Boiling Eggs

Copyright © Karl Dahlke, 2023

There are many temperatures in this chapter; I'll list fahrenheit first, then celsius in parentheses. Similarly, I'll give elevation in feet, then meters.

Here is an apparent contradiction. Of course there are no contradictions in science or math, just things we don't yet understand.

One day I was making hard boiled eggs and took them out too early. The yolks were well cooked but the whites were a bit runny. Oops. The heat conducts through the egg from the outside in, from shell to center, so the yolk might have been one or two degrees cooler than the white, but since I started with cold water, the heating process was slow and gradual across 25 minutes, thus we would expect the egg to be approximately the same temperature throughout at any given time. For simplicity, let's say the yolk and white were the same temperature. However, the yolk was cooked and the white was not. Therefore, yolks cook at a cooler temperature than whites.

I confirmed this on the internet. Yolk cooks at 158 (70) degrees, and white cooks at 176 (80) degrees. perhaps I impatiently took the eggs out when they were only 167 (75) degrees, thus the yolks were cooked and the whites were not. So far so good.

Now for the confusion. It is possible, even desirable, to poach or soft boil an egg so that the white is cooked but the yolk is a bit runny. Wouldn't the yolk harden first? How is this possible?

When the egg is dropped on to or in to boiling water, the white reaches the boiling point quickly, and becomes solid. The yolk is a larger mass, more like a ball, and takes longer to heat throughout. for a short time, the white is hotter than the yolk. Lift the egg out of the water at just the right time, and you have the perfect poached or soft boiled egg. Wait too long, and the yolk cooks through as well, and is hard.

Elevation

Any form of cooking that involves boiling water changes as you move up the side of a mountain. Atmospheric pressure decreases at higher elevations, because there is less air above, and less air pressing down. It is easier to boil water when there is less air pressing upon it and keeping it liquid. In other words, water boils at a lower temperature in the mountains than at sea level. In the vacuum of space, even ice water boils away.

I spent summers with my grandparents in Kamas Utah, elevation 6,500 feet. Cooking was sometimes modified accordingly. For example, a box might instruct: “boil the noodles for 12 minutes, 15 minutes mountain.” A lower boiling point means the noodles must cook longer, 5% longer for each additional thousand feet of elevation.

Here is a table showing the boiling point of water as you climb up a mountain.

The Shirpa live in Napal, at elevations ranging from 15 thousand to 18 thousand feet, where atmospheric pressure is half that of sea level. Most of us would pass out from lack of oxygen, but the Sherpa have spent generations acclimatizing to the thin air. As you see from the table, boiling water is just barely hot enough, or perhaps not hot enough, to cook egg whites, depending on elevation. It is not practical to cook eggs by boiling at these heights. How about scrambled eggs and bacon for breakfast?

Pressure Cooker

Going in the other direction, a pressure cooker traps the steam inside, until the pressure builds up to 15 psi, that is, one atmosphere pressing outward against the walls of the vessel. Thus the water experiences a total pressure of two atmospheres, one from the internal steam and one from the air around us, whence it boils at 250 (120) degrees. This is far above the normal boiling point of water, 212 (100) degrees. Not surprisingly, food cooks faster at these temperatures.

Papin invented the first pressure cooker in 1679. It was basically a sealed pot with a tight fitting lid. There was no release valve, and explosions were not oncommon. A hapless bystander could be sprayed with hyper-boiling water. This was certainly not practical, hence various safeguards were introduced. By the 1700's, a safety valve would prevent the pressure from exceeding 15 psi, the mechanical tolerance of the vessel. It allows steam to escape if the pressure gets to high. A modern cooker, with digital controls, may also turn the heat off until the pressure returns to an acceptable level.

Critical Temperature

Every liquid has a critical temperature, where it will always boil. No amount of pressure can contain it. If water is heated to 707 (374) degrees, it becomes steam, regardless of the pressure around it. (Don't try this at home.) The steam may be under tremendous pressure, even hundreds of atmospheres, but it is a gas, or perhaps a supercritical fluid, not a liquid.