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

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conditions, in management procedures and in the

breed types available. At one end of the climatic

scale, there are the fat-tailed desert sheep, which sur-

vive and thrive where other sheep would perish; at

the other extreme are mountain breeds like the

Scottish Blackface of the UK, which can stand harsh

conditions, high rainfall, snow, and indifferent

herbage. Most sheep feed outside continuously with

only limited supplementation of available pasture,

whereas a few, mainly in Europe and North America,

are housed from birth and fed carefully formulated

diets. There is little mechanization or intensification.

0004 In some countries, sheep-farming systems can

cover wide ranges of climatic condition, as in the

UK, where there is a stratification system involving

the crossing of breeds adapted to particular environ-

ments: crosses from hill and upland areas are the

female breeding stock for lowland areas where they

are mated with sire breeds selected specifically for

meat production.

0005 Genetic improvement in the world’s sheep popula-

tion has been very uneven. There are pioneering

examples of application of genetics, particularly in

Norway, Australasia, and the UK. Published infor-

mation on sheep breeding is much more a review of

the technology about to be applied, rather than a

record of achievement.

0006 Progress is slow because environmental, technical,

and economic conditions prevailing in areas where

sheep are kept are frequently difficult. Breeding ob-

jectives are often not clear: indeed, there may be

direct conflicts between adaptation for survival and

for productivity. Relatively low reproduction rates

and long generation intervals do not help. The multi-

purpose use of sheep for meat, wool, and milk also

reduces the selection pressure for meat characteristics

under some circumstances.

0007 Sheep meat production is dominated by two

factors: first, there are the biological and agricultural

considerations of resource use; second, there are the

marketing considerations, in particular the ability of

farmers to adapt output to conform more closely

with changing market demand. In developed coun-

tries, this demand is for an increasingly lean product.

Carcass Quality

0008 The value of sheep carcasses depends on several

factors, namely weight, conformation (carcass shape),

proportion of the main tissues (muscle, fat, and bone),

distribution of thesetissues throughthe carcass,muscle

thickness, and meat quality. (SeeMeat: Structure.)

0009 The weight and size of the carcass have a major

influence not only on the quantity of the various

tissues but also on the size of the muscles exposed

on cutting and of the individual joints prepared from

it. This is of importance particularly in relation to a

retailer’s ability to provide cuts of a suitable size

to meet customer requirements. Over generations,

wholesalers and retailers from different regions

of countries and different parts of the world have

become accustomed to handle certain weight ranges

of carcasses; abattoir practices and cutting methods

have been developed accordingly. Most whole-

salers state the desired weight ranges in buying sched-

ules and apply discounts to carcasses outside these

ranges. In some cases, these discounts are severe and

are a constraint on the use of improved breeds and

production systems. There has, for example, historic-

ally been a demand in the UK for small legs suitable

for the Sunday roast, which has slowed the movement

to heavier carcasses that are likely to be more efficient

to produce and process. This demand has also had a

major effect on the type of carcass produced in New

Zealand for the British market. (See Meat: Slaughter.)

0010Although conformation (the shape of the carcass)

provides a poor indication of carcass composition, it is

still regarded by many in the meat industry as valu-

able in this respect. Carcasses with good conform-

ation normally command higher prices, and most

national classification and grading schemes include

conformation as a factor. It is particularly important

in some European markets.

0011Among carcasses of similar weight, the percentage

formed by each tissue varies considerably depending

on the breed type and level of feeding. The proportion

of lean meat in the carcass is of major importance,

since this is the principal determinant of yield and

commercial value in many countries. Taken as a gen-

eralized ideal, the best carcasses should have an

optimum level of fatness, sufficient to ensure that

carcasses do not dry out and to ensure good eating

quality, and minimum bone.

0012The general trend is now towards the production of

leaner carcasses because of consumers’ increasing

demand for meat with a low fat content. Table 1 lists

the typical composition for carcasses in different

fat classes of the National Carcass Classification

Scheme operated in the UK (this can be linked directly

to the developing EC scheme). (See Meat: Nutritional

Value.)

Definition of Retail Cuts

0013Traditionally, lamb is sold by butchers largely bone-

in, so about 90% of the carcass is ‘saleable meat’ and

goes across the counter.

0014The lamb carcass is typically separated into the

sides by cutting it lengthwise through the spine. The

main cuts (using British terminology) are as follows.

SHEEP/Meat 5197

0015 The forequarter is a large cut which includes the

neck, shoulder, and part of the breast. It makes an

economical family roast but is more difficult to carve

than a leg. Boned and rolled or stuffed, it is easier to

carve.

0016 The shoulder is smaller than the leg and is easier to

carve after boning. Shoulder chops or cubed shoulder

meat should be well trimmed to remove excess fat

before grilling or stewing.

0017 Neck chops may be stewed, braised, or casseroled.

0018 Breast is an economy cut. The ends of the rib bones

can be cut out to simplify carving or rolling.

0019 The best end of neck or rack is made up of six or

seven rib chops. When it is to be cooked whole, it may

be boned and rolled, or chined (backbone removed)

for easy carving. The rib chops are trimmed to make

cutlets.

0020 The loin can be cut into about seven or eight meaty

chops, each with a short T bone; alternatively, it can

be left in one piece and rolled or trimmed, boned,

rolled, and cut into noisettes before cooking.

0021 Chump chops (or leg chops) are lean, meaty chops

cut from the end of the leg nearest the loin.

0022 The leg is a large, lean roast which may be cooked

with the bone in, or boned and stuffed. Meaty steaks

can be cut from the thick part of the leg.

0023 Consumer attitudes indicate that lamb is often

regarded as fatty, not versatile, and difficult to carve.

Major developments have, therefore, taken place in

new cutting methods. This work produces boneless

cuts which are convenient in size, easy to carve, and

lead to minimal waste on the plate.

