Why Can Curry Left Overnight Cause Food Poisoning?
― Bacteria That Survive Boiling Wake Up as the Pot Cools
"I boiled it hard, so the bacteria must be dead." That is a natural thing to think. But some bacteria in stews survive by hiding inside a heat-resistant shell. And a big pot spends many hours slowly passing through exactly the temperatures those bacteria like best.
At the weekend, you make a big pot of curry. You eat about half for dinner, then turn off the heat and leave the rest on the stove with the lid on.
The next day at lunchtime, you heat it up again. It smells and looks just as it always does.
But in the evening, several people in the family get stomach pains and diarrhoea. "I reheated it properly, so why?" The answer lies in the temperature and the air inside the pot.
There are only two reasons
The most common culprit is a bacterium called Clostridium perfringens (Welch's bacillus). When conditions turn harsh, it puts on a hard shell and goes dormant. In this form it can withstand the heat of cooking. It also likes places with little air.
A large quantity of stew takes a very long time to cool down. It stays for hours at the lukewarm temperatures where bacteria multiply fastest. If it sits at room temperature, it never gets out of that range.
In other words, "I boiled it, so it's safe" and "it's fine to leave overnight" are two separate questions. Let's take them one at a time.
Even after boiling, shell-dwelling bacteria survive
Clostridium perfringens is found in very ordinary places: soil, water, and the guts of people and animals. A small number are normally present on meat and vegetables too.
What is strange about this bacterium is that it has two forms. In its everyday, active form it is weak against heat, and cooking kills almost all of it. But when conditions become unpleasant, it builds a thick shell inside itself and shuts itself in. This shelled form is called a "spore". A spore is like a seed: it neither grows nor feeds, but it is very resistant to heat and drying. Some spores are said to survive even boiling at 100°C.
This bacterium also dislikes oxygen. While a stew simmers, bubbles rise steadily from the pot, and they drive out the air that was dissolved in the liquid. Curry and stew are thick, so after the heat is off, air is slow to soak in from the surface. Look at Figure 1. The bottom of the pot becomes a perfect place for bacteria that dislike oxygen.
A big pot stays lukewarm for hours
As the temperature falls, spores break out of their shells and return to their active form. That is where the trouble starts. Clostridium perfringens multiplies especially fast at temperatures in the 40s °C. Under the right conditions, its numbers are said to double about every 10 minutes. That is far faster than most other food-poisoning bacteria.
Meanwhile, a potful of stew cools down surprisingly slowly. A watery dish stores a great deal of heat. And heat can escape only through the surface and sides of the pot. Double the amount of food, and the exits for heat do not double. So the bigger the pot, the slower it cools.
Figure 2 gives a picture of how the temperature falls. A big pot left at room temperature spends hours in the range where bacteria multiply easily. The room temperature itself lies inside this range, so even by morning the pot has not left it. If the food is divided into small portions and cooled quickly, the time spent in the range is much shorter.
Why isn't reheating enough?
Most of the active bacteria that have multiplied are weakened by thorough reheating. But in a thick stew, heat struggles to reach the middle. The surface may boil while the bottom and centre stay lukewarm. And spores survive reheating. So the idea of "let them multiply overnight, then wipe the slate clean by reheating" cannot be relied on. What matters is not letting them multiply in the first place.
Even when this bacterium has multiplied, the smell, appearance and taste are said to change very little. "It smelled fine" is no basis for judging.
There are well-known cases where many people fell ill at the same time after meals made in bulk, such as school lunches, catered boxed meals and community cook-outs. The reason is the very mechanism in this article: the more food there is, the more slowly the pot cools.
So what should we do?
- Divide leftovers into shallow containersThe thinner you spread it, the faster it cools
- Once cooled, put it straight in the fridgeDon't leave it in the pot on the stove overnight
- Before eating, stir from the bottom and heat it thoroughlyEven if the surface boils, the inside may still be lukewarm
To cool food quickly, divide it into shallow containers, or stand the pot in a bowl of water or ice water. Put it somewhere cool with good airflow and stir now and then to cool it faster. Once the worst of the heat is gone, put it in the fridge.
