http://www.api4animals.org/articles?p=361&more=1
When selecting a commercial food for your animal companion, make sure the label has an “AAFCO guarantee,” preferably one that references “feeding tests” or “feeding protocols” rather than Nutrient Profiles.
Never buy a food containing “by-product meal” or “meat and bone meal.” These rendered products are the most inexpensive sources of animal protein. The contents and quality of these meals can vary tremendously from batch to batch, and are not a reliable source of nutrition for your animal.
In general, avoid foods that rely on by-products as the sole source of animal protein. By-products consist of organs and parts either not desired, or condemned, for human consumption. An occasional can of by-product-based food may be okay, since, in the wild, carnivores do consume the whole prey including the organs, but these foods are not acceptable as a steady diet.
Look for a named meat or meal (“lamb” or “chicken meal,” for example, instead of the generic term “meat”) as the first ingredient.
Avoid generic or store brands. These may be repackaged rejects from the big manufacturers, and generally contain cheaper — and consequently poorer quality — ingredients.
Unless specifically recommended by your veterinarian, avoid “light,” “senior,” “special formula,” or “hairball formula” foods. These foods may contain acidifying agents, excessive fiber, or inadequate fats that can result in skin, coat and other problems. In general, select brands promoted to be “natural.” While they are not perfect, they may be better than most. Several brands are now preserved with Vitamins C and E instead of chemical preservatives (such as BHA, BHT, ethoxyquin and propyl gallate). While synthetic preservatives may still be present, the amounts will be less.
Check the expiration date to ensure freshness.
When you open a bag of dry food, give it a sniff — if there is any rancid odor at all, return it immediately for an exchange or refund.
Store dry pet food in a sealed non-porous container (a large popcorn tin is ideal) in a cool, dry place. Canned food is best removed from the can and refrigerated in a glass or ceramic container.