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

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(24,25(OH)

), which are instrumental in gener-

ating a wide array of cellular responses. These aspects

are detailed elsewhere in this volume.

Enzymes involved in Steroid Metabolism

0013 Table 1 summarizes some general properties of the

key enzymes involved with conversion of cholesterol

into the six key steroid hormones. The various classes

of enzymes that participate in the biosynthesis of

different steroids include: (1) lyases, which cleave

the carbon–carbon bonds bearing vicinal hydroxyl

groups in the cholesterol side chain; (2) dehydro-

genases and isomerases which convert the 5-ene-3b

hydroxysteroid to 4-ene-3-oxosteroid; (3) aromatase

enzyme (present in the microsomes of placenta and

ovaries), which mediates the aromatization of the A-

ring with a phenolic 3-hydroxyl group in the estro-

gens (Figure 1); and (4) hydroxylases, which play a

key role either in the sequential hydroxylation of

C22, C20, C17 of the cholesterol side chain or in

the stereospecific hydroxylation of the ring structure

to yield the different steroids (Figure 1). Thus, for

example, the three hydroxylases 17a-, 21-, and 11b-

hydroxylase are required for cortisol synthesis, and

the 1a and 24-hydroxylases are involved in the pro-

duction of bioactive vitamin-D metabolites.

0014All these apparently different hydroxylation reac-

tions are mediated by a family of enzymes called

tbl0001 Table 1 Key enzymes concerned with production of steroid hormones

Enzyme

reaction

Enzyme Reaction(s) mediated Subcellularlo-

cation

Gene location

Long name Short name

a P450 side-chain

cleavage;

formerly known

as desmolase

(P450scc) 20R-hydroxylation;

22R-hydroxylation;

scission of C20–C22 carbon bond

M15

b 3b-Steroid

dehydrogenase-

, D

-isomerase

(Isom) Oxidizes 3b-OH to oxo group;

isomerizes D

ene to D

ene

c 21-Hydroxylase (P450c21) 21-hydroxylation ER 6p

def 11b-Hydroxylase(P450c11)

d11b-hydroxylation; e18-hydroxylation;

f oxidation of C18 hydroxyl to an

aldehyde

M8q

gh 17a-Hydroxylase/

C17–C20 lyase

(P450c17)

g17a-hydroxylation; hscissionofC17–C20

carbon bond

ER 10

i 17-Ketosteroid

reductase

(17-oxido

reductase)

Reduces 17-oxo to a 17b-hydroxyl as in

androstenedione!testosterone;

estrone!estradiol;

DHEA!androstenediol

j Aromatase (P450aro) Mediates the aromitization of the A-ring ER 15q 21.1

11 Vitamin D25-

hydroxylase

(P450c25) 25-hydroxylation of vitamin D secosteroids M, ER

12 25(OH)D

-1a-

hydroxylase

(P450c1) 1a-hydroxylation of vitamin D secosteroids M

13 25(OH)D

-24-

hydroxylase

(P450c24) 24R-hydroxylation of vitamin D secosteroids M

14 5a-Reductase Reduction of D

-ene as in

testosterone!dihydrotestosterone

15 Ferredoxin-

oxidoreductase

In the mitochondria transfers electrons from

NADPH to ferredoxin

M 17 con!q25

16 P450 reductase In the endoplasmic reticulum transfers

electrons from NADPH to a P450 enzyme

17 11b-Hydroxy steroid

dehydrogenase

Oxidation of 11b-hydroxyl to an oxo as in

cortisol!cortisone

18 12a-Hydroxylase 12a-Hydroxylation of bile acids ER

19 6b-Hydroxylase 6b-Hydroxylation of bile acids ER

20 7a-Hydroxylase 7a-Hydroxylation of bile acids ER

This table summarizes the key enzyme reactions in steroid hormone production (see numbers in circles). Note that, in some instances, the same

enzyme catalyzes more than one reaction. (See footnotes 3 and 4.)

Subcellular location coded as M, mitochondria; ER, endoplasmic reticulum; NM, nuclear membrane.

Reactions d, e, and f can be mediated by the same enzyme.

Reactions g and h are mediated by the same enzyme.

DHEA, dehydroepiandrosterone; NADPH, reduced form of nicotinamide adenine dinucleotide phosphate.

