Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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Metabolic Requirements

S C Dennis and T D Noakes,UniversityofCapeTown

MedicalSchool,Observatory,SouthAfrica

Anaerobic and Aerobic Metabolism

0001 It has become traditional to group energy metabolism

during exercise into that which is independent of

oxygen (O

) supply, so-called anaerobic metabolism,

and that which requires the utilization of O

, so-

called aerobic metabolism. Anaerobic metabolism in-

cludes net muscle creatine phosphate hydrolysis and

conversion of muscle glycogen or blood glucose to

lactate. Aerobic metabolism includes the mitochon-

drial oxidation of the endproducts of the breakdown

of muscle glycogen, blood glucose and circulating

fatty acid.

Anaerobic (Oxygen-Independent) Metabolism

0002 Anaerobic metabolism is particularly important in

short-duration, high-intensity exercise. With extreme

exertion, most of the adenosine triphosphate

(ATP) for contraction is generated from a net break-

down of creatine phosphate and an acceleration of

the conversion of glycogen or glucose to lactate. (See

Glycogen.)

000 3 The metabolic pathway leading to lactate forma-

tion is called anaerobic glycolysis but, strictly speak-

ing, it should be termed O

-independent glycolysis.

Contrary to popular opinion, blood lactate accumu-

lation during intense exercise is not a ‘threshold’

response to inadequate O

delivery. Rather, muscle

lactate production increases as a continuous function

of work rate. Blood lactate accumulates only when

the rate of lactate efflux from the working muscles

into the blood stream exceeds the rate of lactate clear-

ance from the blood by oxidation to carbon dioxide

(CO

) in skeletal and heart muscle, and by conversion

to glucose in the liver.

000 4 During exercise in which the work rate is increased

progressively, there are a number of factors that pro-

mote muscle lactate production. One is the hormonal

acceleration of muscle glycogen breakdown by the

rising concentrations of epinephrine (adrenaline) in

the blood stream. Another is the greater recruitment

of fast glycolytic (type IIb) muscle fibers at high

exercise intensities. A third is the metabolic acidosis

arising from the increasing reliance on carbohydrate

oxidation at high work rates.

000 5 In contrast to the general belief, hydrogen ions

) do not come from lactic acid production

(eqn 1).

Glucose or

glycogen

2 Lactate

−

+ 2H

)

When ATP formation is taken into consideration and

the likely electrical charges at intracellular pH are

summed, the reactions of the O

-independent glyco-

lytic pathway do not produce a net gain of H

ions

(eqn 2).

Glucose + 2MgADP

−

+ 2P

2−

2Lactate

−

+ 2MgATP

2−

(2)

where ADP ¼ adenosine diphosphate; P

¼ inorganic

phosphate; and Mg ¼ magnesium.

0006Instead, metabolic acidosis during progressive ex-

ercise is more a consequence of the increased rate of

glycolytic ATP turnover. When ATP is resynthesized

by oxidative phosphorylation or by phosphate trans-

fer between creatine phosphate and ADP, the H

ions

produced by ATP breakdown are utilized in its

resynthesis (Figure 1a, b).

0007However, when ATP is resynthesized by glycolysis,

the H

ions arising from its hydrolysis are not recon-

sumed (Figure 1c). Net proton production therefore

occurs irrespective of whether lactate is formed or

pyruvate is delivered to the mitochondria for oxida-

tion.

MgATP

2−

MgATP

2−

(a)

(b)

(c)

MgADP

−

MgADP

−

+ P

2−

+ H

MgATP

2−

MgADP

−

+ P

2−

Oxidative phosphorylation

Creatine−P

2−

hydrolysis

Pyruvate

−

lactate

−

Glucose

glycogen

fig0001Figure 1 H

ionproductionfromglycolyticadenosinetriphos-

phate(ATP)turnover.UnlikewhenATPisresynthesizedbyoxi-

dativephosphorylationorcreatinephosphatebreakdown,theH

ionsarisingfromATPhydrolysisarenotreconsumedwhenATP

isresynthesizedbytheconversionofglycogenorglucoseto

lactateorpyruvate.H

ionsarethereforeproducedwhenever

carbohydrateismetabolized.ADP,adenosinediphosphate.Re-

producedfromExercise:MetabolicRequirements,Encyclopaedia

of Food Science, Food Technology and Nutrition, Macrae R, Robin-

son RK, Sadler MJ (eds), 1993, Academic Press.

EXERCISE/Metabolic Requirements 2217

0008 Since any acceleration of carbohydrate utilization

will increase H

ion production, lactate formation is

more a consequence of, than a cause of, metabolic

acidosis. Increases in intracellular H

concentration

shift the lactate dehydrogenase and lactate permease

equilibria towards lactate production and H

plus

lactate



coefflux (Figure 2). Thus muscles ‘dump’

fuel (lactate) to remove H

ions into the blood stream

whenever carbohydrate utilization is increased to

provide energy for exercise of very high intensity.

0009 Once H

ions accumulate, however, further acidi-

fication by glycolytic ATP turnover is prevented by a

slowing of muscle contraction. The H

ions combine

with the P

2

ions from creatine phosphate break-

down to form P



ions, which inhibit P



release

from myosin heads (Figure 3). In terms of muscle cell

survival, this metabolite-induced ‘mechanical arrest’

during intense, ‘anaerobic’ muscle activity is an

important protective mechanism. If muscles were

to become truly ‘anaerobic,’ energy demand would

exceed energy supply and ATP depletion would lead

to irreversible muscle rigor and cell death.

