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Time to boil an egg: how to adjust for cold eggs, small eggs, large eggs, high mountains, low pressure.

Times for a fresh standard 60g egg

Initial temperatures °C		Cooking times
Egg	Water	Soft-boiled	Set	Hard-boiled
4	100	4m3s	9m25s	10m15s
10	100	3m46s	9m5s	9m55s
20	100	3m17s	8m25s	9m15s
20	95	3m46s	8m55s	9m55s
20	90	4m16s	9m40s	10m30s

Thermal properties
of some foods

	k	Cp
Water	0.6	4.22
Ice	2.18	1.96
Yolk	0.35	2.8
White	0.55	3.5
Beef	0.48	3.0
Apple	0.45	3.075

Water boils at 100°C—pure water at standard pressure. The white of an egg begins to coagulate at 60°C, and is fully set at 65°C, at which point the yolk begins to set; by 70°C the egg is cooked through. We say an egg is soft-boiled once the white has reached at least 65°C, set once the centre has reached 65°C, and hard-boiled once the centre has reached 70°C

Salt water boils hotter; at altitude water boils cooler. The boiling point of water is raised by 0.512 °C for each mol per kg of solute—for example, 29.25 grams of salt, NaCl, dissolved in each kg of water: 0.5 mol, 11.5g Na⁺ and 0.5 mol, 17.75g Cl^-, since salt dissolved in water dissociates into these two ions (the freezing point of the same solution is depressed by 1.86 °). Raising or lowering the pressure by about 28 mmHg (37mBar) will change the boiling point by 1°C. Some eggs are larger, some smaller. How long does it take to boil an egg, and how do these factors affect the answer?

The times given here come from a model egg. A standard egg is just over 10% shell, 30% yolk and just under 60% white, by weight. For the standard 60g egg, the figures are 7g shell, 18g yolk, 35g white. Adding salt (in palatable quantities) to the cooking water makes little difference to the boiling temperature. Pressure can make a significant difference—in Denver, CO, the boiling point of water is about 5 °C lower than at sea-level. Extreme weather can have a similar effect, and these two effects are additive.

The specific gravity of all four parts of the egg are different (shell, 2.325; yolk, 1.032; albumen, 1.038; shell membranes, 1.075). We approximate an egg by two concentric spheres with radii 2.2 and 1.5cm. The yolk (14.1ml) occupies the interior of the inner sphere; the white (30.5ml) occupies the space between the spheres.

The key physical quantities relevant to this model are

k thermal conductivity W m^-1 °K^-1

A measure of the ability of a material to conduct heat, quantified in Watts per metre per degree. The linear measure here is area divided by thickness, so the power transmitted through a plate of material with conductivity k, area A and thickness d, is kA/d Watts per degree of temperature difference between its faces.

C_p specific heat J cm^-3 °K^-1

A measure of the energy required to heat 1 cm³ of a given substance through 1 °K, quantified in Joules per cm³ per °K. A Joule is one Watt-second.

The rate at which food heats through, in a given environment, depends on the ratio, k/C_p, of these two quantities.

Approximate values for these quantities are known for various foodstuffs: A.J. Fontana et. al. Thermal properties of selected foods using a dual needle heat-pulse sensor. Institute of Food Technologists Annual Meeting, Chicago 1999, American Society of Agricultural Engineers (ASAE) Paper 996063.

Latent heat of melting for ice 334J g^-1

Quail 10g Hen 60g Duck 100g Emu 1000g Ostrich 1300g

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