Chong Lee, age 13, of McAllen, Tex., for his question:

CAN YOU EXPLAIN FERMENTATION?

Fermentation is a change that takes place in animal or vegetable matter when certain chemicals called ferments, or enzymes, act upon it. Fermentation changes the chemicals which make up animal and vegetable matter, and also changes the taste, smell and form.

Some examples of fermentation include the souring of milk, the ripening of cheese, the curing of silage, the change of apple juice to hard cider and of the alcohol in hard cider to vinegar. The ferments which cause these changes are built up by the cells of plants and animals.

Fermented foods include buttermilk, chocolate and sauerkraut.

The action of yeast on bread dough is as useful kind of fermentation. The ferment from the yeast changes the starch in the flour to sugar. It then breaks down the sugar into carbon dioxide gas and alcohol. The gas forms bubbles which honeycomb the mass of the dough and puff it out until it rises.

Fermentation also aids in the digestion of food. The gastric juice of the stomach contains two ferments: rennin and pepsin. Rennin curdles mik and pepsin softens the albumen of food so it will dissolve in water and pass into the blood.