The usage of whey proteins as a source of nutrition in foods and beverages is ever increasing, creating a demand for highly concentrated and functional forms of protein. Their use as an ingredient to increase protein content in foods is growing, and the ability to withstand thermal treatment while concentrated is paramount to reducing process instability, e.g., high viscosity, precipitation and/or fouling of heat exchangers. The objective of this thesis was to investigate the effect of protein concentration on the heat-induced aggregation of whey proteins and subsequent increases in viscosity in dispersed and emulsified systems. Increasing the protein content of whey solutions from 1 to 12% (w/w) was accompanied by a reduction in denaturation temperature and increase in both viscosity and the shear rate at which onset of turbulent flow was observed. On heating to 85 °C for 30 s, the particle size (Dynamic light scattering, DLS) of aggregates was significantly (P < 0.05) affected by both protein concentration (1 to 12 % w/w) and pH (6.2, 6.7 and 7.2). Aggregates of ~ 50 nm were formed at higher pH (7.2) and protein concentration (12%), contributing to greater heat stability upon subsequent heating compared to aggregates of ~ 100 nm formed from the heat-treatment of 1% whey at pH 6.2. In an emulsified system, simulating that of a 1st stage infant formula, increasing the ratio of α-lactalbumin to β-lactoglobulin significantly (P < 0.05) reduced the level of large soluble aggregates formed on heating, improved emulsion stability and reduced apparent viscosity during concentration. Differences in these physicochemical properties are attributed to underlying molecular changes in the mechanism(s) of aggregation due to a reduction in the total number free thiol groups in the system as β-lactoglobulin is replaced by α-lactalbumin. To better understand the complex interactions involving β-lactoglobulin, 1H13C HSQC solution NMR (Nuclear Magnetic Resonance [13C,1H]–Heteronuclear Single Quantum Correlated spectroscopy) and Fourier Transform Infrared (FTIR) spectroscopy were used to observe molecular rearrangement at low (1% (w/w)) and high concentration (12% (w/w)) at either 62 or 85 °C. FTIR showed that there was formation of antiparallel inter-bonded β-sheets in both 1 and 12% samples heat-treated at 85 °C for 10 minutes, indicating protein denaturation and subsequent aggregation. Partial reversible denaturation was observed by NMR after heat-treatment at 62 °C; however, at 85 °C, denaturation and the subsequent aggregation of β-lactoglobulin monomers was extensive. Results suggest that although β-lactoglobulin monomers and dimers are denatured, the aggregates formed are comprised of unfolded β-lactoglobulin monomers that behaved “native-like”. This thesis demonstrates that by controlling the extent and size of aggregates through reduction in β-lactoglobulin and soluble ‘seed’ aggregates, it is possible to modulate viscosity in concentrated whey protein dispersions and emulsified systems.