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

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layer. This structure allows gaseous exchange, which

is essential for embryo development.

0012 The spongy and mammillary layers are constructed

in such a manner that there are numerous oval-to-

circular openings through the shell. These are called

pores. The normal egg has a greater concentration of

pores on the large end, where the air cell is normally

located, fewer in the equatorial region of the shell and

fewest pores on the pointed end of the shell. The size

of pores varies among shells of different hens,

whereas eggs of a single hen are quite uniform in

number, size, and distribution of the pores.

0013 The outer surface of the shell is covered with a

mucin protein which acts as a soluble plug for the

pores in the shell. The cuticle is permeable to gas

transmission.

Abnormal Eggs

0014 The most common abnormality seen in freshly laid

eggs is the double-yolked egg. Such eggs are produced

when two yolks are released into the oviduct at about

the same time. These eggs are most often produced by

pullets just commencing egg production.

0015 Meat spots in eggs are frequent. A meat spot is a bit

of tissue from the lining of the oviduct. In white-

shelled eggs, the spots are usually white and not no-

ticed unless extremely large. Such large meat spots are

removed during candling of market eggs. In brown-

shelled eggs, most meat spots are brown and are thus

noticed floating in the albumen. Again, large meat

spots are removed during the candling operation. A

less frequent abnormality is the blood spot. Blood

spots are usually removed during the candling oper-

ation of shell-egg processing. Blood spots are most

frequently found on the surface of the yolk and are

the result of a drop of blood being lost from the

capillaries of the yolk follicle at the time of ovulation.

Blood-stained albumen is also possible if more than a

drop of blood is lost during ovulation.

0016 There are also abnormally shaped eggs owing to

the malfunction of part of the uterus where the shell is

formed. Rough-shelled and thin-shelled eggs are con-

sidered to be abnormal and are generally removed

from a retail egg pack. Shell-less, thin-shelled, or

soft-shelled (egg membrane only) eggs lack proper

calcification. Many of these defective eggs are laid

prematurely. The incidence of such abnormalities

increases in aging hens that have been in production

for more than a year.

Composition of the Egg

0017 The avian egg is composed of all of those compounds

needed for animal life, as evidenced by the hatching of

a normal chick from a fertile egg. As a human food the

egg contains all of the nutrients needed by humans

with the exception of ascorbic acid (vitamin C).

0018The egg is naturally divided into the shell, egg

white or albumen, and yolk. The shell is about 96%

inorganic materials, largely calcium carbonate, with

4% organic material, largely protein, on a dry-weight

basis. The egg albumen is primarily water, about

87%. The solids material is largely protein with

small amounts of minerals and carbohydrates. The

yolk is one of the most concentrated of all biological

materials, with only 48% water in fresh egg yolk. The

solids of egg yolk are made up of two-thirds lipid

material and one-third proteins with small amounts

of minerals and carbohydrates. The shell membranes

are primarily proteins. Hens’ eggs vary in relative

amounts of the various parts but a general average is

10% shell, 1% shell membrane, 59% egg albumen,

and 30% yolk.

0019Average proximate chemical composition of the

edible portions of eggs is shown in Table 1. Refer to

individual nutrients.

Egg Albumen Composition

0020There are a number of proteins in the albumen.

Table 2 lists the identified proteins and gives some

of the characteristics of each. The antibacterial nature

of several of the proteins constitutes part of the de-

fense against microbial invasions during the incuba-

tion of the egg. These defenses work equally well

for shell eggs used as human food. These proteins

have been separated and characterized. (See Protein:

Chemistry).

Egg Yolk Composition

0021The egg yolk is a mixture of lipoproteins. An excep-

tion is phosvitin, which contains no lipid material and

is a phosphoprotein. The protein content of yolk is

about 16% and the lipid content varies from 32% to

35%, depending mainly on the strain of bird. The

yolk lipid fraction is made up of 66% triglycerides,

tbl0001Table 1 Ranges in chemical composition of the edible portions

of eggs

Ingredient Wholeegg (%) Egg white (%) Eggyolk (%)

Water 72.8–75.6 87.9–89.4 45.8–51.2

Protein 12.8–13.4 9.7–10.6 15.7–16.6

Lipid 10.5–11.8 0.03 31.8–35.5

Carbohydrate 0.3–1.0 0.4–0.9 0.2–1.0

Ash 0.8–1.0 0.5–0.6 1.1

From Li-Chan ECY, Powrie WD and Nakai S (1995) The chemistry of eggs

and egg products. In: Stadelman WJ and Cotterill OJ (eds) Egg Science and

Technology, 4th edn. Binghamton, New York: Haworth Press with

permission.

