|
One of the most important considerations when buying eggs
is freshness. A good quality, fresh egg will have:
- An upstanding yolk
- A thick albumen layer surrounding it
- A more liquid outer albumen layer
But as the egg ages, changes occur eg:
- Air sac increases in size due to diffusion of water vapour
through the pores in the shell
- Water is transferred to the yolk from the white by osmotic
pressure, so the yolk membrane weakens
- Yolk becomes displaced and settles against the shell instead
of being firmly suspended in the white, due to weakening
of chalazae strands
- CO2 is released from the egg white
so the pH rises and shelf life is shortened
The following tasks will help you determine the freshness
of an egg, what effect this has on the egg itself and the
consequences of this for use in cooking.
| Task
A - |
How can freshness
be tested?
|
| Task B - |
Is it possible to determine the age of the egg by
examining the air sacs?
|
| Task C - |
Does the height of the
yolk also change with age?
|
| Task D - |
What are the effects of age on the aeration of egg
white
|
| Task E - |
What about the freshness
of yolks?
|
| Task F - |
In your opinion, which
eggs are of good quality? |
Work in pairs. Each pair should have:
- An egg stored for two weeks in a domestic airing cupboard
in which the cistern is kept hot.
- An egg stored for two weeks in the fridge
- A newly purchased egg
- 3 sheets of greaseproof paper, each 20cm x 20cm
- 3 sheets cm graph paper, each 20cm x 20cm
- 3 sheets carbon paper (A4)
- 3 chopping boards, each at least 20cm x 20cm
- Chinagraph pencil
- Kitchen knife
- Skewer
|
 |
Arrange
the chopping board so that the surface is flat. |
|
|
Place a sheet of graph
paper on the chopping board, squared side up. Place
a sheet of carbon paper, sensitive side down, on top
of the graph paper. Place a sheet of greaseproof paper
over the carbon paper.
|
 |
|
Take the oldest egg.
Crack the shell carefully with a kitchen knife so that
the blunt end of the shell, with the air sac inside,
remains intact. Empty the egg onto the centre of greaseproof
paper. Label the greaseproof paper with the age of the
egg. Retain the shell for Task B, marking
it with the chinagraph pencil to denote the age of the
egg.
|
 |
|
Using the skewer, trace
the perimeter of the egg, pressing down firmly but without
tearing the greaseproof paper. |
 |
|
Repeat stages 1-4 with
the remaining eggs. |
 |
|
For each egg, carefully
remove the egg and sheet of greaseproof paper from the
board. Remove carbon paper. Put the eggs carefully to
one side, and retain for Task C. Count
the cm squares which fall within the perimeter line;
this gives the approximate area of the egg. Record areas.
|
 |
|
Examine the texture
and smell of all three eggs. |
Egg |
Area(cm2)
|
Texture |
Smell |
|
|
|
|
|
|
|
|
|
|
|
|
|
- What can you deduce about the spread of the egg in relation
to the age of the egg?
- Does texture or consistency also alter with age?
- Does smell alter with age?
- Egg shells from the three eggs used in Task A
- Felt pen
- Sharp, pointed knife
- 20ml measuring cylinder
|
|
Take the
half of the shell of the oldest egg containing the air
sac. |
|
|
Using the felt pen, draw around the perimeter of the
air sac so that the line is clearly visible on the inside
of the shell.
|
|
|
Using the knife, pierce
the membrane and then cut it away with the point. |
|
|
Fill the air sac space
with water up to the line. Empty the water into the
measuring cylinder. Record the volume. |
|
|
Repeat using the shells
of the remaining two eggs. Record water volumes. |
|
Egg |
Water Volume (ml)
|
|
2 weeks in airing cupboard
|
|
|
2 weeks old in fridge
|
|
|
Newly purchased
|
|
Comment on the changes which occur in eggs as they age.
Consider changes in:
(a)
air sac
(b) spread
(c) texture
or consistency
(d) smell
| |
2 weeks in an airing cupboard
|
Storage
for 2 weeks in a fridge |
Newly
purchased |
|
Students |
Area (cm2)
- from A
|
Volume
(ml)
- from B |
Area
(cm2)
- from A |
Volume
(ml)
- from B |
Area
(cm2)
- from A |
Volume
(ml)
- from B |
1 |
|
|
|
|
|
|
| 2 |
|
|
|
|
|
|
| 3 |
|
|
|
|
|
|
| 4 |
|
|
|
|
|
|
| 5 |
|
|
|
|
|
|
| 6 |
|
|
|
|
|
|
| 7 |
|
|
|
|
|
|
| |
|
|
|
|
|
|
| Average
results |
|
|
|
|
|
|
Yolk
Index = |
Height
of yolk |
Width
of yolk |
A fresh, good quality egg has a yolk index of around 0.45.
