Billy Munden, age 10, of Tekoa, Wash., for his question:

WHY DOES YEAST MAKE BREAD RISE?

A French scientist by the name of Charles Cagniard de Latour discovered in 1857 there were tiny plants called yeasts which increased in number by budding. He also discovered that the yeasts acted on sugar to change it into alcohol, by a process known as fermentation. The scientist found that as the yeast plants grow, they produce substances known as enzymes.

Yeast that bakers use for breads and rolls is made up¬of tiny one celled plants that are among the simplest on earth. The yeasts actually belong to a group of plants called fungi.

Both dry and compressed yeast is used by bakers. commercially produced compressed yeast has enough starch added so that fermentation can start quickly. It must be stored in cool places and doesn't keep too long. Dry yeast, on the other hand, can keep indefinitely without spoiling. It is inactive or dormant in this form and becomes active when mixed with certain materials.

Yeast mixed with bread dough produces a type of ferment called leavening. After standing for a few hours or overnight, the sponge, as the mixture of dry yeast, flour and water is called, the baker adds more liquid and kneads the dough thoroughly. It is then covered and allowed to rise. With the use of compressed yeast, it is not necessary to prepare a sponge.

Enzymes in the yeast cells actually attack the starch in flour and change it to sugar  and then the sugar changes to alcohol and carbon dioxide gas. The gas moves through the dough and forms the familiar looking bubbles we see in bread that make it porous and light.

As the bread is being baked, the alcohol evaporates completely and the tiny yeast plants are destroyed.

You can now buy dry yeast or cakes of compressed yeast in your supermarket, but it wasn't too long ago when it had  to be made at home. A batter of flour, potato water, salt and sugar was left uncovered for several hours and yeast cells in the air furnished the enzymes. This method didn't always work because sometimes types of yeast not suitable for bread lodged in the batter.  

Yeast is made commercially by grinding corn and rye to a mash and mixing it with filtered water. Adding sprouted barley or malt, the .starch in the grain is changed by the malt to malt sugar. A culture of bacteria, which is used to turn milk sour, is then added and the entire mash filtered. This is called the wort and is the food for living yeast cells. Yeast increases rapidly. When fermentation takes place, the yeast is skimmed off, the water pressed out and the mass is molded and cut into cakes.