Aerobic Metabolism

0010Because metabolite-induced mechanical arrest de-

velops during high-intensity exercise in which there

are high rates of muscle glycogen and/or blood glu-

cose metabolism, such exercise cannot be sustained

indefinitely. For continued physical activity over

several hours, the contribution to energy production

from carbohydrate utilization must be reduced from

nearly 100% to around 60% or 2–3 g min

1

. At these

rates of carbohydrate oxidation, most of the H

ions

arising from glycolytic ATP turnover leave the muscle

cells in exchange for extracellular sodium ions (Na

(See Carbohydrates: Digestion, Absorption, and

Metabolism.)

0011Since the rate of ATP production from fatty acid

utilization is approximately 50% slower than from

carbohydrate metabolism, exercise requiring 40% of

the energy to come from fat oxidation has to be

performed at intensities corresponding to 70–85%

of maximum O

uptake (Vo

2max

). Typically, an ath-

lete runs a 42-km marathon (126–240 min) at 85% of

2max

and a 100-km ultramarathon (360–540 min)

at 70–75% of Vo

2max

0012During exercise at 70–75% of Vo

2max

, the

average rate of muscle glycogen utilization is around

15 g min

1

. At this rate of depletion, an athlete’s

prerace working muscle glycogen content of around

(30 g kg

1

)  15 kg or 450 g would be sufficient for

approximately 5 h of exercise.

0013In practice, however, depletion of the carbohydrate

stores in the liver may cause premature fatigue. About

135 g of glycogen is stored in the liver after a meal and

these stores are sufficient only to sustain exercise

at ultramarathon pace (70–75% of Vo

2max

) for

around 3 h. As the initially rapid breakdown of

muscle glycogen gradually slows from around 2.1 to

1.0 g min

1

, conversion of liver glycogen to blood

glucose progressively rises to a maximum rate of

approximately 1.1 g min

1

. At this point, further

increases in blood glucose oxidation can no longer

compensate for the decline in muscle glycogen utiliza-

tion, and work rates must be decreased to allow more

energy to come from fat metabolism. Unlike carbo-

hydrate stores, fat reserves could provide energy for

many days of exercise.

0014If an athlete fails to slow down when the demand

for blood glucose exceeds liver glucose output, circu-

lating glucose concentrations fall, producing a condi-

tion known as hypoglycemia or, in common sporting

parlance, the ‘bonks.’ Because the brain depends on

an adequate glucose supply, hypoglycemia leads to a

lack of coordination, an inability to concentrate

Pyruvate

−

Lactate

−

+ Lactate

−

+ NADH + NAD

+ H

fig0002 Figure 2 Muscle lactate formation and efflux. Increases in

intracellular H

ion concentration, arising from rapid carbo-

hydrate oxidation, promote lactate production and efflux from

muscle cells. Lactate formation is therefore more a consequence

of, rather than a cause of, metabolic acidosis. NADH, reduced

nicotinamide adenine dinucleotide; NAD

, oxidized nicotinamide

adenine dinucleotide. Reproduced from Exercise: Metabolic

Requirements, Encyclopaedia of Food Science, Food Technology

and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds),

1993, Academic Press.

2−

ATP

2−

−

+ ADP

−

Contraction

Glycolysis

fig0003 Figure 3 Inhibition of contraction by H

ion accumulation. H

ion accumulation from glycolytic adenosine triphosphate (ATP)

turnover is self-limiting in that H

ions þP

2

ions form P



ions,

which slow contraction by inhibiting P



release from the myosin

heads. This ‘mechanical arrest’ is an important protective mech-

anism to prevent ATP depletion during intense muscle activity.

ADP, adenosine diphosphate. Reproduced from Exercise:

Metabolic Requirements, Encyclopaedia of Food Science, Food

Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ

(eds), 1993, Academic Press.

2218 EXERCISE/Metabolic Requirements

and, ultimately, collapse. This collapse prevents an

individual from dangerously depleting the body’s

carbohydrate stores. (See Glucose: Maintenance of

Blood Glucose Level.)

0015 For prolonged 70–75% of Vo

2max

exercise, an

athlete must therefore ingest carbohydrates and

spare liver glycogen. Provided that enough carbohy-

drate is ingested to satisfy the eventual 2 g min

1

demand for blood glucose by the muscles, an athlete

can continue exercising for some time after muscle

glycogen has been completely exhausted.

0016 Despite the maintenance of high rates of carbohy-

drate oxidation and normal blood glucose concentra-

tions, however, a point is eventually reached where an

athlete can no longer sustain an exercise intensity of

70–75% Vo

2max

. Under these circumstances, there is

an increased perception of fatigue. Some believe that

this fatigue is due to the slow utilization of plasma

branched-chain amino acids by skeletal muscle during

prolonged activity. Since the contribution to energy

production from branched-chain amino acid metabol-

ism is trivial, it has been suggested that branched-chain

amino acid oxidation might provide a communication

link between working muscles and the brain.

0017 Over the course of a marathon, plasma branched-

chain amino acid concentrations have been shown to

decline from 650 to 500 mg l

1

. At the same time, the

increases in circulating free fatty acid concentrations

during exercise were found to raise the free plasma

tryptophan amino acid concentration from 1.4 to

2.9 mg l

1

, by competing with tryptophan for binding

to blood albumin.

