Fred Toole, age 12, of Wilmington, Del., for his question:

HOW IS YOGURT MADE?

Yogurt is a smooth, semisolid dairy product made from milk. It has ranked as a popular food in Iran, Turkey and some other countries of the Middle East for thousands of years.

Commercial yogurt in North America is made from cows' milk. Two types of bacteria are added: lactobacillus bulgaricus and streptococcus lactis.

The bacteria multiply at carefully controlled temperatures and cause the milk to ferment or ripen. During fermentaion, the bacteria change milk sugar into a syrupy substance called lactic acid. Lactic acid causes milk to thicken, resulting in yogurt.

The high acid content of yogurt gives the product a sour taste that many people enjoy. Others eat yogurt that has been sweetened with fruit flavoring.

Yogurt has about the same nutritional elements as milk. Some unflavored yogurt contains only a few calories per serving and is very popular for low calorie diets.