ExtrusionExtrusion

Nitesh Nitesh YadavYadav

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ExtrusionExtrusion

• Extrusion cooking is the process of cooking Extrusion cooking is the process of cooking moistened starchy and/or proteinous food moistened starchy and/or proteinous food material by a combination of heat, pressure material by a combination of heat, pressure and mechanical shear.and mechanical shear.

• Extrusion is a process which combines Extrusion is a process which combines several unit operations including mixing, several unit operations including mixing, cooking, kneading, shearing, shaping and cooking, kneading, shearing, shaping and forming.forming.

• Extrusion provides the foundation for Extrusion provides the foundation for continuous production.continuous production.

• Food extrusion machines use single or twin screws to Food extrusion machines use single or twin screws to transport, mix, knead, shear, shape, and/or cook transport, mix, knead, shear, shape, and/or cook multiple ingredients into a uniform food product by multiple ingredients into a uniform food product by forcing the ingredient mix through shaped dies to forcing the ingredient mix through shaped dies to produce specific shapes and lengths.produce specific shapes and lengths.

• Food extruders are used to produce pasta and other cold Food extruders are used to produce pasta and other cold formed products, cereals, snacks, pet food, feed, formed products, cereals, snacks, pet food, feed, confectionery products (including licorice), modified confectionery products (including licorice), modified starches for soup, baby food, and instant foods, starches for soup, baby food, and instant foods, beverage bases, and texturized vegetable proteins. beverage bases, and texturized vegetable proteins.

• Single-screw extruders have been in use for continuous Single-screw extruders have been in use for continuous cooking and in the forming of ready-to-eat (RTE) cereals cooking and in the forming of ready-to-eat (RTE) cereals as a one-step process since the 1960s. Twin-screw as a one-step process since the 1960s. Twin-screw extruders were in common use in food production by the extruders were in common use in food production by the 1980s.1980s.

Principles of operationPrinciples of operation

• Raw materials are fed into the extruder Raw materials are fed into the extruder barrel and the screw(s) they convey the barrel and the screw(s) they convey the food along it.food along it.

• Further down the barrel, a smaller flight Further down the barrel, a smaller flight restricts the volume and increase the restricts the volume and increase the resistance to movement of the food.resistance to movement of the food.

• As a result, it fills the barrel and the space As a result, it fills the barrel and the space between the screw flights and becomes between the screw flights and becomes compressed.compressed.

• As it moves further along the barrel, the As it moves further along the barrel, the screw kneads the material into a semi-screw kneads the material into a semi-solid, plasticized mass.solid, plasticized mass.

• Here, frictional heat and any additional Here, frictional heat and any additional heating that is used cause the temperature heating that is used cause the temperature to rise rapidly.to rise rapidly.

• The food is then passed to the section of the The food is then passed to the section of the barrel having the smallest flights, where barrel having the smallest flights, where pressure and shearing is further increased.pressure and shearing is further increased.

• Finally, it is forced through one or more Finally, it is forced through one or more restricted openings (dies) at the discharge restricted openings (dies) at the discharge end of the barrel.end of the barrel.

• As the food emerges under pressure from the As the food emerges under pressure from the die, it expands to the final shape and cools die, it expands to the final shape and cools rapidly as moisture is flashed off as steam.rapidly as moisture is flashed off as steam.

• A variety of shapes, including rods, spheres, A variety of shapes, including rods, spheres, doughnuts, tubes strips, squirals or shells can doughnuts, tubes strips, squirals or shells can be formed. be formed.

• Typical products include a variety of low Typical products include a variety of low density, expanded snack foods and ready to density, expanded snack foods and ready to eat (RTE) puffed cereals.eat (RTE) puffed cereals.

Classification of Classification of Extruders Extruders

• Method of operation Method of operation

cold extruders cold extruders

extruder cookers extruder cookers

• Method of construction Method of construction

single screw extruderssingle screw extruders

twin screw extruders twin screw extruders

• Cold extrusion, in which the temperature of Cold extrusion, in which the temperature of the food remains at ambient is used to mix the food remains at ambient is used to mix and shape foods such as pasta and meat and shape foods such as pasta and meat products.products.

• Low pressure extrusion, at temperature below Low pressure extrusion, at temperature below 100ºC, is used to produce, for example, 100ºC, is used to produce, for example, liquorice, fish pastes, surimi and pet foods.liquorice, fish pastes, surimi and pet foods.

• Extrusion cooking is a high-temperature short-Extrusion cooking is a high-temperature short-time (HTST) process which reduces microbial time (HTST) process which reduces microbial contamination and inactivates enzymes. contamination and inactivates enzymes.

