Why does milk curdle when sour?
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Answer: Acid denatures milk proteins
Acid denatures milk proteins ✓ — Correct! Protein denaturation! Milk curdles because: (1) Fresh milk: casein proteins suspended (colloidal). (2) Bacteria produce lactic acid (fermentation—lactose → lactic acid). (3) pH drops (acidic environment). (4) Acid denatures casein—proteins unfold, lose charge. (5) Proteins clump together (coagulate)—visible curds. Buttermilk: intentional curdling (controlled). Cheese: rennet enzyme + acid curdles milk. Also curdles: adding lemon juice/vinegar. Cottage cheese, ricotta: acid-set curds. Pasteurization slows but doesn't prevent souring!
Souring creates gas bubbles — Wrong. No gas creates curds—lactic acid from bacterial fermentation denatures proteins, causing them to clump visibly.
Temperature change causes lumps — Wrong. Heat can curdle milk, but souring specifically from acid (bacterial fermentation) denaturing proteins at room temperature.