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

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.0036 Scale insects (Aspidiotus lataniae)

0037 Fig flies (Lonchaea aristella).

0038 In addition, dried figs have been found to be dam-

aged by fig moth (Ephestia cantelia) and the Indian

meal moth (Plodia interpunctella). Some of these

pests can cause more than 50% losses in yield. The fig

leaf roller (Phycodes mina) has been reported in Hary-

ana (India), for which excellent control was obtained

with trichlorphon (0.05%) and chlorpyrifos (0.03%).

0039 Figs are also susceptible to root-knot nematodes,

which can be controlled by using DD-mixture.

0040 Leaf rust is an important disease of figs in all the

fig-growing countries of the world that is caused by

Cerotelium fici (Syn. Uredo fici). Rust-affected leaves

show small, round, brownish to black eruptive lesions

mostly on the lower surface. The affected leaves drop

off prematurely and, in severe cases, affect the yield of

fruits. This disease can be controlled by dusting with

sulfur or spraying with zineb or Bordeaux mixture.

0041 Anthracnose (caused by Sphaceloma fici-caricae), a

leaf spot disease (caused by Cylindrocladium scopar-

ium) and mosaic disease (caused by virus) of fig have

also been reported from different parts of India.

0042 Some important diseases of fig fruits are caused by

Botrytis cinerea, Alternaria fici, Aspergillus niger,

Fusarium moniliforme, and Cladosporium sicophi-

lum, which can be controlled by using some effective

fungicides. Rhizopus rot (caused by Rhizopus stolo-

nifer) of fig is also quite common among storage

rots and can be controlled by dipping the fruits in a

solution of aureofungin and copper sulfate.

0043 Sunburn and fruit splitting are the important dis-

orders of fig plants. Newly planted young trees are

mostly affected by sunburn in which the affected parts

crack, and the bark peels off. Heavy pruning exposing

the trunk and branches is responsible for sunburn.

Whitewashing of exposed parts can prevent this

malady.Fruitsplittingusuallyoccursifthereisashower

during ripening of fruits. Fruit splitting damages the

fruit, making it completely unfit for consumption.

Uses

0044 Figs are consumed fresh, dried, preserved, canned, or

candied. Fresh figs are delicious and can be used as a

dessert or for jam. The great bulk of the crop is con-

sumed as dried fruit. Better grades of dried fig are used

for making fancy packs, whereas other grades are

used for the production of alcohol and wine. Figs

are also roasted like coffee beans and can be used as

a substitute of coffee. Other fig products are spiced or

pickled figs, fig bread, fig meat, and fig brownies.

0045 Fresh figs generally consist of 84% pulp and 16%

skin. The chemical composition varies with type. The

average nutritional composition of dried figs (per

100 g) has been recorded as follows:

0046Energy: 220 cal

0047Protein: 3.3 g

0048Carbohydrate: 48.0 g

0049Fat: 1.5 g

0050Fiber: 12.4 g

0051Sodium: 62 mg

0052Potassium: 970 mg

0053Vitamin C: 1 mg

0054Thiamin: 0.08 mg

0055Riboflavin: 0.1 mg

0056Nicotinic acid: 0.8 mg

0057Pantothenic acid: 0.5 mg

0058Folic acid: 9 mg

0059Vitamin B

: 0.26 mg

0060Calcium: 250 mg

0061Iron: 4.2 mg

0062Phosphorus: 89 mg

0063Magnesium: 80 mg

0064Copper: 0.3 mg

0065Zinc: 0.7 mg

0066Figs owe their food value chiefly to their mineral

and sugar contents. The total mineral content is two

to four times that of most other fresh foods, and only

cheese and a few types of nuts have a higher calcium

content. They are richer in iron and copper than

nearly all fruits and vegetables and most other dried

fruits. The nutritive index of fig in comparison with

that of other fruit is reported to be as follows: fig, 11;

apple, 9; resin, 8; date, 6; pear, 6.

0067The fruit, fresh or dried, is valued for its laxative

property. It is diuretic, demulcent, emollient, and

nutritive, and is used in the form of confection and

syrups. Figs are considered useful in the prevention

of nutritional anemias. The ash of figs is highly alka-

line.

0068The fig tree yields a latex that is used as an anthel-

mintic. It is toxic when administered parenterally to

animals but has no toxic effect when administered

orally. The anthelmintic action has been traced to

ficin, a proteolytic enzyme that has the remarkable

power of digesting living helminths. This ficin also

reacts with protein of the skin, causing dermatitis.

Some individuals are adversely affected by such

materials and cannot ingest fresh figs without skin

eruptions around the mouth. Many field workers in

fig orchards suffer from dermatitis on the arms and

body if they brush against the limbs and leaves, or as

they handle the fresh fruits. Gloves and protective

aprons are generally provided for such workers.

