Marie Wong, age 15, of Springfield, Ill., for her question:
WHAT IS FERMENTATION?
Fermentation is a change that takes place in certain animal and vegetable matter when chemicals called ferments, or enzymes, act on them. The change is produced by tiny living plants or animals.
It isn't completely clear how enzymes bring about fermentation.
For a long time almost everybody thought that sugar fermented, meat spoiled and wine soured by themselves. Then a Dutch scientist named Anton van Leeuweahoek in 1680 found that yeast was present in sugar solutions that became alcoholic after standing for a time. And a French scientist named Louis Pasteur discovered that yeasts are one celled plants that produce enzymes, and that no fermentation can take place without living cells.
Three main types of fermentation occur: alcoholic, putrefactive and acetic. Each is caused by the presence of bacteria (either molds or yeast) that produce the particular enzyme responsible for the chemical change.
Examples of fermentation include souring of milk, ripening of cheese, curing of silage, the change of apple juice to hard cider and the alcohol in hard cider to vinegar.
Responsible for these changes are fezments built up by the cells of plants and animals. Many come from bacteria and various other low forms of plants. Fermented foods include buttermilk and sauerkraut.
Meat and animal matter decay in putrefactive fermentation after certain molds have formed on them. An enzyme called "ptyalin" in saliva changes the starch of foods in soluble sugar, which is the first step in digestion. Enzymes are also present in the stomach, the intestines, the pancreas and in various other organs.
Sometimes fermentation is harmful. Foods must be kept in cool places or in refrigerators where the temperatures are low to keep them from spoiling. Low temperatures will check the growth of bacteria, yeasts and molds.
The action of yeast on bread dough is an example of a very useful type 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 dough and puff it out until it expands, or "rises."
Fermentation also helps growing plants. Plants get all of their food from the ground and air. When a plant or animal dies, bacteria act on it cad cause it to decay. In decay, the chemical compounds of its body are broken down into elements that return to the soil cad air. Then living plants can use them over again.
The breakdown of dead matter sometimes gives off bad odors and the products may be poisonous. This is part of putrefaction.
In addition to the use of low temperatures to preserve foods, foods can also be preserved by killing the fermenting agents with heat and then sealing the foods in containers in a process called canning.