**5. Technologies to study the starch retrogradation**

Many analytical techniques have been developed to monitor the starch retrogradation based on the TPS property changes. These methods include differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray diffraction (XRD), and enzymatic susceptibility, and others. Karin et al. (2000) summarized some of the methods. For TPS crystallinity study, DSC and X-ray diffraction have proven to be extremely sensitive and therefore valuable tools to quantify retrograded starches.

#### **5.1 Differential scanning calorimetry**

Differential scanning calorimetry (DSC) is a thermo-analytical technique. It measures temperatures and heat flows associated with thermal transitions in a material sample. A reference with a well-defined heat capacity over the range of temperature is identically

Retrogradation and Antiplasticization of Thermoplastic Starch 125

Application of DSC on the starch retrogradation study has a limitation. As mentioned by Biliaderis (1990), starch retrogradation consists of two processes: the rapid gelation of amylose solubilized during gelatinization and the slower recrystallization of amylopectin. DSC is only used to determine the latter phenomenon, since the re-organized region in amylopectin melts while the re-organized region in amylose does not (Gidley & Bulpin,

Fig. 9. DTA curves for (a) the gelatinized and (b) the 35-day retrograded rice starch containing 1.9% amylose. To is the onset temperature, Tp the peak temperature, and Tc the

<sup>0</sup>

0

(2)

100% *DR t T T <sup>t</sup> T T*

An alternative technique, which shares much in common with DSC, is differential thermal analysis (DTA). In this technique it is the heat flow to the sample and reference that remains the same rather than the temperature. When the sample and reference are heated identically, phase changes and other thermal processes cause a difference in temperature between the sample and reference. The differential temperature is then plotted against temperature or against time as a DTA curve that provides data on the physical and chemical transformations, such as crystallization, gelatinization (Wada et al., 1979), melting, and sublimation (Morita, 1956). Fig. 9 shows a DTA curve for retrogradation of rice starch. Tian et al. (2011) used DAT technology to study rice starch retrogradation, which contained 29.3%, 13.5%, and 1.9% amylose. They found the storage time from 0 to 35 days slightly decreased the onset temperature (*T0*), the peak temperature (*Tp*), and the conclusion temperature (*Tc*) for the retrograded starch. This was attributed to the fact that the number of the perfect crystal nuclei was reduced with storage time and the imperfect crystalline nuclei were increased during the retrogradation. By using equation 2, crystalline fraction of

conclusion temperature, respectively. (From Tian et al., 2011)

**5.2 Differential thermal analysis** 

the retrograded starch was calculated.

1987; Russell, 1987).

scanned. The difference in the heat flow between the sample and reference is detected and recorded. A plot of the differential heat flow between the reference and sample as a function of temperature is developed. When the sample undergoes a physical transformation, such as glass transition, melting, crystallization, more (or less) heat will need to flow to it than the reference to maintain both at the same temperature, resulting in a significant deviation in the difference between the two heat flows. When using DSC for starch retrogradation, the deviation between the heat flows results in a peak in the DSC curve. An example of this is illustrated in Fig. 8, using wheat starch.

Fig. 8. Observed DSC curves for gelatinization of native wheat starch (upper) and retrograded wheat starch after storage time of 5, 17, and 129 weeks. (From Kohyama et al., 2004)

The first run in Fig. 8 represents gelatinization of wheat starch. The peaks after 5, 17, and 129 weeks were broader and shallower than that of the first run, and became deeper and more pronounced with increasing storage time.

Tian et al. (2011) used DSC to determine the retrogradation degree of rice starch by equation 1.

$$DR\left(t\right) = \stackrel{\Delta H\_t - \Delta H\_0}{\bigwedge} \stackrel{\Delta H\_0}{\Delta H\_\infty - \Delta H\_0} \times 100\% \tag{1}$$

where *DR(t)* represents the crystalline fraction (%) developed at storage time *t*, *ΔH0* is the enthalpy change (*J/g*) at storage time zero, *ΔHt* is the enthalpy change at storage time *t*, and *ΔH∞* is the enthalpy change at unlimited time (35 days in the study). From 0 to 21 days, rice starch retrogradation went from 10% to 50%.