0069Fresh fig latex also shows milk-clotting activity,

which may be 30–100 times greater than that of the

animal rennet prepared from calf stomach mucosa.

2398 FIGS

See also: Date Palms; Jams and Preserves: Methods of

Manufacture; Methods of Manufacture

Further Reading

Anonymous (1956) The Wealth of India. Raw Materials,

vol. IV, pp. 26–32. New Delhi: Council of Scientific and

Industrial Research.

Condit IJ (1941) Fig Culture in California. California Agri-

cultural Extension Service Circular 77, pp. 1–67.

Berkeley, CA: University of California.

Condit IJ (1947) The Fig. A New Series of Plant Science

Books, vol. XIX, pp. 1–222. Waltham, MA: Chronia

Botanica Company.

Condit IJ (1955) Fig Varieties: A Monograph. Hilgardia

23 (No. 11), pp. 323–538. Berkeley, CA: California

Agriculture Experimental Station.

Eisen G (1901) The Fig: Its History, Culture and

Curing. USDA, Division of Pomology Bulletin No. 9,

pp. 1–317. Washington, DC: Government Printing

House.

Hill AF (1952) Economic Botany, pp. 420–422. Bombay:

Tata McGraw-Hill.

Nagpal RL (1966) Fig Cultivation in India. Farm Bulletin

No. 42. New Delhi: Indian Council of Agricultural

Research.

Sadhu MK (1996) Fig. In: Bose TK and Mitra SK (eds) Fruits:

Tropical and Subtropical, pp. 650–666. Calcutta: Naya

Prakash.

Samson JA (1980) Tropical Fruits. London: Longman.

Simpson BB and Conner-Ogorzaly M (1986) Economic

Botany. Plants in Our World, pp. 129–130. New York:

McGraw-Hill.

Song Q, Yang D, Zhang G and Yang C (2001) Volatiles

from Ficus hispida and their attractiveness to fig wasps.

Journal of Chemical Ecology 27(10): 1929–1942.

Young JH (1999) The early regulation of figs in the United

States. Technology 6: 229–234.

FILTRATION OF LIQUIDS

J Keifer, Formerly of Filtrox-Werk AG, St Gallen,

Switzerland

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001 Filtration and separation play an important role in

the preparation of beverages, especially beer and

wine, which, as the products of a fermentation pro-

cess, require an extensive reduction in microorgan-

isms and haze-forming substances at the end of the

production cycle. The aim of this clarification step is

to obtain a product which does not contain any harm-

ful microorganisms, is visually bright, and has the

required shelf-life.

0002 Selection of the clarification process necessitates a

careful analysis of the separation problems involved.

Not only is the result of the clarification process itself

a criterion, but additional factors also have to be

taken into account.

0003 The clarification processes commonly used in the

brewing and wine-making industries are kieselguhr fil-

tration, sheet filtration, cross-flow microfiltration (for

wine) and dead-end membrane filtration. The first two

processes are the dominant filtration technologies.

Kieselguhr Filtration

0004 Kieselguhr or diatomaceous earth filtration in the

brewing and wine-making industries is a precoat

filtration with the addition of further kieselguhr as

body-feed. In brewing, it is always a pressure filtra-

tion to maintain the carbonation of carbon dioxide

content of the beer; in wine-making, it may be pres-

sure or vacuum filtration.

0005The application of kieselguhr filtration is restricted

toamaximumsolidscontentofabout0.2%.Feedswith

higher solids contents always require pretreatment.

Basic Technology of Kieselguhr Filtration

0006Microorganisms and haze particles are much smaller

than the sieve aperture of that part of the filter’s

structure used as a mechanical support for the kiesel-

guhr cake. Therefore, several precoat layers of coarse

kieselguhr are applied to obtain a tight cake which

retains the body feed and beer/wine solids. Continu-

ous dosing of body feed prevents a rapid pressure

increase by dilution of the solids and, therefore,

results in longer filtration runs.

0007The relationship between filtration pressure, fil-

trate flux, or specific flow rate (flow per unit area),

cake thickness, and cake permeability is obtained

from Darcy’s law and its derivations.

0008One derivation of Darcy’s law, containing the

parameters of cake porosity, specific gravity, and

thickness, is:

U ¼

P

ars 1 

ðÞL

: ð1Þ

As rs and S are constants for a specific cake, the

expression can be simplified to:

FILTRATION OF LIQUIDS 2399

U ¼

P

ac

, ð2Þ

where U is the filtrate flux (m s

1

or m

1

2

), m is

the filtrate viscosity (N s m

2

), DP is the differential

pressure (N m

2

), a is the specific filtration resistance

(m kg

1

), rs is the specific gravity of the cake-forming

particles (kg m

3

), S is the filter cake porosity, c

the filter cake constant (kg m

3

)(¼rs(1  S)), and

L is the cake depth (m). This expression can be

applied to model the different means of operating

a kieselguhr filter, that is:

0009 constant flow, variable pressure;

0010 constant pressure, variable flow; and

0011 variable flow, variable pressure.

