Bovine milk has been processed into a variety of dairy products that provide different nutritional and functional properties. The physicochemical properties of milk protein play an important role in dairy products, impacting on viscosity, emulsifying properties, solubility, texture and heat stability. Understanding the impact of milk protein interactions on the functional properties of the milk proteins can be beneficial to tailoring the properties of dairy foods. This project aimed to investigate the impact of protein modifications on the physicochemical and functional properties of dairy proteins. Experiments involved three protein modification approaches: dephosphorylation, succinylation and transglutaminase-­‐modified protein. αs1-­‐Casein as a key ingredient in dairy products was chosen for the experimental material. The level of modifications was controlled, and the physicochemical and functional properties of the modified proteins were investigated. This study investigated how the changes in the physicochemical properties affected the functionalities of αs1-­‐protein. In addition, the relationship between different measurement methods was studied. Results demonstrated that the physicochemical and functional properties of αs1-­‐casein can be manipulated by controlling the level of modification. The correlation between the physicochemical properties and functionalities of αs1-­‐casein was established. It was shown that dephosphorylation decreased net charge and increased surface hydrophobicity of αs1-­‐ casein, whereas, succinylation increased net charge and decreased surface hydrophobicity αs1-­‐casein. Succinylation had a significant effect on the secondary structure of αs1-­‐casein; it also decreased the self-­‐association behaviour. However, dephosphorylation did not change the secondary structure of αs1-­‐casein, but enhanced the self-­‐association behaviour of αs1-­‐ casein. The surface tension of protein solutions was affected by the net charge and surface hydrophobicity of αs1-­‐casein. The water binding capacity of αs1-­‐casein was influenced by the amino acid groups on the proteins. The foam stabilising ability of αs1-­‐casein was affected by the combined effect of net charge and surface hydrophobicity, whereas, the emulsion stability was mainly dependent on the net charge of the protein. On the other hand, molecular weight was found to have a significant effect on the surface tension and emulsion stability of αs1-­‐casein. The findings reported in this study could be tested in model food systems, which may inform the development of dairy products by manipulating the functional properties of dairy products. Moreover, the understanding of protein interactions and the ability to control the physicochemical parameters of proteins may advise an optimised process design for dairy products.