To be fair. Lenomnom ninja'd me. I was responding 3 posts back :-) But I'm still not quite sure what it means ;-)
As for steric repulsion:
https://www.physicsforums.com/threads/steric-repulsion-simple-explanation.883156/ Lenomnom, why do you think steric repulsion is involved?
Here's a nice explanation of how the isoelectric point is achieved:
https://www.khanacademy.org/test-prep/mcat/biomolecules/amino-acids-and-proteins1/v/isoelectric-point-and-zwitterions.I *think* that diagram is actually conflating 2 concepts. Casein micelles are not composed of only casein. They are bound up into balls by calcium phosphate. This is where we get into a theoretical area because as far as I can tell nobody actually knows how they are composed. However, as the pH decreases (referring back to the video), the casein (statistically) starts picking up protons (hydrogen ions) on the carboxilic acid group. If I understand correctly, this liberates the calcium phosphate from the micelle. This is why the diagram is saying "Dissociation of Ca from the micellar complex".
Incidentally, the release of the calcium phosphate "relaxes" the micelle, which increases its size. It also allows it to stretch, which is important in how cheese melts/stretches.
I think the discussion of "casein salts" and "caseinates" is a bit of a red herring, however in saying that It made me realise something. My assumption was that the casein micelles are suspended in the solution. This is, of course, incorrect. The key to understanding that is that the isoelectric point is the point where the protein *precipitates*. In other words, the casein exists in a dissociated form and when it hits the isoelectric point (average charge 0 across the protein), it precipitates out.
It's also interesting that at the isoelectric point, all of the bound up calcium phosphate is discharged, which makes a certain amount of sense to me if it was relying on the negative charge to stay bound up in the micelle. I still can't quite wrap my head around how it's composed in there, but... oh well.