Onsen Tamago is a traditional soft-boiled egg that has runny egg white on the outside and almost hard egg yolk inside. Yes, it is the other way around to the standard soft-boiled egg, where the egg white is hard and the yolk is runny.
Onsen Tamago is usually served with a special dashi-based clear sauce.
It is quite fascinating to learn the science behind making a soft-boiled egg such as Onsen Tamago.
Science behind Onsen Tamago
Before I talk about how the name of this boiled egg came about, I need to explain to you how you can make such a boiled egg – hard inside (yolk), soft and runny outside (white).
This is only possible because of the difference in the temperature at which egg white and egg yolk get hardened.
The egg white starts hardening at 58°C/136°F, but it does not harden completely until it reaches to 80°C/176°F. On the other hand, the yolk hardens at around 65-70°C/149-158°F.
So, if you keep the temperature of the water to 65-70°C/149-158°F, the egg yolk will harden while egg white is still runny.
Why is it called Onsen Tamago?
Onsen Tamago (温泉卵) literally means hot spring (onsen, 温泉) egg (tamago, 卵). There are many hot springs in Japan as it is a volcanic island. The temperature of the spring of water that is called ‘onsen’ varies from 25°C/77°F to 98°C/208°F.
Some famous hot spring places such as Nozawa Onsen (where I visit every year) are lucky enough to have the springs that are the perfect temperature for making Onsen Tamago, i.e. around 70°C/158°F.
Village folks come with the bags of fresh eggs and put them in the spring of water to make boiled eggs.
Because the eggs are boiled in onsen, it is called Onsen Tamago. See the photo below where the spring of 70°C/158°F water constantly coming out in Nozawa Onsen. This place is called ‘Ogama’ (麻釜). Only the village people can come in to cook eggs and blanch vegetables.
I posted Steak with Japanese Garlic Steak Sauce last week. This dish was a copycat of the steak lunch from Nozawa Onsen ski resort. Nozawa Onsen is also known for Onsen Tamago and my ryokan of course served it in the traditional way at breakfast. I thought I’d better post it while I still have a fresh memory of this delicious dish.
The Temperature is the Key
Based on the scientific facts, it should be simple to make Onsen Tamago – cook eggs at 70°C/158°F for 30 minutes. Right? But it is a bit tricky to make it at home without a 70°C/158°F spring of water, like Nozawa Onsen.
The most challenging thing is to keep the water temperature consistent while cooking the eggs. And it is not easy unless you have an immersion circulator or something like that. But you can keep the temperature of the water to an average 70C°/158°F while cooking the eggs.
My method is to start at 80°C/176°F hot water in a thick pot and leave the eggs in it with a lid on for 25-30 minutes. The temperature of the water gradually drops and by the time 30 minutes passes, it will average out to the right temperature for Onsen Tamago.
To make about 80°C/176°F hot water, mix 1000ml/4 cups of boiling water (100°C/212°F) and 250ml/1 cup of cold water chilled in the fridge for a couple of hours (4°C/39°F).
My fridge is set to 4°C so it was easy. But if your chilled water is warmer than that, you need to increase the volume of the chilled water. Here are the guidelines for the volume of water required per the temperature of the chilled/cold water to add to 1000ml boiling water. You could even use tap water.
How I Make Onsen Tamago
The steps to cook the eggs in my method are quite simple and require minimal effort, although you need to be accurate.
- Leave 4 eggs out of the fridge for a couple of hours before cooking. This is to prevent the accurately set temperature of the hot water from dropping quickly.
- Leave 250ml/1 cup of water in the fridge (about 4°C/39°F) for a couple of hours.
- Bring 1000ml/4 cups of water to a boil in a thick pot. Do not use a thin aluminium pot since it will release the heat quickly. If you don’t have a thick pot, you need to wrap the pot with towels/newspapers to keep it warm while cooking the eggs.
- Remove the pot from the heat and add the chilled water to it.
- Gently drop the eggs into the pot. I use a ladle to do this.
- Place a lid on and leave it for 25-30 minutes.
- Take the eggs out of the pot and cool them down quickly so that they do not get cooked further.
Note that the method above is suitable for making 4 eggs. If you need to make more or less eggs you need to increase or decrease the cooking time slightly.
Unfortunately, the number of eggs, size of the eggs, type of the pot you use and the air temperature impacts on the speed at which the temperature drops. So it can be trial and error if these factors are quite different from my recipe.
Facts about My Onsen Tamago Making
- I used 2 x 62g eggs and 2 x 53g eggs.
