Preservatives cause cancer and other diseases.
There is no scientific evidence to support the often repeated claim that preservatives cause cancer. In fact, just the opposite may be true, as preservatives added to pet foods can help prevent the formation of cancer-causing compounds within the food. Pet foods contain fat and fat-soluble vitamins that readily oxidize when exposed to air. Fat oxidation produces toxic compounds called peroxides that can disrupt cell membranes and loss of cell membrane integrity. There is also no scientific evidence that preservatives cause food allergy. A pet may be intolerant of a preservative (have GI upsets) but that does not involve the immune system. Preservatives protect the fat and vitamins from oxidizing in the presence of air. These are in fact very expensive food additives and so generally the lowest possible effective amount to preserve the food is used.
Dry pet foods must have a preservative system because they are constantly exposed to air. Canned foods do not need a preservative added because the canning process (using heat and vacuum) alone will protect the food. Homemade diets do not have a preserves added and hence must be kept cool in the refrigerator (for 2-3 days) or frozen (up to 3 months).
If you have more specific questions - Ask the Nutritionist.