The Food Lab

Unraveling the mysteries of home cooking through science.

The Food Lab, Ramen Edition: How to Make a Marinated Soft Boiled Egg (Ajitsuke Tamago)

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[Photographs: J. Kenji Lopez-Alt]

Last week at The Food Lab, we plumbed the fascinating depths of homemade tonkotsu ramen broth and came up with a pretty great recipe. But great ramen is not built upon broth alone. What better to add to ultra-rich and creamy pork broth than some extra creaminess in the form of a soft boiled egg yolk?

If you've got a fancypants sous-vide water oven, then you've already got what it takes to create those awesomely tender, custard-like onsen tamago that you get in fancypants restaurants like Momofuku Noodle Bar. Just cook your egg at 145°F for about 45 minutes or so, and there you go. (And of course, you can always do it in a beer cooler).

If, on the other hand, you are like me and prefer ajitsuke tamago—perfectly soft-boiled eggs marinated in a sweet and salty soy-based sauce, split in half and resting, ready to enrich and flavor your broth—well, my friends, you're in luck, because all it takes is a pot, a few ingredients, a bit of know-how, and a bit of time.

Step 1: Perfect Soft Boiled Eggs

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Long time Food Lab readers might recall the very first installment of the series back in 2009. The subject tackled? Perfect boiled eggs. Since then, not much has changed in the basic technique, though I've made a few minor refinements and adjustments here and there to take out a bit of the fiddliness of the old technique, which required a thermometer. Here's the basic gist of it.

For perfect soft-boiled eggs, the goal is to get the white to completely set, while keeping the yolk liquid, creamy, gold, and just warmed through. Egg whites begin to set at around 155°F, egg yolkds at 158°F. So in order to get a soft boiled egg exactly how we want it, we have to simmer the egg just until all of the white has reached at least 155°F, but before any of the yolk has come up to 158°F.

Simple, right? All we have to do is figure out exactly how long that takes. I cooked eggs at two-minute intervals ranging from one minute to 15 minutes and cut them in half.

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The correct timing is somewhere in between the 5-minute egg (too soft) and the 7-minute egg (too hard). A simple binary search led me to an ideal cooking time of 5 minutes 45 seconds. If you're the kind of person who keeps track of their spending to the penny or must find every single heart container in a Zelda game, then you'll probably want to set your timer to 5:45. If, however, you're the kind who forgets to mark down the 65¢ you spend on that can of Coke at the Chinese grocer and you don't really care, then six minutes is good enough for you.

The only other element at play when soft boiling is the maximum temperature to which your egg white is cooked. See, above 180°F, and your egg white starts to become tough, dry, and rubbery. With ajistuke tamago, this problem is compounded by the salty marinade it rests it—the salt will actually cause it to toughen up even more (more on this below). It's vitally important not to overcook your eggs at this stage if you want them to remain tender.

The solution? Just don't use boiling water. By keeping the water at around 190°F—at sea level that's a bare, quivering simmer—you can ensure that your eggs stay tender. I found that by bringing two quarts of water to a boil before dropping in a half dozen from-the-fridge eggs, the temperature dropped down to exactly where I wanted it to be. After that, it was just a simple matter of maintaining the heat at a bare simmer.

Step 2: Marinating

Once you've got your eggs boiled and peeled, the rest is a simple bath in a sweet soy and mirin based marinade. The easiest recipes are just that: a mix of soy sauce and mirin (sweet Japanese wine). I prefer to cut my marinade with a good amount of sake with some added sugar to compensate for the dilution of the mirin.

If you happen to have made a batch of tender, sweet Japanese-style pork belly chashu, you can use that leftover porky broth for an extra-tasty egg.*

*Huh? Don't have a recipe for tender, sweet Japanese-style pork belly chashu? No worries—we'll have you covered later this week.

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Now, you could just pour your marinade into a bowl and add your eggs. That'll work. Sort of. The problem is that hard boiled eggs are more buoyant than the sweet-salty marinade and thus float to the top and poke their heads out, resulting in uneven marination. Restaurants usually have meshed devices intended to hold the eggs under the liquid while they marinate. Home cooks solve this problem through other methods.

One common technique is to put the eggs and the marinade into a plastic zipper-lock bag and carefully remove all the air from it, forcing the liquid to spread around the eggs. It works, but it's a little messy to do. Here's a much easier technique:

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Just cover the tops of the eggs with a paper towel. The towel wicks liquid up and around the eggs, making sure that all sides get even exposure to marinade. It's a technique I use all the time for all kinds of preparations—keeping vegetables submerged in their pickling liquid, for example, or keeping peeled artichokes submerged in lemon water to prevent discoloration.

The Limits of Marinating

When I was in college and living in a house shared by 50 people, I remember getting an email once from a resident offering free chicken breasts to whoever wanted them. His claim, "they've been marinating for three whole days, so they're going to be super tender and tasty as f*&k."

I don't know any college student who'd turn down free food, so I took them, grilled them, and served them to some friends for dinner. The consensus? They were awful. Mushy and mealy with a chalky texture that was completely off-putting. The lesson I learned that day? With marinades, longer does not necessarily equal better. Marinades can be great for seasoning the outer layers of a food, but let your food sit in a marinade too long, and it can wreak chemical havoc on its texture.

With acidic marinades—like the Italian-style dressing those chicken breasts had been marinated in—denaturation of proteins can cause foods to turn mushy and rapidly give up their moisture when heated.

With ajitsuke tamago, there's another culprit: salt.

We all know that salt can have a powerful effect on food, right? In the case of bacon or ham, for instance, salt not only draws moisture out from the interior of the food, it also dissolves some of the proteins in the muscle, causing it to tighten and change in texture (ever notice how different bacon feels from fresh pork belly?).

So it is with ajitsuke tamago. A few hours in a marinade, and you'll get an egg with a delightfully sweet-and-salty flavor on its outer layer. The flavor is powerful enough to season the whole bite, despite the fact that it's only penetrated a milimeter or two. Let the egg sit in that salty marinade for too long, however, and you'll see the marinade slowly work its way into the center of the egg. Eventually, it'll even reach the yolk, causing it to firm up and set into an almost fudge-like texture. Not what we're after.

Here's an egg that I marinated for three whole days before slicing in half.

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As you can see, nearly all of the yolk has been hardered (a small amount of liquid remains in the very center—give it another day or two, and it would have been hard all the way through). Eating this egg is also quite unpleasant. The white is hard, dry, and extremely rubbery, and the parts of the yolk that have been cured are hard set, sticky, and chewy in a manner most unpleasant. This process is taken to the extreme to make the infamous Chinese thousand-year-old eggs, in which raw duck eggs are buried in a salty mixture of tea ashes until cured all the way through to the center. The resultant eggs are as hard as a hard-boiled eggs, but have never seen heat.

If you ever go to a ramen-ya and get horribly tough eggs, most likely they either a) overcooked them; check for a greenish tinge around the yolk to confirm this, or b) over-marinated them (tough, but not green). Either way, it's a sign that you should think twice about going back to that particular shop.

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Of course, once we're through with this whole ramen-at-home business, you'll probably think twice about going back to any ramen-ya. Ya?

Get the Recipe!

Ajitsuke Tamago (Marinated Soft-Boiled Egg) »

More for Your Ramen

Tonkotsu Ramen Broth »
Chashu Pork Belly »

About the author: J. Kenji Lopez-Alt is the Managing Editor of Serious Eats where he likes to explore the science of home cooking in his weekly column The Food Lab. You can follow him at @thefoodlab on Twitter, or at The Food Lab on Facebook.

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