资料题：Why is the heating and cooking done（　）。

Inside a can, the food is protected from the things that would make it spoil.These are bacteria, the tiny little living plants that are everywhere around us.Bacteria, sometimes called germs, are so tiny that we can Bee them only under a microscope.But they are so powerful.Like all living things, bacteria need food to grow.When they grow, they multiply.A few bacteria become many in a matter of minutes.When bacteria settle on food, they make changes in it.These changes are what we call spoiling.To keep food from spoiling, we must first kill any bacteria that may already be in it.Heat and cooking do his.Next, we must keep any bacteria in the air away from the food.So we pack it into cans as airless as we can make them.We seal the cans tight.With no bacteria in the food and no air with bacteria able to each it, fruit, fish, meat and vegetables will stay unspoiled in a can for years.