Food left outside on the counter overnight

It happened to everybody, you left a cooked meal outside and forgot to put it in the fridge. The next day you’re wondering if it’s still safe to eat the food left out overnight for lunch or dinner? Check out what you need to know to stay on the safe side.

How bacteria grow

The answer, as usual, is not straightforward when it comes to nutrition. Let’s face it, most of us have eaten leftovers that weren’t refrigerated and didn’t get sick. But also most of us had an experience with food poisoning. That’s why there are certain food safety rules you should follow when storing food. Different organisms can spoil food including:

  • viruses
  • bacteria
  • parasites
  • fungi

If any of these organisms come in touch with your food left out overnight you could get sick. The most common reason for food spoilage are bacteria. There are two types of pathogenic bacteria:

  • Infective microorganisms e.g. Salmonella
  • Toxicogenic microorganisms e.g. Staphylococcus aureus or Listeria

It is important to make a distinction between these microorganisms. That’s because there is no safe level of infective microorganisms. Already one bacteria can lead to disease. With one bacteria the risk is low because it has to survive natural obstacles our body puts on it. Including the acid environment of the stomach or the immune response. But still, one organism can be enough to make you sick.

Toxicogenic microorganisms produce toxins in food while growing. These toxins can make you feel sick. The bacteria need to grow at a certain level to get to a concentration of toxin that can make you ill. For these microorganisms, a safe level can be set. Which means that up to a certain amount of bacterial growth, there will be no adverse effect.

An unsuspected lunch incident


In July 2012, 13 people had a military work lunch party. The lunch was “perlo”, a chicken, sausage, and rice dish. Two-three hours after dinner some people started to feel sick. They vomited and had abdominal problems. A lot of people who had eaten lunch got ill and went to the hospital for treatment. The food they ate didn’t taste spoiled but the “perlo” dish was found to be the source of contamination.  The bacteria Staphylococcus aureus was found in the stool of some of the patients as well as in the rice dish.

This bacteria is often present on the hands of human beings. Some Staphylococci transferred to the food by touching shouldn’t have been a problem. Since no one would get sick by only a few bacteria. But the food handlers removed the chicken from the bones by hand. They then placed it back into the stock pot to finish the meal. At the end, they placed the finished meal in an unheated oven. The food was left outside for approximately 8 hours overnight.

Food handlers preparing food

What might have happened here is that the food handlers were the source of contamination. They probably contaminated the chicken when separating it from the bones. Storing of the “perlo” overnight outside the refrigerator helped the bacteria to reproduce. This were ideal conditions for the bacteria to grow and produce enterotoxins overnight. Rewarming of the dish for one hour the following day did not destroy the heat-stable toxin. This might have even further increased toxin load.

When does food become dangerous


Let’s assume that the caterers in the example above added only 5 cells per gram of food. Also, assume the bacteria were able to divide every 20 minutes outside of the refrigerator.

  • After one hour: 40 cells per gram – no health problem
  • Two hours: 320 cells per gram – no health problem
  • Three hours: 2.500 cell per gram – still no problem
  • Four hours: 20.000 cells per gram – now it’s becoming dangerous because at 100,000 cells per gram Staphylococci start to produce toxins.
  • Five hours: 160.000 cells per gram – Food with toxins at a level that will make you sick. The food still doesn’t look, taste or smell spoiled.
  • Six  hours: 1.280.000 cells per gram
  • Seven hours: 10.240.000 cells per gram
  • Eight hours: 82.000.000 cells per gram
  • Nine hours: 655.000.000 cells per gram
  • 10 hours: 5 billion cells per gram
  • 11 hours: 41 billion cells per gram – Spoiled food

So after five hours at room temperature, the food left out overnight contains toxins but is not spoiled yet. In this stage, you can’t see, smell or taste that the food has gone bad. Spoilage occurs after about 10 hours. The real danger here is that the count of bacteria for food to be spoiled is very high. But toxin load that can make you sick happens already at much lower bacteria count. So by the time there is toxin you don’t notice spoilage yet. 

Conditions bacteria needs to grow in cooked food


One of the reasons we cook food is because we want to prevent food poisoning. The high temperature will kill many of the food poisoning bacteria and their toxins. Only if you give the bacteria optimal growth conditions it can spoil your food. Bacterial growth is subject to a combination of factors, including:

  • Initial bacterial cultures found in food
    Food inspectors don’t analyze every product for its microbiological safety. But they do inspect the production process for most foods found in the supermarket. So most of the products in the supermarket are microbiologically safe. Also, don’t forget that the cook itself has a crucial role. The cook makes food safe by killing most present bacteria in the cooking process.
  • Temperature and duration of cooking/heating food
    Bacteria grow best at 98 °F (37 °C) which is body temperature. They are able to reproduce quickly between 40 °F- 140 °F (4.4 °C – 60 °C). This is called the danger zone. Cooking at 140 °F (60°C) will kill a lot of harmful bacteria. Heating at 250 °F (120 °C) for a few minutes will kill all bacteria.
    Store your food in the fridge at 36 °F- 38°F ( 2 °C – 3°C) to slow down bacterial growth after cooking. Whenever you’re heating your food it should be above 140 °F (60 °C).  
  • Contamination after cooking
    Food handlers need to be very careful to prevent cross-contamination after cooking. This can happen when raw food touches cooked or raw juices drip onto cooked food. This can happen if you’re storing fresh meat and produce above cooked food in the fridge. Clean your hands, knives and chopping boards between the use of cooked and fresh produce. Don’t use dirty cloths to wipe with or dirty towels to dry hands.
  • Moisture
    All bacteria need water to survive and thrive. Very salty and sugary foods are less prone to spoilage. That’s because they bind water and leave less available water for bacteria to thrive.
  • pH-level
    Most bacteria thrive at neutral pH, but many can reproduce in a pH range from 4.5 – 10.0. That’s why spoilage of high acidic foods is less likely to happen. For example, vinegar has a pH of 3.5 and is used to preserve pickled foods.
  • Environmental condition
    Aerobic bacteria need oxygen to grow, whereas anaerobic bacteria can grow without it. Many disease-causing bacteria are aerobic. That’s the reason why manufacturers use vacuum packs for foods to slow down the growth of bacteria.

What can you do to keep your food safe after cooking


  • Make sure your cooking ingredients are not a source of contamination
    Only use ingredients before their date of expiration. Don’t use ingredients that have been sitting in your fridge for weeks. Check these storage times for common ingredients to be safe.
  • Refrigerate your food immediately after cooking
    Refrigerate cooked food within one hour after cooking. Dividing leftovers into several clean, shallow containers, will allow them to chill faster.
  • Avoid contamination after cooking
    Don’t pour leftovers back into the pot. Don’t touch your meal with unclean hands. Use clean serving equipment. Don’t sneeze into your meal and wash hands before serving. Store cooked food above fresh produce, especially meat and poultry, in your fridge.
  • Reheat you meal at temperatures higher than 140°F (60°C)

So can I eat food left out overnight then?

After all this talk about food safety, you’re probably still wondering. There is no way of knowing if during the night some harmful bacteria has contaminated your food. If you want to stay on the safe side don’t eat food left outside overnight. Definitely, don’t serve this kind of food to sensitive groups. That includes small children, pregnant women, sick people and people with weakened immunity.
If you still want to take the risk, use common sense. Toss the food in case the meal contains very perishable foods like fish. If you want to eat an acidic food like tomato sauce make sure that there was no cross contamination after cooking. You could reheat it above 60 °C for a few minutes and are probably going to be fine. But strictly taken, there is no guarantee for it.