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

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cost-effective to preheat the liquid in a separate heat

exchanger. This can be either a stand-alone system

or, more likely, one that is integrated into the overall

evaporator plant – for example as a preheating

coil located in the steam jacket around the main tube.

0016 Falling-film evaporators in the food industry

normally operate under vacuum, thus reducing the

product boiling point and minimizing heat damage.

Other Types of Evaporator

0017 The thin-film (or wiped-film) evaporator can be used

for very viscous, scaling, or crystallizing materials.

The tubular body of this evaporator is fitted with a

central rotor, which has blades along its length. These

blades almost touch the tube wall (with a clearance

of typically about 1 mm) so that, when they rotate

during the normal operation of the evaporator, a very

thin and well-agitated film of liquid is formed on the

surface of the tube wall. In some equipment models,

the outer edge of the blade is hinged, so that this edge is

forced by centrifugal action on to the wall, and conse-

quently, the thickness of the film so produced is deter-

mined by the balance between this centrifugal force

and the hydrodynamic forces produced in the liquid

film on which the blade rides. This version of the thin-

film evaporator is usually known as a swept-surface

evaporator. Another configuration of this evaporator

group is the scraped-surface evaporator. Here, as the

name suggests, the blades (made from a synthetic ma-

terial or perhaps rubber) actually scrape against the

wall. Owing to the relatively high capital cost of this

evaporator type, it has relatively limited applications.

0018 Strictly speaking, most driers are forms of evapor-

ators, where the evaporation process is taken to the

extreme where effectively all the solvent is removed,

leaving the dissolved solids in dry form.

0019 As mentioned at the beginning of this article, evap-

orator classification is usually based on the way in

which the liquor circulates. However, it is also pos-

sible to classify evaporation plants according to the

mechanical configuration, the most common types

being tubular or plate. Although tubular systems are

most widespread, plate systems have certain attrac-

tions. As the plant height is relatively low, they can

operate in process areas where there is limited head-

room. Hence, building and installation costs can be

modest. It is also often possible to increase their heat-

transfer surface at relatively little additional expense.

(See Drying: Theory of Air-drying.)

Evaporator Selection

0020 Once the need for an evaporator for a particular

product application has been established, it is neces-

sary for a specification to be prepared, which will be

obviously helpful to both purchaser and supplier to

identify plant configuration and component selec-

tion. In carrying out this exercise, the following

points should be borne in mind:

0021Throughput:

0022hourly rate and/or daily rate;

0023length of production run (hours/days).

0024Feed specification:

0025total solids (or equivalent, e.g., degrees Brix);

0026other relevant parameters (e.g., suspended

solids, pH, volatile components).

0027Product specification:

0028temperature;

0029total solids;

0030other requirements or characteristics (e.g., vis

cosity).

0031Utility requirements:

0032steam, electricity, air, cooling/chilled water (in

cluding temperature);

0033existing on-site availability, and proximity to

proposed evaporator location.

0034Plant dimensions, building requirements (or out-

door location), foundation loadings, installation

aspects (e.g., access for cranage, etc.).

0035Plant access for operation and cleaning (e.g.,

ladders, platforms, inspection ports) and main-

tenance (e.g., extra headroom and/or additional

space for unhindered removal of individual plant

components). Manways should be easy to use

(power operated, if necessary).

0036All cleaning requirements (‘cleaning in place’

(CIP) and also manual cleaning operations –

both routine and emergency) must be specified.

Additional site facilities identified and procured

as necessary.

0037Effluent loadings and associated treatment facil-

ities quantified.

0038Specify individual components (e.g., choice of

pumps, electronic controls, etc.). Materials of

construction and surface finishes confirmed.

10.

0039Manufacturing and equipment standards to be

laid down. Plant testing (e.g., pressure and

vacuum). The need for acoustic and thermal

insulation should be examined.

11.

0040Control and automation to be specified, including

links to adjacent plant, e.g., drying equipment.

12.

0041Obtain recommended spares list, and order as

required.

13.

0042Determine pretreatment process which might

affect evaporation process. Identify storage/

buffer tanks needed, for both feedstock and end

product.

14.

0043Identify training requirements, including oper-

ator manuals, maintenance schedules, plant

EVAPORATION/Uses in the Food Industry 2207

drawings, etc. Plant inspection, delivery, instal-

lation and commissioning activities, and arrange-

ments for plant handover.

Applications

0044 A list of established evaporation duties in the food

and related biological industries would include the

materials listed in Table 1. A few of the applications

in Table 1 are considered in more detail in the remain-

der of this article.

Dairy Products

0045 -Milk From the first recorded use of evaporation, as

a means of enhancing the shelf-life of milk, batch

vacuum pans were almost universally employed up

until the 1920s. However, after stainless steel came to

replace copper and aluminum as the material of con-

struction, more sophisticated evaporator configur-

ations (requiring CIP techniques, in turn employing

stronger chemicals – now feasible with the new ma-

terial of construction) became possible. The natural

circulation (i.e., rising film) evaporators that replaced

the earlier pans were mainly tubular systems and

were used up until the early 1950s, when falling film

evaporators became more common, and eventually

ousted the older styles of evaporator. All three types

of plant employed a vacuum, so as to lower the

boiling point of the milk, and hence minimize thermal

damage (and burn on). Single-stage plants had given

way to multieffect units before 1940. All modern-day

milk evaporators are multieffect plants, and they

almost always employ either thermal (TVR) or mech-

anical (MVR) vapor recompression. (See below for

further details of vapor recompression systems.)

