How does milk stay fresh day after day?

You pour milk on your cereal at breakfast. You drink it again for dinner. How does milk stay fresh day after day?

Milk needs to be kept cold or else it will “go bad”. Why does that happen? Milk sours because of a chemical change.

A chemical change happens when one substance changes into another substance. Food contains germs, called bacteria. When milk is left out, the germs in the milk cause the milk to spoil. The fresh milk changes into a new substance - sour milk. And once milk sours, you can’t make it fresh again.

If you keep fresh milk in the refrigerator, though, it might last for a couple of weeks. How? In the 1800s, French scientist Louis Pasteur found a way to kill germs in food. He developed a process – later named for him – called pasteurization. In this process, food is heated at a high temperature for a period of time. The heat kills many of the germs that cause food to spoil. The milk you buy is the grocery is pasteurized. This means the milk is safe from harmful bacteria.

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"How does milk stay fresh day after day?" was posted on Wednesday, September 19th, 2007 at 11:44 pm.