Vol. XVII · Eggs and Dairy Plate 1 of 10egg, shell, white, yolk, air cell, freshness, float test, pH, thick white, storage, refrigerationM

The Egg.

A fresh egg sinks and lies flat in water; an old one stands on end or floats. The difference is a few grams of water that left through the shell.

A hen's egg is about 10 % shell, 60 % white and 30 % yolk. The shell is porous — some 7 000 pores — and lined with two membranes; at the broad end they part to form the air cell. The white is about 88 % water and 10 % protein, in a thick gel that holds the yolk centred and a thinner, runnier layer outside; the yolk is about half water, a third fat and a sixth protein, the source of an egg's emulsifying power (X.2).

Eggs age through their shell. Water evaporates, so the air cell grows and the egg becomes lighter for its size; carbon dioxide escapes, so the white's pH climbs from about 7.6 to over 9 within days at room temperature, and its thick gel thins. That is why a fresh egg poaches into a tight oval while an old one spreads into wisps (XVII.3), and why older eggs peel more easily when boiled (XVII.2). Cold slows all of it. The workshop ages an egg.

Below: the density of an egg against days stored, in the fridge and at room temperature (teaching rates of water loss), with the float point marked, and the white's pH (teaching curve).

What you will learn
Workshop
Teaching model: a 60 g egg of 55 mL losing water through the shell at about 0.08 g a day at 4 °C and 0.3 g a day at 22 °C; density = remaining mass ÷ volume, floating below about 1.00 g/mL. White pH rises from 7.6 towards 9.3 as carbon dioxide escapes (faster at room temperature). Real eggs vary with shell quality and humidity.

Water out, air in

In this model an egg kept at room temperature would float after about 17 days, one kept in the fridge after about 62; long before that, the room-temperature egg's white has reached pH 8.7 in three days, against 7.9 in the fridge, and its thick white has begun to thin. A floating egg is old, not necessarily bad: the float test reads its water loss, not its safety. Crack doubtful eggs into a cup and smell them.

Store eggs in the fridge, in their carton (it slows water loss and keeps out smells), pointed end down so the yolk stays centred under the air cell. In many countries, including the US and Japan, eggs are washed before sale, which removes the protective cuticle, so they must be refrigerated; in much of Europe they are left unwashed and sold unchilled. For poaching and frying use the freshest eggs; for boiling and peeling, eggs a week or two old.

At the stove

Put an egg bought today and one that has been in the fridge for three weeks into a glass of water. Then crack each onto a flat plate: the fresh one has a tall yolk and a thick white that stands up; the old one spreads.

The Chinese 皮蛋 (century egg) takes ageing to an extreme: weeks in an alkaline clay of lime, ash and salt raise the egg's pH above 12, turning the white to an amber jelly and the yolk dark green and creamy.

Terms

Air cell
The pocket of air at the broad end of an egg, which grows as it ages.
Thick white
The gel layer of white that holds the yolk; it thins with age.
Cuticle
A protective coat on the shell, removed by washing.
Float test
Old eggs float because they have lost water and gained air.
Eggs age through the shell: water leaves (the air cell grows, old eggs float) and CO2 leaves (the white thins as its pH rises). Refrigerate them.

Sources

  1. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.
  2. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.