Can you think of making a baked product containing between 10 - 20% fat which you place into a paper sack and leave around for 12 months at room temperature AND expect it to be edible at the end of that time period?
I guess the answer is 'No!' because the food would have gone rancid by that time. That's why you need preservatives/antioxidants to stop that rancidity or at least slow it down. Fat that has gone rancid (that slight well-used cooking oil smell you sometimes notice when you're picking up your fries from the chip shop) produces ketones and aldehydes which are not particularly good for health!
ALL dried pet foods need antioxidants/preservatives to stop the food going off before the Best Before Date.
Any product which contains fat, and which has contact with the air stands a good chance of 'going off' after a while, as the oxygen reacts with the fat molecules - this process is called oxidation, or we tend to think of it as fat going rancid. (Canned dog food is protected from oxidation by its airtight storage)
Rancidity is the oxidation (decomposition) primarily of unsaturated fatty acids(containing one or more bonds, such as oleic, linoleic, and arachidonic acids) resulting in disagreeable flavors and odors in fats and oils (stale chip fat smell). This occurs slowly and spontaneously and may be accelerated by light, heat and certain minerals. Once the reaction is initiated, it becomes autocatalytic (i.e. it just keeps on going!) and proceeds unabated until the reactants are completely exhausted.
Oxidation of the fat decreases the nutritional quality of the food, makes it less palatable to the pet, and can even make it unsafe to eat (Oxidation of unsaturated fatty acids yields ketones and free fatty acids that are usually unpalatable and potentially injurious. It takes as little as 0.05% of the fat to react with oxygen to produce rancidity.)
Since most pet food contains high level of fat and fat-soluble ingredients (including vitamins A and E), it is critical to prevent oxidation to maintain quality, nutritional value and palatability.
The meat meal and fats used in pet food are subject to a lot of processing and mechanical damage before they get to the factory (During rendering, the inherent animal cellular defense mechanisms are disrupted or destroyed), and during that process and storage they will start to deteriorate due to oxidation - they will go rancid - unless something is done to stop or slow this change from happening.
Rancidity is usually expressed as a 'Peroxide Value' or PV, although there is debate as to what is the point at which a fat goes rancid.
As far as the majority of the pet food industry is concerned, your pet food will be preserved with one or more different antioxidants - either natural (Vitamin E, C or Tocopherols) or not (Ethoxyquin, Butylated hydroxyanisole (BHA), Butylated hydroxytoluene (BHT) or similar) or a mixture of both types.
These can either be added to the raw materials before they get to the factory, during manufacture, or a mix of both. From a labelling perspective, we understand that if the preservatives have been added to the raw materials before supply to the manufacturer, then these do not have to be declared on the final product declaration.
Bear in mind that not all manufacturers that state that they add natural antioxidants to their products are necessarily insisting on the natural ones being added to the raw materials by their supplier.
No-one is suggesting that manufacturers are being dishonest in how they declare antioxidants, as they are complying with current legislation - but it does leave the consumer a little unclear!
Dr. Lisa Freeman, DVM writes 'The problem is that there's no legal definition of "all natural," "preservative free," or similarly named products. Manufacturers define products by what they believe these terms mean. In general, the implication is that no preservatives or artificial colors have been added and that natural preservatives, such as vitamin C or vitamin E, have been used, but this can vary from brand to brand. In addition, although these products may not contain added artificial preservatives, they may still contain low low levels of artificial preservatives that were already in the meat or fat-soluble vitamins obtained from suppliers.'