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The gelatinization and retrogradation of cornstarch gels in the presence of citric acid
Food Hydrocolloids 27 (2012) 390e393
Contents lists available at SciVerse ScienceDirect
Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd
Short communication
The gelatinization and retrogradation of cornstarch gels in the presence of citric acid Madoka Hirashima a, *, Rheo Takahashi b, Katsuyoshi Nishinari c a
Faculty of Education, Mie University, 1577 Kurima-machiya-cho, Tsu, Mie 514-8507, Japan Graduate School of Engineering, Gunma University, Kiryu, Gunma, Japan c Graduate School of Human Life Science, Osaka City University, Sumiyoshi, Osaka, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 June 2011 Accepted 20 October 2011
The effects of citric acid on the gelatinization and retrogradation for 20 wt% cornstarch gels were studied by differential scanning calorimetry (DSC) measurements, uniaxial compression tests and dynamic viscoelastic measurements. The pH value of cornstarch dispersion was adjusted to between 5.0 and 3.0 by adding citric acid. The pH of gels without citric acid (control) was 6.3. The cornstarch gels with pH value adjusted to 3.0 were brittle, while the values of Young’s modulus and dynamic viscoelasticity were higher than those of other samples. It is thought that hydrolysis of amylose and amylopectin chains by adding citric acid occurred and the length of elastically active chains decreased, also leading to the formation of new networks. It was found that when cornstarch gels with added citric acid were stored, they became brittle, but became hard to deform against small deformation. However, when cornstarch gels with or without citric acid were stored for longer time at 5 C, the retrogradation ratio estimated by thermal analysis was not affected by the presence of citric acid. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Cornstarch Gelatinization Retrogradation Gel strength DSC
1. Introduction
2. Material and methods
It is well-known that amylose and amylopectin (glucose) chains of starch are hydrolyzed by adding acids. Consequently, there are many reports on the gelatinization and retrogradation of starches by adding acids, which have been examined by means of thermal analysis, amylograms or RVA measurements, rheological measurements or texture analysis (Chamberlain & Rao, 2000; Mali et al., 2003; Russell & Oliver, 1989; Sriburi & Hill, 2000; Sriburi, Hill, & Mitchell, 1999; Thirathumthavorn & Charoenrein, 2005; Wang, Sun, Zeng, & Lu, 2000; Wang, Truong & Wang, 2003). However, there are few reports regarding when the hydrolysis of glucose chains occurs and how hydrolyzed glucose chains affect the physical properties of starch gels. Our objective in this study was to examine the gelatinization and retrogradation of cornstarch gels in the presence of citric acid and how the hydrolyzed glucose chains worked on the rheological property of cornstarch gels.
2.1. Materials
* Corresponding author. Tel./fax: þ81 59 231 9301. E-mail address: [email protected] (M. Hirashima). 0268-005X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2011.10.011
The concentration of the cornstarch was fixed at 20 wt%. The pH was adjusted to between 5.0 and 3.0. The cornstarch sample, the citric acid (anhydrous) and Sorbic acid potassium salt (0.05 wt%) were the same as used in the previous study (Hirashima, Takahashi, & Nishinari, 2003; 2004; 2005). 2.2. Preparation of cornstarch gels Cornstarch powder (20 wt%) was dispersed in distilled water or in citric acid aqueous solution with pH adjusted beforehand in a spherical glass reaction flask (500 mL). Total sample weights were 400 g. A control sample of pH 6.3 was made of distilled water and cornstarch. Cornstarch dispersions were stirred at 200 rpm for 60 min at 25 C using an anchor-shaped stirring bar equipped with a stirrer MAZELA Ze1210 (Tokyo Rikakikai Co., Ltd., Tokyo, Japan). Then the dispersions were heated to 60 C, stirring at the same rate in an oil bath, and heated to 97 C in 20 min, stirring at 400 rpm, and maintained at 97 C for 40 min. The values of the temperature and
M. Hirashima et al. / Food Hydrocolloids 27 (2012) 390e393
0.5
100 T
0.4
80
