Onsen eggs are a Japanese egg dish with a tender, wobbly white and a rich, jammy yolk. Unlike a regular soft-boiled egg, where the white is firm and the yolk is runny, an onsen egg sets almost the other way around. This difference is not some mysterious power of onsen water, but simple cooking science: egg white and yolk set at different temperatures.

In short, onsen eggs can be made by leaving eggs in water at about 65-70°C for a fairly long time (20-30 minutes as a guide). The yolk starts to set at about 65°C, while the white does not fully set until a higher 70-80°C or so. By holding the eggs in the range between the two, the yolk cooks while the white stops short of setting completely. It does not have to be onsen water; all you need is to keep the temperature moderately low rather than setting the egg quickly at high heat.

This article explains how onsen eggs work from the standpoint of temperature, along with how to enjoy them in onsen towns, how to make them at home, and the food safety points to watch because they are cooked at a low temperature. For what to eat and drink after an onsen or sauna as a whole, see Meals and Hydration After Onsen and Sauna.

Onsen Eggs vs. Soft-Boiled Eggs and Poached Eggs

Onsen eggs, soft-boiled eggs and poached eggs all have soft yolks, but they set differently and are cooked differently. Here is how they compare.

What truly sets an onsen egg apart is that the white is softer than the yolk. Soft-boiled eggs cooked in boiling water and poached eggs cook quickly from the outer white inward, so the white sets; an onsen egg never reaches the temperature at which the white fully sets, so the white stays soft.

Why Do the White and Yolk Set the Other Way Around?

Eggs set when their proteins are changed by heat. The key point is that the white and yolk set at different temperatures.

Egg white begins to set at around 60°C and is said to set fully at about 70-80°C. The first protein to change is ovotransferrin, whose heat-denaturation temperature is reported to be about 61°C. Egg yolk, on the other hand, begins to set at about 65°C and becomes thick and jammy at roughly 65-70°C. The Chemical Society of Japan also explains that the yolk sets when the egg is held in water at 65-70°C for a long time, while the white does not fully set.

That temperature gap is the heart of the onsen egg. By keeping eggs for a long time at about 65-70°C, a range where the yolk sets but the white does not fully set, the yolk cooks while the white stays soft. A study in the Journal of Cookery Science of Japan, “The Effects of Heating Temperature and Holding Time on the Coagulation State of Onsen Eggs,” also examines how the combination of heating temperature and holding time shapes the result. A soft-boiled egg cooked briefly in boiling (100°C) water sets white first, while an onsen egg sets yolk first. That reversal is what creates its unique texture.

Enjoying Onsen Eggs in Onsen Towns, and the Traditional Way of Cooking Them in the Source

Onsen eggs are a local specialty in onsen towns because the water rising from underground often falls within just the right temperature range for making them. We explain where the heat of the source comes from in How Onsen Water Rises to the Surface, but in short, egg making took root in onsen towns because water at a suitable temperature is naturally right there.

In onsen towns, you can buy ready-made onsen eggs at shops and eateries, and there are also spots where you can lower the eggs into the water and make them yourself. However, water fresh from the source can exceed 90°C, so it is not always 65-70°C. In the traditional method of placing eggs directly in the source, water that is too hot sets even the white, and water that is too cool leaves the yolk uncooked, so each onsen town adjusts the water temperature and soaking time by experience. The big difference from making them at home in a pot, where you control the temperature, is whether you can choose the water temperature at all.

At places where you can make them yourself, watch out for burns, steam and crowds. With children especially, do not get too close to the hot water or steam. At shared cooking spots, it is good manners to follow the posted guidance on time and hygiene. Check in advance whether there is a place where you can try it. If you want to work onsen eggs from an onsen town into your trip, you can find places to visit in our list of places.

How to Make Them at Home: Keeping the Temperature Matters More Than Lowering It

You can also make onsen eggs at home. The key is not onsen water, but holding the water steadily at about 65-70°C. There are three main methods.