Basic Components of a Basic Components of a Food ExtruderFood Extruder

• A feeding unit or storage bin A feeding unit or storage bin above the extruder that above the extruder that meters the raw ingredients into the extruder.meters the raw ingredients into the extruder.

• Pre-conditioner :Pre-conditioner : assembly sometimes used to adjust the assembly sometimes used to adjust the moisture content and temperature of ingredients (may partially moisture content and temperature of ingredients (may partially or completely cook them before entering the extruder).or completely cook them before entering the extruder).

• Extruder barrel: Extruder barrel: a pipe-like retainer in which the screw turns. a pipe-like retainer in which the screw turns. The barrel may be built in sections that contain their own The barrel may be built in sections that contain their own heating or cooling sleeve.heating or cooling sleeve.

• Screw:Screw: the core of any extruder, the screw conveys the the core of any extruder, the screw conveys the product through the extruder. The flight of the screw pushes product through the extruder. The flight of the screw pushes product forward. The screw determines not only the quality of product forward. The screw determines not only the quality of the product but also the output of the extruder. Extruders can the product but also the output of the extruder. Extruders can use either a single screw or twin screws.use either a single screw or twin screws.

• Vent:Vent: an opening in the barrel before the die an opening in the barrel before the die plate that allows pressure and steam to be plate that allows pressure and steam to be removed.removed.

• Die plate: Die plate: final assembly for shaping the product final assembly for shaping the product as it leaves the extruder.as it leaves the extruder.

• Cutting station: Cutting station: ensures precise and consistent ensures precise and consistent shape and size of product.shape and size of product.

• Gear box, motor, and controlsGear box, motor, and controls

Most extruders used for food Most extruders used for food processing are constructed of processing are constructed of stainless steel. stainless steel.

Popularity Gained in Popularity Gained in ExtrusionExtrusion

• Versatility.Versatility. A very wide variety of A very wide variety of products are possible by changing the products are possible by changing the ingredients, the operating conditions of ingredients, the operating conditions of the extruder and the shape of the dies. the extruder and the shape of the dies. Many extruded foods cannot be easily Many extruded foods cannot be easily produced by other method.produced by other method.

• Reduced costs.Reduced costs. Extrusion has lower Extrusion has lower processing costs and higher productivity processing costs and higher productivity than other cooking or forming processes. than other cooking or forming processes.

• High production rates and automatic High production rates and automatic production.production. Extruders operate continuously and Extruders operate continuously and have high throughputs. For example, production rates have high throughputs. For example, production rates of up to 315 kgh-1 for snack foods, 1200kgh-1 for low of up to 315 kgh-1 for snack foods, 1200kgh-1 for low density cereals and 9000 kgh-1 for dry expanded pet density cereals and 9000 kgh-1 for dry expanded pet foods are possible.foods are possible.

• Product quality.Product quality. Extrusion involves high Extrusion involves high temperatures applied for a short time and the limited temperatures applied for a short time and the limited heat treatment therefore retains many heat sensitive heat treatment therefore retains many heat sensitive components.components.

• No process effluents.No process effluents. Extrusion is a low moisture Extrusion is a low moisture process that does not produce process effluents. This process that does not produce process effluents. This eliminates water treatment costs and does not create eliminates water treatment costs and does not create problems of environmental pollution.problems of environmental pollution.

Food BenefitsFood Benefits

• Starch gelatinization Starch gelatinization

• Protein denaturation Protein denaturation

• Inactivation of raw food enzymes Inactivation of raw food enzymes

• Destruction of naturally occurring Destruction of naturally occurring toxins toxins

• Diminishing of microorganisms in the Diminishing of microorganisms in the final product final product

Operating characteristicsOperating characteristics

•TemperatureTemperature

•PressurePressure

•Diameter of die apparatusDiameter of die apparatus

•Shear rateShear rate

Advantages of Extrusion Advantages of Extrusion in Food Processingin Food Processing

• The advantage of extrusion is that it The advantage of extrusion is that it produces a more homogeneous and produces a more homogeneous and consistent cooking process, which leads to consistent cooking process, which leads to a final product of higher quality with a final product of higher quality with minimum waste.minimum waste.

• Extrusion is a very efficient process, since Extrusion is a very efficient process, since all steps can be done in-line: mixing, all steps can be done in-line: mixing, cooking, forming, cooling, and cutting. cooking, forming, cooling, and cutting.