scanned. The difference in the heat flow between the sample and reference is detected and recorded. A plot of the differential heat flow between the reference and sample as a function of temperature is developed. When the sample undergoes a physical transformation, such as glass transition, melting, crystallization, more (or less) heat will need to flow to it than the reference to maintain both at the same temperature, resulting in a significant deviation in the difference between the two heat flows. When using DSC for starch retrogradation, the deviation between the heat flows results in a peak in the DSC curve. An example of this is

Fig. 8. Observed DSC curves for gelatinization of native wheat starch (upper) and retrograded

The first run in Fig. 8 represents gelatinization of wheat starch. The peaks after 5, 17, and 129 weeks were broader and shallower than that of the first run, and became deeper and more

Tian et al. (2011) used DSC to determine the retrogradation degree of rice starch by equation 1.

100% *DR t H H <sup>t</sup>*

where *DR(t)* represents the crystalline fraction (%) developed at storage time *t*, *ΔH0* is the enthalpy change (*J/g*) at storage time zero, *ΔHt* is the enthalpy change at storage time *t*, and *ΔH∞* is the enthalpy change at unlimited time (35 days in the study). From 0 to 21 days, rice

0

(1)

*H H*

wheat starch after storage time of 5, 17, and 129 weeks. (From Kohyama et al., 2004)

<sup>0</sup>

illustrated in Fig. 8, using wheat starch.

pronounced with increasing storage time.

starch retrogradation went from 10% to 50%.

Application of DSC on the starch retrogradation study has a limitation. As mentioned by Biliaderis (1990), starch retrogradation consists of two processes: the rapid gelation of amylose solubilized during gelatinization and the slower recrystallization of amylopectin. DSC is only used to determine the latter phenomenon, since the re-organized region in amylopectin melts while the re-organized region in amylose does not (Gidley & Bulpin, 1987; Russell, 1987).

Fig. 9. DTA curves for (a) the gelatinized and (b) the 35-day retrograded rice starch containing 1.9% amylose. To is the onset temperature, Tp the peak temperature, and Tc the conclusion temperature, respectively. (From Tian et al., 2011)

#### **5.2 Differential thermal analysis**

An alternative technique, which shares much in common with DSC, is differential thermal analysis (DTA). In this technique it is the heat flow to the sample and reference that remains the same rather than the temperature. When the sample and reference are heated identically, phase changes and other thermal processes cause a difference in temperature between the sample and reference. The differential temperature is then plotted against temperature or against time as a DTA curve that provides data on the physical and chemical transformations, such as crystallization, gelatinization (Wada et al., 1979), melting, and sublimation (Morita, 1956). Fig. 9 shows a DTA curve for retrogradation of rice starch. Tian et al. (2011) used DAT technology to study rice starch retrogradation, which contained 29.3%, 13.5%, and 1.9% amylose. They found the storage time from 0 to 35 days slightly decreased the onset temperature (*T0*), the peak temperature (*Tp*), and the conclusion temperature (*Tc*) for the retrograded starch. This was attributed to the fact that the number of the perfect crystal nuclei was reduced with storage time and the imperfect crystalline nuclei were increased during the retrogradation. By using equation 2, crystalline fraction of the retrograded starch was calculated.

$$DR\left(t\right) = \binom{\Delta T\_t - \Delta T\_0}{} \Big/ \Delta T\_{\alpha} - \Delta T\_0 \Big/ \times 100\,\%\tag{2}$$

Retrogradation and Antiplasticization of Thermoplastic Starch 127

As a result of retrogradation, starch forms resistant starch which is resistant to the enzyme hydrolysis. Enzymatic method has been developed which is bases on measuring the resistant starch level to determine the starch retrogradation degree. Tsuge et al. (1990) gave a detailed procedure on how to conduct enzymatic measurement on the retrogradated starch. Tian et al. (2010) repeated it with rice starch and α-amylase from *Bacillus subtilis*.