The yolk index for an older egg would be less.
- Eggs broken out onto greaseproof paper used in Task A
- Straw
- Ruler
Begin with the oldest egg:
|
|
With eyes
at bench level, look at the egg profile. |
|
|
Place the ruler vertically
at the side of the bench so that 0cm is level with the
surface of the bench. |
|
|
Using the knife, pierce
the membrane and then cut it away with the point. |
|
|
Take the straw and place
it horizontally across the centre of the yolk so that
the straw just touches the surface. Read off the height
of the yolk and record. |
|
|
Repeat with the remaining
two eggs. |
Egg |
Height
of yolk |
Width of yolk
|
Yolk
Index
(height/width) |
|
2 weeks in airing cupboard
|
|
|
|
2 weeks old in fridge
|
|
|
|
newly purchased
|
|
|
|
- Compare yolk index results with results from task A and
task B. Remember that a fresh, good quality egg has a yolk
index of around 0.45 and an older yolk would have a lower
index.
- Eggs from Task C (replace greaseproof paper onto chopping
boards)
- Egg cup
- 3 measuring jugs of same size, labelled A, B, C
- Chinagraph pencil
- Electric hand mixer
- Stop watch
|
|
Place the
egg cup over the yolk of the two week old egg stored
in the airing cupboard. Carefully tip the white into
measuring jug A.
|
|
|
Repeat using the two
week old egg kept in the fridge, pouring the egg white
into jug B. |
|
|
Repeat using the newly
purchased egg, pouring the egg white into jug C. |
|
|
Whisk the first egg
white in jug A until a stiff foam is produced and the
jug can be inverted. Time the whisking procedure.
|
|
|
Repeat with the other
two eggs, using the consistency of the egg foam in jug
A as the standard. |
Egg |
Volume
(ml) |
Time taken
to obtain foam
|
|
2 weeks in airing cupboard
|
|
|
2 weeks old in fridge
|
|
|
newly purchased
|
|
|
- Does egg freshness have any effect on foam formation?
- If so, why?
In the briefing, it was indicated that the yolk membrane
weakens on ageing.
- At least five basins
- Ruler
- Dishcloth, teatowel
|
|
Carefully
slide the yolk from the oldest egg into a basin. |
|
|
Using the ruler to measure
height, drop the egg yolk into a second basin held 10cm
above the bench top. |
|
|
If the yolk breaks,
record the height. If the yolk fails to break, drop
the yolk into another basin, increasing the 'drop height'
by 2cm. Continue this procedure until the yolk breaks.
Record the height.
|
|
|
Wash all used basins
but leave the broken yolk in its final basin.
|
|
|
Repeat using the remaining
two eggs. |
Egg |
Height needed to break yolk
|
|
2 weeks in airing cupboard
|
|
2 weeks old in fridge
|
|
newly purchased
|
|
- How is the strength of the yolk membrane affected by age?
Use the eggs you have selected to make a soufflé
omelette
- Basin containing yolks
- Basin containing whisked whites
- 1 teaspoon water for each egg yolk
- 15g margarine
- Seasoning
- Savoury filling (optional) - grated
- Cheese or chopped cooked ham or peeled and chopped tomatoes
- Tablespoon
- Whisk
- Omelette pan (15-18cm)
- Fish slice
- Serving plate
|
|
Heat grill. |
|
|
Whisk egg yolks and
water together until pale and creamy. |
|
|
Fold white into yolk
mixture with tablespoon.
|
|
|
Melt margarine in omelette
pan. Heat gently. |
|
|
Spread in egg mixture. |
|
|
Cook without moving
until bottom is set and pale golden brown. |
|
|
Place omelette under
hot grill until top is set. Add filling (optional). |
 |
|
Fold omelette in half
and slide onto a warm plate. |
Examine Your Results
- Look at the volume and texture of the omelette. Describe
it.
|
