What are onsen eggs? Explaining the difference between raw eggs

How to Cook Perfect Soft-Boiled Eggs at Home”

Following the publication of a previous article about raw eggs, we received a tremendous amount of access, indicating a high level of interest in egg dishes.
While raw egg dishes, such as Tamago Kake Gohan (TKG, rice mixed with raw egg) and others, are enjoyed in Japan, eating raw eggs is generally considered risky in many other countries.

So this time, we will focus on “Onsen Tamago”, a familiar topping at beef bowl restaurants and curry shops, also known as “Ontama”.
How is that unique texture created?
We’ll introduce a simple recipe you can make at home, and delve deeply into the most pressing question: “If Onsen Tamago is made with eggs from overseas, are Salmonella bacteria killed?” from the perspectives of food safety and science.

Why is Onsen Tamago Special?

What is Onsen Tamago?

“Onsen Tamago”, often abbreviated as “Ontama”, is a type of boiled egg whose name originates from the fact that it was traditionally made using hot springs or their steam.
Its biggest characteristic is that the yolk solidifies into a creamy, semi-liquid state, while the egg white doesn’t completely solidify, remaining a very soft, wobbly, jelly-like consistency.
This finish is clearly different from a typical “soft-boiled egg” where the white is firm and the yolk is semi-liquid, or a “poached egg” which is cooked out of its shell.

This exquisite texture enriches the flavor of dishes when used as a topping for rice bowls (like Donburi or Curry), noodles, or salads.

The Science Behind the Melty Texture: A Tale of Two Proteins

That unique texture of Onsen Tamago is, so to speak, a “gift of culinary science,” utilizing the property that the proteins contained in the egg white and yolk solidify (heat coagulate) at different temperatures.

• Egg White Coagulation
  The egg white begins to solidify gradually at around 57°C (135°F), becoming a white, soft jelly between 60°C and 65°C (140-149°F).
  It fully solidifies between 70°C and 80°C (158-176°F).
• Egg Yolk Coagulation
  The yolk, on the other hand, begins to solidify at a slightly higher temperature than the white, starting around 65°C (149°F).
  It solidifies firmly between 68°C and 70°C (154-158°F). It is said to fully solidify at 75°C (167°F) or above.

The ‘Sweet Spot’ for Onsen Tamago

Onsen Tamago is made by cleverly utilizing this difference in coagulation temperatures, maintaining the hot water temperature at around 65°C to 70°C (149-158°F), where the yolk solidifies but the white doesn’t solidify too much.
In this temperature range, the yolk approaches a thick, custard-like state, while the white is not yet fully solidified, resulting in that uniquely loose consistency.
This delicate temperature control is the key to creating the distinctive texture of Onsen Tamago.

Understanding this difference in coagulation temperatures is not only crucial for following a recipe, but also for understanding why Onsen Tamago has such a texture and the meaning of heating temperatures in the context of Salmonella countermeasures, which will be discussed later.

Egg Protein Coagulation Temperatures

The Mystery of Raw Eggs: The Difference Between Japan and Overseas

Why are Raw Eggs a Staple in Japan?: A Culture of Safety and Trust

In Japan, a food culture of eating raw eggs, typified by Tamago Kake Gohan (TKG), is deeply rooted.
This is possible because Japanese eggs are produced and distributed “with the premise of being eaten raw,” and extremely high hygiene management standards are established for this purpose.

• Thorough Management at the Farm Level
  • Health management of chickens, implementation of vaccination programs.
  • Strict biosecurity measures are implemented, such as restricting access to outsiders, preventing the entry of rodents and wild birds, and regular cleaning and disinfection of poultry houses.
  • Hygiene management incorporating the concept of HACCP (Hazard Analysis and Critical Control Points) is also progressing.
• Processing at GP Centers (Egg Grading and Packing Facilities)
  • Eggs are sometimes transported from farms to GP centers by conveyor to minimize human contact.
  • Washing and Sterilization
    The surface of the eggshell is washed to remove dirt using brushes, and then sterilized with warm water or sodium hypochlorite solution (also used for sterilizing tap water and vegetables).
  • Drying and Re-sterilization
    Eggs are dried promptly and then the eggshell surface is sterilized again using ultraviolet light.
  • Systems to automatically detect and remove cracked eggs are also introduced.
• Management at Distribution and Retail Stages
  • Refrigerated storage at 10°C (50°F) or below is the standard for shell eggs.
  • A “best before date for raw consumption” is clearly indicated, which is generally about 21 days from the laying date (including 7 days of refrigeration at home). Eggs past this date are recommended to be thoroughly cooked.

