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

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lactating pigs also have a minimum fish meal level of

about 4% in most diets. Finisher pig diets (> 50 kg)

probably have no minimum constraint. However,

in circumstances of large availability of supplies of

fish meal at relatively low prices compared with

vegetable proteins, such as in the main fish meal-

producing countries of South America, when it is

economically advantageous to include fish meal, a

constraint is normally placed upon the maximum

level of fish meal to be used in order to ensure that

no fishy taint is imparted to the pig flesh. For a high-

fat fish meal containing > 10% of fat is is not unusual

to see a maximum constraint of 5% in finishing diets.

(See Pork.)

Fish

0011 The principal cultivated fish species using fish meal in

their diet are prawns, salmon, eels, trout, and yellow

tails, i.e., mainly the marine carnivorous species.

These do not use vegetable proteins efficiently, espe-

cially if they are fast-growing. Fish meal content of

salmon diets is believed to average 45%, with slightly

higher inclusion in diets for Atlantic salmon than for

Pacific salmon. In prawns a typical fish meal content

in the diet would be about 30%; the level of incorpor-

ation for trout would be 35% and for eels about

50%. (See Fish Farming.)

Fur-Bearing Animals

0012 Fish meal is commonly used in mink feeding. The

amount of fish meal used varies, but up to 60% of

the protein can originate from fish meal provided that

the meal is of high quality. Antioxidant-stabilized fish

meal, or low-fat fish meal prepared from fresh fish, is

preferred.

Ruminants

0013 Fish meal fed at relatively low levels (0.75 kg day

1

)

to high-yielding dairy cows (over 251 per day) has

been shown to increase the milk yield significantly.

The effect has been particularly pronounced with

high-forage diets, especially with grass silage as the

main forage source. Probably of greater economic

significance is the fact that dietary fish meal increases

the reproductive performance of the dairy cow. For

example, conception rates have been shown to

increase from 44% to 64%. This increases calving

frequency and milk production.

0014 Fish meal also has a role in the diets of sheep,

improving productivity and product quality and re-

ducing production costs. It has been found to be

particularly important during the last 2–3 weeks of

pregnancy for ewes given mainly roughage diets,

during the first 5–6 weeks of lactation when body

fat in the ewes can help to sustain a large milk pro-

duction, as a feed supplement in high-energy diets for

fattening of early weaned lambs, and for older lambs

on high-roughage diets. Finally, it has been found to

be useful in manipulating body fat stores, thus redu-

cing excess body fat prior to going to market.

0015In diets for beef cattle, especially those on high-

forage diets, fish meal (200–400 g day

1

) increases

growth rate. It has been recently shown to provide a

means of increasing lean growth at the expense of

body fat, which decreases in over-fat finishing cattle

on only poor roughage plus fish meal. (See Beef.)

Source of Raw Material

0016The main producing and exporting countries of fish

meal are shown in Figure 1. South America, Japan,

the former Soviet Union and the Scandinavian coun-

tries are the main producers. These same countries,

with the exception of Japan and the former Soviet

Union, are the major exporters.

0017The principal species of fish processed into meal

and oil in these countries are listed in Table 2, to-

gether with the name commonly used (sometimes

erroneously) in the fish meal trade to describe these

types of fish meal.

0018One of the distinguishing features of the fish meal

industry, compared with other sectors of the fishing

industry, is the ability to process large volumes of fish

in a short time. A company in South America could be

processing 250 000 t of fish in a year and, because of

the seasonal nature of the fishing, the storage capacity

of the plant would be greater than 1000 t of fish.

0019Because ambient temperatures in the north of Chile

and in Peru are relatively high, it is necessary to have

the capacity to process the stored fish in 24 h or less.

Fish stored at 30



C will soon turn into a hydrolyzed

liquid, resulting in considerable losses of raw material

and reducing the yield of the meal. In addition, the

quality of the meal produced by such stale fish is

poorer than that produced from fresh fish, resulting

in poorer performance from livestock fed on the

resultant meal. Most manufacturers now monitor

the freshness of raw material by measuring its volatile

nitrogen content. The lower this is, the fresher

the raw material. Special-product fish meals for

farmed fish, early weaned pigs, and ruminants are

made from fish with less than 90 mg of volatile nitro-

gen per 100 g of fish.