The first of the above is commonly used in practice

for beer and wine filtration.

Kieselguhr Filtration of Beer

0012 A typical beer plant consists of the components

shown in Figure 1. The process starts with the kiesel-

guhr filter unit filled with water. Then, after a two-

step precoat, as specified in Table 1, the water is

pushed out by incoming beer; a portion of the

resulting diluted beer is collected and blended back

to the unfiltered beer during the production run. (See

Beers: History and Types.)

0013The beer in the filter plant is then recirculated for

several minutes to stabilize the filtrate haze level. At

the end of this step, the beer should reach a clarity of

no more than 0.6 European Brewery Convention

(EBC) units. At the end of recirculation, the collection

of filtered or bright beer can start.

0014Quality Parameters of the Filtrate

Microbiology Normal beer before filtration con-

tains about 1–10 million yeast cells per milliliter.

Kieselguhr filtration can reduce this figure to a max-

imum of 5 yeast cells per 100 ml – a reduction by a

factor of 2 10

to 2 10

. Bacteria – if present – are

reduced by a factor of 10

–10

0015Haze The total haze is formed by the optical effects

resulting from the presence of yeast cells, proteins,

and a lot of other high-molecular-weight substances.

In brewing, the haze level is measured in EBC units. A

level of 0.6 EBC or less is acceptable; less than about

1 EBC is detectable only by accurate measurement of

the portion of a light source scattered at a 90



or 25



angle by the suspended particles.

0016Oxygen pick-up A low dissolved oxygen content of

the packaged product is important to maintain its

shelf-life, that is, to avoid changes in its appearance

or sensory characteristics within the consumption

period. Normally, an oxygen pick-up of less than

0.1 mg l

1

beer is acceptable.

0017Process conditions Only typical ranges of values for

the process parameters are given below since beer is a

natural product which is available in many varieties.

Batch sizes, for example, show variations by a factor

of about five.

0018Filtration speed and batch size The filtration speed

(or flux) strongly influences batch size; the higher the

speed, the smaller the batch, the volume of which is

often measured in hectoliters (hl).

0019The filtration speed cannot be standardized.

Normal values – depending on the filter system and

the type of beer – vary from 2.5 to 10 hl m

2

1

. The

higher filtration speeds are normally obtained by

using vessel filter systems, preferably candle filters

or horizontal leaf filters. The traditional plate-and-

frame filter is operated at lower filtration rates.

0020The filter is precoated at 7–10 hl m

2

1

0021Batch sizes normally range from 15 to 70 hl m

2

filter area. Large batch sizes can be obtained by the

CDEG

fig0001 Figure 1 Components of a typical beer filtration plant: A, stor-

age tank for unfiltered beer; B, buffer tank for unfiltered beer;

C, kieselguhr filter including dosing unit; D, polyvinylpolypyrroli-

done stabilization unit, including dosing unit; E, buffer tank for

bright beer; F, carbonation unit; G, bright beer tank; H, cleaning-

in-place (CIP) station.

tbl0001 Table 1 Kieselguhr permeabilities for different process steps

Processstep Permeability Examples

First precoat 1.2–1.5 Darcy Manville Hyflo

Ceca Dic B

Dicalite Speedplus

Second precoat 100–250 m Darcy Manville Standard

Supercel

Ceca CBR

Dicalite Speedflow

Bodyfeed 30–250 m Darcy Manville Filtercel

Ceca CBL

Dicalite 215

Also types from second

precoat

2400 FILTRATION OF LIQUIDS

use of enzymes to degrade b-glucan, thus reducing

bodyfeed requirements. b-Glucan in its gel form

leads to beers of extremely poor filtrability. The

filtrability can be measured by the so-called Esser

test, a membrane filtration test.

0022 Temperature The filtration temperature is less than



C, ideally 1



C. Lower temperatures are better, as

so-called chill hazes comprising proteins and other

high-molecular-weight substances are precipitated

and, therefore, removed by the filtration.

0023 Additional treatment Together with kieselguhr fil-

tration, beer is treated sometimes by the addition of

silica gel or polyvinylpolypyrrolidone (PVPP) to im-

prove the colloidal or haze stability. The space con-

sumed within the filter by this additional processing

aid leads to smaller batch sizes. Therefore, the PVPP

treatment is mostly performed in a second, separate

filter unit with a regeneration station for the PVPP.

