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Physical aging of glassy normal and waxy rice starches: effect of aging time on glass transition and enthalpy relaxation
Food Hydrocolloids 17 (2003) 855–861 www.elsevier.com/locate/foodhyd
Physical aging of glassy normal and waxy rice starches: effect of aging time on glass transition and enthalpy relaxation Hyun-Jung Chung, Seung-Taik Lim* Graduate School of Biotechnology, Korea University, 5-1 Anam-dong, Sungbuk-ku, Seoul 136-701, South Korea Received 4 December 2002; accepted 28 February 2003
Abstract Physical aging and glass transition characteristics of amorphous normal and waxy rice starches (11 and 15% moisture contents) were investigated under differential scanning calorimetry as function of aging time. Normal rice starch showed higher Tg than waxy rice starch. The Tg and DCp at glass transition gradually increased with aging time, whereas fictive temperature was slightly reduced regardless of moisture content and starch type. The relaxation enthalpy and relaxation peak temperature increased with aging time until structural equilibrium was reached. Enthalpy increase was more significant in the early stage of aging whereas temperature increase was constant during the aging period tested (120 h). Aging kinetic analysis using Cowie and Ferguson model revealed that the amorphous normal and waxy rice starches behaved in different modes for the physical aging. Relaxation distribution parameter ðbÞ of both starches was in a range of 0:3 , b , 0:7; but higher at a lower moisture content, and for normal starch than for waxy starch. Maximum relaxation enthalpy for normal starch (1.10 and 2.69 J/g, respectively, at 11 and 15% moistures) was higher than those of waxy starch (0.77 and 2.48 J/g). Based on the characteristic time ðtc Þ; normal starch has slower progression toward an equilibrium than waxy starch. Overall results proved that physical aging kinetics were highly dependent on starch structure and composition. q 2003 Elsevier Ltd. All rights reserved. Keywords: Physical aging; Glass transition; Enthalpy relaxation; Rice starch
1. Introduction Many cereal-based food products are manufactured by thermal processing with limited moisture, and thus consumed in a glassy state. These glassy products exist in metastable state, and undergo physical aging process during storage that is responsible for the changes in physical property such as texture, and brittleness. Long-term stability and retention of freshness of the cereal-based foods thus depend on the physical aging (Blanshard & Lillford, 1993; Borde, Bizot, Vigier, & Buleon, 2002a,b; Champion, Le Meste, & Simatos, 2000; Slade & Levine, 1995). Starch is a major biopolymer in most cereal products, and thus physical aging characteristics of starch should be understood to control physical changes of the cereal products during storage. The term ‘physical aging’ is a structural transformation (relaxation) toward an equilibrium as a function of storage * Corresponding author. Tel.: þ 82-2-3290-3435; fax: þ82-2-927-5201. E-mail address: [email protected] (S.-T. Lim). 0268-005X/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0268-005X(03)00106-1
time. It usually occurs at a constant temperature at zero stress and under no influence from any other external conditions (Hutchinson, 1995). Physical aging of a glassy polymer matrix is observed at a temperature below glass transition temperature ðTg Þ: The structural relaxation of a glass is time-dependent with change in specific volume, enthalpy, and mechanical and dielectric responses (Hodge, 1994; Hutchinson, 1995). Thus, physical aging may be quantitatively measured as the changes in specific volume or relaxation enthalpy (Hodge, 1994). One of the most frequent techniques for analysis is differential scanning calorimetry (DSC). The aged samples exhibit endothermic peak (enthalpy relaxation) in DSC thermogram usually before glass transition (Cowie & Ferguson, 1986). Several research groups have studied the physical aging of starch: amylopectin in cornstarch (Kalichevsky, Jaroskewicz, Ablett, Blanshard, & Lillford, 1992), cornstarch (Shogren, 1992), waxy cornstarch (Borde et al., 2002a,b; Yuan & Thompson, 1994), potato starch (Thiewes & Steeneken, 1997), boll-milled potato starch (Kim, Suzuki, Hagiwara, Yamaji, & Taki, 2001), heat-treated corn and
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potato starches (Lim, Chang, & Chung, 2001), and native and gelatinized rice starches (Chung, Lee, & Lim, 2002). Recently, Borde et al. (2002a,b) systematically studied the physical aging of waxy maize starch, in comparison to synthetic polymers. Their studies were based on the phenomenology commonly used for synthetic polymers. They suggested that the physical aging kinetics depended on cooling rate, storage temperature, and chemical structure of polymer. It was found that waxy maize starch underwent the aging process at a faster rate than did synthetic polymers. In this study, physical aging kinetics of amorphous rice starches were investigated, and differences between normal and waxy rices were examined.
the measurement of the aged sample, rescanning was done under the same cooling and heating conditions for the measurement of unaged sample. The glass transition temperature was determined as the peak of the first derivative curve of the heat capacity thermogram. The fictive temperature ðTf Þ; defined as the temperature at which the enthalpy of material was equal to its equilibrium enthalpy, was determined according to the procedure of Montserrat (1994). Enthalpy of relaxation during aging was measured from the DSC thermogram as the function of aging time and the kinetics were analyzed using Cowie and Ferguson (1986) model.
3. Results and discussion 2. Materials and methods
3.1. Glass transition temperature
2.1. Amorphous rice starches
In the amorphous state resulted from the isolation process used, normal rice starch exhibited higher Tg values than waxy rice starch (Fig. 1). Bizot et al. (1997) reported that pure amylose exhibited higher Tg than branched polyanhydroglucose compounds. The a(1 ! 6) linkages theoretically offer three rotational degrees of freedom whereas the a(1 ! 4) linkages offer two degrees. Thus, amylopectins that has higher degree of branching have greater chain flexibility than relatively linear amyloses (Bizot et al., 1997). Additionally, when starch was isolated in amorphous state, linear amyloses might form denser structure by chain associations than did branched amylopectins. The greater molecular rigidity and the tendency of dense matrix formation of amyloses in normal rice starch led to the higher Tg for the starch. At 11% moisture content, the Tg of amorphous rice starches could be readily measured from the DSC thermogram because the relaxation endotherm did not overlap the glass transition (thermogram not shown). At this moisture content, there was a trend of Tg increase as aging time increased (Fig. 1). The Tg increase by aging has
Normal and waxy rice starches were isolated from Japonica-type rice flours by following the procedure of Lim, Lee, Shin, and Lim (1999), and the starches were further purified by using 90% dimethyl sulfoxide (DMSO) and ethanol (Jane & Chen, 1992). The purification was repeated three times, and the pure starch was dried in a convection oven (40 8C) overnight. 2.2. DSC sample preparation For physical aging analysis, DSC samples of the amorphous rice starch were prepared at 11 or 15% moisture content. Starch (10 ^ 0.1 mg dry solid basis) was placed in a silver DSC pan (Seiko Instrument, Chiba, Japan) and hydrated excessively in a humidity chamber over distilled water. Then moisture in the sample was allowed to evaporate in a balance until desired moisture content was obtained. The starch sample pan was then hermetically sealed and equilibrated by leaving at room temperature for 24 h prior to the DSC measurement. Temperature and enthalpy in DSC (Seiko Instrument, DSC 6100, Chiba, Japan) were calibrated with indium and mercury (156.6 and 2 39.0 8C for melting, respectively), and heat capacity was calibrated with sapphire. An empty pan was used as a reference. 2.3. DSC measurements To erase the thermal history of starch, DSC samples were heated from 5 to 160 8C at a heating rate of 2 8C/min, held at 160 8C for 10 min, and then immediately cooled to an aging temperature (Ta ; 25 8C) at a cooling rate of 20 8C/min. The aging at 25 8C was performed for different periods (ta ; 1– 10 days), and then the enthalpy relaxation and glass transition were measured under DSC at a heating rate of 2 8C/min up to 160 8C. Immediately after
Fig. 1. Glass transition temperature ðTg Þ change at different aging times up to 240 h at 11% moisture content.