0018 Decreases in the circulating branched-chain amino

acid concentration and rises in plasma free trypto-

phan concentration with sustained physical activity

promote tryptophan entry into the brain. Passage

of tryptophan across the blood–brain barrier is nor-

mally limited by its low free concentration and by a

competition from branched-chain amino acids for

access to the neutral amino acid transporters.

0019 Once in the brain, tryptophan is converted to the

neurotransmitter 5-hydroxytryptamine (serotonin) in

the serotonergic nerve terminals of the hypothalamus.

Serotonin biosynthesis is unusual in that it is exclu-

sively regulated by the rate of entry of tryptophan

into the brain.

0020 Since serotonin is involved in the control of sleep,

food intake, pain sensitivity, pituitary hormone re-

lease and mood, it is tempting to speculate that the

elevated serotonin levels in the brain might contribute

to the perception of fatigue in prolonged exercise,

when muscle fuel supply appears to be adequate.

Certainly, excessive serotonin release would account

for the improvement in mood and increased drowsi-

ness after long-duration, low-intensity exercise.

The Athlete’s Diet and Supplementation

Additional Protein

0021Whether increased amino acid utilization during and

following exercise significantly increases an athlete’s

daily dietary protein requirement is not known. Des-

pite considerable research on this subject, there are

still two schools of thought. Some argue that the 78%

safety margin in the 0.8 g per kg of body weight per

day recommended dietary allowance (RDA) for pro-

tein is sufficient to cover the increased amino acid

requirements of both endurance- and strength-trained

athletes. Others feel that frequent exercise elevates

the daily need for dietary protein. The balance of

evidence suggests that an athlete should probably

ingest around 1.2 g of protein per kg of body weight

per day. (See Protein: Requirements.)

0022Daily protein intakes of 1.2 g per kg of body

weight, however, do not necessitate protein supple-

mentation. An athlete’s larger than normal daily

kilojoule ingestion more than adequately covers any

increase in his or her protein requirements. Further-

more, there is no evidence that protein intakes in

excess of 1.2 g per kg of body weight per day improve

sporting performance. Provided that an athlete eats

enough to meet his or her daily energy expenditure,

protein is not a major fuel for exercise. A signifi-

cant use of protein for energy occurs only in starva-

tion, when body protein is converted to carbohydrate

to maintain an adequate supply of glucose to the

brain.

0023Higher than recommended (1.2 g per kg of body

weight per day) protein intakes are also unlikely to

help increase muscle size. Muscle hypertrophy is

under much more subtle control than the supply of

amino acids for protein synthesis.

Additional Vitamins and Minerals

0024Another question frequently asked by athletes is

whether or not they should increase their intake of

vitamins and essential minerals. Unless an athlete is

malnourished or follows an extreme vegetarian diet,

the answer is probably no. Vitamin and mineral defi-

ciencies are extremely uncommon among athletes and

there is no evidence to show that extra vitamins and

minerals improve performance. Vitamin and mineral

supplements are not being promoted to meet the

unique nutritional requirements of athletes, but

rather to satisfy commercial interests.

0025However, there is growing interest in the possibility

that very heavy training and competition may sup-

press the immune response and thereby increase an

athlete’s risk of infection. Since vitamin A and per-

haps also vitamins C and E are thought to improve

EXERCISE/Metabolic Requirements 2219

resistance to infection, an athlete might consider an

increased intake of these vitamins during periods of

intense training and competition. (See Ascorbic Acid:

Physiology; Retinol: Physiology; Tocopherols: Physi-

ology.)

Additional Carbohydrate

0026 With heavy training, it is essential that an athlete eats

sufficient carbohydrate. Studies of the effects of daily,

2-h, 70% of Vo

2max

training runs on leg muscle

glycogen contents have shown that a standard 40–

45% carbohydrate diet is not sufficient to maintain

body carbohydrate stores. For the last 4 or 5 days’

intensive training, the subjects’ preexercise muscle

glycogen contents progressively declined from around

22 to 5–6gkg

1

0027 Even a high-carbohydrate (70%) diet failed to

replenish completely the subjects’ muscle glycogen

stores between exercise bouts. With the ingestion of

560 g of carbohydrate per day, muscle glycogen levels

still fell from approximately 22 to 17 g kg

1

0028 Three days before an endurance race, therefore, an

athlete should stop training and follow what is known

as a ‘carbohydrate-loading’ regime. During carbohy-

drate loading, the optimum strategy is to eat 500–

600 g of simple carbohydrates for the first 24 h and

then consume the same quantity of mainly complex

carbohydrates on the following 2 days. By ingesting

large amounts of carbohydrate and not exercising,

athletes can increase their muscle glycogen content

from 22 to 36 g kg

1

and their liver glycogen stores

from 70 to 90 g kg

1

0029 As mentioned above, it is also important for an

athlete to drink carbohydrate solutions during pro-

longed exercise to prevent hypoglycemia. Provided

that the volumes described below are ingested, it

does not matter whether the carbohydrate is glucose,

sucrose, maltose or a commercial 7–22-chain-length

glucose polymer. In each case, peak rates of exogen-

ous carbohydrate oxidation occur 75–90 min after

ingestion and are 0.7–0.9 g min

1

.(See Carbohy-

drates: Metabolism of Sugars.)

