The Preparation Of Extruded Foods

Extruded foods. Snacks of assorted shapes and sizes produced by extrusion.
Photo by lightwise, c/o www.123-rf.com

Extrusion is now a standard processing method for preparing a wide variety of consumer products.  Extruded foods are one particular application of this particular technology and there is plenty of business opportunities for anyone developing these type of products. The technology is used to produce snacks, breakfast cereals, ready-to-eat foods, a host of starch-based crispy foods and pasta.

Extruded Foods: What We Like About Them!

Extruded fabricated foods are composed mostly of cereals and starches in particular. In a number of cases protein from both plants and meat (to some extent) are incorporated. Consumer acceptance of these foods is based on the role of these ingredients in producing food structure, mouthfeel, texture, bulk and a host of other characteristics that define the specification of the finished extruded product (Launay & Lisch, 1983).

The consumer acceptance of extruded foods is based on their nutritional value, convenience, attractiveness, texture and range of flavours. This is especially true of snack foods which are highly dependent on the extrusion cooking process (Anton & Luciano, 2007).

Types of Food Produced

(1) Cereal-based foods. The list is endless including expanded snack foods, ready-to-eat and puffed breakfast cereals, soup and beverage bases, instant drinks, weaning foods, modified and pre-gelatinized starches and dextrins, pasta foods, crispbreads and croutons.

(2) The sugar-based foods. These include liquorice, chewing gum, caramels, confectionary, fruit gums and  nut brittles.

(3) Protein-based foods. The production of texturized vegetable protein (TVP), semi-moist and expanded petfoods, animal feeds and protein, supplements, sausages such as hot dogs and frankfurters, surimi, processed cheeses and caseinate based foods.

The Extrusion Process

Extrusion as a process involves a number of unit operations which include kneading, shearing, mixing, cooking to varying degrees, shaping and forming. We discuss the fundamentals of extrusion in a related article which discusses the physical processes occurring during extrusion.

The principles of operation in extrusion are similar in all types: raw materials are fed into an extruder barrel and the screw (single-screw) or screws (twin-screw) conveys the food along it. Further down the barrel, smaller flights restrict the volume and increase the resistance to movement of the food. As a result, the mixture  fills the barrel and the spaces between the screw flights which becomes compressed. As it moves further along the barrel, the screw kneads the material into a semi-solid, plasticized mass. This becomes the extrudate.  

Extrusion Processes: Cold Versus Hot Extrusion

There are two categories of extruded foods based on the temperature of processing – we have extrusion cooking or cold extrusion processing. 

Extrusion cooking

Extrusion cooking is the most common and probably most extensively researched process method. All food extruders of this type work on two basic principles. They modify a food by delivering specific mechanical energy using a main drive motor and where extrusion cooking is concerned, specific thermal energy using direct steam and water injection during processing (Riaz, 2000).

Extrusion cooking is usually a  high shear extrusion which is also HTST (high temperature – short time) process where high heat is needed for a short time. In the extrusion process, the food must be heated above 100ºC when it enters the extruder barrel. Here, frictional heat and any additional heating that is used cause the temperature to rise rapidly. The food is then passed to the section of the barrel having the smallest flights, where pressure and shearing is further increased. 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 as moisture is flashed off as. 

This is used to produce ready-to-eat products and involves either a single or twin screw system. Typical examples of foods using this process include expanded snacks, flat bread and flakes, various breakfast cereals, pillow snacks, certain types of meat analogues (TVP), snack pellets or collets, even rice and dal analogues.

For the product developer, extrusion cooking alters dietary fiber, amino acid and protein profiles, vitamin contents and various other nutrients. Generally this can be a benefit as well as an issue for the food (Singh et al., 2007). Indeed when it comes to nutritional quality, extrusion cooking can have ambiguous effects. The positive benefits of extrusion cooking come from destroying or inactivating undesirable compounds and increasing their digestability. When it comes to vitamins, none will escape some form of destruction because of both heat and mechanical forces employed. The level of degradation depends on the particular vitamin and the process parameters of cooking.

