Presentation on theme: "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."— Presentation transcript:
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)
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 hens 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 hrs to create shell.
Yolk and white functions Egg yolk (~1/3 of eggs 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 eggs 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 F) –Dried egg whites –Liquid eggs