Effects of Physical Aging on Thermal and Mechanical Properties of Glassy Normal Corn Starch

Transcription

1 354 DOI /star Starch/Stärke 57 (2005) Hyun-Jung Chung a Byoungseung Yoo b Seung-Taik Lim a Effects of Physical Aging on Thermal and Mechanical Properties of Glassy Normal Corn Starch a School of Life Sciences and Biotechnology, Korea University, Seoul, Korea b Department of Food Science and Technology, Dongguk University, Seoul, Korea Physical aging of glassy corn starch and its effect on the physical properties of compressed starch bars were investigated by using differential scanning calorimetry (DSC), dynamic mechanical thermal analyzer (DMTA), and Instron texturometer. Amorphous corn starch samples were prepared by drying a starch paste, followed by compressing the dried powders to bar-type specimens under heat (1007C, Pa). These bars were then aged at 257C for up to 22 days, after which thermal and mechanical characteristics were determined. There were progressive increases in glass transition temperature and relaxation enthalpy during the aging, which reached limiting values as structural equilibrium of the amorphous matrix was approached. The peak temperature of the relaxation endotherm (DSC thermogram) changed with time in a trend similar to the relaxation enthalpy. The storage modulus (DMTA) increased with aging time, but the peak intensity of tan d decreased. The breaking strength (Instron texturometer) increased gradually with time of aging and reached a maximum. It has been demonstrated the relaxation kinetics for glassy amorphous corn starch can be characterized by both thermal and mechanical properties, such as relaxation enthalpy, storage modulus, and breaking strength. Keywords: Physical aging; Structural relaxation; Corn starch Research Paper 1 Introduction Many processed foods are commercially manufactured by thermal treatment that has induced a rubbery state formation in the food matrices, but by consecutive cooling or drying they may transform back to a glassy state. This glassy food matrix usually remains in a non-equilibrium state and its physical properties such as volume and enthalpy are greater than those in equilibrium. With time, the glass usually transforms toward its equilibrium by molecular rearrangement, and thus the physical and mechanical properties also tend to change [1 3]. This process is called physical aging. Cereal-based foods containing relatively limited moisture, such as breakfast cereals, snacks, crackers and biscuits, are consumed in a glassy state, usually after thermal processing. The texture of some of these cereal-based foods changes gradually with physical aging. Champion et al. [4] claimed that loss of crispness from cereal products resulted from physical aging. Thus, to control aging so as to reduce the physical property changes and to maintain the quality of the glassy foods during storage is important [5 8]. In the cereal-based food products, starch is the major component responsible for the physical aging. Correspondence: Seung-Taik Lim, School of Life Sciences and Biotechnology, Korea University, 5 1 Anam-dong, Sungbuk-ku, Seoul , Korea. Phone: , Fax: , limst@korea.ac.kr. Recently, we investigated the physical aging of glassy rice starch using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMTA) and an Instron texturometer, and found that the relaxation kinetics for the starch could be explained by thermal transition enthalpy as well as mechanical properties [4]. In the present study, physical aging of corn starch, which is the major constituent in many breakfast cereals and snacks, was characterized by thermal and mechanical analyses, by using DSC, DMTA, and an Instron texturometer. 2 Materials and Methods 2.1 Sample preparation Normal corn starch was supplied by Samyang Genex, Inc. (Seoul, Korea). Glassy corn starch was prepared by gelatinizing starch in boiling water (10% dry solids, w/w), and subsequently dehydrating the paste in a convection oven at 507C. The dried starch samples were ground and sieved (50 mesh) into powders. For moisture adjustment, the starch powders were rehydrated to 20,25% (w/w) moisture content in a humidity chamber, and then compressed at Pa for 10 min at 1007C to a rectangular bar (20 mm650 mm63 mm). This bar was then aged up to 22 days at 257C, over a saturated NaCl solution to minimize moisture change. Moisture content of the starch bar was determined by drying at 1057C to constant weight.