HORMONES/Steroid Hormones 3169

cytochrome P450 hydroxylases which have a cyto-

chrome moiety at the catalytic center. The cyto-

chrome P450 enzymes are present in either the

mitrochondrial or microsomal fraction of cells pre-

sent in the liver, adrenal cortex, ovary, testis, kidney,

placenta, lungs, and intestinal mucosa. The mito-

chondrial cytochrome P450s are comprised of a

flavoprotein dehydrogenase, which accepts electrons

from the reduced form of nicotinamide adenine

dinucleotide phosphate (NADPH), and the nonheme

Cholesterol

7-dehydrocholesterol Vitamin D

25-OH-D

24,25(OH)

1α,25(OH)

Progesterone DOC

17OH-Progesterone 11-Deoxycortisol

Corticosterone 18OH-Corticosterone Aldosterone

Cortisol

Androstenedione Testosterone Estradiol

Pregnenolone

17OH-Pregnenolone

Dehydroepiandrosterone

(DHEA)

A, O, T, P

T, O T,O O

3 4

4 5

9 10

fig0002 Figure 2 Principal pathways of human steroid hormone synthesis. The details of the enzyme reactions (circled numbers) are

summarized in Table 1. Reactions a, b, g, and h are found universally in the adrenals, ovary, testes, and placenta. Reactions i

and j are found principally in the testes and ovaries. Reactions k ! m describe the pathway of production from cholesterol of the

prohormone vitamin D and its hormonal metabolite products 1,25-dihydroxyvitamin D

(1a,25(OH)

) and 24,25-dihydroxyvitamin D

(24,25(OH)

). Reaction k is P450c25 present in the mitochondrial and/or endoplasmic reticulum of the liver. Reactions l and m:

P450c1a and P450c24 are present in the mitochondria of the kidney proximal tubule. Tissue codes: A, adrenals; O, ovaries; T, testes;

P, placenta; S, skin; L, liver; K, kidney; DHEA, dehydroepiandrosterone; DOC, deoxycorticosterone.

Tropic

hormone

ACTH

Steroid

hormone

Endocrine

gland

Adrenal cortex

(zona

fasciculata)

Glucocorticoids

(cortisol)

Mineralocorticoids

(aldosterone)

Adrenal cortex

(zona

glomerulosa)

Renin−

angiotensin

Luteinizing

hormone (LH)

Testes

(interstitial cells)

(Male) (Female) (Female)

Ovaries

(follicles)

Estrogens

(estradiol)

Progestins

(progesterone)

Vitamin D

metabolites

[1α,25(OH)

]

Kidney

(proximal

tubule)

Ovaries

(corpus

luteum)

(Pregnant

female)

Placental

chorionic

gonadotropin

(hCG)

Parathyroid

hormone

(PTH)

Follicle-

stimulating

hormone

(FSH)

Androgens

(testosterone)

fig0003 Figure 3 Relationship between stimulatory peptide hormones and the production of steroid hormones. For each of the six classes of

steroid hormones, the specific stimulatory or tropic peptide hormone is indicated as well as specification of the organ or cell upon

which it acts. Also, in males, follicle-stimulating hormone is known to act upon the Sertoli cells of the seminiferous tubules to increase

the production not of a steroid hormone but of several sperm proteins, androgen-binding protein, and inhibin, a peptide hormone.

ACTH, adrenocorticotropic hormone.

3170 HORMONES/Steroid Hormones

iron protein adrenodoxin, which accepts electrons

from flavoprotein and transfers them to the cyto-

chrome P450 protein. The microsomal hydroxylase

system lacks the adrenodoxin component. Cyto-

chrome P450 hydroxylases (also known as mixed-

function oxidases) catalyze oxidation-reduction

reactions in which one atom of the substrate (O

)

ends up on the steroid as a part of the hydroxyl and

the other oxygen atom as part of water (H

O); the

necessary reducing equivalents are derived from

NADPH.

