Extrusion Equipment Design And Selection: Part 1 - Introduction, Extrusion Equipment

The process of extrusion has been around for nearly a century, beginning in the rubber industry to produce items such as hoses and belting. Extrusion has been used nearly that long in the production of pasta by means of a batch extrusion concept. The use of continuous extrusion in food found its first application in the 1940’s to produce puffed cereals and snacks from corn meal or grits and pasta from semolina.

Extrusion processing of dietary ingredients and finished feeds for animals began in the 1950’s to produce foods for dogs. That industry has grown to nearly a $13 billion industry in the US alone and nearly that large in Europe.

Today, the extrusion process is considered a high-temperature, short time "bioreactor" that can transform any number of raw materials into intermediate or finished products that have high consumer appeal. Food examples of items include an infinite variety of snacks, pasta, textured vegetable protein, breakfast cereals and the like. In terms of tonnage, there is little doubt that pet food reigns at the top. Extrusion allows the continuous cooking of the starch fraction of a formula necessary for the digestive requirements of companion animals and gives the processor the ability to create nearly any shape that might appeal to the pet owner.

A significant application of the extrusion process to animal feeds is in the production of aquafeeds. In some cases, it is desirable or necessary to produce a floating feed to accommodate the feeding habits of the target species. In other cases, extrusion provides a method of agglomerating a variety of ingredients into a sinking food that is water stable and is able to stay in the water column for several hours without disintegrating.

A third widely used application of extrusion to animal feed ingredients is in the processing of raw soy beans into full-fat soybean meal that can be used in non-ruminant feeds without causing digestive upset and that is easily digested. The application of the extrusion to processing raw soybeans expanded dramatically with the availability of low-cost equipment that could adequately heat ground raw soybeans to denature trypsin inhibitors and other anti-nutritional factors. The conditions necessary to warrant the use of extruded soybeans in non-ruminant feeds were reviewed by Hancock (1992), Hancock, (1989) and Hancock, et al (1991). Hancock, et al (1991) demonstrated improved soybean protein utilization in nursery pigs when dry-roasting of the soybeans was replaced by extrusion processing.

Extrusion Equipment

Extrusion equipment used today to process animal feeds generally fall into two categories: Single screw extruders and twin-screw extruders (Harper, 1989).