0020In colder climates, such as Iceland, it is possible to

store the raw fish for much longer periods than in

tropical or semitropical climates.

0021Over the years, a number of chemical agents

have been added to the raw fish in order to extend

its shelf-life. However, this practice is currently not

2488 FISH MEAL

very common and physical means of preservation,

such as icing or holding in refrigerated sea water, are

Further Reading

Barlow SM and Windsor ML (1983) Fishery by-products.

CRC Handbook of Nutritional Supplements VII:

253–272.

Bowman SJ (1990) Digest of Selected Statistics.

Potters Bar, UK: International Association of Fish

Meal Manufacturers.

FAO, Fishery Industries Division (1986) The Production of

Fish Meal and Oil. FAO Fish. Technical Paper 142

(rev. 1). Rome: Food and Agriculture Organization.

Pike IH (1984) Recent developments in feeding fish meal to

farm animals. The Feed Compounder September: 10–13.

Pike IH, Andorsdottir G and Mundheim H (1990) The Role

of Fish Meal in Diets for Salmonids. IAFMM Technical

Bulletin no. 24. Potters Bar, UK: International

Association of Fish Meal Manufacturers.

Windsor M and Barlow S (eds) (1981) Introduction to

Fishery By-Products. Farnham, UK: Fishing News

Books.

FISH OILS

Contents

Production

Composition and Properties

Dietary Importance

Production

G M Pigott and B W Tucker, Sea Resources

Engineering, Inc., Kirkland, WA, USA

Production of Fish Oils

0001 The importance of fish oils has increased greatly

during recent years as the nutritive value of certain

components is being emphasized. Prior to this time,

unrefined foul-smelling oxidized fish oils known as

‘cod liver oil’ were spooned as a vitamin supplement

for vitamins A and D. Later, partially refined and

hydrogenated fish oil was used for the manufacture

of margarine. However, most of the oil was, and is,

used for industrial purposes such as components in

paints, varnishes, and lubricants, where the quality

for human acceptance as a food receives little

consideration.

0002Highly refined fish oils are now in demand for

their o-3 fatty acids, which have been shown to

have a profound effect on the cardiovascular and

neurological systems. During the last few years, be-

ginning with menhaden oil, fish oil is being approved

for human consumption as food supplements and

as an ingredient in foods. This creates a demand

for better quality control through improved handling

of oil in conventional meal plants and in other

commercial processes in which fish oil is a by-

product. Great improvements have been, and are

being, made in maintaining the condition of raw

materials and primary products so that fish oil does

not have any major oxidation problems prior to

extracting and/or refining oil for human and animal

consumption.

FISH OILS/Production 2491

0003 The term ‘fish oil’ or ‘marine oil’ is normally used

for the class of lipids that originates from the marine

and freshwater bodies of the world, as opposed to

those from land plants and animals. Any plant or

animal raised in these aquatic environments has the

high profile of o-3 fatty acids not found in most land

plants and animals. Hence, by practice, since the large

majority of oils from this aquatic environment come

from fish, marine oils, fish oils, and aquatic plant and

animal oils are used interchangeably throughout the

world and are commonly called fish oils.

0004 Fish to be used as a source of oil must be cooled

and stored at refrigerator temperatures to minimize

enzymatic breakdown of certain components. Unfor-

tunately, this is often not possible in tropical and

subtropical areas, and the resulting quality of both

the fishmeal and oil is significantly reduced. Not only

does spoilage and oxidation occur, but also such

degradation results in the formation of compounds,

such as sulfur, which are catalyst poisons.

Extraction of Fish Oils

0005 Although both extraction and refining technology is

advancing rapidly, most of the fish oil being produced

in the world is as a byproduct of the conventional

fish meal process, a wet rendering technique. Other

processes include rendering, hydrolysis, silage pro-

duction, solvent extraction, critical extraction, and

ion exchange. Other new techniques are rapidly

being developed to ensure a high-quality product for

human and animal food. The superiority of high o-3

fish oils is creating a demand for these oils to become

a common component in engineered and formulated

foods consumed by humans.

