There is an increasing interest in the food industry to find and utilize alternative protein sources other than animal in order to meet the increasing demand for high quantities of protein. However this is a difficult task due to the inherent functional differences between plant proteins and animal proteins. When plant proteins are used alone, they typically lead to textural/sensorial properties which are not acceptable to the consumer. Alternatively, plant proteins can be used as partial replacers of (traditional) animal proteins which can lead to tailored properties. Furthermore, plant proteins can be mixed with other proteins (heteroprotein association) to form new ingredients with novel functionalities. We first explored the influence of the partial replacement of animal proteins with plant proteins on the rheological properties of gelled systems. For this purpose, we used a model system based on rapeseedproteins and the widely utilized whey proteins. Rapeseed proteins were used as an alternative to the more commonly used plant proteins from soy. In this thesis, we tried to understand which factors lead to changes in rheological properties of mixed protein gels opposed to single gels. Single gels and mixed protein gels were analyzed for their rheological and microstructural properties. Upon mixing rapeseed proteins with whey proteins at certain protein mixing ratios synergistic stiffening was evident in mixed gels. This was attributed to microstructural modifications of whey protein gels, in terms of an increase of gel homogeneity.Another factor that seemed to play a role in synergistic stiffening was the minimization of the differences between the rheological moduli of single gels at the relevant concentrations. Particularly maximum synergistic stiffening was observed when the rheological moduli of single gels overlapped. In the second part of this thesis, we explored the ability of one rapeseed protein fraction (napin) to form new ingredients with the major protein fraction in whey (β-lactoglobulin). Here we focused on fundamentally understanding the process of hetero-protein complexation; a relatively new research area. This was done in comparison to exploring the complexation process in a second binary system using β-lactoglobulin and a napin-like protein (lysozyme) from egg white. Using electrostatic potential modeling we found that napin and lysozyme differed in protein surface charge anisotropy which enabled us to understand the importance of this parameter on hetero-protein selectivity. It was evident that β-lactoglobulin had a preference to form complexes with lysozyme; the less anisotropic protein in terms of the distribution of charges on the protein surface. Furthermore, with the combined use of isothemral titration calorimetry and turbidimetric titrations, we identified that precursors played a role in the assembly process during hetero-protein complexation in both binary systems. Overall this thesis provided new insights in understanding the factors thatlead to different rheological properties in mixed animal protein/plant protein gels and led to a better understanding of the importance of precursors and surface charge anisotropy on hetero-protein association.