EGGS/Structure and Composition 2007

28% phospholipid, 5% cholesterol, and small

amounts of other lipids. The major component of

the phospholipids is phosphatidylcholine (73%),

and other compounds are present in the following

amounts: phosphatidylethanolamine, 15%; lyso-

phosphatidylcholine, 6%; sphingomyelin, 2.5%;

lysophosphatidylethanolamine, 2%; plasmalogen,

1%; traces of inositol phospholipid. The lipoproteins

are classified as high-density lipoprotein (HDL), low-

density lipoprotein (LDL), and very-low-density

lipoprotein (VLDL). Density is determined by the

percentage of lipid material in the molecule. The

HDL fraction of the egg yolk consists of phosvitin

and the livetins, a-, b-, and w-livetin. These com-

pounds all have less than 10% lipids. The LDL

compounds are a- and b-lipovitellin and contain

about 20% lipids. The VLDL lipovitellenin contains

about 40% lipid. (See Lipids: Classification; Lipopro-

teins; Phospholipids: Properties and Occurrence;

Triglycerides: Structures and Properties.)

0022 Cholesterol in the egg is found only in the yolk. The

concentration of cholesterol has been found to vary

among hens The content of cholesterol per gram of

yolk is relatively constant for a hen. For this reason,

as the egg yolk size increases, the amount of choles-

terol also increases. (See Cholesterol: Properties and

Determination.)

Modifying Egg Composition

0023 Composition of the egg varies slightly with egg size

and to a greater degree among eggs from different

hens, especially as hens of different strains are com-

pared. The total solids content can be altered signifi-

cantly by varying the proportion of yolk and white.

Genetic selection has apparently made some change

in the composition of hens’ eggs, as evidenced by

differences between the values listed in the US De-

partment of Agriculture Handbook 8.1, published in

1973 and revised in 1989. One significant difference

was in the saturated fat content of the egg which was

found to be reduced by about 0.5%. Part of the

difference might be attributed to changes in the diets

fed to the laying hens. All nutrients in the egg, with

the exception of zinc and choline, may be altered by

feeding management or genetics, as reported by one

or more research groups.

Modifying Shell Composition

0024The composition of the shell remains relatively

constant, but shell thickness can be altered by redu-

cing the calcium content of the diet. A vitamin D

deficiency will also result in thin-shelled eggs. The

composition of the shell does not change greatly, but

the percentage of shell as part of the total egg may

vary from less than 8% to over 12%. Shell membranes

have not been found to vary significantly as a result of

genetics, management, or nutrition of the hen.

Modifying Albumen Composition

0025The vitamin content of the albumen can be altered

greatly by varying the vitamin content of the hens’

diet. The mineral content can also be altered. The

protein content remains constant for any hen but

varies among hens from 9.7% to 10.6%. Specific

amino acid content can be altered by dietary modifi-

cation, but not sufficiently to affect total protein.

Modifying Yolk Composition

0026The fatty acid composition of hens’ eggs can be

altered by feeding fats of various composition to the

hen. In addition, the vitamin and mineral content of

the yolk can be changed by feeding.

tbl0002 Table 2 Proteins in egg albumen

Protein Relative amount (%) Characteristics

Ovalbumin 54 Phosphoglycoprotein

Conalbumin or ovotransferrin 13 Binds metallic ions, iron, copper, and zinc

Ovomucoid 11 Inhibits trypsin

Ovomucin 3.5 Sialoprotein, viscous

Lysozyme or G

globulin 3.4 Lyses some bacterial cell walls

globulin 4.0 ?

Ovoinhibitor 1.5 Inhibits serine proteases

Ovoglycoprotein 1.0 Sialoprotein

Ovoflavoprotein 0.8 Binds riboflavin

Ovomacroglobulin 0.5 Strongly antigenic

Cystatin 0.05 Inhibits thiol proteases

Avidin 0.05 Binds biotin

From Li-Chan ECY, Powrie WD and Nakai S (1995) The chemistry of eggs and egg products. In: Stadelman WJ and Cotterill OJ (eds) Egg Science and

Te c hn o l o g y , 4th edn. Binghamton, New York: Haworth Press with permission.

2008 EGGS/Structure and Composition

See also: Cholesterol: Properties and Determination;

Fats: Classification; Lipoproteins; Phospholipids:

Properties and Occurrence; Protein: Chemistry;

Triglycerides: Structures and Properties

Further Reading

Li-Chan ECY, Powrie WD and Nakai S (1995) The

chemistry of eggs and egg products. In: Stadelman WJ

and Cotterill OJ (eds) Egg Science and Technology, 4th

edn. Binghamton, New York: Haworth Press.

Romanoff AL and Romanoff AJ (1949) The Avian Egg.