In many instances the majority of ready-to eat snack foods are produced using extrusion. They are generally high energy but lack the nutritional value (Brennan et al., 2013) and some examples are presented below.

Cold Extrusion (Low-Shear Extrusion) 

Foods using low shear extrusion or a cold process are mainly for ready-to-cook products. It is not widely applied and is a process that uses room temperature or temperatures that are slightly elevated. There is no cooking involved so temperatures are below 100 Centigrade. It is commonly used in the metal extrusion technologies.

A single screw extrusion process is often employed where foods of this type of manufactured. Many pastas, usually the hollow-tube types are prepared this way including macaroni, plain noodles, vermicelli and penne. We also find liquorice, meat foods, surimi, pet food and fish pastes prepared this way.

The Expansion Ratio (ER)/Expanion Volume

The expansion ratio (ER) or the equivalent measure of expansion volume is a measure of how much the volume of the extruded product has increased compared to the volume of the same amount of starting material. It is often calculated as the cross-sectional diameter of the extrudate divided by the diameter of the die opening (Alvarez-Martinez et al., 1988; Ding et al., 2005).

Most analytical methods rely on using calipers and photographic evidence to measure various aspect ratios.

The expansion ratio is an important functional quality parameter especially of a food’s texture and mouthfeel. It is associated with product crispiness, water absorption, water solubility and crunchiness (Vickers, 1988) in the sensory sense. It is perhaps the one factor that dictates the success of how the consumer perceives the quality of a puffed extruded snack (Owusu-Ansah et al., 1984).

Radial expansion is a good measure of extrudate expansion although expansion occurs in both a longitudinal and lateral direction during extrusion cooking (Launay & Lisch, 1983). [Corn grits were used in this study on expansion].

Breaking Force

The breaking force (kN) is that required to fracture the extrudates. It helps producers decide how strong the final product might be. A typical instrument for this type of study would be a    Lloyd LRX/plus Universal Testing Machine usinga 2.5 kN load cell. 

Pieces of extrudate are randomly chosen, cut to about 8cm in length and placed across the bottom of a Kramer Type Shear Cell. The samples are compressed perpendicular to the length of the extrudate at a compression speed of say 60 mm/min. The compression fore is recorded and the observed peak force taken as the breaking force of the samples. 

Bulk Or Product Density

A parameter closely related to the expansion ratio. Both parameters are used to explain the degree of puffing. As the expansion ratio rises the bulk density of the product is meant to drop. A low bulk density is a desirable property as is a high expansion ratio. At high moisture levels, the bulk density is also high. This occurs when the extrusion cooking has not been high enough to cause vapourization of the moisture so puffing is reduced. A denser product is generated (Asare et al., 2004).

The other parameter to consider is hardness – the lower the better from a sensory point of view.

Advantages Of Food Extrusion

Extrusion technology offers many advantages over the more traditional methods of preparing breakfast cereals and snack foods. These are:-

(1) A wide range of product characteristics. It is possible to generate foodstuffs of various shapes, textures, colour and appearance which would not be possible using other methods.

(2) Creation of shelf-stable foods. Whatever the process of extrusion, the water activity of the product is usually extremely low. It will be between 0.1 and 0.4. These are extremely dry foods and generally microbially stable. If the food is semi-moist and thus having a higher water activity, a protective packaging is needed to maintain stability.

(3) Extrusion is highly adaptable. Changes in ingredient types, process parameters and the pre-extrusion and post-extrusion conditions means a large range of foods are possible. They can also be tailored to consumer demand.

(4) Energy efficiency is possible. Extruders work at relatively low moisture levels when cooking their food, so minimal drying is needed.

(5) Minimal wastage. The technology is used in such a way that process effluent is kept to a minimum.

(6) Extrusion is often the low-cost option. Extrusion generally costs less to do that cooking and other forming processes. It is estimated that there is just under a 20% saving on raw materials, a 14% reduction in labour and capital investment is about 44% less (Darrington, 1987).

(7) Extruders take up less space than other pieces of equipment than other types of cooking processes.