0024 Filter types The traditional filter for kieselguhr

filtration, that is, the plate-and-frame filter, is being

replaced more and more by modern vessel filters

which can be easily automated and require less

manual work.

0025 Vessel filters can be classified by the kind of kiesel-

guhr support: candle filters with vertical, cylindrical

filter elements, hanging from a perforated plate

into the vessel; horizontal leaf filters with centrifugal

sludge discharge; and, uncommonly, vertical leaf

filters. Candle filters are preferred, because of their

excellent filtration characteristics and the absence of

any moving parts.

0026 Performance ranges from 30 to 1000 hl h

1

for the

horizontal leaf filter and from 100 to 1000 hl h

1

for

the candle filter.

Kieselguhr Filtration of Wine

0027 For wine filtration, the same principles as for beer

filtration are valid. The process parameters differ: the

filtration speed is higher, ranging from 10 to 20 hl

2

1

, due to the better filtrability of wine; the

temperature is higher, at up to 20



C; and different

grades of kieselguhr are used. (See Wines: Production

of Table Wines; Production of Sparkling Wines.)

0028 Several stages of filtration can be differentiated in

wine production: the first filtration after the main

fermentation, a second after the use of finings, and a

possible third as a prefiltration stage prior to mem-

brane filtration just before bottling. The kieselguhr

types vary from coarse, at the first filtration, to very

fine, at the last.

0029 The filter types normally used are the plate-and-

frame, the horizontal leaf filter and, for smaller units,

the vertical leaf filter. Candle filters are normally not

used, because of the numerous types of wine and the

risk of mixing taking place upon batch change. The

horizontal leaf filter can be emptied and filled again

with the new variety; this reduces the extent of

mixing.

0030The performance of typical plants ranges from 10

to 600 hl h

1

. The plants are much smaller than beer

filters and normally are operated manually.

Sheet Filtration

0031Sheet filtration is normally used as a second filtration

for polishing or obtaining a further reduction of

microorganisms. In the wine industry, it is also used

for prefiltration. For these purposes, a wide range of

filter sheets is available, from a coarse prefiltration to

a final filtration with bacterial reduction up to a

factor of 10

. Figure 2 shows an electron micrograph

of a filter sheet.

0032In brewing, filter sheet grades vary only over a

narrow range; for polishing (reduction of haze level),

for obtaining yeast-free beer and for reduction of

bacteria. Tighter filter sheets, with bacterial reduction

by a factor of 10

–10

, are not used in the brewing

industry, mainly for economic reasons; the presence

of colloids shortens sheet life.

0033The process conditions for sheet filtration depend

on the type of sheet. A survey is presented in Table 2.

0034The permeability value for filter sheets is given in

liters per square meter per minute (1 m

2

min

1

measured at 20



C and 100 kPa with prefiltered

water. Filter sheets with permeabilities lower than

30 l m

2

min

1

are only used as pharmaceutical

grades.

0035The typical life span of a filter sheet depends on

its porosity, the quantity and kind of solids to be

removed, and the filtration speed. Typical values

for beer filtration vary from 50 to 200 hl m

2

0036The filter sheets commercially available consist

of cellulose, kieselguhr, and, sometimes, perlite.

Asbestos-containing sheets are no longer available,

due to the well-known problems caused by the use

of asbestos.

0037The disadvantages of filter sheets include so-called

drip losses through the exposed edges of the sheets,

especially with high-value products, and the high

manual input required in handling.

0038Recent developments of vessel filter systems, in-

corporating the use of filter sheet material as the

filtration medium, eliminate these problems. Filter

sheets formed in so-called lenticular cartridges permit

a filtration free from drip losses. The handling of such

cartridges is very easy; up to 3.6 m

are combined in a

single unit, as opposed to an absolute maximum of

FILTRATION OF LIQUIDS 2401

1.4 m

per individual sheet. Under typical process

conditions, it is possible to attain about double the

filtration speed of the normal sheet filter.

Final Filtration of Wine

0039 As a last step before bottling, wine can be treated by

membrane filtration. The removal of all harmful bac-

teria requires membranes with a pore size of 0.45 mm.

Such a membrane is expensive and therefore requires

a very effective prefiltration. This can be achieved by

using a lenticular cartridge with its deep-bed filtration

medium, or by cartridge filters, constructed from

wound cotton, glass, or polypropylene fibers or

nonwoven fibrous polymeric media, which may be

pleated for optimal filter surface per volume.

0040Integrity checks of the final membrane are possible.