New York: John Wiley.

Stadelman WJ and Cotterill OJ (1995) Egg Science

and Technology, 4th edn. Binghamton, New York:

Haworth Press.

Stadelman WJ and Pratt DE (1989) Factors influencing

composition of the hen’s egg. World Poultry Science

Journal 45: 247–266.

USDA (1976) Composition of Foods: Dairy and Egg Prod-

ucts. US Department of Agriculture, ARS, Agriculture

Handbook 8.1. Washington, DC: US Government

Printing Office.

Dietary Importance

W J Stadelman, Purdue University, West Lafayette, IN,

USA

Introduction

0001 Eggs are a complete food for a developing embryo of

a chick. The only vitamin not found in eggs is vitamin

C. Most nutrients in the egg are present in variable

amounts depending on breeding, feeding, and man-

agement of the hens. The amino acids in the proteins

of the egg are relatively uniform among eggs so that

egg proteins are often used as the standard against

which other protein food sources are evaluated in

biological value studies.

Nutrient Distribution in the Egg

0002 The proximate analyses of the edible portion of

whole eggs, albumen, and yolk are given in Table 1

of the previous article Eggs: Structure and Compos-

ition. Values are included for pure yolk and commer-

cially available yolk. The difference is that some

albumen is included with the commercial yolk, as

evidenced by a change in solids from 51.2% to

44%. The percentages of yolk and albumen vary

among eggs from different hens and change with age

of the hen, with the percentage of albumen increasing

with age of the hen. For rough calculation the egg is

composed of about 10% shell, 30% yolk, and 60%

albumen.

0003The overall composition of the egg is influenced by

breeding and age of the hens. Nutrition of the hen has

little effect on the proximate analysis but can signifi-

cantly modify the content of some of the nutrients in

eggs.

Protein and Amino Acids

0004The egg white is a protein system consisting of ovo-

mucin fibers in an aqueous solution of globular

proteins. The thick and thin layers of egg white differ

only in their ovomucin content. The protein fractions

of egg white consist of ovalbumin, conalbumin or

ovotransferin, ovomucoid, lysozyme, ovomucin,

avidin, ovoglobulins, ovoinhibitor, and flavoprotein.

The molecular structure and amino acid mapping

of several of these proteins have been documented.

(See Protein: Chemistry.)

0005The absence of lipid materials from the albumen

makes possible the formulation of foods with the high

protein quality of eggs and a complete absence of fats.

The amino acid composition of the several compon-

ent parts of the egg is listed in Table 1. It is apparent

that whole egg, egg yolk, and albumen each have a

good balance of amino acids. (See Amino Acids:

Properties and Occurrence.)

tbl0001Table 1 Amino acids in the edible portion of eggs (per 100 g)

Yolk

Aminoacid(g)Whole

Albumen

Pure

Commercial

Alanine 0.696 0.608 0.861 0.818

Arginine 0.750 0.572 1.000 0.904

Aspartic acid 1.256 1.072 1.639 1.559

Cystine 0.290 0.272 0.301 0.301

Glutamic acid 1.632 1.400 2.126 2.005

Glycine 0.420 0.368 0.518 0.497

Histidine 0.296 0.237 0.434 0.402

Isoleucine 0.682 0.596 0.849 0.790

Leucine 1.068 0.886 1.470 1.364

Methionine 0.390 0.362 0.416 0.416

Phenylalanine 0.664 0.614 0.717 0.696

Proline 0.498 0.410 0.699 0.645

Serine 0.930 0.725 1.432 1.307

Threonine 0.600 0.479 0.892 0.819

Tryptophan 0.152 0.129 0.199 0.191

Tyrosine 0.510 0.410 0.747 0.686

Valine 0.762 0.671 0.934 0.885

Based on data from USDA (1989) Composition of Foods: Dairy and Egg

Products. USDA Handbook 8.1. Washington, DC: US Department of

Agriculture.

Based on data from Cotterill OJ and Glauer JL (1979) Nutrient values for

shell, liquid/frozen and dehydrated eggs derived by linear regression

analysis and conversion factors. Poultry Science 58: 131–134.

EGGS/Dietary Importance 2009

0006 There are four major proteins in egg yolk. These

are livetins, phosvitin, lipovitellins, and lipovitellenin.

The livetins, (a, b, and w) and phosvitin are classified

as high-density lipoproteins; the lipovitellins (a and b)

are classed as low-density lipoproteins and lipovitel-

lenin is a very-low-density lipoprotein.

0007 The amino acid balance of egg protein is almost

perfect to meet human requirements. The amino acid

content of eggs can be modified slightly by feeding

and management programs. When hens are molted

(loss of feathers), the cystine content of the eggs will

be slightly reduced because of the high demand for

this amino acid in the growth of new feathers.