0.3 60 0.2
pH4.0
2.3. Differential scanning calorimetry (DSC) measurement
control
0.1
2.4. Uniaxial compression tests The cornstarch gels were formed into cylinders 20 mm in diameter 20 mm in height. The samples, which were stored at 5 C for various periods in silicone oil, were then warmed in a water bath at 25 C for 60 min. Uniaxial compression tests were conducted using a Rheoner RE33005 (Yamaden Co. Ltd., Tokyo, Japan) with a 40 mm diameter disc plunger. The compression velocity was 0.5 mm/s. All tests were made at 25 C. From these tests, stressestrain curves were obtained. It was confirmed by video observation that the shapes of sample gels kept cylindrical shapes with no volume change, indicating that Poisson’s ratio was 0.5, and true fracture stress (sf) and fracture strain (gf) were calculated. From the initial slopes of stressestrain curves, Young’s modulus (E) was determined. 2.5. Dynamic viscoelastic measurements The cornstarch gels were formed into cylinders 20 mm in diameter 30 mm in height. Dynamic viscoelastic measurements were carried out using a Rheolographegel (Toyo Seiki Seisaku-sho, Ltd., Tokyo, Japan) (Nishinari et al., 1980). The tops and bottoms of the sample gels were fixed on the sample table with adhesive (Aron Alpha, Toagosei Co. Ltd. Osaka, Japan). The frequency was 3 Hz and the amplitude was 100 mm. The temperature was controlled in silicone oil at 25 C. From these measurements, storage modulus (E’) was determined. 3. Results and discussion
40
pH3.0
0.0 0
2000
4000
20 8000
6000
t (s) Fig. 1. Torque (N) and temperature (T) changes on the preparation of samples as a function of time (t). The pH was adjusted by adding citric acid.
values of N for the sample at pH 4.0 were similar to those of the control. When the pH decreased to lower than 3.4, it was found that the ratio of acid hydrolysis increased from intrinsic viscosity measurements (Hirashima et al., 2003; 2004; 2005). Therefore, lowering the pH to 4.0 did not affect the gelatinization of cornstarch. However, the values of N of the sample at pH 3.0 were smaller than those of the control at temperatures higher than 80 C, which was higher than T1c though they were similar to those of the control at temperatures lower than 80 C. According to Simonin, Guyon, Orlowska, de Lamballerie, and Le-Bail, 2011, no significant effect of pH was observed on the swelling power of waxy cornstarch granules. It is thought that the swelling rate of cornstarch granules with adjusted pH value to 3.0 remained the same as that of the control up to 80 C. From these results, it was found that lowering the pH of cornstarch dispersions did not affect the gelatinization temperatures and DH1, but the hydrolysis occurred while heating the samples at higher temperatures. Fig. 2 shows the pH dependence of sf, gf, E and E’ for cornstarch gels immediately after preparing. The values of sf for cornstarch gels did not depend on pH. The values of gf for samples were similar at pH >3.0, but became smaller at pH3.0. It was found that lowering pH did not change the value of sf but the gels became brittle. However, the values of E and E’ for the sample of pH3.0 became
80
0.6
σf f
E E'
60
σf ,E, E' (kPa)
DSC measurements were carried out using a DSC6100S or a DSC100P (Seiko Instruments Inc. Chiba, Japan). Cornstarch and distilled water or citric acid aqueous solutions were mixed in 70 ml silver (Ag) pans to make 20 wt% samples, which were added to the total sample weights of 50 mg, then sealed. Distilled water was used for a reference. Samples were heated from 25 C to 130 C at a heating rate of 1.0 C/min. From DSC curves, onset temperature (T1o), peak temperature (T1p), conclusion temperature of amylopectin gelatinization (T1c) and gelatinization enthalpy (DH1) could be obtained. To investigate the re-gelatinization of starch, the gelatinized samples, which had been heated by a DSC apparatus, were stored at 5 C for 3e45 days. The stored samples were heated from 5 C to 100 C at the same heating rate. From re-heated DSC curves, regelatinization enthalpy (DH2) could be obtained. The retrogradation ratio (DH2/DH1) was calculated (Kohyama & Nishinari, 1991) and the retrogradation behaviors were estimated by determining this ratio.