With Clostridium perfringens food poisoning, stomach pain and diarrhoea appear about half a day after eating, and most people recover in a day or two. However, small children and older people become dehydrated easily, so take care. If someone cannot keep fluids down, is limp, or has bloody stools, do not wait and see on your own judgement: go to a medical facility. If they are not fully conscious or respond slowly when spoken to, call the emergency number (119 in Japan). When cleaning up vomit or stools, wash your hands thoroughly before leaving the spot.
Summary
Food poisoning from curry left overnight does not happen because the stew was under-cooked. It happens because shell-dwelling bacteria that survive cooking wake up and multiply at the air-poor bottom of the pot while it cools slowly. The key to preventing it lies less in the heating than in how you cool.
Simmering is not a job that wipes out every bacterium.
The danger is the lukewarm hours that follow.
We also covered how bacteria change food in What's the difference between fermentation and spoilage?. For heating that 100°C cannot achieve, see Why does rice cook badly on Mt. Fuji, yet a pressure cooker cooks fast?. For an example of food where bacteria cannot grow, try Why doesn't honey go bad?. And for a case where the food itself is toxic, and cooking doesn't remove the poison, there is Why shouldn't you eat potato sprouts or green skin?.
- Pour the same amount of water at the same temperature into a deep pot and a shallow dish. Water at around 50°C, not too hot, is enough.
- With a cooking thermometer, measure and record both temperatures every 10 minutes. In the deep pot, measure at about the middle depth.
- After 1 hour, compare the two temperatures. Although the amount is the same, you will see that the shallow one has cooled much faster.
* Please don't try this by leaving real food at room temperature. Hot water alone is enough to show the difference in cooling.
For those who want to know more ― terms, formulas and links to textbooksEach part is labelled with its level, from middle school science to university courses
- Middle schoolCovered in middle school science
- High schoolCovered in high school "Basic Biology / Basic Home Economics"
- High school+Advanced high school material, or treated as a textbook sidebar
- UniversityNot taught in high school; university-level content (microbiology, food hygiene)
- ResearchNot yet taught even at university as settled fact; what researchers are investigating now
Middle schoolTerms: this phenomenon has names
- Clostridium perfringens (Welch's bacillus): A bacterium widespread in nature, including soil and the gut. It multiplies where oxygen is scarce.
- Spore: A dormant, shelled form that some bacteria make inside themselves in harsh conditions. It resists heat and drying, and returns to its original form when conditions improve.
- Anaerobic: Describes a preference for places without oxygen, or difficulty growing when oxygen is present.
Middle schoolHigh schoolChecking with a formula: how many times do bacteria multiply in 3 lukewarm hours?
Consider the case where the food stays for 3 hours at the temperature where bacteria multiply fastest. The time it takes for the number of bacteria to double is called the "generation time", and its unit is minutes. We use the value for ideal conditions, so in a real pot the growth is often slower than this.
| In symbols | N ＝ N0 × 2^( t ÷ g ) |
| In words | Number of bacteria now ＝ starting number × 2 multiplied by itself "elapsed time ÷ generation time" times |
| Where it comes from | One bacterium divides into two. The number doubles every generation time, so you multiply by 2 once for each doubling (exponential growth) |
| Symbol | Meaning and unit |
| N0 | Starting number of bacteria (per gram, individuals) |
| N | Number of bacteria after time t (per gram, individuals) |
| t ・ g | Elapsed time and generation time (minutes). Here t ＝ 180 minutes and g ＝ 10 minutes, so the number of doublings is 18 |
| Generation time (ideal conditions) | Said to be about 10 minutes |
| Doublings per hour | 6. Multiplying 2 by itself 6 times gives 64× |
| Bacteria per gram at the start (assumed value) | 10 |
| Number regarded as a guide for food poisoning (per gram) | 100,000 or more |
| How many times after 2 hours? | 64 × 64 ＝ 4096 |
| How many times after 3 hours? | 4096 × 64 ＝ 262144 |
| Number per gram after 3 hours | 10 × 262144 ＝ 2621440 |
| How many times the guide number? | 2621440 ÷ 100000 ≒ 26 |
Even if there are only 10 bacteria per gram at the start, under ideal conditions the count reaches about 2.6 million in 3 hours, roughly 26 times the guide number. In a real pot the temperature is not optimal the whole time, so growth is not this fast. Even so, if the food sits inside the band for many hours, as the solid line in Figure 2 does, you can see that it could reach a dangerous number overnight.
