Extrusion Extrusion is a process used to create objects of a fixed cross-sectional profile by pushing material through a die of the desired cross-section. Its two main advantages over other manufacturing processes are its ability to create very complex c ...

food processing Food processing is the transformation of agricultural products into food, or of one form of food into other forms. Food processing includes many forms of processing foods, from grinding grain to make raw flour to home cooking to complex industr ...

consists of forcing soft mixed ingredients through an opening in a perforated plate or

die Die, as a verb, refers to death, the cessation of life. Die may also refer to: Games * Die, singular of dice, small throwable objects used for producing random numbers Manufacturing * Die (integrated circuit), a rectangular piece of a semicondu ...

designed to produce the required shape. The extruded food is then cut to a specific size by blades. The machine which forces the mix through the die is an extruder, and the mix is known as the extrudate. The extruder is typically a large, rotating screw tightly fitting within a stationary barrel, at the end of which is the die. Extrusion enables

mass production Mass production, also known as flow production or continuous production, is the production of substantial amounts of standardized products in a constant flow, including and especially on assembly lines. Together with job production and batch ...

of food via a continuous, efficient system that ensures uniformity of the final product. Food products manufactured using extrusion usually have a high starch content. These include some

pasta Pasta (, ; ) is a type of food typically made from an unleavened dough of wheat flour mixed with water or eggs, and formed into sheets or other shapes, then cooked by boiling or baking. Rice flour, or legumes such as beans or lentils, are som ...

bread Bread is a staple food prepared from a dough of flour (usually wheat) and water, usually by baking. Throughout recorded history and around the world, it has been an important part of many cultures' diet. It is one of the oldest human-made f ...

s (

crouton A crouton is a piece of rebaked bread, often cubed and seasoned. Croutons are used to add texture and flavor to salads—notably the Caesar salad— or eaten as a snack food. Etymology The word crouton is derived from the French ''croûton' ...

s, bread sticks, and

flat bread A flatbread is a bread made with flour; water, milk, yogurt, or other liquid; and salt, and then thoroughly rolled into flattened dough. Many flatbreads are unleavened, although some are leavened, such as pizza and pita bread. Flatbreads ra ...

s), many

breakfast cereal Cereal, formally termed breakfast cereal (and further categorized as cold cereal or warm cereal), is a traditional breakfast food made from processed cereal grains. It is traditionally eaten as part of breakfast, or a snack food, primarily in ...

s and ready-to-eat

snack A snack is a small portion of food generally eaten between meals. Snacks come in a variety of forms including packaged snack foods and other processed foods, as well as items made from fresh ingredients at home. Traditionally, snacks are p ...

confectionery Confectionery is the art of making confections, which are food items that are rich in sugar and carbohydrates. Exact definitions are difficult. In general, however, confectionery is divided into two broad and somewhat overlapping categories ...

, pre-made

cookie dough Cookie dough is an un-cooked blend of cookie ingredients. Cookie dough is normally intended to be baked into individual cookies before eating, however edible cookie dough is made to be eaten as is, and usually is made without eggs to make it safer ...

, some

baby food Baby food is any soft easily consumed food other than breastmilk or infant formula that is made specifically for human babies between four and six months and two years old. The food comes in many varieties and flavors that are purchased ready-mad ...

s, full-fat soy,

textured vegetable protein Textured or texturized vegetable protein (TVP), also known as textured soy protein (TSP), soy meat, or soya chunks is a defatted soy flour product, a by-product of extracting soybean oil. It is often used as a meat analogue or meat extender ...

, some

beverage A drink or beverage is a liquid intended for human consumption. In addition to their basic function of satisfying thirst, drinks play important roles in human culture. Common types of drinks include plain drinking water, milk, juice, smoo ...

s, and dry and semi-moist

pet food Pet food is animal feed intended for consumption by pets. Typically sold in pet stores and supermarkets, it is usually specific to the type of animal, such as dog food or cat food. Most meat used for animals is a byproduct of the human food ind ...

Process

DEMACO 1,000 lbs-hr Short Goods Pasta Press, 1958

In the extrusion process, raw materials are first ground to the correct particle size, usually the consistency of coarse flour. The dry mix is passed through a pre-conditioner, in which other ingredients are added depending on the target product; these may be liquid

sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or double ...

fat In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food. The term often refers spec ...

s, dyes,

meat Meat is animal flesh that is eaten as food. Humans have hunted, farmed, and scavenged animals for meat since prehistoric times. The establishment of settlements in the Neolithic Revolution allowed the domestication of animals such as chic ...

s or water.

Steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...

is injected to start the cooking process, and the preconditioned mix (extrudate) is then passed through an extruder. The extruder is a large, rotating screw tightly fitting within a stationary barrel, at the end of which is the die. The extruder's rotating screw forces the extrudate towards and through the die. The extrudate is in the extruder for the residence time. Many extruded products puff and change texture as they are extruded because of the reduction of forces and release of moisture and heat. The extent to which it does so is known as the expansion ratio. The extrudate is cut to the desired length by blades at the output of the extruder, which rotate about the die openings at a specific speed. The product is then cooled and dried, becoming rigid while maintaining porosity. Cooking takes place within the extruder, where the product produces its own friction and heat due to the

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...

generated (10–20 bar). The process can induce both

protein denaturation In biochemistry, denaturation is a process in which proteins or nucleic acids lose the quaternary structure, tertiary structure, and secondary structure which is present in their native state, by application of some external stress or comp ...

and

starch gelatinization Starch gelatinization is a process of breaking down the intermolecular bonds of starch molecules in the presence of water and heat, allowing the hydrogen bonding sites (the hydroxyl hydrogen and oxygen) to engage more water. This irreversibly d ...

under some conditions. Many food extrusion processes involve a high temperature for a short time. Important factors of the extrusion process are the composition of the extrudate, screw length and rotating speed, barrel temperature and moisture, die shape, and rotating speed of the blades. These are controlled based on the desired product to ensure uniformity of the output. Moisture is the most important of these factors, and affects the mix

viscosity The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

, acting to plasticize the extrudate. Increasing moisture will decrease viscosity, torque, and product temperature, and increase bulk density. This will also reduce the pressure at the die. Most extrusion processes for food processing are carried out at low to intermediate moisture (moisture level below 40%). High-moisture extrusion is known as wet extrusion, but it was not used much before the introduction of twin screw extruders (TSE), which have a more efficient conveying capability. The most important

rheological Rheology (; ) is the study of the flow of matter, primarily in a fluid (liquid or gas) state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an appli ...

factor in the wet extrusion of high-starch extrudate is temperature. The amount of salt in the extrudate may determine the colour and texture of some extruded products. The expansion ratio and airiness of the product depend on the salt

concentration In chemistry, concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: '' mass concentration'', ''molar concentration'', ''number concentration'', an ...

in the extrudate, possibly as a result of a chemical reaction between the salt and the starches in the extrudate. Colour changes as a result of salt concentration may be caused by "the ability of salt to change the water activity of the extrudate and thus change the rate of browning reactions". Salt is also used to distribute minor ingredients, such as food colours and flavours, after extrusion; these are more evenly distributed over the product's surface after being mixed with salt.

History

Automatic short cut dry pasta production machine built by Consolidated Macaroni Machine Corporation 001

The first extruder was designed to manufacture

sausage A sausage is a type of meat product usually made from ground meat—often pork, beef, or poultry—along with salt, spices and other flavourings. Other ingredients, such as grains or breadcrumbs may be included as fillers or extenders. ...

s in the 1870s. Dry pasta and

s have been produced by extrusion since the 1930s, and the method has been applied to pet food production since the 1950s (first extruded dog food: Purina Dog Chow in 1957, and first extruded cat food: Purina Friskies in 1962). Some domestic

kitchen appliance A home appliance, also referred to as a domestic appliance, an electric appliance or a household appliance, is a machine which assists in household functions such as cooking, cleaning and food preservation. Appliances are divided into three ty ...

s such as

meat grinder A meat grinder (also called a "meat mincer" in the UK) is a kitchen appliance for fine chopping ('mincing') and/or mixing of raw or cooked meat, fish, vegetables or similar food. It replaces tools like the mincing knife (which are also use ...

s and some types of pasta makers use extrusion.

Pastry bag A pastry bag (or piping bag in the Commonwealth) is an often cone- or triangular-shaped bag made from cloth, paper, plastic, or the intestinal lining of a lamb, that is squeezed by hand to ''pipe'' semi-solid foods by pressing them through a na ...

s (piping bags), squeezed by hand, operate by extrusion.

Effects

Extrusion enables mass production of food via a continuous, efficient system that ensures uniformity of the final product. This is achieved by controlling various aspects of the extrusion process. It has also enabled the production of new processed food products and "revolutionized many conventional snack manufacturing processes". The extrusion process results in "chemical reactions that occur within the extruder barrel and at the die". Extrusion has the following effects:Riaz, page 82. * Destruction of certain naturally occurring

toxin A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849– ...

s * Reduction of

microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...

s in the final product * Slight increase of

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

bioavailability In pharmacology, bioavailability is a subcategory of absorption and is the fraction (%) of an administered drug that reaches the systemic circulation. By definition, when a medication is administered intravenously, its bioavailability is 100%. Ho ...