(8) Extrusion cooking destroys heat-labile toxins and microorganisms

(9) Iron bioavailability is raised to a certain degree.

(10) An improved glycemic index.

Extruded Food Example (1): Using Low Shear Extrusion/Cold Extrusion To Produce A Pasta

A dough made of semolina flour and durum wheat flour is produced. The dough mix is passed through a cold extruder. The pasta is shaped using various die. It is dried, cooled and then packaged.

Extruded Food Example (2): Using HTST High Shear Extrusion To Produce A Snack Food

Various flours (maize, rice, wheat, oat, BG flour) and salt is weighed and blended. There is a usually a pre-conditioning step followed by high shear extrusion. The extrusion process relies on a mix of energy from both thermal and mechanical pressure. The snack is cut using a rotary cutting device, dried and then flavours and oils added. The product is packed appropriately.

The process parameters for a HTST extrusion process are:-

  • moisture content of 10 to 20 per cent weight
  • temperature of processing between 150 and 200 Centigrade
  • retention time of 1 minute

The product bulk density can be anywhere between 30 and 600 g/litre.

The biochemical mechanisms involve mainly starch gelatinization, protein denaturation to denature enzymes in particular and microbial sterilization.

Snack Foods Produced By Extrusion

Snack producers use various specific methods to manufacture their product but there are three main types of snack.

(1) First generation snacks. These are very simply extruded snacks and include popcorn, potato chips. Other 1st generation snacks would also include nuts which are not extruded foods.

(2) Second generation snacks. These are the expanded snacks and most fall into this category. They include single ingredient snacks, simple shaped products including puffed corn curls and corn tortilla chips.

(3) The third generation snacks. These are pellets or par-baked or semi-snack foods. These are often composed of many ingredients. They are prepared by extrusion cooking using low pressure to prevent unwanted excessive expansion. They are dried to a final moisture content of about 10 per cent to a form a glassy pellet.

Preparation Of Extruded Foods

Corn curls are the original direct expanded extruded snack food (Riaz, 2006). There are two types of corn curls: baked and fried.  

(A) Fried Collets

Fried collets are the most common extruded snacks in the market place. A special die arrangement produces a product with a twisted and puffed up shape.

Moistened corn meal is the ideal ingredient for this type of snack although other cereal grains have been used. Collets like these are prepared using specific extruders known as collet extruders. The selection of grains other than corn is limited by the narrower capabilities of a collet extruder.

What is atypical of this type of extrusion process is that the collet extruder only concerns the pumping of the corn meal through the die. Most of the cooking process occurs in the die assembly which is unique. The assembly itself involves a rotor and a stator. The corn meal passing through the die is cooked where it is heated to around 180°C.

The main things happening in this special die are:

  • the corn meal is subjected to high shear rates and pressure. These physical conditions create most of the heat which cooks the corn.
  • a rapid pressure loss causes the superheated water in the corn mass to turn to steam. Loss of steam puffs up the cooked corn.
  • the flow of corn between one rotating plate and the stationary plate twists the expanding corn leaving it twisted and collapsed in places. The effect produces the standard shape and texture.

The extruded products are fried in vegetable oil and then coated in cheese or other flavours. During frying, the moisture level in the product reduces from 8 per cent down to  a level of between 1 and 2 per cent by weight which influences both stability and ultimately texture. During frying, the corn collet picks up 20 to 25% oil. Following frying the collets are dried.

A Cheesy Puff for example is produced from a fried corn curl. The mixture that goes into an extruder will be 96% corn meal, 4% water. The moisture content is between 13 and 16%.

Moisture control is a major influence on quality. If the collet is puffing up to much or the texture is too soft, then the moisture level can be raised to reduce viscous dissipation which then reduces temperature and starch dextrinization. In another case, the stator-rotor gap can be made smaller to increase the shear rate to which the corn meal is exposed. This generates more heat with greater starch breakdown which the corn meal is exposed to. Corn meal granulation also has an impact on collet texture and cell size.