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been reported with a variety of materials. Schmidt and co-workers (1996, 1999, 2001) reported the Tg increases for aged maltose and glucose – fructose mixture, and suggested that it was from the shift of relaxation endotherm to higher temperature. Liao, Quan, and Lu (2002) proposed that perfection in inter-chain packing in segmental relaxation process occurred during aging of poly(DL-lactide). Aging caused chain stiffness of cured epoxy resin under a dynamic thermal analysis (Barral et al., 1999, 2000). Likewise, conformation changes proceed during aging in a direction to decrease free volume and mobility of the glassy starch chains, raising the Tg : Temperature range for glass transition in DSC thermogram was also affected by aging (Fig. 2). Reduction in glass transition temperature range has been observed with aged maltose (Lammert, Lammert, & Schmidt, 1999), glucose – fructose mixture (Wungtanagorn & Schmidt, 2001), and poly(DL-lactide) (Liao et al., 2002). In this study, transition range decreased mostly in the early stage of aging, up to 50 h, because the segmental mobility was more restricted as free volume decreased rapidly in the early stage of aging. The starch samples used in this experiment included precedent thermal history. Fig. 3 shows the two DSC thermograms of the normal rice starch samples in the absence and presence of thermal history. By erasing the thermal history the rice starch exhibited a thermogram ((b) in Fig. 3) with a noticeable reduction in Tg (93.9 versus 99.3 8C) and DCp (0.245 versus 0.741 J/g 8C). This result proved that the Tg increases with aging (similar to thermal history). The relaxation endotherm, which represents the structural and conformational changes of glassy polymers, affects the glass transition.
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Fig. 3. DSC thermogram of normal rice starch with thermal history (a), and rescanned sample (b) at 11% moisture content.
The heat capacity increment ðDCp Þ at Tg ; which was a major attribute for glass transition, increased with aging
time (Fig. 4). The DCp values of unaged normal and waxy rice starches at 11% moisture were 0.245 and 0.229 J/ g deg.C, respectively. These values were similar to that reported for waxy cornstarch by Borde et al. (2002b) (0.201 J/g 8C). At 15% moisture content, the DCp was higher (0.277 and 0.255 J/g 8C, respectively, for normal and waxy rice starches), and this DCp increase was from the increased free volume of starch chains by raising moisture content (Roos, 1995). As discussed with the Tg results, amylose chains are less flexible than amylopectin chains. Based on this theory, the amylose presence in normal starch leads to depress DCp at glass transition. In accordance, Bizot et al. (1997) reported that amylose had a smaller DCp than amylopectin. However, our experimental data showed an opposite trend to the theoretical expectation, showing higher DCp for normal starch (Fig. 4). It was assumed that three-dimensional network conformation in the starch matrix was more important than the molecular structure based on degree of branching. The matrix structure might be more critical when moisture is limited as in this experiment. Based on the composition, normal rice starch may form a heterogeneous
Fig. 2. Glass transition range change at different aging times up to 240 h at 11% moisture content.
Fig. 4. Change of heat capacity increment ðDCp Þ at Tg at different aging times up to 240 h at 11 and 15% moisture contents.
3.2. Heat capacity change
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mixture because two different starch chains (amylose and amylopectin) exist, whereas waxy rice starch composed only by amylopectin chains, and thus forms a more homogeneous matrix structure. Therefore, the homogeneity of waxy starch could result in the low DCp : The DCp increase at glass transition by aging was reported also for glucose, fructose, and their mixture by Wuntanagorn and Schmidt (2001), whereas the DCp for polymers has been reported to be relatively constant with aging time (Barral et al., 1999; Cook, Mehrabi, & Edward, 1999; Montserrat, 1994). The DCp depends mainly on the changes in glassy state during aging. Borde et al. (2002b) and Tsukushi, Yamamuro, and Suga (1994) found that the glass heat capacity ðCp Þ of hydrated polysaccharides decreased with aging time. With the decreased heat capacity of glass but relatively constant Cp of rubber during aging, the heat capacity difference ðDCp Þ results in an increase ðDCp ¼ rubber Cp 2 glass Cp Þ: The experimental results may suggest that the glass Cp is different between the amorphous normal and waxy rice starches. But additional experiments should be done to understand the mechanism. 3.3. Fictive temperature Fictive temperature ðTf Þ is a hypothetical temperature at which a glass structure remains in a thermodynamic equilibrium. The Tf could be obtained by intersecting extrapolated glassy enthalpy-temperature line with liquid enthalpy-temperature line. It was also theoretically expressed by the following approximate equation (Montserrat, 1994): Tf ðTa ; ta Þ < Tg 2 DH=DCp
ð1Þ
The DH and DCp in the equation are relaxation enthalpy and heat capacity change at Tg at an aging time ta ; respectively. The DCp varies with aging time as shown in Fig. 4. The fictive temperatures measured at different aging times are shown in Fig. 5. Theoretically it decreases as a glass transforms toward an equilibrium by aging. Our result was consistent with the theory. Tf decrease for the amorphous rice starches was significant in the early stage of aging, and gradually reached plateau (Fig. 5), in accordance with Montserrat (1994). This trend was related to the relaxation enthalpy ðDHÞ increase that occurred more substantially during the early stage of aging (Fig. 6). Normal rice starch showed higher Tf values than did waxy rice starch. This may relate to the glass transition temperature that was higher for normal rice starch (Fig. 1). The effect of moisture content on the Tf was obvious. For instance, the Tf difference between the initial and the aged (120 h) starches was about 4 8C when moisture content was 15%, which was more than double that found at 11% moisture. Borde et al. (2002b) reported that the Tf decrease was greater when aging temperature was closer to initial fictive temperature. Our result agreed with them because the aging temperature
Fig. 5. Change of fictive temperature ðTf Þ at different aging times up to 240 h at 11 and 15% moisture contents.
