 for the samples containing about 13% moisture. The microstructure and phase transition were used to explain this phenomenon, in particular the multiphase transitions that occur in high-amylose starches at higher temperatures, and the gel-ball structure of gelatinized amylopectin. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Starch Glass transition temperature Amylose/amylopectin High-speed DSC
1. Introduction It is well known that the glass transition temperature (Tg) represents the narrow temperature range of the transition of polymers from a hard and brittle glass into a softer, rubbery state. The origin of the Tg is the onset of large-scale cooperative motion of polymer chain segments (of the order of 20e50 consecutive carbon atoms) (Ebewele, 2000). Polymer Tg generally depends on chain flexibility, stiffness, geometric configuration, copolymerization, molecular weight, branching, cross-linking, crystallinity, plasticization, pressure, and rate of testing (Stevens, 1999). In practice, the most popular and conventional techniques to detect the Tg are differential scanning calorimeter (DSC) and dynamic mechanical analysis (DMA). However, the measurement of Tg for starch and starch-based materials by DSC is difficult since the change of heat capacity or the signal from heat flow is usually much weaker than that of conventional polymers (Liu et al., 2006; Yu and Christie, 2001). Furthermore, the multiple phase transitions that starch undergoes during heating and the instability of water (such as evaporation) contained in starch make it more difficult to study * Corresponding author at: Commonwealth Scientific and Industrial Research Organisation Materials Science and Engineering, Melbourne, Vic. 3169, Australia. Tel.: þ61 3 9545 2777; fax: þ61 3 9544 1128. E-mail address: [email protected] (L. Yu). 0733-5210/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jcs.2010.02.007
the thermal behavior of starch materials using DSC (Liu et al., 2009b; Yu and Christie, 2001; Zeleznak and Hoseney, 1987). The moisture evaporation during heating is also the reason that other techniques such as DMA cannot be simply used to measure the Tg of starch. Recently a high-speed differential scanning calorimetry (Hyper-DSC), has attracted much attention for observing glass transitions using its extremely high heating rate to enlarge the weak signal and minimize the effect of losing moisture (Gramaglia et al., 2005; Katayama et al., 2008; Liu P. et al., 2009; McGregor and Bines, 2008; McGregor et al., 2004; Saunders et al., 2004). It is well known that most of the granular native starches are a mixture of amylose, a linear structure of alpha-1,4 linked glucose units, and amylopectin, a highly branched structure of short alpha1,4 chains linked by alpha-1,6 bonds. Most native starches are semicrystalline with crystallinity about 20e45% (Whistler et al., 1984; Zobel, 1988). Amylose and the branching points of amylopectin form the amorphous regions. The short branching chains in amylopectin are the main crystalline component in granular starch. The molecular weight of amylopectin is about 100 times larger than that of amylose. Study of the effect of the amylose/amylopectin ratio on Tg has both scientific and commercial importance. Arvanitoyannis et al. (1994) and Psomiadou et al. (1996) reported that the Tg of potato starch was slightly higher than that of rice starch, and the Tg was decreased by various plasticizers. Based on the results of extracted, then acetated amylose and amylopectin
P. Liu et al. / Journal of Cereal Science 51 (2010) 388e391
from native potato starch, previous studies (Bizot et al., 1997; Gowie and Henshall, 1976; Oford et al., 1989) have deduced that the Tg of amylose is higher than that of amylopectin, with the explanation that linear chains appear to favor chainechain interactions and induce-partial crystallinity. Branched chains, on the other hand, have chain end effects and flexibility of branch points. However, starches used for previous work with different amylose content came from different sources, or the amylose used in these studies was extracted from starch granules by n-butanol. There is no systematic report about the Tg of the same kind of starch with different amylose/amylopectin ratios. Furthermore, because of the problems from conventional DSC mentioned before, the results of the measured Tg from previous reports are significantly different and sometimes contradictory (Liu P. et al., 2009). In a previous work, we have successfully detected the Tg of cornstarch film using a high-speed DSC (Liu P. et al., 2009). In this work, cornstarch with different amylose/amylopectin ratios (waxy 0; maize 23, G50 50 and G80 80) are used as model materials to study their Tg by high-speed DSC with heating rate up to 300 C/ min. The true Tg of samples with different moisture content will be calculated by the method of linear regression equation through plotting the measured results from different heating rates. The effect of amylose content on Tg will be discussed based on their microstructures and phase transitions. 2. Experimental 2.1. Materials and sample preparation Cornstarches with different amylose/amylopectin ratios were used in the experimental work. All the starches are commercially available and were kindly supplied by Penford (Australia). Table 1 shows details of the starches studied. Starch film was prepared by casting. For waxy and maize starch, a 3% starch suspension was first stirred and then heated to 98 C, and kept for 2 h to fully gelatinize the starch (Chen et al., 2007). For G50 and G80, the suspension was heated to 125 C for 1 h (Liu et al., 2006). Then the solution was cast on glass and dried by airflow at 50 C for 24 h. The thickness of the films is about 0.10e0.12 mm. The film was stored in different relative humidity environments (24%, 33%, and 54%), which were maintained by saturated CH3COOK, MgCl, and Mg(NO3)2 solutions respectively for 48 h to equilibrate the moisture content. The actual moisture contents of the film samples were measured by drying samples in a vacuum at 130 C for 24 h.
