The Melting Process in Thermoplastic Starches

The heating-shearing behavior of potato starch/glycerol mixtures with limited water addition (below 30%) was investigated by Wojtowicz and van der Goot and by Wojtowicz (paper in print). The goal of the treatment was to use Brabender Mixograph simulating mixing-kneading and a specially designed shearing device-shear cell to obtain a starchy molten phase under well-defined thermomechanical processing conditions.

This new equipment is based on the cone and plate rheometers approach, with the capability to isolate singular parameters, such as temperature, rotation speed, or shear stress in the equipment, during processing. The base material was potato starch with addition of glycerol (20%, 22.5%, and 25%), together with added water (5, 10, 15, and 20% w/w). To compare the behavior of the starch, different starches were also tested under the same conditions (unpublished data).

In the Brabender Mixorgaph and shear cell device, the mechanical shearing and heating are simultaneously delivered to treated raw materials and transformation of starch/glycerol/water mixtures occurred. Wojtowicz (in print) reports on mixing tests performed in a two-bladed counter-rotating batch mixer (Do-Coder E330, Brabender OHG, Germany) interfaced with a computer and control unit.

Samples of starch/glycerol/water mixtures were placed inside the chamber and mixed for 2 minutes with 5 rpm screw rotation speed. The mixer temperature was set to 85°C, heating was started directly after closing of the chamber, and the screw rotation was increased from 5 to 100 rpm over 3 minutes. Comparison of starch origins was performed with the same temperatures and times, but the screw rotation was increased from 5 to 80 rpm. Samples were treated for 10 minutes in total. During mixing, the torque was recorded continuously.

The second shearing device used in the study was a pilot-scale shear cell based on the cone and plate rheometer. Further details on this device can be found in the works of Einde et al and Veen et al. To ensure accurate results, the sample mixture was loaded onto the plate at a uniform rate, and the apparatus was closed precisely due to the pressure increase during the heating-shearing process. The treatment temperature varied with the added water levels, with 85°C selected for samples with 15% and 20% added water, 88°C for samples with 5% and 10% added water, and 115°C for starch/glycerol mixtures. The rotation speed started at 10 rpm for the first two minutes and gradually increased to 100 rpm over three minutes. During treatment, changes in torque were recorded with a calibrated measurement device and used for further calculations. The shear stress (τ), shear stress multiplied by time (τ.t), and SME values were calculated and analyzed as a function of glycerol and water levels in the tested mixtures. The intrinsic viscosity (η) was measured as a method for molecular weight measurement, as it is very sensitive to thermomechanical treatment and the degradation of molecular weight compounds in starch. The intrinsic viscosities of native starch and of sheared-heated samples were measured using the Ubbelohde viscometer method [20] at 25°C. The microstructures of the samples were also analyzed using an optical microscope (Axioskop, Zeiss, Germany) with polarized light.

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