Transport

0015 All steroid hormones, which are quite water-

insoluble, circulate in blood, bound to plasma pro-

teins called steroid hormone-binding globulins. The

androgens and estrogens are both transported by a

plasma b-globulin, which is a hepatic glycoprotein

with a molecular weight of 94 000. This carrier pro-

tein has a high affinity (K

1.5  10

10

mol l

1

) for

testosterone, 5a-DHT, and estradiol but not for

progesterone or cortisol. The glucocorticoids are

systemically transported via the corticosteroid-bind-

ing protein (CSBP) or transcortin, which is also a

hepatic protein. Similarly, the vitamin D sterols are

bound to the vitamin D-binding protein (DBP), and a-

globulin. The primary role of all these different sterol-

binding proteins is to provide a reservoir of bound

hormone which can effectively dampen oscillations in

the ‘free’ concentrations of hormones.

Catabolism and Excretion of Steroid Hormones

0016 The catabolic pathways that result in the inactivation

of the steroid hormones occur mainly in the liver.

Conjugation of the hydrophobic steroids either with

sulfate or glucuronides renders the hormone more

hydrophilic for easy excretion in the urine. The

excreted species are usually polyhydroxyl forms

with the particular glucuronide or sulfate: these con-

jugations are mediated by sulfokinase and glucuronyl

transferase enzymes.

Functions of Steroid Hormones

0017 Steroid hormones secreted by the endocrine glands

exert profound influences on cell growth, develop-

ment, and physiology, chiefly by controlling the

synthesis of particular proteins. All steroid hormones

modulate cellular responses by either inducing or

repressing the transcription of specific proteins at

the genetic level.

0018 A general result of glucocorticoid action is a

direct effect of increased glucogen, especially in the

liver, and increases in the levels of circulating glucose.

Prolonged high levels of glucocorticoids can lead to

the death of susceptible cells which accounts for

muscle wasting and immunodeficiency. As a powerful

antiinflammatory agent, glucocorticoids may act

on many cells to induce the synthesis of the protein(s)

lipomodulin. Lipomodulin acts as an inhibitor of

phospholipase A

and thereby prevents the release

of prostaglandins, prostacyclins, and leukotrienes,

some of which mediate processes of inflammation

and pain.

0019Mineralocorticoids such as aldosterone promote

sodium reabsorption by the kidney tubules, usually

followed by changes in water balance; in terms of

function they mesh with the vasopressin system and

atrial natriuretic factor which also control water

absorption and excretion by the kidney and also

participate in the regulation of blood pressure.

0020The biological responses of androgens are divisible

into four categories: (1) growth-promoting or andro-

genic effect on the male reproductive tract; (2) stimu-

lating or anabolic effect on body weight (skeletal

growth, skeletal muscle growth, subcutaneous fat

distribution, growth of prostate and seminal vesicle);

(3) development of male secondary sexual character-

istics (growth of external genitalia, enlargement of

larynx and thickening of vocal cords, hair growth

distribution); and (4) actions in the central nervous

system and brain (differentiation of brain cortex,

hypothalamus, initiation of puberty).

0021The dominant effect of estrogens occurs in the

female reproductive tract. Significant biological activ-

ity of estrogens also occurs in the hypothalamus,

brain, and mammary tissue. Estrogens primarily

initiate the transcription of specific proteins in the

uterus, and affect uterine cell proliferation and differ-

entiation. A classical developmental role of estrogen

in the liver of the adult hen is in the induction

of vitellogenin, which is transported to the ovary

and cleaved to yield egg proteins, phosvitin, and

lipovitellin.

0022The biological activity of progesterone is restricted

to the female reproductive tract and mammary tissue.

Some of the effects mediated by progesterone include

thermogenesis in women, regulation of egg move-

ment through the fallopian tubes, preparation of

uterus to receive the blastocyst, alteration of electrical

activity in the brain, control of uterine contraction

(at parturition), and generation of the secretory

system of breast during pregnancy.

0023The steroid hormone 1,25(OH)

is principally

involved in the regulation of mineral (calcium

and phosphorus) metabolism, bone growth, and

differentiation. A detailed account of the manifold

effects of vitamin D is given in a subsequent article.

(See Cholecalciferol: Properties and Determination;

Physiology.)

HORMONES/Steroid Hormones 3171

Mechanism of Action of Steroid Hormones

0024 The receptors for all the steroid hormones,

1a,25(OH)

, thyroid hormone, and retinoic acid,

are all proteins which exist exclusively in the cell

nucleus or are partitioned between the cytoplasm

and nucleus. Each of these receptor proteins functions

as DNA-binding proteins, where they regulate the

expression of genes related to the biological response

of the hormone in question.