Cross-Flow Microfiltration

0041All the techniques described above are based on con-

sumable materials such as kieselguhr, filter sheets, or

fig0002 Figure 2 Electron micrograph of a filter sheet.

tbl0002 Table 2 Typical filter sheets

Permeability (1 m

2

min

1

)Flux(hlm

2

1

) Use Examples

300 6 Prefiltration of wine Seitz K200

Filtrox K30

Carlson XE150

150 2–3 Polishing and filtration of beer Seitz K150

Filtrox 71H

Carlson XE280

130 4 Polishing and filtration of wine Seitz K150

Filtrox K70

Carlson XE280

80–90 1.5 Removal of yeast from beer Seitz K100

Filtrox St 110

Carlson XE1200

80–90 3 Removal of yeast from and reduction of bacteria in wine Seitz K100

Filtrox St 110

Carlson XE1200

30 2 Removal of bacteria from wine Seitz EK1

Filtrox St 140

2402 FILTRATION OF LIQUIDS

different cartridges. A technique that can combine

several filtration stages into a single operation, hence

reducing wastage, is cross-flow microfiltration, based

on the well-known cross-flow or tangential flow

principle. The application of pore sizes of 0.1–1 mm

allows an alternative to the systems used until now.

0042 Cross-flow microfiltration systems are based either

on capillary or tubular membranes (internal diameter

0.8–4 mm), or flat sheet membranes in conjunction

with a special turbulence-promoting mesh. Various

polymeric materials are used and ceramic membranes

have been tested.

0043 The process conditions should provide a turbulent

flow, to prevent concentration polarization and,

hence, rapid membrane clogging.

0044 In wine filtration, cross-flow units have found

some application (Figure 3). Typical values for the

filtrate or permeate flux are 30–80 l m

2

1

, and

run times vary from 6 to 30 h. The main problem is

the prevention of a rapid flux decay. The reduction in

flux can be prevented to an extent by backflushing or

other fouling-reducing techniques.

0045 In beer filtration, the cross-flow technique is still

not economic due to low process temperatures and

the high colloidal fraction of the solids, which results

in flux rates of only 20–30 l m

2

1

; and the large

volumes of beer involved. Further investigations into

the use of membranes will lead to the attainment of

better fluxes in the future.

Prospects and Conclusion

0046The long-term preservation of a food or beverage is a

critical requirement of the future consumer. Addition-

ally, chemical or thermal treatments of food are no

longer accepted by an increasing number of con-

sumers. For the microbiological stability of beer and

wine, filtration in the submicro-meter range is gaining

more importance.

0047Filter media with guaranteed properties, guaran-

teed solids- or dirt-holding capacities, and micro-

organism separation characteristics are required as

the final step of treatment.

0048Environmental concern will cause the substitution

of waste-producing techniques by others; for example,

cross-flow technology and filter media will replace

kieselguhr and sheet filtration.

0049Economic implementation of new techniques will

probably arise from the increasing costs of waste

disposal, not from any additional advantages in qual-

ity, handling or energy consumption. In contrast, the

cost savings associated with a reduction in waste will

be balanced by a higher energy consumption.

Further Reading

Dickenson C (ed.) (1987) Filters and Filtration Handbook,

2nd edn. Morden: Trade and Technical Press.

Kiefer J (1990) Kieselgurfiltration – Anwendungsgrundla-

gen fu

r den Kesselfilter. Brauwelt 44(130): 1964–1970.

Matteson MJ and Orr C (eds) (1987) Filtration – Principles

and Practices, 2nd edn, pp. 133–162. New York: Marcel

Dekker.

Rushton A, Ward AS and Holdich RG (2000) Solid–liquid

Filtration and Separation Technology, 2nd edn. New

York: Wiley-VCH.

Scheuermann EA (1989) Kla

rfiltration mit Filterschichten.

Filtrieren und Separieren 2(3): 69–76.

100

120

140

0 5 10 15 20 25

Flux rate (I m

−2

−1

)

With backflushing

Without backflushing

Time (h)

fig0003 Figure 3 Typical behavior of a cross-flow microfiltration plant

used in wine production.

FILTRATION OF LIQUIDS 2403

FISH

Contents

Introduction

Catching and Handling

Fish as Food

Demersal Species of Temperate Climates

Pelagic Species of Temperate Climates

Tuna and Tuna-like Fish of Tropical Climates

Demersal Species of Tropical Climates

Pelagic Species of Tropical Climates

Important Elasmobranch Species

Processing

Miscellaneous Fish Products

Spoilage of Seafood

Dietary Importance of Fish and Shellfish

Introduction

Q Bone, The Marine Laboratory, Plymouth, UK

Kinds of Fish used as Food

0001 An extraordinary diversity of different kinds of fish

and fish products are used as food, from jawless

lampreys to lungfish (and, at times, even coela-

canths), but the most abundant kind of fish, the

bony ray-finned teleosts (like carp and cod), make

up the great majority of fish used directly as food.

