Apr. 03, 2015

The 1-2-3s of Hard-Cooked Eggs

by Jeff Potter

Click to enlarge images
Tune in to Science Friday on April 3, 2015, to hear more of Jeff Potter's egg cooking tips. 
Eggs are complicated.
Let me revise that: Eggs are really complicated. One could teach an entire semester-long course about food science using nothing but the egg. Most of us who cook regularly know that egg proteins are binders, that they give custards and ice cream rich flavors, and that, when whisked up, they impart a delicious texture to goodies like macarons and angle food cake. But did you know that eggs are one of only two common ingredients in the kitchen that are alkaline? (Here is my contribution to egg jokes: That’s why cooks always say eggs are a basic ingredient! I know, I’m funny.)
Even when eggs are cooked by themselves, it’s said that there’re a hundred ways to do it. I’m going to talk about just one of those ways: hard-cooked, a.k.a “hard-boiled.”
Last summer, I received one of my more delightful reader emails:

From: Dan

Subject: I can't cook easy-peel hard-boiled eggs!

I know there is some secret to cooking eggs that are easily peeled…I’ve tried a lot of things: starting with boiling water, starting with cold water, adding a little baking soda, adding little salt (NaCl), adding a lot of baking soda (the whole container), adding a lot of salt, cooling them quickly with ice, letting them cool slowly…

I want a fool-proof formula that takes the following precise initial parameters: volume of water (ml), air pressure, air temperature, water temperature, water pressure, egg white temperature, egg yolk temperature, egg pressure, salinity of water, pH of water, precise amount of baking soda and/or salt, exact dimensions of the cooking vessel along with it's material properties...

HELP! I need an eggspert!


These are the sorts of emails I love: from someone who’s done a lot of homework, tells me what they’ve done, and is still stumped. I was stumped, too, so I started digging into the research.
When posed with a question like Dan’s, I start by looking on books.google.com and scholar.google.com—both great resources for information. Based on what I find there, I’ll try to reproduce what I read in the kitchen.
In the case of an easy-to-peel, hard-cooked egg, the pH of the egg white matters, according to the papers. If the pH is below 9.2, eggs are hard to peel. Above that, and they become blissfully easy, or so one would believe from the research.

How, you might be wondering, do I get my eggs to have a pH above 9.2? Eggs are not perfectly sealed vessels. The shells are porous, with one paper claiming somewhere between 7,000 and 17,000 microscopic holes in them to allow for gas exchange while the chick develops before hatching. Those same holes allow for carbon dioxide, initially deposited in the white by the chicken as it lays it, to escape, which in turn raises the pH. The rate at which carbon dioxide leaves the egg depends on both time and temperature. Stored at fridge temperatures, that point should be around four or five days after an egg was laid.

I took my quest seriously. May I present Exhibit A, at left, in which I steal eggs from some chickens and stick a pH meter in their egg whites.

The pictured egg had a pH of 8.6—a little high for a fresh egg, but not unreasonable if the egg had been laid 12 hours earlier and taken in the warm summertime air. (I might also blame my too-cheap pH meter; I keep meaning to do something about that.)

According to the literature, the egg should have been hard to peel. But after I cooked it, it actually peeled pretty easily.

This is the bane of anyone writing about food science: In theory, there’s no difference between theory and reality; in reality, there is. Things just do not always reproduce (invariably, I’m not doing something right).
The “Ah-ha!” moment came six months later. (The other bane of writing about food science is that, often, the solution just takes a while to present itself.) I realized that consumer egg cookers—these are $30 unitaskers sold on Amazon—work by steaming eggs. And that’s how I’d been cooking these “should have been hard to peel” eggs. 
But the studies that I’d been looking at never listed steaming as a cooking method. (Some of these studies are quite entertaining, by the way—someone spent lots of time creating tables of cooking method, number of seconds to peel, and number of visual defects.) By steaming the eggs, I presumably heated the white in ways that prevented it from adhering to the inner membrane.
One other thing came up during my research, with a hat tip to Kenji Lopez-Alt of Serious Eats: Shocking the cooked eggs in ice water keeps the shape of the egg nice and round. Here’s why: The egg white, as it cooks, expands. Allowing it to cool down slowly causes the egg white to contract back toward the top of the egg shell—but shocking it in cold water prevents that from happening, leading to nice, beautiful, round eggs.
Based on all this, I present to you (and Dan) my solution for easy-to-peel, hard-cooked eggs.
Jeff Potter’s 1-2-3s for Hard-Cooked* Eggs
*Hard-Cooked, because there’s no water-boiling
1: Make sure your eggs are at least a few days old—or at least that the eggs have a pH on the higher side of things. If you’re lucky enough to have really fresh eggs, they can be harder to peel.
2: Cooking method matters. Don’t boil them; steam them! Boil a quarter-inch of water in the bottom of the pan, add the eggs, cover with a lid, wait 12 minutes.
3: Before peeling them, let them cool down. You don’t need to shock them in cold water to make them easy to peel. Shocking them stops the cooking—why not just pull them out a minute sooner? If you like, dropping them in cold water—no ice necessary—will make the shape of the egg rounder (no "flat bottom"). You can peel them when they’re still steaming hot, but you need a little bit of water to unstick the inner membrane and the outer white.
P.S. Poking holes in the shell to prevent them from cracking doesn’t make any sense when eggs are boiled in water—the water gets into the shell, and the egg whites spew out of the hole. But in steaming, poking a hole on the bottom of the shell can make sense: As the air pocket inside the egg heats up, it will expand, and poking a hole provides an outlet for the expanded air so the shell won’t crack. My guess is that somewhere along the way we lost the idea of steaming eggs, but the idea of poking a hole in the shell survived.

About Jeff Potter

A science and food geek, Jeff Potter is the author of Cooking for Geeks: Real Science, Great Hacks, and Good Food, which the Washington Post called “one of the most useful books on understanding cooking."

The views expressed are those of the author and are not necessarily those of Science Friday.
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