You know milk turns sour when it is a day old, sometimes. But you can buy milk that is a year old, or more, that is just as sweet as it was when it came from the cow. It is condensed, or evaporated milk, sealed in air-tight tins. When the can is opened the milk in it will soon turn sour. That is because it is exposed to the air. The air is full of microbes, or little living plant cells like yeast and mould. They are too small for you to see. Like all plants they need soil to grow in. The soil they like best is a liquid with sugar or starch in it like milk, fruit juices and flour batters. In growing, these tiny plants change sugar and starch to acids. So milk becomes sour and fruit juices ferment. After a time the fruit juices stop fermenting and turn to vinegar or wine. But because milk is an animal product it decays.

If milk is cooled quickly, and kept on ice, the microbes find it much harder to live and multiply in it. Iced milk can be kept sweet for two or three days. It can be kept still longer if it is first heated in a corked bottle to a temperature that kills the microbes, then cooled quickly. This is called Pasteurizing. In cities, people have to Pasteurize milk to make it safe for babies. In evaporating, or condensing milk, the milk is heated in steam tanks to the boiling point, and kept there until most of the water passes off in vapor. Then it is sealed, boiling hot, in air-tight tins. Milk is a favorite food of many little plant and animal cells that harm us if we drink them. So, to keep as many of them out as possible, cows should be healthy, and stables and milkmen clean. All pails and strainers and bottles used should be boiled. The milk should go into the bottles as quickly as possible, be sealed up with waxed paper caps, cooled and shipped at once. The ice box must be kept clean, and the bottles sealed until the milk is wanted. Never allow milk to stand in a warm place, or in an open vessel. If you do, millions of microbes will move right in and begin to grow.