Polysaccharides and proteins are two key components in both natural and processed foods. The interactions between polysaccharides and proteins determine the final structure, texture, and stability of the food materials. Coacervates, formed through attractive and nonspecific interactions such as electrostatic, van der Waals, and hydrophobic interactions between proteins and polysaccharides, have already served as important materials in food delivery applications because they create a barrier between food ingredients and food matrices. The overall goal of this dissertation is to generate a clear description of the dynamics and intermolecular interactions of biopolymeric complex fluids at multi-length scales, ranging from macro- to nano- scale.

Atomic force microscopy (AFM) has been used to determine the nanoscale mechanical properties of a series of negatively-charged polysaccharides, such as κ-carrageenan and Na-alginate through the measurement of the pull-off forces between the AFM tip and the immobilized polysaccharide films. The nanoscale mechanical properties obtained have been compared with the bulk rheological properties measured by rheometer.

The interactions of bovine serum albumin (BSA) with negatively-charged surfaces, such as mica and κ-carrageenan, in saline solutions of different pH values have been studied using chemical force microscopy (CFM). For BSA interactions with mica surface with the increase of pH, the statistical analysis of the CFM results shows that the averaged pull-off force for the elongation monotonously decreases. Such decrease results from the decrease of the strength of hydrogen bonding and the number of interaction pairs, as well as the slight increase of the strength of van der Waals interaction, suggesting that the force-extension curve is mainly contributed by the van der Waals interaction. Similar results are also found in BSA interactions with κ-carrageenan. CFM results indicate at least two pull-off events occur in the force-extension curves of BSA/κ-carrageenan mixtures. The lower force one is interpreted as due to non-specific binding, while the higher-force one is due to the specific binding between BSA-modified tip and κ-carrageenan. The larger specific binding at low pH suggests the existence of stronger electrostatic interaction at low pH, consistent with our titration and rheology results.

The rheological properties of complex coacervates formed by BSA/κ-carrageenan, BSA/gum arabic and BSA/pectin have shown significant correlations with sodium chloride concentration (C_NaCl) and initial protein/polysaccharide ratio (r). The higher storage modulii (G') than loss modulii (G") for all protein/polysaccharide coacervates suggests the formation of highly interconnected gel-like structure. The self-aggregation properties of a protein may have significant impact on the final rheological properties of coacervates. With the increase of salt concentration, coacervates formed by BSA and polysaccharides (κ-carrageenan, gum arabic, and pectin) show only salt-reduced effects.