100 ( ) *b c DR*

where *DR* is the crystalline fraction (%), *a* represents absorbance read of total starch fraction in a spectrophotometer, *b* the absorbance of starch fraction to be tested, and *c* the absorbance of the complete digestion of starch. Wave length of 625 nm was selected for the absorbance measurement. Tian et al. (2010) measured rice starch retrogradation and found 7 – 28%

Size-exclusion high performance liquid chromatography (SE-HPLC) is usually used to determine the molecular weight (*MW*) of starch components, according to the different retention time (*TR*) for its components. Tian et al. (2010) used it to estimate amylose retrogradation in rice starch and compared the results with that of enzymatic measurement and DSC. Equation 5 was used to calculate the amylose recrystalline

> 0 ( ) <sup>0</sup> ( ) 100% ( ) *<sup>t</sup> <sup>t</sup>*

Where *DR(t)* represents the recrystalline fraction of amylose (%) developed at time *t*, *Tt* is retention time of the sample stored for *t* hours, *T0* is the retention time of the gelatinized sample, *T∞* is limiting retention time of 24 h storage sample, and *A* is the amylose area (%)

Tian et al. (2010) found two peaks in SE-HPLC plot with retention time 13.2 min and 16.0 min respectively. The first peak was ascribed to the amylopectin retrogradation and the second one amylose retrogradation. Analysis on retrogradation degree of rice starch showed 7 – 25% crystallinity during storage of 1 – 14 hours. This data corroborated that obtained by enzymatic measurement, but not with DSC data. DSC is reported to suitable for amylopectin retrogradation study, not for amylose crystallization. Tian et al. (2010) further concluded that SE-PHLC is an effective method to measure the amylose retrogradation during short-

While many factors may affect the retrogradation of starch, the botanical source is the most important one. Different botanical sources produce starches which are different with respect to properties such as amylose/amylopectin ratio (Gudmundsson & Eliasson, 1990; Klucinec

*ATT DR T T*

*a c*

(4)

(5)

**5.4 Enzymatic susceptibility** 

Equation 4 was used to estimate the retrogradation degree.

**5.5 Size-exclusion high performance liquid chromatography** 

**6. Factors which can affect the retrogradation of starch** 

crystallinity during storage of 1 – 14 hours.

fraction in the rice starch.

occupied of the peak (Tain et al., 2010).

term storage time, i.e. 0 – 24 h.

where *DR(t)* is the crystalline fraction (%) formed at storage time t, *ΔTt* is the maximum differential temperature for the *t* day retrograded starch, *ΔT0* is the maximum differential temperature for the gelatinized starch, and *ΔT∞* is the maximum differential temperature for the long-term, for example 35 days, retrograded starch. With equation 2, Tian et al. (2011) calculated the rice starch crystallized from around 10% to 50~60% with increasing of the storage time from 0 to 21 days. The *DR* data obtained from DTA was slightly higher than that evaluated from the DSC, but no significant difference between the two data was found.

#### **5.3 X-ray diffraction**

The application of X-ray diffraction (XRD) on the study of starch crystallinity has shown that three X-ray diffraction crystal patterns exist in native starch granules, namely A-type, B-type, and C-type. The typical XRD patterns of TPS are characterized by sharp peaks associated with the crystalline portion area and an amorphous area (Fig. 10). The amorphous fraction of the sample can be estimated by the area between the smooth curve drawn following the scattering hump and the baseline joining the background within the low and high-angle points. The crystalline fraction can be estimated by the upper region above the smooth curve (Mali et al., 2006). Equation 3 was used to estimate crystallinity of the TPS.

$$DR = \frac{I\_c}{I\_c + I\_a} \times 100\,\%\tag{3}$$

where *DR* is the crystalline fraction (%), *Ic* is the crystalline area on the X-ray diffractogram and *Ia* is the amorphous area on the X-ray diffractogram (Kalichevsky et al., 1993; Yoo & Jane, 2002). With the X-ray method, Zhang & Han (2010) studied the anti-plasticization of pea starch film. They announced that native pea starch film contained about 6.0% crystalline structure, and the crystallinity increased with plasticizer content increasing until 25%. The X-ray method is one of most popular methods for studying TPS structure and its application has been widely studied (Garcia et al., 2000; Smith et al., 2003; Zhang & Han, 2010).