• Many extruders with modular designs Many extruders with modular designs allow changing from one product to allow changing from one product to another product, color, or shape to be another product, color, or shape to be done within minutes-a significant process done within minutes-a significant process efficiency. efficiency.

• Twin-screw extruders are largely self-Twin-screw extruders are largely self-cleaning, an advantage from the cleaning, an advantage from the viewpoint of both sanitation and labor viewpoint of both sanitation and labor savings.savings.

• Work continuouslyWork continuously

Twin-Screw Food Twin-Screw Food ExtruderExtruder

• Twin screw extruder two screws, side by side, are Twin screw extruder two screws, side by side, are placed within the extruder barrel; they are either placed within the extruder barrel; they are either counter-rotating or co-rotating. counter-rotating or co-rotating.

• The co-rotating units are used for compounding The co-rotating units are used for compounding materials where thorough mixing and high output materials where thorough mixing and high output rates are important.rates are important.

• Twin screw extruder unit resembles a positive Twin screw extruder unit resembles a positive displacement screw pump. displacement screw pump.

• It conveys the material at low speeds with It conveys the material at low speeds with controlled shear. controlled shear.

• Positive action assures that all portions of the Positive action assures that all portions of the material experience a uniform residence time. material experience a uniform residence time.

1.1.Counter-rotating twin-screw extruders Counter-rotating twin-screw extruders (move in opposing directions)(move in opposing directions)

2.2.Co-rotating twin-screw extrudersCo-rotating twin-screw extruders

(move in same direction)(move in same direction)Counter-rotating twin-screw extruders Counter-rotating twin-screw extruders are commonly are commonly

used for processing relatively non-viscous materials used for processing relatively non-viscous materials requiring low speeds and long residence times, such requiring low speeds and long residence times, such as gum, jelly, and licorice confections. as gum, jelly, and licorice confections.

Co-rotating twin-screw extruders Co-rotating twin-screw extruders have broadened the have broadened the variety of products that can be made with extrusion variety of products that can be made with extrusion technology and are commonly used in the snack technology and are commonly used in the snack food industry. They provide a high degree of heat food industry. They provide a high degree of heat transfer but not forced conveyance. transfer but not forced conveyance.

Single-Screw Food Single-Screw Food ExtruderExtruder

• Single-screw extruders typically consist of Single-screw extruders typically consist of three zones: feeding zone, kneading zone, three zones: feeding zone, kneading zone, cooling zone.cooling zone.

• In general, single-screw extruders have In general, single-screw extruders have poor mixing ability and therefore materials poor mixing ability and therefore materials should be pre-mixed or pre-conditioned.should be pre-mixed or pre-conditioned.

• A single-screw extruder’s processing A single-screw extruder’s processing conditions can be controlled to achieve a conditions can be controlled to achieve a variety of effects on temperature and variety of effects on temperature and residence time.residence time.

• The residence time in the barrel can The residence time in the barrel can vary from 15 to 300 seconds, vary from 15 to 300 seconds, determined by increasing or decreasing determined by increasing or decreasing the speed of the shaft.the speed of the shaft.

• Single-screw extruders are limited in Single-screw extruders are limited in their ability to transport sticky and/or their ability to transport sticky and/or gummy raw materials, and materials gummy raw materials, and materials that become sticky during heat that become sticky during heat compression.compression.

• A variety of fried and baked snacks are A variety of fried and baked snacks are processed using single-screw extruders.processed using single-screw extruders.

Low Shear Stress Low Shear Stress ExtrudersExtruders

• Low shear stress extruders (forming Low shear stress extruders (forming extruders) are used to densify material extruders) are used to densify material that is generally high in moisture, such that is generally high in moisture, such as pasta.as pasta.

• They operate at a slow speed and They operate at a slow speed and feature a long length-to-diameter ratio, feature a long length-to-diameter ratio, imparting low levels of mechanical imparting low levels of mechanical energy per unit of throughput. energy per unit of throughput.

Medium Shear Stress Medium Shear Stress ExtrudersExtruders

• Medium shear stress extruders Medium shear stress extruders handle materials with a lower handle materials with a lower moisture level and mechanical moisture level and mechanical energy inputs are higher.energy inputs are higher.

• Typical products processed are pet Typical products processed are pet food, aquatic feeds, and textured food, aquatic feeds, and textured vegetable proteins. vegetable proteins.

High Shear Stress ExtrudersHigh Shear Stress Extruders

• High shear stress extruders are used High shear stress extruders are used for highly expanded products with for highly expanded products with low moisture and bulk density levels.low moisture and bulk density levels.