0024 The new techniques only work satisfactorily on

lambs which are quite lean at the outset, since it

is difficult to trim the intermuscular fat without

adopting a complete muscle seaming technique. If

there is increasing use of these boneless techniques,

it will create further pressure towards the production

of heavier and leaner lambs.

0025 Lamb and mutton are used less in processed form

than other meats. One of the main reasons is the high

bone-to-meat content of the carcass and the small size

of the carcass. These two factors make it more labor-

intensive for boning-out and processing and therefore

more costly relative to other meats. Lamb fat is also

more saturated and less suitable for processing.

0026The main area of development in the UK at the

moment is in kitchen-ready products which offer var-

iety, convenience, and novelty to consumers. The

range of products now being introduced in the UK,

particularly by independent butchers, includes lamb

burgers, marinated chops and steaks, and more exotic

dishes such as lamb bengali, lamb pasanda, and lamb

italliene. Lamb is the basis for many spiced dishes on

a worldwide basis, and quite a number of these are

being introduced into Europe by the major supermar-

ket chains.

0027Recently, there has been renewed interest in the

curing of lamb because of its cheapness in some

countries relative to pigmeat. (See Meat: Preservation.)

See also: Meat: Structure; Slaughter; Preservation;

Nutritional Value

Further Reading

Food and Agriculture Organization of the United Nations

(1990) Production Yearbook, 1989, vol. 43. Rome: FAO.

Kempster AJ, Cook GL and Grantley-Smith M (1986) Na-

tional estimates of body composition of British cattle,

sheep and pigs with special reference to trends in fatness.

A review. Meat Science 17: 107–138.

Milk

M Juarez, Instituto del Frio (CSIC), Madrid, Spain

M Ramos, Instituto de Fermentacinos Industriales

(CSIC), Madrid, Spain

Background

0001Although the production of sheeps’ milk (approxi-

mately 8 million tonnes) is of marginal importance

tbl0001 Table 1 Composition of carcasses in different fat classes of the British classification scheme

Fat class

1 2 3L 3H 4 5

Lean meat in carcass (%) 62.4 58.7 56.1 54.3 51.7 48.1

Separable fat in carcass (%) 16.5 21.7 25.6 28.2 32.1 37.3

Total protein in carcass (%) 14.2 13.3 12.7 12.2 11.4 10.2

Lipid in lean (%)

3.6 5.1 6.2 6.9 8.1 9.6

Most carcasses fall into fat class 3L. Fat classes 4 and 5 would be too fat for most requirements.

Intramuscular fat.

Data from Kempster AJ, Cook GL and Grantley-Smith M (1986) National estimates of body composition of British cattle, sheep and pigs with special

reference to trends in fatness. A review. Meat Science 17: 107–138.

5198 SHEEP/Milk

compared to cows’ milk in quantitative terms (2% of

the total), it is of major economic relevance in a

numbers of areas such as the Mediterranean and

Middle East, where the climatic conditions are not

favorable for raising cattle. The number of sheep does

not fully reflect the amount of milk produced, since

sheep are often used for other purposes, such as meat

and wool. In general, sheeps’ milk is used essentially

for cheesemaking, but in some countries, some is

made into yogurt or whey cheeses.

Chemical Composition

0002 Milk is a characteristic secretion of mammals

designed to meet the complete nutritional require-

ments of the neonate of the species. It is a mixture,

in equilibrium, of proteins, fat, lactose, and minerals

in different states of dispersion in water, such as

emulsions, colloidal suspensions, and true solutions.

0003 Sheeps’ milk is more viscous than cows’ milk and is

more resistant to the proliferation of bacteria during

the first hours after milking, owing to its greater

immunological activity. It has a unique smell, charac-

teristic of the animal that produces it, and is an

opaque white in color.

0004 The variability in the composition of sheeps’ milk

is due to genetic, physiological, and environmental

factors. The principal factor is breed; breeds selected

for a high milk yield produce milk with a lower total

solids content. Fat shows more variation between

breeds than proteins. Another factor to consider is

the stage of lactation. Colostrum contains high levels

of fat (10.7–17.2%), proteins, mainly as immuno-

globulins (7.1–19.5%), and low levels of lactose

(1.6–4.0%). The fat levels of protein in the milk

decline during the first weeks postpartum and then

increase gradually after the first month until lactation

ends, and the yield falls off. At the end of 7 months’

lactation, fat values may be as high as 8.1–10% and

protein as high as 6.8–8.9%. The lactose content falls

slightly throughout lactation and more sharply at the

end (3.4–4.1%). Another factor having a notable

influence on milk composition, especially the fat con-

tent, is the feeding of the livestock. Other influencing

factors include the age and health of the animal,

season, and milking times and procedures. Table 1

shows the gross composition of sheeps’ milk. Sheeps’

milk contains higher concentration of fat, proteins,

mineral salts, and total solids than cows’ milk. The

lactose content is comparable in the two species. Fat

and protein are the principal components of the, total

solids, constituting 69% in sheeps’ milk, compared to

56% in cows’ milk. The fat-to-protein ratio is higher

in sheeps’ than in cows’ milk, and the cheese yield

expressed as kilograms of cheese per 100 l of milk is

approximately 18–20 kg, compared to 10 kg for

cows’ milk.

Nitrogen Compounds

0005Sheeps’ milk contains 0.7–1% nitrogen. This is dis-

tributed in fractions the importance of which varies in

terms of dairy technology and nutrition. Proteins ac-

count for approximately 95% of total nitrogen, while

5% is nonprotein nitrogen. Milk proteins occur in

two distinct phases. One is an unstable micellar

phase composed of caseins, which is suspended as

micelles averaging about 190 nm in diameter, inter-

linked by calcium phosphate and small amounts of

magnesium, sodium, potassium, and citrate, which

diffuse light and give the milk its opaque white

appearance. The other is a soluble phase composed

of whey proteins. The caseins precipitate at pH 4.6

at room temperature, whereas under the same

conditions, the whey proteins (b-lactoglobulin,

a-lactoalbumin, and serumalbumin) remain soluble.

