I always thought caesin was just a common protein in milk. If you process it by partial proteolysis (with pancreatic extracts or whatever), you have a mixture of small peptides and amino acids you add to culure media as a source of amino acids. Unprocessed caesin is just the intact protein, which probably isn’t as useful as a nutritional additive for culture media.

I think that casein is a bit too ‘large’ as a substrate in a culture medium, because it’s a protein which also contains calcium.

So what? It is a large protein that need to be hydrolyzed to be used, hence it is a good selective agent for specific bacteria. So if your medium require casein, do not use the hydrolysate because it won’t have the right effect. But casein is easy to find, it is skim milk 🙂