0030 The exception to this pattern is fructose. Because

ingested fructose is (1) slowly absorbed from the

intestine and (2) converted to glucose in the liver

before being oxidized, its maximum rate of use is

only 0.4–5 g min

1

Maintaining Water and Electrolyte

Balance

0031 Interestingly, the first evidence that carbohydrate

ingestion can enhance performance during prolonged

exercise was published in the 1930s but was ignored

by athletes, coaches, and scientists until the mid-

1980s. Even the idea that fluids should be ingested

by athletes during prolonged exercise is of recent

origin. Prior to the early 1970s, the prevailing opinion

was that any fluid ingested during exercise, including

marathon running, adversely affected performance.

This fact is frequently overlooked by today’s athletes

and their advisors who have emphasized the per-

ceived value of fluid replacement during exercise,

sometimes to excess.

Factors Influencing Fluid Loss

0032The important principles underlying fluid and elec-

trolyte balance during exercise are that the rate at

which sweat is lost is determined principally by the

athlete’s rate of energy expenditure (metabolic rate)

which is largely determined by his or her mass and

speed of movement. Thus sweat rate is highest in

activities of short duration and high intensity, such

as competitive running over distances of 3–10 km,

and is slower in prolonged, lower-intensity exercise,

such as marathon and ultramarathon running or

ultratriathlon events. Sweat rate also tends to be

greater in heavier than in lighter athletes. At the

same speed, the heavier cyclist or runner has to expend

more energy in moving a larger mass.

0033When cyclists and runners exercise at the same

metabolic rate, the sweat rate of cyclists is lower

than that of runners. Cyclists travel faster than

runners and therefore lose more heat by convection

to the passing air than by sweat evaporation.

0034Sweat rate is also influenced by the ambient tem-

perature but, in practice, environmental conditions

may affect sweat rates less than expected. When

forced to exercise in severe heat, athletes usually

choose to exercise at a lower exercise intensity. Ath-

letes are well aware that they will fatigue rapidly

when exercising in the heat, and a lower metabolic

rate reduces their sweat rates.

0035Contrary to popular opinion, however, ‘heat ex-

haustion’ is not caused by excessive body heat accu-

mulation. In very severe environmental conditions,

the rate of energy expenditure appears to be regulated

specifically to prevent the development of high sweat

rates, severe dehydration, and heat injury.

0036There is also some uncertainty as to whether or not

all the fluid lost during exercise must be replaced if

serious dehydration is to be prevented. Up to 1.8 l of

fluid can be stored in association with the glycogen

contained in a carbohydrate-loaded athlete’s working

muscle and liver. Thus, in activities that largely de-

plete liver and muscle glycogen contents, a mass loss

of up to 1.8 kg can be incurred without the athlete’s

fluid status being seriously compromised.

2220 EXERCISE/Metabolic Requirements

Fluid Replacement

0037 In athletes losing more than 1.8 kg of mass, the prin-

cipal aims of fluid ingestion during exercise are to

replace the fluid and electrolytes lost during exercise

and to provide carbohydrate in solution to supple-

ment the body’s limited carbohydrate stores. Usually,

adequate fluid replacement has been considered of

greater importance than carbohydrate supplementa-

tion, but more modern studies indicate that an ad-

equate carbohydrate supply is at least as important as

optimum fluid and electrolyte replacement.

0038 Failure to replace adequately the sweat losses in-

curred during exercise leads to progressive dehydra-

tion. Symptoms of dehydration are: (1) an increase in

heart rate with a fall in cardiac stroke volume; (2) a

reduction in sweat rate; (3) a decrease in blood flow

to the skin; and (4) a progressive rise in body core

temperature.

0039 It is not yet clear to what extent the symptoms of

dehydration adversely influence athletic perform-

ance. A small weight loss resulting from unreplaced

sweat losses during exercise might enhance perform-

ance by reducing the mass which the athlete must

carry. Eventually, however, there must be a crossover

point at which the negative cardiovascular and other

effects of more severe dehydration outweigh the

beneficial effects of weight loss.

Electrolyte Replacement

0040 In considering fluid replacement, it is important to

realize that, in health, it is not the fluid content of the

body that is regulated but the electrolyte concentra-

tion in the extracellular and intracellular compart-

ments. In the 15-l extracellular compartment of a

70-kg man, the major electrolyte is sodium (Na

)

and, in the 30-l intracellular compartment, the main

electrolyte is potassium (K

0041 As Na

is the principal electrolyte in sweat, the

initial site of fluid loss is from the extracellular com-

partment, which includes the circulating plasma

volume. The extent to which dehydration of the

extracellular compartment occurs is determined by

the amount of Na

lost in sweat.

0042 An important consequence of exercise training,

especially in the heat, is a reduced sweat Na

content.

The result is that fluid losses from the extracellular

compartment and circulation are decreased in

trained, heat-acclimatized athletes. This might ex-

plain why highly trained athletes are better able to

cope with dehydration during exercise, because more

of their fluid loss will come from the intracellular

compartment than from the plasma.

0043 Because the concentration of electrolytes regu-

lates the volumes of the extra- and intracellular

compartments, it follows that rehydration of these

compartments can only occur when their electrolyte

losses are corrected. Consequently, there is the need

to ingest Na

and, perhaps, some K

during and after

exercise.

