Hot Spring Eggs are a traditional method of boiling eggs in the hot springs of Japan. When you break open the eggshell, the cooked egg will slide out whole, and you will be surprised to find the egg white still soft, and the egg yolk solid. This phenonmenon is due to the egg yolk and egg white each having different solidification temperatures.
Speaking of Hot Spring Eggs (the Japanese name is Onsen Tamago), some people may confuse this dish with soft boiled eggs, but they are completely different for the following reason:
Coagulation – the egg yolk in soft boiled eggs is a little runny, but in Hot Spring Eggs, the egg yolk is solidified, and the egg white is a semi-solid state which is very soft and similar to the texture of bean curd. Today I will let you in on the secret of making perfect Hot Spring Eggs.
Hot Spring Eggs are not common to Chinese cuisine, nor in Western cuisine (though it is similar to the “semi-cooked egg” or soft boiled egg in Western food). Hot Spring Eggs is the ingenuity of the Japanese.
Japan is located at the junction of the Asia-Europe and Pacific plates, where volcanic and seismic activity is frequent and is rich in geothermal resources with hot springs scattered all over the country.
Some of the hot springs maintain a water temperature of around 68-70 degrees Celsius. The locals found they could sit a basket of eggs in the hot springs for around 30-40 minutes to cook them.
The result was a solidified egg yolk with the egg white still very soft and only just cooked. This strange phenomenon, the most wonderful part of which, is the experience on your palate: distinct flavored, waxy egg yolk together with the silky soft white is a real delight and different to soft boiled eggs.
Well, that’s enough babbling, let’s talk about the science of the matter and how you can achieve Hot Spring Eggs at home!
Let’s start with the composition of the egg. The outer most layer is, of course, the eggshell, but after that is a layer of dense film, which is divided into three parts. From the outside to the inside are loose protein, compact protein, and egg yolk.
The loose protein and compact protein appear to be transparent, and when using eggs for baking, etc., you will not distinguish the two. But when cooking the egg alone, the state of these proteins can be seen more clearly.
Different cooking temperatures produce different effects on the three parts of the egg. We intuitively think that the temperature required for egg yolk coagulation is higher than that for the protein because in soft boiled eggs the protein is fully condensed and the egg yolk is still runny.
In fact, the contrary is true. The egg protein requires a higher temperature to coagulate than the egg yolk, and the reason that the yolk in soft boiled eggs does not fully cook is due to a short heating time. The heat did not have time to reach the egg yolk to cook it.
The properties of protein and egg yolk at different temperatures are shown in the following table:
It’s complicated, is it not? More complicated still is that this table lists only the initial state of the egg at those temperatures.
The protein and egg yolk coagulation is a complex and slow process. The protein molecules in the normal state are like a ball of wool and as the temperature increases the molecules slowly connect to each other over time.
The table above shows that the degree of solidification of the egg yolk and egg white increases the longer it is heated at a constant temperature.
From the above table, it’s easy to see that as long as the whole egg is heated to 66-68 degrees Celsius, you can get the same effect as the original Hot Spring Eggs.
Well, hello, we don’t have a hot spring at home! But, hey, don’t worry, I’m going to let you in on the secret of replicating original Hot Spring Eggs at home. Since we don’t have a hot spring handy, we will make one using the common soup pot.
You can find many places online teaching you how to make Hot Spring Eggs, but they are not all reliable. For example, putting the eggs in a bowl covered with plastic wrap and microwaving for 45 seconds will certainly not give you the same results as the traditional method, nor the same as I have in this recipe.
The other example is when you put the eggs in boiling water, cook for a while, turn off, cover and stand for a while in the warm water, is actually Western-style half-cooked eggs or soft boiled eggs.
The outermost egg white cooks completely to a solid state in boiling water in a matter of 30-40 seconds, but it does not have the same texture or taste as Hot Spring Eggs.
In one way, the principle above is correct, but it is difficult to master. For example, bringing the pot of water to the boil, and then adding cold water will cause the temperature to drop to 70-80 degrees Celsius. You then pop the eggs into the water, cover and let them cook at this temperature for half an hour.
The problem with this is that there are a few variables affecting the results such as room temperature, the pot material (aluminum pots have better thermal conductivity than cast iron or stainless steel pots), the color of the pot (dark pots heat better than light colored pots), the amount of water, and how well the lid seals, will all affect the actual water temperature inside the pot.
The result will be inconsistent temperature and will not replicate the stable temperature of a hot spring. The way forward here is to completely eliminate these uncertainties, and as long as you follow my instructions, you can be assured of 100% success in making Hot Spring Eggs.
List of Ingredients:
- Servings: 6
- 6 eggs, average weight 63 g each
- 1.8 L water
- 2 teaspoons Japanese kelp soy sauce
- 1/8 teaspoon Japanese seven flavor chili powder
- 1 teaspoon chopped chives
Step by step to make Onsen Tamago:
You will need the following equipment for this recipe:
20 cm soup pot
The best is a cast iron pot due to its large heat capacity. You could also use a heavy based stainless-steel pot, but do not use a tin wok, aluminum pot or cheap stainless-steel pot. A thin based pot will not heat uniformly on the stove, and it will be more difficult to control the temperature. The thin base will also mean the eggs will sit closer to the heat source.
There are two kinds of kitchen thermometer on the market: the bimetallic dial thermometer and the digital thermocouple thermometer. Bimetallic thermometers have the advantage of not needing a battery, however, the disadvantage is that they are slow to detect the temperature.
Thermocouple thermometers detect the temperature quickly, but the disadvantages are they require a battery, the digital display part is not heat proof and may not be waterproof.
