This dynamic forms the core action of cheesemaking prior to aging. Let's review.
- Casein, a milk protein, consists of micellar structures formed of casein subtypes grouped together, with a k-casein end that has a negative charge due to the carbohydrate end that tends to protrude out, giving each micellar structure stability to be liquid.
- As you add acid, it degrades both that hydrophilic carb "leg" of micelles, and the calcium phosphate bonds within micelles. The first helps with the set, and the second helps with the stretch and plasticization upon heating.
- Rennet works faster at lower pH levels, with an optimal or around 5.5.
So it's like you said. Acid helps to start the process, and rennet fosters enzymatic cleaving.