0006Caseins Caseins are the principal proteins in sheep’s

milk (76–83% of total proteins) and are present in

most types of cheese. The caseins are of four types of

polypeptide chain, as1-CN, as2-CN, b-CN, and

tbl0001 Table 1 Composition of ewes’ milk

Component Average content (%) Range (%) Average of dry matter (%)

Water 81.60 79.27–83.80

Lactose 4.61 4.10–4.95 25.00

Fat 7.09 5.10–8.70 38.50

Crude protein (total nitrogen  6.38) 5.72 4.75–6.60 31.10

Casein 4.44

Whey proteins 0.98

Nonprotein nitrogen 0.05

Ash 0.91 0.70–1.10 4.90

Total solids 18.40 16.20–20.73

Solids–nonfat 11.31

Average values have been calculated from the results given by different authors.

SHEEP/Milk 5199

k-caseins. The heterogeneity of caseins is due either to

the presence of genetic variants or other factors such

as a discrete phosphorylation level, variation in the

extent of glycosylation of the k-casein fraction, and

the coexistence of protein forms having different

chain lengths. It has been demonstrated that four

ovine casein genes, as1-CN, as2-CN, b-CN, and

k-casein, are polymorphic and are linked on the

both the ovine and the bovine genome. In recent

years, five variants of as1-CN have been described,

designated A, B, C, D, and E, in line with the nomen-

clature proposed for cow or goat caseins. as1-CN D

was called the Welsh variant in view of its discovery

in the milk of Welsh mountain sheep by King in 1966.

This is the least phosphorylated variant. The lower

number of phosphate groups explains the higher mi-

gration time of this variant by capillary electrophor-

esis at acid pH and the slow migration by alkaline

polyacrylamide gel. Variant as1-CN C differs from

the A variant by the substitution of Ser for Pro at

position 13, which explains the loss of the phosphate

group on site 12 of the protein chain. Two variants of

as2-CN, A and B, have been described, differing in

that Asn

and Lys

200

are replaced by Asp

and

Asn

200

. In addition, a variant with a high electrophor-

etic mobility and a low molecular weight has been

found in the milk of the Manchega breed. Ovine

b-CN consists of 209 amino acid residues. There is

nongenetic polymorphism due to varying degrees of

phosphorylation, with six and five phosphate groups

in b1 and b2, respectively. In addition, three genetic

variants, designated A, B, and C, have been described

recently. The only sequence difference found between

A and C is the substitution of Glu at position 2 in

variant A for Gln in variant C; no sequence data for

the B variant are yet available. Both the stability of

the casein micelle and the availability and distribution

of Ca are affected by the extent of phosphorylation of

the caseins. Sequencing of ovine k-casein has shown

that it consists of 171 amino acid residues. No genetic

variants of k-casein have been found, but it shows

nongenetic polymorphism due to varying degrees of

glycosylation at three Thr residue sites (positions 135,

137, and 138) and two phosphorylation sites (Ser

151

and Ser P

168

). The casein fraction also contains

g-caseins, the products of the breakdown of b-caseins

by plasmin.

0007 Whey proteins Sheeps’ milk whey proteins repre-

sent 17–22% of total protein. The principal proteins

are b-lactoglobulin and a-lactalbumin. Immuno-

globulins, serum albumin, and proteose peptones are

present at lower concentrations. The latter are prod-

ucts of the breakdown of b-casein by plasmin.

Another soluble protein found in small amounts and

possessing antibacterial properties is lactoferrin. In

the case of rennet whey, caseinomacropeptide is pre-

sent also, produced by the chymosin action on the

bond 105–106 of k-casein. Serum albumin is not

specific to milk, and it is considered to be the same

as that found in the blood. The IgA in milk is synthe-

sized in the mammary gland, i.e., it is milk-specific.

0008b-Lactoglobulin, the principal protein in the whey,

consists of a polypeptide chain of 162 amino acids.

Three genetic variants have been described in sheep’s

milk: b-Lg A, b-Lg B, and b-Lg C. Ovine b-Lg B and A

differ only by a single amino acid exchange, His for

Tyr at position 20. b-Lg C is a subtype of ovine b-Lg

A with a single exchange, Arg for Gln at position

148. a-Lactalbumin is closely homologous to bovine

a-lactalbumin. It is a metalloprotein, containing one

atom of Ca per molecule and is important from a

biological standpoint in that it is involved in lactose

synthesis. Two genetic variants, A and B, have been

described, although the B variant is very uncommon.

0009Nonprotein nitrogen Nonprotein nitrogen accounts

for between 5 and 6.8% of total nitrogen. Nonpro-

tein nitrogen compounds are: urea (45%), amino

acids (16%), creatine (2.4%), creatinine (1.7%), am-

monium (1%), uric acid (2.1%), and other unidenti-

fied compounds (32.3%). Sheeps’ milk contains more

urea and uric acid than cows’ milk.

Lipid Fraction

0010Lipids are one of the most important components of

milk in terms of cost, nutrition, and the physical and

sensory characteristics they impart to dairy products.

They are present in the form of globules and are

characteristically abundant in sizes of less than

4.5 mm. No appreciable differences have been found

in the mechanism of fat-globule secretion in sheep

and cows; the structure and composition of the mem-

brane are similar in both species. The phospholipid

profile in both species is similar to that of the plasma

membrane, which would tend to confirm their

common origin. Along with triglycerides, the lipid

fraction of sheeps’ milk contains other simple lipids

(diglycerides, monoglycerides, and cholesterol

esters), complex lipids (phospholipds) and liposoluble

compounds (sterols, alcohols, and hydrocarbons).