0046For an example of a typical installation, consider a

multiple-effect TVR unit. Prior to the milk entering

the first effect, it is usual practice for the milk to be

pasteurized. This is generally effected by ensuring

that the milk-feed line, which passes through each of

the jackets so as to preheat the milk (final effect jacket

first, first effect last), goes through a special section in

the first effect jacket. Here, the temperature is

boosted by extra steam. After regenerative cooling,

the milk then enters the first effect or is diverted for

additional heat treatment (for example, holding at

elevated temperatures, as required for milk to be

used for high-heat powder production). The milk

then passes through each of the calandria in turn, its

concentration obviously increasing (and boiling

tbl0001 Table 1 Materials in the food and related biological industries that require evaporation in their manufacture

Dairy products Food products Other biological liquids

Baby foods Agar Amino acids

Buttermilk Beef stock Calcium formate

Fat-enriched milk Beer Corn steep liquor

Icecream mix Brewers’ yeast Distillery pot ale

Reconstituted milk Chestnut extract Distillery spent wash

Skim milk Chicken stock Fish stick liquor

Sweetened condensed milk Clam broth Formic acid

Whey Coffee extract Gelatine

Whole milk Dextrose Gluconic acid

Yogurt milk Glucose Glycerine

Golden syrup Hydrolyzed protein

Fruit juices Hopped wort Itaconic acid

Apple Invert sugar Lactone solution

Apricot Jam Lignin

Blackcurrant Jelly Liquorice water

Cherry Lactic acid Methanol extracts

Grape Malt extract Molasses effluent

Grapefruit Meat broth Monosodium glutamate

Lemon Meat extract Pentaerythritol

Lime Soy sauce Phenol formaldehyde resin

Mandarin Sucrose Tannin extract

Orange Tea extract Wheat starch effluent

Passion fruit Turkey stock

Pear Vegetable extract

Pineapple Whole egg

Plum Yeast cream

Raspberry Yeast extract

Strawberry

Tangerine

Tomato

2208 EVAPORATION/Uses in the Food Industry

temperature decreasing) in each subsequent effect.

The last effect is often termed the ‘finisher’ and oper-

ates at a higher temperature (and temperature differ-

ence) than the penultimate effect, due to an additional

steam supply. One or two thermal vapor recompres-

sion units are used. The internal design of the finisher

is usually configured to handle the higher viscosity

associated with the end product.

0047 Alternatively, an MVR system could be employed.

Depending on the overall plant capacity, and final

product solids, the system would typically comprise

a main calandria, where most of the water would be

removed, followed by a finisher (possibly TVR), to

remove the remainder of the water. (See Milk:Pro-

cessing of Liquid Milk.)

0048 -Whey Whey is often characterized by its level of

acidity, and hence can be either low-acid ‘sweet’ whey

(from hard or semihard cheese) or high-acid ‘acid’

whey (from soft cheese, e.g., cottage cheese).

0049 The whey (typical 6% total solids) is normally

evaporated after pretreatment to remove fat (and/or

cheese fines) up to between 40 and 60% total solids,

the choice of the final product solids depending on

subsequent processing needs. If the whey concentrate

has to be transported, 40% is preferred, to avoid

crystallization taking place en route and causing

unloading difficulties. Alternatively, when the whey

is to be spray-dried (on-site), 60% is better. Both TVR

and MVR falling-film equipment can be used, the

latter configuration usually giving – in this applica-

tion as well as others – better energy economy. (See

Whey and Whey Powders: Production and Uses.)

Fruit Juices

0050 -Citrus juices High-quality orange juice has been

concentrated to between 65 and 75% total solids.

Some types of comminuted orange can only be

evaporated to about 45%.

0051 Grapefruit juice is typically concentrated to 65%

total solids, whilst lemon and lime juices are pro-

cessed to 45 and 40% total solids, respectively. At

these final evaporator concentrations, color quality is

maintained intact.

0052 In order to maximize flavor retention, an aroma

recovery system should be incorporated into the plant

design.

0053 -Apple juice Most apple juice plants concentrate to

72–75% total solids. Aroma recovery should be used

to ensure maximum retention of the original fruit

flavor characteristics.

0054 -Tomato juice It is possible to use film-type eva-

porators to preconcentrate tomato juice up to

approximately 20% total solids. To obtain higher

solids, a swept-surface unit can be considered.

0055-Other juices Typical concentrations that are pos-

sible from commercial evaporation systems are black-

currant 65–70%, grape 70–75%, pineapple 65%,

raspberry and strawberry 70–75%.

