Why milk turn sour?

Pasteurized milk production:

a. Temperature treatment: < 100 degree C.

b. Packaging is not sprayed by peroxide chemical.

c. Packaging structure without aluminum foil.

d. Filling process may be open.

UHT milk production:

a. Temperature treatment: 145 degree C.

b. Packaging is sprayed by peroxide chemical.

c. Packaging is laminated with aluminum foil.

d. Filling process is close.

The above explained why UHT milk does not turn sour.