Pellets of dry dog food, called kibble, is primarily produced by one of two methods: extrusion and baking. During the extrusion process, cut dough or a mixture of raw materials is fed into an expander, while pressurized steam or hot water is added. When removed from the high pressure that results, the pellets puff like popcorn. The resultant kibble is allowed to dry, then sprayed with vitamins, grease, or any other ingredients that are not heat-tolerant.
If extruded kibble is exposed to air for too long or not properly stored, the fats and oils added after cooking can become rancid, and vitamins and minerals in the food may be destroyed by heat during storage or shipping.
Baked kibble is often sought after by pet owners seeking a more natural or more digestible food, although the effect of baking on either of these factors is the subject of much discussion (see below). Because the heating process is slower, kibble being prepared this way is subjected to a lower pressure for a much longer period of time. Because fat can be cooked in rather than sprayed on, baked kibble will tend to have a slightly longer shelf life compared to extruded kibble, especially when exposed to air, or after its package has been opened by the consumer.
Because of the longer shelf life and less efficient cooking methods, baked kibble will tend to have a notably higher price tag, and is generally only available in pet and specialty stores, rather than supermarkets. There is persistent debate within the pet care industry as to which method is better for the dog nutritionally. Some proponents of baked food assert that baking aids in the digestibility of raw proteins, and that extrusion requires the use of harmful preservatives, such as BHT or Ethoxyquin. However, it has been pointed out that the canine digestive system is tailored to eating raw meat in the natural environment, and that many extruded foods use natural preservatives, such as tocopherols.
As with any cooking procedure, the simple act of preparing kibble destroys many nutrients in raw ingredients. It has been suggested that either baking or extrusion is less harmful to the nutrient content of a food, but no conclusive evidence has been presented in either direction, and the argument continues.