Food and Related Products

0056-Starch industry The main evaporator duties in the

corn starch industry are associated with the concen-

tration of corn steep liquor (e.g., for animal feed and

antibiotic production medium purposes). Because

corn steep liquor is primarily a byproduct, it contains

many ingredients, including mainly sugars and

proteins, and their derivatives. It shows, therefore,

relatively high fouling characteristics, particularly

at higher concentrations. Its evaporation requires

the use of a plate or tubular falling-film evaporator

for the lower concentration stages, and a forced-

circulation arrangement for the final effect. Similar

byproduct recovery systems, using evaporation tech-

nology, are employed in the wheat starch industry.

(See Starch: Sources and Processing.)

0057-Sugar industry The beet and cane sugar industries

use evaporation to concentrate the dilute sugar steam,

as extracted from the beet or cane, up to the final

concentration prior to crystallization, itself a form of

evaporation. Such systems are typically multieffect

tubular systems, incorporating vapor recompression.

(See Sugar: Refining of Sugarbeet and Sugarcane.)

0058The production of sugar syrups (glucose, maltose,

fructose) also requires evaporation: the raw syrup

(often a product of the acid, alkali, or enzymatic

hydrolysis of starch) is typically generated at

20–30% total solids and will need concentration up

to 70–80% final solids. The evaporator is almost

always a falling-film unit, incorporating either ther-

mal or mechanical vapor recompression. Unlike the

byproducts of starch processing (see above), sugar

syrups show very low fouling factors on evaporation.

0059-Gelatin industry The current method is to employ a

multiple-effect TVR or single-stage MVR (with or

without finisher). The feed material will usually be a

dilute gelatin material at around 5% total solids. It

will be concentrated to typically 20% total solids,

at which point, there is an option of sterilizing the

product by live steam injection (at 135–145



C) and

holding the product for up to 8 s. Flashing into

a vacuum chamber cools the product down to

100–105



C, and then the gelatin is further concen-

trated to a final level of 30% total solids or there-

abouts, usually by flash evaporation.

EVAPORATION/Uses in the Food Industry 2209

0060 -Lactic acid Food-grade material is usually gener-

ated by fermentation, and the resultant acid – after

primary removal of impurities – will be at around

10% total solids. This can be concentrated by evap-

oration to 80–82% total solids. A typical installation,

operating at a feed rate of 5 tonnes per hour, consists

of three effects (including thermal vapor recompres-

sion) and a finisher unit. The boiling temperatures in

these three effects are, respectively, 87, 75, and 50



and 50



C in the finisher. Grade 316 stainless steel is

required due to the acidity of the product.

0061 -Beverage industry Most fermented beverage indus-

tries that employ distillation to produce a concen-

trated alcoholic product (e.g., whisky, gin, cognac)

generate large volumes of low-alcohol ‘waste’

streams, i.e., the remaining fraction after the alcohol

has been removed by distillation.

0062 The Scotch whisky industry produces two effluent

streams: ‘pot ale’ from malt distilleries and ‘spent

wash’ from grain distilleries. Although these mater-

ials closely resemble each other, spent wash is nor-

mally more difficult to evaporate to high solids due to

a higher level of suspended solids and a greater degree

of fouling.

0063 An example of an evaporator to handle, say, 60

tonnes of feed stock (pot ale) per hour at 4% total

solids, and concentrate to a final value of 50%

total solids, would be configured as follows: a

double-effect falling-film MVR system (with turbine

fans to recompress the vapor, and optionally a double

calandria for the first effect to handle the volume of

feed material), supplemented by a forced-circulation

finisher. The feed temperature depends on prior hand-

ling and storage. The boiling temperature in the two

MVR effects would be, respectively, 96 and 91



The finisher would operate at 91



C also. The main

fans would draw around 500–550 k Wh, which, to-

gether with other motors on the evaporator, would

give an overall power consumption of some 12 kWh

per tonne of feed stock. TVR systems are still used;

however, most new installations incorporate MVR.

0064 Other whisky distilleries (in Ireland and the USA)

yield similar byproducts, which can be evaporated in

a similar manner.

0065 The neutral spirits industry (gin, vodka, etc.) gen-

erates byproducts of a related composition, albeit

higher in inorganic salts (e.g., calcium sulfate), which

have inverse solubility curves, leading to increased

levels of fouling at higher concentrations. Again a

forced-circulation layout will be the preferred design

for the finisher.

0066 Effluent from the production of cognac requires

pretreatment to remove tartaric acid and tartrates

before evaporation can be carried out.

Energy Economy

0067The vapor removed in a single evaporator stage

(effect) basically equals the amount of steam fed to

that effect. The temperature of the evaporated vapor

is, however, lower than the temperature of the heating

steam and cannot therefore be used as such to reheat

the same stage; instead, the vapor in such a system is

led to the condenser, where the remaining heat con-

tent is lost. The specific steam consumption (i.e.,

steam used/vapor removed) of a single-effect evapor-

ator is thus unity.

0068The specific steam consumption can be halved by

the addition of a second effect, which is heated

by the vapor from the first effect but is operated at a

lower temperature because the heating medium is at

a lower temperature.

0069Addition of further effects thereby reduces (i.e.,

improves) pro rata the specific steam consumption.

