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Nov 1, 2013

An eggciting designed eggsperiment

IMG_0328.jpgWhat's the best method for getting hard-boiled eggs that are easy to peel and attractive? (Photos by Caroll Co)

A typical scene in my kitchen: I make a batch of hard-boiled eggs with the hope of an easy peel and a beautifully cooked center. But when I sit down to enjoy my egg, I find that, sadly, it’s not so easy to peel – or I have discoloration around the yolk (or worse yet, sometimes both occur).

Here's how I've been preparing my hard-boiled eggs: I start with the eggs in a pot of cold water. Then, I bring the pot to a boil, remove it from the heat and cover the pot for 12 minutes. After a recent disappointing experience with both overcooked and hard-to-peel eggs, I decided to investigate further in a quest to make better hard-boiled eggs.

My Internet search revealed that almost everyone claims to have a foolproof way to make hard-boiled eggs, but a quick browse through comments shows mixed results. Some common themes and questions appear, so it sounded like the perfect opportunity to use a designed experiment to separate fact from folklore.

For a first try at this eggsperiment, my budget for runs was two dozen eggs – same size/brand, purchased two weeks apart. Perhaps in a future experiment, I will use more eggs, but I wanted the peeler (my wife) to be blinded from knowing how the egg was prepared. Since I wasn’t going to be doing the peeling, 24 eggs seemed to be the limit of asking for help from my wife. I also quailed at the thought of having to eat so many egg salad sandwiches in a short period of time.

While most cooking methods for hard-boiled eggs start with cold water, a recent blog post had me intrigued about putting the eggs directly into boiling water.

So I ultimately decided on the following factors to study:

  • Cooking method (start with cold water or put into boiling)
  • Age of the egg (purchased two weeks ago or newly purchased)
  • Cooling method (ice bath or cold tap water)
  • Pre-cool crack (yes or no)

The pre-cool crack indicates whether I cracked the egg before using the cooling method in 3. If you’re familiar with design of experiments, you may recognize that not all of these factors are equally easy to change. For factors 2-4, I can assign these on an egg-by-egg basis (that is, they’re easy to change). For the cooking method, it is much more convenient if I cook more than one egg at a time. Thus, cooking method is a hard-to-change variable, or whole plot variable in the parlance of split-plot designs.

This means that the estimate of the effect of the cooking method is based on the number of batches I cook rather than the number of eggs. I ultimately decided on six batches of four eggs, or six whole plots. While this gives me only three batches for each cooking method, I hoped that I would get at least some indication whether changing the cooking method mattered. For the easy-to-change factors, I’m more likely to detect the important effects because of the number of eggs I have.

For the cooking method, I cooked one batch at a time in the same pot. I used the same amount of water in each batch (2 cups). The start with cold water was heated on medium until the water reached 188 degrees Fahrenheit, at which point I turned off the heat and covered the pot for 10 minutes. For the boiling method, I waited until the water just started boiling and put the eggs in for 11 minutes, while reducing the temperature to medium so that the water was simmering.

The Responses

My main purpose here was to find out about ease of peeling, but there is still the aspect of whether or not a peeled egg is aesthetically pleasing. The final responses measured were:

  • Peel time (in seconds)
  • Attractiveness of the egg (rating from 1 to 5)
  • Ease of peel (rating from 1 to 5)

While 1 and 3 seem similar, the peel time is likely to be very noisy and may not always pick up on frustration that can arise while peeling, which ease of peel should capture.


Hard_boiled_eggs_peeled.jpgThe responses measured were peel time, attractiveness of the egg and ease of peel.

The Experiment

Now it’s time to design the experiment. The first step is to enter my responses and factors in the Custom Design platform, which is the first item under the DOE menu. We get something that looks like this:




Notice that all of the factors are set to “Easy” under the Changes column in the Factors table. To change cook start to be hard-to-change, click on the “Easy” under the Changes column for the cook start factor and select “Hard” from the list that comes up.




If we click the Continue button at the bottom, it’s time to set up the rest of the design. By default, the model is set to be able to estimate the main effects. With 24 eggs, we should be able to look at two-factor interactions, so I select Interactions -> 2nd to have the Custom Designer ensure the design can estimate all the main effects and two-factor interactions.




Finally, we need to set up the appropriate run size. Recall that we want six batches of four eggs (24 eggs total). Under the Design Generation tab, this means we set the Number of Whole Plots to 6, and the Number of Runs to 24.




Clicking the Make Design button, and the experiment is ready to go. The design will look something like this:




Any predictions as to the results? I’ll reveal the results next week.

Community Member

Inna wrote:

Great application, talking about practical! :-)

I would suggest two more factors: putting salt in the water (yes/no) and cooking time in boiled water. I have my secret recipe, but will only share once you reveal your results; we don't want any bias... :-)

Ryan Lekivetz wrote:

Great suggestions! Doing this experiment definitely has me thinking about a follow-up - I'll look into those in the sequel. I'm looking forward to hearing your secret recipe =)

Cheers, Ryan

Community Member

Lou V wrote:

Ryan..Great blog!