T (oC)
N (N/m)
the torque (N) during the sample preparation were recorded by a thermo recorder TRe81 (T & D Co., Nagano, Japan) and a stirrer MAZELA Ze1210, respectively. After that, the hot dispersions were poured into molds coated with PTFE to make cylindrical gels, and they were cooled to 5 C in an ice water bath. The samples, which were soaked into silicone oil, were stored at 5 C for 1e45 days for examining the process of the retrogradation.
391
0.5
40
0.4
20
0.3
3.1. Gelatinization and gel strength of cornstarch gels with added citric acid 0
T1o, T1p, T1c and DH1 for 20 wt% cornstarch obtained from heating DSC measurements did not depend on pH used in this study (data not shown). This result was in agreement with previous reports (Mali et al., 2003; Wang et al., 2003). Fig. 1 shows the values of the torque (N) for the stirrer during the preparation of the cornstarch samples with adjusted pH values. The
0.2 2
3
4
5
6
7
pH Fig. 2. pH dependence of sf, gf, E and E0 of cornstarch gels. The pH was adjusted by adding citric acid. The pH of the control was 6.3. Gels were used immediately after preparation. Measurements were made at 25 C.
392
M. Hirashima et al. / Food Hydrocolloids 27 (2012) 390e393 200
1000
0.6 0.5
800
100
E (kPa)
0.4
γf
σ f (kPa)
150
0.3 0.2
control pH5.0 pH4.0 pH3.0
50
0 0
10
20
30
40
control pH5.0 pH4.0 pH3.0
0.1
400 control pH5.0 pH4.0 pH3.0
200
0.0
50
600
0 0
10
20
30
40
50
t (day)
t (day)
0
10
20
30
40
50
t (day)
Fig. 3. sf (left), gf (center) and E (right) of cornstarch gels as a function of storage time (t). The pH was adjusted by adding citric acid. The pH of the control was 6.3. Samples were stored at 5 C. Measurements were made at 25 C.
much larger than those of other samples, though those for samples with 5.0 pH >3.0 decreased slightly. From these results, it was found that decreasing pH made cornstarch gels harder to deform against a small deformation. This result mainly reflects the acid hydrolysis of the cornstarch samples. Consequently, the length of amylose and amylopectin (glucose) chains in the sample with pH value adjusted to 3.0 must be shorter than that of other samples. Therefore, it is difficult for glucose chains to entangle due to their shorter length. Thus, brittle gels were formed. However, short glucose chains within the gels increase through the hydrolysis of long glucose chains. Thus, the number of elastically active network chains increases leading to the increase in the elastic modulus. Lowering pH leads to the formation of gels, which are hard to deform in a small deformation range (Treloar, 1975). 3.2. Retrogradation of cornstarch gels with added citric acid Fig. 3 indicates the time dependence of sf, gf and E for cornstarch gels for which the pH was adjusted by adding citric acid. sf and E of all samples increased, and gf decreased with increasing storage time. These are general tendencies for the retrogradation of starch (Keetels, van Vliet & Walstra, 1996a; 1996b). The increasing tendency of sf for all samples was the same in all storage time examined. The value of gf for sample of pH3.0 was smaller when the
0.6
test was conducted immediately after being prepared, and remained smaller compared to that of the control even after storage. The values of gf for samples with pH >3.0 were the same as those for the control when the test was conducted immediately after being prepared. However, those values for stored samples with 5.0 pH > 3.0 became smaller than those for the control. It was found that cornstarch gels with 5.0 pH > 3.0 became brittle for samples that were stored. The values of E for samples with 5.0 pH > 3.0 were larger than those for the control at shorter storage time until 30 days. However, the values of E for