0025 Figure 4 presents a generic model of a typical ster-

oid receptor. The protein consists of a single poly-

peptide chain divided into six domains. Starting at

the N-terminus, the domains are respectively (1, 2)

the A/B or variable (sequence and length) domain;

(3) the C or DNA-binding domain which contains

two zinc fingers; (4) the D or variable hinge region;

(5) the E or ligand-binding domain; and (6) the F or

variable domain.

0026 Figure 5 illustrates the striking structural hom-

ology between all the domains of the steroid super-

family. All these proteins are related evolutionarily.

The ancestor of these proteins may be the oncogene

product v-erb A.

0027 The steroid receptors are labile soluble proteins

with molecular weight ranging between 50 and

94 kDa. The molecular cloning of steroid receptors

has greatly facilitated the delineation of the various

functional domains of these receptor proteins

(Figures 4 and 5). All members of steroid receptors

bear a short DNA-binding domain comprising about

70 amino acids containing many conserved cysteine

residues. Eight of the cysteines are organized into two

so-called zinc fingers, each containing four cysteine

residues tetrahedrally coordinating a zinc ion. Based

on the amino acid sequence within the DNA-binding

region of steroid receptors, they can be classified into

two subfamilies: (1) glucocorticoid receptor is the

prototype of the smaller subfamily which includes

progesterone, androgens, and mineralocorticoid re-

ceptors; (2) the estrogen receptor is the prototype

of the larger subfamily which includes the vitamin D

receptors, thyroid hormone receptors, and receptors

for retinoic acid. Steroid receptors exhibit a large

C-terminal domain, responsible for binding hormone,

known as hormone-binding domain (HBD). The

amino-terminal region contains hypervariable amino

acids and certain transactivation sequences. Recent

studies have shown that functional properties of

steroid receptors are modulated by phosphorylation,

dimerization, and association with other proteins

such as heat shock proteins (HSPs).

0028A model of the steroid hormone mode of action is

provided in Figure 6. In step 1, the hormone (H)

dissociates from the plasma transport protein. In

step 2, the hormone enters the target cell by diffusing

through the outer cell membrane. Then, depending

upon the subcellular localization of the unoccupied

receptor, the hormone will either interact with the

receptor in the cytoplasmic compartment (glucocorti-

coid and aldosterone receptors) or continue through

A/B

Transcription

Variable

(immunogenic)

DNA Hinge

Ligand

binding

Transcription

Molybdate

HSP

Activation

domain

COOH

Zinc fingers

Homologies: 60−95%

Nuclear translocation

CD E F

COOH

65−75% 30−60%

fig0004 Figure 4 Model of a typical steroid hormone receptor illustrating the six functional domains A ! F. HSP, heat shock protein.

DNA-binding

domain

Hormone-binding

domain

ALDO

PROG

ANDR

CORT

erb-A3⬘5⬘

fig0005Figure 5 Steroid receptor gene superfamily. T

, triidothyro-

nine; RA, retinoic acid; D

, 1,25-dihydroxyvitamin D

, estra-

diol; CORT, cortisol; ANDR, androgen; PROG, progesterone;

ALDO, aldosterone. The N-terminal domain (open rectangle to

the left of the DNA-binding domain) represents the sequences of

least homology among the receptors.

3172 HORMONES/Steroid Hormones

the cytoplasm and cross the perinuclear membrane to

interact with a receptor in the nucleus (thyroid, estro-

gen, progesterone, retinoic acid, and 1,25(OH)

receptors).

0029 The unoccupied form of the steroid receptor is

believed to exist in the cell as a conjugate with a

dimer of the 90-kDa HSP; in Figure 6 the HSP

dimer is indicated by a pair of ovals. It has been

suggested that the HSP’s function is to occlude the

DNA-binding domain of the receptor which prevents

the unoccupied receptor from binding to DNA.

0030 In steps 3 and 4, the receptor becomes transformed

or activated as a consequence of release of the 90-kDa

HSP and binding of its cognate ligand, so that the

DNA-binding domain is now exposed. In step 5,

the cytoplasmic activated receptor translocates to

the nucleus, probably through a nucleopore. Then

in step 6, the activated receptor, or more likely a

homodimer of the receptor, seeks out the correct

sequence of DNA that will allow it to form a high-

affinity complex between the receptor and the

hormone response elements (HRE) of the promoters

of a selected set of genes and also any required

transcription factors.