389
The specimen with about 2 mg was sealed in an aluminium pan (PE No. 0219-0041). Tg was taken as the half variation in heat capacity occurring at the transition. All results were the average of triplicate parallel experiments. All of the measured results, such as sample temperature, time, Tg, and heating rate, were imported into Excel software to obtain the linear regression equation and linear dependent coefficient (R2). The R2 was used to evaluate the linearity of variable X vs. Y, with the value of R2 decreasing from 1 as the linear fit decreases. 3. Results and discussions Figs. 1 and 2 show the typical DSC thermograms of gelatinized cornstarch of pure amylopectin (waxy) and high amylose (G80) starches containing similar moisture content at different heating rates. It can be seen that there are no gelatinization endotherms being detected in the curves, which indicates that the starches have been fully gelatinized. Furthermore, there is no clear step change being detected when the heating rate is lower for all starches, and the size of the step change increased with increasing heating rate for both samples. The medial point of the step change (Tg) increased with increasing heating rate for all samples. Due to moisture evaporation, there is an upward excursion on each curve when the temperature exceeds 95 C. Similar curves for other starch samples with different moisture content have also been observed. Fig. 3 shows the plots of measured Tg vs. heating rate for different starch films with different moisture content. It can be seen that the Tg increases linearly with increasing heating rate for all starches (with R2 > 0.90). Measured Tg increased with decreasing moisture content for all starches, which is expected. In this work, the extrapolation method was used to calculate the extrapolated Tg. Theoretically, the heating rate can only change the kinetics of a thermal event when all other heating conditions are constant, such as film thickness, DSC pans, and the headspace between sample film and cover of the pan, which results in a linear relationship between Tg and heating rate (see Fig. 3). The extrapolated Tg can be obtained through a linear regression equation (Liu et al., 2009a,b), which represents the theoretical Tg under an extremely slow heating rate (approaching 0 C/min). Based on the results from Fig. 3, the extrapolated Tg was calculated for various starches. The slope and extrapolated Tg calculated from the regression equation are listed in Table 2. As expected, it is observed that the extrapolated Tg increased with decreasing moisture content. It has been noted that the slopes are not parallel for a starch containing different moisture content. This could be the
2.2. High-speed DSC A PerkineElmer DSC Diamond-1 with an internal coolant (Intercooler 1P) and nitrogen purge gas were used in the experimental work. The instrument was calibrated for temperature and heat flow using indium and zinc as the standards. A baseline using an empty pan was established for each corresponding heating rate.
Table 1 List of starches studied and their characterization. Starch
Amylose content (%)
Molecular weight (MW)a
Crystal patternb
Crystallinity (%)b
Waxy Maize G50 G80
0 26 50 80
20,787,000 13,000,000 5,115,000 673,000
A A B B
42.3 38.3 31.3 28.3
a b
Molecular weight measured by GPC (supplied by Penford (Australia)). Measured by wide-angle X-ray diffraction (see Liu et al., 2009a).
Fig. 1. DSC thermograms of waxy film with 13.2% moisture content under different heating rates ( C/min): 270, 250, 230, 200, 180, 160, 120 and 20 (from top to bottom).
390
P. Liu et al. / Journal of Cereal Science 51 (2010) 388e391 Table 2 The linear regression equation and extrapolated Tg of films from G80, G50, maize starch and waxy cornstarches. Moisture Regression equation content (%) G80
9.3 11.5 13.3 G50 9.1 11.8 13.4 Maize 8.7 11.6 13.3 Waxy 8.7 11.2 13.2
y y y y y y y y y y y y
¼ 0.0723x ¼ 0.0643x ¼ 0.0416x ¼ 0.0591x ¼ 0.0559x ¼ 0.0476x ¼ 0.0749x ¼ 0.0594x ¼ 0.0515x ¼ 0.0488x ¼ 0.0534x ¼ 0.0612x
þ 73.518 þ 69.133 þ 60.368 þ 70.163 þ 62.957 þ 60.747 þ 67.328 þ 61.363 þ 57.424 þ 59.565 þ 57.191 þ 53.894
Slope
Extrapolate Dependent coefficient (R2) Tg ( C)
0.0723 0.0643 0.0416 0.0591 0.0559 0.0476 0.0749 0.0594 0.0515 0.0488 0.0534 0.0612
0.9847 0.9576 0.9117 0.9711 0.9555 0.9780 0.9596 0.9912 0.9571 0.9534 0.9933 0.9229
73.5 69.1 60.4 70.2 63.0 60.7 67.3 61.4 57.4 59.6 57.2 53.9
Fig. 2. DSC thermograms of G80 film with 13.8% moisture content under different heating rates ( C/min): 300, 250, 200, 180, 160, 140 and 20 (from top to bottom).