The teleosts also make up the greater part of fish used

as fish meal, which we eat indirectly after it has been

used as animal feed or plant fertilizer, for most fish

meal is made from small pelagic teleosts. Some

20 000 teleosts are known presently, but new species

are still being described from both marine and fresh-

waters, and the final total may well exceed the present

estimate of 30 000 (with around 12 000 from fresh-

water). Many of this grand total are naturally too

inaccessible, too rare, or otherwise unsuitable for

use as food, but most of the different teleost families

contain food fish species, and some families like the

Clupeidae, Gadidae, and Salmonidae are of special

importance. In contrast, the other major fish group,

the sharks and rays (elasmobranchs), has only some

800 species, almost all entirely marine. Relatively few

are of significant importance as food, though flesh

from small sharks (dogfish) is much used in the UK

as rock salmon, whilst other elasmobranchs provide

liver oils. Apart from the teleosts and elasmobranchs,

the only other food fish important for their flesh (and,

in particular, their eggs) are the ray-finned sturgeons,

one of the small number of surviving Chondrostean

fishes.

Elasmobranchs and Teleosts

0002The general structural features of the two main

groups of food fish are shown in Figure 1. The

elasmobranch body is covered with hard denticles,

often variously ridged or pointed; in the jaws, these

form formidable successions of teeth. In contrast,

teleosts are covered with bony scales (sometimes lost

or minute, as in eels), and these too may be variously

sculptured and ridged, although they are usually more

or less circular. In both groups, the general body plan

is the same, with two sets of paired fins as well as

dorsal and ventral unpaired fins. The paired fins are

attached to the pectoral and pelvic girdles, the former

linked to the central vertebral column. On either side,

serial myotomal muscle blocks (the main edible por-

tion of most) are separated by tough connective tissue

myosepta, which attach to the vertebral column and

to the connective tissue layer underlying the skin.

0003However, the design of the fins and axial skeleton is

different in the two groups: in sharks, the skeleton

is cartilaginous (strengthened by granular or pris-

matic calcifications where necessary); the vertebral

column is simpler than in teleosts, and the ribs are

small. The unpaired fins are attached to basal cartil-

ages. Shark fins are supported by jointed cartilagin-

ous elements at their bases and by thin elastoidin rods

(ceratotrichia) along their length, which stiffen the

outer border of the fin. Lacking the bony rays of

teleost fins, the supporting elements of shark fins

2404 FISH/Introduction

can be sufficiently softened by cooking to be eaten as

an expensive delicacy. They are much less flexible

than teleost fins and, unlike the latter cannot be furled

and used for delicate fanning and flapping move-

ments. The pectoral fins act in almost all sharks as

lifting foils, like aeroplane wings, and they can adjust

their angle of attack and be swung back and forth to

some extent; the other fins of sharks can only bend

stiffly from their bases. Rays differ from sharks

because their pectoral fins (which they use for swim-

ming) are greatly enlarged and supported by jointed

cartilages to their tips, so that they are much more

flexible and can make delicate flapping movements.

Naturally, the fins are highly muscular; hence, it is the

pectoral muscles rather than the reduced myotomal

muscles that are used as food, and they are easily

separated from the supporting cartilages when

cooked.

0004In teleosts, the skeleton is much more prominent

than in elasmobranchs: the vertebrae have neural

and hemal spines; in addition, the unpaired fins are

attached to bony, spinous pterygiophores, and there

may also be intramuscular bones in the myosepta, as

in clupeids such as herring (Clupea harengus) and the

shads (Alosa spp.), which are irritatingly obvious

when they are eaten. Teleost fins are very different

to those of elasmobranchs, for they are supported

by flexible, jointed rays (lepidotrichia) controlled by

Brain

Basal cartilage

Vertebral

column

Kidney

Ovary

Vertebral column

Swim bladder

Stomach

Red muscle

Red

muscle

Heart

(a)

(b)

Stomach

Pancreas

Spiral valve

White muscle

Spiny ray

Pterygiophore

Soft rays

White muscle

Gill

arches

Heart

Liver

fig0001 Figure 1 General features of (a) a shark and (b) a teleost fish. Note the absence of a swim bladder and less prominent skeletal

elements in the shark, and the presence of a large liver and spiral valve. In both, the myotomal muscle consists of an outer layer of

dark flesh and an inner white layer (see text). (a) After Lagler KF, Bardach JE, Miller RR and Passino DRM (1977) Ichthyology, 2nd edn,

pp. 506. Chichester, UK: Wiley. (b) After Dean B (1895) Fishes living and fossil. An outline of their forms and probable relationships.

In: Columbia University Biology Series, vol. 3. London and New York: Macmillan and Co.

FISH/Introduction 2405

muscles at their bases moving individual rays, and

hence can be furled or make most delicate movements

(e.g., in seahorses or knifefishes).