0031As a consequence of the activated receptor binding

to the promoters, either induction or repression of

that gene will occur, leading to more or less of the

mRNA coded for by that gene.

0032The newly transcribed mRNAs are translocated to

the cytoplasm where they become incorporated into

polysomes and undergo translation (step 7). In the

final step, step 8, the increased or decreased amount

of new proteins generates more or less of the bio-

logical response(s) dictated by that hormone in that

target cell. For any given hormone, a wide array of

biological responses can be modulated depending

upon the phenotype of the target cell that possesses

the cognate receptor. In any given target cell pheno-

type, only a small subset of genes will have their DNA

chromatin in an active or open configuration. Thus,

while a hormone may modulate as many as 300 genes

in a given organism, in a specific target cell perhaps

only a few genes will be available for regulation.

Summary

0033Steroid hormones impact profoundly on cellular

metabolism, development, and physiology. The

STEP 1

New protein

STEP 7

STEP 5

STEP 6

DNA

STEP 4

STEP 3

STEP 2

Protein synthesis

mRNA

Translation

Transcription

Activated

Activation

Translocation

of cytoplasmic

receptor complex

Biological

response

Non-DNA-

binding

cytoplasmic

receptor

Plasma

transport

protein with

bound

steroid

hormone

Heat shock

protein

Free steroid

hormone +

Target cell

Nucleus

Nuclear

receptor

fig0006 Figure 6 Model of steroid hormone mode of action. A detailed description is given in the text.

HORMONES/Steroid Hormones 3173

repertoire of cellular responses elicited by the various

steroid hormones is mediated by specific intracellular

receptor proteins localized in target tissues. Cloning

of the receptor genes has allowed detailed structure–

function analysis of these physiologically important

molecules. Greater insight and appreciation of the

complex molecular details of steroid hormone action

should be possible with the application of modern

cellular and molecular biological techniques.

See also: Cholecalciferol: Properties and Determination;

Physiology

Further Reading

Bouillon R, Okamura WH and Norman AW (1995) Struc-

ture–function relationships in the vitamin D endocrine

system. Endocrinology Review 16: 200–257.

Chambon P (1996) A decade of molecular biology of

retinoic acid receptors. FASEB Journal 10: 940–954.

Darimont BD, Wagner RL, Apriletti JW et al. (1998) Struc-

ture and specificity of nuclear receptor–coactivator

interactions. Genes and Development 12: 3343–3356.

Jenster G, Spencer TE, Burcin MM et al. (1997) Steroid

receptor induction of gene transcription: a two-step

model. Proceedings of the National Acadomy of Science

of the USA 94: 7879–7884.

Mangelsdorf DJ, Thummel C, Beato M et al. (1995) The

nuclear receptor superfamily: the second decade. Cell

83: 835–839.

Norman AW and Litwack G (1997) Hormones, 2nd edn.

Florida: Academic Press.

Weatherman RV, Fletterick RJ and Scanlon TS (1999)

Nuclear receptor ligands and ligand-binding domains.

Annual Review of Biochemistry 68: 559–582.

HORSE MEAT

E Rossier, Formerly of CEREOPA, Paris, France

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001 The custom of eating horse meat is probably as old as

the world itself and, without venturing into prehis-

toric times, the majority of the nations of Antiquity

(Persians, Greeks, Romans, Chinese, Cossacks, etc.)

all ate horse meat.

0002 Paradoxically, in European countries Christianity

has strived to abolish the eating of horse meat since

the seventh century ad. This ban was based on reli-

gious beliefs and was only dispelled after 1870. The

eating of horse meat then developed quickly thanks to

the vigorous action of several well-known personal-

ities in scientific and, in particular, medical circles.

0003 This article explains the situation of the world

market as well as the nutritional and hygienic

qualities of horse meat.

The World Horse Meat Market

0004 Horse meat is produced in several countries, although

the level of production is often marginal when com-

pared to other types of meat.

