Investigating two food hydrocolloid based-systems under extreme stresses: (I) The non-linear rheology of Gelatin-Multiwalled Carbon Nanotubes composites, and (II) Starch dispersions under high hydrostatic pressure

Abstract

This thesis reported two food hydrocolloid based-systems under two extreme stresses: (I) The non-linear rheology of pure gelatin and gelatin-Multilwalled carbon nanotubes (MWNT) nanocomposites, and (II) Starch dispersions under High Hydrostatic Pressure (HHP). Various gelatin networks, namely gelatin physical gel, chemically cross-linked gelatin gel, and a hybrid gel made of a combination of the former two, have been prepared and their nonlinear behavior under large deformation were investigated using the pre-stress, strain ramp, and large oscillatory shear. The Blatz, Sharda, Tschogel (BST) (Blatz, Sharda, & Tschoegl, 1974)-scaling equation was employed to fit the stress-strain curves and fractal dimensions were extracted and compared to those obtained from Small Angle Neutron Scattering (SANS) study. The results demonstrate that the BST-scaling equation describes well the strain hardening of gelatin gels and the fractal dimensions obtained using BST fitting are in very good agreement with those obtained using SANS. The incorporation (up to 1wt%) of MWNTs into the gelatin networks resulted in a slight increase in the complex modulus G* of the gelatin-MWNTs nanocomposites. This could be due to the poor dispersibilty of the MWNTs in the gelatin gels. The linear viscoelastic region of these nanocomposites also decreased linearly with the increase in MWNTs concentration. The pre-stress results demonstrated that the addition of MWNTs can either change the strain hardening behavior of the gelatin chemical gel and hybrid gel or have little effect on the gelatin physical gel. The gelatinization of waxy corn, waxy potato, and high amylose corn starches by HHP was investigated in situ using synchrotron Small Angle X-ray Scattering (SAXS) and X-ray powder diffraction on samples held in a Diamond Anvil Cell (DAC). During HHP treatment, both SAXS peak area (corresponding to the lamellae phase) of waxy corn and potato starches decreased suggesting starch gelatinization increased with pressure. For both waxy potato and corn starches, the long period length and the average thickness of amorphous layers decreased when the pressure increased. While for both of waxy starches, the thickness of crystalline layers first increased, then decreased when the pressure increased. Both waxy and high-amylose corn starches can be fully gelatinized at 5.9 GPa and 5.1 GPa, respectively. In the case of waxy corn starch, upon release of pressure (to atmospheric pressure) crystalline structure appeared instantaneously as a result of amylopectin aggregation. Further the change in the supramolecular structure of corn starches suspended in water or ethanol investigated ex situ after HHP treatment (0-600MPa), showed that all starch dispersed in water showed a decrease in lamellae area as revealed by SAXS. The long period increased for pressurized waxy, corn, and Gelose80 corn starch, suggesting water is forced into starch lamellae during HHP. However, for Gelose50 starch, the long period remained constant over the whole pressure range and light microscopy showed no granule swelling. Wide Angle X-ray Scattering (WAXS) studies demonstrate that HHP partially converted A-type starches (waxy and normal corn) to starches with a faint B-type pattern while starches with a B+V-type patterns (Gelose50 and Gelose80), were not affected by HHP. All corn starches suspended in ethanol showed no detectable changes in either granule morphology, or the fractal, the lamellae, and the crystalline structures, under HHP conditions used in this thesis. Finally, the retrogradation behavior of HHP (600 MPa, at 25 °C for 15min) gelatinized corn starches with different amylose contents were investigated using rheology and Fourier Transformed Infrared Spectroscopy (FTIR). The effect of crystallization on the mechanical properties of starch gel network were evaluated in terms of dynamic complex modulus (G*). The crystallization induced increase of short-range helices structures were investigated using FTIR. It was found that the pressure release rate does not affect starch retrogradation, but the rate and extent of retrogradation depends on the amylose content. Minimal retrogradation was observed for HHP treated waxy corn starch. The rate of retrogradation is higher for HHP treated high amylose corn starch than that of normal corn starch. A linear relationship between the extent of retrogradation measured by FTIR and G* is found.