Pet foods can be divided into three main forms based on the way they are processed:, dry, semi-moist and canned.

Dry

To make extruded dry foods, the dry ingredients are finely ground, weighed out according to the formula, and then mixed together. This blend of raw materials is then mixed with water and steam in a preconditioner (a metal tube with paddles to mix the water and dry ingredients) to ensure that all the dry particles are properly moistened. This is important, as the proper amount of moisture is necessary to cook the carbohydrates. From the preconditioner, the wet dough enters the extruder. In the extruder, the dough is mixed with steam and pressure is applied to the product. This is where the actual cooking of the product occurs. At the end of the extruder, the cooked doughy material is pushed through a die (a series of holes at the end of the extruder) and cut by a knife blade. As it emerges from the die, the food expands due to a pressure drop to form small, porous nuggets the shape of which is determined by the die. These nuggets are then conveyed to an oven where they are dried. After drying, the nuggets are dusted with canola oil (to provide energy) and other ingredients to enhance the taste of the product. These ingredients include natural flavors, brewer's yeast, whey or other ingredients. After coating, the product passes over a screening system to remove small and broken particles. The food is then sent to the packaging line to be put in any one of a variety of packages.

Semi-Moist

Semi-moist foods are made using the same process as that described for extruded foods until the point of drying. Rather than being dried after cooking, the product is cooled (which allows a skin to form on the outside of the piece), and the nuggets are coated with fat (to provide energy) and other ingredients to enhance the taste of the product. After coating, the product passes over a screening system to remove small and broken particles and then is sent to the packaging line to be put into any one of a variety of packages.
Four critical differences between dry and semi-moist foods relate to the higher moisture content of semi-moist foods and efforts to keep microorganisms from growing on them:

• First, semi-moist foods must be acidic (low pH) to discourage the growth of certain bacteria and molds. This is usually accomplished through the use of phosphoric acid or in some cases hydrochloric acid.
• Second, semi-moist foods usually contain humectants such as glycerin or propylene glycol, which helps keep the food soft due to their water-holding properties. Propylene glycol also helps kill mites, which would otherwise grow on semi-moist foods. Propylene glycol is not permitted in cat foods because of concerns about abnormal red blood cell formation. It should be noted that the active compound in anti-freeze, ethylene glycol, which is very toxic to all animals is quite different from propylene glycol, the ingredient in semi-moist pet foods.
• Third, because of their high water content, semi-moist foods use additional agents to bind the excess water in such a way to make it unavailable to the bacteria. This is accomplished using various sugars such as sorbitol, molasses, or high fructose corn syrup.
• Fourth, semi-moist foods also contain a variety of preservatives to inhibit the growth of a variety molds. The most common preservative is potassium sorbate.  

Wet (includes cans, pouches and aluminum and plastic trays)

Canned foods are prepared by blending ingredients together with water to form a slurry type mixture.The ingredients are usually various meats, poultry, and fish.

The next step in the process depends on whether the products is a "loaf style" product or a "chunks and gravy" style product. If the product is a "loaf style", the ingredients are cooked in a mixture, and while still hot, placed in a can or tray, which is then sealed under vacuum. For a "chunks and gravy" type product, the slurry usually passes through a forming oven, much like an oven which bakes dog biscuits, which converts the products into a long thin sheet. The heat from the oven cooks the ingredients and gives them a tough skin so they can then be cut into small pieces or chunks. The chunks are then mixed with a gravy, composed of water and various gums or starches, and placed in a can, tray or pouch and then sealed.

Once sealed, the product is sterilized by heating it at a temperature necessary to kill any remaining microorganisms in the can. The sterilization process is carefully controlled and is regulated by the FDA. The cans are then cooled, dried and a label is applied.

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