0005 The USA is the major producer. The other principal

countries are Mexico, Argentina, Canada, China,

Mongolia, Ethiopia, Chile, and Brazil, as well as

several European countries – Poland, Yugoslavia,

France, and Italy. The Commonwealth of Independ-

ent States (former Soviet Union) is also a large

producer although few data are available.

0006The majority of these countries are exporters of

carcass meat, semicarcass meat, quartered or boned

meat, or else of live animals destined for slaughter in

the importing country. In practice, a few countries

dominate trading, the USA being the lead producer,

followed by Argentina and Canada.

0007There are also a few main importing countries and

in these consumption exceeds production which has

diminished as a result of mechanization of agriculture

and transport. This is particularly so in France, Japan,

Italy, Belgium, and the Netherlands. The other mar-

kets are much more limited: Switzerland, Austria,

Sweden, Norway, and Denmark.

0008The internal production of horse meat is adequate

for domestic consumption in Mongolia, China,

Mexico, Ethiopia, and the Commonwealth of

Independent States.

0009In many countries there is a considerable horse

population, but little information is available reg-

arding the final use to which the animals are put,

except in the production of dog and cat food.

0010Except for the countries mentioned above, Italy,

France, Japan, Belgium, the Netherlands, Spain,

Switzerland, and Sweden are the principal consumers

(Table 1). Only small proportions of these

3174 HORSE MEAT

populations eat the meat. This attitude might

change significantly if the meat were promoted with

a specific image in view of the qualities mentioned

below.

0011 There are several restraints on consumption of this

type of meat, in particular cultural or religious pro-

hibitions, the ‘noble animal’ symbolism of horses, and

the often specific distribution channels of horse

butchers, although more widespread distribution is

gradually taking over.

Butcher Qualities

Carcass Characteristics

0012 As horses were not intended to produce meat for

consumption, unlike other species, the meat that has

been eaten has come mainly from grazing animals,

working horses and, more recently, sport and racing

horses.

0013 The growth in production of young meat, espe-

cially from the Trait breed, is new in several European

countries and Japan. The horse is in fact a good meat

producer. Carcass yields are high (real yield is equal

to weight of warm carcass divided by live empty

weight), bordering on 70% for young stallions born

from the Trait breed and comparable with those for

cattle meat.

0014 The carcass contains a very high proportion of

muscle (70%) and little fat (10–14%). The distribu-

tion of adipose tissue is not the same as in cattle, as

horse carcasses contain more subcutaneous and in-

ternal adipose deposits. On the other hand, there is

a smaller proportion of intermuscular fat. Horse

carcasses therefore look more ‘glazed,’ but studies

on the relative growth of different tissues, organs,

and body regions have confirmed that the horse is a

lean animal. (See Meat: Structure.)

Changes to the Meat after Slaughter

0015The physiological principles of postmortem change

are identical for the horse as for other species. How-

ever, horse meat differs from cattle meat in its speed

of maturation. Rigor mortis sets in rapidly (in several

hours), and the pH quickly drops to 5 or 6, which

limits microbial proliferation and allows autolysis of

the muscle. (See Meat: Slaughter.)

0016Horse meat contains a large amount of glycogen

and adenosine triphosphate (ATP) and therefore

retains a degree of plasticity and elasticity for a

long time.

0017Maturation occurs quickly in horse muscle. Most

of the maturation is achieved 4–5 days after slaughter,

allowing the meat to be eaten very fresh. As for pre-

servation, several factors combine to give horse meat

an excellent sanitary quality, including short storage

period for maturation, and low pH. But the large

proportion of glycogen, lactic acid, and nonprotein

nitrogen encourages microbial development. The

meat must therefore be sold quickly and stored at

low temperature before being eaten.

Organoleptic Qualities

Tenderness

0018This criterion largely overshadows other quality

parameters since the consumer’s satisfaction when

tasting the meat depends on this. (See Sensory Evalu-

ation: Texture.)

0019It is the insoluble protein constituents of the meat

which more or less determine its tenderness: the col-

lagen of connective tissue and its degree of polymer-

ization, myofibril proteins, and muscular fibers.

0020It seems that, with horses as with other species,

there are important variations between animals, age,

and different muscles of the same carcass.