(B) Baked Collets

As well as fried collets, baked collets are a typical extruded snack food. Classic examples include onion rings, potato sticks and baked corn curls. The ingredients used include various cereal grains and flours from roots and tubers like potato. Healthy snacks are often generated exploiting the protein, fibre, bran and cellulose that come from many other ingredients which are mixed with cereal grains up to at least 20 per cent (by weight). A potato stick is made using potato flour and corn or corn flour.

The Impact Of Extrusion Processing Parameters On The Quality Of Food

The expansion of an extrudate depends on the pressure difference between the die and the atmosphere. It also depends on the ability of the exiting product to sustain expansion and that relates directly to the viscosity of the feed material, to the initial moisture content and various processing parameters such as temperature, barrel speed etc. All these have been examined to varying levels.

Expansion of a food depends on temperature 

There is no expansion if steam cannot be generated to produce puffing. That implies that the temperature must be at least 100 ºC. Expansion is a highly desirable characteristic.

Expansion itself increases with an increase in temperature especially when the moisture content of the extrusion mix is close to 20%. That level is optimal for reducing viscosity sufficiently to allow for a rapid expansion of the molten mass and/or an increase in water vapour pressure.

Expansion happens in both radial and axial directions, to differing degrees. It all depends on the viscoelastic properties of melt. 

The reduction or even loss of expansion at very high temperatures is due to an increase in dextrinization which subsequently weakens the starch structure. Dextrinization is the process where starches simply breakdown into their constituent parts, especially dextrins.

We mentioned earlier a study by Launay and Lisch (1983) on expansion of corn grits. In that study overall and radial expansion increased with both screw speed and temperature but axial expansion decreased.

The removal of moisture through vapourization and cooling of the extrudate serve to change the product from a molten to rubbery state. Further drying creates a brittle food which is also capable of fracturing.

Expansion of Extrudates And Feed Moisture Content

Feed moisture content has a significant effect on expansion of snacks. Expansion ratios tend to decrease inversely as the feed moisture increases irrespective of barrel temperature and screw speed.

An example of this phenomenon was examined in a purple rice and soybean flour snack (Suksomboon et al., 2010). In their example, increasing the feed moisture from 15g/100g to 19g/100g produced a decrease in expansion ratio in the range of 7.7 to 10.9% with a constant barrel temperature and screw speed of 350rpm. The researchers reasoned that the effect could be put down to the elasticity of the dough and the following plasticization of the melt. Increasing moisture content produced a a reduction in starch gelatinization (seen as a change in plasticization) and thus a decrease in expansion (Ding et al., 2006).

A high feed moisture usually offers less viscosity than one at lower feed moisture content. The pressure difference is higher for a low feed moisture content which produces a higher expansion of extrudate (Harper, 1981; Suknark et al., 1997).

Increasing the feed moisture content reduces the friction between the feed material, the screw and the barrel (Liu et al., 2000). The specific mechanical energy is reduced and this too probably has a bearing on the expansion ratio but that is not so well defined.

Bubble growth

The primary factor influencing expansion is bubble growth and it in turn is affected by those factors such viscosity, starch gelatinization etc. It affects both longitudinal and radial direction during the extrusion process. Bubbles are able to grow through flash evaporation when the extrudate leaves the die where the high pressure of superheated steam generated by moisture at nuclei overcomes the mechanical resistance of the viscoelastic melt.

Sectional expansion index considers expansion only in the radial direction, while bulk density considers expansion in all directions. Several research groups believe that the expansion ratio of the extruded foods depends on the degree of starch gelatinisation. 

The Effect Of Extrusion Conditions On The Physical And  Sensory Characteristics Of Extruded Foods.

Basic snacks are good models for assessing the effect of extrusion on the physicochemical and sensory properties of a process.

A major sensory attribute is crunchiness which is associated with expansion ratio or volume but as we’ve already referenced, that in turn depends on microstructure which relates directly back to starch gelatinization. This is discussed more comprehensively in the next section.