* Creation of insulin-desensitizing starches (a potential risk-factor for developing

diabetes Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ap ...

) * Loss of

lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. It contains an α-amino group (which is in the protonated form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −C ...

, an

essential amino acid An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life form ...

necessary for developmental growth and nitrogen management * Simplification of complex

starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human diets ...

es, increasing rates of

tooth decay Tooth decay, also known as cavities or caries, is the breakdown of teeth due to acids produced by bacteria. The cavities may be a number of different colors from yellow to black. Symptoms may include pain and difficulty with eating. Complicatio ...

* Increase of

glycemic index The glycemic (glycaemic) index (GI; ) is a number from 0 to 100 assigned to a food, with pure glucose arbitrarily given the value of 100, which represents the relative rise in the blood glucose level two hours after consuming that food. The GI of ...

of the

processed food Convenience food, also called tertiary processed food, is food that is commercially prepared (often through processing) to optimise ease of consumption. Such food is usually ready to eat without further preparation. It may also be easily por ...

, as the "extrusion process significantly increased the availability of carbohydrates for digestion" * Destruction of

Vitamin A Vitamin A is a fat-soluble vitamin and an essential nutrient for humans. It is a group of organic compounds that includes retinol, retinal (also known as retinaldehyde), retinoic acid, and several provitamin A carotenoids (most notably bet ...

( beta-carotene) * Denaturation of

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

s. The material of which an extrusion die is made can affect the final product. Rough

bronze Bronze is an alloy consisting primarily of copper, commonly with about 12–12.5% tin and often with the addition of other metals (including aluminium, manganese, nickel, or zinc) and sometimes non-metals, such as phosphorus, or metalloids such ...

dies on pasta extruders produce a rougher surface than smooth

stainless steel Stainless steel is an alloy of iron that is resistant to rusting and corrosion. It contains at least 11% chromium and may contain elements such as carbon, other nonmetals and metals to obtain other desired properties. Stainless steel's corros ...

dies, considered to make more liquid pasta sauces adhere better; pasta made this way is labelled "bronze die" pasta to indicate a premium product. The effects of "extrusion cooking on nutritional quality are ambiguous", as extrusion may change carbohydrates, dietary fibre, the protein and amino acid profile, vitamins, and mineral content of the extrudate in a manner that is beneficial or harmful. High-temperature extrusion for a short duration "minimizes losses in vitamins and amino acids". Extrusion enables mass production of some food, and will "denature antinutritional factors", such as destroying

s or killing

s. It may also improve "protein quality and digestibility", and affects the product's shape, texture, colour, and flavour. It may also cause the fragmentation of proteins,

es, and non-starch polysaccharides to create "reactive molecules that may form new linkages not found in nature". This includes

Maillard reaction The Maillard reaction ( ; ) is a chemical reaction between Amino acid, amino acids and Reducing sugar, reducing sugars that gives browned food its distinctive flavor. Searing, Seared steaks, fried dumplings, cookies and other kinds of biscuits, b ...

s which reduce the nutritional value of the proteins. Vitamins with heat

lability Lability refers to something that is constantly undergoing change or is likely to undergo change. Biochemistry In reference to biochemistry, this is an important concept as far as kinetics is concerned in metalloproteins. This can allow for th ...

may be destroyed. , little is known about the stability or bioavailability of

phytochemical Phytochemicals are chemical compounds produced by plants, generally to help them resist fungi, bacteria and plant virus infections, and also consumption by insects and other animals. The name comes . Some phytochemicals have been used as poisons ...

s involved in extrusion. Nutritional quality has been found to improve with moderate conditions (short duration, high moisture, low temperature), whereas a negative effect on nutritional quality of the extrudate occurs with a high temperature (at least 200 °C), low moisture (less than 15%), or improper components in the mix. A 2012 research paper indicates that use of non-traditional cereal flours, such as amaranth,

buckwheat Buckwheat (''Fagopyrum esculentum''), or common buckwheat, is a flowering plant in the knotweed family Polygonaceae cultivated for its grain-like seeds and as a cover crop. The name "buckwheat" is used for several other species, such as '' Fago ...

millet Millets () are a highly varied group of small-seeded grasses, widely grown around the world as cereal crops or grains for fodder and human food. Most species generally referred to as millets belong to the tribe Paniceae, but some millets al ...