Fig. 10. X-ray diffraction patterns of peas starch films plasticized by 0, 10, and 25 % glycerol, respectively. (From Zhang & Han, 2010)

#### **5.4 Enzymatic susceptibility**

126 Thermoplastic Elastomers

where *DR(t)* is the crystalline fraction (%) formed at storage time t, *ΔTt* is the maximum differential temperature for the *t* day retrograded starch, *ΔT0* is the maximum differential temperature for the gelatinized starch, and *ΔT∞* is the maximum differential temperature for the long-term, for example 35 days, retrograded starch. With equation 2, Tian et al. (2011) calculated the rice starch crystallized from around 10% to 50~60% with increasing of the storage time from 0 to 21 days. The *DR* data obtained from DTA was slightly higher than that evaluated from the DSC, but no significant difference between the two data was found.

The application of X-ray diffraction (XRD) on the study of starch crystallinity has shown that three X-ray diffraction crystal patterns exist in native starch granules, namely A-type, B-type, and C-type. The typical XRD patterns of TPS are characterized by sharp peaks associated with the crystalline portion area and an amorphous area (Fig. 10). The amorphous fraction of the sample can be estimated by the area between the smooth curve drawn following the scattering hump and the baseline joining the background within the low and high-angle points. The crystalline fraction can be estimated by the upper region above the smooth curve (Mali et al.,

*<sup>c</sup>* 100%

(3)

*c a*

has been widely studied (Garcia et al., 2000; Smith et al., 2003; Zhang & Han, 2010).

*I I <sup>I</sup>*

where *DR* is the crystalline fraction (%), *Ic* is the crystalline area on the X-ray diffractogram and *Ia* is the amorphous area on the X-ray diffractogram (Kalichevsky et al., 1993; Yoo & Jane, 2002). With the X-ray method, Zhang & Han (2010) studied the anti-plasticization of pea starch film. They announced that native pea starch film contained about 6.0% crystalline structure, and the crystallinity increased with plasticizer content increasing until 25%. The X-ray method is one of most popular methods for studying TPS structure and its application

Fig. 10. X-ray diffraction patterns of peas starch films plasticized by 0, 10, and 25 % glycerol,

respectively. (From Zhang & Han, 2010)

*DR*

2006). Equation 3 was used to estimate crystallinity of the TPS.

**5.3 X-ray diffraction** 

As a result of retrogradation, starch forms resistant starch which is resistant to the enzyme hydrolysis. Enzymatic method has been developed which is bases on measuring the resistant starch level to determine the starch retrogradation degree. Tsuge et al. (1990) gave a detailed procedure on how to conduct enzymatic measurement on the retrogradated starch. Tian et al. (2010) repeated it with rice starch and α-amylase from *Bacillus subtilis*. Equation 4 was used to estimate the retrogradation degree.

$$DR = \begin{array}{c} 100 \times (b - c) \Bigvee\limits\_{a - c} \\ \end{array} \tag{4}$$

where *DR* is the crystalline fraction (%), *a* represents absorbance read of total starch fraction in a spectrophotometer, *b* the absorbance of starch fraction to be tested, and *c* the absorbance of the complete digestion of starch. Wave length of 625 nm was selected for the absorbance measurement. Tian et al. (2010) measured rice starch retrogradation and found 7 – 28% crystallinity during storage of 1 – 14 hours.

#### **5.5 Size-exclusion high performance liquid chromatography**

Size-exclusion high performance liquid chromatography (SE-HPLC) is usually used to determine the molecular weight (*MW*) of starch components, according to the different retention time (*TR*) for its components. Tian et al. (2010) used it to estimate amylose retrogradation in rice starch and compared the results with that of enzymatic measurement and DSC. Equation 5 was used to calculate the amylose recrystalline fraction in the rice starch.

$$DR\_{(t)} = A \times (T\_t - T\_0) \Big/ \begin{pmatrix} T\_t - T\_0 \end{pmatrix} \times 100\,\%\tag{5}$$

Where *DR(t)* represents the recrystalline fraction of amylose (%) developed at time *t*, *Tt* is retention time of the sample stored for *t* hours, *T0* is the retention time of the gelatinized sample, *T∞* is limiting retention time of 24 h storage sample, and *A* is the amylose area (%) occupied of the peak (Tain et al., 2010).

