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Summary Biology of Domestic Animals - Case 1 Eggs
Summary of case 1 (eggs) of course Biology of Domestic Animals
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Biology of Domesticated animalsCase 1:
1.0 Eggs
1.1 Function:
o Reproduction important for survival of the individual and the species. Starts among
others in the egg cell (ovum)
o Eggs of every species are very different. Size of yolk varies. Chickens have large egg
cells. Therefore, we refer to the cell body as ‘the yolk’ ignoring small quantity of
cytoplasm also in it.
o Most animals are oviparous. This means that the female lays the eggs from which the
young emerges. egg has to be large to contain all nutrients needed. most species
live in water
o Most of remaining animals are viviparous: giving birth to live young. Develop inside
uterus so eggs are small.
o Certain animals are ovoviviparous: produce eggs with a lot of yolk that develop later
within female’s reproductive organs
1.2 Egg production
o Mainly chickens, but ducks and geese can also be used for egg production. Breeds
were crossed for large laying capacities but product had lower acceptance than the
chicken egg but same taste and shape.
o 1960’s chicken housing system was developed in US (battery cage). Made possible to
mechanise and automate chicken farming for egg production. More chickens and
fewer people. Ducks weren’t able to be housed. Duck egg farming declined sharply.
Battery cage banned in European Union since 2012.
o 2010 10.1 billion chickens eggs produced in NL. Domestic market consumed
approx. 3.0 billion eggs 185 eggs per person per year
1.3 Structure of the egg
o 4 components: the yolk, the egg white (albumen = protein), shell membranes & shell.
o Chicken egg: 1/3 yolk and 10% of egg is shell. Proportions vary widely for different
bird species.
o Egg white: has nutritional & protective function; elastic, cushions & insulates. Has
also an antibacterial effect. Four layers
Outermost thin layer: thin white (23%)
Think layer: Thick white (57%)
Innermost thin layer (17%)
Layer thick white around yolk, extends into chalazas (3%)
o Chalazas: strands of egg white, keeps yolk centred. Two at rounded end and three at
pointed end. With age, lose attachment yolk rises.
, o Egg shell: 93-98% CaCO3. Porous so ±10.000 pores for gas exchange. Covered by thin
film cuticula, dries after laying and reduces gas exchange + bacteria entering. During
incubation (high temp), permeability cuticula increases again and 15% water
evaporates from egg.
o Two thin shell membranes inside shell lie close together, except for rounded end: air
chamber. Shell most porous in this area. Size increases overtime, especially in dry and
warm environment.
o Shell structure strong enough to bear contents yet broken by chicken beak. ±0.35
mm thick and varies from 0.25 mm – 0.45 mm.
1.4 Chemical composition of the egg
o Common proteins in egg white:
Ovalbumin: 75% -> lends firm structures as coagulates due to heat.
Ovomucoid (13%) & ovomucin (7%): glycoproteins. Stabilise egg white foam.
Ovoglobulin: Able to beat egg white into foam.
o Yolk: mostly lipids, source of nutrition:
Triglycerides (62%), mostly saturated fatty acid chains;
Phospholipids (33%);
Sterols (5%), primarily cholesterol (0.4%). Reason to limit egg consumption:
cardiovascular disease.
o Yolk proteins: (lipids bind to proteins and form lipoproteins = emulsifying effect.
Vitellin (phosphoprotein) (77%)
Livetin (23%) contains sulphate
o Many pigments are fat soluble and so occur in the yolk of the egg (for example
carotene, xanthophyll and riboflavin). These pigments are essential for the colour of
the products such as mayonnaise and egg noodles. Riboflavin also occurs in egg white
1.5 Quality of eggs
o Larger egg represents higher quality. Egg trade can assess freshness by candling:
Fresh: translucent. Relative small air chamber and the yolk is dimply visible in
the centre of the egg
Older: Enlarged air chamber
o Other methods to asses the freshness:
Shaking the egg: no sound? -> fresh
In bowl of salt water: Remains at the bottom? -> Fresh. Older floats up due to
increasing air chamber.
Fresh egg has lots of thick white with flat and firm yolk, won’t spread a lot.
o Partially genetic (thickness egg white and firmness yolk). Temperature and age of hen
can also influence. Height is measured on flat surface.
o Shape is mainly determined genetically. Each hen lays practically same shape: round,
cylindrical, pointed, pear shaped etc. Small = round, large = conical, elliptical, oval or
biconical.
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