In the broadest of terms, food additives are substances intentionally added to food either directly or indirectly with one or more of the following purposes: 1. to maintain or improve nutritional quality; 2. to maintain product quality and freshness; 3. to aid in the processing or preparation of food; and 4. to make food more appealing. Some 2,800 substances are currently added to foods for one or more of these uses. During normal processing, packaging and storage, up to 10,000 other compounds can find their way into food. Today more than ever, additives are strictly regulated. Manufacturers must prove the additives they add to food are safe. This process can take several years and includes a battery of chemical studies as well as tests involving animals, the latter to determine whether the substances could have harmful effects such as cancer and birth defects. The results of these comprehensive studies must be presented to the Food and Drug Administration (FDA), which then determines how the additive can be used in food. There are two major categories of food that are exempt from this testing and approval process: 1. a group of 700 substances categorised as gras ("generally recognised as safe"), which are so classified because of extensive past use without harmful side effects; and 2. substances approved before 1958 either by the FDA or the USDA. An ongoing review of many of these substances is in effect, however, to make sure they're tested against the most current scientific standards. It's interesting to note that about 98 percent (by weight) of all food additives used in the United States are in the form of baking soda, citric acid, corn syrup, mustard, pepper, salt, sugar and vegetable colorings.