One comprehensive study looked at the production of a simple rice-based snack (Ding et al., 2005). In this study the variables examined were feed rate (20–32%), feed moisture content (14–22%), screw speed (180–320 rpm), and barrel temperature (100–140 °C). The physical parameters affected were the density, expansion, water absorption index (WAI), and water solubility index (WSI). The sensory measures were textural – hardness and crispness. The key findings were:-

  • Screw speed had really no bearing on any of the parameters examined !
  • Increasing feed rate produced extrudates with a higher expansion ratio, a lower water solubility index and higher hardness.
  • Increasing feed moisture content resulted in extrudates with a higher density, lower expansion, higher WAI, lower WSI, higher hardness and lower crispness. 
  • Higher barrel temperature increased the extrudate expansion but reduced density, increased the WSI and crispness of extrudate.

An almost repeat study was conducted with wheat-based snacks (Ding et al., 2006) with the same range in process variables and measures. The same results were obtained except in the case of the wheat snack the puncture energy rather than crispness was measured. The main difference appears to be with the WAI and WSI values. They were reversed when increasing feed moisture was raised.

A chickpea flour-based snack was examined in a similar fashion too. Lowering the feed moisture from 18% to 16%w/w, raising the screw speed from 250 rpm to 320 rpm, and having a barrel temperature between 160 and 170 C produced a higher expansion ratio, lower bulk density and hardness (Meng et al., 2010).

A recent study with a red lentil snack shows different effects (Luo et al., 2020). Red lentils have a higher fibre and protein content than the two preceding rice and wheat examples. In their example an increase in screw speed from 150 rpm to 200 rpm produced an large increase in expansion. However, like the rice and wheat snacks the expansion and crispness were reduced as the feed moisture increased (from 18% w/w to 22% w/w).

Issues With Producing Extruded Foods

Cereal products in a variety of shapes are produced using extrusion. They can be formed using direct expansion or for puffing in a later process step. When designing extrusion dies for product shaping, a number of effects produce distortion of the extruded shape. Puffing produced by whatever method that is suitable always tends to produce the most rounded shape possible. A sphere is the most natural product to be formed. the flat surfaces of extruded pellets are weaker than sharply curved surfaces which allows them to bulge outwards during inflation.

An additional feature is that elasticity of the flowing cereal mass within the die causes the product shape to rebound when the applied stresses in viscous flow are relieved on emergence from the extruder. The phenomenon of elastic rebound causes the product thickness and trailing cut-off surfaces to swell which adds further to product rounding even before expansion starts.

Shape distorting factors can be altered. The application of slippery die surfaces or heat transfer in particular sections of the die are possible. Miller (1991) describes various mechanisms of distortion. These include:-

  • velocity distribution in the die ——affects product thickness
  • viscosity variations———————affect velocity
  • thermal effects————————-affect viscosity
  • heat transfer—————————-causes temperature variation
  • viscous dissipation——————— friction generates surface heat
  • pseudoplasticity———————— flattens velocity distribution
  • end and edge effects——————-drag reduces velocity at die surfaces
  • pressure variations——————–  affect velocity profile
  • viscous swell—————————- die-face cut emerging stream spreads outward
  • Slip—————————————  separation of product from die surface reduces end/edge effects
  • Elasticity—-                                    Product shape rebounds from stressed state in die.
  • Elasticity swell ————————- Cross -section enlarges from compressed state.
  • Elastic rebound————————- Continuous extruded strand outer layers stretch to cancel flow velocity profile in die.
  • Puffing                                             Tends to round-out all extruded shapes.

Extrusion Effects On Vitamins

Extrusion cooking has generally degradative effects on vitamins (Killeit, 1994; Camire, 1998). Vitamin stability varies with vitamin structure, extrusion conditions, and food matrix composition.

Vitamin B1 (Thiamin) is heat sensitive and even though it is added to cereals it must be at a high enough level before extrusion where nearly 50% might be loss in the process.

The Properties Of The Raw Materials And The Effect On Extrusion

As in all processing the quality of the raw materials, how they are treated and prepared has a profound influence on extrusion. Essentially it comes down to the quality of the starch but associated with this at the beginning of the process is the level of lipids, fibre and proteins. All these additional components affect expansion as well as the other functional properties.

