Sunday Yogurt with Grandpa: Precise Words in a How-To Explanation

Grandpa's Kitchen Science

1Every Sunday, my grandpa and I make a big jar of yogurt. Grandpa says that yogurt is a little bit of kitchen science. People in many parts of the world have made yogurt at home for a very long time. All you need is milk, a spoonful of yogurt, a pot, a thermometer and a lot of patience.

2First, Grandpa pours milk into a pot and heats it slowly. He clips a thermometer to the side of the pot so we can read the temperature. He heats the milk until it is steaming and almost boiling. Heating the milk changes its proteins, which helps the yogurt turn out thick. The heat also kills unwanted germs that might be in the milk.

3Next, we wait for the milk to cool until the thermometer shows that it is warm, but not hot. This step matters because yogurt is made by bacteria, tiny living things far too small to see. Too much heat would kill them. When the milk is warm enough, I stir in a spoonful of plain yogurt from our last batch. This spoonful is called a starter. Store-bought plain yogurt works too, if its label says it has live cultures.

4Then Grandpa pours the milk into a clean glass jar, puts on the lid and wraps the jar in a thick towel. We set it in the oven with only the oven light turned on, which keeps it just warm enough. Grandpa calls this step incubating. To incubate something means to keep it warm so that living things inside can grow. The jar incubates for several hours, often overnight.

5While the jar stays warm, the bacteria are busy. They feed on lactose, the natural sugar in milk. As they feed, they make lactic acid. This change is called fermentation, and we say that the milk ferments. The lactic acid makes the milk proteins clump together, so the milk thickens into yogurt. The acid also gives yogurt its tangy, slightly sour taste.

6In the morning, the milk has become soft, creamy yogurt. We put the jar in the refrigerator, where the cold slows the bacteria down. Then we save one spoonful as the starter for next week.