Look on the back of the can and see if it has undergone feeding trials. Call the 1-800 number on the back or look at the manufacturers web site. Keep pet food as you would your own food. Keep it in a cool dry environment. Do not feed cans of food that have been open for a week.
Preservatives prevent oxidation of fat.
Fat is needed by dogs and cats. Oxidation of fat makes food smell bad and causes toxicity. Preservatives prevent this. Vitamin E, for example, is a preservative. It prevents oxidation of fat in the body and plays a similar role in pet foods.