0044Solutions containing sodium chloride (NaCl) are

best ingested together with carbohydrate. Carbohy-

drate digestion leads to Na

–glucose cotransport

across the intestinal brush border and that accelerates

the absorption of NaCl andH

O into the blood stream.

Practical Implications

0045The present opinion is that an athlete should ingest

between 300 and 800 ml of fluid per hour. The high

(800 ml) value is for heavy athletes exercising at fast

speeds; the low (300 ml) value is for light athletes

exercising slowly. Ideally, the fluid should contain

NaCl at concentrations of 5–6g l

1

but salt concen-

trations closer to 2 g l

1

are more palatable.

0046To provide carbohydrate at rates equal to that at

which glucose is absorbed into the blood stream and

oxidized by the working muscles (approximately 1 g

min

1

), a mono-, di-, or oligosaccharide should also

be added to the solution. For an athlete drinking

600 ml h

1

or 10 ml min

1

, it can be calculated that

the optimum concentration of carbohydrate after

1–2 h of exercise is around 0.1 g ml

1

See also: Ascorbic Acid: Physiology; Carbohydrates:

Digestion, Absorption, and Metabolism; Metabolism of

Sugars; Glucose: Maintenance of Blood Glucose Level;

Glycogen; Protein: Requirements; Retinol: Physiology;

Tocopherols: Physiology

Further Reading

Burke LM and Deakin V (eds) (1994) Clinical Sports Nutri-

tion. Sydney: McGraw Hill.

Clarke N (1991) Sports Nutrition Guidebook. Campaign,

Illinois: Human Kinetics.

Coyle EF (1991) Timing and method of increased carbohy-

drate intake to cope with heavy training, competition

and recovery. Journal of Sports Science 9: 29–52.

Hawley JA and Burke LM (1998) Peak Performance:

Training and Nutrional Strategies for Sport. Sydney:

Allen and Unwin.

Hawley JA, Dennis SC and Noakes TD (1995) Carbohy-

drate, fluid and electrolyte replacement during pro-

longed exercise. In: Keis CK and Driskell JA (eds)

Sports Nutrition: Minerals and Electrolytes, pp.

233–263. Boca Raton, CRC Press.

Noakes TD (1991) Lore of Running. Champaign, Illinois:

Human Kinetics.

Noakes TD, Hawley JA and Dennis SC (1995) Fluid and

energy replacement during prolonged exercise. In: Tory

EXERCISE/Metabolic Requirements 2221

SS and Shepherd RJ (eds) Current Therapy in Sports

Medicine, 3rd edn, pp. 517–520. Philadelphia: Mosby-

Year-Book.

Peterson K (1988) Eat to Compete. A Guide to Sports

Nutrition. Chicago: Year Book Medical Publishers.

EXTRUSION COOKING

Contents

Principles and Practice

Chemical and Nutritional Changes

Principles and Practice

R C E Guy, Campden & Chorleywood Food Research

Association, Chipping Campden, UK

Extrusion Cooking

0001 Extrusion cooking technology was invented in the

1940s by the Adams Company to manufacture

snack foods from maize grits. It is derived from the

earlier extrusion technologies used to form pasta at

temperatures of 70–80



C. These processes involved

the conveying, mixing, and compressing of moist

wheat semolina to form dough and to extrude it as

well-defined shapes.

0002 In extrusion cooking, a feedstock of moist powders

is also conveyed, mixed, and compressed before

shaping at a die (Figure 1). However, the dough tem-

perature is raised to much higher levels in the range

from 110 to 200



C on the screws within a barrel.

This means that any water present is superheated

within the dough but remains in the liquid state.

The high temperature also causes changes in the

structures and form of natural biopolymers such as

starch and soya globulins. Physical bonds can be

broken at high temperatures, and the new forms of

the polymer can be used to create novel structures for

foodstuffs. A particular feature of extrusion cooking

is the ability of the process to manipulate these poly-

mers at low moistures and to form doughs or fluid

systems from them with the minimum of moisture.

0003 The basis of the technology is a screw system

within a tube or barrel, which conveys the dough

towards small openings called dies. In the confined

space of the barrel, the dough is compressed and

heated to high temperatures at high pressures before

being extruded through the dies into the atmosphere.

An extrusion cooking process has a number of key

features, the feeding devices supplying the raw mater-

ial feedstocks, the design of the screw system and its

barrel, the dimensions and number of the dies, and

the devices that handle the extrudates.

0004Extrusion cooking is a continuous process in which

the raw material feedstock is metered in at a constant

rate, and the machinery maintains a steady-state equi-

librium. This is achieved by balancing the forward

flow produced by the screws against the pressure at

the die. It is a multivariate process depending on all

the independent variables of the machinery and the

raw materials to produce a range of dependent vari-

ables in the machinery and a further range of depend-

ent variables in the extrudates and products. The

consequence of varying one independent variable

may be to change several of the product characteris-

tics. Therefore, a good systems approach must be

used to control such a process.

0005The range of products manufactured by extrusion

cooking can be divided into two groups: animal prod-

ucts and human foods. The animal products them-

selves include basic feeds for stock animals, marine

feeds for freshwater and saltwater fish, shrimps or

prawns. They also include large markets for both

dry and canned dog and cat foods and treats, and a

number of products for other pets and for wild and

tame birds. Human foods include the original maize

snacks and many new forms including modern ver-

sions of prawn crackers and other traditional snacks.

