Many chemicals harmful in large quantity are used in small. Though ill results may not be detected, there is reason to doubt whether constant consumption of even small quantities is not ultimately harmful, especially as those that eat any foods so treated usually eat many. Such food-dangers are to be avoided.

Dangerous residues in food of chemicals added, or of any created by bacterial life, and deterioration of food-quality through the effect of these, are the dangers of commercial food-preservation and food-storage and of home delay in use of food.

Different kinds of food need to be kept apart. Some give off odors; fruits do. Others absorb odors; milk and butter do, and have their own flavors destroyed thus. Cold compartments need to be aired and kept completely clean.

Though cooking usually destroys bacteria, cooked starchy foods such as potatoes are decomposed more readily than uncooked, as cooked (not raw) starch readily supports germ-life.

Refrigeration retards decay and reinstates appearance of freshness. Cooled air (about 400 F.) is circulated around whatever is to be preserved from decomposition, or kept fresh, or freshened by cold. See pp. 220-221.

Ripening of fruit is affected by heat, moisture, air, and light. By control of these it may be hastened or delayed. Some fruits, as apples, may be kept in ripened condition for a number of months. Others, as bananas, may be stored green and allowed to ripen in storage. This is possible because many of the changes in ripening are carried on by unorganized ferments (or enzymes) in the fruit. If this is too long continued, the overripe fruit becomes unfit food. Fruits in storage are living. They consume oxygen and produce carbon dioxid. As this happens, part of their carbohydrate is oxidized and heat is generated.