Synthetic polymers derived from petroleum products are used widely on a daily basis. Unfortunately, they are not biodegradable. They have posed an increasingly severe environmental problem on this planet. In addition, the petroleum supply is not without limit. Thus, there is a real need to explore inexpensive, renewable, biodegradable materials, including natural materials produced in large quantity by the agriculture industry.
Wheat gluten protein is made directly from wheat flour. The wheat grain is milled into flour, and the flour is further processed into gluten through wetting, kneading, and washing.1 Wheat gluten has some interesting features that make it a promising candidate for nonfood applications. For example, wheat gluten is insoluble in water, has a high modulus, and is viscoelastic, which is unique among plant proteins.2 However, the challenge is that wheat gluten is brittle after thermal processing. One solution to this problem is a chemical modification. Previous work indicated that a thiol-terminated molecule could be used to chemically modify the wheat gluten structure; the resulting product was less brittle and had a much higher elongation at break.3 During this research, thermal analysis proved to be an indispensable characterization tool. In this work, the thermal behavior of a new wheat gluten/thiol additive material was characterized by different thermal analysis techniques.
Materials and methods
Materials
Wheat gluten was obtained from Arrowhead Mills (Hereford, TX). The wheat gluten/thiol additive materials were prepared at the University of Connecticut (Storrs).
TGA of wheat gluten was performed on a Pyris 1 TGA (PerkinElmer, Shelton, CT). The instrument was calibrated with Curie standards. The scanning rate was 10 °C/min.
Conventional DSC, modulated temperature DSC (StepScan DSC, PerkinElmer), and fast scan DSC (HyperDSC, PerkinElmer) were all performed on the PerkinElmer Diamond DSC. The instrument was calibrated with indium at the appropriate scanning rate. The StepScan DSC method used steps of 1 °C and a scanning rate of 10 °C/min. The HyperDSC measurements used a variety of scanning rates up to 500 °C/min.
Organic elemental analysis
The organic elemental analysis was performed on a PerkinElmer 2400 CHNS/O elemental analyzer.
Results and discussion
Figure 1 - TGA of wheat gluten.
As a natural material, wheat gluten contains some moisture even though it looks dry. TGA analysis, as shown in Figure 1, indicated that there was about 7% moisture in the material and the degradation started above 200 °C. The results provide guidance for the processing, and show that it is necessary to stay below 200 °C during the thermal processing of wheat gluten.
Figure 2 - Conventional DSC study of wheat gluten at 10 °C/min in a standard pan.
Wheat gluten protein is an amorphous material as shown by an X-ray diffraction test. It has a weak glass transition (Tg) around 170 °C, and is strongly plasticized by moisture.4 DSC is generally used to study the Tg of wheat gluten. From the TGA result, it is known that the sample contains about 7% water. Since 1% moisture will decrease the Tg of wheat gluten protein by 10 °C,4 we anticipate the plasticized Tg around 100 °C. However, a DSC test at the conventional temperature of 10 °C/ min using a standard aluminum pan (Figure 2) does not allow the detection of the plasticized Tg due to the interference of the Tg with the broad endothermal peak of water evaporation.
Figure 3 - Illustration of StepScan method.
Modulated temperature DSC was applied to separate these two overlapping events: the water evaporation, which is a kinetic, and the glass transition, which is a thermodynamic, event. StepScan DSC, as one of the modulated temperature DSC methods, was used for this analysis. StepScan applies a series of linear heating and true isothermal holding steps to the sample. The red curve in Figure 3 is the sample temperature profile and the blue curve is the measured heat flow. Two signals can be generated from the StepScan approach. One is the thermodynamic Cp signal, which represents the reversible heat capacity change of the sample (thermodynamic events). Another is the IsoK signal, which reflects irreversible aspects (kinetic events) of the sample during heating. The kinetic events such as the enthalpy relaxation during Tg or the cold crystallization after Tg will be seen in the IsoK baseline. The thermodynamic Cp curve records the specific heat capacity of the sample. The StepScan demonstrates that a thermodynamic event and a kinetic event can be well separated. The benefits associated with this separation include the enhanced characterization of Tg and the more accurate Cp value measurement.
Figure 4 - StepScan DSC of wheat gluten in a standard pan.
The StepScan result of wheat gluten in a standard pan is shown in Figure 4. The endothermal peak of water evaporation was registered in the IsoK signal as expected. However, the mass loss associated with the water evaporation resulted in the Cp signal decrease, which masked the upward change for the glass transition. The Tg of the dry wheat gluten can be detected in the thermodynamic Cp curve at higher temperatures. Also, note that the IsoK signal is bumpy at higher temperatures. This indicates that some kinetic events are occurring during the two-hour experiment. The long-time exposure to heat induced some changes in the material; thus slow heating rates are not recommended in this case.
As shown, modulated temperature DSC did not help to detect the Tg of the moist wheat gluten due to the overlapping weight loss. The way to overcome this problem is to prevent water evaporation before the glass transition. StepScan could be run in a sealed pan to prevent the weight loss. However, the measurement takes a long time and induces further changes to the material.