Everyone cooks eggs differently. If you want to get a perfect egg, use our calculator and stick to the following rules:
- Bring the water to a boiling point.
- Adding half a spoon of table salt may help preventing the eggs from cracking. For the same reason, it's better to use eggs that aren't taken straight out of the fridge.
- Be generous with the amount of water: inserting the egg lowers the temperature, we want to be sure we're at the boiling point! Once the eggs are in the water, start a timer.
- When the time is over, take the eggs out of the pot and rinse them with cold water to stop the boiling process. This is the recipe in a nut(egg?)shell, but what's the science behind it?
The science behind the boiling of an egg
When we place the eggs in hot water the, eggs get warmer. The heat of water warms shells of eggs and then their interior. The eggs consist of white and yolk. A soft-boiled egg has firm white and runny yolk. Both whites and yolk coagulate, change from liquids to gels in hot water. However, whites coagulate at slightly lower temperatures then yolk. So for a soft-boiled egg, we should warm the egg to a temperature when whites have already thickened and yolks not quite yet. The calculator assumes that we get a perfect soft-boiled egg when it reaches a temperature of 65C (149F). So how much time does it take to heat an egg to such temperature?
Once eggs are in the water, the shell has a temperature of the boiling water. The heat then starts to spread through an egg. This process is controlled by a heat diffusion equation which depends on two parameters: the heat conductivity and heat capacity. The heat conductivity specifies how quickly the heat spreads, while the heat capacity tells us how much heat do we need to warm up the substance by one degree. From the heat diffusion equation, we can find a time
t when the temperature between the white and yolk reaches our 65C. That's when we stop warming the eggs. Because the whites are still at a noticeably higher temperature (the shell has a temperature of 100C/212F) even if we take the eggs out of the water, there will be still some heat transferred from whites to the yolk, and the yolk might coagulate. That's why we should cool down the eggs immediately after taking the eggs out of the water - we want to stop the boiling process.
A precise calculation of the time required to cook a perfect soft-boiled egg is difficult. The whites and yolk are complicated substances involving various types of proteins and fat molecules. Also, every egg is different, and the ratio of whites to yolk varies. Charles Williams of the University of Exeter came up with the formula involving all the main factors. The time for the region between the white and yolk to get to a temperature
t = m * K * log(ywr * (Tegg - Twater)/(T - Twater))
mis the mass of an egg
Kis the factor describing thermal properties of an egg
Teggis the initial temperature of the egg
Twateris the temperature of the water
Tis the tempreature in the region between whites and yolk.
ywris a ratio of white to yolk
For a soft-boiled egg, the temperature
T should be 65 degrees Celsius (149 F). And that's the value set in our calculator. For a hard-boiled egg, the temperature inside the egg should be higher, so the yolk coagulates. However, it cannot be too high because then the sulfur in whites reacts with the iron in the yolks and creates the greenish ferrous sulfide covering the yolk. To avoid it we should keep the temperature
T bellow 77 Celsius (170.6 F).
The altitude? Is it a joke?
Why there is an altitude in our calculator? When we go higher above the sea level, the air pressure lowers, and the boiling point of water decreases. That makes the cooking process more time-consuming. If you are high enough, like Mount Everest high, you won't get hard-boiled eggs. Water boils at 8848m (29029 ft) above the sea level at a disappointingly low temperature of 68 degrees Celsius (154.5 F). For a more real-world example, a Bolivian city of El Alto, a home to over 1 million people, is 4150 meters above the sea level. Water boils at 85.9 Celsius (186.6 F), which translates to approximately 2.5 minutes longer soft-boiling time. That's crazy!
The Egg Calculator was created by Miłosz Panfil, Omni Calculator chief scientist, who happens to have a PhD in Quantum Physics at University of Warsaw.