Vegetables have value in human diet only as they serve directly or indirectly some food-need of humanity. The condition of vegetables affects their food-usefulness as much as does their kind. All kinds do not serve alike; nor do all qualities. Inferior quality of the right kind for the purpose may even cause disease. All food should always be a health-help, strength-giver, work-aid. To make it so, it must be selected with knowledge of the food-need and quality of the food eaten.

Selection of vegetables suitable for human use is a daily occupation of those determining the food of humanity. Food may through manipulation in preparation be made to appear well irrespective of its actual quality. This is to be avoided. It menaces health and may life. Safe and unsafe food, sound and unsound food, need to be easily distinguishable. Overripe tomatoes have developed in them acid not present earlier. This makes them undesirable and may dangerous. Preparation with seasoning, as in catsup, may make such tomatoes a palatable food, but does not overcome the result in that food of the overripeness of the tomatoes. Such food-preparation is to be discouraged by disuse.

Digestion is hindered by selection of unfit food. Malnutrition instead of nutrition results. Underripe food sometimes contains undeveloped substances not ready for human use. Green apples do. Foods picked green rarely ripen naturally. Choose those gathered ready for use, and use promptly. Overkept food may have lost what it was desirable it should retain or may have developed what it is essential it should not have. Such food is both more exposed to contamination and less able to resist it. Vegetables may carry human disease from the soil, receptacles, or persons. They may also be diseased themselves. This destroys their value as food.