Although it would be possible in theory to keep

adding effects ad infinitum so as to reduce the specific

consumption to an almost negligible level, in practice,

this possibility is limited for two reasons. First, the

increase in capital cost associated with adding extra

effects has to be balanced against the reduction in

operating cost (i.e., steam); for example, in the dairy

industry, it was found that additional effects over and

above seven did not pay for themselves in terms of

savings in operating costs. Second, between each

effect, there is a drop in temperature of perhaps 5



(because of the above-mentioned difference in tem-

perature between heating medium and vapor driven

off), and coupling this with the fact that, for most

food and related products, there is a maximum and

minimum evaporation temperature (for reasons of

thermal and bacteriological stability, respectively),

there is thus another limit to the maximum number

of effects permissible in industrial practice.

0070An alternative method of improving the energy

consumption is by the use of vapor recompression.

This technique involves taking the product vapor

from the effect, compressing it to increase its tem-

perature, and then returning it for reuse in that same

effect as the heating medium; the vapor condenses as

a result and is subsequently led away for disposal.

However, in the process, the specific steam consump-

tion for that effect has been nominally halved. It must

be remembered, of course, that additional energy is

required to recompress the vapor, but this is small

when compared to the energy content of the com-

pressed vapor generated.

0071The vapor can be compressed either thermally or

mechanically: in practice, this means, respectively,

either by fresh, high-pressure steam or by a form of

mechanical compressor. Where steam is employed, the

2210 EVAPORATION/Uses in the Food Industry

system is known as TVR, whilst mechanical systems

are known as MVR. This latter system will usually

involve a fan-based compressor rather than a conven-

tional centrifugal-type unit, but individual applica-

tions and circumstances will dictate the final choice.

0072 The relatively small additional energy costs re-

quired for this vapor recompression are hence in the

form of either extra steam or extra electricity costs.

The main components of a vapor-recompression

evaporator are shown in Figure 4.

0073 As can be seen, once the evaporation process has

been brought up to a steady-state operating condition

(that is, from start-up), no further addition of primary

heating steam is needed (i.e., excluding steam to the

steam ejector, for thermal vapor compression). Evap-

oration is brought about solely by recompressing and

‘recycling’ vapor separated from the product. In prac-

tice, however, heat losses – and often the requirements

of (steady-state) control systems – mean that a small

but finite quantity of additional steam is required.

Energy Consumption

0074 The power demand of the compressor depends on

the vapor quantity and the pressure difference

(equating to the temperature difference, T). This

means that the power demand is inversely propor-

tional to the heat-transfer surface of the evaporator.

When the heat-transfer surface of the evaporator

is increased, the T required and the power demand

decrease.

0075With evaporating temperatures between 50 and



C, the power demand of the compressor can be

generally calculated using the following equation:

P ¼ CMT, ð1Þ

where P is the power demand (kWh

1

), M is the

vapor mass flow (tonne h

1

), C is a constant, which

depends mainly on the size of the compressor and

is usually between 2.5 and 3.0 (kW tonne

1



1

and T is the temperature difference across the com-

pressor.

0076Usually, the T is between 3 and 8



C, which corres-

ponds to a specific power consumption of between

8 and 20 kW per 1000 kg of water evaporated.

See also: Drying: Theory of Air-drying; Milk: Processing of

Liquid Milk; Starch: Sources and Processing; Sugar:

Refining of Sugarbeet and Sugarcane; Whey and Whey

Powders: Production and Uses

Further Reading

Coulson JM and Richardson JF (1976) Chemical Engineer-

ing, vol. 2, 2nd edn. Oxford: Pergamon Press.

Kay JM (1968) An Introduction to Fluid Mechanics and

Heat Transfer, 2nd edn. Cambridge: Cambridge Univer-

sity Press.

To LC, Tade MD and Kraetzl M (1999) Robust Nonlinear

Control of Industrial Evaporation Systems, 340 pp.

London, UK: World Scientific.

Concentrate

Separator

Vapor recirculation

Distributor

Calandria

Condensate

Feed

Compressor

or fan

Concentrate

(a) (b)

Separator

Steam

ejector

Vapor recirculation

Distributor

Calandria

Condensate

Feed

fig0004 Figure 4 Main components of vapor recompression evaporators: (a) TVR and (b) MVR. Reproduced from Evaporation: Uses in the

Food Industry, Encyclopaedia of Food Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993,

Academic Press.

EVAPORATION/Uses in the Food Industry 2211

EXERCISE

Contents

Muscle

Metabolic Requirements

Muscle

S C Dennis and T D Noakes, University of Cape Town

Medical School, Observatory, South Africa

Introduction

0001 Muscles in the human body can be broadly classified

into three main types: smooth, cardiac, and skeletal.

Smooth muscles surround many of the body’s internal

organs and control such functions as the motility of

the gastrointestinal tract and the flow of blood

through a vascular bed. Cardiac (heart) muscle regu-

lates the rate at which blood is pumped around the

body. Skeletal muscles account for around 40% of

body weight and are responsible for posture and

movement. In this article, attention is focused on the

physiology of skeletal muscle.