, may be used to reduce the

of breakfast cereals produced by extrusion. The extrudate using these cereal flours exhibits a higher bulk and product density, has a similar expansion ratio, and has "a significant reduction in readily digestible carbohydrates and slowly digestible carbohydrates". A 2008 paper states that replacing 5% to 15% of the

wheat flour Wheat flour is a powder made from the grinding of wheat used for human consumption. Wheat varieties are called "soft" or "weak" if gluten content is low, and are called "hard" or "strong" if they have high gluten content. Hard flour, or ''bread ...

and white flour with

dietary fibre Dietary fiber (in British English fibre) or roughage is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fibers are diverse in chemical composition, and can be grouped generally by th ...

in the extrudate breakfast cereal mix significantly reduces "the rate and extent of carbohydrate

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

of the extruded products", which increased the level of slowly digested carbohydrates and reduced the level of quickly digested carbohydrates.

Products

Extrusion has enabled the production of new processed food products and "revolutionized many conventional snack manufacturing processes". Cheese puffs

The various types of food products manufactured by extrusion typically have a high starch content. ''Directly expanded'' types include

s and corn curls, and are made in high temperature, low moisture conditions under high shear. ''Unexpanded'' products include

, which is produced at intermediate moisture (about 40%) and low temperature. ''Texturized'' products include

meat analogue A meat alternative or meat substitute (also called plant-based meat or fake meat, sometimes pejoratively) is a food product made from vegetarian or vegan ingredients, eaten as a replacement for meat. Meat alternatives typically approximate qua ...

s, which are made using plant proteins ("textured vegetable protein") and a long die to "impart a fibrous, meat-like structure to the extrudate", and

fish paste Fish paste is fish which has been chemically broken down by a fermentation process until it reaches the consistency of a soft creamy purée or paste. Alternatively it refers to cooked fish which has been physically broken down by pounding, grin ...

. Confectionery made via extrusion includes

chewing gum Chewing gum is a soft, cohesive substance designed to be chewed without being swallowed. Modern chewing gum is composed of gum base, sweeteners, softeners/ plasticizers, flavors, colors, and, typically, a hard or powdered polyol coating. Its t ...

liquorice Liquorice (British English) or licorice (American English) ( ; also ) is the common name of ''Glycyrrhiza glabra'', a flowering plant of the bean family Fabaceae, from the root of which a sweet, aromatic flavouring can be extracted. The liqu ...

, and

toffee Toffee is a confection made by caramelizing sugar or molasses (creating inverted sugar) along with butter, and occasionally flour. The mixture is heated until its temperature reaches the hard crack stage of . While being prepared, toffee is ...

. Some

processed cheese Processed cheese (also known as process cheese, cheese food, prepared cheese, cheese product, or plastic cheese) is a food product made from cheese and unfermented dairy ingredients mixed with emulsifiers. Additional ingredients, such as vegeta ...

s and cheese analogues are also made by extrusion. Processed cheeses extruded with low moisture and temperature "might be better suited for manufacturing using extrusion technology" than those at high moisture or temperature. Lower moisture cheeses are firmer and chewier, and

cheddar cheese Cheddar cheese (or simply cheddar) is a natural cheese that is relatively hard, off-white (or orange if colourings such as annatto are added), and sometimes sharp-tasting. Cheddar originates from the English village of Cheddar in Somerset. Ched ...

with low moisture and an extrusion temperature of 80 °C was preferred by subjects in a study to other extruded cheddar cheese produced under different conditions. An extrudate mean residence time of about 100 seconds can produce "processed cheeses or cheese analogues of varying texture (spreadable to sliceable)". Other food products often produced by extrusion include some

s (

s, bread sticks, and

s), various ready-to-eat

s, pre-made

, some

s, some

s, and dry and semi-moist

s. Specific examples include cheese curls, macaroni,

Fig Newtons Newtons are a Nabisco-trademarked version of a pastry filled with sweet fruit paste. "Fig Newtons" are the most popular variety (fig rolls filled with fig paste). They are produced by an extrusion process. Their distinctive shape is a character ...

jelly bean Jelly beans are small bean shaped sugar candies with soft candy shells and thick gel interiors (see gelatin and jelly). The confection is primarily made of sugar and sold in a wide variety of colors and flavors. History It has been clai ...

sevai Sevai (Hindi : सेवई), shavige (Kannada: ಶಾವಿಗೆ), saemia and santhakai (Tamil: சந்தகை) is a type of rice vermicelli popular in India. While typically made from rice, varieties made out of other food grains like ...

, and some french fries. Extrusion is also used to modify starch and to pellet animal feed.

Process

History

Effects

Products

See also

References

Further reading