Transcript Eggs - PDST

Eggs
© PDST Home Economics
Nutritive value of eggs
 Protein: 13%, HBV, albumin &
globulin in white, livetin &
vitellin in yolk.
 Fat: 12%, saturated, in yolk, in
fine emulsion because of
lecithin, easy to digest, high in
cholesterol.
 Carbohydrate: 0%, serve with
complex carbohydrates, no fibre
present.
 Vitamins: A & D in yolk, B2,Niacin and B12 ( more
in free range). Lack vitamin C.
 Minerals: 1%,Calcium and phosphorus useful
amounts, iron in yolk.
 Water: 74%, more in white than yolk.
 Energy: 147 kcal, mostly in yolk.
Dietetic value of eggs
• Cheap & nutritious, low
budgets
• Versatile
• Protein alternative to
meat/fish
• Source of HBV protein
for lacto-vegetarians
• Restrict on low calorie
or low cholesterol
diets
• Easily digested
therefore good for
invalids, children,
elderly
• Should be served with
food rich in complex
carbohydrates, fibre
and vitamin C
Egg structure
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Shell = calcium carbonate, shell is
porous so air, bacteria, smells
and flavours can get in and water
vapour can get out
Shell pores are covered by a
natural varnish when laid
2 membranes - one stuck to the
shell and one surrounding the
white
Air space at rounded end of the
egg between shell and membrane
White/ albumin - 2 layers thin
and thick
Yolk membrane
Chalazae hold yolk in centre of
white for protection
Yolk
Fresh eggs
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Heavy for size
Rough shell
Domed yolk
Thick white
Sinks in water
Date stamp
Small air space
No bad smell when
cracked
Grading eggs
• Graded by weight and quality
• Weight from size 1-7, size 1=70g
size 7=45g
• Quality decided by candling
• Extra sticker = within 7 days of
laying should be removed when 7
days is over.
• Class A = best, small air space,
poaching frying boiling
• Class B = large air space, staler,
yolk off centre, scrambling,
baking, sauces.
• Class C = similar to class B but
sold only to food manufacturers.
Buying and storing eggs
Storage
Buying
• Free range cost more. • Store at 7-13ºC e.g.
fridge door, for 1 month.
• Quick turnover
• Away from strong
• Class A, B or extra
smelling food
• Size 1-7
• Rounded end up
• Check shells
• Left over egg white, air
tight container in fridge
• Check date stamp
• Heavy with rough shell • Left over yolk, covered
with water in fridge
• Remove from fridge 1
hour before use
Quality assured eggs
• Ireland has an EU approved salmonella plan to maintain
the health of the country’s laying stock.
• Incoming hens must be certified as salmonella free.
• Laying hens are checked for salmonella on a monthly
basis.
• All feed for hens is heat treated.
• Management systems ensure full traceability of eggs.
• Eggs carry best before date and house code and logo.
• All producers and suppliers are inspected and
approved.
• All systems are independently inspected before QA
mark is awarded.
Properties of eggs
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1. Coagulation
Egg protein coagulates and sets when heated.
White protein @ 60-65ºC. Yolk protein @ 65-70
ºC.
Coagulation causes the protein chains to untwist
and straighten (denature) and bond together
around small pockets of water.
When overheated the protein clumps
together squeezes out the water and
this causes curdling.
Coagulation is used when eggs are boiled,
poached, scrambled, fried or turned into
omelettes or custards. When eggs are
used to thicken, bind, glaze, coat the
principle of coagulation is used.
2. Aeration/Entrapping air
• Whisking egg or just egg white
brings bubbles of air into a mixture.
• Whisking also causes heating of the
egg protein (by friction) which
slightly sets the protein chains and
makes them unravel and line up
around the air bubbles.
• This causes a temporary foam to
form.
• To keep the foamy texture
permanently in place the mixture
must be further heated or a setting
agent like gelatine must be added.
• Aeration is used to make meringues
and souffles.
3. Emulsification
• Lecithin in egg yolk is an
emulsifier.
• An emulsifier is a substance
that causes mixtures that
would normally separate (e.g.
vinegar and olive oil) to stay
mixed together.
• This property is used in making
mayonnaise and hollandaise
sauce.
Uses of eggs in food
preparation
• Binding: sticking ingredients
together e.g. burgers, fish
cakes (coagulation).
• Coating: protects food when
frying, egg is used to stick a
layer on outside of food e.g.
breaded fish (coagulation).
• Glazing: beaten egg brushed
on baked foods to make them
brown and shiny e.g. scones
(coagulation).
• Thickening: e.g. custard
sauce (coagulation).
• Enriching: increasing the
nutritive value of a dish
e.g. brown bread.
• Garnishing: hard boiled egg
sliced or chopped for
decoration e.g. salad or
dressed crab.
• Emulsifying: e.g.
mayonnaise, hollandaise
sauce.
• Aerating: e.g. sponge cake,
meringue.
• Clarifying: egg whites used
to make jelly, stock or wine
clear (coagulation).
Effects of heat on eggs
• Egg protein denaturates and coagulates causing it to set
and harden, egg white becomes opaque
• Shrink
• Sulphur in egg white protein reacts with iron in yolk to
form iron sulphide which causes the greenish colour on
outside of the yolk of hardboiled eggs
• Destroys pathogenic bacteria like salmonella.
• Loss of B group vitamins especially vitamin B1
• Too much heat causes curdling
• If overcooked white becomes tough and rubbery, yolk
becomes dry and crumbly
• Egg albumin (white) which is soluble in cold water
becomes insoluble
Pasteurisation of eggs
• This involves slowly heating whole raw eggs to kill
harmful bacteria without cooking the egg
• Effects: White is cloudy
Harder to whisk
Cost more
Kills harmful bacteria
Allows children, pregnant women,
elderly, invalids to eat raw egg products
without risk of food poisoning