(25 8C) was closer to initial fictive temperature at 15% moisture (93.7 8C in rice starch) than that at 11% (54.1 8C). 3.4. Analysis of physical aging The rate of relaxation process ðdDH=dta Þ; based on relaxation enthalpy and aging time, gradually decreased with aging time (Fig. 6). This trend appeared in both normal and waxy starch samples regardless of moisture content. Polymeric system loses its chain mobility when the free volume decreases by approaching toward an equilibrium state by physical aging (Montserrat, 1992). Starch used in our experiment behaved accordingly with many other polymer materials (Cowie & Ferguson, 1986; Montserrat, 1994). During the aging period up to 5 days (120 h), the amorphous waxy rice starch continually exhibited higher enthalpy for relaxation than did the normal rice starch (Fig. 6). This difference was possibly from the difference in polymeric matrix conformation, due to the presence of
Fig. 6. Relaxation enthalpy change at different aging times up to 120 h at 11 and 15% moisture contents.
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activation energy. On the other hand, non-exponentiality is a consequence of relaxation time distribution, which may be introduced by Kohlrausch – Williams – Watts (KWW) response function fðtÞ (Hodge, 1994; Hutchinson, 1995; Williams & Watts, 1970):
fðtÞ ¼ exp{ 2 ðt=tÞb };
ð3Þ
where b is a non-exponentiality parameter which is inversely related to the width of relaxation time distribution. To describe the rate of relaxation with our experimental data, as a function of physical aging time, Cowie and Ferguson equation, as follows, was used (Cowie & Ferguson, 1986): DHðta ; Ta Þ ¼ DH1 ðTa Þ{1 2 fðta Þ}; Fig. 7. Peak temperature change of relaxation at different aging times up to 120 h at 11 and 15% moisture contents.
amylose, as discussed with DCp data. The relaxation enthalpy change was greater at 15% than that at 11% (Fig. 6). Several researches (Chung et al., 2002; Gidley, Cooke, & Smith, 1993; Shogren, 1992) have reported that the relaxation enthalpy in starch increased proportionally with moisture content. It was also suggested that the free volume increase by increasing moisture content facilitated the relaxation process. Unlike the enthalpy, peak temperature of the relaxation endotherm appeared not significantly dependent upon starch type (Fig. 7). But both starches showed linear increases in the peak temperature with aging time. Similar results have been reported with synthetic polymers (Barrel et al., 1999; Montserrat, 1994), maltose (Schmidt & Lammert, 1996), amorphous starches (Shogren, 1992; Thiewes & Steeneken, 1997). Although the peak temperature increased, temperature range for relaxation endotherm decreased as aging time increased (data not shown). This may indicate that the chain mobility becomes more uniform as the glassy starch reaches an equilibrium. 3.5. Kinetics of physical aging
ð4Þ
where DH is enthalpy relaxed by aging, DH1 is the maximum enthalpy at an infinite aging time, and fðta Þ describes the kinetics for the system to approach the equilibrium state as shown in the Eq. (3). The three parameters DH1 ; tc ; and b were determined by using a nonlinear curve-fitting analysis, and Cowie and Ferguson model in which KWW parameter t was replaced by tc : Table 1 shows the physical aging parameters obtained from Eqs. (3) and (4) for the amorphous rice starch samples tested. The maximum relaxation enthalpy ðDH1 Þ calculated from the equations was higher for normal rice starch than for waxy rice starch, at both moisture contents (Table 1). But our experimental data obtained in the aging up to 120 h revealed that the relaxation enthalpy was higher for waxy rice starch (Fig. 6). This result was related to the characteristic time ðtc Þ; which represents an average relaxation time. The tc values for normal and waxy rice starches at 11% moisture content were 295 and 112 h, and those at 15% moisture content were 198 and 80 h, respectively. The tc data revealed that the rate of relaxation process was much slower in normal rice starch than in waxy rice starch. The presence of amylose in normal rice starch made structural relaxation slower, and the retarding tendency resulted in the raised the maximum relaxation enthalpy. Shogren and Jasberg (1994) compared gelatinized starches from normal and high amylose maizes, and found that high amylose maize starch had a slower physical aging (relaxation). Borde at al. (2002b) analyzed that the extruded normal potato starch in comparison with amylopectin and claimed that the heterogeneous structure of amylose and amylopectin in potato starch could modify the aging behavior. Therefore, the heterogeneity by amylose presence in the matrix tested might cause the slower
Structural relaxation is a non-exponential and non-linear behavior (Hodge, 1994; Hutchinson, 1995). Interpretation of experimental results is usually based on a phenomenological model. The relaxation time depends on aging temperature and system structure, and the relation is often described by Too– Narayanaswamy– Moynihan equation (Hodge, 1994; Hutchinson, 1995; Moynihan, Easteal, Wilder, & Tucker, 1974):
Table 1 Parameters for enthalpy relaxation kinetics
t ¼ t0 exp{xDhp =RT þ ð1 2 xÞ=Dhp =RTf }
ð2Þ
Samples
DH1 (J/g)
tc (h)
b
In the equation, Tf is a fictive temperature which characterizes glass structure, x is a non-linearity parameter which defines relative contribution of temperature and structure to the relaxation time, and Dhp is an apparent
RS11 WRS11 RS15 WRS15
1.1010 0.7702 2.6924 2.4799
294.9 112.0 198.3 79.7
0.6780 0.4671 0.3401 0.3719
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relaxation in normal rice starch, which led to the lower DH1 for normal rice starch than the value for waxy rice starch. However, further study is still needed to understand detailed mechanism of the relaxation behavior in terms of starch structure. Additionally, we observed tc dependence on Tg 2 Ta : The Tg 2 Ta for normal and waxy rice starches at 11% moisture content was 68.7 and 63.3 8C, respectively. Such a small difference in Tg 2 Ta might be sufficient to yield the difference in the relaxation behavior observed in this experiment. Ediger, Angell, and Nagel (1996) suggested that for a fragile material, a change of 3 –5 8C in Tg could lead to a change in the rate of relaxation about one decade. So, it is reasonable to assume that the higher Tg 2 Ta for normal starch may result in the slower relaxation at an aging temperature far below Tg ðTa , Tg 2 20 8CÞ: This trend also was shown at 15% moisture content. The significant difference in tc between the two moisture contents could result from the difference in Tg 2 Ta : The Tg decreased by raising the moisture content to 15% caused the decrease in Tg 2 Ta ; and consequently it resulted in the increase of relaxation rate (decreased tc ). Compared to the DH1 and tc reported for synthetic polymers and sugars, the DH1 for the rice starches measured in this experiment was relatively lower, whereas tc value was higher (Table 1). Hancock, Shamblin, and Zografi (1995) reported that DH1 and tc for sucrose at an aging temperature of 61 8C (Tg 2 Ta reported as 17 8C) were 10.9 J/g and 12 h, respectively. The