Introduction

Extrusion cooking is the process of forcing a material to flow under a variety of conditions through a shaped hole (die) at a predetermined rate to achieve various resulting products (Dziezak, 1989). Extrusion cooking of foods has been practiced for over fifty years. Initially, the role of an extruder was limited to mixing and forming macaroni and ready to eat (RTE) cereal pellets. Today, the food extruder is considered a high temperature-short time bioreactor that transforms raw ingredients into modified intermediate and finished products (Harper, 1989). Extrusion cooking technology today is used for the production of pasta, breakfast cereals, bread crumbs, biscuits, crackers, croutons, baby foods, snack foods, confectionery items, chewing gum, texturized vegetable protein (TVP), modified starch, pet foods, dried soups and dry beverage mixes (Linko et al, 1983).

There are three major types of extruders used in the food industry; piston extruders, roller-type extruders and screw extruders (Thorz, 1986). Figure 1 shows examples of both the piston and roller extruders. The piston extruder can consist of a single piston or multiple sets of pistons that deposit a precise amount of product onto conveyers or trays. Piston extruders are primarily used for forming product shape and are used in confectionery as well as bakery production facilities. One example of the function of a piston extruder is cake, cookie or muffin dough being deposited onto a sheet with the use of a wire cutter, or into individual cups in an already shaped pan, and being conveyed to an oven for baking. Another example of a piston extruder is the depositing of fillings into doughnuts, cupcakes or chocolate type products.

Roller extruders are used to form the shape of a product. A roller extruder consists of two counter-rotating rollers that either turn at similar or differential speeds. This process is also refered to as calendaring in the dough industry. The roller surfaces can be smooth to create a long thin strip or can be perforated to form the dough into shaped products. The roller extruder can be altered to control the width of the layer of product moving in between them. Products such as crackers and hard cookies can be formed by creating the desired shape within the rollers and conveying the dough in between the rollers. The dough is forced into the pattern on the roller and is then conveyed to an oven for baking. Excess dough can be collected and reused. Products such as graham crackers or saltines are created using smooth roller systems to form thin layers.

Screw extruders utilize single, twin or multiple screws rotating within a metal cabinet called the barrel. The screws convey the material forward and through a small orifice called a die which can take on many shapes and sizes. Several external parameters such a screw

speed and configuration, temperature of the barrel, die size and shape, and the length of the barrel effect the properties of the final product. The first food application for extrusion occurred in the 1800's for the production of ground sausage and meats stuffed in natural casings. The common meat grinder is a screw extruder that forces meat down a barrel with a single screw and through a multi-holed die plate. The size openings in the die plate give various widths to the final ground product. The piston extruder or a "ram type" extruder was used to stuff the ground sausage into casings (Dziezak, 1989). The same principle is used today for meat emulsions for the production of frankfurters and bologna-type products (Rust, 1987).

A brief history of the screw extruder shows that the pasta press was introduced in 1935 for forming and shaping pasta dough. Screw extruders providing both cooking and forming capabilities came in around 1950 for the production of animal feeds. The Collet extruder which is a short barreled cooker extruder was developed in 1946 and was used then and now for grain-based snack products (Dziezak, 1989). Because of the demand for pre-cooked cereals and starches in the 1960's, larger machines were required. These larger cooker extruders led to new applications in RTE cereals, snack products, as well as expanded the dry pet food market. Precooked infant foods were also developed (Harper, 1989). Improvements to the cooker extruder in the 70's led to the development of soft-moist pet foods, co-extrusion and the use of two extruders, one for cooking and the second for forming, were developed. The 80's has seen expanded use of twin screw extruder due to their versatility and productivity (Harper, 1989).

Making Pet Food at Home

Making Pet Food at Home

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