Triglycerides constitute the largest group (nearly

98%), including a large number of esterified fatty

acids, and so the composition is complex.

0011Fatty acids Fatty acids may be saturated or unsatur-

ated (having from one to four double bonds). Most

acids, from acetic to arachic acid, contain an even

number of carbon atoms, of while approximately

2% are saturated with an odd number of atoms, and

5200 SHEEP/Milk

roughly the same percentage of saturated fatty acids

with branched methyl chains with an odd or even

number of carbon atoms. Sheeps’ milk contains

more caproic, caprylic, and capric acid than cows’

milk. These fatty acids are associated with the char-

acteristic flavor of sheeps’ milk cheeses and can also

be used to detect mixtures of milk from different

species (Table 2).

0012 The most important of the factors that affect fatty

acid composition is the feed. The addition of animal

fats to the diet produces a decrease of C4–C14 acids

and an increase of C16, C18, and C18:1 acids. If the

intake of forage is reduced, the proportion of C4 to

C16 fatty acids decreases accordingly.

0013 Triglycerides The triglyceride structure of the milk

fat is responsible for its rheological properties and its

behavior during melting and crystallization. The tri-

glyceride composition is useful to verify the authenti-

city of milk fat.

0014 Triglycerides are almost invariably accompanied

by small amounts of di- and monoglycerides, mainly

at positions 1 and 2, which are therefore probably

intermediates in the biosynthesis of triglycerides. The

distribution of fatty acids in the triglyceride molecule,

as determined by stereospecific analysis, differs

slightly from cows’ milk fat, but the distribution of

acids in the molecule is not random. However, in both

sheeps’ and cows’ milk, butyric acid and other short-

chain fatty acids (C6–C8) are esterified mainly at

position 3 of the glycerol molecule; the distribution

of the other fatty acids (C10 or greater) exhibits no

such marked specificity for positions 1 or 2.

0015Table 3 shows the average triglyceride composition

of sheeps’ milk fat compared with cows’ milk fat. The

triglycerides in sheeps’ milk show a wide range of

molecular weights when distributed according to the

number of carbon atoms (taking into account the

carbon atoms of the three acyl radicals), with two

peaks at C38–C40 and C50–C52 and a minimum at

C44–C46. Sheeps’ milk fat has a higher percentage of

short-chain triglycerides (C26–C36) than cows’ milk

fat (18% versus 11%). Percentages of medium-chain

triglycerides (C38–C44) are also higher in sheeps’

milk fat (33% versus 25%), and the percentages of

unsaturated trglycerides are lower than in cows’ milk

fat (51% versus 55%). These differences relate to the

need for a triglyceride composition with the appro-

priate melting point to allow the fat to be secreted. It

has been established that the principal triglycerides in

sheeps’ milk fat are composed largely of three fatty

acids (C14, C16, and C18:1), combined with the

short-chain fatty acids C4 and C6.

0016Unsaponifiable lipids The unsaponifiable fraction

of the milk fat is composed largely of sterols, with a

smaller proportion of hydrocarbons, mainly squa-

lene, and trace amounts of a number of normal- and

branched-chain hydrocarbons in the range C17–C48

with odd and even numbers of carbon atoms, lipo-

soluble vitamins and aliphatic alcohols. Ovine milk

contains virtually no tocopherol or b-carotene.

0017Sterols are a minor fraction of total milk fat, the

main component being cholesterol (270–350 mg per

100 g of fat, equivalent to approx. 20 mg per 100 ml

of sheeps’ milk), but small amounts of other sterols

tbl0003Table 3 Triglyceride composition of ewes’ and cows’ milk fat

Triglyceride Cow (wt%) Ewe (wt%)

C26 Trace 0.60–1.00

C28 0.30 1.30–2.50

C30 0.80 2.00–3.40

C32 1.90 3.10–5.00

C34 4.70 5.70–7.00

C36 9.80 9.20–10.30

C38 13.00 12.50–14.00

C40 10.80 11.00–12.50

C42 6.40 8.00–9.70

C44 5.80 7.30–8.60

C46 7.00 6.50–6.90

C48 9.10 6.10–7.80

C50 13.10 4.70–9.10

C52 12.10 5.00–9.40

C54 5.30 3.10–5.30

C56 < 0.50

Triglycerides are identified by the number of acyl carbon atoms per

glyceride molecule.

Ranges have been calculated from the results given by different authors.

tbl0002 Table 2 Fatty acid composition of ewes’ milk fat

Fatty acid (long chain) Percentage

Butanoic acid (C4) 3.00–5.80

Hexanoic acid (C6) 2.10–4.00

Octanoic acid (C8) 1.50–3.60

Decanoic acid (C10) 5.00–9.00

Decenoic acid (C10:1) 0.10–0.30

Dodecanoic acid (C12) 2.90–5.20

Tetradecanoic acid (C14) 7.00–13.40

9-Tetradecenoic acid (C14:1) 0.40–1.00

Pentadecanoic (C15) 0.60–1.50

9-Pentadecenoic (C15:1) 0.20–0.60

Hexadecanoic (C16) 20.00–28.50

9-Hexadecenoic (C16:1) 1.00–2.80

Heptadecanoic (C17) 0.20–1.00

9-Heptadecanoic (C17:1) 0.20–0.70

Octadecanoic (C18) 6.20–13.10

9-Octadecenoic

(C18:1) 16.60–27.70

9,12-Octadecadienoic

(C18:2) 2.80–4.30

9,12,15-Octadecatrienoic (C18:3) 0.60–2.00

All isomers.