higher DH1 and lower tc than those for starch are because sucrose is a smaller compound and Ta used is higher. The aging of poly(ethylene terephthalate) at a Ta 9 8C below Tg was investigated by Bailey, Hay, and Price (2001), and the DH1 and tc values were 3.20 J/g and 301 min. Cowie and Ferguson (1986) reported the values for poly(vinyl methyl ether) aged at a Tg 2 Ta of 10 8C, 3.48 J/g and 19.4 min. The Tg 2 Ta used for the starch samples in our experiment was significantly higher than those in most literature, and thus comparison with the literature data was difficult. But the nature of tested materials is another key parameter for relaxation characteristics. A study for epoxy resin investigated Montserrat (1994) used an aging temperature at a Tg 2 Ta of 48.9 8C similar to that in our experiment. In the result, however, the lnðtc Þ was 19.7, much longer than the value for starch. This could be proof that starch has different kinetics for relaxation from those of most synthetic polymers. The b values for the normal and waxy rice starch samples aged were 0.6780 and 0.4671 at 11% moisture, and 0.3401 and 0.3719 at 15% moisture, respectively. The b values for other amorphous materials have been reported: 0.4 – 0.8 for sucrose (Hancock et al., 1995), 0.41 for maltose (Schmidt & Lammert, 1996), 0.32 – 0.44 for maltose (Lammert et al., 1999), 0.322 – 0.403 for poly(ethylene terephthalate) (Bailey et al., 2001), 0.179 – 0.222 for epoxy resin (Montserrat, 1994), and 0.33 – 0.45 for poly(vinyl acetate) (Cowie, Harris, & McEwen, 1998). The b values reported in
the literature were not much different from those in our result. Shogren (1992) reported that a cornstarch aged for 7 days at 13.8% moisture had a b value of 0.25. But it was not obtained from actual measurement with starch, but from calculating with poly(vinyl chloride) which had similar relaxation parameters. The b value ranges from 0 to 1, and the smaller b indicates the broader relaxation time distribution. For the starch sample, as the moisture content increased, the relaxation time distribution in aging process became broad (Table 1). In our experiment, the b value was increased as the Tg 2 Ta increased (11% moisture). Cowie and Ferguson (1993) studied with poly(methyl methacrylate), and reported that the relaxation time distribution was broadened as the system reached the glass transition. Our experimental data agreed with their finding. Hancock et al. (1995) also reported that the poly(vinyl pyrrolidone) and indomethacin showed a slight reduction in b as the Tg 2 Ta decreased. However, Cowie and Ferguson (1989) reported that poly(vinyl methyl ether) showed the b value approaching unity when the aging temperature became close to Tg : On the other hand, Aref-Azar, Arnoux, Biddlestone, and Hay (1996) claimed that the b of poly(ethyl terephthalate) was independent of aging temperature. Therefore, the relation of b value to the aging conditions is still under controversy. But it is certain that b depends not only on the aging conditions but on the nature of material tested. Comparing normal and waxy starch samples, the b was higher for normal rice starch at 11% moisture content, but the difference became insignificant at 15% moisture. The b difference between normal and waxy rice starches may result form the amylose presence in normal starch. However, the data is insufficient to draw a firm conclusion. Further study in relaxation kinetics in terms of amylose and amylopectin composition is necessary.
4. Conclusions During aging the Tg of amorphous rice starch was slightly raised, whereas Tf was decreased. The heat capacity increment ðDCp Þ of the starch at Tg slightly increased with aging time, unlike those of synthetic polymers, possibly indicating the inherent nature of starch was different from that of synthetic polymers. Based on Cowie– Ferguson model, the distribution of relaxation time depends on the Tg 2 Ta regardless of moisture content and starch type. The maximum enthalpy and the characteristic time were higher in normal rice starch than waxy rice starch, indicating that waxy rice starch aged more rapidly than normal rice starch. The differences in relaxation kinetics between the two rice starches could suggest different aging behaviors of amylose and amylopectin. The higher structural homogeneity in waxy starch made the relaxation faster, whereas the heterogeneous matrix structure formation in normal starch due to the amylose
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presence might cause the relaxation rate slow. Physical aging kinetics could be used in controlling the processing and storage of glassy starch-based products. But more researches are needed to understand the detailed mechanisms.
Acknowledgements This research was financially supported by Korea Research Foundation (KRF-2001-041-G00062).
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Gidley, M. J., Cooke, D., & Smith, S. W. (1993). Low moisture polysaccharide systems. In J. M. V. Blanshard, & P. J. Lillford (Eds.), The glass state in foods (pp. 303 – 316). Leicestershire: Nottingham University Press. Hancock, B. C., Shamblin, S. L., & Zografi, G. (1995). Molecular mobility of amorphous pharmaceutical solids below their glass transition temperatures. Pharmaceutical Research, 12, 706– 799. Hodge, I. M. (1994). Enthalpy relaxation and recovery in amorphous materials. Journal of Non-Crystalline Solids, 169, 211– 266. Hutchinson, J. M. (1995). Physical aging of polymers. Progress in Polymer Science, 20, 703–760. Jane, J., & Chen, J. (1992). Effect of amylose molecular size and amylopectin branch chain lengths on paste properties of starch. Cereal Chemistry, 69, 60–65. Kalichevsky, M. T., Jaroszkiewicz, E. M., Ablett, S., Blanshard, J. M. V., & Lillford, P. J. (1992). The glass transition of amylopectin measured by DSC, DMTA and NMR. Carbohydrate Polymers, 18, 77–88. Kim, Y. J., Suzuki, T., Hagiwara, T., Yamaji, I., & Taki, R. (2001). Enthalpy relaxation and glass to rubber transition of amorphous potato starch formed by ball-milling. Carbohydrate Polymers, 46, 1–6. Lammert, A. M., Lammert, R. M., & Schmidt, S. J. (1999). Physical aging of maltose glasses as measured by standard and modulated differential scanning calorimetry. Journal of Thermal Analysis and Calorimetry, 55, 949–975. Liao, K., Quan, D., & Lu, Z. (2002). Effects of physical aging on glass transition behavior of poly(DL-Lactide). European Polymer Journal, 38, 157–162. Lim, S. T., Chang, E. H., & Chung, H. J. (2001). Thermal transition characteristics of heat-moisture treated corn and potato starches. Carbohydrate Polymers, 46, 107– 115. Lim, S. T., Lee, J. H., Shin, D. H., & Lim, H. S. (1999). Comparison of protein extraction solutions for rice starch isolation and effects of residual protein content on starch pasting properties. Starch, 51, 120–125. Montserrat, S. (1992). Vitrification and further structural relaxation in the isothermal curing of an epoxy resin. Journal of Applied Polymer Science, 44, 545–554. Montserrat, S. (1994). Physical aging studies in epoxy resins. I. Kinetics of the enthalpy relaxation process in a fully cured epoxy resin. Journal of Polymer Science: Part B: Polymer Physics, 32, 