In this way, the safety of Japanese raw eggs is supported by a multi-layered management system at each stage from farm to table.
This is not simply because “Japanese chickens are special,” but rather a result of the thorough hygiene management system, and the trust in this system makes the Japanese raw egg culture possible.

The Risk of Salmonella: Why Raw Eggs are Considered Risky Overseas

Outside of Japan, eating raw eggs is generally not recommended.
The main reason for this is the risk of contamination by Salmonella bacteria, particularly Salmonella Enteritidis, which can cause food poisoning.

Routes of Salmonella Contamination:

• Internal contamination (transovarian infection)
  This occurs when Salmonella bacteria infect the chicken’s ovaries or oviduct, and the yolk or white becomes contaminated during egg formation.
  In this case, it is difficult to remove the bacteria even by washing the eggshell.
• Eggshell surface contamination
  As eggs are laid through the chicken’s cloaca (the same opening for both eggs and feces), fecal-derived Salmonella bacteria may adhere to the eggshell during this process.
  Contamination from the environment after laying is also possible.
  Since the eggshell is porous, bacteria are more likely to invade the interior, especially when temperature changes cause moisture to condense on the eggshell surface (a state referred to as “sweating”).

Differences in Egg Handling Overseas:

• In many countries, a comprehensive egg management system specifically for raw consumption is not legally mandated to the same extent as in Japan.
• Egg washing methods vary by country.
  In the U.S., eggs are typically washed and sanitized, and then refrigeration is mandatory.
  In contrast, in parts of Europe, washing was sometimes not recommended, as it was thought to damage the cuticle layer (a natural protective coating) on the eggshell surface, potentially making it easier for bacteria to enter.
  However, in recent years, measures such as mandatory vaccination of chickens and setting target levels for Salmonella contamination rates have progressed across the EU.
• Some producers vaccinate their chickens against Salmonella, such as the “Blije Kip” brand in the Netherlands.
  In the UK, since 2017, eggs bearing the “British Lion Mark” are produced under strict control, including vaccination, and are considered safe for raw consumption even by high-risk groups such as pregnant women and the elderly.
• Regional differences in contamination rates
  The Salmonella contamination rate of eggs varies greatly by country and region.
  Some countries have very low rates, while others report relatively high contamination rates ranging from a few percent to over ten percent.
  This is thought to reflect differences in production environments and hygiene management levels.
  In other words, the risk is not uniformly the same for all eggs overseas; the potential risk varies depending on where and how the eggs were produced.
• General International Advisory
  Food safety agencies in many countries recommend that eggs be thoroughly cooked until both the yolk and white are firm, especially for individuals at higher risk of food poisoning, such as infants, the elderly, pregnant women, and those with weakened immune systems.

While not all overseas eggs are dangerous, the same level of raw consumption safety as Japanese eggs cannot always be guaranteed.
Therefore, it is generally wise to assume that overseas eggs without specific safety labeling should be cooked thoroughly.

Easy Recipe for Making Onsen Tamago at Home

To make perfect Onsen Tamago, some adjustments may be necessary depending on the size of the eggs, their initial temperature, and the cooking equipment used.
For consistent results, it’s recommended to use eggs straight from the refrigerator, as their initial temperature is stable.

