THE EGG

• Eggs are found in many food preparations including:– Candy, ice cream, soups, pastas, cake/bread

batters, pastry glazes– Used to clarify meat stocks and wine– Vital for emulsifying (bringing water and oil

together)

Cooking preps

• Boiling (hard and soft)

• Frying

• Baking

• Pickling

• Fermented

Egg-laying animals

• Chicken (Gallus gallus)

• Duck

• Pigeon

• Quail

• Penguin

• Turtle

• Crocodile

Origin/Evolution of the Egg

• Discuss at your table

Origin of the Egg

• Vessel to carry female genetic contribution to next generation (follows the origin of sex).

• Must carry enough nutrients to feed growing embryo.

• First eggs layed in water (soft exterior).• During evolutionary transition from water to land

– Later eggs utilized a leathery coating (reptiles; 300 mya)

– finally a hard shell (birds; 100 mya)

Origin of the Chicken

• Genus Gallus is ~8 mya

• Species Gallus gallus ~3-4 mya

• Immediate ancestors most likely ‘jungle fowl’ native to tropical and subtropical SE Asia and India.

• Probable first domestication from 7500 BC, found in Egypt by 1500 BC and Greece by 800 BC.

Indeterminate egg layers

• Domesticated chickens are ‘indeterminate layers’.

• Far more useful as source of eggs than of meat.

• Wild Indian jungle fowl (predecessors of modern chickens) produce clutches of ~12 eggs a few times a year)

• ‘Industrial’ hens will lay an egg a day for a year or more.

White Leghorn –Best egg layer

Cornish – for meat only

Plymouth Rock – brown eggs and meat

Rhode Island Red – brown eggs and meat

Factory chickens (250 – 290 eggs/yr)

Pros and Cons of Industrialized Chickens

• Pros– 2 lbs of feed produces

1 lb of meat– 3 lbs of feed produces

1 lb of eggs– Improved reliability of

product– Year round availability

• Cons– Flavor has suffered?– ‘Spent’ hens recycled

into feed leading to increase in salmonella

– Humane issues

How eggs are made by hens

• Every egg is ~3% of hen’s weight (1000x the investment of humans)

• Over the course of a year hen converts 8x body wt. into eggs.

• ¼ of daily energy use put into egg production (for duck ½)

Egg yolk production

• Each chicken ovary contains several thousand germ cells (‘egg cells’)

• As chicken matures germ cell accumulates yellow yolky material (fats and proteins) that is synthesized in liver.– Color depends on feed: corn/alfalfa leads to

yellow, grasses lead to orange)

• Yolk grows very large with germ cell floating on surface

Egg white production

• Egg white (albumin) is added to yolk over the course of about a day, as the yolk moves down the oviduct protein secreting cells layer on albumin.

• First layer of albumin coils to form ‘chalaza’ which will eventually hold yolk to shell. (keeps yolk and germ cell cushioned in center of egg).

Egg shell

• Last step before laying egg is creation of shell.

• Occurs in uterus/’shell gland’ (over ~20 hrs).

• First 5 hours – water and salt pumped into egg to plump it up.

• Calcium carbonate added over the next 14-15 hrs to create shell.

Yolk and white functions

• Egg yolk (~1/3 of egg’s weight) is almost exclusively nutritive (McGee, p. 75)– Holds ¾ of calories, iron, thiamine, vit. A and

amylase and LDLs (surrounded by protein, phospholipids and cholesterol).

• Egg white (~2/3 of egg’s weight) is nearly 90% water. The rest is protein, minerals, vitamins and a little glucose.

Egg white proteins

• Ovalbumin – nourishment, blocks digestive enzymes

• Ovotransferrin – binds irons

• Ovomucoid – blocks enzymes

• Globulins – plug defects in shell/ membranes

• Ovomucin – thickens albumin

• Etc…

Cholesterol

• Average egg contains 215 mgs, compared to 50 mgs for an equivalent protion of meat (McGee, p.78)

• Why so much cholesterol?– Essential component of cell membranes that

developing embryo must build.

• Saturated fats increase cholesterol more than cholesterol itself.

Egg grades

• Generally only AA and A sold in stores.

• Grade required depends on usage.– As whole eggs AA– In custards, pancakes, etc. A is fine.

Salmonella

• ~ 1985 Salmonella enteritidis identified as cause of rash of European food poisonings.

• Eggs should be cooked to 140F for 5 mins or 160F for 1 minute

• Alternatives to fresh eggs include:– Pasteurized eggs (raised to 130-140F)– Dried egg whites– Liquid eggs