Tian et al. (2010) found two peaks in SE-HPLC plot with retention time 13.2 min and 16.0 min respectively. The first peak was ascribed to the amylopectin retrogradation and the second one amylose retrogradation. Analysis on retrogradation degree of rice starch showed 7 – 25% crystallinity during storage of 1 – 14 hours. This data corroborated that obtained by enzymatic measurement, but not with DSC data. DSC is reported to suitable for amylopectin retrogradation study, not for amylose crystallization. Tian et al. (2010) further concluded that SE-PHLC is an effective method to measure the amylose retrogradation during shortterm storage time, i.e. 0 – 24 h.

#### **6. Factors which can affect the retrogradation of starch**

While many factors may affect the retrogradation of starch, the botanical source is the most important one. Different botanical sources produce starches which are different with respect to properties such as amylose/amylopectin ratio (Gudmundsson & Eliasson, 1990; Klucinec

Retrogradation and Antiplasticization of Thermoplastic Starch 129

Storage time increases starch retrogradation. Fig. 11 shows the relationship between degree of retrogradation of waxy rice starch gels with storage time. Rice starch becomes increasingly retrogradated with storage time, until the 9th day when it levels off (Baik et al., 1997). Mali et al. (2002) found as storage time increased, the width of the X-ray diffraction peak of starch samples decreased but its peak intensity increased, showing an increase in

In the rubbery state, high relative humidity (RH) favors starch macromolecular mobility which in turn facilitates the development of retrogradation (Delville et al., 2003). Glycerol content slows the crystallization kinetics in starch (Delville et al., 2003). According to Mali et al. (2006), glycerol limited the crystal growth and recrystallization by interacting with the polymeric chains and interfering with polymer chain alignment due to steric hindrances. Controversially, Garcia et al. (2000) reported that plasticizers (including glycerol and water) favored polymer chain mobility and allowed the development of a more stable crystalline structure during shorter periods of storage. Similarly, Smits et al. (2003) found that starch films without plasticizers formed less recrystallinity than the plasticized starch films. They attributed this phenomenon to the mobility of starch polymer chains, because plasticized starch polymers could easily vibrate and align up to form crystallites, while the unplasticized starch polymers interact with each other strongly and lose their mobility. Zhang & Han (2010) found that plasticizer concentration plays a critical role in starch retrogradation. When plasticizer content is greater than 25%, the plasticizer limits the starch polymer retrogradation. Otherwise, the plasticizer will favour the crystallization of the starch chains. Fig. 12 shows the relationship of

Fig. 12.Relationship of crystallinity of pea starch film and the plasticizer concentration. Crystallinity of the starch films increases with plasticizer concentration increasing until 25%.

Bars indicate mean ± standard deviation. (From Zhang & Han, 2010)

**6.2 Storage time** 

crystallinity of starch.

**6.3 Storage relative humility and plasticizer content** 

starch film crystallinity and the plasticizer concentration.

& Thompson, 2002), lipid content (Gudmundsson & Eliasson, 1990), and amylopectin fine structure (Kalichevsky et al., 1990). These properties will strongly influence the starch retrogradation kinetics. Ottenhof et al. (2005) stated that potato TPS (34 ± 1% water content, w/w) showed the highest rate of retrogradation (~0.17 h-1) followed by waxy maize (~0.12 h-1), while wheat TPS was the slowest (~0.05 h-1). In addition of the botanical source, factors such as storage time, environmental temperature, and TPS moisture and plasticizer content, influence starch retrogradation also (Mali et al., 2002).