Lipid Components

0008 The lipids of the egg are all contained in the yolk in

a fresh egg. There may be some migration into the

albumen in stale eggs. Nearly all of the yolk lipids are

present as lipoproteins. As mentioned above, they are

classified as high-density, low-density, and very-low-

density lipoproteins. The classification is based on

percentage of the lipoprotein that is lipid material.

The high-density lipoproteins contain very small

amounts of lipid, usually less than 10%. Most

human nutritionists consider the high-density lipo-

proteins to be desirable components of a diet. The

low-density lipoproteins, a- and b-lipovitellin, con-

tain about 20% lipids; lipovitellenin, the very-low-

density lipoprotein, has 40% or more lipid content.

This latter material is considered to be a harmful

ingredient in the human diet as its presence tends to

increase the serum cholesterol level.

0009 A summary of lipid content is given in Table 2. The

saturated fatty acids make up 38% of the total,

monounsaturated fatty acids account for 46%, and

the polyunsaturated fatty acids comprise the remain-

ing 16%. The amounts of different fatty acids can be

dramatically altered depending upon the fatty acids

in the diet of the hen. (See Fatty Acids: Properties.)

0010 A lipid component that has received much atten-

tion is cholesterol. According to the value given in

Table 2, one large egg with a 17-g yolk has 213 mg of

cholesterol. This is now considered to be the value,

but a number of studies indicate a wide range of

values, apparently the result of breeding, feeding, or

management. Age of the hen will also influence the

cholesterol content of an egg. This change is due to

the increase in size of the yolk in eggs from older hens.

The cholesterol level of an egg, on a per-gram-of-yolk

basis, is relatively constant for any hen as long as the

feed remains the same. The method of analysis for

cholesterol in egg yolk can also influence results, as

colorimetric methods generally give higher values

than high-pressure liquid chromatography. Extensive

works have been reported indicating that eggs in the

diet of humans had slight or no effect on serum chol-

esterol levels in the blood.

Minerals in Eggs

0011The egg is an excellent source of many of the trace

minerals. The mineral content of egg components is

listed in Table 3. The exact mineral content of the egg

depends on the mineral content of the hen’s diet. The

content of all minerals analyzed for could be varied,

with the exception of zinc. The minerals of the shell

are not considered in the values given in Table 3 as the

shell is not a normal item in the human diet. The shell

is primarily calcium carbonate and could be a readily

available source of calcium. Refer to individual

minerals.

Vitamins in Eggs

0012The vitamins found in the egg vary with the level

of vitamins in the layer’s feed. The average levels of

vitamins are given in Table 4. The fat-soluble

tbl0002Table 2 Lipid components of the edible portion of eggs (per

100 g)

Yo lk

Fattyacids (g) Whole

Pure

Commercial

Saturated (total) 3.100 9.554 7.663

8:0 0.004 0.012 0.010

10:0 0.004 0.012 0.010

12:0 0.004 0.012 0.010

14:0 0.034 0.102 0.077

16.0 2.226 6.861 5.524

18:0 0.784 2.416 1.915

20:0 0.004 0.012 0.010

Monounsaturated (total) 3.810 11.741 9.444

14:1 0.010 0.030 0.030

16:1 0.298 0.916 0.717

18:1 3.472 10.699 8.623

20:1 0.028 0.084 0.067

22:1 0.004 0.012 0.010

Polyunsaturated (total) 1.364 4.205 3.374

18:2 1.148 3.356 2.842

18:3 0.034 0.102 0.077

20:4 0.142 0.440 0.352

20:5 0.004 0.012 0.010

22:6 0.036 0.114 0.092

Cholesterol (mg) 426.0 1,283.0 1,038

Lecithin (g) 2.30 6.687 5.396

Cephalin (g) 0.46 1.319 1.068

Based on data from USDA (1989) Composition of Foods: Dairy and Egg

Products. USDA Handbook 8.1. Washington, DC: US Department of

Agriculture.

Based on data from Cotterill OJ and Glauert JL (1979) Nutrient values for

shell, liquid/frozen and dehydrated eggs derived by linear regression

analysis and conversion factors. Poultry Science 58: 131–134.

2010 EGGS/Dietary Importance

vitamins, A, D, and E, are found only in the yolk.

Water-soluble vitamins are found in both albumen

and yolk components. Refer to individual vitamins.