A number of quality parameters have been examined but one clear factor is the starch amylose-amylopectin ratio which affects extrusion most of all. It appears that the amylose content is the leading component in the amylose-amylopectin mix.

Mercier and Feillet (1975) found starches with a low or waxy content expanded best at a relatively low extrusion temperature of 135ºC. A higher amylose content was shown to expand better at higher extrusion temperatures and levels of 225ºC were appropriate. 

Bhuiyan and Blanshard (1982) found corn flours and grits with a 35% amylose content at least expanded extremely well. These researchers found that the higher the amylose content the better the level of expansion. Waxy corn starch expands better than normal corn starch according to studies by Bhattacharya and Hanna (1988).

Chinnaswamy and Hanna looked at the amylose contents of various corn starches and found that a 50% amylose starch expanded best of all (Chinnaswamy & Hanna, 1988a,b).

The expansion volume using cereals and their starches falls as the amount of protein increases in the feedstock.

The impact of lipids in the feedstock is important. Bhuiyan and Blanshard (1982) found in their grits and corn meal study the following:

  • the poorest expansion occurred with corn meal containing relatively high lipid contents.
  • the contradictory element was that for a maize grit a slightly higher level of lipid produced the best expansion
  • the fatty acid composition of the extracted lipids for the various corn meals and grits were not significantly different. In this case the endogenous lipids were not as significant to expansion as amylose content.  

Starch Gelatinization During Extrusion

In the early years of studying extrusion, considerable attention was paid to starch. After all it is the main carbohydrate component in the feed to an extruder. Whilst gelatinization was an important phenomenon to be examined, most cereal chemists attributed the properties of the extrudate (physical and chemical) to the extent of starch degradation.

In the early years studying extrusion cooking of corn at the end of the 1960s and very early 70s, researchers thought that low-moisture extrusion allowed for high temperatures and shear rates. This enhanced degradation of starch with the formation of dextrins (Conway et al., 1968; Anderson et al., 1969; Conway, 1971a&b).

A few years later Mercier and Feillet (1975) looked at the extrusion of various starches and found that the amount of soluble starch increases as feed moisture decreased. No maltodextrins were formed.

Starch gelatinization is an important chemical process which specifically affects the behaviour of this particular polymer and has enormous bearing on extruded food production.  Gelatinization is also a complex process and has been discussed widely on this web-site at various times. What is so special about extrusion is it is one means of causing gelatinization. It provides both heat and shear in the presence of moisture to cause release of starch polymers from the granule and then modify them.

At the start of extrusion, starch granules are swelling as water enters the granule. The process of gelatinization begins – it may of course be before extrusion by occurring as a pre-gelatinization phase in the starting mixture. However as the starch mixture progresses down the barrel, heat and shear eventually produces a melt called thermoplastic starch (TPS).

Too much heat and shear destroys the gelatinized starch – a situation called thermo-mechanical degradation). This is often the case in processing of starch as low moisture contents. The products are dextrins and in the more extreme cases smaller carbohydrate oligomers and even sugar (Gomez & Aguilera, 1983, 1984). As we mentioned earlier this is the situation that earlier researchers were contemplating. 

To improve matters, the starch granules can be modified so that when they are disrupted, the gelatinized starch is then mixed with a plasticizer. This reduces the melting temperature and improves processability but also the quality of the extrudate. This new material is thermoplastic starch (TPS).

Plasticizers such as water are used but it appears there is a dependency of the final product and its properties on the humidity in the environment. Glycerol, various polyols, nitrogen compounds have also been tried but glycerol is used most often.  

The strength of the starch gel generated is based on the micellar network of starch molecules. This is dependent on the degree of association and molecular arrangement of these starch molecules. The strength of a starch gel is also dependent on how water behaves as the starch molecules interact.

Most of our knowledge to date about starch gelatinization is still based on the heating of dilute starch solutions with no shear applied. This is a far cry from what normally happens in an extruder.

