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)
Mar 31, 2015
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