nature of the thermal behaviors for the samples measured by DSC. Different thermal conductivity or heat capacity of different samples containing different moisture content could cause this. This phenomenon will not affect the extrapolated results. Fig. 4 shows the plot of extrapolated Tg of various starches vs. different moisture content. The extrapolated Tg increased with decreasing moisture content for all starches. Furthermore, the linear regression equation and the true Tg can also be calculated from Fig. 4. The sequence of true Tg of the cornstarch is G80 > G50 > maize > waxy when samples contain the same moisture content, which corresponds to their amylose/amylopectin ratio. This is the first confirmation that the higher the amylose content is, the higher the Tg is for the same kind of starch. Previous studies (Bizot et al., 1997; Gowie and Henshall, 1976; Oford et al., 1989) considered that linear chains appear to favor chainechain
interactions and induce-partial crystallinity. Branched chains, in contrast, have chain end effects and flexibility of branch points. We would like to use the crystal structure and the microstructure of gelatinized starch to further explain this phenomenon. The starches with B type crystal structure (G50 and G80) show higher Tg than that with A type crystal structure (waxy and maize) (see Table 1). It has also been noted that high amylopectin starches have higher molecular weight and higher crystallinity but lower Tg. Our previous study (Yu and Christie, 2005) has shown that the double helical, crystalline structures formed by the short branched chains in amylopectin are torn apart during gelatinization. However, these short-branched chains remain in a regular pattern by retaining a certain “memory”. These short branched chains form gel-balls that are comprised mainly of chains from the same sub-main chain. In addition, one amylopectin molecule may form a relatively separate super-globe. The molecular entanglements between gel-balls and
90
75
Maize
Waxy 80
Tg (
Tg (
)
)
70
70
8.70% 11.2% 13.20%
65
60 100
150
200
Heating rate (
250
60
300
Series1 8.7% Series2 11.6% Series3 13.3% 90
140
/min)
190
Heating rate (
240
/min)
100
90
G80
G50 90
Tg (
Tg (
)
)
80
70
9.30% ♦ 11.80% ■ 13.40% 150
200
Heating rate (
250
/min)
9.30% 11.50% ♦ 13.30%
70
▲
60 100
80
▲ ■
300
60
90
140
190
Heating rate (
240
290
/min)
Fig. 3. Plots of measured Tg vs. heating rate for different starch films containing different moisture content.
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P. Liu et al. / Journal of Cereal Science 51 (2010) 388e391
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References
Fig. 4. The plots of extrapolated Tg of various starches with different moisture content.
super-globes are much less than those between linear polymeric chains, due to their size and the length of the chains (only 4e6 glucose). These gel-balls require less energy to move than long linear chains, especially when they are lubricated by a plasticizer (water). This explains why high amylopectin materials initially have lower modulus and higher elongation, and lower viscosity during extrusion (Xie et al., 2009; Yu and Christie, 2005). It could also explain why high amylopectin starches exhibit lower Tg. 4. Conclusions The Tg of starch with different amylose/amylopectin ratios was systematically studied using a high-speed DSC with heating rate up to 300 C/min. It was observed that the Tg increases linearly with increasing heating rate for all starches (with R2 > 0.90). Extrapolation was used to calculate the extrapolated Tg. The extrapolated Tg increased with decreasing moisture content for all starches. From the plot of extrapolated Tg vs. moisture content, the true Tg was obtained. It was found that Tg was increased from about 52 to 60 C with increasing amylose content from 0 to 80 for the samples containing about 13% moisture. This confirms for the first time that the higher the amylose content is, the higher the Tg is for the same kind of starch. The sequence of true Tg of cornstarch is G80 > G50 > maize > waxy when samples contain the same moisture content, which corresponds to their amylose/amylopectin ratio. The unique gel-ball structure of gelatinized amylopectin can be used to explain this phenomenon. Acknowledgement The authors from SCUT, China, would like to acknowledge the research funds NRDPHT (863) (2007AA10Z312, 2007AA100407), NKTRDP (2006BAD27B04) and ASTATFP (2009GB23600523).
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