509– 522. Moynihan, C. T., Easteal, A. J., Wilder, J., & Tucker, J. (1974). Dependence of glass transition temperature on heating and cooling rate. Journal of Physical Chemistry, 78, 2673–2677. Roos, Y. H. (1995). Phase transition in foods. SanDiego: Academic press. Schmidt, S., & Lammert, A. M. (1996). Physical aging of maltose glasses. Journal of Food Science, 61, 870–875. Shogren, R. L. (1992). Effect of moisture content on the melting and subsequent physical aging of cornstarch. Carbohydrate Polymers, 19, 83– 90. Shogren, R. L., & Jasberg, B. K. (1994). Aging properties of extruded highamylose starch. Journal of Environment Polymer Degradation, 2, 99–109. Slade, L., & Levine, H. (1995). Glass transitions and water–food structure interactions. Advances in Food and Nutrition Research, 38, 103 –269. Thiewes, H. J., & Steeneken, P. A. M. (1997). The glass transition and the sub-Tg endotherm of amorphous and native potato starch at low moisture content. Carbohydrate Polymers, 32, 123 –130. Tsukushi, I., Yamamuro, O., & Suga, H. (1994). Heat capacities and glass transitions of ground amorphous solid an liquid-quenched glass of tri-Omethyl-b-cyclodextrin. Journal of Non-Crystalline Solids, 175, 187–194. Williams, G., & Watts, D. C. (1970). Non-symmetrical dielectric relaxation behaviour arising from a simple empirical decay function. Transactions of Faraday Society, 66, 80 –85. Wungtanagorn, R., & Schmidt, S. J. (2001). Phenomenological study of enthalpy relaxation of amorphous glucose, fructose, and their mixture. Thermochimica Acta, 369, 95–116. Yuan, R. C., & Thompson, D. B. (1994). Sub-Tg thermal properties of amorphous waxy starch and its derivatives. Carbohydrate Polymers, 25, 1–6.
Physical aging of glassy normal and waxy rice starches: effect of aging time on glass transition and enthalpy relaxation Hyun-Jung Chung, Seung-Taik Lim* Graduate School of Biotechnology, Korea University, 5-1 Anam-dong, Sungbuk-ku, Seoul 136-701, South Korea Received 4 December 2002; accepted 28 February 2003
Abstract Physical aging and glass transition characteristics of amorphous normal and waxy rice starches (11 and 15% moisture contents) were investigated under differential scanning calorimetry as function of aging time. Normal rice starch showed higher Tg than waxy rice starch. The Tg and DCp at glass transition gradually increased with aging time, whereas fictive temperature was slightly reduced regardless of moisture content and starch type. The relaxation enthalpy and relaxation peak temperature increased with aging time until structural equilibrium was reached. Enthalpy increase was more significant in the early stage of aging whereas temperature increase was constant during the aging period tested (120 h). Aging kinetic analysis using Cowie and Ferguson model revealed that the amorphous normal and waxy rice starches behaved in different modes for the physical aging. Relaxation distribution parameter ðbÞ of both starches was in a range of 0:3 , b , 0:7; but higher at a lower moisture content, and for normal starch than for waxy starch. Maximum relaxation enthalpy for normal starch (1.10 and 2.69 J/g, respectively, at 11 and 15% moistures) was higher than those of waxy starch (0.77 and 2.48 J/g). Based on the characteristic time ðtc Þ; normal starch has slower progression toward an equilibrium than waxy starch. Overall results proved that physical aging kinetics were highly dependent on starch structure and composition. q 2003 Elsevier Ltd. All rights reserved. Keywords: Physical aging; Glass transition; Enthalpy relaxation; Rice starch
1. Introduction Many cereal-based food products are manufactured by thermal processing with limited moisture, and thus consumed in a glassy state. These glassy products exist in metastable state, and undergo physical aging process during storage that is responsible for the changes in physical property such as texture, and brittleness. Long-term stability and retention of freshness of the cereal-based foods thus depend on the physical aging (Blanshard & Lillford, 1993; Borde, Bizot, Vigier, & Buleon, 2002a,b; Champion, Le Meste, & Simatos, 2000; Slade & Levine, 1995). Starch is a major biopolymer in most cereal products, and thus physical aging characteristics of starch should be understood to control physical changes of the cereal products during storage. The term ‘physical aging’ is a structural transformation (relaxation) toward an equilibrium as a function of storage * Corresponding author. Tel.: þ 82-2-3290-3435; fax: þ82-2-927-5201. E-mail address: [email protected] (S.-T. Lim). 0268-005X/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0268-005X(03)00106-1
time. It usually occurs at a constant temperature at zero stress and under no influence from any other external conditions (Hutchinson, 1995). Physical aging of a glassy polymer matrix is observed at a temperature below glass transition temperature ðTg Þ: The structural relaxation of a glass is time-dependent with change in specific volume, enthalpy, and mechanical and dielectric responses (Hodge, 1994; Hutchinson, 1995). Thus, physical aging may be quantitatively measured as the changes in specific volume or relaxation enthalpy (Hodge, 1994). One of the most frequent techniques for analysis is differential scanning calorimetry (DSC). The aged samples exhibit endothermic peak (enthalpy relaxation) in DSC thermogram usually before glass transition (Cowie & Ferguson, 1986). Several research groups have studied the physical aging of starch: amylopectin in cornstarch (Kalichevsky, Jaroskewicz, Ablett, Blanshard, & Lillford, 1992), cornstarch (Shogren, 1992), waxy cornstarch (Borde et al., 2002a,b; Yuan & Thompson, 1994), potato starch (Thiewes & Steeneken, 1997), boll-milled potato starch (Kim, Suzuki, Hagiwara, Yamaji, & Taki, 2001), heat-treated corn and
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potato starches (Lim, Chang, & Chung, 2001), and native and gelatinized rice starches (Chung, Lee, & Lim, 2002). Recently, Borde et al. (2002a,b) systematically studied the physical aging of waxy maize starch, in comparison to synthetic polymers. Their studies were based on the phenomenology commonly used for synthetic polymers. They suggested that the physical aging kinetics depended on cooling rate, storage temperature, and chemical structure of polymer. It was found that waxy maize starch underwent the aging process at a faster rate than did synthetic polymers. In this study, physical aging kinetics of amorphous rice starches were investigated, and differences between normal and waxy rices were examined.
the measurement of the aged sample, rescanning was done under the same cooling and heating conditions for the measurement of unaged sample. The glass transition temperature was determined as the peak of the first derivative curve of the heat capacity thermogram. The fictive temperature ðTf Þ; defined as the temperature at which the enthalpy of material was equal to its equilibrium enthalpy, was determined according to the procedure of Montserrat (1994). Enthalpy of relaxation during aging was measured from the DSC thermogram as the function of aging time and the kinetics were analyzed using Cowie and Ferguson (1986) model.
