Answer to Question 1
Milk has a near-neutral pH of 6.6 . Thickening began around a pH of 5.5, and curd formation was apparent at a pH of 4.6.
Answer to Question 2
Coagulation of casein proteins occurs when negative charges are neutralized by the hydrogen ions from acid. When the pH drops to 4.6, casein becomes very insoluble and precipitates readily into a curd. Milk's isoelectric point is 4.6; this is when the negative and positive charges on the casein balance each other and the overall charge is neutral.