Skeletal Muscle Organization

0002 Unlike smooth and cardiac muscles, which contract

spontaneously and are regulated by hormones and

electrical messages from the subconscious regions of

the brain, skeletal muscle fiber cells are under volun-

tary control. With the exception of certain reflexes,

skeletal muscle fibers contract only when an electrical

signal is sent from the motor cortex of the brain to the

muscle via the spinal and motor nerves.

Motor Units

0003 The response of a skeletal muscle to electrical im-

pulses from the brain depends on the motor nerves

used to send the signal to contract. When signals to

contract are sent via motor nerves that supply as few

as 5–15 muscle fibers, tension is adjusted in small

increments, and movements are precise. However,

when signals to contract are delivered to the muscle

by motor nerves that supply several hundred muscle

fibers, large increases in tension lead to strong, but

less precise, contractions.

0004 Information on the contraction of the skeletal

muscle motor units is relayed back to the central

nervous system from stretch receptors in the muscle

spindles and tension receptors in the Golgi tendon

organs. This feedback allows the initial movement

to be refined by either reducing motor unit activation

or recruiting additional motor units.

Muscle Fiber Types

0005The small to intermediate motor units in skeletal

muscle are generally recruited first during exercise.

They are comprised of either red slow-twitch (type I)

or red fast-twitch (type IIa) muscle fibers, which are

relatively resistant to fatigue. Type I and IIa muscle

fibers contain high concentrations of a red oxygen

transport protein called myoglobin and obtain most

of their energy for contraction from oxidative pro-

cesses in structures called mitochondria.

0006In contrast, large motor units are used only when

either a very heavy weight has to be lifted or an

explosive movement is required. These units are com-

prised of white fast-twitch (type IIb) muscle fibers,

which contain few mitochondria and little myoglo-

bin, and fatigue rapidly. White muscle fibers are

designed to power escape reactions and, as will be

described later, they generate energy for contraction

via an oxygen-independent pathway which maintains

maximum force development for around 20 s.

Skeletal Muscle Ultrastructure

0007When cross-sections of skeletal muscle are viewed

under a microscope, type I, IIa, and IIb fibers are

found to be arranged in parallel and to be mixed

randomly. Ratios of types I:IIa:IIb range from

51:41:8% in untrained persons, to 78:19:3% in elite

endurance athletes. (See Cells.)

Myofibril Arrangement

0008Within the different muscle fibers, there are several

hundred to several thousand smaller parallel fibers

known as myofibrils. Surrounding each myofibril is

a lattice-like network of tubules, which is part of the

sarcoplasmic reticulum. As will be described, the sar-

coplasmic reticulum is involved in the initiation and

termination of contraction.

0009Interspersed amongst the myofibrils are columns of

small semicircular mitochondria which provide

2212 EXERCISE/Muscle

energy in the form of adenosine triphosphate (ATP).

Between the mitochondria are scattered glycogen

granules and triglyceride droplets. The muscle trigly-

ceride and glycogen stores are important intracellular

sources of fuel during prolonged, low-intensity exer-

cise. (See Glycogen.)

0010 When viewed laterally at high magnifications,

myofibrils are seen to be comprised of a series of

units known as sarcomeres. Sarcomeres are separated

by structures called Z disks, which pass from myofib-

ril to myofibril. Adjacent to the Z disks are light I

bands and in the center of each sarcomere is a dark A

band.

Myofilament Arrangement

0011 The I and A bands of the sarcomere are caused by a

partial interdigitation of yet smaller parallel fibers,

called thin and thick myofilaments. Thin myofila-

ments emerge from the Z disks to form the I bands,

and centrally positioned thick myofilaments create

the A band.

0012 Within the A band, around 1500 thick myofila-

ments are surrounded by hexagonal arrays of twice

that number of thin myofilaments. Thick myofila-

ments are anchored by a lattice network of proteins

at the center of the sarcomere. Thin myofilaments are

kept in register by their interdigitation with the thick

myofilaments and their attachment to the Z disks.

Skeletal Muscle Contractile Proteins

0013 When sarcomeres contract, the thin myofilaments

slide between the stationary thick myofilaments and

pull the Z disks closer together. Sarcomeres shorten

whenever the proteins of the thick and thin myofila-

ments are allowed to interact.

Myosin

0014 Thick myofilaments are comprised of around 300

spirally arranged protein molecules called myosin.

Each myosin molecule is composed of two heavy

chains and four light chains. The heavy chains form

fibrous, rigid ‘coiled coils’ in what are known as the

tail and neck regions and then split into globular

heads, on to which the light chains are attached.

Hinges at either end of the neck allow the myosin

heads to protrude from the thick myofilaments and

rotate freely during the cross-bridge cycling (de-

scribed later) with the thin myofilaments.

Actin

0015 In thin myofilaments, chains of globular G actin pro-

tein molecules are assembled into double helical fila-

ments. On each G actin molecule are sites to which

myosin heads will bind and, in resting muscle, these

myosin binding sites are blocked by tropomyosin

protein molecules, which are similar in structure to

the myosin tail.

