资料题：What are nitrates used for（　）。

The food we eat seems to have profound effects on our health.Although science has made enormous steps in making food more fit to eat, it has, at the same time, made many foods unfit to eat.Some research has shown that perhaps eighty percent of all human illnesses are related todiet and forty percent of cancer is related to the diet as well, especially cancer of the colon(大肠).Same cultures are prone to contact certain illnesses because of the food that is characteristic in these cultures.That food is related to illness is not a new discovery.In 1945, government researchers realized that nitrates and nitrites, commonly used to preserve color in meats, and other food additives, caused cancer.Yet, these carcinogenic additives remain in our food, and it becomes more difficult all the time to know which things on the packaging labels of processed food are helpful or harmful.The additives which we 'eat are not all so direct.Farmers often give penicillin to beef and poultry and because of this, penicillin has been found in the milk of treated cows.Sometimes similar drugs are administered (given) to animals not for medicinal purposes,but for financial reasons.The farmers are simply trying to fatten the animals in order to obtain a higher price on the market.Although the Food and Drug Administration has tried repeatedly to control these procedures, the practices continue.