Classic! Method Using a Pot (Stovetop)

1. Pour water (e.g., 1 liter) into a pot and heat it to a boil.
2. After turning off the heat, add cold tap water (e.g., 200ml or 1 cup).
   This will lower the water temperature to approximately 65°C to 70°C (149-158°F), suitable for Onsen Tamago.
   (During times when tap water temperature is high, such as in summer (20°C/68°F or above), it’s a good idea to add a little ice to the water you add to cool it down to around 15°C/59°F before use).
3. Gently place the eggs, taken straight from the refrigerator, into the hot water.
4. Make sure the eggs are completely submerged in the water.
5. Cover the pot with a lid and let it sit as is.
6. The sitting time needs to be adjusted depending on the season and the number of eggs.
   • Guideline: In summer, 8–10 minutes; in winter, 12–13 minutes.
   • Alternatively, after putting the eggs into 65°C to 70°C (149-158°F) water, cover and let sit for 12–15 minutes.
   • Another method is to put the eggs into boiling water, immediately turn off the heat, cover, and let sit for 10 minutes.
7. After the time has elapsed, immediately transfer the eggs to ice water to rapidly cool them and prevent them from overcooking from residual heat.

The Core Question: Is Onsen Tamago Safe When Made with Overseas Eggs?

What are the Conditions for Killing Salmonella Bacteria?

The biggest concern when making Onsen Tamago with overseas eggs is, naturally, Salmonella bacteria.
To kill Salmonella bacteria, both temperature and time are important. It’s not enough to simply reach a certain temperature; that temperature must be maintained for a sufficient duration.
Here are the main temperature and time guidelines for inactivating (killing) Salmonella bacteria.

• Pasteurization Standards
  • According to information from the U.S. Department of Agriculture (USDA) and the Japan Poultry Association, Salmonella bacteria are said to be killed by heating shell eggs at 60°C (140°F) for 3–5 minutes.
  • According to the standards of the Japanese Ministry of Health, Labour and Welfare, for liquid whole eggs (pasteurized liquid eggs), heating at 60°C (140°F) for 3.5 minutes is indicated.
• Killing at Higher Temperatures for Shorter Times
  • As a general measure for preventing food poisoning, heating the core temperature of food to 70°C (158°F) for 1 minute, or 75°C (167°F) for 1 minute, is often recommended (especially for chicken, etc.).
  • Some views suggest that even for making soft-boiled eggs, heating to a core temperature of 65°C (149°F) for 3–5 minutes provides sufficient bactericidal effect.
• D-value (A More Scientific Indicator)
  • The D-value is the time required to reduce the number of microorganisms by 90% (1/10) at a specific temperature.
  • For example, the D-value for Salmonella is reported to be approximately 2.0–2.2 minutes at 60°C (140°F), about 0.5–0.6 minutes at 63°C (145.4°F), and about 0.2–0.3 minutes at 65°C (149°F).
  • As a food safety goal, to significantly reduce the number of microorganisms (e.g., by 99.999% or more), heating must be maintained for several times the D-value.

The killing of Salmonella bacteria is a gradual process, and longer heating times are required at lower temperatures. What is important is that the core of the egg yolk also meets these killing conditions (temperature and time).

Ministry of Health, Labor and Welfare: Prevention of Salmonella food poisoning caused by eggs

Approximate Temperatures and Times for Salmonella Inactivation (Killing)

Onsen Tamago Cooking Conditions vs. Salmonella Killing Conditions

Typical Onsen Tamago is cooked by immersing eggs in 65°C to 70°C (149-158°F) water for about 8–10 minutes (in summer) up to 20–40 minutes for longer methods.
What’s important here is that the water temperature doesn’t directly translate to the internal temperature of the egg, especially the core temperature of the yolk.
It takes time for heat to pass through the eggshell and white to reach the center of the yolk.

From the perspective of D-value, even if the core temperature of the yolk reaches 65°C (149°F), to significantly reduce Salmonella bacteria, that temperature needs to be maintained for 7 times the D-value (approx. 0.2–0.3 minutes), which is 1.4–2.1 minutes.
In typical home Onsen Tamago cooking methods, especially the pot method where the water gradually cools down, there is doubt whether the center of the yolk reliably meets these conditions.
A 1997 study also reported that Salmonella Enteritidis in liquid whole egg survived for 6 minutes at 55°C (131°F) and survived for less than 30 seconds even at 70°C (158°F), suggesting the importance of ensuring that the egg contents reach the killing temperature.

