Edible eggsAbeer Abdel-Naser Awad Abdel-AllAss. Prof. of milk hygiene

Introduction• Eggs are produced for reproduction not for man.• Shell eggs have many uses at home and food processing establishments, they are used either alone or as ingredients in other foods.

• Eggs provide a unique, well-balanced, low caloric value, easily digestible nutrient for persons.• In addition, eggs are inexpensive, delicious, and easily to be served.• For public: use only properly refrigerated, clean, sound shell, fresh and high-grade eggs.

Egg formation• Yolk enclosed in the vitelline membrane and containing the germ cell is formed in chicken’s ovaries.• The yolk drops into the mouth of the oviduct and passes slowly down.• As it does, it is covered with layers of egg white from the albumen secreting cells.• Then with membranous tissue from other protein secreting cells.• Finally it is covered by calcium and other minerals from minerals secreting cells.• Before laying the shell is coated with a colloidal liquid which on drying forms the cuticle or bloom.

Female Reproductive Tract• Ovary• Infundibulum - site of fertilization• Magnum - albumin addition• Isthmus - membranes• Uterus - shell gland and pigments• Vagina - transport to exterior• Sperm storage occurs at various sites in tract in some species

Approximate time for egg Approximate time for egg formationformation Part of the oviduct Approximate time Function Funnel hour ¼- receives yolk from ovary- fertilization Magnum 3 hours - secretion of albumen Isthmus 1 hour ¼- shell membranes Uterus (shell gland) 21 hours - shell & pigments Vagina/ cloaca Less than 1 min. - egg pass to the outside

Color Color of shell and yolk may vary but has no effect on egg: quality, nutritive value, cooking characteristics or shell thickness.

Color of the shell• Color of the shell comes from ooporpherins pigments deposited on shell surface. The color may range in various breeds from white to deep brown “the breed of hen determines the color of the shell”

Color of the shell

Color of egg albumen• In raw eggs the albumen is opalescent and does not appear white until it is beaten or cooked. • Yellow or greenish cast in raw white may indicate presence of riboflavin.• Cloudiness of the raw white is due to the presence of CO2, which has not had time to escape through the shell and thus indicates a very fresh egg.

Color of yolk• Yolk color depends on the diet of the hens, if they get plenty of yellow-orange plant pigments known as xanthophylls, they will be deposited in the yolk.• Hens fed mashes containing yellow corn and alfa alfa meal lay eggs with medium yellow yolks, while those eating wheat or barley yield lighter-colored yolk.• Yolk pigments are relatively stable and are not lost or changed in cooking

Color changes due to food preparationGreenish-black discoloration on the surface of

yolk of hard-cooked eggs, due to sulphur and iron compounds in the egg reacting at the surface of the yolk.

This case is due to over-cooked or high amount of iron in the cooking water and hydrogen sulphide from sulphur containing proteins.

The alkalinity of the white and slow cooling after cooking may favor the reaction.

Although the color may be a bit unappealing, the eggs are still wholesome and nutritious and their flavor is unaffected.

Prevention:By using the proper cooking time and

temperature.By rapidly cooling and peeling the cooked eggs.

Caramel shade in a hard-cooked egg white due

to a high amount of iron in the cooking water

or to a carbonylamine-type reaction (reaction

between glucose and amine groups of certain

proteins of the white, the longer the egg

remains at elevated temperature and more

alkaline the egg, the greater discoloration).

Prevention:Using fresh eggs.Cooling eggs quickly after cooking.

Structure and composition of the eggComposition

Egg componentRelation to the weight

of the egg

shell11%

white58%

yolk31%

1 -Shell

• Forms 9-12% of egg weight.• It is porous, a greater No. of pores is present at the

broad end.• Composed of: Calcium carbonates (95%),

magnesium carbonate, Calcium phosphate and other organic matter including protein.

• Deficiency of these elements or Vit. D in the ration will lead to production of thin or soft shell

• It is covered by a protective coating called the cuticle or bloom.

2-Cuticle or bloom

• A thin film of mucoid protein found on the shell of fresh egg.