Tropomyosin–Troponin Complex

0016At 40-nm intervals along the thin myofilaments, tro-

pomyosin molecules are attached to troponin com-

plexes. In the troponin complexes, there are three

proteins: troponin I, troponin T, and troponin C.

Troponin I anchors the complex to the thin myofila-

ment; troponin T attaches to the tropomyosin mol-

ecules, and troponin C binds calcium. As will be

described, an elevated intracellular calcium ion

(Ca

2þ

) concentration is the mechanism by which elec-

trical signals lead to muscle contraction.

Skeletal Muscle Electromechanical

Coupling

0017Electrical signals to contract are delivered to the mid-

points of muscle cells by branches of the motor nerve.

As the motor nerve enters the muscle, it divides into a

number of nerve endings that come to lie in troughs in

the outer membranes of each of the muscle cells of the

motor unit.

Neuromuscular Transmission

0018Within the nerve endings are about 300 000 small

vesicles, which contain the chemical neurotransmit-

ter, acetylcholine. When a wave of electrical excita-

tion arrives at the nerve ending, some Ca

2þ

ions flow

into the nerve and cause around 300 vesicles to fuse

with the nerve cell membrane. Acetylcholine mol-

ecules released from the vesicles then diffuse across

a20–30-nm cleft and bind to receptors on the under-

lying muscle cell membrane. At the membrane, the

binding of acetylcholine opens receptor-operated

channels which allow positively charged sodium

ions (Na

) to flow into the electronegative interior

of the muscle cell. In the millisecond or so before the

acetylcholine is broken down, the entry of Na

ions

decreases the electrical potential across the muscle

cell membrane and triggers a subsequent wave of

electrical excitation along the muscle cell.

Electrical Conduction Along Muscle Cells

0019Electrical excitation arises when the electrical poten-

tial across the muscle cell membrane decreases to a

threshold at which voltage-gated Na

and potassium

) channels open. First, Na

channels open and a

rapid influx of Na

ions causes the inside of the cell to

become positive for 1–2 ms. Then, as Na

channels

close, K

channels open and a rapid efflux of K

ions

EXERCISE/Muscle 2213

returns the interior of the muscle cell to its resting

electronegative state.

0020 With the transient depolarization of the muscle cell

membrane at the neuromuscular junction, ions are

pulled away from adjacent muscle regions which de-

creases their membrane electrical potential. This sets

off further voltage-activated Na

influx. In this

manner, waves of depolarization propagate outwards

from the center of the motor unit towards the ends of

the muscle fibers.

Voltage-dependent Sarcoplasmic Reticulum Ca

2þ

Release

0021 Waves of depolarization on the muscle cell surface

descend to the myobrils via transverse T tubules,

which are continuous with the surface cell membrane

and pass into the muscle cell at the junctions between

the thick and thin myofilaments. On either side of the

T tubules, the cisternae or ‘chests’ of the sarcoplasmic

reticulum are attached to the T tubules by structures

known as foot processes.

0022 These foot processes are comprised of four proteins

which open Ca

2þ

channels in sarcoplasmic reticulum

cisternae by altering their shape during waves of T

tubule membrane depolarization. Under these cir-

cumstances, the rapid release of Ca

2þ

from the cister-

nae of the sarcoplasmic reticulum exceeds the rate at

which Ca

2þ

is pumped back into the sarcoplasmic

reticulum via the previously described network of

longitudinal tubules. As a result, the concentration

of Ca

2þ

ions around the myofilaments rises. (See

Calcium: Physiology.)

2þ

Activation of Thin Myofilaments

0023 As the free Ca

2þ

concentration inside the muscle cell

increases from its resting value of around 4 mgl

1

40–400 mgl

1

, the troponin–tropomyosin protein

complexes on the thin actin myofilaments rotate to

expose the binding sites for the myosin heads. The

mechanism by which this occurs is not precisely

known, but one possibility is that the binding of Ca

2þ

to troponin C tugs the tropomyosin into the grooves

between the double helical actin myofilaments.

Cross-bridge Cycling

0024 Once thin actin myofilaments are ‘activated’ by Ca

2þ

ions, they are pulled towards the center of each sar-

comere by a process known as cross-bridge cycling.

Cross-bridge cycles are formed by repeated attach-

ments and detachments of the myosin heads of the

thick myofilaments to the actin of the thin myofila-

ments.

0025 Energy for cross-bridge cycling comes from the

binding of ATP to the myosin head, which causes

the myosin head to be cocked like the trigger of a

gun at a ‘strained’ 90



angle to the thick myofilament.

At this angle, the bound ATP is cleaved to adenosine

diphosphate (ADP) and inorganic phosphate (P

) and

that cleavage creates an actin-binding site on the

myosin head. Attachment of the myosin head to

actin then leads to a release of P

which causes the

‘strained’ 90



myosin head to move to an ‘unstrained’



angle to the thick myofilament. This movement

pulls the actin thin myofilament and the attached Z

disk some 12 nm towards the center of the sarcomere.

At full excursion, the ADP is then replaced by ATP

which causes a rapid detachment from the actin and a

return of the myosin head to the ‘strained’ 90



angle.