Conventional Fish Meal Process

0006 Other than improving the machinery and equip-

ment made possible by technical developments, the

conventional fish meal process has not had any major

procedural changes for the past 50 years. The process,

as depicted in Figure 1, involves cooking, pressing to

separate the liquid from the solids, and drying the

solids. The liquid phase contains water, oil, water

solubles, and suspended solids. Oil is removed from

the mixture by centrifuging, after which it is pumped

into large storage tanks to await bulk shipment to

market. The large fish meal operations are designed

for processing several hundred tonnes of industrial

fish per day. Some of these plants can handle 50–60

tonnes of fish per hour and are located in areas of the

world where large schools of fish such as menhaden,

herring, sardines, and anchovy are located.

0007 Until recently, a large portion of the world’s fish

meal products were produced from the large resource

of industrial fish in Peru and Chile. However, with the

growing market for high-quality fish oils and the

decrease in numbers of wild fish in South America,

there is a steadily increasing amount of oil being

produced in other areas. Often this is related to grow-

ing government environmental regulations limiting

the amount of processing waste that can be discarded

into bodies of water.

0008Cooking Cooking the raw fish is necessary to co-

agulate the proteins, thus allowing the separation of

oil from the solids. There are several types of cookers

used in fish meal production. The most often used is a

steam-jacketed cooker where the heat is supplied to

the fish from steam passing through a jacket that

surrounds the cooker. Many cookers also have a

steam-heated augur in hollow flights that agitate the

cooking fish as well as supplying additional heat.

0009Batch cookers are used in some processing plants,

but the large volumes of fish passing through a con-

ventional meal plant require a continuous process.

This improves the economics, consistency, and

quality control of the fish oil.

0010Pressing and drying Cooked fish solids are separ-

ated from the liquid phase by a press, normally a

single screw or a multiscrew press. Wet solids

emerging from the press are called ‘the press cake.’

The liquid phase, or press liquor, as it comes from the

press contains water, oil, water solubles, and sus-

pended solids. From the press, the press cake is passed

through a continuous direct or indirect dryer to

remove the moisture. The resulting fishmeal is the

Raw fish storage

Cooker

Dryer

Fish

meal

Press

Waste

Centrifuge

Miscella

Oil

Stickwater

fig0001Figure 1 Conventional fish meal process.

2492 FISH OILS/Production

primary product of this process, and oil is considered

a byproduct that varies in composition and yield.

0011 Separating oil from the press liquor Oil is separated

from the other liquid components by first passing

the press liquor over a vibrating screen and then

centrifuging. The screen removes the solids that

have been carried over during the processing oper-

ation. An oil quality-control operation, called ‘polish-

ing,’ is often carried out whereby steam or hot water

is used to wash or strip the oil. This final washing and

then centrifuging procedure removes many of the

contaminants that contribute to degradation during

storage. Still quite warm from the cooking process,

the oil is pumped into large holding tanks, where it

is held for shipment. The solids recovered during

screening of the press liquor are added to the press

cake prior to drying. The remaining water phase of

the press liquor is often sprayed on to the fish being

dried in the dryer. This recovers the dissolved solids

and reduces the environmental impact of disposing of

them in the wastewater.

0012 With the increasing importance of preventing oxi-

dation of the o-3 fatty acids used for human foods

and health, oil should be cooled before being stored

and should be introduced into the tank near the

bottom and removed from the top. In fact, the quality

of oil from conventional fishmeal plants can be sig-

nificantly improved if it is immediately cooled after

being extracted from the hot cooked fish residue and

before being pumped into the storage tanks. In order

to keep the free fatty acid content low during storage

of oil, sludge from the bottom, which contains bac-

teria and other microorganisms, and water on the

surface should be regularly removed. Also, an anti-

oxidant is usually added to the oils after centrifuging,

or during other phases of the processing, to further

reduce oxidative degeneration during storage.

Dry Rendering

0013 Dry rendering is carried out in a single cooker–dryer

in which the fish are cooked and dried to the final

product in one operation. This process is quite popu-

lar in meat plants where both bone meal and meat

scrap meal is made from the low-fat continuing

byproduct materials. However, since the fat or oil is

not removed in the dry rendering process, the tech-

nique is used only in small fish processing plants that

utilize low-fat fish. The normally utilized industrial

fish containing over 5% oil would render a meal

containing 20% or more oil. This would be a gummy,

highly oxidized product of little commercial value.

0014 There have been numerous efforts to modify dry

rendering techniques for fishmeal, but the economics

of processing have prevented viable commercial

application. Certainly, in the future, as fish oil be-

comes a normal component of human foods, some

of the processes abandoned in the past will be modi-

fied and utilized.

