NARRATOR：

Listen to part of a talk in a food science class.

FEMALE PROFESSOR：

Everyone knows food can become spoiled—go bad and become inedible.

Usually, the cause is harmful bacteria that grow naturally in food.

So, one of the best ways to prevent food from spoiling is to slow down bacterial growth.

But how do you do that?

Well, what influences how fast bacteria grow?

Mainly, it’s temperature and moisture.

So if you want to prevent food spoilage, you need to slow bacterial growth by controlling these factors.

Why’s controlling temperature so important?

Because the bacteria that cause food spoilage grow fast at warm temperatures and more slowly at cooler temperatures.

When your food is kept cool, it lasts longer, right?

For example, fresh fish left out in the sun on a warm day will spoil in a few hours.

But if you freeze the fish, it will keep for months and months, right?

That’s because low temperatures drastically slow down bacterial growth.

Now, I said that moisture’s another factor you need to control.

And every living thing needs moisture, and bacteria are no exception.

Food that’s high in water content provides the moisture that bacteria need to grow.

Let’s, uh, let’s compare two different kinds of milk, okay?

Let’s compare regular liquid milk with dried, powdered milk.

Regular milk spoils quickly, right?

But milk in powder form can be stored quite a long time…in fact, for years.

Of course, bacteria are present in powdered milk too.

But what’s different between powdered and regular milk is the water content.

Powdered milk has had the water, the moisture removed, and without moisture bacteria can’t grow.

And this isn’t true just for milk; just about any fresh food will spoil more quickly than a dried food.