Designer Eggs

0014 Niche markets for special eggs have resulted in some

producers of eggs modifying the nutrient content of

the ‘normal’ egg by enriching the diet of the hen with

specific fatty acids, vitamins, or minerals. An example

is the ‘omega egg.’ This egg is obtained by adding a

rich source of omega-3 fatty acids (menhaden fish oil,

specific algae products, or canola oil). The omega egg

producers also increase the vitamin E content of the

diet of their hens to get a higher level of both vitamin

E and omega-3 fatty acids in the egg. The inclusion

of omega-3 fatty acids in the human diet has

been reported to reduce the incidence of heart-related

problems. Other designer eggs can be produced

with increased levels of vitamins, minerals, or other

fatty acids. A likely future designer egg is one with

increased conjugated linolenic acid.

General Considerations

0015The exact nutrient composition of eggs is dependent

on many factors, including the breed or strain of

the hen, age of the hen, diet of the hen, and other

management factors.

0016The per capita consumption of eggs in the USA has

declined significantly from a high of about 400 eggs

per year in 1945 to a level of about 233 in 1990. The

annual consumption of eggs then increased slowly to

about 245 in 1998. The decrease in egg consumption

has been attributed to three factors: (1) increased

buying power, which allows consumers to purchase

more expensive substitutes for eggs, such as meats

and convenience foods; (2) changing eating patterns,

including lighter breakfasts; and (3) concerns about

the negative health effects of eating eggs – first the

cholesterol issue and more recently the inclusion of

Salmonella enteritidis, a pathogenic microorganism,

in a low percentage of intact shell eggs. In 1990 over

20% of all eggs were converted to egg products prior

to sale to the ultimate consumer in the USA. This

percentage has increased to over 30% by 1998 as

more convenience foods rich in eggs are being offered,

as well as the fear of foodborne pathogen.

See also: Amino Acids: Properties and Occurrence;

Ascorbic Acid: Properties and Determination; Calcium:

Properties and Determination; Cholecalciferol:

Properties and Determination; Cobalamins: Properties

and Determination; Eggs: Structure and Composition;

Fatty Acids: Properties; Fats: Classification; Retinol:

Properties and Determination; Riboflavin: Properties and

Determination; Thiamin: Properties and Determination;

Vitamin K: Properties and Determination; Vitamins:

Overview; Vitamin B

: Properties and Determination

Further Reading

Cotterill OJ and Geiger GS (1977) Egg product yield trends

from shell eggs. Poultry Science 56: 1027–1031.

tbl0004 Table 4 Vitamin components of the edible portion of eggs (per

100 g)

Yolk

Vitamin ( unit) Whole

Albumen

Pure

Commercial

A (IU) 634.0 1946.0 1580.0

D (IU) 49.0 147.6 118.3

E (mg) 1.40 4.217 3.390

(mg) 1.00 0.210 3.132 2.512

Biotin (mg) 19.96 7.006 45.662 37.943

Choline (mg) 430.12 1.257 1301.00 1050.09

Folic acid (mg) 46.0 2.994 144.58 120.17

Inositol (mg) 10.78 4.132 23.795 19.891

Nicotinic acid (mg) 0.074 0.093 0.012 0.013

Pantothenic

acid (mg)

1.254 0.120 3.807 3.899

Pyridoxine (mg) 0.140 0.003 0.392 0.320

Riboflavin (mg) 0.508 0.452 0.639 0.595

Thiamin (mg) 0.062 0.006 0.169 0.133

Based on data from USDA (1989) Composition of Foods: Dairy and Egg

Products. USDA Handbook 8.1. Washington, DC: US Department of

Agriculture.

Based on data from Cotterill OJ and Glauert JL (1979) Nutrient values for

shell, liquid/frozen and dehydrated eggs derived by linear regression

analysis and conversion factors. Poultry Science 58: 131–134.

tbl0003 Table 3 Mineral components of the edible portion of the egg

(per 100 g)

Yo lk

Mineral (mg) Whole

Albumen

Pure

Commercial

Calcium 50.0 5.998 138.552 113.276

Chlorine 174.2 179.64 163.250 163.250

Copper 0.014 0.006 0.024 0.020

Iodine 0.048 0.003 0.133 0.108

Iron 1.440 0.030 3.554 2.861

Magnesium 10.0 11.976 6.024 5.837

Manganese 0.024 0.003 0.072 0.059

Phosphorus 178.0 11.976 487.944 395.080

Potassium 120.0 143.7 96.384 106.022

Sodium 126.0 164.7 42.168 60.008

Sulfur 164.0 167.7 150.600 150.600

Zinc 1.10 0.0 3.132 2.536

Based on data from USDA (1989) Composition of Foods: Dairy and Egg

Products. USDA Handbook 8.1. Washington, DC: US Department of

Agriculture.