#### **6.1 Storage temperature**

At a temperature which is higher than the *Tg*, TPS is in the rubbery state and its starch molecule retrogradation (or recrystallization) in the amorphous phase occurs easily. The rate of starch retrogradation depends on difference between the storage temperature and *Tg* (Mali et al., 2006), with increasing retrogradation rate for higher temperature, especially under the conditions when TPS being stored at high relative humidity or high plasticizer contents (Delville et al., 2003). Conversely, when TPS is stored at temperature below the *Tg*, starch polymers are in a stable glassy state, and retrogradation does not occur or is extremely slow (Baik et al., 1997). Williams-Ferry-Landel (WLF) equation was commonly used to study the kinetic of the retrogradation process (Baik et al., 1997) at different temperature value.

$$\log\_{10}\left(\frac{t}{t\_{\mathcal{g}}}\right) = \frac{-\mathcal{C}\_1(T - T\_{\mathcal{g}})}{\mathcal{C}\_2 + (T - T\_{\mathcal{g}})} \tag{6}$$

where *t* is the time of crystallisation, *tg* the time to crystallise at *Tg*, *C1* and *C2* are constants (17.44 and 51.6K, respectively), and *T* is the temperature (*K*). Currently, there are numerous studies available on native starch, but limited studies are available on TPS retrogradation rate results at different storage temperatures.

Fig. 11. Changes in degree of retrogradation of nonwaxy (A) and waxy (B) rice starch gels during storage at various temperatures; ●, 30; ■, 20; ▲, 4; ▼, 0 °C. (Adapted from Baik et al., 1997)

#### **6.2 Storage time**

128 Thermoplastic Elastomers

& Thompson, 2002), lipid content (Gudmundsson & Eliasson, 1990), and amylopectin fine structure (Kalichevsky et al., 1990). These properties will strongly influence the starch retrogradation kinetics. Ottenhof et al. (2005) stated that potato TPS (34 ± 1% water content, w/w) showed the highest rate of retrogradation (~0.17 h-1) followed by waxy maize (~0.12 h-1), while wheat TPS was the slowest (~0.05 h-1). In addition of the botanical source, factors such as storage time, environmental temperature, and TPS moisture and plasticizer content,

At a temperature which is higher than the *Tg*, TPS is in the rubbery state and its starch molecule retrogradation (or recrystallization) in the amorphous phase occurs easily. The rate of starch retrogradation depends on difference between the storage temperature and *Tg* (Mali et al., 2006), with increasing retrogradation rate for higher temperature, especially under the conditions when TPS being stored at high relative humidity or high plasticizer contents (Delville et al., 2003). Conversely, when TPS is stored at temperature below the *Tg*, starch polymers are in a stable glassy state, and retrogradation does not occur or is extremely slow (Baik et al., 1997). Williams-Ferry-Landel (WLF) equation was commonly used to study the kinetic of the retrogradation process (Baik et al., 1997) at different

1

( ) ( ) *g*

(6)

2

where *t* is the time of crystallisation, *tg* the time to crystallise at *Tg*, *C1* and *C2* are constants (17.44 and 51.6K, respectively), and *T* is the temperature (*K*). Currently, there are numerous studies available on native starch, but limited studies are available on TPS retrogradation

Fig. 11. Changes in degree of retrogradation of nonwaxy (A) and waxy (B) rice starch gels during storage at various temperatures; ●, 30; ■, 20; ▲, 4; ▼, 0 °C. (Adapted from Baik et al.,

*g g t CT T log t C TT* 

10

influence starch retrogradation also (Mali et al., 2002).

rate results at different storage temperatures.

**6.1 Storage temperature** 

temperature value.

1997)

Storage time increases starch retrogradation. Fig. 11 shows the relationship between degree of retrogradation of waxy rice starch gels with storage time. Rice starch becomes increasingly retrogradated with storage time, until the 9th day when it levels off (Baik et al., 1997). Mali et al. (2002) found as storage time increased, the width of the X-ray diffraction peak of starch samples decreased but its peak intensity increased, showing an increase in crystallinity of starch.

#### **6.3 Storage relative humility and plasticizer content**

In the rubbery state, high relative humidity (RH) favors starch macromolecular mobility which in turn facilitates the development of retrogradation (Delville et al., 2003). Glycerol content slows the crystallization kinetics in starch (Delville et al., 2003). According to Mali et al. (2006), glycerol limited the crystal growth and recrystallization by interacting with the polymeric chains and interfering with polymer chain alignment due to steric hindrances. Controversially, Garcia et al. (2000) reported that plasticizers (including glycerol and water) favored polymer chain mobility and allowed the development of a more stable crystalline structure during shorter periods of storage. Similarly, Smits et al. (2003) found that starch films without plasticizers formed less recrystallinity than the plasticized starch films. They attributed this phenomenon to the mobility of starch polymer chains, because plasticized starch polymers could easily vibrate and align up to form crystallites, while the unplasticized starch polymers interact with each other strongly and lose their mobility. Zhang & Han (2010) found that plasticizer concentration plays a critical role in starch retrogradation. When plasticizer content is greater than 25%, the plasticizer limits the starch polymer retrogradation. Otherwise, the plasticizer will favour the crystallization of the starch chains. Fig. 12 shows the relationship of starch film crystallinity and the plasticizer concentration.