samples with 5.0 pH > 3.0 became the same as those for the control at longer storage time of more than 30 days. On the other hand, the values of E for the sample of pH 3.0 were larger than those for the control at all storage time examined. Fig. 4 shows the time dependence of DH2/DH1 obtained from DSC measurements. The values of DH2/DH1 for the samples in the presence of citric acid were larger than those for the control at less than 30 days storage. It was found that adding citric acid led to faster progress of the retrogradation from DSC measurements. However, when the samples were stored for more than 30 days, the differences in the values of DH2/DH1 among the control and samples with citric acid were smaller, and the values of DH2/DH1 for samples with 5.0 pH > 3.0 were slightly smaller than those for the control. It is thought that the retrogradation of starch is also affected by acid hydrolysis. According to Wang et al., 2003, acid-thinned cornstarch showed a slight increase in crystallinity. Therefore, the retrogradation progress of the sample at pH3.0 was faster than those of other samples, because the crosslinks with shorter glucose chains increased. 4. Conclusion
0.4
H2/ H1
It was found that a heating at higher temperatures (> 80 C) in the presence of citric acid led to the acid hydrolysis of amylose and amylopectin chains, and that affected the gel strength and the retrogradation of cornstarch gels. Cornstarch gels in the presence of citric acid became brittle against large deformation and hard to deform against small deformation compared with the control. However, in the case where samples are stored for longer time, it is thought that the effects of adding citric acid are not necessarily influential, because the retrogradation ratio for all samples with or without citric acid converge to a constant value.
0.2
control pH5.0 pH4.0 pH3.0
0.0 0
10
20 30 t (day)
40
50
Fig. 4. DH2/DH1 of cornstarch gels as a function of storage time (t). The pH was adjusted by adding citric acid. The pH of the control was 6.3.
Acknowledgments We thank Sanwa Starch Co. Ltd. for their provision of cornstarch, and Prof. Makoto Hisamatsu and Dr. Takashi Mishima at Graduate
M. Hirashima et al. / Food Hydrocolloids 27 (2012) 390e393
School of Bioresources, Mie University for their cooperation with DSC measurements. This research was supported by Grant-in-Aid for Young Scientists (B) (20700582).
References Chamberlain, E. K., & Rao, M. A. (2000). Effect of concentration on rheological properties of acid-hydrolyzed amylopectin solutions. Food Hydrocolloids, 14, 163e171. Hirashima, M., Takahashi, R., & Nishinari, K. (2003). Effects of taste substances on the physical properties of corn starch pastes. I. sour seasoning (citric acid and acetic acid). Journal of Cooking Science of Japan, 36, 225e233. Hirashima, M., Takahashi, R., & Nishinari, K. (2004). Effects of citric acid on the viscoelasticity of cornstarch pastes. Journal of Agricultural and Food Chemistry, 52, 2929e2933. Hirashima, M., Takahashi, R., & Nishinari, K. (2005). Effects of adding acids before and after gelatinization on the viscoelasticity of cornstarch. Food Hydrocolloids, 19, 777e784. Keetels, J. A. M., van Vliet, T., & Walstra, P. (1996a). Gelation and retrogradation of concentrated starch systems: 2. Retrogradation. Food Hydrocolloids, 10, 355e362. Keetels, J. A. M., van Vliet, T., & Walstra, P. (1996b). Gelation and retrogradation of concentrated starch systems: 3. Effect of concentration and heating temperature. Food Hydrocolloids, 10, 363e368. Kohyama, K., & Nishinari, K. (1991). Effects of soluble sugars on gelatinization and retrogradation of sweet potato starch. Journal of Agricultural and Food Chemistry, 39, 1406e1410.