Based on data from Cotterill OJ and Glauert JL (1979) Nutrient values for

shell, liquid/frozen and dehydrated eggs derived by linear regression

analysis and conversion factors. Poultry Science 58: 131–134.

EGGS/Dietary Importance 2011

Cotterill OJ and Glauert JL (1979) Nutrient values for shell,

liquid/frozen and dehydrated eggs derived by linear

regression analysis and conversion factors. Poultry

Science 58: 131–134.

Ginsberg HN, Karnally W, Siddiqyi M et al. (1994) A dose–

response study of the effects of dietary cholesterol on

fasting and postprandial lipid and lipoprotein meta-

bolism in healthy young men. Artery Thrombosis 14:

576–586.

Hu FB, Stampfer MJ, Rimm EB et al. (1999) A prospective

study of egg consumption and risk of cardiovascular

disease in men and women. Journal of the American

Medical Association 281: 1387–1394.

Li-Chan ECY, Powrie WD and Nakai S (1995) The chemistry

of eggs and egg products. In: Stadelman WJ and Cotterill

OJ (eds) Egg Science and Technology, 4th edn, pp. 105–

175. Binghamton, New York: Haworth Publishing.

McCharen C (1988) Nutrition and cholesterol. In: Stadel-

man WJ, Olson VM, Shemwell GA and Pasch S (eds)

Eggs and Poultry-Meat Processing, pp. 26–35. Chiches-

ter: Ellis Horwood.

Stadelman WJ (1977) Egg proteins. In: Graham HD (ed.)

Food Colloids, pp. 207–239. Westport, Connecticut:

AVI Publishing.

Stadelman WJ and Pratt DE (1989) Factors influencing

composition of the hen’s egg. World Poultry Science

Journal 45: 247–266.

USDA (1989) Composition of Foods: Dairy and Egg

Products. USDA Handbook 8.1. Washington, DC: US

Department of Agriculture.

Vorster HH, Bernade AJS, Barnard HC et al. (1992) Egg

intake does not change plasma lipoprotein and coagula-

tion profiles. American Journal of Clinical Nutrition 55:

400–410.

Watkins BA (1995) The nutritive value of the egg. In:

Stadelman WJ and Cotterill OJ (eds) Egg Science and

Technology, 4th edn, pp. 177–194. Binghamton, New

York: Haworth Publishing.

Yoshida A, Naito H, Niiyama Y and Suzuki T (1990) Nu-

trition: Proteins and Amino Acids. Tokyo: Japan

Scientific Societies Press.

Microbiology

C E Clay, J L Lock, J Dolman, S A Radford and

R G Board, University of Bath, Bath, UK

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001 The microbiology of eggs must be considered in a

biological setting, namely their role in the breeding

biology of birds. At the time of lay, an egg contains

the nutrients, elements, and water required for embryo

development. Natural selection has put in place a

defense system that protects the food stored in the

yolk from attack by saprophytic bacteria and micro-

fungi. This article deals with the defense system of the

eggs of domestic hens, the only species that has been

studied in detail. It deals with the events that culmin-

ate in the breakdown of the defense system such that

the yolk and white become grossly contaminated and,

in most instances, markedly changed in appearance.

Eggs may be endowed also with defense against

pathogenic microorganisms harbored by the parents

or present in the nest environment. This does not

appear to have been studied in detail and will not be

considered further.

The Defense

0002Physical and chemical systems contribute to the

antimicrobial defense of an egg. The former resides

mainly in the shell. Both systems occur in the

albumen. The yolk appears to contain little, if any,

defense against exploitation by saprophytic microor-

ganisms.

0003The physical defense of the egg shell is most easily

understood if its component parts are considered as

resistances in series (Figure 1). The porous shell of

calcite is bounded on its outer surface by a thin layer

of glycoprotein, the cuticle (resistance R

). At the

moment of lay the cuticle is immature but it rapidly

loses its amorphous state and becomes vesicular

fig0001Figure 1 Schematic drawing of hen’s egg shell in radial section

showing the resistances (R

, R

, and R

) to water movement.

C, cuticle; P, pore; S, calcitic shell; SM, shell membrane. For

further details, see Board RG and Sparks NHC (1991) Shell struc-

ture and formation in avian eggs. In: Deeming DC and Ferguson

MWJ (eds) Egg Incubation. Cambridge: Cambridge University

Press. Reproduced from Eggs: Microbiology, Encyclopaedia of

Food Science, Food Technology and Nutrition, Macrae R, Robinson

RK and Sadler MJ (eds), 1993, Academic Press.

