Understanding gelation systems with different gelling mechanisms in foods

We explored several typical food gel systems to understand the structure-rheology relationship. The first one is zein protein in aqueous ethanol solution. We utilized rheology techniques, in effort to understand the aging phenomenon in zein solution. Time evolution of storage and loss modulus of zein solution with and without perturbation is tested. We conclude that the aging of zein solution follows a power law relation with time. The second system is BLG fibril and pectin complex. Fibrillar aggregates of beta-lactoglobulin (BLG) were prepared by heating at pH2 and 90 oC for different time period. Progress of protein hydrolysis was estimated by gel electrophoresis (SDS-PAGE). Morphology of fibril/pectin complex is probed by turbidity titration, atomic force microscopy (AFM) and small angle x-ray scattering (SAXS). Secondary structure of fibrils with and without pectin complexation is studied by circular dichroism (CD). Our results suggest that the complexation process between BLG fibril and pectin is self-facilitated, results in system heterogeneity. BLG fibrils retained their linear structure at large length scale when associate with pectin. At local regions clusters and bundles are formed. The mixture of BLG-pectin and BLG fibril-pectin can be well separated domains that can be resolved by small angle X ray scattering. Addition of pectin can significantly change secondary structure of heated BLG. We validated microrheology technique in our lab. We compared the results from microrheology to that from bulk rheology in Newtonian-fluids and acrylamide gels. We found excellent agreement between bulk and micro- rheology in such "classic" conditions. We further used carrageenan solution, a complex fluid, and found significant difference between the two methods. The result shows that pores with size large than probe particles exist in carrageenan solution. We then used microrheology to test the gelation of KGM solution, and compared the results with bulk rheology. The effect of temperature, KGM concentration and calcium carbonate concentration are tested. We again found significant lower storage and loss modulus obtained from microrheology. The gelation rate of KGM gel was increased with increase of temperature and/or increase of coagulant concentration. The final storage modulus of the KGM gel on the other hand, decreases with increase of temperature and coagulant concentration. We found negative relation between inhomogeneity and storage modulus, which can be explained by cascade theory. We also found monotone increase of storage modulus during heating time in microrheology experiment, further confirmed that the peak in storage modulus in bulk rheology is due to syneresis. The increase of coagulant concentration will improve the storage modulus of cured gel. Microrheology results suggested no significant relation between inhomogeneity and coagulant concentration.Ph.D.Includes bibliographical referencesby Qiuyang Xi