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Mali, S., Ferrero, C., Redigonda, V., Beleia, A. P., Grossmann, M. V. E., & Zaritzky, N. E. (2003). Influence of pH and hydrocolloids addition on yam (Dioscorea alata) starch pastes stability. LWT Food Science and Technology, 36, 475e481. Nishinari, K., Horiuchi, H., Ishida, K., Ikeda, K., Date, M., & Fukada, E. (1980). A new apparatus for rapid and easy measurement of dynamic viscoelasticity for gellike foods. Journal of Japanese Society of Food Science and Technology, 27, 227e233. Russell, P. L., & Oliver, G. (1989). The effects of pH and NaCl content on starch gel ageing. A study by differential scanning calorimetry and rheology. Journal of Cereal Chemistry, 10, 123e138. Simonin, H., Guyon, C., Orlowska, de Lamballerie, M., & Le-Bail, A. (2011). Gelatinization of waxy starches under high pressure as influenced by pH and osmolarity: gelatinization kinetics, final structure and pasting property. LWT Food Science and Technology, 44, 779e786. Sriburi, P., & Hill, S. E. (2000). Extrusion of cassava starch with either variations in ascorbic acid concentration or pH. International Journal of Food Science & Technology, 35, 141e154. Sriburi, P., Hill, S. E., & Mitchell, J. R. (1999). Effects of L-ascorbic acid on the conversion of cassava starch. Food Hydrocolloids, 13, 177e183. Thirathumthavorn, D., & Charoenrein, S. (2005). Thermal and pasting properties of acid-treated rice starches. Starch/Stärke, 57, 217e222. Treloar, L. R. G. (1975). The physics of rubber elasticity (3rd ed.). Oxford: Clarendon Press. Wang, H.-H., Sun, D.-W., Zeng, Q., & Lu, Y. (2000). Effect of pH, corn starch and phosphates on the pasting properties of rice flour. Journal of Food Engineering, 46, 133e138. Wang, Y.-J., Truong, V.-D., & Wang, L. (2003). Structure and rheological properties of corn starch as affected by acid hydrolysis. Carbohydrate Polymers, 52, 327e333.
Contents lists available at SciVerse ScienceDirect
Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd
Short communication
The gelatinization and retrogradation of cornstarch gels in the presence of citric acid Madoka Hirashima a, *, Rheo Takahashi b, Katsuyoshi Nishinari c a
Faculty of Education, Mie University, 1577 Kurima-machiya-cho, Tsu, Mie 514-8507, Japan Graduate School of Engineering, Gunma University, Kiryu, Gunma, Japan c Graduate School of Human Life Science, Osaka City University, Sumiyoshi, Osaka, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 June 2011 Accepted 20 October 2011
The effects of citric acid on the gelatinization and retrogradation for 20 wt% cornstarch gels were studied by differential scanning calorimetry (DSC) measurements, uniaxial compression tests and dynamic viscoelastic measurements. The pH value of cornstarch dispersion was adjusted to between 5.0 and 3.0 by adding citric acid. The pH of gels without citric acid (control) was 6.3. The cornstarch gels with pH value adjusted to 3.0 were brittle, while the values of Young’s modulus and dynamic viscoelasticity were higher than those of other samples. It is thought that hydrolysis of amylose and amylopectin chains by adding citric acid occurred and the length of elastically active chains decreased, also leading to the formation of new networks. It was found that when cornstarch gels with added citric acid were stored, they became brittle, but became hard to deform against small deformation. However, when cornstarch gels with or without citric acid were stored for longer time at 5 C, the retrogradation ratio estimated by thermal analysis was not affected by the presence of citric acid. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Cornstarch Gelatinization Retrogradation Gel strength DSC
1. Introduction
2. Material and methods
It is well-known that amylose and amylopectin (glucose) chains of starch are hydrolyzed by adding acids. Consequently, there are many reports on the gelatinization and retrogradation of starches by adding acids, which have been examined by means of thermal analysis, amylograms or RVA measurements, rheological measurements or texture analysis (Chamberlain & Rao, 2000; Mali et al., 2003; Russell & Oliver, 1989; Sriburi & Hill, 2000; Sriburi, Hill, & Mitchell, 1999; Thirathumthavorn & Charoenrein, 2005; Wang, Sun, Zeng, & Lu, 2000; Wang, Truong & Wang, 2003). However, there are few reports regarding when the hydrolysis of glucose chains occurs and how hydrolyzed glucose chains affect the physical properties of starch gels. Our objective in this study was to examine the gelatinization and retrogradation of cornstarch gels in the presence of citric acid and how the hydrolyzed glucose chains worked on the rheological property of cornstarch gels.