3. Results and discussion 2. Materials and methods
3.1. Glass transition temperature
2.1. Amorphous rice starches
In the amorphous state resulted from the isolation process used, normal rice starch exhibited higher Tg values than waxy rice starch (Fig. 1). Bizot et al. (1997) reported that pure amylose exhibited higher Tg than branched polyanhydroglucose compounds. The a(1 ! 6) linkages theoretically offer three rotational degrees of freedom whereas the a(1 ! 4) linkages offer two degrees. Thus, amylopectins that has higher degree of branching have greater chain flexibility than relatively linear amyloses (Bizot et al., 1997). Additionally, when starch was isolated in amorphous state, linear amyloses might form denser structure by chain associations than did branched amylopectins. The greater molecular rigidity and the tendency of dense matrix formation of amyloses in normal rice starch led to the higher Tg for the starch. At 11% moisture content, the Tg of amorphous rice starches could be readily measured from the DSC thermogram because the relaxation endotherm did not overlap the glass transition (thermogram not shown). At this moisture content, there was a trend of Tg increase as aging time increased (Fig. 1). The Tg increase by aging has
Normal and waxy rice starches were isolated from Japonica-type rice flours by following the procedure of Lim, Lee, Shin, and Lim (1999), and the starches were further purified by using 90% dimethyl sulfoxide (DMSO) and ethanol (Jane & Chen, 1992). The purification was repeated three times, and the pure starch was dried in a convection oven (40 8C) overnight. 2.2. DSC sample preparation For physical aging analysis, DSC samples of the amorphous rice starch were prepared at 11 or 15% moisture content. Starch (10 ^ 0.1 mg dry solid basis) was placed in a silver DSC pan (Seiko Instrument, Chiba, Japan) and hydrated excessively in a humidity chamber over distilled water. Then moisture in the sample was allowed to evaporate in a balance until desired moisture content was obtained. The starch sample pan was then hermetically sealed and equilibrated by leaving at room temperature for 24 h prior to the DSC measurement. Temperature and enthalpy in DSC (Seiko Instrument, DSC 6100, Chiba, Japan) were calibrated with indium and mercury (156.6 and 2 39.0 8C for melting, respectively), and heat capacity was calibrated with sapphire. An empty pan was used as a reference. 2.3. DSC measurements To erase the thermal history of starch, DSC samples were heated from 5 to 160 8C at a heating rate of 2 8C/min, held at 160 8C for 10 min, and then immediately cooled to an aging temperature (Ta ; 25 8C) at a cooling rate of 20 8C/min. The aging at 25 8C was performed for different periods (ta ; 1– 10 days), and then the enthalpy relaxation and glass transition were measured under DSC at a heating rate of 2 8C/min up to 160 8C. Immediately after
Fig. 1. Glass transition temperature ðTg Þ change at different aging times up to 240 h at 11% moisture content.
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been reported with a variety of materials. Schmidt and co-workers (1996, 1999, 2001) reported the Tg increases for aged maltose and glucose – fructose mixture, and suggested that it was from the shift of relaxation endotherm to higher temperature. Liao, Quan, and Lu (2002) proposed that perfection in inter-chain packing in segmental relaxation process occurred during aging of poly(DL-lactide). Aging caused chain stiffness of cured epoxy resin under a dynamic thermal analysis (Barral et al., 1999, 2000). Likewise, conformation changes proceed during aging in a direction to decrease free volume and mobility of the glassy starch chains, raising the Tg : Temperature range for glass transition in DSC thermogram was also affected by aging (Fig. 2). Reduction in glass transition temperature range has been observed with aged maltose (Lammert, Lammert, & Schmidt, 1999), glucose – fructose mixture (Wungtanagorn & Schmidt, 2001), and poly(DL-lactide) (Liao et al., 2002). In this study, transition range decreased mostly in the early stage of aging, up to 50 h, because the segmental mobility was more restricted as free volume decreased rapidly in the early stage of aging. The starch samples used in this experiment included precedent thermal history. Fig. 3 shows the two DSC thermograms of the normal rice starch samples in the absence and presence of thermal history. By erasing the thermal history the rice starch exhibited a thermogram ((b) in Fig. 3) with a noticeable reduction in Tg (93.9 versus 99.3 8C) and DCp (0.245 versus 0.741 J/g 8C). This result proved that the Tg increases with aging (similar to thermal history). The relaxation endotherm, which represents the structural and conformational changes of glassy polymers, affects the glass transition.
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Fig. 3. DSC thermogram of normal rice starch with thermal history (a), and rescanned sample (b) at 11% moisture content.
The heat capacity increment ðDCp Þ at Tg ; which was a major attribute for glass transition, increased with aging
time (Fig. 4). The DCp values of unaged normal and waxy rice starches at 11% moisture were 0.245 and 0.229 J/ g deg.C, respectively. These values were similar to that reported for waxy cornstarch by Borde et al. (2002b) (0.201 J/g 8C). At 15% moisture content, the DCp was higher (0.277 and 0.255 J/g 8C, respectively, for normal and waxy rice starches), and this DCp increase was from the increased free volume of starch chains by raising moisture content (Roos, 1995). As discussed with the Tg results, amylose chains are less flexible than amylopectin chains. Based on this theory, the amylose presence in normal starch leads to depress DCp at glass transition. In accordance, Bizot et al. (1997) reported that amylose had a smaller DCp than amylopectin. However, our experimental data showed an opposite trend to the theoretical expectation, showing higher DCp for normal starch (Fig. 4). It was assumed that three-dimensional network conformation in the starch matrix was more important than the molecular structure based on degree of branching. The matrix structure might be more critical when moisture is limited as in this experiment. Based on the composition, normal rice starch may form a heterogeneous
Fig. 2. Glass transition range change at different aging times up to 240 h at 11% moisture content.
Fig. 4. Change of heat capacity increment ðDCp Þ at Tg at different aging times up to 240 h at 11 and 15% moisture contents.
