Vol. VI · Proteins and Heat Plate 3 of 11egg proteins, ovotransferrin, ovalbumin, yolk, slow-cooked egg, onsen tamago, 温泉蛋MH

The Egg's Proteins.

Hold an egg at one temperature long enough and its centre reaches that temperature exactly. The texture you get is a direct reading of which proteins have set.

Boil an egg and its outside overshoots while its centre catches up, so the white is firm before the yolk sets (I.1). Hold an egg in water at a fixed, lower temperature for an hour instead — a Japanese 温泉玉子 (onsen tamago), a "63-degree egg" — and every part reaches the same temperature. The texture then reads straight off the proteins: ovotransferrin in the white sets near 62 °C, the yolk thickens near 65 °C and sets near 70 °C, and ovalbumin, most of the white, only near 80 °C.

So at 62 °C the white is barely set and milky, the yolk liquid; at 64 °C the yolk is thick and fudgy while much of the white is still loose; at 70 °C the yolk is firm and the white tender; only above 80 °C does the white turn fully firm and rubbery. The workshop holds an egg at your temperature for the time you choose.

Below: the egg cut open after the hold you choose, and how far its white and yolk have set (thermal bench with the course's egg-protein kinetics).

What you will learn

Name the egg proteins and their setting temperatures.
Explain a long low-temperature egg.
Choose a temperature for a texture.
Compare boiled and slow-cooked eggs.

Workshop

A large egg (treated as a 45 mm sphere) from 5 °C held in water at the chosen temperature. Set fractions at the centre of the yolk and in the outer white, from the course kinetics for ovotransferrin, ovalbumin and yolk proteins.

A thermometer you can eat

At 63 °C for an hour the egg's centre reaches the bath after 50 min; the white is 55% set and the yolk 66% — the silky white and runny-to-jammy yolk of an onsen egg. At 64.5 °C the yolk is 89% set and spoonable like fudge; at 70 °C the yolk is 100% set while the white, held back by its ovalbumin, is 66%; at 80 °C the white is 100%.

Boiling cannot give these combinations, because in boiling water the white is always far hotter than the yolk. Low-temperature eggs need time and an accurate bath, and they are safe only if the eggs are pasteurised or the hold long enough at a high enough temperature to deal with Salmonella (XVII.6). Their texture is also the clearest demonstration in the kitchen that temperature, not time, sets the result once everything has come to equilibrium.

At the stove

If you have an accurate water bath or a sous-vide circulator, hold eggs at 63 and 65 °C for an hour and crack them into bowls. The two degrees make a visible difference in the yolk. Use pasteurised eggs, or eggs your local food authority considers safe to eat soft.

Japanese onsen tamago were traditionally cooked in the outflow of hot springs, which sit conveniently near 65–70 °C — a natural water bath.

Terms

Ovotransferrin: Egg-white protein that sets near 62 °C.
Ovalbumin: The main egg-white protein, setting near 80 °C.
Onsen tamago (温泉玉子): A Japanese egg slow-cooked in hot-spring water: silky white, soft yolk.
Equilibrium cooking: Holding food until it reaches the bath temperature throughout.

Held at one temperature, an egg sets only the proteins whose temperature has been reached: 62 °C white begins, 65 °C yolk thickens, 80 °C white firms.

Sources

Egg proteins: ovotransferrin sets near 62 °C, ovalbumin near 80 °C, the yolk thickens near 65 °C and sets near 70 °C (H. McGee, On Food and Cooking, 2004, ch. 2; P. Barham, The Science of Cooking, 2001).
D. E. Baldwin, A Practical Guide to Sous Vide Cooking (2008–2012) and "Sous vide cooking: a review", Int. J. Gastron. Food Sci. 1, 2012.