- Room temperature was around 20°C/68°F.
- I used a thick stainless steel pot (18cm in diameter).
- After mixing the boiling water and chilled water, the hot water temperature became 80°C/176°F.
- I cooked the 53g eggs for 25 minutes, and the 62g eggs for 30 minutes.
- After 30 minutes of cooking the eggs, the hot water temperate reduced to 66°C/151°F.
I don’t usually measure the temperature of the hot water like this, but I thought it might give you some ideas about how Onsen Tamago can be made.
Traditional Way to Serve Onsen Tamago
It is one of the most popular egg dishes served in the morning at ryokan (traditional Japanese inns). It is usually served with a dashi-based clear sauce.
The clear sauce is made of dashi stock, cooking sake, mirin, light soy sauce and a pinch of salt. It is served at room temperature or chilled depending on the temperature of the Onsen Tamago.
Crack open the egg shell and drop the egg into a small bowl. Gently pour about 2 tablespoons of the sauce around the egg white. Eat with a spoon.
Instead of eating Onsen Tamago in the above-mentioned traditional way, you can serve it as a topping too. For example, place it on top of Gyū-don (Japanese Beef Bowl), Pork Shabu-shabu Salad or even on Japanese Curry (Katsu Curry, Vegetarian Curry).
Onsen Tamago is a traditional soft-boiled egg that has runny egg white on the outside and almost hard egg yolk inside. Yes, it is the other way around to a standard soft-boiled egg where the egg white is hard and the yolk is runny. Eat Onsen Tamago in the traditional way with a special dashi-based sauce.
Prep Time includes the time to chill water in the fridge as well as taking the eggs from the fridge to room temperature.
It takes a long time to make Onsen Tamago but you are really not doing anything for most of the 2.5 hours!
Don't forget to see the section 'MEAL IDEAS' below the recipe card! It gives you a list of dishes that I have already posted and this recipe that can make up a complete meal. I hope it is of help to you.
- 4 eggs at room temperature (note 1)
- 1000ml/4 cups water
- 250ml/1cup water chilled to 4°C/40F° (note 2)
- 4 tiny mizuna leaves or any other green leaves
Put all the Sauce ingredients in a saucepan and bring it to a boil.
Turn the heat off and cool the sauce down to room temperature.
Put 1000ml/4 cups water in a thick pot and bring it to a boil. Remove the pot from the heat.
Add the 250ml/1 cup chilled water to the pot.
Using a ladle, gently place each egg in the pot.
Place a lid on and leave for 25-30 minutes (note 5).
Transfer the eggs to a large bowl containing cold water to cool them down quickly and stop further cooking.
Crack open the egg, in the same way as cracking a fresh egg, into a small serving bowl. The egg should slide into the bowl easily because the egg white is runny.
Gently pour two tablespoons of the Sauce around the egg white and place a leaf on top if using. Serve with a spoon.
1. The time required to cook Onsen Tamago varies depending on the size of the egg as well. I used 2 x 62g and 2 x 53g.
2. Leave the water in the fridge for the minimum of 1.5 hours. If you are using tap water, which is not as cold as the chilled water, the quantity of tap water needs to be adjusted. See the table below as a guide.
3. If you are using dashi powder that contains salt, no need to add a pinch of salt.
4. Onsen Tamago is often served with no topping. But I added it for two reasons – to add colour to the dish, and to be able to focus on the centre of the white egg when taking a photo of the dish (Otherwise my camera couldn't focus. It’s true!).
5. If your eggs are just over 50-55g, cook for about 25 minutes. If your eggs are over 60-65g, cook for about 30 minutes.
I have not tested 70g+ large eggs. You might think that extending the cooking time by extra 5 minutes should work. But I am not sure about this. Since the temperature of the hot water after 30 minutes becomes 66°C/151°F, leaving the eggs in the water longer than 30 minutes may not cook the egg yolk further. In this case, you may need to start at a higher temperature.
6. Nutrition per serving.
serving: 80g calories: 83kcal fat: 5g (8%) saturated fat: 1.6g (8%) trans fat: 0.0g polyunsaturated fat: 1g monounsaturated fat: 1.9g cholesterol: 186mg (62%) sodium: 233mg (10%) potassium: 111mg (3%) carbohydrates: 1.4g (0%) dietary fibre: 0g (0%) sugar: 1g protein: 7g vitamin a: 5% vitamin c: 0% calcium: 2.3% iron: 5%