High pressure High shear Compression

Fully

developed

fluid state

Product's

structure

forms on

expansion

Plastic

fluid

Melt

transition

Tightly

packed

powder

Loosely

packed

powder

fig0001Figure 1 Diagram of the development of raw materials in a twin

screw extrusion cooker.

2222 EXTRUSION COOKING/Principles and Practice

There are many breakfast cereal products formed into

balls, hoops, and flakes that have large traditional

markets in the USA and UK and growing markets

in many areas. Similar expanded pieces of rice and

maize are used in chocolate confectionery bars. These

bullets of cereal are also used in other products where

a crisp, expanded-foam particle is required, such as in

granola bars, biscuits, and even icecream.

0006 The range of extruded foods has been extended to

coatings, such as breadings. These are used on fried

foods, particularly frozen fish and chicken portions.

Extruded products have replaced some of the trad-

itional baked crumbs. They are made by expanding

cereal extrudates and slicing them into crumbs. This

process may be taken a step further in the manufac-

ture of baby foods from extruded feedstocks of

cereals and milk powders. For these products, the

extrudates are ground to a fine powder, sieved and

used to replace the powders manufactured by the

more expensive roller drying process.

Principles of Extrusion Cooking: Overall

Balance

0007 The extrusion cooking process is used to transform a

feedstock of raw materials into a fluid dough and to

continue to change the physical nature of this dough

until it is forced out of the die (Figure 2). At that stage,

the dough must have attained the ideal form to create a

structure in the extruded product. The transformation

of the native raw materials has an optimum range for

most product types. For some products, only small

changes are required, whereas for others, a very high

degree of change must be induced. For each individual

product type, the process must be maintained in a

balance between the independent input variables to

obtain the ideal conditions within the barrel to achieve

this level of transformation.

0008 A good view of an extrusion cooking process is

given by the publications of Meuser and van Lenger-

ich. They showed that the relationships between the

independent process inputs from the machinery and

raw materials created a series of dependent variables

within the barrel of the extruder. Most notable of these

are residence time, temperature, pressure, moisture

level, applied mechanical energy, and mass flow rate.

Biopolymers in raw materials, such as starch, were

affected by these variables and changed in physical

form to dominate the physical features of the fluid

dough. As the dough was extruded, the new forms of

biopolymers controlled the expansion process influ-

encing the shape, extent of expansion, and the foam

texture of the extrudates. The important principle in

this section is that the continuous process is in a meta-

stable steady state. Guy and Robert showed that all

extrusion cooking processes fluctuate as the balance is

maintained between the independent inputs of the

process. A fluctuation in an independent input could

upset the balance and cause a larger fluctuation in the

outputs and therefore in the product quality. It is im-

portant, therefore, to minimize any fluctuations in the

input variable such as the powder and liquid feed rate,

feedstock raw material mix, and quality.

Principles of Extrusion Cooking: the

Screw Design

0009The screws of an extrusion cooker are designed to

convey a powder or viscous fluid from the feed port

to the die and to force the material through the die

(Figure 3). Single screws were used in the early

machines leading on from the pasta equipment.

More recently, pairs of corotating intermeshing

screws have been adapted from their use in the plas-

tics industry to become important machines for the

food and feed industries. For conveying materials,

the screws are designed with a pitch of 0.5–1 times

the screw diameter and a small clearance between the

leading edge and the wall. This provides a screw with

excellent conveying power for viscous fluid and

powders. The effect of a simple conveying screw on

the raw materials is minimal, and even with a very

small die, the physical transformation of biopolymers

is small.

Preconditioner

Extruder

20−90 8

C Cool zone 100−160 8C Hot zone

Dryer Cooler

Enrober

Flavor precursors

added or generated

Flavors created60

−80 8C

Some flavors

lost

Flavors may be created or

lost

120

−140 8C30−358C25−35 8C

Flavors added by

spray or dusting

Product

fig0002 Figure 2 Extrusion cooking equipment in a processing line, with a powder feeder, preconditioner, extruder, dryer and enrober.

EXTRUSION COOKING/Principles and Practice 2223

0010 The second feature of design for a screw system is

to apply high levels of mechanical shear to the feed-

stock being conveyed along its length. In the single

screw, this is achieved by reducing the depth of the

flights as the material approaches the die and by

introducing cuts in the flights to increase back-

leakage, thus extending the residence time in the

high temperature/high shear zone for the fluid. In

the twin screw machines, special sections are added

in which the flights are reversed to give a back-

pressure pump acting against the main flow. These

sections are made weaker by cutting gaps in the

flights so that the main pumping screws overcome

them. This achieves a steady flow of the fluids though

the reversing section under shear.

0011 A special form of reverse section can be constructed

from elliptical paddles placed on each shaft in a series.

If successive pairs are placed at an angle to the

upstream pair, a screw-like structure can be formed.

This type of section acts on the fluid as it passes

through, kneading the fluid against walls and the

other elliptical paddle. An important feature of such

a section is that it is set up as a reversing screw to

ensure that all the paddles are full of fluid. If they are

not full, they have little or no shearing effect on the

fluid.

0012 The final role of the screws is to push the fluid melt

through the dies in a controlled manner. Normally,

the extruder is run with the final sections completely

full so that pressure is transmitted from the main

conveying screws to the dies. In those cases where

reverse sections are used, there may be a pressure

drop after the reverse section, and a final section of

conveying screw can be used to pump the fluid

through the die. In most processes where the extru-

sion melt fluid has a temperature above 100



C, the

pressure in the die will need to be higher than the

water vapor pressure. This insures that the fluid

emerges from the die and expands in the air rather

than in the die cavity. Normally, the pressure at the

die for good control of hot melt extrusions is above

40 bar.