Brown versus white eggs would be interesting as well. I'm told that brown eggs peel easier that perhaps could be due to the price and thus shelf age of the egg rather than color. I would also consider temperature as a continuous factor and also include time at temperature for both the cooking and cooling.

Community Member

Jasmine wrote:

This is excellent stuff!

I'm just a bit scared about the ratings as response- by the time your wife gets to peeling the last eggs- her fatigue will start ruling the ratings! But randomization will save the day?


Community Member

Phil Kay wrote:

Important work Ryan. I will be very interested to see the results.

Age of egg is a strong effect when poaching eggs: old eggs are more watery so don't hold their shape well for poaching. I'm not sure how this might affect boiled eggs though. I'm thinking old eggs might be easier to peel.

Ryan Lekivetz wrote:

Very interesting - I'll add those to my list. One of these rainy weekends I'll think about doing the follow-up experiment.



Ryan Lekivetz wrote:

Thanks for the feedback, and a very good point. We did randomize the peel order, and while we didn't notice any effect this time, I would probably try and build peel order into the design as a covariate next time. I think this was more pronounced in the attractiveness of the egg rating - in hindsight, we should have kept previously rated eggs in piles of how they were rated.

Ryan Lekivetz wrote:

That's good to know on poaching eggs. I'll probably stretch the age of the eggs a bit further apart next time - while watching the expiry date of course!

Community Member

Isabel wrote:

Thank you for your experiment!! Would you consider adding salt to the water? I guess it would be easier to peel the eggs if you use salt.

Community Member

Joseph wrote:

My guesses for the most important to least important for peeling:

0) cook start (highly non-uniform heating at t=0), experienced by all eggs

1) pre-cool cracking (egg temperature and solidity seem inversely related)

2) ice bath (same reason as cook start, but now it's experienced by both pre- and post-cracked eggs, which would muddy the signal. I'd expect similar performance in the post-cracked group, though.

For yolk colour, I'd expect cooking time to have the most effect, but that was not measured, so I expect no clear signal from the given explanatory variables.

Community Member

Richard Zink wrote:

That's a great post! And that's no yolk. :)

Ryan Lekivetz wrote:

I've already ran this experiment, but next time around salt will definitely be a factor. I'll plan for more whole plots next time, as I think there's interesting things to look at there.

Ryan Lekivetz wrote:

Very good guesses ;)

And in regards to the ice bath vs. cold water, next time I will possibly use a "warmer than cold tap water" cooling method.

Likewise, I'll want to vary cook time, but in terms of yolk color, the eggs were better-cooked compared to when I do it on my own.

Ryan Lekivetz wrote:

Thanks! Your pun cracks me up.

Community Member

Emil Friedman wrote:

This situation has multiple and possibly competing objectives so the desirability profiler might be worth using during the analysis stage. If so, it might illustrate some inherent subjectivities. For example, I prefer very well-done yolks, even if they're green. Others prefer less well done yolks, even if they're soft or watery.

PS, I noticed that all of your experiments involve quickly chilling the eggs after cooking. Some people just bring them to a boil and then let them cool slowly.

PPS, it's important to remember that they're on the stove. Exploding eggs can reach all the way to a cathedral ceiling. It's even worse if you're in an apartment with a smoke detector and forget about them when leaving. Some people even mistake exploding eggs for gunshots.

Ryan Lekivetz wrote:

Excellent point on the objectives. Unfortunately I didn't vary the cook time for this experiment, hopefully next time. Good catch on the cooling method as well.

Thankfully I've yet to forget about eggs while they're cooking - I think I'd never hear the end of it if I had exploding eggs!

Community Member

Emil Friedman wrote:

I wasn't talking about varying the cooking time. I was talking about letting them cool slowly versus your two methods of quickly chilling. Is there any good subject area knowledge that can be used here?

Community Member

Mike Clayton wrote:

On a serious note,. back in 1970's we found that the Japanese schools were teaching early grade school kids to run RICE COOKING experiments.

That was one of our wakeup calls about the Deming influence there, and their 100 year plan to dominate in product quality by elimination of variation. Remember, after WWII they were famous for junk toys, etc, which humiliated them. Quality in manufacturing gave them back their dignity.

Isikawa is honored as much as Deming for the Japanese quality miracles. Your egg cooking experiments should perhaps be taught in our grade schools? We need to get back our dignity.

Ryan Lekivetz wrote:

Apologies, I understand you now. From what I've read, not putting them in cold water immediately would presumably have them continue to cook (but not as much as leaving them in the covered pot). I'm more convinced now to have an additional cooling method of the slow cool...

Ryan Lekivetz wrote:

Fascinating information, thank you for sharing. I don't know what the solution is, but I'm certainly of the opinion that children should have an idea of experimentation (particularly beyond one-factor-at-a-time) sooner rather than later.

Community Member

Lou V wrote:


Would the randomization in this experiment be considered "scrambled" eggs?

Ryan Lekivetz wrote:

Eggcellent observation! Seems fitting to me.

Community Member

Shannon wrote:

I am driving up your view count for this and the following blog by returning to it every time I make hard-boiled eggs, Ryan! Thanks for posting on this topic, the conditions you found do make a great batch!