High schoolHigh school+Why does a big pot cool slowly?
High schoolThe amount of heat stored is proportional to the volume (weight) of the food. The amount of heat escaping is proportional to the surface area in contact with the air or the pot wall. If you double the size while keeping the shape, the volume becomes 8 times larger and the surface area 4 times larger. Because the exit for heat cannot keep up with the contents, the time to cool grows by more than double.
High school+In addition, in a thick dish the liquid hardly moves, and heat is carried mainly by conduction alone. Unlike a thin soup, where warm liquid rises and cooler liquid sinks and they swap places, this circulation is hard to get going. As a result, the centre cools especially slowly. Dividing the food into shallow containers shortens the distance from the centre to the surface itself, so the effect is large.
UniversitySpores waking, and the toxin released in the gut
It is known that spores can actually be woken more easily by the stimulus of heating, which is called heat activation. In that sense, simmering also gives surviving spores a "wake-up signal". Clostridium perfringens food poisoning is caused by eating large numbers of the multiplied bacteria. When the bacteria form spores again in the gut, they release a toxin that acts on the intestine (an enterotoxin), which is said to cause the stomach pain and diarrhoea. The mechanism differs from the type of food poisoning in which the toxin is made in the food itself.
📖 Further reading: Clostridium perfringens (Wikipedia, in Japanese) / Spore (Wikipedia, in Japanese)
ResearchWhat is still not clear
- The full picture of what wakes spores It is known that temperature, amino acids and other things act as signals, but exactly which receptors do what is still being studied for each type of bacterium.
- Individual differences in the number needed to fall ill A guide number exists, but how much it varies with age or the state of the gut is not clear.
- The share of strains carrying the toxin gene Even among Clostridium perfringens, only some strains are said to be able to make the food-poisoning toxin. How common they are in food and the environment is still being surveyed.
So the content of this article, too, is "an explanation of what is known now". Still, the advice to "divide into portions and cool quickly" is considered a reliable method that will not change, however the details of the mechanism are worked out.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: "Living things and the environment (the work of microorganisms)" | Bacteria in soil and the gut; preference for oxygen |
| High school | Basic Home Economics: "Food safety"; Basic Physics: "Heat" | How to cool; volume and surface area |
| High school+ | Advanced heat transfer | Why the centre of a thick dish cools slowly |
| University | Microbiology; food hygiene | Heat activation of spores; toxin in the gut |
| Research | Research on spore germination and pathogenicity | Wake-up signals; the number needed to cause illness |
| ― | Connection to daily life | Make-ahead cooking, packed lunches, community cook-outs |
- Ministry of Health, Labour and Welfare (厚生労働省), general-audience explanatory material on "Clostridium perfringens food poisoning"
- Ministry of Agriculture, Forestry and Fisheries (農林水産省), consumer explanation of "Food poisoning from Clostridium perfringens in stewed dishes such as curry and stew"
- Tokyo Metropolitan Government Bureau of Public Health and Medical Services (東京都保健医療局), "Shokuhin Eisei no Mado" (食品衛生の窓) explanation of Clostridium perfringens
- National Institute of Infectious Diseases (国立感染症研究所), explanation of "Clostridium perfringens infection"
- Japan Food Hygiene Association (日本食品衛生協会), ed., Shokuchūdoku Yobō Hikkei (『食中毒予防必携』, Essential Handbook for Preventing Food Poisoning)
* This article is a general-audience science explanation. The figures given are guides and rough estimates meant to help you understand the mechanism. For food handling, follow the instructions of public health centres, local authorities, fire services and so on. If you feel unwell, do not rely on your own judgement; consult a medical facility.