0021These variations in toughness result from large

differences in the level of collagen present and its

fluctuation with the age of the animal. The collagen

content is at its maximum at 18 months. The solubil-

ity of collagen, which is linked to its aging and

hardening, seems to reduce rapidly until 24 months

and then stabilizes thereafter. Thus insoluble collagen

reduces the tenderness. Horse meat is generally con-

sidered to be very tender, however, and this quality of

tenderness is one of the consumer’s main reasons for

purchase.

tbl0001 Table 1 National gross production and total consumption of

horse meats in the main consumer countries in 1988

Country National gross

production

(10

Total

consumption

(10

Belgium and Luxembourg 3 28

Denmark 1 1

France 15 60

Germany 5 5

Italy 14 68

The Netherlands 4 21

Spain 7 7

Japan 4 59

Austria 1 2

Finland 1 1

Norway 1 1

Sweden 2 3

Switzerland 1 4

Yugoslavia 9 1

Source: Organisation for Economic Cooperation and Development (1990)

Meat Statistics, with permission.

HORSE MEAT 3175

Color

0022 It is important to consider the color of horse meat, in

particular minced horse meat, which constitutes a

high proportion of the value of horse carcass. The

color is linked to the myoglobin content in the

muscles. (See Sensory Evaluation: Appearance.)

0023 Adult horse meat is very rich in myoglobin and iron

and looks very red. This is a quality sought after by

consumers in some countries (France), whereas others

(Italy) prefer lighter-colored meats. The level of pig-

ment varies according to the particular muscles

and the age of the animal. The content of heme iron

is still very low at 6 months, giving a very clear,

‘brassy’ meat. A strong color hardly appears before

18 months.

Nutritional Qualities

0024 The structures – particularly of the muscular fibers –

of horse meat are classical and identical to those of

other species. On the other hand, the chemical com-

position of horse muscle is highly characteristic and

conveys particularly interesting nutritional qualities.

Average Composition of Horse Meat

0025 At the beginning of the twentieth century the average

composition of horse meat was published as follows:

water, 74.27%; nitrogenous substances, 21.71%;

fats, 2.55%; nonnitrogenous substances, 0.46%;

mineral materials, 1.01%. More recent writers give

very similar figures, shown in Table 2. Globally,

water content is higher in horse meat than in other

species; a very low lipid content in horse meat also

leads to a low energy value. The high level of water in

horse meat can be a handicap for certain processes

and can present technological problems for the salted

horse meat trades.

Carbohydrates

0026 Horse meat is rich in carbohydrates, especially glyco-

gen. It is one of its main characteristics. It contains

from 0.5 to 3 times more than cattle meat. This high

level of glycogen gives horse meat a sweeter flavor

than beef. (See Carbohydrates: Classification and

Properties.)

Lipids

0027The main dietary quality of this meat is its low level of

lipids. However, this meat is interesting not only in

its low lipid content and its distribution of lipids

throughout the carcass at different adipose deposits,

but also in the structure of these fats. They are mainly

comprised of triglycerides with a high proportion of

unsaturated and polyunsaturated fatty acids: palmi-

toleic acid (C

16:1

), oleic acid (C

18:1

), linoleic acid

18:3

) and arachidonic acid (C

). (See Fatty Acids:

Properties; Fats: Classification.)

Proteins

0028The proteins are the major constituents after water.

They can be divided into extracellular proteins, e.g.,

collagen, and intracellular proteins. Horse meat is

interesting not only in its proportion of proteins

but also in the distribution of different amino acids.

The few studies on the amino acid content of horse

meat show a strong percentage of essential amino

acids such as lysine, leucine, and isoleucine. (See

Amino Acids: Properties and Occurrence; Protein:

Chemistry.)

Minerals and Vitamins

0029Principal minerals As shown in Table 3, horse meat

possesses a low sodium content (21 mg per 100 g on

average) and is therefore suitable for those on low-

sodium diets; this meat has a strong image of being

‘good for the health.’ The red color of the adult meat

can darken and turn brown rather quickly as a result

of oxidation, which makes the meat difficult to

present and market. Refer to individual minerals.

0030Trace elements There are heavy metals such as cad-

mium, zinc, and lead in horse meat. Practically all

heavy metals are present but in very small quantities,

with no risk of toxicity. (See Heavy Metal Toxicology.)