So, in the end, is Onsen Tamago made with overseas eggs safe?

To reach a conclusion, the most plausible answer seems to be, “Potentially possible, but with significant caveats and uncertainties.”

If the Onsen Tamago cooking method ensures that the entire yolk reliably reaches 65°C (149°F) or above, and that temperature is maintained for a sufficient time (e.g., 3–5 minutes or more, ideally a time that achieves multiple log reductions based on D-value), then it is possible to significantly reduce Salmonella bacteria.
For example, a method using equipment capable of precise temperature control, such as a yogurt maker or sous vide cooker, to heat at 65°C (149°F) for 30 minutes may be more reliable than the typical pot method where the water cools down.

However, with standard home recipes (especially the pot method), there is no guarantee that Salmonella bacteria will be reliably and highly inactivated, for the following reasons:

• Variability in initial bacterial load of overseas eggs.
  If the initial bacterial count is high, more effective killing is required.
• Difficulty in accurately measuring the internal temperature of the egg at home.
• “Almost killed” is not the same as “all killed.”
  This difference can be problematic, especially for individuals with weakened immune systems.

Therefore, unfortunately, when making Onsen Tamago using eggs commonly distributed in Europe and America that do not have specific safety measures indicated, relying solely on the cooking process to ensure safety is considered high-risk.
While heating Onsen Tamago may reduce Salmonella bacteria to some extent, it is unlikely to provide the same level of reassurance as thorough heating at 75°C (167°F) for 1 minute, or using eggs with known safety measures like pasteurized eggs or those with the British Lion Mark.

The core of this issue is the uncertainty that exists between the mild heating conditions of Onsen Tamago and the potential Salmonella risk of overseas eggs.
It is difficult to definitively state “it is safe” or “it is dangerous” across the board, and cautious judgment based on the situation is required.

How to Enjoy Eggs Safely, Wherever You Are

What are general safety measures when handling eggs?

• Purchase eggs from a reliable source and check that the shells are clean and free of cracks.
• Refrigerate promptly after purchase (at 10°C/50°F or below; in the U.S., 7.2°C/45°F or below). Avoid leaving refrigerated eggs at room temperature for more than 2 hours.
• After handling raw eggs, wash your hands, utensils, and work surfaces thoroughly with soap and hot water.
• If there is any doubt about safety, cook thoroughly until both the yolk and white are firm.
• Eat cooked egg dishes promptly or store them in the refrigerator.

When in Doubt, Cook Thoroughly!

If you have any concerns about egg safety, or if the eggs are for individuals at higher risk of food poisoning, such as infants, the elderly, pregnant women, or those with weakened immune systems, the safest method is to heat until both the yolk and white are completely firm (to a core temperature of 75°C/167°F for at least 1 minute).

By understanding these points, it becomes possible to pursue the deliciousness of Onsen Tamago while managing risks, even when using overseas eggs.

Enjoying Onsen Tamago (Hot Spring Egg) Safely and Deliciously

The uniquely melty texture of Onsen Tamago (Hot Spring Egg) is a delicate culinary art based on the scientific principle that the proteins in the egg white and yolk solidify at different temperatures.

In Japan, eggs produced under a strict hygiene management system are distributed, so raw consumption or gentle heating like Onsen Tamago can be enjoyed with relative peace of mind.
However, with regards to overseas eggs, a more cautious approach is necessary, considering the risk of Salmonella bacteria.

While traditional Onsen Tamago cooking methods can indeed provide some bactericidal effect, there is no guarantee of completely killing Salmonella bacteria, given the variability in initial contamination levels of overseas eggs and the difficulty of temperature control in home cooking.

That being said, there is no need to give up on Onsen Tamago overseas.

• Understand the potential risks of overseas eggs.
• If possible, use pasteurized eggs.
• Utilize sous vide cookers or similar equipment to pasteurize eggs at home before making Onsen Tamago.
• Always prioritize food safety and make safer choices, especially for high-risk individuals.

We hope that with this knowledge and these measures, you can enjoy your culinary exploration wisely and deliciously.

Tags