Ranges have been calculated from the results given by different authors.

SHEEP/Milk 5201

implicated in cholesterol biosynthesis have also been

found in ovine milk: lanosterol (5–15 mg per 100 g of

fat) and, in even smaller proportions, dihydrolanos-

terol, desmosterol, and lathosterol.

0018 Phospholipids The major phospholipids in both

sheeps’ and cows’ milk are: phosphatidylethanola-

mine, phosphatidylcholine, phosphatidylserine, phos-

phatidylinositol, and sphingomyeline. These account

for roughly 0.8% of the total lipid fraction.

Lactose

0019 Lactose is the principal carbohydrate in milk. Chem-

ically, it is a disaccharide consisting of a residue of

d-glucose and d-galactose joined in a b-1, 4-glyco-

sidic linkage. Sheeps’ milk contains between 45 and

50 g of lactose per kilogram of milk, and lactose

accounts for 22–27% of dry matter versus 33–40%

in cows’ milk. The lower lactose content does not

present a problem in cheesemaking, since a sufficient

amount of lactose is available to ensure lactic fer-

mentation.

Enzymes

0020 Enzymes are constituents of the mammary gland

which pass into the milk during the secretion process.

The enzymes found in milk are highly specific and are

chiefly oxidoreductases, transferases, and hydrolases.

Particularly important for technological purposes are

proteases and lipase. Lipase, lysozyme, ribonuclease,

and xanthineoxidase are less active in sheeps’ milk,

whereas alkaline phosphatase, despite having the

same molecular weight and the same properties, is

more active than in cows’ milk. The most abundant

enzyme is lactoperoxidase, which has proved impos-

sible to distinguish from the bovine enzyme but

appears to be more thermolabile. The indigenous

proteases include aminopeptidases, thermolabile neu-

tral acid proteinases, and serine proteinases, which

are heat-resistant. The serine proteinase, plasmin,

is heat-stable and hydrolyzes Lys—X and Arg—X

bonds, becoming more active after thermal treatment.

Renneting Properties

0021 The renneting properties of sheeps’ milk are affected

by a number of factors such as pH, physicochemical

composition, micellar system, salts equilibrium, cal-

cium concentration, and the temperature and time of

heating. Sheeps’ milk coagulates well and is indeed

very suitable for making high-quality cheeses. It co-

agulates faster than cows’ milk, and less rennet is

needed to coagulate it in the same time as cows’

milk. Curd formation is also faster than in cows’

milk, but syneresis takes longer. The reason for this

difference may be that sheeps’ milk contains more

casein, ionic calcium, and colloidal calcium than

cows’ milk. The renneting time and rate of firming

are practically unaffected by the addition of calcium.

Physical Properties

0022The differences in the composition of sheeps’ and

cows’ milk affect their physical properties. Sheeps’

milk has a higher density, viscosity, refractive index,

freezing point, and titrable acidity than cows’ milk.

Sheeps’ milk is slightly more acid (lowest pH) than

cows’ milk, possibly due to its higher protein content.

Nutritive Value

0023The nutritional importance of sheeps’ milk is due to

its composition. Although sheeps’ milk is richer in

nutrients than cows’ milk, it is rarely used as milk

for drinking. In general, it is used for cheesemaking,

but in certain countries, some is made into yogurt or

other fermented milks. Sheeps’ milk is an excellent

source of high-quality protein, calcium, phosphorus,

magnesium, zinc, and several vitamins. Only iron is in

short supply, but its bioavailability is good because it

is bound by lactoferrin, which is involved in the

transport of iron in the body.

0024There is a good balance between the protein, fat,

and carbohydrate components, each being present in

similar amounts. The supply of nutrients is high in

relation to the calorie content of the food. Sheeps’

milk has a higher calorific value (500 kJ per 100 g)

than cows’ milk. Table 4 shows the range of concen-

trations of minerals and vitamins in ewes’ milk.

0025Sheeps’ milk contains mineral salts (around 0.9%).

The most abundant elements are Ca, P, K, Na, and

Mg. Ca and P are the most important of the macro-

elements in nutritional terms and for their role in the

stability of casein micelles, and hence in the behavior

of the caseins during milk processing. The most abun-

dant of the trace elements are Zn, Fe, Cu, and Mn.

With the exception of sodium, the concentrations of

these elements are higher than those found in cows’

milk. Sheeps’ milk contains an average of 2 g of cit-

rate per kilogram, which is slightly higher than that in

cows’ milk.

0026Concerning the distribution between the soluble

and colloidal phases, the percentages of Ca and P in

the soluble phase are, respectively, 20–25% and 35–

40% of the total. These proportions are lower than in

cows’ milk, but the absolute concentrations in the

soluble phase are comparable for the two species.

Therefore, the concentrations of both elements in

the colloidal phase are much higher than in cows’

5202 SHEEP/Milk

milk, given the higher levels of casein found in sheeps’

milk. Na and K are the major elements in the soluble

phase. Of the trace elements Zn, Mn, Fe, and Cu, the

first two are present mainly (around 90%) with the

colloidal fraction of the milk, while Fe and Cu are

present to a lesser extent (70 and 65%, respectively).

0027 The vitamin content is generally higher in sheeps’

milk than in cows’ milk. The level of retinol activity

of ewes’ milk is approximately twice that of goats’

and cows’ milk.

Cheeses

0028 Practically all sheeps’ milk is used to make cheeses,

most of which are craft products. Some have an Ap-

pellation of Origin, so that production is confined to

specific regions, and only a minority are made on an

industrial basis. There are six categories of sheeps’

milk cheese based on the technology used: fresh,

white brined (pickled), blue-veined, semihard, hard,

and whey cheeses.

