Tribology and Rheology of Biopolymer Mixtures

The thesis discusses the field of tribology and its growing recognition in the context of oral processing of structured biopolymer mixtures. The tribological properties of poly-saccharides, particularly starch and non-starch polysaccharides which are often added as thickeners in processed foods can play a critical role in altering the mouthfeel of foods and beverages. To date, there is lack of understanding on lubrication mechanism of biopolymers and combinations thereof have. Therefore, the study started experiments on rheological and tribological properties of biopolymer mixtures having the possibility of designing natural lubricants. Specifically, thermodynamically incompatible biopolymer mixtures with phase separation i.e. water-in-water (W/W) emulsions present the possibility of acting as fat-replacers which have never been investigated for their tribological properties. These W/W emulsions suffer from poor stability, which has led to the use of particles as stabilizers, in-cluding microgel particles. This study explores the potential of using both animal and plant protein-based microgels to design Pickering W/W emulsions for both stability and tribology, which has not been previously explored.
The key finding of this thesis presents the rheological, microstructural and tribolog-ical properties of W/W emulsions formed from gelatinized starch and polysaccharides (κ-carrageenan or xanthan gum) stabilized with protein microgel particles. The results show that the W/W emulsions had different rheological and tribological properties than the indi-vidual biopolymer, i.e., increasing viscosity and lowering friction at specific concentrations of the biopolymers and the microgels present at the interface compared with the corre-sponding weight average of the values of the individual equilibrium phases. The contribu-tion of the water droplets in the emulsion systems, and the sizes and shapes of droplets might affect the rheological and tribological properties. In addition, the presence of micro-gels affected the tribological properties of the emulsions, with plant protein microgels offer-ing higher boundary lubrication properties compared to dairy protein microgels explained by elasticity, size, softness and adhesives interaction with the surface.
Overall, the results from this thesis highlight the importance of tribology in biopol-ymer mixtures and shows the unique properties of sustainable Pickering W/W emulsion system with improved lubrication performance. The findings contribute to the development of eco-friendly water-based systems with potential applications in food, biomedicine, and biocatalysts