For Hot Spring Eggs, a bimetallic thermometer will suffice as the pot of water heated on low will have a very slow temperature change (rise and fall may take 3-5 minutes), so a bimetallic thermometer will work fine in this situation, and you do not have to worry about waning battery power.
You will need to fill the pot with 1.8 L of water. Why so much water for only 6 eggs? The reason is to increase the heat capacity so that the water temperature rises and falls slowly, making it easier to control the temperature between 69-71 degrees Celsius.
When the eggs enter the water, the water temperature does not drop as much as when they are dropped into a smaller amount of water, and so you can better control a more constant cooking temperature, and thus more accurately simulate the characteristics of a natural hot spring.
The thermometer will usually come with a clip so you can clip it onto the edge of the pot. If you are using a bimetallic thermometer, you must submerge the length of the probe in the water for at least 6 cm, otherwise the temperature reading will be low. Unlike a thermocouple thermometer, you can simply push the tip of the bimetallic thermometer probe into the water.
Heat the water to 70 degrees Celsius and then turn the flame down so that the water temperature remains at 70 degrees Celsius.
After the water temperature is stable, take 6 eggs from the refrigerator and, using a soup ladle, carefully put them into the water and start timing.
Some practices require the eggs at room temperature before placing them in the water, but room temperature fluctuates throughout the year, while the refrigerator always maintains 4 degrees Celsius. Using eggs directly from the refrigerator will ensure the consistency of the egg temperature.
Some people worry that putting cold eggs directly into hot water will crack them; the temperature change has nothing to do with why the eggs crack. The Yang Yang family had their eggs at room temperature and then put them into cold water, and even with a very slow heating time of 100 minutes to 70 degrees Celsius, the eggs still cracked.
In most cases, putting eggs from the fridge directly into hot water will remain intact, which shows that the temperature difference is not directly related to whether or not the eggs will crack.
The eggs should be covered with at least 3 cm of water to ensure that the eggs are heated evenly from all directions. Even with the large amount of water, the water temperature will still fall 2-3 degrees Celsius, so you can increase the flame slightly, to get the water temperature back to 70 degrees Celsius as soon as possible.
During the heating process, you need to constantly check the thermometer reading, and adjust the gas flame to stabilize the temperature at 70 degrees Celsius.
It sounds difficult but is really pretty simple. Because the temperature change is very slow, you will have enough time to adjust it.
If you find your gas stove does not have a low enough flame to maintain the low temperature required, you can use a “simmer mat” to further reduce the stove temperature. The mat is a steel plate with raised parts to rest your pot up and away from the direct heat.
Of course, you can save on gas by turning the flame off when the water temperature reaches 71 degrees Celsius, but you will need to turn it back on when it drops to 69 degrees Celsius and then continue heating.
After 30 – 40 minutes take the Hot Spring Eggs out of the pot and immediately immerse them in cold water. If you don’t do this, the temperature of the eggshell will continue to heat the inside of the egg.
The egg white in Hot Spring Eggs is very weak, so you should not peel the eggs. Just crack them into a small bowl and eat them accompanied by Japanese kelp soy sauce, seven flavor chili powder, and chopped chives.
A sprinkling of salt is also very delicious, or like my husband, pouring on a little soy sauce and sesame oil is also great.
The heating time for Hot Spring Eggs is 30 to 40 minutes, which is quite a wide window of time. However, you can mark the time starting at 27 minutes, depending on how you prefer your eggs. After 27 minutes the egg yolk has sealed, but it is still very soft, and the white is fragile and has only just started to change color.
After 33 minutes, the egg yolk is a little firmer but can be easily broken. This is how I like my Hot Spring Eggs.
Cooked for 39 minutes, the egg yolk has become more flexible, and the egg white’s attachment to the egg yolk is stable. My personal conclusion about this result is that the cooking time was a bit long. Of course, everyone has their own preference, and there is no absolute correct time.
The point here is to learn to get consistent results every time, so you need to make up your mind how you like your eggs at the outset.
Notice in the above picture how the egg yolk is still open and detached from the white at between 27- 36 minutes cooking time. Observe the egg yolk at the 30-minute mark.
Its color is lighter, but this is only a natural difference between eggs and is not due to the heating time. The different colors of eggs heated at the same temperature does not affect the taste. However, if the cooking temperature is increased, the color of the egg yolk will be lighter and the taste drier, and the charm of the Hot Spring Egg is gone.
If you heat the eggs for longer than 40 minutes, you will get that gray-green color on the surface of the white and the familiar eggy-sulfur smell.
How Do You Know the Eggs Are Safe to Eat?
While reading through this recipe, some of you may have been wondering if this low cooking temperature is actually safe. Will there be any bacteria? The US Department of Agriculture says the temperature must exceed 70oC to kill egg-related pathogens such as Salmonella. You can instantly kill the majority of Salmonella bacteria if the temperature is slightly lower, but it will take longer to kill it, as shown in the following table:
Salmonella mostly appears in the egg shell and the packaging, which can be polluted in storage. If the laying hens were infected with Salmonella, it might also appear in the egg (this situation is rare, in the United States one in every 100,000 eggs is infected).
Salmonella inside the eggs will continue to reproduce at room temperature, but when the temperature is reduced to 4 degrees Celsius, Salmonella stops breeding, so when you buy eggs, get the ones that are stored in a refrigerator.
Salmonella on the egg shell surface is killed in 70 degrees Celsius hot water. At 27 minutes (or earlier) cooking time, you can see in the photos above the egg yolk has also been formed, indicating that the temperature has reached 66 degrees Celsius.
Heating for another minute will raise the temperature enough to kill any Salmonella, but to be conservative, 30 minutes is safe *.
* The prerequisite is that the cooking temperature must be 70 degrees Celsius. In Japan, some hot springs temperature is only 65 degrees Celsius, meaning poor sterilization.