Ravi boils an egg. The clear egg white turns solid and white, and it stays solid when the egg cools…

Step 1. The egg. Heat broke the weak bonds, so the albumin chains unfolded. Their exposed hydrophobic patches stuck to the patches of neighbouring chains, and the chains clumped into a solid network. Cooling does not undo this, because the chains are tangled together and cannot find their way back to the original fold.

Step 2. The milk. Casein has an isoelectric point near 4.6. At that pH its net charge is zero, so the molecules no longer repel each other. They clump and come out of solution as lumps.

Step 3. The bonds. No peptide bond was broken in either case. Heat and acid only upset the weak bonds and the charges, so the primary structure is intact.