Hydrolysis

0015Fish flesh can be hydrolyzed by proteolytic enzymes,

either endogenous or added, to produce a high-

quality functional protein product. The general

scheme for this process is shown in Figure 2. Excess

oil is removed by centrifugation, or, with high-fat

fish, some of the oil comes to the surface and can be

decanted. Since the resulting liquefied product has

good emulsion abilities, there is a certain amount of

oil left in the finished product. In fact, up to 5% oil

remains in the protein hydrolysate due to the forma-

tion of a water–oil emulsion. If the hydrolysate is

dried to remove water, the oil remains in the dried

product.

0016Depending on the proteolytic enzyme chosen, the

process is carried out under either acid or basic

conditions. The process of hydrolysis carried out by

endogenous enzymes at a low pH (e.g., 2–4) has an

advantage in that the resulting product is quite stable

from spoilage by microorganisms. However, the reac-

tion can be slow and uncontrollable, depending on

the source and portions of fish used as substrate.

The hydrolysis by endogenous enzymes varies quite

widely, depending on the amount of feed in the

Raw fish

Butcher

Skin

Debone

(if for humans)

Homogenizer

Enzyme reactor

Centrifuge

Fertilizer

Hydrolysate

Human

food

Sludge

and

lipid

Water

Animal

feed

Acid, base,

and/or

enzyme

(if necessary)

Viscera

Skin

Bones

fig0002Figure 2 Fish hydrolysate process.

FISH OILS/Production 2493

harvested fish stomachs. This is due to the activation

of stomach enzymes by the amount and type of food

being ingested.

0017 When endogenous enzymes are used for hydrolysis,

with an acid addition that maintains the pH at 4 or

below, the product is known as ‘fish silage.’ The crude

product resulting from this form of hydrolysis is used

for animal feeds or fish fertilizer. As a fertilizer, the

product has many advantages since it is a highly

digested liquid that can be used as an efficient foliage

spray. The resulting hydrolyzed fish product, when

properly processed to the end point, has little fish

odor and is much more efficient than petrochemical

fertilizers, as well as being environmentally friendly.

0018 Hydrolysis can be accomplished at higher pH

values, particularly in the basic range, using enzymes

extracted from vegetables or manufactured by micro-

bial action. Some of these enzymes are much more

efficient than endogenous enzymes in that processes

can be more precisely controlled and completed in

much less time. However, there is a negative factor

in using processes that require near neutral or basic

conditions, since the products have a short shelf-life

unless stabilized after the hydrolysis is complete.

Solvent Extraction

0019 There has been a considerable amount of work in-

volving the extraction of fish oils by solvents. Alcohol

was used in a major development program during the

1960s and 1970s backed by what is now the US

National Marine Fisheries Service, National Oceano-

graphic and Atmospheric Administration. This pro-

gram, an effort to make fish protein concentrate for

human consumption, was a dismal failure due to the

poor economics of the process. Alcohol, a polar solv-

ent, is not an efficient fat extractor since it preferen-

tially removes water and results in the necessity for

carrying out several extractions to ensure recovery of

the oil. The cost of removing water by solvent and

reclaiming the solvent from the emulsified residue

of water, oil, and water solubles is economically

unsound.

0020 Extraction with nonpolar solvents is an efficient

method of removing oil from animal raw material.

A process using ethylene dichloride as the solvent was

used commercially for animal feed. This is an efficient

and safe process if properly supervised and con-

trolled. However, the problems involving the use of

a toxic solvent and the subsequent governmental

restrictions prevented extensive use of this solvent

for preparing human fish protein concentrate.

0021 The recovery of high-quality oil has proven to be a

problem with all solvent extraction processes. The

solvents remove many pigments and other compon-

ents not desired in the oil, and the economics of

recovering pure oil from the solvent have not been

satisfactorily solved. However, newer techniques are

being developed.

Purification and Storage of Oils

Alkaline/Acid Treatment

0022Purification or refinement of fish oils involves a series

of alkaline and acid treatments to remove unwanted

products. Alkaline treatment removes the free fatty

acids and products of oil decomposition, which pro-

duce the strong rancid flavor and odor. It also coagu-

lates the protein to release the oil. The process

involves agitating the oil with alkali for various

times depending on the temperature and fatty acid

content of the oil. Following this treatment, often

involving 12 or more hours of agitation and settling,

the clear oil is washed with hot water to remove the

residuals and the remaining caustic compounds.