2012 EGGS/Microbiology

in structure with a fissured outer surface. Many of the

fissures occur above the outer orifice of pores in the

underlying calcite shell. These pores, which in the

hen’s egg have a straight posthorn morphology,

number some 7–17 10

per shell and contribute

resistance R

(Figure 1). The inner surface of the

calcite shell is lined with two membranes. Both are

formed from anastamosing fibers having a central

core of elastin-like protein and a carbohydrate-rich

mantle. These membranes constitute resistance R

(Figure 1).

0004 If the germfree contents of an egg at oviposition are

to become infected with saprophytic microorganisms,

then such organisms have to penetrate the fissured

cuticle, traverse the pore canal, and pass through the

void spaces in the shell membranes. The agencies that

promote translocation of microorganisms across the

shell are considered below.

0005 Any microorganism that penetrates the albumen

encounters the principal defense system of avian

eggs. The albumen contains upwards of six proteins

having biological properties with the potential to kill

microorganisms (e.g., lysozyme) or impede their

growth (Table 1). Of the proteins cited in Table 1,

ovotransferrin, a glycoprotein constituting about

12% of the total protein of egg white, plays a cardinal

role. It binds two ferric iron atoms to a molecule at

two different sites to produce a salmon-pink com-

plex. The metal complexes dissociate in acid (< pH

6.5), but not in alkaline solutions (pH 9–10). Meas-

urements of binding by equilibrium dialysis have

shown that the binding constants for iron are very

large (about 10

). Changes in the pH of albumen

favor iron sequestration by ovotransferrin as well as

making egg white an unfavorable medium for micro-

bial growth. At the time of lay, the albumen has a pH

of 7.2–7.5 but, owing to the outward diffusion of

carbon dioxide, the pH of the albumen drifts to a

value of 9.0–9.5 within a few days when eggs are

stored at ambient temperatures. The egg white is

made an even more unfavorable medium for micro-

bial growth through its minute content of combined

nitrogen in a nonprotein form.

0006From a theoretical view point, lysozyme (a mura-

midase-N-acetylhexosaminidase, EC 3.2.1.17) would

appear to be an important component of the anti-

microbial defense. It hydrolyzes the b-4,1-glycosidic

bonds in peptidoglycans, the exoskeleton of prokary-

otic bacteria. Once lysed, this exoskeleton cannot

prevent unfettered water uptake by the bacterial cell

and the eventual lysis of its cytoplasmic membrane.

Although lysozyme accounts for 3.4% of the proteins

in the albumen, there is no compelling evidence that it

has an important role in the chemical defense of the

hen’s egg.

0007Avian embryologists contend that the albumen sup-

ports the yolk and young embryo and cushions both

from mechanical damage. The support stems from

the viscous nature of albumen. Ovomucin is largely

responsible for viscosity, and its contribution is

augmented by electrostatic attraction between the

negative charges of the terminal sialic acids of ovo-

mucin and the positive charges of the lysyl S-amino

groups of lysozyme. The viscosity of the albumen in a

newly laid egg insures that the vulnerable yolk is held

at the center of the egg and hence at the furthest

distance from infection should microorganisms pene-

trate the calcareous shell and become lodged in the

shell membranes. With storage, the albumen loses its

viscosity, and the yolk floats upwards until contact is

made with the inner shell membrane. In practice,

therefore, the physical defense that the albumen

contributes to the antimicrobial defense of eggs is

transient. Moreover, there are reasons for believing

that loss of viscosity favors bacterial movement in the

albumen.

tbl0001 Table 1 Properties of the main proteins of hen albumen

Protein Amount in albumen (%) M

Characteristics

Ovotransferrin 12 80 000 Chelation of metal ions, particularly iron

Ovomucoid 11 28 000 Inhibition of trypsin

Lysozyme 3.4 14 600 Hydrolysis of b-1,4-glycosidic bond in peptidoglycans

Electrostatic interaction with ovomucin

Ovomucin 3.5 ND

Ovoinhibitor 1.4 44 000–49 000 Inhibition of several proteases

Ovomacroglobulin 0.5 760 000–900 000

Ovoglycoprotein 1.0 24 400

Ovoflavoprotein 0.8 32 000 Chelation of riboflavin

Avidin 0.05 70 000 Chelation of biotin

Value not determined or reported.

For further details, see Tranter HS and Board RG (1982) The antimicrobial defence of avian eggs: biological perspective and chemical bias. Journal of

Applied Biochemistry 4: 295–338.

EGGS/Microbiology 2013

Course of Microbial Infection

0008 It is generally accepted that the majority of hens’

eggs do not harbor saprophytic bacteria or micro-

fungi at the time of lay. The recent episode of

egg-associated cases of food poisoning caused by

Salmonella enteritidis phage type 4 (PT4) led to the

contention that the egg contents may be infected

during formation, by organisms either harbored by

the ovaries (transovarian infection) or present in the

oviduct (oviductal infection). A survey of upwards of

6000 eggs laid by hens known to harbor S. enteriti-

dis PT4 demonstrated a low incidence (approxi-

mately 0.5%) of contamination of egg contents.