Principles of Extrusion Cooking: the

Transformation of Biopolymers

0013The main feature of extrusion cooking is the trans-

formation of the biopolymers in a raw material

feedstock. In 90% of the extruded products manufac-

tured, the active biopolymer is starch, and for the

remainder is a protein from oilseeds, such as soya or

wheat gluten. Physical transformation is necessary to

obtain a more suitable form of the biopolymer for

structure creation in the postextrusion expansion

processes (Figure 1).

0014Starch is found in the world’s major crops, cereals,

such as wheat, rice and maize, and in important

tubers such as potato and manioc. In all cases, it is

found as partial crystalline granules, 5–80 mmin

maximum dimension. The polymers can be released

from the granules (Figure 4) by melting their crystal-

line regions and applying large shearing forces, or by

dispersing in an excess of water (>30–35%). For

extrusion cooking, low moisture is used, 15–20% in

many cases, and the melted starch must be dispersed

purely by mechanical shear to obtain a melt fluid of

partially or fully dispersed aggregates. The melting

temperature for a low moisture feedstock may be in

the range of 120–160



C. Therefore, it is necessary to

heat the feedstock to this temperature by the action

of the screw and by barrel heating. In low moisture

systems with up to 25% moisture, the action of

the screws on the powders under compression to a

density of 1.4 g ml

1

will normally achieve the re-

quired temperature by frictional heating and the dis-

sipation of mechanical energy from the drive motor.

0015The second part of the transformation of the starch

is more subtle. It has been shown that the major

polymer in starch, amylopectin, has a molecular

weight range of 10

–10

Da. This size range is too

Pressure- and temperature-

sensing probes

Die

zone

12-mm-

pitch

single

start

screw

45 

reversing

Kneeding paddles

90  10-mm-pitch

twin start

screw

45 

forwarding

paddles

12.7-mm-

pitch

single

start

screw

45 

forwarding

paddles

25.4-mm-pitch

twin start

self-wiping

feed screw

Dry feed

Water inletOil inlet

1234

fig0003 Figure 3 Side view of screw system for a twin screw extruder used at CCFRA showing the functions of the elements.

2224 EXTRUSION COOKING/Principles and Practice

large for good flow properties in fluids at the concen-

trations found in the melt fluids. It interferes with the

expansion process and reduces the extensibility of the

starch films in cell walls. However, the application of

high shear inputs to the fluid using reversing screws

and high compression ratios can degrade the starch to a

much smaller range of 1–5 10

Da. This material is

much more extensible and gives a more expanded

range of products.

0016 The principle of extrusion is to transform the bio-

polymers, by heat and shearing forces, to a new form.

This may be a completely dispersed form in which the

native structures have all been destroyed, and the

polymers now form a continuous fluid in water.

Such a processed starch would be suitable for forming

films that can retain gases during expansion to form

fine foams. In examples with proteins, the fluid may

be made to texturize during laminar flow through

long dies.

0017 The processing of starch may be set up to produce

any type of starch structure from the native to the

fully dispersed and degraded forms. If the range is

divided into a number of stages, as in Figure 4, the

type formed at each stage may be associated with a

different product in the list discussed earlier.

Principles of Extrusion Cooking:

Miscellaneous Reactions and Physical

Effects

0018 The extrusion cooker is a chemical reactor that can

cause interactions between some of the ingredients

such as amino acids and reducing sugars to take part

in Maillard reactions and similar degradative and

condensation reactions. These reactions produce

color and flavor compounds and may also reduce

amino acid and vitamin contents. More details of

these reactions are given in the chapter on Extrusion

Nutrition.

0019Extrusion cooking also has a unique action that

serves to sterilize the materials being processed when

their processing temperatures are above 100



C. The

time spent in the extrusion processing zone is short,

only 45–120 s in most processes, but the high shear

fields and temperatures of 140–160



C in the shear

zone help to reduce the viable cell counts to low

levels. Finally, the explosive release of superheated

water from within cells ruptures their structure and

kills off the residual level of viable cells.

Practice of Extrusion Cooking: Animal

Feedstuffs

0020The basic stock feeds are made using cookers and

pelleting presses. This does not gelatinize the starch

and merely pasteurizes and compacts the material

as loose pellets. The second level of processing is

obtained with expanders that add steam to an open

screw system feeding pelleting presses. This process

also fails to gelatinize more than a small amount of

starch, until a die is used, and the equipment becomes

a full extrusion cooker.

0021Feedstuffs with gelatinized and dispersed starch are

made for special weaning foods and for all the marine

feeds for fish and shrimps. In these products, the

pellets are held together by the melted dispersed

starch so that a strong structure is formed.

0022Extruders are also used to improve the nutritional

value of oilseed and other protein stocks by denatur-

ing proteins that inhibit digestion such as lectins and

trypsin inhibitor in soya beans and dough ball

forming elastic glutenins in wheat flours.

