The process of extrusion has been around for nearly a century, beginning in the rubber industry to produce items such as hoses and belting. 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.
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. 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.
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 used today to process animal feeds generally fall into two categories: Single screw extruders and twin-screw extruders (Harper, 1989).
From an engineering point of view, an extruder is simply a pump that provides the pressure necessary to force the process mash through a restrictive die. During the transport through the barrel, it is common that massive amounts of heat are added to the mash through friction generated between the mash and stationary and rotating components of an extruder. Therefore, an extruder is often considered a “heat exchanger” . The pressure and temperature profiles experienced by the process mash can, within limits, be chosen and controlled by variations in screw design and operational conditions. Due to the pressure applied to the barrel, the shape of the final product can be easily controlled through die selection and design.
The major features of the single-screw extruder are shown in Figure 1. In most cases, a preconditioner is used in conjunction with the extruder to increase moisture and heat absorption into the process mash, reduce mechanical power requirements and increase capacity. The conditioner normally operates at atmospheric pressure and provides a means in which either water or steam or both are uniformly incorporated into the process mash. In addition, additives such as vitamins, flavors, colors and even meat slurries may be incorporated. The conditioner provides retention time necessary for the mash to absorb the heat and moisture needed before entering the extrusion barrel.
Conceptually, the barrel of a single screw extruder can be divided into following three separate zones depending on what is happening to the process mash in that zone.
Feed zone: the conditioned mash is simply received from the conditioner and transported forward in the barrel to a point where the cross-section of the barrel is completely full and an elastic plug is formed.
Transition zone of the barrel: it is identified by the fact that the mash changes, rheologically, from a powder to an elastic dough.
Metering zone or cooking zone: extreme pressure is applied to the mash and high levels of heat are induced by friction causing the temperature of the dough to increase to well above 100 c. From a thermodynamic point of view, 75% or more of the work done in the extruder barrel is done in the metering zone.
It is observed in the maintenance records of any extruder that will show that the final screw and barrel sections require replacement much more often than any other component.
Depending upon the specific design if the extruder, various manufacturers use different screw configurations to create elevated compression in the transition and metering zones. In many cases, either decreasing flight height or decreasing screw diameters are used to create compression rations in the range of 1:2 to 1:5. As compression on the extrudate is increased, the mechanical energy created by the screw turning is dissipated as heat into the extrudate.
In many designs, the surface of the barrel is grooved, either in a spiral or straight, so that the barrel “grips” the extrudate so that the rotating screw can force the material forward toward the die. Mixing of ingredients within the extruder barrel is limited by laminar flow within the flight channel. To increase mixing potential, it is sometimes advisable to modify the screw profile to include cut-flight sections that allow backward flow of extrudate.
Single screw operations depend on the pressure requirements of the die, the slip at the barrel extrudate interface and the degree to which the void volume in the barrel id filled. Feed rate, Screw speed and design and the characteristics of the extrudate dictate screw fill. The interaction of all these variables creates the limits in the operating range and flexibility of a single screw extruder.
In order to create a design with greater operating flexibility and with greater operational control, twin-screw machines were developed. Twin-screw extruders can be co-rotating, counter-rotating, intermeshing or non-intermeshing in terms of basic designs. Co-rotating, intermeshing screw designs have dominated that scene as far as these extruders are concerned. This is because of relative ease of design and manufacture compared to counter-rotating designs.
The screw design of twin-screw extruders can dramatically affect operating efficiency and machine capability. Screw components in the feed section of the barrel can be single, double or even triple flight arrangements. With more flights intertwined on the shaft, the conveying capacity of the screw is reduced but the residence time distribution (RTD) is lower. This promotes a first-in, first-out movement of the extrudate. However, double- and triple flighted screws produce more shear across the screw channel and therefore, improved processing uniformity.
Single- verses Twin-Screw Comparison
There is no doubt that twin-screw extruders allow the development and production of a greater array of products but at a significant cost. Twin-screw machines are 1.5 to 2.0 times the cost of a single-screw machine of the same relative cost. The extra expense is due to the relative complexity of the screw design; the complicated drive components and the required heat transfer jackets. This may be somewhat offset by the ability of the twin-screw machine to process drier product thus requiring lower energy for drying. In addition, simply being able to produce a product that is impossible to produce on a single-screw design may justify the additional cost.
A very unique characteristic of a twin-screw machine is the ability to configure a single machine to perform two distinct tasks or functions at the same time. By configuring the first half or so of the machine as a high-shear, high-compression, cooking extruder, providing a vent into the barrel and, then, configuring the last half or the barrel as a low compression, low shear forming extruder, a twin screw can be used to produce high density, cooked products like pre-cooked pasta, sinking aquafeeds or pellets for flaked breakfast cereal. In doing this, a single machine is able to accomplish the functions of two separate single-screw machines producing the same product.
As is often the case, entrepreneurs find a way reduce the cost of expensive technology and produce an item (in this case an extruder) that is much lower in cost, has much less flexibility in terms of application but that is capable of producing an acceptable final product. Such is the case with low cost extruders.
While these machine designs usually produce acceptable FFS, they often fail when used to produce other extruded products, such as floating aquafeeds or pet foods, without significant mechanical and system modification. In most cases, a preconditioner is not needed when processing FFS, however, when attempting to produce pet- or aquafeeds, a preconditioner is necessary to precook and hydrate the starch and protein in the formula for successful production. Because of the relative poor fit (tolerance) between the screw sections and the barrel, much higher back flow tends to occur and production efficiency, product quality and nutrient survival tend to be low. However, there is no doubt that low cost extrusion equipment has a place in the feed industry and provides a feasible way to produce products that otherwise would not be cost effective.
Extrusion has become a common process in the feed industry and allows the production of feed types that simply are not possible with pelleting or any other available process. In general, single screw extruders provide adequate operating ranges and are more economical to operate than twin-screw extruders. With the availability of low cost extrusion equipment, processing of oilseeds such as soybeans is feasible. There is no doubt that the application of extrusion to the development of new and novel feed products will continue and will result in increased usage of the technology in the future of the feed industry.
Source: Kansas State University
by Keith C. Behnke, Kansas State University