Pigments must be removed by bleaching, usually by

natural or activated clays.

Lipase Hydrolysis

0023Over the past decade, the use of microbial lipases to

produce n-3 fatty acid concentrate has been studied.

A major advantage of this technology is that there are

no extremes of pH or high temperatures, which have

an adverse effect on fish oil n-3 highly unsaturated

fatty acids (eicosapentaenoic acid (EPA) and docosa-

hexaenoic acid (DHA)). This technology has tremen-

dous potential for conserving processing energy and

increasing the selectivity of specific products. Further-

more, the resulting acylglycerol/ethyl ester mixture

can be distilled with much less adverse effect than

when fish oil is distilled.

Resin Refinement

0024A cold process utilizing macroporous cation resins

has been developed for improving fish oil chemical,

physical, and sensory quality characteristics. Resins

include both weak and strong cation and anion resins.

Results have been reported indicating that a strong

acid cation resin process results in many characteris-

tics that are superior to molecular distillation and

thawing fractionation.

Special Products from Fish Oils

Liver Oils

0025The chemical structure of fish oils gives them certain

desirable properties for human consumption and

other uses. Certain species of fish with low-fat flesh,

such as cod, store most of their oil in their relatively

2494 FISH OILS/Production

large livers. This oil was a valuable source of vitamins

A and D until the development of less expensive

synthetic forms of these vitamins in the late 1940s

destroyed the fish liver oil market. Oil from livers is

separated by gentle heating with steam. The oil is then

washed and centrifuged.

Concentrating Fatty Acids

0026 Various methods have been developed for extracting

certain components from fish oils for specific medical

or nutritional value. These processing techniques

include distillation, crystallization, various chroma-

tographic methods, and supercritical fluid extraction.

The pharmaceutical value of fish oil components

depends on maintenance of their natural form and

structure. For example, it is important to retain

the n-3 highly unsaturated (long-chain) fatty acids

(HUFA) if they are to be valuable in human health.

Not only are whole body oils a source of these special

fatty acids, but fractionated oils are valuable as a

concentrated source of specific desirable fatty acid

esters such as those of EPA and DHA.

0027 Molecular distillation Molecular stills can be used

for separating oil components based on the molecular

weight of the specific component and its vapor pres-

sure. The large difference in molecular weight of

vitamins A and D allows good separation of the two

components in fish liver oils. Conversely, the small

differences between many fatty acid components in

oil make it difficult to accomplish effective separation

or enrichment of a particular fatty acid.

0028 Chromatographic techniques Fatty acids in fish oils

can be fractionally crystallized at low temperatures

by mixing them with solvents in which certain fatty

acids are more soluble. Separation of components in

fish oil can be made by chromatographic techniques

in which each component will move through at a

different rate in a two-phase system.

0029 Supercritical Separation The potential for commer-

cial scale separation of oil components (e.g., EPA and

DHA) was significantly improved by the development

of fractionation techniques involving supercritical

fluids. Carbon dioxide is the most widely used solvent

for food and pharmaceutical products since there are

no residual problems involving toxicity, flammability,

or environmental pollution. At a pressure of 7384

kPa (1071 psi) and a temperature of 31.1



C, carbon

dioxide becomes a supercritical fluid. As such, it acts

as a solvent capable of enriching certain components

in an oil. By complexing fish oil with certain chemical

compounds (e.g., urea for concentrating EPA and

DHA), continuous countercurrent extraction will

yield highly enriched products and enriched still raf-

finate or bottoms. For example, EPA and DHA can be

separated from urea-complexed fish oil by this

method. In this case, the product is highly enriched

EPA concentrated in the raffinate.

See also: Fatty Acids: Properties; Dietary Importance;

Fish Oils: Composition and Properties; Dietary

Importance

Further Reading

Ackman RG (1982) Fatty acid composition of fish oils. In:

Barlow SM and Stansby ME (eds) Nutritional Evalu-

ation of Long-chain Fatty Acids in Fish Oil, pp. 25–88.

Eckert CA, Knutson BL and Debenedetti PG (1996) Super-

critical fluids as solvents for chemical and materials

processing. Nature 383: 313–318.