0005 Internally, the chief differences between teleosts

and elasmobranchs are as follows: many teleosts

have a gas-filled swim bladder, used for buoyancy

(and also in some for sound production and recep-

tion, and in some freshwater species for respiration);

the gills are of a slightly different design; all elasmo-

branchs and only a few teleosts have a spiral valve in

the intestine, and the reproductive apparatus is differ-

ent. Only a very few teleosts (e.g., damsel fishes,

Pomacentridae) have internal fertilization (universal

in elasmobranchs) and, except in bottom-dwelling

species, store lipid in the large liver for buoyancy.

0006 A notable physiological difference between the two

is that elasmobranchs are close to osmotic equilib-

rium with seawater by virtue of high concentrations

of urea and trimethylamine oxide (TMAO) in the

body fluids, whereas in marine teleosts, the body

fluids are much more dilute than seawater, and

hence they lose water and gain ions across the gills

and other permeable surfaces. In fresh water, of

course, the reverse occurs. The high urea and TMA0

content of elasmobranchs naturally has important

consequences for spoilage.

Fish as Food

0007 Both adult and larval teleosts form the basis of special

fisheries as do the eggs of certain species (e.g., stur-

geons), but elasmobranch fisheries concern adults

only. Even in adults, whilst myotomal muscular tissue

is the chief food source, other nonmuscular tissues,

such as fins, gonads, and livers, may be important

foods (of economic, if not always of much nutritional,

importance, e.g., shark fins). In some instances where

the bones are eaten (e.g., sardines and whitebait, and,

in Japan, the roasted vertebral columns of certain

larger species), these are important sources of dietary

calcium and phosphorus. Fish are also a valuable

source of the vitamins nicotinic acid, riboflavin, and

vitamin B

. Refer to individual nutrients.

0008 Above all, however, fish are important as an excel-

lent protein source, for muscular tissue makes up a

larger part of the fish body than in terrestrial verte-

brates, from 37 to 68% of the total bodyweight.

What is more, in all food fish except rays, the loco-

motor musculature is arranged in serial myotomal

blocks along the body so that, at least in larger fish,

it is an easy matter to gut the fish and separate the

flesh from the skeleton as fillets. The relatively larger

muscular mass in fish, and their importance as a

protein source, is directly related to their life in

water. Because most fish are well streamlined, the

major drag force opposing their forward motion is

skin friction drag, proportional to the square of for-

ward speed. Thus, the power output required from

the musculature is (approximately) proportional to

the cube of swimming speed. Even modest swimming

speeds require a large mass of locomotor muscle, but

only a small weight penalty is incurred by the large

amounts of (edible) musculature needed for locomo-

tion, since the water in which fish live is not much less

dense than the musculature. Even those fish that do

not reduce their density (or achieve neutral buoyancy)

by storing gas or fat for static lift only weigh in water

around 5–7% of their weight in air.

0009The locomotor muscle is divided in almost all fish

into a lateral red component, used during cruising,

and a much larger white or pale yellow mass, which is

active during short bursts of higher speed. A similar

division is seen in the pectoral muscles of skates and

rays. The two types of muscle have a different enzyme

spectrum and metabolism, and also differ in lipid

content and vascular bed. In yellowfin tuna (Thunnus

albacares), for example, the red muscle contains twice

the amount of lipid than the white muscle, as well as a

large amount of myoglobin absent from the white

muscle. The functional division into two muscle

types of different composition (most simply distin-

guished by color) is sometimes reflected in the use of

the muscles as food. For example, the internalized red

muscle mass of the porbeagle shark (Lamna cornu-

bica) is regarded as a delicacy when smoked, whereas

the larger white portion of the myotomes is cooked

and eaten unsmoked.

0010Raw fish muscle contains around 20% protein (the

contractile lattice of actin and myosin and various

sarcoplasmic proteins) so that protein forms the

major energetic component of the fish, as shown in

Table 1.

tbl0001Table 1 Protein energy as percentage of total energy of some

food fish and meat

Food Protein energy (%)

Cod fillet (Gadus callarias)95

Albacore (Seriola lalandi)90

Haddock (Melanogrammus aeglefinus)96

Sole (Solea vulgaris)85

Flounder (Pleuronectes flesus)95

Halibut (Hippoglossus hippoglossus)66

Herring (Clupea harengus), an oily fish 41.5

Chicken (white meat) 76

Lean beef 69

Medium-fat beef (cooked) 27.4

Medium-fat pork (cooked) 13.9

From Geiger E and Borgstrom G (1962) Fish protein – nutritive aspects. In:

Borgstrom G (ed.) Fish as Fo od, vol. 2, p. 128. London: Academic Press.