2.1. Materials
* Corresponding author. Tel./fax: þ81 59 231 9301. E-mail address: [email protected] (M. Hirashima). 0268-005X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2011.10.011
The concentration of the cornstarch was fixed at 20 wt%. The pH was adjusted to between 5.0 and 3.0. The cornstarch sample, the citric acid (anhydrous) and Sorbic acid potassium salt (0.05 wt%) were the same as used in the previous study (Hirashima, Takahashi, & Nishinari, 2003; 2004; 2005). 2.2. Preparation of cornstarch gels Cornstarch powder (20 wt%) was dispersed in distilled water or in citric acid aqueous solution with pH adjusted beforehand in a spherical glass reaction flask (500 mL). Total sample weights were 400 g. A control sample of pH 6.3 was made of distilled water and cornstarch. Cornstarch dispersions were stirred at 200 rpm for 60 min at 25 C using an anchor-shaped stirring bar equipped with a stirrer MAZELA Ze1210 (Tokyo Rikakikai Co., Ltd., Tokyo, Japan). Then the dispersions were heated to 60 C, stirring at the same rate in an oil bath, and heated to 97 C in 20 min, stirring at 400 rpm, and maintained at 97 C for 40 min. The values of the temperature and
M. Hirashima et al. / Food Hydrocolloids 27 (2012) 390e393
0.5
100 T
0.4
80
0.3 60 0.2
pH4.0
2.3. Differential scanning calorimetry (DSC) measurement
control
0.1
2.4. Uniaxial compression tests The cornstarch gels were formed into cylinders 20 mm in diameter 20 mm in height. The samples, which were stored at 5 C for various periods in silicone oil, were then warmed in a water bath at 25 C for 60 min. Uniaxial compression tests were conducted using a Rheoner RE33005 (Yamaden Co. Ltd., Tokyo, Japan) with a 40 mm diameter disc plunger. The compression velocity was 0.5 mm/s. All tests were made at 25 C. From these tests, stressestrain curves were obtained. It was confirmed by video observation that the shapes of sample gels kept cylindrical shapes with no volume change, indicating that Poisson’s ratio was 0.5, and true fracture stress (sf) and fracture strain (gf) were calculated. From the initial slopes of stressestrain curves, Young’s modulus (E) was determined. 2.5. Dynamic viscoelastic measurements The cornstarch gels were formed into cylinders 20 mm in diameter 30 mm in height. Dynamic viscoelastic measurements were carried out using a Rheolographegel (Toyo Seiki Seisaku-sho, Ltd., Tokyo, Japan) (Nishinari et al., 1980). The tops and bottoms of the sample gels were fixed on the sample table with adhesive (Aron Alpha, Toagosei Co. Ltd. Osaka, Japan). The frequency was 3 Hz and the amplitude was 100 mm. The temperature was controlled in silicone oil at 25 C. From these measurements, storage modulus (E’) was determined. 3. Results and discussion
40
pH3.0
0.0 0
2000
4000
20 8000
6000
t (s) Fig. 1. Torque (N) and temperature (T) changes on the preparation of samples as a function of time (t). The pH was adjusted by adding citric acid.