0029 Roquefort cheese, is a blue-veined cheese made in

France from raw sheeps’ milk inoculated with Peni-

cillium roqueforti spores. It undergoes extensive pro-

teolysis, 50% water-soluble nitrogen in total nitrogen

(WSN/TN) and lipolysis, 8–10% of total fatty acids

after 5 months of ripening. Spain produces a blue-

veined variety, Cabrales cheese, with similar charac-

teristics. This cheese is highly appreciated, although

the raw material is a mixture of cows’, sheeps’, and

goats’ milk.

0030Feta cheese is a white-brined cheese originally from

Greece, and is one of the most popular cheeses inter-

nationally. Traditionally, it is made from raw sheeps’

milk and is ripened in barrels with brine (6–8%

NaCl) for around 1 month at 8–10



C; it is then

kept chilled for at least 2 months prior to consump-

tion. Nowadays, it is made mainly from pasteurized

sheeps’ milk. There is a variety similar to feta called

teleme, originally from Romania, which is generally

made from a mixture of milk of all three species. The

white-brined category also includes halloumi, a

cheese produced chiefly in Cyprus from raw sheeps’

milk.

0031Pecorino is an Italian semihard or hard cheese.

There are several varieties with Appellations of

Origin, including Romano, Siciliano, and Fiore

Sardo. The Romano variety accounts for 50% of

total production. This cheese is made with raw or

pasteurized sheeps’ milk, in most cases coagulated

with lamb or kid rennet paste. Proteolysis is moderate

(20% WSN/TN), but there is intense lipolysis due

to the presence of pregastric esterases in the trad-

itional rennet pastes used. Kefaloteri is a variety simi-

lar to pecorino produced in Greece, although rennet

pastes is not normally used for coagulation.

0032Kachkaval has been produced in various European

countries, particularly in the Balkans, at least since

the eleventh century. This is a stretched-curd cheese

which undergoes moderate proteolysis even after 3

months of ripening (13% WSN/TN).

0033The most characteristic Spanish semihard/hard

sheeps’ milk cheeses are Manchego, Zamorano, Ron-

cal, and Idiazabal, traditionally made from raw milk.

These are generally ripened for 3–6 months, although

Manchego may be matured in olive oil for a longer

period. Proteolysis is medium/high, 20–45%WSN/

TN, while lipolysis is low. Idiazabal cheese is ripened

for 1–2 months and then smoked.

0034Some sheeps’ milk cheeses are made in Portugal

and Spain using vegetable rennet extracted from the

flower of the thistle, Cynara cardunculus. Examples

include Serra da Estrela in Portugal and La Serena,

Los Pedroches, and Torta del Casar in Spain. These

are commercialized after relatively short ripening

(30–60 days), but the proteolysis levels are high

(40–50% WSN/TN) due to the strong proteolytic

activity in the curd. Spain produces two highly popu-

lar fresh cheeses, Burgos and Villalo

n. These are now

made from pasteurized milk using animal rennet, but

without a starter culture. They must be kept in chilled

storage and consumed in less than 6 days.

0035Finally, in various countries, the whey from sheeps’

milk in cheesemaking is used on its own or mixed

with milk to manufacture products of high nutritional

value. The whey contains mainly water-soluble

tbl0004 Table 4 Vitamin and mineral content of sheeps’ milk (per 100 g)

Mean value Range

Vitamins

Vitamin A (mg) 50.00

b-Carotene (mg) 5.00 2.00–7.00

Thiamin (B

)(mg) 48.00 28.00–70.00

Riboflavin (B

) (mg) 0.23 0.16–0.30

Nicotinamide (mg) 0.45 0.40–0.50

Pantothenic acid (mg) 0.35

Biotin (mg) 9.00

Vitamin B

(mg) 0.51 0.30–0.71

Vitamin C (mg) 4.25 3.00–6.00

Folic acid (mg) 5.60

Vitamin E (mg) 120.00

Minerals

Ca (g l

1

) 1.98 1.80–2.39

Mg (g l

1

) 0.18 0.10–0.22

Na (g l

1

) 0.50 0.27–0.81

K(gl

1

) 1.20 0.96–1.96

P(gl

1

) 1.30 1.17–1.70

Fe (mg l

1

) 0.76 0.34–1.40

Zn (mg l

1

) 7.50 0.47–1.20

Cu (mg l

1

) 0.07 0.04–0.14

Values have been given by different authors.

SHEEP/Milk 5203

proteins (1%), fat (< 1%), lactose, minerals, non-

nitrogen substances, and vitamins. These whey

cheeses are the result of heat-induced coagulation of

the whey protein (which occludes fat). The best-

known whey cheeses are ricotta from Italy, Manouri

and Myzithra from Greece, and Requeso

n from

Spain.

0036 Besides the cheeses mentioned here, many coun-

tries produce cheeses, generally semihard varieties,

in which the raw material is a mixture of sheeps’,

cows’, and goats’ milk.

Nutritional Aspects of Cheese

0037 Cheese, in its concentrated form, contains much of

the same nutrients as milk, caseins, fat, minerales,

vitamins, colloidal salts, and some whey constituents.

Some (up to 90%) of the hydrosoluble vitamins may

be lost in the whey, but the levels of vitamins in

cheeses depend on the balance on the consumption

and production of the vitamins by microorganisms.

The important nutrients in cheese come mainly from

the high content of biologically valuable proteins

which supply essential amino acids, and the levels of

minerals, mainly calcium and phosphorus. Table 5

shows the average content of fat, protein, calcium,

and phosphorus of some of the principal sheeps’ milk

cheeses produced in different countries, indicating the

type of consistency.

0038 The transformation of lactose to lactic acid occurs

during the first stage of processing, this means that

people who are lactose-intolerant can consume cheese

with impunity.

0039 The protein content of different varieties of ewes’

milk cheese varies between 20 and 30%. In cheese

manufacture, the casein is incorporated into the curd.