Practice of Extrusion Cooking: Pet Foods

0023Dry pet foods are made from an extruded feedstock of

cereals, fortified with protein-rich materials such as

dried meat meal, chicken digest, and similar mater-

ials. They tend to be fairly dense products with a

specific density of 0.35–0.45 g ml

1

. The structural

material is normally starch, which is supplied by a

cereal such as wheat or maize for standard products,

and rice for special dietary feeds. There may also be

higher levels of oils and fats and minerals to provide a

balanced diet for the animals.

0024Many processes incorporate a preconditioning unit

(Figure 2) in which the bulk of the feedstock is

conveyed and mixed with oil and steam to produce

a hot feedstock at 80



C for the extruder. This enables

a higher throughput of materials to be processed by

the extruder.

0025Processes containing high oil contents must be

balanced with materials that absorb oil and prevent

Biopolymers degraded

Biopolymers dispersed

Damage by compression

Melting of crystallites

Native crystalline granules

Changes to the starch

fig0004 Figure 4 Stages in the transformation of starch in an extruder.

EXTRUSION COOKING/Principles and Practice 2225

slippage in the machinery. Alternatively, the oil can be

sprayed on to the extrudates postextrusion.

Practice of Extrusion Cooking: Snack

Foods

0026 The snack foods are made in several forms by either

direct expansion or expansion of dense cooked

pellets. In direct expansion, the feedstock may be

formed from a cereal or potato base mix containing

75–85% starch. The extrusion process is high shear at

a low moisture to give highly degraded starch with no

granular structure and a large amount of molecular

degradation. This allows an expansion to a very low

specific density of 0.14–0.20 g ml

1

. Directly ex-

truded snacks may be formed as tubes at the die and

injected with a cream or jam through the center to

manufacture a coextrusion or coextruded snack. The

simple products are coated with oil and flavorings to

complete the products.

0027 For the pellet process, a similar feedstock is pro-

cessed at low shear to melt the starch crystalline

regions but to retain its granular structure. The cooked

melted fluid is cooled to 95–100



C after cooking,

before extruding to the final shape. This soft dense

extrudate is cut into a pellet and dried to < 12%

moisture. As it cools to ambient temperatures, it

assumes a glassy structure and is known as an inter-

mediate pellet or half-product.

0028 A snack is formed by heating the half-product for a

few seconds in hot oil or hot air at 190



C. Recent

developments have seen the production of such prod-

ucts in two layers with a half-product. On frying, this

forms a large central cavity and is sometimes called

a ‘3-D snack’.

Practice of Extrusion Cooking: Breakfast

Cereals

0029 A simple form of breakfast cereal is made for a cereal

feedstock with wheat, maize, or rice flours and some

small additions of sugar, salt, and proteins. The feed-

stock is processed at 20–25% moisture to a medium

expansion at 150



C, to give a specific density of

0.22–0.28 g ml

1

. The desired products are formed

with a coarse cellular texture and thick cell walls, so

that they remain crisp in milk for a few minutes. This

is achieved by melting the starch granules and degrad-

ing about 50% of the granular structures.

0030 The recipes for directly expanded breakfast cereals

may be varied to form single cereal varieties such as

crisp rice or corn pops, or to form multigrain cereals

from blends of wheat, rice, maize, barley, and oats.

0031 A unique form of the expanded breakfast cereal is

the high bran product in which wheatbran may be used

at levels of 70–75%. In this product, a small amount of

starch (15–20%) is used to bind the bran fibers to-

gether ina cellular structure. The starch must be melted

and completely dispersed to achieve this effect.

0032The second major form of extruded breakfast cereal

is the flaked product, such as the cornflake or multi-

grain flake. This product is similar in processing re-

quirements to the half-product snack in that it is made

from a cereal feedstock at 120–130



C at low shear.

The starch granules are melted, but not dispersed to

any significant level. In the process, the cooked cereal

feedstock is extruded as a dense fluid at 90



C and cut

into small beads. Each bead forms the basis for a flake

when rolled out to a thickness of 0.5–0.7 mm. This

process occurs under temperature control at 40–



C and is followed by a toasting process at high

temperature 240



C for a few seconds to blister and

dry out the flakes. The degree of processing of the

starch is very important for the quality of the flakes

when theyare consumed inmilk as a breakfast cereal. If

the starch is sheared too much, the flakes take up water

too quickly and become limp and flabby in the bowl.

Practice of Extrusion Cooking: Breadings

and Coatings

0033Traditional breadcrumbs are manufactured by

making loaves of bread, and shredding and drying

their crumb to produce a small range of products.

Several extrusion cooking processes have been set

up to make similar products from the same materials

and a wider range from other ingredients, which

cannot be used in traditional bread. The extrusion

of a wheat-flour feedstock at 28–35% moisture at

temperatures of 110–120



C at low shear produces

expanded extrudates with cellular textures similar to

that of breadcrumbs. Starch granules within the struc-

ture should be melted to lose their crystalline struc-

ture, but only dispersed to a small degree so as to

provide material for expansion. The extrudates may

be cut while still warm and moist to form crumbs,

and then dried to <10% to form the glassy structure

required for coating crumbs. The recipe for coating

crumbs may be based on wheat flour and modified

with some chalk to increase the number of cells.

Other cereals such as maize and rice may be used

with, or instead, of wheat flour to give a variety of

products. Maize adds an attractive yellow color to

products, whereas rice give whiter crumbs.

Practice of Extrusion Cooking: Baby

Foods

0034Extrusion cooking has been used to produce

expanded baby foods from rice and vegetable flours

2226 EXTRUSION COOKING/Principles and Practice