0026Myosin head detachments do not occur in unison.

While some myosin heads are hydrolyzing ATP to

reestablish actin-binding sites for further cross-bridge

cycling, other myosin heads are attached. Thus, while

free intracellular Ca

2þ

concentrations remain high, a

sarcomere shortens continuously and that continuous

shortening, repeated in millions of sarcomeres in

series, is what we see as muscle contraction.

Muscle Movement

0027The slowest step in a muscle contraction depends on

the load opposing the movement of the myosin heads

to their 45



state. When a muscle shortens against a

near-zero load, contraction velocity is limited by the

rate at which the myosin heads can hydrolyze ATP to

recreate an actin-binding site. However, as the load

opposing shortening is increased, control of contrac-

tion velocity shifts away from the rate of ATP hy-

drolysis towards the speed at which the myosin

heads can detach from the actin filament.

0028A useful analogy is a man pulling on a rope tied to

an increasingly heavy weight. As the weight becomes

heavier, less of his time is spent in changing hands and

more is spent in hauling on the rope.

Maximum Shortening Velocity

0029In unloaded contractions at high velocity, where less

than 5% of the myosin heads are attached at any one

time, maximum rates of shortening depend almost

exclusively on the speed at which the myosin heads

can hydrolyze ATP to form actin-binding sites. Since

fast-twitch muscle fibers hydrolyze ATP at more than

twice the rate of slow-twitch muscle fibers, muscles

with a high proportion of fast-twitch motor units are

able to contract more rapidly against a minimal load

than are muscles with mainly slow-twitch motor units.

Maximum Power Output

0030A high proportion of fast-twitch muscle fibers is also

of advantage for maximum power output. Optimum

2214 EXERCISE/Muscle

power output is achieved by adjusting the number of

motor units recruited so that each shortens against

one-third of its maximum load. Against such loads,

muscles contract at around one-third of their max-

imum velocity, and about 20% of the myosin heads

are attached. Maximum power output is therefore

also largely determined by the rate at which myosin

heads can hydrolyze ATP to recreate binding sites

for actin.

Maximum Load

0031 As the load opposing shortening is further increased,

however, rates of ATP hydrolysis become less import-

ant. Once a muscle can only restrain a load, all of the

myosin heads are attached and the maximum load is

more a function of the size of the muscle than of its

fiber-type composition.

Energy Supply for Muscle Movement

0032 Irrespective of the mode of muscle contraction, the

ATP hydrolyzed by the myosin heads has to be imme-

diately resynthesized to allow the cross-bridges to

detach. In working muscles, the half-life of an ATP

molecule can decrease to a few seconds.

Phosphocreatine Hydrolysis

0033 With a sudden increase in muscle work, ATP for

relaxation and contraction is regenerated from ADP

and P

via a number of mechanisms. The most imme-

diate source of ATP is from a reservoir of chemical

energy stored in the form of creatine phosphate. From

measurements of muscle creatinine phosphate break-

down to creatine and P

during severe exercise, it

has been calculated that net creatine phosphate hy-

drolysis can sustain maximal contractile activity for

about 4 s.

Oxygen-independent Glycogen Utilization

0034 Following the initial hydrolysis of creatine phos-

phate, the next major source of energy for ATP

resynthesis is from the breakdown of the glucose

molecules in muscle glycogen to lactate ions via a

metabolic pathway known as oxygen-independent

glycolysis. Because it takes time for muscle blood

flow to increase at the start of exercise, the oxygen-

independent glycolytic pathway is particularly im-

portant in the white fast-twitch (type IIb) muscle

fibers that power escape reactions. Type IIb muscle

fibers generate almost all of their ATP for contraction

by an acceleration of glycogen to lactate conversion.

0035 During sprinting, glycogen breakdown is increased

by activation of an enzyme called glycogen phosphor-

ylase. Glycogen phosphorylase activity is increased by

two rather complex mechanisms. Both occur simul-

taneously but, for the sake of simplicity, they are

described separately.

0036One mechanism of glycogen phosphorylase acti-

vation is via a hormonally controlled conversion of

the enzyme from a ‘b’ form to a more active ‘a’ form.

If a need to escape is anticipated, circulating epineph-

rine (adrenaline) concentrations rise and stimulate the

intramuscular production of cyclic adenosine mono-

phosphate (cAMP). Increases in cAMP concentration

cause a rapid conversion of glycogen phosphorylase b

to a when free Ca

2þ

concentrations increase for con-

traction. (See Hormones: Adrenal Hormones.)

0037Another mechanism of glycogen phosphorylase ac-

tivation arises from the sudden increase in ATP hy-

drolysis at the myosin heads. This ATP is initially

resynthesized by a breakdown of creatine phosphate

to creatine and P

and by a condensation of two ADP

molecules to form one of ATP and one of adenosine

monophosphate (AMP). A reduction in creatine

phosphate concentration and a rise in P

, and AMP

concentrations act in concert to stimulate glycogen

phosphorylase activity.