• They feature the shortest length-to-They feature the shortest length-to-diameter ratio and extruder speeds diameter ratio and extruder speeds and mechanical energy inputs are and mechanical energy inputs are high.high.

Effect on FoodsEffect on Foods

1)1) Sensory CharacteristicsSensory Characteristics

• Extent of changes to starch determined by Extent of changes to starch determined by the operating conditions and feed the operating conditions and feed materials, produces the wide range of materials, produces the wide range of product textures that can be achieved.product textures that can be achieved.

• The HTST conditions in extrusion cooking The HTST conditions in extrusion cooking have only minor effects on the natural have only minor effects on the natural colour and flavour of foods.colour and flavour of foods.

• Fading of colour due to product expansion, Fading of colour due to product expansion, excessive heat or reactions with proteins, excessive heat or reactions with proteins, reducing sugars or metal ions may be problem reducing sugars or metal ions may be problem in some extruded foods.in some extruded foods.

• Added flavours are mixed with ingredients Added flavours are mixed with ingredients before cold extrusion, but this is largely before cold extrusion, but this is largely unsuccessful in extrusion cooking as the unsuccessful in extrusion cooking as the flavours are volatilesed when the food flavours are volatilesed when the food emerges from the die.emerges from the die.

• Flavours are therefore more often applied to Flavours are therefore more often applied to the surface of extruded foods in the form of the surface of extruded foods in the form of sprayed emulsions or viscous slurries. sprayed emulsions or viscous slurries. However, this may cause stickiness in some However, this may cause stickiness in some products and hence require additional drying.products and hence require additional drying.

Effect on Foods Effect on Foods

2)2) Nutritional valueNutritional value• Vitamin losses in extruded foods vary Vitamin losses in extruded foods vary

according to type of foods, the moisture according to type of foods, the moisture content, the temperature of processing and content, the temperature of processing and the holding time.the holding time.

• Losses are minimal in cold extrusionLosses are minimal in cold extrusion

• The HTST conditions in extrusion cooking, and The HTST conditions in extrusion cooking, and the rapid cooling as the product emerges from the rapid cooling as the product emerges from the die, cause relatively small losses of most the die, cause relatively small losses of most vitamins and essential amino acids.vitamins and essential amino acids.

• Losses of ascorbic acid and β-carotene are up Losses of ascorbic acid and β-carotene are up to 50% depending upon the time that the to 50% depending upon the time that the food is held at elevated temperaturesfood is held at elevated temperatures

• High temperature and presence of sugar High temperature and presence of sugar cause Maillard browning and a reduction in cause Maillard browning and a reduction in protein quality.protein quality.

• Low temperatures and low concentration of Low temperatures and low concentration of sugars results in increase in protein sugars results in increase in protein digestibility, owing to rearrangement of the digestibility, owing to rearrangement of the protein structure.protein structure.

• Destruction of anti-nutritional components in Destruction of anti-nutritional components in soya products improves the nutritive value of soya products improves the nutritive value of texturized vegetable proteins.texturized vegetable proteins.

Type of Snack FoodsType of Snack Foods• Cheese puffs/Cheese curlsCheese puffs/Cheese curls

• Cheese ballsCheese balls

• Corn chips and Tortilla chipsCorn chips and Tortilla chips

• Crispy riceCrispy rice

• CrackersCrackers

• ConfectionaryConfectionary

• Extruded breadExtruded bread

• Co-extruded snackCo-extruded snack

• Bugles snackBugles snack

• FryumsFryums

• Pet snackPet snack

• Dog chews/treatsDog chews/treats

• Fish feedFish feed

• Bread crumbsBread crumbs

Uses of Extrusion in Uses of Extrusion in Industry- PastaIndustry- Pasta

• Pasta production relies mainly on the process of Pasta production relies mainly on the process of extrusion, e.g. macaroniextrusion, e.g. macaroni

• Low temperatures and pressure are needed to Low temperatures and pressure are needed to keep the pasta from cooking.keep the pasta from cooking.

Uses of Extrusion in Uses of Extrusion in Industry- SnacksIndustry- Snacks

• The production of snack products is a rapidly The production of snack products is a rapidly growing area. High temperatures and pressures growing area. High temperatures and pressures are necessary to produce the snacks. are necessary to produce the snacks.

• The gelatinised starch ensures that when the The gelatinised starch ensures that when the mixture expands by going from high pressure to mixture expands by going from high pressure to low pressure it holds its expanded shape. low pressure it holds its expanded shape.