Native starch can be transformed into a processable material through gelatinization, which involves heating starch in water to create a disordered state. The process can be characterized using various techniques, including differential scanning calorimetry (DSC), which is commonly used due to its convenience and accuracy. Shear treatment during processing can physically tear apart starch granules and disrupt molecular bonds, causing loss of crystallinity. Glycerol and water content can affect the multiphase transition pattern of starch, and ionic liquids are being studied as novel plasticizers for starch processing. The torque curve can provide practical information for extrusion processing.
Phase Transition of Starch
To transform native starch into a processable material, it is essential to understand the process of gelatinization. Gelatinization is the phase transition of starch from its original ordered granule structure into a disordered state through heating in water. This irreversible process involves granule swelling, loss of birefringence, and solubilization of molecules. Full gelatinization of starch requires excess water, while with limited water content, a higher temperature is needed to facilitate the mobility of starch molecules and the destructuration of crystalline regions. In starch-based materials, the process of thermally induced gelatinization under low water content conditions could accurately be defined as the “melting” of starch.
Characterization of Starch Phase Transition Under Shearless Condition
Starch’s thermal phase transition can be characterized using various techniques, including light microscopy, X-ray diffraction, nuclear magnetic resonance, and differential scanning calorimetry (DSC). DSC is the most commonly used technique due to its convenience and accuracy. However, to avoid water loss during measurement, it is necessary to mix starch with water and use a stainless steel pan to maintain high pressure inside the sample compartment for high-amylose starches with thermal transitions above 100°C. Heating rates should be appropriate for different pans to obtain accurate results. Liu et al. studied the thermal phase transition behaviors of maize starches and found that amylose/amylopectin ratio and moisture content affect the temperature positions, number, and enthalpies of the endotherms. Water can act as a plasticizer by lowering Tm of starch, but it is not used as the sole plasticizer in practical processing due to its volatility and resulting poor mechanical properties. Nonvolatile plasticizers, such as glycerol, are used instead. Glycerol and water content can greatly affect the multiphase transition pattern of starch, and higher glycerol/water ratios tend to shift the transitions to higher temperatures. Ionic liquids have been attracting great interest as novel plasticizers for starch processing, and their effects on starch’s thermal phase transition must be studied.
Characterization of Starch Phase Transition Under Shear Condition
When subjected to shear treatment, the gelatinization/melting behavior of starch displays significant differences (Xie, Liu et al., 2006). Since most processing techniques for starch polymers involve shear treatment, this finding is crucial. Shear forces during extrusion processing can physically tear apart starch granules, allowing water to penetrate the interior molecules more quickly (Burros, Young, & Carroad, 1987). As a result, loss of crystallinity during extrusion is caused not only by water penetration but also by the mechanical disruption of molecular bonds due to intense shear fields within the extruder (Barron, Bouchet, Della Valle, Gallant, & Planchot, 2001; Wang & Zheng, 1995; Zheng & Wang, 1994; Zheng, Chiang, & Wang, 1995).
Xie, Yu, Chen, and Li (2008) attempted to investigate the phase transition of rice starch with low moisture content (45%) and under shear stress using dynamic mechanical analysis (DMA) with a shear sandwich mode. The results revealed that the variations in physical properties, as indicated by damp tanδ in DMA, occurred both before and after the thermal transition (indicated by heat flow in DSC). However, the shear treatment used in DMA was weak, and there was little to no effect of shear on the thermal transition of rice starch. Xue, Yu, Xie, Chen, and Li (2008) and Wang et al. (2010) used a twin-rotor mixer (Haake Rheomix) to investigate the thermal phase transitions of different starches under strong shear treatment, similar to an extruder. The torque curve could reflect the viscosity change and estimate the time required to achieve the steady molten starch phase, providing practical information for extrusion processing.