0038At the same time, changes in creatine phosphate, P

and AMP concentrations also accelerate the rate of

glycolysis by increasing the forward reaction and de-

creasing the backward reaction of a substrate cycle

involving two enzymes known as phosphofructo-

kinase and fructose diphosphatase. Fructose dipho-

sphatase is unique to type IIb fibers and inhibition of

its backward reaction allows the forward rate of gly-

colysis in these fibers to be increased more than 1000-

fold.

0039In response to intense muscle activity, glycogen

utilization in type IIb fibers rises from around 0.01

to 11 g min

1

fresh weight. Since the glycogen

content of human muscle is roughly 15 g kg

1

fresh

weight, oxygen-independent glycolysis could in theory

provide enough energy for 80 s of maximum activity.

In practice, however, the mobilization of glycogen is

limited to around 5 g kg

1

fresh weight and max-

imum power output cannot be maintained for much

longer than 20 s.

0040When ATP is resynthesized by glycolysis, rather

than by mitochondrial oxidative phosphorylation or

creatine phosphate breakdown, the hydrogen ions

) produced by its hydrolysis are not reconsumed

and H

accumulation may be an important factor in

the rapid development of fatigue under these circum-

stances (Figure 1). When carbohydrates are metabol-

ized at high rates to fuel intense muscle activity,

hydrogen ions accumulate from the turnover of

glycolytic ATP. Hydrogen ions buffered by P

2

ions form P



ions, which inhibit P



release from

the myosin heads and slow cross-bridge cycling.

EXERCISE/Muscle 2215

This feedback inhibition of cross-bridge cycling by



accumulation insures that the muscles’ demand

for energy cannot exceed the energy supply. If that

were to occur, falling ATP concentrations would lead

to an irreversible muscle rigor and cell death.

Oxygen-dependent Metabolism

0041 Oxygen-independent metabolism is therefore of little

use to the endurance runner. For sustained exercise,

the H

ions arising from ATP hydrolysis have to be

used in its resynthesis and this means that most of the

ATP must be derived from mitochondrial oxidative

phosphorylation.

0042 The major fuels for mitochondrial oxidative phos-

phorylation are muscle glycogen, blood glucose,

and circulating fatty acids. Muscle glycogen and

blood glucose are particularly important fuels during

high-intensity exercise. As work rate is increased

beyond 70% of an individual’s maximum oxygen

) uptake (Vo

2max

) to 95% of Vo

2max

, the contri-

bution to energy production from carbohydrate oxi-

dation rises to 100%.

0043 For some reason, fatty acid oxidation can only

provide enough ATP for around 50% of maximum

power output. Fats are therefore mainly used, to-

gether with carbohydrate, in prolonged low-intensity

(30–65% of Vo

2max

) exercise. During such exercise,

the contribution to energy production from fat oxi-

dation climbs from 37–39% to 62–67% as rises in

blood glucose utilization increasingly fail to compen-

sate for muscle glycogen depletion. (See Fatty Acids:

Metabolism.)

0044 The advantage of muscle fatty acid oxidation in

prolonged exercise is that it spares some liver glyco-

gen breakdown for tissues such as the brain which are

totally dependent on glucose. Without fat oxidation,

it can be calculated that the carbohydrate reserves of

the liver would be exhausted within an hour or so of

exercise, while the remaining fat cell triglyceride

stores would be sufficient for another 2–3 days.

0045 Consequently, muscle fuel utilization is ultimately

regulated by the delivery of glucose from the liver into

the blood stream. Release of liver glucose is promoted

by decreases in circulating glucose concentration and

rises in circulating glucagon and epinephrine concen-

trations. Glucagon and epinephrine binding to the

liver cell membrane increase liver glycogen phosphor-

ylase b to a conversion via a mechanism similar to

that in muscle fibers. The only differences in the liver

are that glucagon binding accelerates intracellular

cAMP formation, and epinephrine binding raises

free intracellular Ca

2þ

concentrations. (See Glucose:

Function and Metabolism; Maintenance of Blood

Glucose Level.)

0046Glucagon and epinephrine binding also increase

the provision of fatty acids to the muscles by activat-

ing the cleavage of triglyceride stores to fatty acids

and glycerol. In fat cells, both hormones act via a

stimulation of cAMP production to convert a less

active triglyceride lipase b enzyme to a more active

triglyceride lipase a enzyme. Release of fatty acids

extends endurance by decreasing blood glucose

utilization.

0047Use of blood glucose and fatty acids by contracting

muscles is regulated by the rate of mitochondrial

ATP resynthesis. Mitochondrial ATP resynthesis is

coupled to a metabolic pathway known as the elec-

tron transport chain. In the electron transport chain,

ions and electrons from carbohydrate and fat

breakdown are oxidized to water and this oxidation

only proceeds when ADP is available for phosphoryl-

ation. Thus, until the carbohydrate stores become

depleted, muscle metabolism is controlled by the

demand for energy rather than the supply of fuel.

See also: Calcium: Physiology; Cells; Fatty Acids:

Metabolism; Glucose: Function and Metabolism;

Maintenance of Blood Glucose Level; Glycogen;

Hormones: Adrenal Hormones