values of N for the sample at pH 4.0 were similar to those of the control. When the pH decreased to lower than 3.4, it was found that the ratio of acid hydrolysis increased from intrinsic viscosity measurements (Hirashima et al., 2003; 2004; 2005). Therefore, lowering the pH to 4.0 did not affect the gelatinization of cornstarch. However, the values of N of the sample at pH 3.0 were smaller than those of the control at temperatures higher than 80 C, which was higher than T1c though they were similar to those of the control at temperatures lower than 80 C. According to Simonin, Guyon, Orlowska, de Lamballerie, and Le-Bail, 2011, no significant effect of pH was observed on the swelling power of waxy cornstarch granules. It is thought that the swelling rate of cornstarch granules with adjusted pH value to 3.0 remained the same as that of the control up to 80 C. From these results, it was found that lowering the pH of cornstarch dispersions did not affect the gelatinization temperatures and DH1, but the hydrolysis occurred while heating the samples at higher temperatures. Fig. 2 shows the pH dependence of sf, gf, E and E’ for cornstarch gels immediately after preparing. The values of sf for cornstarch gels did not depend on pH. The values of gf for samples were similar at pH >3.0, but became smaller at pH3.0. It was found that lowering pH did not change the value of sf but the gels became brittle. However, the values of E and E’ for the sample of pH3.0 became
80
0.6
σf f
E E'
60
σf ,E, E' (kPa)
DSC measurements were carried out using a DSC6100S or a DSC100P (Seiko Instruments Inc. Chiba, Japan). Cornstarch and distilled water or citric acid aqueous solutions were mixed in 70 ml silver (Ag) pans to make 20 wt% samples, which were added to the total sample weights of 50 mg, then sealed. Distilled water was used for a reference. Samples were heated from 25 C to 130 C at a heating rate of 1.0 C/min. From DSC curves, onset temperature (T1o), peak temperature (T1p), conclusion temperature of amylopectin gelatinization (T1c) and gelatinization enthalpy (DH1) could be obtained. To investigate the re-gelatinization of starch, the gelatinized samples, which had been heated by a DSC apparatus, were stored at 5 C for 3e45 days. The stored samples were heated from 5 C to 100 C at the same heating rate. From re-heated DSC curves, regelatinization enthalpy (DH2) could be obtained. The retrogradation ratio (DH2/DH1) was calculated (Kohyama & Nishinari, 1991) and the retrogradation behaviors were estimated by determining this ratio.
T (oC)
N (N/m)
the torque (N) during the sample preparation were recorded by a thermo recorder TRe81 (T & D Co., Nagano, Japan) and a stirrer MAZELA Ze1210, respectively. After that, the hot dispersions were poured into molds coated with PTFE to make cylindrical gels, and they were cooled to 5 C in an ice water bath. The samples, which were soaked into silicone oil, were stored at 5 C for 1e45 days for examining the process of the retrogradation.
391
0.5
40
0.4
20
0.3
3.1. Gelatinization and gel strength of cornstarch gels with added citric acid 0
T1o, T1p, T1c and DH1 for 20 wt% cornstarch obtained from heating DSC measurements did not depend on pH used in this study (data not shown). This result was in agreement with previous reports (Mali et al., 2003; Wang et al., 2003). Fig. 1 shows the values of the torque (N) for the stirrer during the preparation of the cornstarch samples with adjusted pH values. The
0.2 2
3
4
5
6
7
pH Fig. 2. pH dependence of sf, gf, E and E0 of cornstarch gels. The pH was adjusted by adding citric acid. The pH of the control was 6.3. Gels were used immediately after preparation. Measurements were made at 25 C.
392
M. Hirashima et al. / Food Hydrocolloids 27 (2012) 390e393 200
1000
0.6 0.5
800
100
E (kPa)
0.4
γf
σ f (kPa)
150
0.3 0.2
control pH5.0 pH4.0 pH3.0
50
0 0
10
20
30
40
control pH5.0 pH4.0 pH3.0
0.1
400 control pH5.0 pH4.0 pH3.0
200
0.0
50
600
0 0
10
20
30
40
50
t (day)
t (day)
0
10
20
30
40
50
t (day)
Fig. 3. sf (left), gf (center) and E (right) of cornstarch gels as a function of storage time (t). The pH was adjusted by adding citric acid. The pH of the control was 6.3. Samples were stored at 5 C. Measurements were made at 25 C.
