Sodium caseinate is much easier to work with in my experience. NaoH does help with casein solubility but be careful not to cause structural changes which increase protease susceptibility.
hammarsten casein disolves in Tris-HCl pH 7.5 with the help of heat. Get the protein in the buffer and heat the flask in a microwave oven for a while and then stirr. The pH is usend to analyze serine peptidase activity with or without inhibitors.
It has to be dissolved in alkaline buffer as isoelectric pH of casein is 4.6. Buffers like tris or phosphate buffers of pH 7.5 to 8.0 can be used to dissolve for its use as a substrate to check activity of proteolytic enzymes but for other uses it can be dissolved in NaOH as sodium caseinate