The more biologically valuable whey proteins are lost

in the whey, but the biological value of the proteins in

cheese is higher than that of casein alone. The bio-

logical value of proteins is improved when the whey

proteins are incorporated into the cheese by mem-

brane processes such as ultrafiltration. During

ripening, caseins are hydrolyzed to peptides and

amino acids by the action of rennet and microbial

proteases. The nutritional value of the proteins in

cheese is not affected by the above-mentioned pro-

cesses, and the bioavailability of lysine is comparable

to that of casein in milk.

0040The fat is hydrolyzed during ripening by microbial

lypases, which produce free fatty acids. These com-

ponents contribute to the development of the aroma

of the cheeses. The level of fat in sheeps’ milk cheeses

varies from 12 to 30%, depending on the length of

time and degree of ripening. The digestibility of the

fat of different varieties is between 88 and 94%. The

content of Ca and P in cheese is higher than that in

milk, four to five times higher in fresh cheeses, seven

to eight times higher in semihard cheese, and 10 times

higher in hard cheeses. Only in acid-coagulated

cheeses is the content of Ca and P lower than in

milk. The bioavailability of calcium cheese is compar-

able to that from milk and is not affected by the

ripening process.

See also: Casein and Caseinates: Methods of

Manufacture; Uses in the Food Industry; Cheeses: Types

of Cheese; Starter Cultures Employed in Cheese-making;

Enzymes: Functions and Characteristics; Fats:

Requirements; Fatty Acids: Properties; Lactose;

Phospholipids: Properties and Occurrence;

Triglycerides: Structures and Properties; Whey and

Whey Powders: Protein Concentrates and Fractions

Further Reading

Alichanidis E and Polychroniadou A (1996) Special features

of dairy products from ewe and goat milk from the

physicochemical and organoleptic point of view. In:

Production and Utilization of Sheep and Goat Milk,

pp. 21–43. Brussels: International Dairy Federation.

Amigo L, Recio I and Ramos M (2000) Genetic polymorph-

ism of ovine milk proteins: its influence on technological

properties of milk. A review. International Dairy Journal

10: 135–149.

Anifantakis EM (1986) Comparison of the physico-

chemical properties of sheeps’ and cows’ milk. In:

Bulletin 202, pp. 42–53. Brussels: International Dairy

Federation.

Assenat L (1985) Le lait de brebis. In: Luquet FM (ed.) Laits

et Produits Laitiers. Vache, Brebis, Che

vre, pp. 279–

392. Paris: Technique et documentation Lavoiser.

tbl0005 Table 5 Average content of fat, protein, calcium, and phosphorus in cheeses made from ewes’ milk

Country Name Type of cheese Fat content (percentage total solids) Protein (%) Ca (%) P (%)

France Roquefort Blue-veined 50.00 21.00 0.62 0.42

Greece Feta Brined 40.00 18.00 0.65 0.40

Italy Pecorino Hard 42.00 29.00 0.40 0.32

Portugal Serra da Estrela Semihard 56.00 20.00 0.65 0.53

Romania Teleme Brined 50.00 18.00 0.53 0.40

Spain Manchego Semihard 50.55 23.00 0.68 0.54

Average values have been calculated from the results given by different authors.

5204 SHEEP/Milk

De la Fuente MA, Olano A and Juarez M (1997) Distribu-

tion of calcium, magnesium, phosphorus, zinc, manga-

nese, copper and iron between the soluble and colloidal

phases of sheeps’ and goats’ milk. Lait 77: 515–520.

Juarez M and Ramos M (1984) Dairy products from

sheeps’ and goats’ milk. Dairy Industries International

49: 20–24.

Kalantzopoulos GC (1993) Cheeses from sheeps’ and goats’

milk. In: Fox PF (ed.) Cheese: Chemistry, Physics and

Microbiology Vol. 2 Major Cheeses Group, 2nd edn, pp.

507–553. London: Chapman & Hall.

ez M, Medina M and Gaya P (1989) Sheeps’ milk

cheese: technology, microbiology and chemistry. Journal

of Dairy Research 56: 303–321.

Rainal K and Remeuf F (1998) The effect of heating on

physicochemical and renneting properties of milk: a

comparison between caprine, ovine and bovine milk.

International Dairy Journal 10: 135–149.

Ramos M and Jua

rez M (1981) The composition of sheep’s

and goats’ milk. In: Bulletin 140, pp. 1–19. Brussels:

International Dairy Federation.

Ramos M and Jua

rez M (1986) Chromatographic, electro-

phoretic and immunological methods for detecting

mixtures of milk from different species. In: Bulletin

202, pp. 175–190. Brussels: International Dairy Feder-

ation.

Ramos M and Juarez M (in press) Sheep’s milk. In: Encyclo-

pedia of Dairy Science. London: Academic Press.

Renner E (1987) Nutritional aspects of cheese. In: Fox PF

(ed.) Cheese: Chemistry, Physics and Microbiology Vol.

1 General Aspects, 1st edn, pp. 345–363. London:

Elsevier Applied Science.

Scott KJ (1989) Micronutrients in milk products. In:

Renner E (ed.) Micronutrients in Milk and Milk-based

Food Products, pp. 71–123. London: Elsevier Applied

Science.