0031Vitamins The few reported findings in the literature

suggest the presence of few fat-soluble vitamins and

tbl0003Table 3 Mineral content of horse meat

Minerals Content (mg 100 g

1

freshmatter)

Phosphorus 200

Calcium 4

Sodium 21

Potassium 176

Iron 2.7

Magnesium 4

Sulfur 10

tbl0002 Table 2 Average composition of horse meat (per 100 g of

consumable meat)

Composition Averagevalue

Water (g) 75

Protein (g) 23

Lipid (g) 2

Carbohydrate (g) 1

Energy (kJ) 462–504

3176 HORSE MEAT

more water-soluble vitamins in horse meat. (See Vita-

mins: Overview.)

Hygienic Qualities

0032 Horse meat is reputed for its hygienic qualities as it is

consumed very fresh. It is even common for minced

horse meat to be eaten raw.

Parasitic and Microbial Contaminations

0033 Horse meat is naturally not very contaminated. Al-

though it is claimed to be safe, it must be fresh to be

eaten raw or lightly cooked. In fact, its richness in

glycogen can encourage the growth of microorgan-

isms. Contamination often occurs at the surface and

can easily spread, especially as the meat is often

minced. A strict code of hygiene is necessary. (See

Contamination of Food.)

0034 Parasitic infections are rare but must be given

special attention. Lesions of the digestive tract must

be watched for as well as similar signs of the effects of

parasites in both living and dead animals. (See Para-

sites: Occurrence and Detection.)

0035 Bacteria such as Salmonella are the main agents of

contamination of horse meat. They are easily des-

troyed by cooking heat, but as the meat is often

consumed either without being fully cooked through

or even raw, they can survive in the food. A very strict

code of hygiene is therefore required when working

with horse meat in the slaughterhouse and in the

butcher’s shop.

0036 The transmission of the parasitic disease trichinosis

through the consumption of horse meat was un-

known until roughly 1975. The disease can be spread

amongst carnivores (e.g., dogs, cats, foxes) rodents

(e.g., badgers, rats) and omnivores (e.g., pigs, boars,

bears, humans), and infestation in horses is quite

unusual.

Processing

0037 Horse meat lends itself to butchering and salting

processes, either for typical horse meat products

(e.g., sausages, saveloy, andouilles, smoked hams) or

else in conjunction with other meats. This process has

been developed in Belgium, Italy, and France as well

as in Japan, where hams are reconstituted, often

mixed with other meats. Also in Japan, horse meat

is frequently incorporated into corned beef. (See

Meat: Preservation.)

Conclusion

0038 Horse meat shows many advantages in terms of pro-

cessing and storage, and organoleptic, nutritional,

and hygienic qualities.

0039Some of the special characteristics of this meat are

carried over from generation to generation: it is the

meat to treat anemia; it is meat suitable for tubercu-

losis diets; it is the meat for energy; these are special

characteristics familiar to nutritional biochemists.

0040These characteristics, in the current context of

competition between meats and other available

sources of proteins, and as a result of changes in

eating habits in recent years, make horse meat, at

the very least, a most interesting meat with the

following advantages: high butcher yields; high levels

of glycogen and good-quality proteins (essential

amino acids); low contents of salt and lipids; tender-

ness associated with collagen characteristics and their

distribution; a characteristic sort of fatty tissue with a

predominance of unsaturated fatty acids; safe in

terms of residues or contamination provided that the

conditions of slaughter, transport, storage, and distri-

bution are satisfactory in order to reduce as much as

possible the dangers of microbial infection.

0041Ultimately, horse meat not only possesses obvious

dietary qualities for people in good health and par-

ticularly for children, but also is very suitable for

those on special diets or everyday diets, e.g., diabet-

ics, obese subjects, those with gout, hypercholesterol-

emia, hypertriglyceridemia, gastrectomy or kidney

insufficiency, and those following a residue-free diet

or a low-sodium diet.

See also: Amino Acids: Properties and Occurrence;

Carbohydrates: Classification and Properties;

Contamination of Food; Fats: Classification; Fatty

Acids: Properties; Heavy Metal Toxicology; Meat:

Structure; Slaughter; Preservation; Parasites:

Occurrence and Detection; Protein: Chemistry; Sensory

Evaluation: Appearance; Texture; Vitamins: Overview