much larger than those of other samples, though those for samples with 5.0 pH >3.0 decreased slightly. From these results, it was found that decreasing pH made cornstarch gels harder to deform against a small deformation. This result mainly reflects the acid hydrolysis of the cornstarch samples. Consequently, the length of amylose and amylopectin (glucose) chains in the sample with pH value adjusted to 3.0 must be shorter than that of other samples. Therefore, it is difficult for glucose chains to entangle due to their shorter length. Thus, brittle gels were formed. However, short glucose chains within the gels increase through the hydrolysis of long glucose chains. Thus, the number of elastically active network chains increases leading to the increase in the elastic modulus. Lowering pH leads to the formation of gels, which are hard to deform in a small deformation range (Treloar, 1975). 3.2. Retrogradation of cornstarch gels with added citric acid Fig. 3 indicates the time dependence of sf, gf and E for cornstarch gels for which the pH was adjusted by adding citric acid. sf and E of all samples increased, and gf decreased with increasing storage time. These are general tendencies for the retrogradation of starch (Keetels, van Vliet & Walstra, 1996a; 1996b). The increasing tendency of sf for all samples was the same in all storage time examined. The value of gf for sample of pH3.0 was smaller when the
0.6
test was conducted immediately after being prepared, and remained smaller compared to that of the control even after storage. The values of gf for samples with pH >3.0 were the same as those for the control when the test was conducted immediately after being prepared. However, those values for stored samples with 5.0 pH > 3.0 became smaller than those for the control. It was found that cornstarch gels with 5.0 pH > 3.0 became brittle for samples that were stored. The values of E for samples with 5.0 pH > 3.0 were larger than those for the control at shorter storage time until 30 days. However, the values of E for samples with 5.0 pH > 3.0 became the same as those for the control at longer storage time of more than 30 days. On the other hand, the values of E for the sample of pH 3.0 were larger than those for the control at all storage time examined. Fig. 4 shows the time dependence of DH2/DH1 obtained from DSC measurements. The values of DH2/DH1 for the samples in the presence of citric acid were larger than those for the control at less than 30 days storage. It was found that adding citric acid led to faster progress of the retrogradation from DSC measurements. However, when the samples were stored for more than 30 days, the differences in the values of DH2/DH1 among the control and samples with citric acid were smaller, and the values of DH2/DH1 for samples with 5.0 pH > 3.0 were slightly smaller than those for the control. It is thought that the retrogradation of starch is also affected by acid hydrolysis. According to Wang et al., 2003, acid-thinned cornstarch showed a slight increase in crystallinity. Therefore, the retrogradation progress of the sample at pH3.0 was faster than those of other samples, because the crosslinks with shorter glucose chains increased. 4. Conclusion
0.4
H2/ H1
It was found that a heating at higher temperatures (> 80 C) in the presence of citric acid led to the acid hydrolysis of amylose and amylopectin chains, and that affected the gel strength and the retrogradation of cornstarch gels. Cornstarch gels in the presence of citric acid became brittle against large deformation and hard to deform against small deformation compared with the control. However, in the case where samples are stored for longer time, it is thought that the effects of adding citric acid are not necessarily influential, because the retrogradation ratio for all samples with or without citric acid converge to a constant value.
0.2
control pH5.0 pH4.0 pH3.0
0.0 0
10
20 30 t (day)
40
50
Fig. 4. DH2/DH1 of cornstarch gels as a function of storage time (t). The pH was adjusted by adding citric acid. The pH of the control was 6.3.
Acknowledgments We thank Sanwa Starch Co. Ltd. for their provision of cornstarch, and Prof. Makoto Hisamatsu and Dr. Takashi Mishima at Graduate
M. Hirashima et al. / Food Hydrocolloids 27 (2012) 390e393
School of Bioresources, Mie University for their cooperation with DSC measurements. This research was supported by Grant-in-Aid for Young Scientists (B) (20700582).
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