Question

Why does the milk that comes in packets not spoil? My mother told me that milk is pasteurized. What is pasteurization?

Answer 1

Packet milk does not spoil because before packaging it undergoes the process where all the microorganisms are destroyed or deactivated.

Complete answer:
Pasteurization is the process in which substances are processed under the mild heat and cold treatment to kill or destroy the microorganisms which can cause the spoilage to the product and also done to extend shelf life.
Milk can be pasteurised by heating the milk at the temperature $145^\circ F$ ($63^\circ C$) for $30$ minutes. Milk can also be pasteurised by another method where milk is heated for $160^\circ F$ for$15$ minutes followed by immediately cooling at $50^\circ F$ at which temperature it is stored.
Pasteurization can destroy all the pathogens and microbes except the spore. But when the second pasteurization is done it can kill spores that have germinated.
The pasteurization can also be done on wine, fruit juices, cheese, egg products and beer.

Additional Information:
Keeping quality: Pasteurization is done to improve the quality of the milk and milk products. Hence, this leads to an increase in the shelf life of milk and its products by 10-16 days. It kills the unwanted enzymes which can spoil the milk and products.
For public health: This process is also done to make the milk and its products safe for consumption.

Note:
The process of pasteurization was discovered by Louis Pasteur. He identified that spoilage microbes can be inactivated in wine by heating it below its boiling point. After that process is applied to all the liquid substances to kill the microbes or enzymes that lead to the spoilage.
The medical equipment like respiratory or anesthesia equipment also gets sterilised or disinfected by using hot water and alternative chemicals. The temperatures should rise to $70^\circ C$ for 30 minutes.