Liu Y (1995) Production of Fish Oil from Proteolytic

Hydrolysis of Fish Processing By-products. Thesis,

Master of Science,University of Washington, Seattle,WA.

Nicklason P and Pigott GM (1989) Simultaneous heating,

forming and extrusion of surimi-based products. Journal

of Food Engineering 9(3): 210–229.

Pigott GM (1999) A New Look at Fish Processing Techno-

log. Symposium for Aquaculture of Central America.

Aquaculture and Environment: Together in the New

Millenium. Honduras: Associacion Nacional de Acuicul-

tores de Honduras, San Pedro Sula, pp. 109–115.

Pigott GM and Tucker BW (1990) Seafood: Effects of

Technology on Nutrition. New York: Marcel Dekker.

Composition and Properties

B W Tucker and G M Pigott, Sea Resources

Engineering Inc., Kirkland, WA, USA

Background

0001Knowledge of the chemistry of fish oils has expanded

rapidly during the past few decades, beginning with

the development of gas chromatography (GC). More

recently, interest in the healthful effects of o-3 highly

unsaturated fatty acids (HUFAs) has spurred identifi-

cation and characterization of the fatty acid constitu-

ents of triglycerides (triacylglycerols).

Composition

0002Lipids occurring in fish oils are mostly the saponifiable

triacylglycerols, with far lesser amounts of di- and

monoglycerides. Also present are the unsaponifiable

FISH OILS/Composition and Properties 2495

phospholipids, sterols, wax esters, hydrocarbons,

and, in certain species, diacyl glyceryl ethers, also

known as alkoxydiglycerides. Additionally, minor

fat-soluble components such as vitamins, pigments,

and contaminants may be present as well as oxidation

products. (See Fats: Classification.)

Fatty Acids

0003 The fatty acid component of the triglycerides and other

lipids are saturated, monounsaturated (monoenoic),

or polyunsaturated (polyenoic) chains with lengths

generally of 12–24 carbon atoms (Figure 1). More

than 95% of these chains have even numbers of

carbon atoms and are cis-oriented. Up to 5% may

be odd numbered (Table 1), branched chain, and/or

furanoids (F acids). Although 60–80 fatty acids have

been identified from a single species, there are four

pairs that comprise 80–85% of the fatty acids in fish

oil: C

14:0

and C

16:0

16:1

and C

18:1

20:1

and C

22:1

and C

20:5

and C

22:5

. The nomenclature specifies first

Squalene

Cholesterol

Fatty acids

HOOC

(CH

)

HOOC

(CH

)

(CH=CH

HOOC

(CH

)

(CH

)

CH=CH

Saturated

PUFA

Monoene

16:0

20:5ω

−3

22:1ω

−11

C(CH

)

C(CH

)

C(CH

)

Triglyceride

(CH

)

N(CH

)

−

(CH

)

(CH

)

Lecithin

fig0001 Figure 1 Examples of component compounds found in fish oils. Three representative fatty acids (saturated, monounsaturated, and

polyunsaturated) typify those found in fish oil triglycerides. Lecithin is the major phospholipid present; cholesterol is the major sterol.

Squalene is a hydrocarbon often present at low levels. Reproduced from Fish Oils: Composition and Properties, Encyclopaedia of Food

Science, Food Technology and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds), 1993, Academic Press.

2496 FISH OILS/Composition and Properties

the number of carbon atoms in the chain and next the

degree of unsaturation (or number of double bonds)

present. (See Fatty Acids: Properties; Metabolism.)

0004 Compared with other oils or fats, fish oils contain

not only a greater variety of fatty acids, but also

considerably more long-chain HUFAs. The basic pat-

tern of fish triglycerides usually finds polyenoic fatty

acids preferentially bound in the middle (2) position,

with monoenes at the ends (1 and 3 positions), al-

though not all species conform to this pattern. Satur-

ated fatty acids comprise 20–35% of the total fatty

acids in most fish oils, although herring oil may be up

to 50% saturated. The C

16:0

content is between 10

and 25%, that of C

14:0

is 3–13%, and that of C

18:0

1–4%. Proportions of fatty acids vary considerably

between and within species. Herring oil is character-

ized by its high percentages of long-chain monoenes,

usually about 30%, but often more. Menhaden oil

has more C

14:0

and less C

22:6

than other fish oils.

