Many dry kibble pet foods are mixed with "palatants", which are flavor-enhancers added to encourage pets to eat the food. Many times, pet food has been cooked at high temperatures multiple times, all but eliminating the flavors of the original ingredients. While the manufacturers state the product is still considered nutritious, complete and balanced, your pet may not be interested in eating something bland. (Would you rather eat plain, tasteless lettuce or lettuce with salad dressing?)
Palatability enhancers may be mixed in the food or sprayed on after the food is processed. Millions of dollars are spent on research to determine flavors, smells, and textures that are most desirable to pets and consumers. Cheese and yogurt powders, yeast extracts, animal digest, and fats are among the products that are used to encourage pets to consume the products.
One of the biggest problems with pet food palatants is shelf-life. When fats and oils are incorporated to make the food taste better, they are prone to oxidation. Pet food bags have linings that help prevent oxidation, as long as the bag is sealed. Once the bag is opened, air reacts with the fats in the food, causing oxidation. This oxidation makes the fats become rancid. Many pet owners remove the kibble from the bag, pouring it into a storage container, which can speed up oxidation.
If you are still feeding kibble to your pets, please buy small bags that will be used within two to three weeks. I once had a client tell me he purchased one fifty-pound bag of kibble each year to feed his four-pound Chihuahua. I cannot imagine how rancid and stale that food became.