2 Starch/Stärke 57 (2005) Effects of Aging on Properties of Glassy Corn Starch DSC measurement A portion (,50 mg) of the aged starch bar was placed in a DSC silver pan, and thermal transitions were determined over a temperature range of 0 to 1407C at a heating rate of 57C/min, using a differential scanning calorimeter (Seiko Instruments DSC 6100, Chiba, Japan). Rescanning was done immediately after cooling at a rate of 207C/min to examine the thermal transition of the unaged starch sample. The DSC instrument was calibrated using indium and mercury, and an empty pan was used as reference. 2.3 DMTA measurement A dynamic mechanical thermal analyzer (DMTA, Seiko DMA 6000, Chiba, Japan) was used to determine dynamic storage modulus (E ), loss modulus (E ), and tan d of the aged starch bar. The measurements were carried out in a three-point bending mode at a frequency of 1 Hz. While being tested, the sample was heated from 20 to 1407C at a rate of 27C/min. In order to avoid water loss, silicon oil was applied on the surface of the starch sample bar. 2.4 Texturometer measurement Breaking strength of the starch bar was determined in a three-point bending mode using a texturometer (Instron 4465, Canton, MA). A cross-head speed of 10 mm/min was employed. 3 Results and Discussion 3.1 Calorimetric behavior Moisture content of the compressed bar of the amorphous corn starch quickly reached % in the chamber containing the saturated NaCl solution, and remained nearly constant during the aging period (up to 22 days at 257C). The unaged corn starch specimen, in which the previous thermal history was erased by heating, displayed dual glass transitions (Fig. 1). Using the first derivative curve of the thermogram (dc p /dt) (small box in Fig. 1), dual glass transitions were clearly seen at temperatures of 57.3 and 73.47C. Dual glass transitions of some synthetic polymers have been reported by several researchers [9 11]. The dual glass transitions could indicate the presence of two structurally different amorphous regions, in which the structural differences result in differences in molecular mobility. It was also reported that dual glass transitions could appear when moderate crystallinity is present, whereas only single transitions were observed in specimens of very low or no crystallinity [9, 10]. All the corn starch specimens tested in the experiment, exhibited melting endotherm (DSC thermogram), with a peak temperature and enthalpy of 1587C and 3.10 J/g, respectively (Fig. 2). The presence of the melting endotherm indicates that recrystallization might occur during the drying process (24 h at 507C) of the gelatinized starch paste. In our previous study [8], unaged normal rice starch exhibited Fig. 1. DSC thermogram of unaged corn starch (first derivative curve in the small box).

3 356 H.-J. Chung et al. Starch/Stärke 57 (2005) Fig. 2. DSC thermogram for melting of recrystallized corn starch. dual glass transitions at 56.7 and 71.47C, and also a melting endotherm at about 1507C with an enthalpy of 2.21 J/g, whereas unaged waxy rice starch had a very low crystallinity and a single glass transition. Therefore, the moderate crystallinity, induced by drying, could lead to the formation of heterogeneous amorphous regions in the starch sample, revealed by the two glass transition temperatures. Chung and Lim [12] reported that rice starch at a low moisture content (11%) displayed its T g far apart from the relaxation endotherm. In the present experiment, however, the lower T g was overlapped the relaxation endotherm, whereas the upper T g appeared at much higher temperature, apart from the relaxation endotherm. Thus, only the change in the upper T g could be observed from analysis of aged samples. The upper T g of the corn starch specimen increased with aging. The T g increase was significant in the early stage of the aging, but the rate increase decreased gradually as aging continued (Fig. 3). A similar trend has been already reported with various sugar and polymeric materials [12 14]. During aging, conformational changes proceed, decreasing the free volume and mobility of the glassy starch chains, but as the system approaches an equilibrium state, the conformations change less and less because of the rigidity of the matrix increases. Therefore, the glass transition temperature reaches a maximum when the matrix system reaches the most stable equilibrium state. The relaxation enthalpy also increased with aging time, and the rate of increase gradually decreased as with the glass transition temperature (Fig. 4). This trend is universally observed with many materials undergoing physical aging. According to the same theory, relaxation decreases as the matrix reaches equilibrium. The peak temperature of the relaxation endotherm also increased with aging time (Fig. 5), and the increasing rate gradually decreased as observed with the glass transition and relaxation enthalpy. For normal and waxy rice starches, it was found that the relaxation temperature change was very similar to the relaxation enthalpy change [15]. Therefore, conformational changes brought about by aging are well explained by restricted mobility and reduced free volume of the constituent starch chains. The Cowie and Ferguson model [16] has been used to explain the relaxation kinetics: DH(t a, T a )=DH? (T a )[1-exp {-(t a /t c ) b }], where DH? represents maximum equilibrium enthalpy, t c average relaxation time, and b the width of relaxation time distribution. These parameters were calculated using non-linear curve fitting analysis. The maximum relaxation enthalpy (DH? ) (0.983 J/g) for the aged corn starch was substantially lower than that for the normal rice starch (2.064 J/g), measured in our previous study [8]. Although the corn starch specimen used in this study was prepared in the same way as the rice starch specimen used in the

