Extrusion Extrusion is a process used to create objects of a fixed cross section (geometry), cross-sectional profile by pushing material through a Die (manufacturing), die of the desired cross-section. Its two main advantages over other manufacturing pro ...

food processing Food processing is the transformation of agricultural products into food, or of one form of food into other forms. Food processing takes many forms, from grinding grain into raw flour, home cooking, and complex industrial methods used in the mak ...

consists of forcing soft mixed ingredients through an opening in a perforated plate or die 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. In some cases, "extrusion" is taken as synonymous with extrusion cooking, which cooks the food with heat as it is squeezed through the die. Extrusion enables

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

of food via a continuous, efficient system that ensures uniformity of the final product. Products made through extrusion (without simultaneous cooking) include

pasta Pasta (, ; ) is a type of food typically made from an Leavening agent, unleavened dough of wheat flour mixed with water or Eggs as food, eggs, and formed into sheets or other shapes, then cooked by boiling or baking. Pasta was originally on ...

bread Bread is a baked food product made from water, flour, and often yeast. It is a staple food across the world, particularly in Europe and the Middle East. Throughout recorded history and around the world, it has been an important part of many cu ...

s ( croutons, bread sticks, and flat breads), pre-made cookie dough, and

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. Products made through extrusion cooking include many

breakfast cereal Breakfast cereal is a category of food, including food products, made from food processing, processed cereal, cereal grains, that are eaten as part of breakfast or as a snack food, primarily in Western societies. Although warm, cooked cereals li ...

s and ready-to-eat

snack A snack is a small portion of Human food, food generally Eating, eaten between meals. Snacks come in a variety of forms including Food packaging, packaged snack foods and other processed foods, as well as items made from fresh ingredients at ho ...

confectionery Confectionery is the Art (skill), art of making confections, or sweet foods. Confections are items that are rich in sugar and carbohydrates, although exact definitions are difficult. In general, however, confections are divided into two bro ...

, some baby foods, full-fat soy flour, textured vegetable protein, 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, smoothie ...

s, and dry and semi-moist pet foods. Food products manufactured using extrusion usually have a high starch content.

Process

Extrusion cooking

In the extrusion cooking 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 Glucose is a sugar with the Chemical formula#Molecular formula, molecul ...

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 specif ...

s, dyes,

meat Meat is animal Tissue (biology), tissue, often muscle, that is eaten as food. Humans have hunted and farmed other animals for meat since prehistory. The Neolithic Revolution allowed the domestication of vertebrates, including chickens, sheep, ...

s or water.

Steam Steam is water vapor, often mixed with air or an aerosol of liquid water droplets. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization. Saturated or superheated steam is inv ...

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 eve ...

generated (10–20 bar). The process can induce both protein denaturation and starch gelatinization 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 Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

, 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 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'', ...

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

s in the 1870s. Dry pasta has been produced by extrusion since the 1930s, and the method has been applied to

tater tots Tater tots are grated potato formed into small cylinders and deep-fried, often served as a side dish. "Tater" is a dialect form of the word ''potato''. The name "tater tot" is a registered trademark of the American frozen food company Ore-Ida, b ...

(first extruded potato product: Ore-Ida in 1953). Some domestic kitchen appliances such as meat grinders and some types of pasta makers use extrusion. Pastry bags (piping bags), squeezed by hand, operate by extrusion. The first extrusion cooking machine was the expanding pelleting machine from Wenger Mixer Manufacturing from 1954. Its first mentioned use seems to be with Purina in 1957, which developed extruded food for dogs, monkeys, and fish. In 1963, the USDA and UNICEF tested a full fat soy flour produced from extrusion-cooked soybeans as a source of nutrients for children. Milk substitutes were later developed from this flour. In 1966, the US government started providing a CSM (Corn-Soya-Milk) formula to protein-deficient children in the Third World. The later Meals for Millions project also prominently featured soy flour in its Multi-Purpose Food (MPF), a high-protein food supplement that could be made for just three cents per meal. The idea of using extrusion cooking to produce

has been mentioned since the Wegner patent of 1960. In 1970, the Israeli Shefa Protein Industries introduced a line of breakfast cereal called Krunch, made from cereal flour and full-fat soy flour. It's unclear whether there has been an earlier breakfast cereal made from extruded products. Meat analogues have been made through extrusion since 1969.

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".

Chemical changes with cooking

Extrusion cooking results in "chemical reactions that occur within the extruder barrel and at the die" like most other forms of

cooking Cooking, also known as cookery or professionally as the culinary arts, is the art, science and craft of using heat to make food more palatable, digestible, nutritious, or Food safety, safe. Cooking techniques and ingredients vary widely, from ...