3.2. Heat capacity change
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mixture because two different starch chains (amylose and amylopectin) exist, whereas waxy rice starch composed only by amylopectin chains, and thus forms a more homogeneous matrix structure. Therefore, the homogeneity of waxy starch could result in the low DCp : The DCp increase at glass transition by aging was reported also for glucose, fructose, and their mixture by Wuntanagorn and Schmidt (2001), whereas the DCp for polymers has been reported to be relatively constant with aging time (Barral et al., 1999; Cook, Mehrabi, & Edward, 1999; Montserrat, 1994). The DCp depends mainly on the changes in glassy state during aging. Borde et al. (2002b) and Tsukushi, Yamamuro, and Suga (1994) found that the glass heat capacity ðCp Þ of hydrated polysaccharides decreased with aging time. With the decreased heat capacity of glass but relatively constant Cp of rubber during aging, the heat capacity difference ðDCp Þ results in an increase ðDCp ¼ rubber Cp 2 glass Cp Þ: The experimental results may suggest that the glass Cp is different between the amorphous normal and waxy rice starches. But additional experiments should be done to understand the mechanism. 3.3. Fictive temperature Fictive temperature ðTf Þ is a hypothetical temperature at which a glass structure remains in a thermodynamic equilibrium. The Tf could be obtained by intersecting extrapolated glassy enthalpy-temperature line with liquid enthalpy-temperature line. It was also theoretically expressed by the following approximate equation (Montserrat, 1994): Tf ðTa ; ta Þ < Tg 2 DH=DCp
ð1Þ
The DH and DCp in the equation are relaxation enthalpy and heat capacity change at Tg at an aging time ta ; respectively. The DCp varies with aging time as shown in Fig. 4. The fictive temperatures measured at different aging times are shown in Fig. 5. Theoretically it decreases as a glass transforms toward an equilibrium by aging. Our result was consistent with the theory. Tf decrease for the amorphous rice starches was significant in the early stage of aging, and gradually reached plateau (Fig. 5), in accordance with Montserrat (1994). This trend was related to the relaxation enthalpy ðDHÞ increase that occurred more substantially during the early stage of aging (Fig. 6). Normal rice starch showed higher Tf values than did waxy rice starch. This may relate to the glass transition temperature that was higher for normal rice starch (Fig. 1). The effect of moisture content on the Tf was obvious. For instance, the Tf difference between the initial and the aged (120 h) starches was about 4 8C when moisture content was 15%, which was more than double that found at 11% moisture. Borde et al. (2002b) reported that the Tf decrease was greater when aging temperature was closer to initial fictive temperature. Our result agreed with them because the aging temperature
Fig. 5. Change of fictive temperature ðTf Þ at different aging times up to 240 h at 11 and 15% moisture contents.
(25 8C) was closer to initial fictive temperature at 15% moisture (93.7 8C in rice starch) than that at 11% (54.1 8C). 3.4. Analysis of physical aging The rate of relaxation process ðdDH=dta Þ; based on relaxation enthalpy and aging time, gradually decreased with aging time (Fig. 6). This trend appeared in both normal and waxy starch samples regardless of moisture content. Polymeric system loses its chain mobility when the free volume decreases by approaching toward an equilibrium state by physical aging (Montserrat, 1992). Starch used in our experiment behaved accordingly with many other polymer materials (Cowie & Ferguson, 1986; Montserrat, 1994). During the aging period up to 5 days (120 h), the amorphous waxy rice starch continually exhibited higher enthalpy for relaxation than did the normal rice starch (Fig. 6). This difference was possibly from the difference in polymeric matrix conformation, due to the presence of
Fig. 6. Relaxation enthalpy change at different aging times up to 120 h at 11 and 15% moisture contents.
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activation energy. On the other hand, non-exponentiality is a consequence of relaxation time distribution, which may be introduced by Kohlrausch – Williams – Watts (KWW) response function fðtÞ (Hodge, 1994; Hutchinson, 1995; Williams & Watts, 1970):
fðtÞ ¼ exp{ 2 ðt=tÞb };
ð3Þ
where b is a non-exponentiality parameter which is inversely related to the width of relaxation time distribution. To describe the rate of relaxation with our experimental data, as a function of physical aging time, Cowie and Ferguson equation, as follows, was used (Cowie & Ferguson, 1986): DHðta ; Ta Þ ¼ DH1 ðTa Þ{1 2 fðta Þ}; Fig. 7. Peak temperature change of relaxation at different aging times up to 120 h at 11 and 15% moisture contents.
amylose, as discussed with DCp data. The relaxation enthalpy change was greater at 15% than that at 11% (Fig. 6). Several researches (Chung et al., 2002; Gidley, Cooke, & Smith, 1993; Shogren, 1992) have reported that the relaxation enthalpy in starch increased proportionally with moisture content. It was also suggested that the free volume increase by increasing moisture content facilitated the relaxation process. Unlike the enthalpy, peak temperature of the relaxation endotherm appeared not significantly dependent upon starch type (Fig. 7). But both starches showed linear increases in the peak temperature with aging time. Similar results have been reported with synthetic polymers (Barrel et al., 1999; Montserrat, 1994), maltose (Schmidt & Lammert, 1996), amorphous starches (Shogren, 1992; Thiewes & Steeneken, 1997). Although the peak temperature increased, temperature range for relaxation endotherm decreased as aging time increased (data not shown). This may indicate that the chain mobility becomes more uniform as the glassy starch reaches an equilibrium. 3.5. Kinetics of physical aging
ð4Þ
where DH is enthalpy relaxed by aging, DH1 is the maximum enthalpy at an infinite aging time, and fðta Þ describes the kinetics for the system to approach the equilibrium state as shown in the Eq. (3). The three parameters DH1 ; tc ; and b were determined by using a nonlinear curve-fitting analysis, and Cowie and Ferguson model in which KWW parameter t was replaced by tc : Table 1 shows the physical aging parameters obtained from Eqs. (3) and (4) for the amorphous rice starch samples tested. The maximum relaxation enthalpy ðDH1 Þ calculated from the equations was higher for normal rice starch than for waxy rice starch, at both moisture contents (Table 1). But our experimental data obtained in the aging up to 120 h revealed that the relaxation enthalpy was higher for waxy rice starch (Fig. 6). This result was related to the characteristic time ðtc Þ; which represents an average relaxation time. The tc values for normal and waxy rice starches at 11% moisture content were 295 and 112 h, and those at 15% moisture content were 198 and 80 h, respectively. The tc data revealed that the rate of relaxation process was much slower in normal rice starch than in waxy rice starch. The presence of amylose in normal rice starch made structural relaxation slower, and the retarding tendency resulted in the raised the maximum relaxation enthalpy. Shogren and Jasberg (1994) compared gelatinized starches from normal and high amylose maizes, and found that high amylose maize starch had a slower physical aging (relaxation). Borde at al. (2002b) analyzed that the extruded normal potato starch in comparison with amylopectin and claimed that the heterogeneous structure of amylose and amylopectin in potato starch could modify the aging behavior. Therefore, the heterogeneity by amylose presence in the matrix tested might cause the slower
