Extrusion processing is a high-temperature-short time processing technique employed in the food industry to manufacture products like pasta, macaroni, snacks, pet food, meat analogs, ready-to-eat cereals, and baby foods. The machine performing extrusion is called the extruder, and it is a single unit machine offering the functionality of a heat exchanger, pressure reactor, temperature reactor, and as a whole as a bioreactor.
Figure 1. A Typical twin-screw extruder employed in the food industry
Extrusion processing has been employed in the plastics and metallurgy industry since the early 19th century. The application of food extruders has been relevant since the early 1940’s in manufacturing food materials. The earlier known food extruders were single-screw extruders, it had had one screw as the name suggests which transformed raw materials from solid to melted form and finally forced this melted material outside the die to form puffed products.
Machinery and working of extruders:
An extruder is a multi-operational unit that simultaneously mixes, kneads, cooks and transforms the raw materials and pushes outside the die, and forms the puffed products. The raw material is subjected to high-pressure and temperature inside the extruder. When the material leaves the die, there is a sudden drop in temperature and pressure which causes the loss of moisture and immediate puffing of the product. The product cools to ambient temperature within a few seconds after leaving the die and attains the puffed texture. The extrudates, the products which are collected after extrusion processing can be further dried, and seasoned and packaged. The shelf-life of extruded products is about six-eight months and is shelf-stable, due to the low water activity.
Extruders can exist as single-screw and twin-screw extruders depending upon its functionality. In the current time, the twin-screw extruders are largely used in the food industry, due to its high efficiency and ability to extrude different materials. The extruder is a piece of versatile equipment and can handle raw materials from a variety of sources such as starches, proteins, lipids, and material with varying moisture content and chemical composition.
The extruder consists of a motor, barrel, screws, die head, and the die, the screws and die heads are removable and could be customized based on the desired outcome of the product. The barrel houses the screws and the heating zones, which provide thermal energy input to the extruder. The energy for transforming or cooking raw material is offered by thermomechanical forces, the source of this force is from the motor and shear imparted by the rotation of the screws.
Figure 2. Screws of extruder
The operator of the extruder can control the extrusion variables which include temperature, screw configuration, screw speed, length/diameter ratio of the barrel, die diameter, die type, feeding rate, moisture content, and the raw materials. These variables offer liberty to the operator to design products based on the data collected from the previous experiments (formulations) to design products with greater textural attributes. The process parameters include pressure and torque, these measurements are recorded by the extruder software and depend largely upon the material chemistry and variables set.
It is essential to understand the chemistry of materials before extruding them, so as to understand their behavior during extrusion processing. Extrusion is a quick transformation or cooking of raw materials under unusual processing conditions of high temperature and shear, that transforms the raw material within 60-100 seconds. The most extruded material in the food industry includes corn meal and corn starch, they are considered as the ideal material for extrusion processing due to the high availability of corn and the easy reach of data on these materials. The behavior of corn starch during extrusion has been widely studied and reported in the literature.
Advantages of extruder processing:
- The versatility of handling a variety of raw materials
- Low labor cost and energy-efficient processing
- Extruders have a self-wiping mechanism that reduces the energy and cost associated with cleaning-in-place protocols
- The equipment and accessories can be modified based on the application and functionality of the desired product
- It can be used for inactivating antinutritional factors present in cereal and legume flours
- Loss of color and nutrients is low as compared to other heat processing techniques such as baking and frying
- The products have a high-shelf life and not susceptible to microbial attack
- Harper, J. M. (1981). Extrusion of starches and starches materials. Extrusion of food, 2, 41-60.
- Riaz, M. N. (Ed.). (2000). Extruders in food applications. CRC press.
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