Fig. 12.Relationship of crystallinity of pea starch film and the plasticizer concentration. Crystallinity of the starch films increases with plasticizer concentration increasing until 25%. Bars indicate mean ± standard deviation. (From Zhang & Han, 2010)

Retrogradation and Antiplasticization of Thermoplastic Starch 131

Mechanism of anti-plasticization effect of plasticizer on TPS has not been clearly understood, but some efforts have been made to elucidate its mechanism. Zhang & Han (2010) found that starch retrogradation results in antiplasticization phenomena. The crystallinity of the pea starch films increased with plasticizer content increasing from 1% to 20%, leading to decrease in MC, OP, WVP, and E, and increase in EM. Plasticizer performs plasticization or antiplasticization depending on its concentration. Addition of plasticizers at the range of low to intermediate concentration level (1% to 25%) facilitates the formation of crystallites in the starch films, leading to the antiplasticization. Zhang & Han (2010) further proposed an anti-plasticization model. Due to the movement or vibration of the starch polymer chains, water and plasticizer molecules were pushed aside gradually from starch polymers. D-glucosyl residues of the amylose or amylopectin, which used to be separated by water or plasticizers molecules, interacted to form strong hydrogen bonds causing retrogradation or recrystallization. However, when starch polymer was plasticized by high content plasticizer (>25%), the plasticizer molecules could not be pushed aside completely from the starch polymers. Then, the plasticizers performed their plasticization effect, which was to interrupt interaction between the hydrogen bondings of starch polymers, increase the

Thermoplastic starch (TPS) has attained more attention for its potential to replace the conventional polymers. Retrogradation occurs in TPS with time and affects its properties and applications. TPS contains normally 10% crystallinity, but this value changes with storage time, temperature, atmosphere relative humidity, and plasticizer content. Starch retrogradation mechanisms are discussed at molecular level. Methods to measure the retrogradation degree, such as differential scanning calorimetry, differential thermal analysis, X-ray, etc. are also reviewed. Changes in TPS property, such as tensile strength, elongation, gas

Baik, M.Y., Kim, K.J., Cheon, K.C., Ha, Y.C., Kim, W.S. (1997). Recrystallization kinetics and

Biliaderis, C. G. (1990). Thermal analysis of food carbohydrates. In: *Thermal Analysis of Foods*,

Chang, Y.P., Abd Karim, A., Seow, C.C. (2006). Interactive plasticizing-antiplasticizing

Debeaufort, F., Quezada-Gallo, J.A., Voilley, A. (1998). Edible films and coatings:

Delville, J., Joly, C., Dole, P., Bliard, C. (2003). Influence of photocrosslinking on the retrogradation of wheat starch based films. *Carbohydrate Polymers*, 53, pp.373-381. Farris, S., Schaich, K.M., Liu, L.S., Piergiovanni, L., Yam, K.L. (2009). Development of

glass transition of rice starch gel system. *Journal of Agricultural and Food Chemistry*,

Harwalkar, V.R. and Ma, C.Y. pp. 120-134. Elsevier Applied Science, ISBN:

effects of water and glycerol on the tensile properties of tapioca starch films. *Starch*,

tomorrow's packagings: a review. *Critical Reviews in Food Science and Nutrition*. 38,

polyion-complex hydrogels as an alternative approach for the production of bio-

permeability, are due to the retrogradation of starch polymers and these are described.

TPS elongation, reduce its *Tg*, and prevent retrogradation of starch chains.

**8. Summary** 

**9. References** 

45, pp. 4242-4248.

55, pp.304-312.

pp.299-313.

9781851664368, London.
