Why does milk turn sour?

Answer: Bacteria produce lactic acid

Bacteria produce lactic acid ✓ — Correct! Bacteria (mainly Lactobacillus) naturally present in milk feed on lactose (milk sugar) and produce lactic acid as a byproduct. This acid lowers the pH, making milk taste sour and causing proteins to curdle. Pasteurization kills most bacteria to delay this, but eventually some survive and multiply. That's why milk has an expiration date!

Milk proteins start decomposing — Wrong. Protein decomposition happens later in spoilage and creates foul odors, not sourness. The sour taste comes specifically from lactic acid produced by bacteria consuming milk sugar, not from protein breakdown.

Sugar ferments to alcohol — Wrong. While fermentation is involved, milk bacteria produce lactic acid, not alcohol. Alcohol fermentation requires different microorganisms (yeasts) and conditions. The sourness in milk is specifically from lactic acid fermentation.