2406 FISH/Introduction

0011 In comparison, even lean beef has relatively less

protein energy. It is the high protein content of fish

flesh that permits the preparation of increasingly im-

portant surimi products (See Fish: Miscellaneous Fish

Products). Some fish also offer a useful lipid source,

because, as well as the liver oils and lipid in ovaries,

there can be much lipid in the muscles themselves,

especially in scombroids and clupeids. In mackerel

(Scomber), adipose cells between the muscle fibers

and lipid droplets within the fibers make up 17% of

the cross-sectional area of the lateral strip of red

muscle. Unsurprisingly, fresh mackerel fillets contain

some 16% lipid (protein: 19%), so that lipids make

up the greater part of the total energy. As important

as content is the nature of the lipid, for fish lipids

contain o-3 polyunsaturated fatty acids, shown in a

number of studies to confer significant benefits

in the prevention of cardiovascular diseases. Some

fish contain lipids that are unsuitable for human

consumption; for example, the castor oil fish, Ruvet-

tus, gains its name from the purgative qualities of the

low-density oil it stores for buoyancy purposes, and

since it is abundant in oil sacs within the skeleton,

naı

ve consumers are warned not to suck the bones.

(See Fatty acids: Dietary Importance.)

0012 Both fisheries and aquaculture are crucial for pro-

viding protein in many communities, some 5% of all

protein eaten per capita in low-income countries. In

addition, fish protein fed to animals provides indir-

ectly for further human protein food. It is hardly

surprising that fish form an important part of the

diet in many countries, and that as well as being

hunted with ever more sophisticated equipment, fish

are farmed on an increasing scale. The annual per

capita consumption ranges from over 20 kg to less

than 5 kg, being highest in countries with a coastline

facing a productive sea, such as Iceland or Japan, and

lowest in such countries as Switzerland and Turkey.

The average annual per capita consumption in 1996

was some 13.3 kg (excluding China). The most recent

worldwide statistics available (2000) show that fish

continue to increase in importance as a food source.

0013 Of these different fish, the overwhelming majority

are teleosts, as they are in numbers of species. The

total elasmobranch catch is less than 1% of the

marine fish caught.

History of Fish as Food

0014 Fish have been used as food from ancient times, and

quite a wide variety of fish skeletal remains are

known from middens and spoil heaps, including

both freshwater and marine species. The presence of

marine fish remains in inland Late Old Stone Age

refuse heaps (around 40 000 bc) in the Dordogne,

France, implies that, even at that date, some means

of preservation (possibly drying over smoky fires)

must have been used to counter the rapid perishability

of fish flesh. Air-drying, smoking, salting, and pick-

ling in brine were probably developed around

4000 bc and permitted the wider availability and

use of fish as food. The preservation treatments used

varied according to the kind of fish: air-drying and

salting were suited to leaner fish, whilst oily fish, such

as herring or salmonids, had to be covered in brine to

prevent rancidity owing to lipid oxidation. Today,

more rapid transport of fresh fish and improved tech-

niques of chilling and freezing are of much greater

importance than these other means of preserving fish

flesh, although curing in various ways may be import-

ant in increasing monetary value, as manifested, for

example, in the price difference between fresh herring

and kippers. (See Preservation of Food.)

Preparation

0015Different storage, preparation, and cooking methods

may profoundly alter the value of fish as food, since

they greatly influence important constituents of fish

flesh, such as lipid and vitamin content. These effects

will not be considered here. (See Fish: Processing.)

Problems in using Fish as Food

0016Ensuring the safety of seafood (fish and fish products

and shellfish) requires government regulation and

expense, but several Hazard Analysis Critical Control

Point programs are now being put in place to minim-

ize and avoid illness and disease caused by seafood,

and consequent difficulties in import and export of

seafood products. Two special cases of the problems

with fish as food are where some or all of the fish may

be dangerously poisonous. The puffer fishes (Tetra-

odontidae) contain high concentrations of the Na

channel blocker, tetrodotoxin (TTX), which is an

exceedingly powerful nerve- and muscle-blocking

agent. Such fish (a delicacy in Japan) are carefully

prepared by licensed chefs to remove the tissues

containing the highest concentrations of TTX before

consumption in special Fugu restaurants; neverthe-

less, between 1987 and 1996, there were 32 deaths

from eating puffer fish. Connoisseurs apparently

require the initial symptoms of low doses of TTX

(tingling of the lips) to enjoy the delicacy. The second

special case is that of tropical marine fish. Herbivores

may accumulate ciguatera toxin, apparently of algal

origin, which also passes to carnivores such as barra-

cuda, Sphyraena, when they eat the contaminated

herbivores. The liver and viscera are particularly

liable to be poisonous, and as yet, there is no reliable

FISH/Introduction 2407