Structural relaxation is a non-exponential and non-linear behavior (Hodge, 1994; Hutchinson, 1995). Interpretation of experimental results is usually based on a phenomenological model. The relaxation time depends on aging temperature and system structure, and the relation is often described by Too– Narayanaswamy– Moynihan equation (Hodge, 1994; Hutchinson, 1995; Moynihan, Easteal, Wilder, & Tucker, 1974):
Table 1 Parameters for enthalpy relaxation kinetics
t ¼ t0 exp{xDhp =RT þ ð1 2 xÞ=Dhp =RTf }
ð2Þ
Samples
DH1 (J/g)
tc (h)
b
In the equation, Tf is a fictive temperature which characterizes glass structure, x is a non-linearity parameter which defines relative contribution of temperature and structure to the relaxation time, and Dhp is an apparent
RS11 WRS11 RS15 WRS15
1.1010 0.7702 2.6924 2.4799
294.9 112.0 198.3 79.7
0.6780 0.4671 0.3401 0.3719
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relaxation in normal rice starch, which led to the lower DH1 for normal rice starch than the value for waxy rice starch. However, further study is still needed to understand detailed mechanism of the relaxation behavior in terms of starch structure. Additionally, we observed tc dependence on Tg 2 Ta : The Tg 2 Ta for normal and waxy rice starches at 11% moisture content was 68.7 and 63.3 8C, respectively. Such a small difference in Tg 2 Ta might be sufficient to yield the difference in the relaxation behavior observed in this experiment. Ediger, Angell, and Nagel (1996) suggested that for a fragile material, a change of 3 –5 8C in Tg could lead to a change in the rate of relaxation about one decade. So, it is reasonable to assume that the higher Tg 2 Ta for normal starch may result in the slower relaxation at an aging temperature far below Tg ðTa , Tg 2 20 8CÞ: This trend also was shown at 15% moisture content. The significant difference in tc between the two moisture contents could result from the difference in Tg 2 Ta : The Tg decreased by raising the moisture content to 15% caused the decrease in Tg 2 Ta ; and consequently it resulted in the increase of relaxation rate (decreased tc ). Compared to the DH1 and tc reported for synthetic polymers and sugars, the DH1 for the rice starches measured in this experiment was relatively lower, whereas tc value was higher (Table 1). Hancock, Shamblin, and Zografi (1995) reported that DH1 and tc for sucrose at an aging temperature of 61 8C (Tg 2 Ta reported as 17 8C) were 10.9 J/g and 12 h, respectively. The higher DH1 and lower tc than those for starch are because sucrose is a smaller compound and Ta used is higher. The aging of poly(ethylene terephthalate) at a Ta 9 8C below Tg was investigated by Bailey, Hay, and Price (2001), and the DH1 and tc values were 3.20 J/g and 301 min. Cowie and Ferguson (1986) reported the values for poly(vinyl methyl ether) aged at a Tg 2 Ta of 10 8C, 3.48 J/g and 19.4 min. The Tg 2 Ta used for the starch samples in our experiment was significantly higher than those in most literature, and thus comparison with the literature data was difficult. But the nature of tested materials is another key parameter for relaxation characteristics. A study for epoxy resin investigated Montserrat (1994) used an aging temperature at a Tg 2 Ta of 48.9 8C similar to that in our experiment. In the result, however, the lnðtc Þ was 19.7, much longer than the value for starch. This could be proof that starch has different kinetics for relaxation from those of most synthetic polymers. The b values for the normal and waxy rice starch samples aged were 0.6780 and 0.4671 at 11% moisture, and 0.3401 and 0.3719 at 15% moisture, respectively. The b values for other amorphous materials have been reported: 0.4 – 0.8 for sucrose (Hancock et al., 1995), 0.41 for maltose (Schmidt & Lammert, 1996), 0.32 – 0.44 for maltose (Lammert et al., 1999), 0.322 – 0.403 for poly(ethylene terephthalate) (Bailey et al., 2001), 0.179 – 0.222 for epoxy resin (Montserrat, 1994), and 0.33 – 0.45 for poly(vinyl acetate) (Cowie, Harris, & McEwen, 1998). The b values reported in
the literature were not much different from those in our result. Shogren (1992) reported that a cornstarch aged for 7 days at 13.8% moisture had a b value of 0.25. But it was not obtained from actual measurement with starch, but from calculating with poly(vinyl chloride) which had similar relaxation parameters. The b value ranges from 0 to 1, and the smaller b indicates the broader relaxation time distribution. For the starch sample, as the moisture content increased, the relaxation time distribution in aging process became broad (Table 1). In our experiment, the b value was increased as the Tg 2 Ta increased (11% moisture). Cowie and Ferguson (1993) studied with poly(methyl methacrylate), and reported that the relaxation time distribution was broadened as the system reached the glass transition. Our experimental data agreed with their finding. Hancock et al. (1995) also reported that the poly(vinyl pyrrolidone) and indomethacin showed a slight reduction in b as the Tg 2 Ta decreased. However, Cowie and Ferguson (1989) reported that poly(vinyl methyl ether) showed the b value approaching unity when the aging temperature became close to Tg : On the other hand, Aref-Azar, Arnoux, Biddlestone, and Hay (1996) claimed that the b of poly(ethyl terephthalate) was independent of aging temperature. Therefore, the relation of b value to the aging conditions is still under controversy. But it is certain that b depends not only on the aging conditions but on the nature of material tested. Comparing normal and waxy starch samples, the b was higher for normal rice starch at 11% moisture content, but the difference became insignificant at 15% moisture. The b difference between normal and waxy rice starches may result form the amylose presence in normal starch. However, the data is insufficient to draw a firm conclusion. Further study in relaxation kinetics in terms of amylose and amylopectin composition is necessary.
4. Conclusions During aging the Tg of amorphous rice starch was slightly raised, whereas Tf was decreased. The heat capacity increment ðDCp Þ of the starch at Tg slightly increased with aging time, unlike those of synthetic polymers, possibly indicating the inherent nature of starch was different from that of synthetic polymers. Based on Cowie– Ferguson model, the distribution of relaxation time depends on the Tg 2 Ta regardless of moisture content and starch type. The maximum enthalpy and the characteristic time were higher in normal rice starch than waxy rice starch, indicating that waxy rice starch aged more rapidly than normal rice starch. The differences in relaxation kinetics between the two rice starches could suggest different aging behaviors of amylose and amylopectin. The higher structural homogeneity in waxy starch made the relaxation faster, whereas the heterogeneous matrix structure formation in normal starch due to the amylose
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presence might cause the relaxation rate slow. Physical aging kinetics could be used in controlling the processing and storage of glassy starch-based products. But more researches are needed to understand the detailed mechanisms.
Acknowledgements This research was financially supported by Korea Research Foundation (KRF-2001-041-G00062).
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