Onsen Tamago (温泉卵)
Onsen tamago (onsen eggs) are a type of soft boiled egg, or chicken eggs with the yoke soft and the white only half done.
There are also soft boiled eggs called 'ondo tamago.'
Also, eggs which have been steamed in the steam of hot springs (onsen) or boiled in the hot water are called onsen tamago regardless of whether they are hard or soft.
Outline
In contrast to the usual soft boiled egg, the characteristic of the onsen tamago is that the egg white is softer than the egg yoke. This is because they are cooked in water that is between 65 - 68℃ for 30 minutes, to take advantage of the fact that the temperature at which the yoke hardens (70℃ is lower than the temperature at which the egg white hardens 80℃).
If the water temperature of water gushing from a hot spring is close to this range, the onsen eggs can be made just by placing eggs in the spring water, so they are often served at hotels in hot spring (onsen) resorts and this is thought to be the origin of the term onsen egg.
They are often cracked and put in a bowl with dashi (stock) and soya sauce.
They are also used as a topping for noodle dishes and donburi dishes (fish or meat and vegetables served on rice).
They are far more digestible than eggs cooked in any other way.
Specialized 'Onsen egg cookers' are available however they can be made by using a container that retains heat such as a polystyrene container (see 'How to make onsen eggs at home'). They can also be made without using water, by using a rice cooker on the warm setting.
The onsen eggs on sale at some restaurants (such as Yoshinoya) under the name soft boiled egg are products called 'Cupid's eggs' manufactured by Q.P.Corporation.
Like soft-boiled eggs, onsen eggs may be a source of food poisoning if the eggs are not fresh.
Onsen eggs in hot spring resorts
Chicken eggs boiled in hot spring water or steamed in the hot spring steam are called onsen eggs regardless of whether they are soft or hard boiled. At many hot spring resorts, shops demonstrate the cooking of onsen eggs in hot spring steam or water and the onsen eggs are sold as specialties. If the onsen has a high level of sodium chloride the eggs will often taste salty.
Radium eggs
The onsen eggs of Iizaka hot springs in Fukushima City, Fukushima Prefecture are called radium eggs as it is the first onsen in Japan at which the presence of radium was detected in the water.
Black eggs
The specialty eggs of Owakudani hot springs in Hakone and Goshogake hot springs are black. This is because the iron in the hot springs turns the egg shells black.
It is only the shells that are black, and the inside of the egg is the same as a normal boiled egg.
Arayu onsen eggs
The source of the hot springs of Yumura hot springs (Hyogo Prefecture) is the hottest spring water in Japan (98℃) and it does not need to be heated up. The onsen eggs can be cooked in 10 minutes. Most people who visit put a raw egg in the Arayu spring water.
Unzen-jigoku eggs
At Unzen hot springs, eggs are available which have a slightly sulfurous smell and have been cooked in a steamer using steam from Jigokudani valley. The salt added is not the usual salt but salt made from the hot springs. Most people drink Ramune lemonade (soft drink) when eating the eggs.
Onsen eggs of Beppu Jigoku Meguri (Beppu Hell Tour)
Each of the Jigoku spring areas on the Beppu Jigoku Meguri (Beppu Hell Tour) of Beppu hot springs has specialty onsen eggs which are steamed or boiled in the hot spring water. Apart from jigoku steamed eggs which are steamed in a stone steamer which utilizes steam from a hot spring, at Umi jigoku hot springs on the Beppu Jigoku (Hell) Tour, the specialty is jigoku boiled eggs which are eggs that have been put in bamboo baskets and put directly into the 98℃ cobalt blue hot springs.
How to make onsen eggs at home
Put an egg into a polystyrene container (such as a cup noodle container) that will retain heat and add hot water. Put in just enough hot water to cover the egg. Put a lid on and leave it for 30 to 40 minutes. If you suddenly cover an egg which has just been taken out of the refrigerator with hot water it will crack, so it is a good idea to warm up the egg a little first in warm water before putting in the hot water. Furthermore, if you boil an egg for 20 to 25 minutes at between 70 to 75 degrees, you can make an onsen egg without the necessity for a container, however it is necessary to have a way of maintaining the boiling and temperature.
Also, it is possible to make an onsen egg using a paper drip type coffee machine. This is a simple way as you just put a raw egg into the coffee pot and turn it on without adding the filter paper or coffee beans. Even with this method, if the egg is cold it will crack so it is a good idea to let it return to room temperature.
Other
Yoshinoya added onsen eggs to its menu in 2003 under the name soft boiled egg.