Typical raw materials used in pet food processing include cereal flour, vegetable proteins, minerals and vitamins and, in some cases, meat or fish flour and fresh meat. The dry ingredients enter the the pre-conditioner and are steam-hydrated. The counter-rotary twin shaft unit blends the ingredients and ensures proper mixing of steam, moisture and ingredients. Preconditioning produces partial gelatinization of starches and texturing of polymers. The partially cooked mixture is then fed to the twin-screw extruder. Shearing, internal temperature, pressure, moisture and residence times are accurately controlled to achieve optimum starch gelatinization without loss of food quality. Liquids or steam may be injected through ports in the extruder barrel at various processing stages. Degassing is possible at this stage using a vent-stuffer.
Granules are then shaped when they are extruded through the die. They are then put through a dryer, which reduces moisture (generally between 8 and 10%). The product is then coated, and flavoring and fat is added before cooling and packaging.