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To Boil an Egg
Time to boil an egg: how to adjust for cold eggs, small eggs, large eggs, high mountains, low pressure.
|Initial temperatures °C||Cooking times|
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
The rate at which food heats through, in a given environment, depends on the ratio, k/Cp, 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.
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