4 Starch/Stärke 57 (2005) Effects of Aging on Properties of Glassy Corn Starch 357 Fig. 3. Glass transition temperature (T g ) changes at different aging times up to 22 days for glassy corn starch. Fig. 4. Relaxation enthalpy changes at different aging times up to 22 days for glassy corn starch. previous study, there was a difference in moisture content (14.5 and 13.5% for rice and corn starches, respectively). The higher moisture content of the specimen used in the previous study provided more free volume and chain mobility, and resulted in the higher DH?. Another possible basis of differences is the melting enthalpy (DSC thermograms), which was higher for the corn starch specimen than for the rice starch specimen (3.10 and 2.21 J/g for corn and rice starch specimens, respectively). This suggests that there was a difference in residual crystallinity. As reported by Mukherjee and Jabarin [17], the degree of relaxation is negatively affected by the residual crystallinity, because the crystalline regions connect the amorphous regions and decrease the mobility of starch chains. The t c (average relaxation time) was 10.4 days, and the b (relaxation time distribution) for the normal con starch specimen was For the normal rice starch specimen, the t c and b were 26.0 days and 0.336, respectively [8]. The smaller b value indicates a broader distribution of relaxation times and a slower approach to equilibrium [18]. The result thus suggests that the normal corn starch specimen relaxed faster and had a narrower relaxation time distribution than did the normal rice starch specimen, although the corn starch sample had less moisture and higher crystallinity. The normal corn starch has longer amylopectin chains but shorter amylose chains compared to normal rice starch [19]. Possibly, the shorter amylose chains in corn starch allowed the relaxation

5 358 H.-J. Chung et al. Starch/Stärke 57 (2005) Fig. 5. Peak temperature changes of relaxation at different aging times up to 22 days for glassy corn starch. to occur faster. In addition, the difference in chain length between amylopectin and amylose is less in corn starch than that in rice starch, perhaps allowing the corn starch sample to relax in more homogeneous fashion. For a clear understanding of the relaxation kinetics, more study is required. 3.2 Viscoelastic behavior The viscoelastic behaviors of the amorphous corn starch specimen, examined by DMTA, are shown in Figs Storage modulus (E ) of the glassy state (,507C) increased during aging for 22 days (Fig. 6), indicating a process of chain stiffening [20, 21]. However, the modulus dropped at the glass transition temperature (50,607C). At a temperature of about 1207C, a slight increase in E was observed, possibly due to a minor water loss. For loss modulus (E ), a major peak was observed between 40 and 807C (Fig. 7), which might originate from the glass transition. Along with the E increase, the E also increased during aging due to the increased stiffness of the starch chains. Above 1007C, multiple but small peaks are observed in the thermograms. The minor peak around 1207C was possibly a result of water loss, but the peak at C corresponded to the crystal melting that was observed in DSC thermograms around 1507C. The normal corn starch specimen tested in this study exhibited multiple peaks in the loss factor (tan d) in the DMTA thermograms (Fig. 8). The major tan d peak that occurred over a broad temperature range from 607C to 1007C was related to the glass transition. However, there was a minor peak or shoulder at about 607C prior to the major peak, perhaps indicating an additional glass transition as shown in the DSC thermogram. In our previous study [8], dual peaks in tan d were found in similar temperature range for a normal rice starch specimen. The tan d peak intensity decreased with aging time. The intensity and temperature of the tan d peak, storage modulus, and loss modulus for the amorphous corn starch specimen at different aging times are summarized in Tab. 1. The intensity of the tan d peak decreased, but the peak temperature increased with aging time. The decrease of the peak intensity can be used to identify the structural relaxation in which the starch chains lose their mobility and free volume as the equilibrium glassy state is approached. Furthermore, the increase of the tan d peak temperature (glass transition temperature) was in accord with the DSC result and findings by other researchers [8, 22]. The E and E increase by aging was also attributed to stiffening of the starch chains, but it was gradually minimized as the system approached the equilibrium glassy state (Tab. 1). Tab. 1. Tan d peak temperature and intensity, storage modulus at 307C (E 30 ) and maximum loss modulus (E max ) and peak temperature at E max for glassy corn starch. Aging time [day] Tan d intensity [peak temperature, 7C] E 30 [Pa610 8 ] E max [Pa610 7 ] [peak temperature, 7C] (89.0) (57.5) (90.1) (58.3) (97.0) (58.5) (91.0) (59.5)