With a pot, bring water to a boil, take it off the heat, add cold water to lower the temperature, then put in the eggs, cover, and leave for 20-30 minutes. The more water there is, the more slowly the temperature drops and the steadier it stays, so use plenty of water for the number of eggs. With insulated cooking (a thermal pot or a styrofoam box), the water cools less from the outside air, so the temperature is easier to hold. The most reliable method is a sous vide cooker, which lets you set a constant temperature and rarely fails.

Whichever method you use, the hard part is not lowering the temperature but holding it. If the temperature rises too high along the way, the white sets as well and the result is closer to an ordinary boiled egg; if it is too low, the yolk does not cook. Eggs straight from the refrigerator give uneven results, so bring them to room temperature first for more consistent eggs.

Food Safety for a Low-Temperature Dish

Because onsen eggs are made at a relatively low temperature of about 65-70°C, it is worth knowing the basics of food safety. There is no need to be overly afraid, but there are facts you should know.

A well-known food-poisoning bacterium associated with eggs is Salmonella. Salmonella is said to be killed when the center is heated at 75°C for 1 minute or more, but onsen eggs are made at lower temperatures, so the safe approach is to treat them much like raw eggs. In practice, the basics are enough to lower the risk considerably: use fresh eggs sold for eating raw and within their best-before date, choose eggs with no cracks or dirt on the shell, eat them soon after making them (refrigerate if you keep them), and do not leave them at room temperature for long. Also note that eggs are meant to be kept refrigerated at 10°C or below, and the best-before date is set as the period during which they can be eaten raw.

Also, there is no basis for claiming that onsen eggs have health benefits. Onsen eggs are simply part of the food culture of Japan’s onsen towns, not a dish to eat in hope of recovery or beauty benefits.

FAQ

How are onsen eggs different from soft-boiled eggs?

They set the other way around. A soft-boiled egg has a firm white and a runny yolk, while an onsen egg has a soft white and a jammy, set yolk. They are also made differently: a soft-boiled egg is cooked briefly in boiling water, while an onsen egg takes 20-30 minutes in water at about 65-70°C.

At what temperature are onsen eggs made?

About 65-70°C is the guide. The yolk begins to set at about 65°C and the white sets fully at about 70-80°C, so holding the eggs between the two for a long time sets the yolk while the white stays half-set.

Can I make them at home without an onsen?

Yes. What you need is not onsen water, but water held steadily at about 65-70°C. You can mix boiling and cold water in a pot, use an insulated container, or use a sous vide cooker; the sous vide cooker is the least likely to fail.

Are onsen eggs safe to eat?

If you use fresh eggs sold for eating raw, within their best-before date, and eat them soon after making them, there is little problem. However, because they are cooked at a low temperature, treat them much like raw eggs and do not leave them at room temperature for long.

Do the minerals in onsen water change the taste?

The minerals in onsen water do not greatly change the egg itself. The result depends mainly on water temperature and time, and differences in taste come largely from how each region serves them, for example with soy sauce or dashi.

Summary

An onsen egg uses the gap between the setting temperatures of the yolk (which begins to set at about 65°C) and the white (which sets fully at about 70-80°C), and is made by leaving eggs in water at about 65-70°C for 20-30 minutes. Its signature is a soft white and a jammy yolk, the reverse of a soft-boiled egg. In onsen towns you can eat them as a local specialty or make them yourself, and you can recreate them at home if you keep the temperature steady. Because they are cooked at a low temperature, follow the basics of using fresh eggs sold for eating raw and eating them soon, and you can enjoy this food tradition of Japan’s onsen towns with peace of mind.

Sources

• The Chemical Society of Japan: Hard-Boiled Eggs, Soft-Boiled Eggs, and Onsen Eggs — Which Do You Prefer?
• Journal of Cookery Science of Japan: The Effects of Heating Temperature and Holding Time on the Coagulation State of Onsen Eggs — How to Make Your Ideal Onsen Egg
• Ministry of Health, Labour and Welfare: Preventing Salmonella Food Poisoning from Eggs