Moreover, the majority of these eggs appeared to

contain the contaminants in the albumen rather

than the yolk. As yet there is insufficient evidence

to draw conclusions about the behavior of salmonel-

lae in eggs infected by the transovarian route during

storage at chill or ambient temperatures. However,

there is abundant information on the course of

infection of eggs infected postoviposition with

saprophytic bacteria or microfungi. (See Food

Poisoning: Economic Implications; Spoilage: Bacter-

ial Spoilage.)

0009 With microfungi, contamination is confined to the

shell’s surface unless eggs are stored under very humid

conditions. Under such conditions, hyphal develop-

ment gives the shell a ‘whiskery’ appearance. Hyphae

that penetrate the pore canals (Figure 1) spread pro-

gressively throughout the egg contents.

0010 The available evidence suggests that free water and

some agency to overcome the water resistance of R

and R

in Figure 1 are prerequisites for bacterial

infection of the egg contents. If a warm egg is

submerged in a cold suspension of bacteria, the de-

crease in volume of the yolk and white is greater than

that of the shell. The partial negative pressure sucks

water across the shell and lodges bacteria in the

underlying shell membranes. The water resistance of

the cuticle and pores (R

and R

in Figure 1) can also

be overcome by imposing a positive pressure on eggs

submerged in water or by suddenly releasing a nega-

tive pressure on eggs submerged in water.

0011 Many studies have shown that Gram-positive bac-

teria, especially those of a coccal morphology, are the

dominant contaminants on the surface of the shell.

In contrast, the microflora of addled eggs is almost

invariably dominated by Gram-negative bacteria

(Table 2). The inference that selection of the latter

at the expense of the former in the shell mem-

branes has been supported by laboratory studies.

(See Aeromonas.)

0012 The behavior of contaminants in the shell mem-

branes is determined by storage temperature of eggs

and the level of iron contamination of the water in

which the contaminants were translocated across the

shell. Storage of eggs at 4



C selects psychrotrophic

bacteria, such as Pseudomonas putida. At ambient

temperatures, psychrotrophic organisms as well as

mesophilic ones, such as Proteus vulgaris, occur in

addled eggs.

0013When relatively small numbers (1.0 10

per ml)

of bacteria suspended in water containing less than

1 mg of iron (Fe

3þ

) per ml are inoculated on to the

shell membranes of recently laid eggs, very few or-

ganisms invade the albumen, and those that do

appear to be in a quiescent state. At some stage in

storage there is a rapid increase in the level of con-

tamination of the egg contents and, when certain

organisms are present, macroscopic changes in the

white and yolk become evident (Table 3). The con-

sensus of opinion is that a switch from the quiescent

to active growth phase of contaminants in the albu-

men is the result of (1) chance collision of the con-

taminants with the yolk, or (2) contact of the yolk

with an infected shell membrane. In either case, the

yolk appears to short-circuit the chemical defense of

the albumen by providing the contaminants with the

3þ

which they require for growth.

0014If the water used to suspend bacteria contains

>1mgofFe

3þ

per ml, then there is a rapid build-up

in the level of contamination of the albumen under-

lying the inoculated shell membrane, and a rapid

onset of rotting of the egg contents, if appropriate

organisms are present. In this situation, iron present

in the water used to suspend the organisms is taken up

by the mantles on the fibers of the shell membranes

and used for growth by bacteria contained in the

membranes. It is the continuous escape of organisms

from the membranes that leads to a heavy contamin-

ation of the underlying albumen.

0015It was inferred above that only some of the organ-

isms recovered from the contents of eggs cause macro-

scopic changes in the yolk or white. It is evident from

tbl0002Table 2 Types of organisms recovered from contents of eggs

Organisms Type of egg

Rotten Tainted

Coli-aerogenes þþ

Proteus þ

Aeromonas þ

Pseudomonas þþ

Alcaligenes þþ

Achromobacter þþ

Gram-positive ++

For details, see Board RG (1969) The microbiology of the hen’s egg.

Advances in Applied Microbiology 11: 245–281.

2014 EGGS/Microbiology

Table 3 that an organism needs one of four attributes

in order to change the appearance of the yolk or

white. Microorganisms lacking all of the attributes

listed in Table 3 form populations of > 1.0 10

cells per ml of albumen without causing perceptible

changes in the appearance of the yolk or white.

See also: Aeromonas; Food Poisoning: Economic

Implications; Spoilage: Bacterial Spoilage