In the modern world of globalization, consumers are looking forward to a healthy nutrition at an affordable price. Extrusion systems meet the current, constantly changing requirements in a very flexible and economical manner. Food Extrusion plays an important role in the manufacture of pasta, ready-to-eat cereals, snacks and pet foods. The process involves a combination of several unit operations including mixing, cooking, kneading, shearing, shaping and forming. Food Extrusion cooking has gained popularity over the last two decades for a number of reasons:
- Product Quality
The principles of operation are similar in all types: raw materials are fed into the extruder barrel and the screw(s) then convey the food to it. Further down the barrel, smaller flights restrict the volume and increase the resistance to movement of the food. As a result, it fills the barrel and the spaces between the screw flights and becomes compressed.
As it moves further along the barrel, the screw kneads the material into a semi-solid, plasticized mass. Hot extrusion involves heating above 100ºC. Here, frictional heat and any additional heating cause temperature to rise rapidly.Further, the smaller flight section of the barrel increases pressure and shearing.
Finally, it is forced through one or more restricted openings (dies) at the discharge end of the barrel. As the food emerges under pressure from the die, it expands to the final shape and cools rapidly flashing off moisture as steam . A variety of shapes, rods, spheres, doughnuts, tubes, strips or shells can be formed. Typical products include a wide variety of low density, expanded snack foods and ready-to-eat (RTE) puffed cereals.
For pasta and meat products, an implication of cold extrusion technique which involves ambient temperature to mix and shape food. Low-pressure extrusion, at temperatures below 100ºC, is used to produce, for example, liquorice, fish pastes, surimi and pet foods.
Extrusion cooking involves high-temperature-short-time (HTST) process which reduces microbial contamination and inactivates enzymes. The main method of preservation of both hot- and cold-extruded foods is by the low water activity of the product (0.1–0.4), and for semi-moist products in particular, by the packaging materials that are used.
Classification of Extruders
Extruders are classified into two types according to operation: Hot and cold extruders
Based on type of construction : Single screw and twin screw extruder
|Parameters||Single Screw||Twin Screw|
|Transport mechanism||Friction between metal and food material||Positive displacement|
|Minimum water content||10%||8%|
|Mechanical power dissipation||Large shear forces (550-6000kpa)||Small shear forces (2000-4000)|
|Heat Distribution||Large temperature difference||Small temperature difference|
Physiochemical changes during Food Extrusion
Major changes occurs during extrusion process are:
Changes in starches
The major difference between extrusion processing and conventional food processing is that in the former starch gelatinization occurs at much lower moisture content(12-22%). Once inside the extruder, and at relatively high temperatures, the starch granules melt and become soft, besides changing their structure compressing to a flattened form. The application of heat, the action of shear on the starch granule and water content destroy the organized molecular structure, also resulting in molecular hydrolysis of the material. The starch polymers are then dispersed and degraded to form a continuous fluid melt. The fluid polymer continuum retains water vapour bubbles and stretches during extrudate expansion until the rupture of cell structure. The starch polymer cell walls recoil and stiffen as they cool to stabilize the extrudate structure. Finally, the starch polymer becomes glassy as moisture is removed, forming a hard brittle texture.
Changes in Protein
Proteins are biopolymers with a great number of chemical groups when compared to polysaccharides and are therefore more reactive and undergo many changes during the extrusion process, with the most important being denaturation. Electrostatic and hydrophobic interactions favour the formation of insoluble aggregates. The creation of new peptide bonds during extrusion is controversial. High molecular weight proteins can dissociate into smaller subunits. Enzymes, also proteins, lose their activity after being submitted to the extrusion process due to high temperatures and shear.
Changes in lipids
Fats and oils can be described as lipids. Lipids have a powerful influence in extrusion cooking processes by acting as lubricants because they reduce the friction between particles in the mix and between the screw and barrel surfaces and the fluid melt.
Changes in Fibres
Research has shown that cooking fibres by extrusion can produce changes in their structural characteristics and physicochemical properties, with the main effect being a redistribution of insoluble fibre to soluble fibre. This effect would be the result of the rupture of covalent and noncovalent bonds between carbohydrates and proteins associated to the fibre, resulting in smaller molecular fragments, that would be more soluble.