The increasing global demand for protein rich dairy products and dairy products with low content of sugars and fat provides novel possibilities for the use of whey protein ingredients due to their favourable nutritional and functional properties. Tailor-made whey protein aggregates are highly promising to provide a desired functionality in dairy products. The present PhD project aims at gaining fundamental understanding of the molecular interactions that occur between whey protein ingredients and inherent milk components during processing of acidified milk model systems. This can thus be used to tailor the physical, chemical and functional characteristics of fermented dairy products. Liquid casein concentrate (LCC) and liquid whey protein concentrates (LWPC) were produced from micro-/ ultra-filtration, respectively, and used as sources of inherent milk proteins. Frozen storage of LCC were investigated applying different thawing procedures. Significant differences were mainly observed in acid gelation. A prolonged thawing procedure of initially storing at 4 °C for 3 d and then keeping at 30 °C for 5 h demonstrated increased G’ with an earlier gelation time, as well as decreased pH and increased free Ca2+ activity, compared to fresh LCC (paper I). Frozen LCC and LWPC were used as a basis for construction of non-fat milk model systems, combined with commercial whey protein ingredients, i.e. nano-particulated whey protein (NWP), micro-particulated whey protein (MWP), and whey protein concentrate (WPC) in different ratios. Addition of NWP resulted in increased surface hydrophobicity, accessible thiol groups, gelation pH and G’, with drawbacks of increased graininess. Systems with added MWP caused a very weak gel. The physical properties after WPC addition were similar to LWPC, characterized by a large quantity of smaller aggregates (<1 μm) (paper II). Low-field nuclear magnetic resonance (LF-NMR) was used to investigate water mobility in the designed milk model systems during both acidification and subsequent storage of stirred acidified gels. The system containing the highest amount of MWP showed higher mobile water fraction during acidification, but lower in the corresponding stirred acidified gel during storage. The acidified gel structure measured by confocal laser scanning microscopy (CLSM) was found to be more open and coarser and other analyses showed a higher proportion of free water, induced syneresis, instability index, and lower water-holding capacity (WHC) compared to other systems. Higher proportion of mobile water was found in the stirred acidified gel containing the highest amount of NWP. The system with added WPC presented a similar proportion of free water and WHC as those containing NWP, however, displaying lower induced syneresis, which was related to a more homogeneous and denser gel structure (paper III). Structural changes of milk model systems, induced by heat treatment and acid gelation, were monitored using small-angle X-ray scattering (SAXS). Major changes in the scattering were seen in the intermediate-q range (0.009-0.04 Å-1) with increasing heating time (2, 5, and 10 min) for the systems containing whey proteins, and more pronounced with a high amount of LWPC (from 0.5 to 4%). The most striking changes with decreasing pH were the reduction of overall scattering intensity, and high-q (0.08-0.1 Å-1) features washing out for systems containing casein, and more markedly with high amount of LCC (i.e., 2.8 and 4%). In the lower-q range (0.004-0.005 Å-1), the systems containing NWP exhibited distinguishable decreased intensity with decreasing pH compared to the other mixed systems (paper IV). Microstructure of acidified milk gels was characterized using a high resolution microscopy technique (stimulated emission depletion, STED) combined with quantitative image analysis, where the relative spatial distributions of both casein and whey proteins were separately monitored. The gelation behavior of WPC and NWP were found to be similar to LWPC by both intra-and inter-chain crosslinking when either self-aggregating or attaching to casein assemblies. Besides, NWP could form larger aggregates and associate with the formed gel network. Scattered MWP particles were observed in its acidified gels, indicating that MWP did not interact with any other proteins. The systems containing the highest amount of LWPC demonstrated the highest co- localization level from the casein and whey protein channels, and lowest for the systems with added MWP. Meanwhile, the systems containing MWP also presented the highest coarseness, corresponding to the low G’ observed. Stronger gel network were found in the systems containing either LWPC or NWP, correlating to their relatively thicker aggregate strands, when comparing to other systems (paper V). Therefore, the obtained results gained new insights on understanding of the interaction mechanisms between whey protein ingredients and inherent milk proteins in milk matrices, enabling the dairy industry to further tailor whey protein ingredients with desired functionality and develop new and sufficiently stable fermented products.