Much is known about the changes in the physical and chemical properties of casein micelles during processing in skim milk. Membrane technologies have become increasingly widespread as a means to prepare concentrated casein suspensions as ingredients, and the interactions occurring among the milk proteins as a function of their volume fraction is yet to be fully understood, especially during processing. Of particular interest is the soluble fraction, known to affect the final processing functionality of the milk matrix. In untreated skim milk, the soluble phase is mostly constituted of whey proteins. Heating induces the formation of soluble aggregates containing caseins and whey proteins, and these aggregates strongly affect, for example, the texture of acidified milk products. While we know how to control the processing history of skim milk to modulate the properties of dairy products, this is much less understood in concentrated milk systems. Extensive work is needed to characterize the type and concentration of soluble aggregates in casein suspensions depending on casein volume fraction and processing conditions. As the volume fraction increases, there are profound changes in the composition of the serum phase. This paper will discuss the importance of soluble protein complexes on rheological properties and texture formation of dairy matrices. Small changes in composition may strongly affect the physical chemical properties of the concentrates used as ingredients. Understanding how to control compositional changes in milk concentrates will unravel the development of a new generation of functional ingredients from milk.