You're actually describing a different process than what happens when milk goes bad. Here's a breakdown of the key points:

1. Milk Spoilage and Acidity:

* Oxygen is NOT the primary factor: The main culprit in milk spoilage is bacteria, not the depletion of oxygen.

* Lactic acid: Bacteria in milk convert lactose (milk sugar) into lactic acid. Lactic acid is what gives spoiled milk its sour taste and increases its acidity.

* Oxygen and spoilage: While oxygen is present in milk, it can promote the growth of some bacteria. However, this is not the main reason for the increase in acidity.

2. Importance of Pasteurization:

* Killing bacteria: Pasteurization is a heat treatment designed to kill most of the bacteria naturally present in milk, significantly slowing down the spoilage process.

* Longer shelf life: Pasteurization allows milk to remain safe and palatable for a longer period.

In short, the increase in acidity in milk is primarily due to the formation of lactic acid by bacteria, NOT the depletion of oxygen.