Madge Todd, age 13, of Dover, Del., for her question:
HOW DOES FOOD SPOIL?
Food can spoil in one of two ways: as a result of chemical changes and from the growth of tiny forms of life called microorganisms.
Chemical spoilage of foods usually takes place during the time the foods are placed in storage. Stored foods may change color or develop bad odors and flavors as chemical changes occur in their proteins, fats and carbohydrates.
When fats and oils are exposed to air, they oxidize, and the foods turn rancid and develop an "old" taste. Chemical combinations of sugars and proteins cause browning, turning foods to a dark color. Browning often occurs in dried foods.
Chemical substances called enzymes cause much food spoilage. All living things produce enzymes. These substances are important in digestion and also ripen fruits and vegetables and tenderize meats.
Enzymes continuously split or break down starches, proteins, fats and similar materials into new substances. If their action is not stopped, they make foods overripe. The action of some enzymes may also cause a loss of flavor compounds.
Microorganisms, such as bacteria, yeast and molds, grow in all kinds of food. Some produce substances that make food decay.
Molds that grow in bread and other moist foods often change the flavor of the food. Some bacteria form acids that turn food sour. Others produce poisons that make people ill.
Yeasts often ferment food, or make it alcoholic. Some bacteria produce hydrogen sulfide, a gas that may turn food black if it combines with normal amounts of iron in the food. This gas also has the ability to give food the odor of rotten eggs.
Bacteria can also form so much carbon dioxide or hydrogen gas in canned foods that the pressure may swell or even burst the containers.
Food preservation makes it possible to store food for future use without spoiling.
The chief methods of food preservation include cold storage, canning, freezing, drying, freeze drying and curing. In addition, treatment with antibiotic drugs or exposure to ultraviolet rays helps preserve some foods.
Cold storage keeps food fresh at low temperatures. These temperatures, usually ranging from 30 to 50 degrees Fahrenheit, do not actually stop spoilage but they do slow down microorganism growth and enzyme action. The food must not dry out or become too moist.
Canning has two aims: to sterilize food or make it germ free and to keep air away from it. Heating food to high temperatures destroys microorganisms and stops enzyme action. Making the can or jar airtight helps prevent oxidation of the food.
Freezing ranks as one of the best methods of food preservation. Drying or dehydration removes most of the moisture from food. Curing slows the growth of microorganisms inside and on food.