. Extrusion enables mass production of some food, and will "denature antinutritional factors" while destroying

toxin A toxin is a naturally occurring poison produced by metabolic activities of living cells or organisms. They occur especially as proteins, often conjugated. The term was first used by organic chemist Ludwig Brieger (1849–1919), derived ...

s or killing

microorganism A microorganism, or microbe, is an organism of microscopic scale, microscopic size, which may exist in its unicellular organism, single-celled form or as a Colony (biology)#Microbial colonies, colony of cells. The possible existence of unseen ...

s. It may also improve protein quality and digestibility and affects the product's shape, texture, colour, and flavour. Changes associated with extrusion include: * Destruction of certain naturally occurring

s and antinutrients (including trypsin inhibitors, haemagglutinins,

tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and Precipitation (chemistry), precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' is widel ...

s and phytates) ** All four listed antinutrients reduce the absorption of protein. Phytate and tannins also reduce the absorption of minerals. * Reduction in the level of

s in the final product. * Partial destruction of heat-liable vitamins (A, B, C, and E). * Moderate increase in

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residue (biochemistry), residues. Proteins perform a vast array of functions within organisms, including Enzyme catalysis, catalysing metab ...

digestibility, due to protein protein denaturation and the inactivation of antinutrients. * Maillard reactions, which reduce the available amounts of certain amino acids, including the

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 forms ...

lysine Lysine (symbol Lys or K) is an α-amino acid that is a precursor to many proteins. Lysine contains an α-amino group (which is in the protonated form when the lysine is dissolved in water at physiological pH), an α-carboxylic acid group ( ...

. Lysine loss can be reduced by using wetter mixtures. * Breakdown of complex carbohydrates (

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 diet ...

es and non-starch polysaccharides) into simpler components. Part of this action is caused by amylase from the cereal themselves. ** This increases

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 ...

and creates starches more likely to cause and insulin resistance. The "extrusion process significantly increased the availability of carbohydrates for digestion". ** This may also lead to higher tooth decay. ** On the other hand, this breakdown converts insoluble fibers into soluble fibers. * Binding and volatization of flavor compounds. * Gelatinization of starch.Riaz, page 82. "Extrusion cooking has been defined as "the process by which moistened, expansile, starchy, and/or proteinaceous materials are plasticized in a tube by a combination of moisture, pressure, heat and mechanical shear. This results in elevated product temperatures within the tube, gelatinization of starchy components, denaturization of proteins, the stretching or restructuring of tractile components, and the exothermic expansion of the extrudate" * An increase in iron content due to the wearing of machine components. No significant change in zinc absorption. * , little is known about the stability or bioavailability of phytochemicals involved in extrusion. Phenols appear to be decreased. Overall, 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. 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. High-temperature extrusion for a short duration "minimizes losses in vitamins and amino acids". A 2012 research paper indicates that use of non-traditional cereal flours, such as

amaranth ''Amaranthus'' is a cosmopolitan distribution, cosmopolitan group of more than 50 species which make up the genus of annual plant, annual or short-lived perennial plants collectively known as amaranths. Some names include "prostrate pigweed" an ...

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. Buckwheat originated around the 6th millennium BCE in the region of what ...

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 millets belong to the tribe Paniceae. Millets are important crops in the Semi-arid climate, ...

, 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 and white flour with dietary fibre 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.

Texture

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 metalloid ...

dies on pasta extruders produce a rougher surface than smooth

stainless steel Stainless steel, also known as inox, corrosion-resistant steel (CRES), or rustless steel, is an iron-based alloy that contains chromium, making it resistant to rust and corrosion. Stainless steel's resistance to corrosion comes from its chromi ...

dies, considered to make more liquid pasta sauces adhere better; pasta made this way is labelled "bronze die" pasta to indicate a premium product. File:Elbow macaroni die front.jpg, alt=White plastic disk with five ring-shaped holes in it, Front of a die for making elbow macaroni File:Elbow macaroni die back.jpg, Back of the same plastic die File:Vintage Pasta Dies 002.tif, alt=Four corroded metal dies are shown on the left, and their other sides on the right. The bottom side has small holes and the top sides have larger ones. Each die has 40 to 50 matching holes in it. Each die is slightly different, to produce different sizes of spaghetti or similar pasta noodles., Old metal dies for pasta, showing bottom and top of the dies

Products

Extrusion has enabled the production of new processed food products and "revolutionized many conventional snack manufacturing processes". 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 analogues, 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. File:Cheetos (14225295386).jpg, Cheese curls, a directly expanded snack food File:HK food ingredient texture of Alberto Poiatti Italian brand 直通粉 penne rigate yellow September 2021 SS2 04.jpg, Pasta, an unexpanded food File:Beyond Burger 1.jpg, Meat alternative, a texturized food Some processed cheeses 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. It originates from the English village of Cheddar, Somerset, Cheddar in ...

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)". 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 tex ...

liquorice Liquorice ( Commonwealth English) or licorice (American English; see spelling differences; ) is the common name of ''Glycyrrhiza glabra'', a flowering plant of the bean family Fabaceae, from the root of which a sweet, aromatic flavouring is ...

, and toffee. Other food products often produced by extrusion include some

s ( croutons, bread sticks, and flat breads), various ready-to-eat

s, pre-made cookie dough, some baby foods, some

s, and dry and semi-moist pet foods. Specific examples include cheese curls, macaroni, Fig Newtons, jelly beans, sevai, and some french fries. Extrusion is also used to modify starch and to pellet

animal feed Animal feed is food given to domestic animals, especially livestock, in the course of animal husbandry. There are two basic types: fodder and forage. Used alone, the word ''feed'' more often refers to fodder. Animal feed is an important input ...