Lets consider gelatinization in relation to extrusion in detail:- 

The process of gelatinization starts as the more accessible and amorphous intermicellular areas of the granule open up because bonding in this area is at its weakest. No starch is the same. The degree of association in these amorphous regions is different for each type of starch and is dependent on the species from which it is derived.

When granules gelatinize they do so over a range of temperatures rather than a single one and so this has a great bearing on foods produced by extrusion.

An amylose-lipid complex is said to form during extrusion. Its formation is claimed to be a key component in the flow behaviour of pastes and in the rupture strength of extrudates (Launay & Lisch, 1983).

The kinetics of starch gelatinization during extrusion cooking has been extensively explored (Lawton et al., 1972; Harper, 1981; Bhattacharya et al., 1986; Bhattacharya & Hanna,1987). These studies have examined starch gelatinization as a function of time, shear-rate, temperature of cooking and moisture content. Corn extrusion is a good model for observing the changes in starch.

Lawton et al., (1972) noted that the moisture content of the starting material and the barrel temperature of the extruder had the biggest effect on gelatinization of corn starch. A study by Chaing and Johnson (1977) then examined various extrusion variables on gelatinization of starch in wheat flour. When the temperature rose above 80°C, gelatinization sharply increased. They also noticed that a higher moisture content in the starting sample produced a higher degree of gelatinization but this effect was less pronounced than that due to a rise in temperature. Increasing the screw speed produced lower levels of gelatinization because the residence time in the unit was less.

Gomez and Aguilera (1983, 1984) showed that decreasing the moisture content produced a higher degree of gelatinization for both corn starch and white corn. The reduced extrusion moisture content produced a change from gelatinized to dextrin-like properties in the extruded product.

Bhattacharya et al., (1986) at the University of Nebraska established that the textural characteristics of of extruded corn samples depended on their degree of cooking. As the degree of cooking rises they noticed the puff ratio rose and they had higher water holding capacities although their shear strengths started dropping. That study showed that a higher level of cooking implied a higher degree of starch gelatinization.

McPherson et al., (2000) realised that the viscosity of extruded corn starches increases as the starch moisture content increases but in a range from 30% to 40% MC. Samples extruded with 30% moisture content showed a slight increase in viscosity with an increase in temperature from 60 to 80 C. From 80 to 100 C, there was no further change in viscosity. These researchers believe the difference is due to starch with a lower moisture content having a higher glass transition temperature. When starch is extruded at higher temperatures, the starch becomes more rubbery resulting in less friction and less degradation.

Corn and wheat starch have all the necessary features for producing highly acceptable extruded snack foods but they do not have the sort of appeal in terms of nutritional value. Health-conscious consumers do not find extruded foods that exciting (Rampersad et al., 2003).

The Effect Of Extrusion On Fiber Quality

The functional quality as well as the nutritional quality of foods has been altered by the addition of fiber. Dietary fibre is an extremely important material for our health and well-being. It is usually quoted as total dietary fibre and inlcudes both insoluble and soluble fibre.

A number of reports on the subject have yet to come up with any firm findings on the subject. Every study published so far seems to produce different conclusions

Take this example where two sources of the same potato peel but generated differently are affected in different ways by the same extrusion process. Abrasion peeling produces a peel with more starch and less dietary fibre than a steam peeling process. Back in 1997, an article on extrusion of these two types of potato peel showed this process had some significant effects on the total dietary fibre (Camire et al., 1997) but it depended on how the potato peel had been derived. If the peel came from steam peels, extrusion cooking reduced the starch content and increased the total dietary fibre. The total dietary fibre of the abrasion peel was not affected. More of the sugar, glucose, could be recovered from insoluble fibre of the steam peel extrudate which indicated resistant starch was produced. Extrusion of both peel types raised the amount of soluble nonstarch polysaccharides. The amount of lignin in the extruded steam peels rose but dropped in the abrasion peels.