Monoenoic fatty acids, often present in significant

quantities include C

16:1

20:1

, and C

22:1

, the last

two being mainly exogenous, originating as dietary

components. Cetoleic acid, C

22:1 o-11

, is the major

isomer, whereas erucic acid, C

22:1 o-9

, is a relatively

minor component. (See Triglycerides: Structures and

Properties.)

0005 More than 90% of the polyunsaturated fatty acids

(PUFAs) in fish oils are of the o-3 configuration: an

ethylene bond at the third carbon from the nonpolar

or o end of the molecule. These PUFAs make up

30–40% of the total fatty acids, whereas o-6 fatty

acids constitute less than 10%. In contrast, PUFAs

in vegetable oils are almost all o-6, with less

than 5% being o-3. (See Vegetable Oils: Dietary

Importance.)

0006Fish oils are characterized by o-3 HUFAs with five

or six ethylenic bonds per acid, and their presence

reflects the composition of the diet and other envir-

onmental factors, especially water temperature. Fish

from colder waters contain larger amounts of HUFAs,

which allows more cell membrane flexibility. As re-

lated to human health, the most important HUFAs

are eicosapentaenoic acid (EPA), C

20:5 o-3

, and doc-

osahexaenoic acid (DHA), C

22:6 o-3

. Although up to

50% of the total fatty acids in certain species may be

made up by these fatty acids, the usual concentration

found is 17.5–33% with a considerable variation

between and within species. Pink salmon contains

about 32% HUFAs, whereas the sablefish content is

only about 7%.

0007The influence of environmental factors on HUFA

content is illustrated by the difference in DHA levels

in menhaden oil. In one study of menhaden, large

samplings over 2 years at various times from the

Atlantic coast of North America showed a DHA con-

tent ranging from 7.3 to 13.1%, whereas the content

in menhaden from the US Gulf of Mexico coast was

4.2–8.2%. Another study of herring oil indicated that

the EPA contents from Alaskan fish ranged between

11.4 and 15.2%, whereas herring from the east coast

of Canada contained just 3.9–8.8%.

0008Farmed fish (aquaculture) have fatty acid profiles

reflecting the fatty acid composition of their feed.

Fish oil containing high levels of EPA and DHA is

included in feeds used in production of both fresh-

water and marine species, not only to provide essen-

tial o -3 fatty acids for fish health, but also to generate

products nutritionally similar to wild species.

Unreliability of Data

0009This general discussion of fish oil composition must

be qualified. Much available data on fish and fish oil

composition are taken from single samplings, small

numbers of fish, or samples from specific days. Un-

fortunately, such data have come to be considered

true for all oil from a specific fish species. There exists

a difference between species as well as a significant

variation in a single species that is influenced mainly

by season, but also by catch year, geographical loca-

tion, spawning cycle, maturity or age of the fish,

water temperature, and salinity. The composition of

available feed that is determined by the foremen-

tioned variables contributes the most significant

factor. Even analyses of commercial fish oil from

many thousands or millions of fish will vary

(Table 1). Statistically reliable data on fatty acid

content in fish oil are lacking for nearly all species

analyzed. Because of the wide fluctuation in oil

content in a single species owing to these variables,

data on the average oil content are not meaningful.

tbl0001 Table 1 Range of selected fatty acids in commercial fish oils

Range of total fatty acids (%)

Fatty acid Menhaden oil North American herring oil Mullet oil

14:0

7.2–12.2 4.6–8.4 4.6–11.5

16:0

15.3–25.6 10.1–18.6 20.2–33.7

16:1

9.3–15.8 6.2–12.0 13.4–29.3

0.2–3.0 0.0–8.2

18:0

2.5–4.1 0.7–2.1 1.8–5.4

18:1

8.3–13.8 9.3–25.2 7.1–13.6

18:2

0.7–2.8 0.1–0.6 0.7–2.7

18:3

0.8–2.3 0.0–1.1 0.3–1.3

18:4

1.7–4.0 1.1–2.8 0.8–4.5

0–2.5

20:5

11.1–16.3 3.9–15.2 4.6–8.1

22:1

0.1–1.4 6.9–30.6

22:5

1.3–3.8 0.3–1.3 1.7–4.6

22:6

4.6–13.8 2.0–7.8 0.7–3.9

Source: Stansby ME (ed.) (1990) Fish Oils in Nutrition. New York: AVI, Van

Nostrand Reinhold.

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