6 Starch/Stärke 57 (2005) Effects of Aging on Properties of Glassy Corn Starch 359 Fig. 6. Storage modulus (E ) changes at different aging times up to 22 days for glassy corn starch. Fig. 7. Loss modulus (E ) changes at different aging times up to 22 days for glassy corn starch. 3.3 Mechanical behavior Fig. 9 shows that the breaking strength of the glassy corn starch specimen in the stress-strain curve increased with aging time and approached a plateau after 15 days of aging. This behavior was consistent with results from the glassy rice starch specimen [8] and epoxy resin [23]. Lourdin et al. [20] suggested that the strength at breakage of a starch-sorbitol specimen increased but the deformation at break decreased,during aging. It indicates that there was a progressive stiffening and embrittlement for the molded specimen during aging, which are common physical changes induced by physical aging. In our previous study [8], it was also found that the breaking strength of the molded normal rice starch specimen increased up to 10 MPa, and thereafter remained constant. The time for reaching the equilibrium state for the corn starch specimen was similar, but the breaking strength at the plateau was different (14 and 10 MPa for corn and rice starches, respectively). The structural densities of the two samples might not have been the same because they had different moisture contents (14.5% for

7 360 H.-J. Chung et al. Starch/Stärke 57 (2005) Fig. 8. Tan d changes at different aging times up to 22 days for glassy corn starch. Fig. 9. Breaking strength changes at different aging times up to 22 days for glassy corn starch. rice and 13.5% for corn) and crystallinities (2.21 J/g for rice and 3.10 J/g for corn, as melting enthalpy). The lower moisture content and higher crystallinity of the corn starch specimen possibly induced a higher structural density, which led to the higher breaking strength. In addition, the higher amylose content in corn starch [19] could contribute to a more rigid structure. Yoshida [24] suggested that the enthalpy relaxations of poly(ether imide) and poly(aryl ether ether ketone) were well correlated to the dynamic mechanical relaxation, but the periods over which they reached their equilibrium glassy states were not identical. Cook et al. [23] reported that the kinetics for the changes in yield stress and relaxation enthalpy of epoxy resin were similar, but the times for reaching equilibrium were different. Fig. 10 shows the changes in the thermal and mechanical values responsible for the relaxation behavior, observed by the different methods. The progressive changes toward the equilibrium state were highly correlated among the tests by DSC, DMTA and texturometer. However, the times for reaching the equilibrium were slightly different. The t c

8 Starch/Stärke 57 (2005) Effects of Aging on Properties of Glassy Corn Starch 361 Fig. 10. Relaxation enthalpy, storage modulus, and breaking strength changes against aging time for glassy corn starch. values calculated by nonlinear curve fitting from the Cowie and Ferguson equation, as in Fig. 10, were 10.4, 11.2 and 37.8 days, respectively, as calculated from the relaxation enthalpy, storage modulus, and breaking strength. The relaxation rates measured by DSC and texturometer were approximately similar, whereas the rate by the dynamic relaxation was significantly lower. As shown in Fig. 10, the storage modulus continued increasing in the late stage of aging, whereas other values almost reached plateaus. The relaxation measured by DMTA is from small-ranged changes in the sample matrix, and thus could be progressive during the aging without significant changes in enthalpy and large-deformation property. Structural relaxation by physical aging is accompanied by simultaneous changes in physical and mechanical properties, and fundamental correlation between structural relaxation and the changing properties of the glass would be of great importance to understanding physical aging phenomenon. Although the relaxation rates measured by different instruments differ, the understanding of calorimetric and mechanical relaxations is useful for prediction of physical changes during the storage of low-moisture glassy food products. 4 Conclusions The physical aging behavior of glassy corn starch specimen, molded by compression, could be characterized by both thermal and mechanical instruments (DSC, DMTA, and Instron texturometer). The relaxation enthalpy, storage modulus, and breaking strength of glassy corn starch gradually increased with aging, and reached the limits for structural equilibrium. These progressive changes imply that the starch chains become less mobile and the matrix of amorphous glass slowly become more dense during aging. The relaxation rates measured by relaxation enthalpy and breaking strength were similar, whereas that by dynamic modulus was lower. The calorimetric and mechanical behaviors of glassy starch are predictive for describing the time-dependent physical changes during the aging of low-moisture glassy food products. References [1] M. R. Tant, G. L. Wilkes: An overview of the nonequilibrium behavior of polymer glasses. Polym. Eng. Sci. 1981, 21, [2] J. M. Hutchinson: Physical aging of polymers. Progr. Polym. Sci. 1995, 20, [3] S. J. Livings, C. Breach, A. M. Donald, A. C. Smith: Physical ageing of wheat-flour-based confectionery wafers. Carbohydr. Polym. 1997, 34, [4] D. Champion, M. Le Meste, D. Simatos: Towards an improved understanding of glass transition and relaxations in foods: molecular mobility in the glass transition range. Trends Food Sci. Technol. 2000, 11, [5] B. Borde, H. Bizot, G. Vigier, A. Buleon: Calorimetric analysis of the structural relaxation in partially hydrated amorphous polysaccharides. I. Glass transition and fragility. Carbohydr. Polym. 2002, 48,

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