Orange pulp is an interesting example (Larrea et al., 2005) because of how extrusion affects pectin. Pectin is an important fibre in pulp. Using a Brabender laboratory single-screw extruder with a length to diameter ratio of 20:1, the process reduced the amount of insoluble dietary fibre by 39% whilst increasing the soluble dietary fibre level by 80%. Extrusion also reduced the total dietary fibre level because smaller fragments were generated. Changes to pectin quality produced by solubilization through extrusion can be achieved.

Extrusion improves the amount and proportion of soluble dietary fibre (SDF)

For example, value added materials can be obtained from soybean residue using extrusion cooking to release soluble dietary fibre (Jing & Chi, 2013).  

Various factors such as extrusion temperature (90 to 130°C), feed moisture (25 to 35% w/w) and screw speed (160 to 200 rpm) were examined on the type of soluble dietary fibre generated. Once the optimum extrusion parameters were obtained, soluble dietary fibre from soybean residue was 12.6% which was approximately an improvement of 10.6% on the unextruded soybean residue. The extrusion cooking modified the dietary fibre which from a product development point of view was useful. The fibre had higher water retention, oil retention capacity and swelling capacity compared to any dietary fibre extracted from unextruded soybean residue.

It is possible to incorporate fibre into extracted foods. One example was the creation of a functional fibre packed puffed ingredient using defatted soy flour and oat bran with some corn starch (Lobato et al., 2011). The corn starch was added to generate various textural properties. The optimum product from a sensory and nutritional point of view contained  250 g/kg corn starch, 375 g/kg soy flour, and 375 g/kg oat bran. The one key parameter of the extrusion process was the temperature of extrusion at 160 C. This produced the best radial expansion ratio whilst decreasing hardness the most. The extruded product contained 213 g/kg fibre, 281 g/kg protein with a calorific value of 319 kcal/100g.

A dog food containing various fibre was evaluated in terms of extrusion properties and kibble formation (Monti et al., 2016).

The Effect Of Extrusion On Bioactive Compounds.

Extrusion generally causes losses of bioactive compounds (Brennan et al., 2011). Antioxidants for example are often degraded. The extent is dependent on the degree of cooking.

Interestingly, there are a few exceptions when the antioxidant capacity of a food is increased because the cooking process produces changes in the structures of certain compounds.

Examples of adding nutritionally important nutrients has been tried by adding curcuminoids to an extruded snack prepared from oat fibre and corn flour which was fortified with 1.5% w/w curcuminoids (Sayanjali et al., 2019)

Procyanidins are flavonoids which have various biochemical roles. They possess antibacterial, antioxidant and anti-inflammatory properties. They also come as monomers through to large polymeric forms. One study assessing these compounds in sorghum found extrusion reduced their size. More lower molecular weight procyanadins were generated whilst the higher molecular weigh variants and the polymers were degraded (Awika et al., 2003).

A similar study was conducted with blueberry pomace and decorticated white sorghum flour. Whatever the extrusion process, the levels of monomeric, dimeric and trimeric procyanadins increased at the expense of higher polymers – about a 40% reduction (Khanal et al., 2009). Anthocyanins are also reduced using extrusion with the total content being reduced by 33 to 42% in total.

Extrusion And Anti-Nutritional Factors

 The extrusion process denatures undesirable enzymes such as polyphenol oxidases and pectinases. It also inactivates and destroys various anti-nutritional factors such as the trypsin inhibitots, haemagglutinins, tannins and phytates based on inositol.

One example relates to the inactivation of a variety of anti-nutritional factors associated with bambara nut. In that process it was heat rather than specific mechanical energy that destroyed an antio-trypsin inhibitor in a blend of yam starch and Bambara nut (Oluwelo et al., 2013). 

Extrusion Of Films

As well as producing puffed foods, films can be produced by extrusion as an alternative to casting (Fishman et al., 2000).

Nitrogen Injection

Nitrogen gas injection has potential in changing extrudate expansion, texture and even colour. One study shows it can produce a red lentil snack when blown into the mixture (Luo et al., 2020). Injection of nitrogen gas at 300 kPa generated the greatest expansion.

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