Shelf-life See Chilled Storage: Principles; Attainment of Chilled Conditions; Quality and Economic

Considerations; Microbiological Considerations; Use of Modified-atmosphere Packaging; Packaging Under

Vacuum; Spoilage: Chemical and Enzymatic Spoilage; Bacterial Spoilage; Fungi in Food – An Overview; Molds in

Spoilage; Yeasts in Spoilage; Storage Stability: Mechanisms of Degradation; Parameters Affecting Storage

Stability; Shelf-life Testing; Shelf-life Testing

SHELLFISH

Contents

Characteristics of Crustacea

Commercially Important Crustacea

Characteristics of Molluscs

Commercially Important Molluscs

Contamination and Spoilage of Molluscs and Crustaceans

Aquaculture of Commercially Important Molluscs and Crustaceans

Characteristics of Crustacea

D A Jones, University of Wales Bangor, Gwynedd,

Wales, UK

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001 As shellfish or shelled aquatic invertebrates, the

Crustacea share important features, such as bilateral

symmetry, body segmentation, and the possession of

a well developed exoskeleton, with the other groups

within the phylum Arthropoda. However, they may

be separated at the subphylum level from the Cheli-

ceriformes (scorpions, spiders, etc.), Trilobitomorpha

(fossil trilobites) and Uniramia (insects and myria-

pods) by the features listed below:

0002Appendages are uniramous or biramous.

0003Brain is tripartite (with deutocerebrum).

0004Body is divided into cephalon and trunk, or latter

subdivided into thorax and abdomen.

SHELLFISH/Characteristics of Crustacea 5205

4.0005 There are five pairs of cephalic appendages: pre-

oral first antennae and four pairs of postoral ap-

pendages – second antennae (which migrate to

preoral position in adults), mandibles, maxillules,

and maxillae.

0006 Mandible is gnathobasic (mandible arises from

limb base); endopod and exopod are reduced in

adults.

0007 There are at least 39 000 species of crustaceans,

ranging in size from less than 1 mm in length to spider

crabs with leg spans of over 4 m, and these are sub-

divided into six classes and 38 orders. Although most

of these are marine, small groups have exploited the

fresh-water and terrestrial environments, so that

crustaceans can be found from mountain tops and

deserts to abyssal depths. Despite the large number

of species, many are either too small or too dispersed

to be exploited by humans. The forms most com-

monly exploited commercially are crabs, lobsters,

and shrimps. Planktonic copepods (10 000 species),

Cladocera (450 species), planktonic and benthic

Ostracoda (5000 species), and mainly benthic

Amphipoda and Isopoda (10 000 species) often

form an essential link in food webs which support

fisheries, but such forms are only rarely themselves

the subject of fisheries. However, a key problem in

the management of crustacean fisheries concerns

euphausiids (krill) which play a central role in the

Antarctic ocean food web as food for many whales,

seals, and birds, yet are also increasingly being

fished directly as food for humans and domestic

animals.

Taxonomy

0008 A brief outline classification of crustaceans utilized

directly by humans is given in Table 1; for more

detailed descriptions, refer to the Further Reading

section. It is clear that, apart from larger planktonic

species such as euphausiids, mysids, and sergestids,

which swarm in sufficient concentration to be har-

vested economically, and gourmet species such as

stalked barnacles (Pollicipes) and stomatopods, the

majority of edible crustaceans are found in order

Decapoda, which comprises some 10 000 species.

The possession of branched or dendrobranch gills,

external fertilization, and release of eggs into the sea

separates the penaeid and sergestid shrimp from the

rest of the Decapoda.

0009 The Pleocyemeta have unbranched gill filaments

and brood their eggs, which hatch at a later stage

than the nauplius produced on hatching by the Den-

drobranchiata. This group contains the majority of

the familiar shrimps or prawns (the terms are now

synonymous), crayfish, lobsters, squat lobsters, and

crabs. Caridean shrimps (Palaemon, Macrobra-

chium) possess phyllobranchiate or flattened, plate-

like gills, separating them from the larger and more

robust Astacidea (which possess trichobranchiate or

unbranched, tubular gill filaments). All the fresh-

water crayfish (Astacidea) and marine lobsters

(Homarus and Nephrops) have greatly enlarged

chelae or claws on the first pair of walking legs,

distinguishing them from the Palinura (spiny and slip-

per lobsters), which have similar gill structures, but

lack the enlarged claws. This marine group tends to

be more mobile and possesses a flattened abdomen

and large tail fan used in swimming.

0010The Anomura contain forms in which the abdomen

is either soft and twisted asymmetrically to fit into

gastropod shells, or flexed beneath the cephalothorax

as in galatheid lobsters. The coconut crabs (Birgus)

and lithodids represent extreme groups in which the

asymmetrical abdomen is closely applied to the

underside of the thorax, giving the appearance of

true crabs or Brachyura. This final group is distin-

guished by the lateral expansion of the cephalo-

thorax, and reduction of the abdomen to form a

symmetrical flap, lacking uropods, which is flexed

under the thorax.

Crustacean Structure and Function

0011As a predominantly aquatic group, crustaceans have

been able to exploit the arthropod chitinized exoskel-

eton fully without the weight restrictions imposed by

such a system on land. In the classes under consider-

ation (Table 1), fusion of the head and one to three

thoracic segments has occurred to produce a cara-

pace, or protective shield, which often extends for-

wards in the form of a rostrum and posteriorly to

protect the cephalothorax (Figure 1). The abdomen

usually retains flexibility by means of soft cuticular

joints between each segment. Even barnacles

(Figure 1) retain most of these features, although

much modified for their sedentary existence.

0012Each segment typically bears a pair of jointed ap-

pendages, anteriorly. The two pairs of antennae are

often elongated and mobile, and carry esthetascs

which are chemosensory hairs. Other head append-

ages usually form mouthparts concerned with feed-

ing and include the mandibles or jaws, maxillules,

and maxillae, and, in malacostracans, additional

maxillipeds.

0013The rest of the thoracic appendages are modified

into pereopods (Figure 1) specialized for walking,

swimming, respiration, feeding, or defense. The

abdominal segments (pleonites) typically bear bira-

mous pleopods, paddle-like appendages, used for

5206 SHELLFISH/Characteristics of Crustacea