• It gives fresh egg its dull appearance• It acts as a protective layer.• It is resistant to damage• It is removed by bad handling or washing• It is normally disintegrated after 14 days of

laying.

3 -Shell membranes

• Inner and outer shell membranes.• Formed from insoluble proteins (keratin).• Act as barrier against invading microorganisms.

Shell Membranes

– Inner and Outer Membranes

– Envelope yolk and albumin

– Contiguous with each other except at one end where they separate to form air cell.

4 -Vitelline membrane

• Covers the yolk and protects it from breaking• It is weakest at the germinal disc.

5 -Air cell

• It is located at the broad end between the 2 shell membranes.

How it is formed?• It is used by Candler for determining the grade of

eggs.• Its depth in fresh eggs does not exceed 7 mm.• As the egg ages, moisture and Co2 leave through the

pores of the shell and air enters to replace them leading to enlargement of the air cell.

Air cell

Air cell Radiology

6 -Albumen• Accounts for most of an egg liquid weight about 57%.

• It is alkaline having pH 7.5-7.9• It contains more than 50% of: total proteins, niacin, riboflavin,

chlorine, magnesium, potassium, sodium and sulfur.• The main proteins in the white are: ovalbumin, conalbumin

and ovomucoid.• It also contains: Ovoglobulin (related to lysozymes),

Ovomucin (responsible for thickness of thick white) and Avidin (binds with iron making unavailable biotin)

• It is consisting of 4 alternating layers of thick and thin layers from the yolk outwards:

Albumin

• Four distinct layers– Chalaziferous - inner

thick– Inner thin layer– Outer thick layer– Outer thin layer

• Protects yolk from invasion by microorganisms and provides water, protein and minerals to the embryo

Albumen

A- (inner thick white) Chalaziferous white: around the vitelline membrane and ends with the chalazae (2 coiled strands of insoluble protein).

B- thin whiteC- thick whiteD- outer thin white

• Function of chalazae • Foaming property of white

Chalazae

• Twisted fiber-like structures at each pole

• Hold yolk in place inside the egg– permit limited rotation– inhibit lateral

displacement

7 -yolk

• Makes up to 33% of the liquid weight of egg.• Its pH is about 6.• It consists of latebra (concentric layers) ,

germinal disc and a small sphere of white yolk surrounded by the yellow yolk.

• It is roughly half water and half solids• Protein account for 1/3 and fat 2/3• The main protein in the yolk is vitellin.

Yolk

• Yolk contains all of the fat in the egg.• The fat of yolk consists of triglycerides,

phospholipids and lipoproteins.• With the exception of riboflavin and niacin,

yolk contains a higher proportion of the egg vitamins than the white.

• All of the vitamins A, D and E are in the yolk

Yolk

• Yolk is one of the few foods naturally containing Vit. D.

• Yolk contains more phosphorus, manganese, iron, iodine, copper and calcium than white

• It contains all zinc.• Yolk is responsible for the emulsifying

properties of the eggs.

Nutritive value of eggs

1- Eggs contain all dietary supplements required by the body.

2- They are rich source of high quality proteins.

3- They are important source of:- unsaturated fatty acids- iron, phosphorus, and trace elements.- Vit’s A, E, K and B complex including

B12

N. V. of eggs

4- eggs are good source of Vit. D (they rank second to fish-liver oil).

5- Excellent food for growing children.6- Their high nutrient content, low caloric value

and ease of digestibility make eggs valuable in many therapeutic diets and for persons with weight problems.

Some misconceptions about the nutritive value of eggs

• The color of the shell (white or brown) is directly related to the breed or strain of the hens and has no effect N.V. of the eggs.

• The deep yellow color of the yolk is not better than the light one, as it is due to Xanthophylls pigment which has no N.V.

• Some persons consider raw eggs more digestible than cooked. In fact cooked eggs are more readily digested than raw, however both are very completely digested and absorbed.

Organic eggs

• Are eggs from hens fed on ration that is free from antibiotics, pesticides, fungicides, herbicides or commercial fertilizers.

• Organic eggs are more expensive than eggs from hens fed on conventional ration, however no difference in N. V. exist.

Thank you