How Fast Can Bacteria Multiply in Food?

Bacteria can double every 20 minutes on food.
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Are you wondering if that food you left out is safe to eat? Bacteria double every 20 minutes in food, depending on the temperature, and can quickly spoil your food if not handled properly. Get the scoop on food safety tips, including storage and reheating, to prevent food poisoning from bacteria.



Bacteria grow quickly in food, especially if it is left in the "danger zone" of temperatures, between 40 degrees and 140 Fahrenheit. To avoid food poisoning, don't let food sit out longer than two hours.

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Bacterial Contamination Can Spread Quickly

Bacteria and other microorganisms can spread at a fast pace, especially at certain temperatures, warns the USDA. If you leave food out at room temperature for too long, microbes like Staphylococcus aureus, Salmonella Enteritidis and Escherichia coli can quickly multiply and cause food poisoning.

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How long is too long to leave food out? The USDA says that bacteria doubles every 20 minutes when food is in the "danger zone" of temperatures, which is defined as between 40 and 140 F.


As a rule of thumb, never leave your food out for more than two hours before refrigerating it. If you have the food at temperatures above 90 F, such as during a warm summer picnic, then you should not leave it out for more than one hour.

Bacteria are found everywhere in nature, but when kept at low levels, they are not harmful, states the USDA. When the conditions are right, with warm temperatures, moisture and the nutrients in food, then bacterial contamination can spread quite quickly.


To give you an idea of how fast bacteria can grow after that two-hour window, the University of Missouri shares specific numbers.

If you have just four bacteria in your food and leave it for 30 minutes or longer in the "danger zone," then by the two-hour mark you will already have 256 germs. If you leave it sitting out for just one more hour, the number of bacteria will reach 4,096. This is why that 2-hour window is so important.


How Food Can Be Contaminated

There are five ways food can be contaminated with bacteria and lead to food poisoning, says the CDC.

The first way is during production, such as when the plants are being grown or animals are being raised. If fields are sprayed with contaminated water, for example, then the vegetables or fruits will be contaminated at this point.


The second way is through processing, which involves changing the animals or plants into a form we are accustomed to seeing at the store. If an animal is being slaughtered and germs from its intestines get on the final meat product, it can result in bacterial growth.


The third way food can be contaminated with bacteria is during the distribution phase. For example, if refrigerated food is left out on the loading dock too long and gets too warm, bacteria will grow and contaminate it.


The fourth way is during food preparation. For instance, if the chef is sick and doesn't wash her hands carefully, then germs can spread to the food. Or if a cutting board is used to cut both raw meat and fresh vegetables, then the vegetables can be contaminated with bacteria from the raw meat.

Lastly, contamination may occur at several points in the food production chain, resulting in bacterial overgrowth. It is important to be aware of the five ways food can be contaminated and ensure that you are taking the proper steps in the areas that you can control.


Preventing Food Poisoning From Bacteria

Pathogens multiply at a rapid rate. In fact, the Academy of Nutrition and Dietetics reports that a single microorganism can multiply to trillions (yes, trillions!) in just 24 hours when left too long in the "danger zone" between 40 F and 140 F.

The best way to prevent food poisoning from bacterial growth is to promptly refrigerate foods and avoid letting them sit out for longer than two hours. It is also important to thoroughly wash fruits and vegetables before eating. Make sure you throw out any canned goods if the cans are dented, bulging or damaged as that could contribute to bacterial growth.


As the Academy of Nutrition and Dietetics notes, a refrigerator that is properly set at 32 F will slow down the growth of bacteria but won't stop it completely. Foods that are frozen will not have any more bacteria growth. However, the existing bacteria on your food will survive. If you have left food sitting out for longer than two hours, freezing it won't kill the harmful bacteria, and it is best to discard the product.


According to the same source, most healthy adults can handle small amounts of bacteria without any issues. However, when bacteria multiply to larger numbers, they can be quite harmful.

Certain people are at higher risk of food poisoning. People who are pregnant, young children, older adults and those with a weakened immune system have to be more careful and ensure their food is properly handled and refrigerated.

Read more:What Can I Eat After Having Food Poisoning?

Foods Prone to Bacterial Growth

The Ontario Ministry of Health reports that some foods are more likely to develop harmful bacteria. These may include:

  • Dairy products, including milk, cheese, yogurt, cream and foods containing dairy
  • Eggs
  • Meat and meat products
  • Poultry
  • Fish and seafood

The following foods have a higher salt, sugar or acid content or are low in water. For this reason, they are less prone to bacterial growth. In general, these foods don't need to be refrigerated until they have been opened or washed.

  • Fresh fruits and vegetables
  • Bread
  • Most baked goods
  • Candies
  • Pickles
  • Honey
  • Jam and preserves
  • Syrups
  • Vinegar

It America, it's not safe to leave your eggs sitting out on the counter, unlike in some European countries that rarely refrigerate their eggs. According to the Egg Safety Center, chickens in some European countries are vaccinated against the bacteria Salmonella, while the chickens in America are not.

Additionally, eggs in Europe are not washed before distributing, making them less vulnerable to germs. Washing the eggs removes a protective layer that keeps them safe from bacteria.


The CDC states it's important to follow the two-hour rule when it comes to eggs. Refrigerate them promptly to avoid Salmonella poisoning. Make sure you are using pasteurized eggs, especially in recipes that use raw or undercooked eggs, such as eggnog and Caesar salad.

Furthermore, the CDC warns against eating raw batter or dough, no matter how good it tastes. Unfortunately, you can get Salmonella poisoning from eating raw cookie or cake dough that contains raw eggs.

Read more:Can You Eat Meat After Sell-By Date?

Safely Reheating and Storing Food

It is crucial to properly store and reheat leftovers to prevent harmful bacterial growth. As the USDA points out, bacteria can grow in food even when it has been cooked thoroughly. To prevent bacterial growth, leftovers need to be placed in shallow containers and stored in a refrigerator set at 40 F within two hours.

Do not thaw foods on the counter as this may cause unsafe bacterial growth, warns the CDC. Foods must be thawed in the refrigerator or the microwave. You can also safely thaw them in a sink with cold water — just remember to change the water every 30 minutes.

When reheating leftovers, make sure they are heated to an internal temperature of 165 F or until "hot and steaming." If food has been left out longer than two hours, reheating or boiling it will not kill the harmful bacteria and make it safe to eat again. It is best to throw that food out to avoid getting food poisoning.

If you're planning a party, avoid having your food in the "danger zone." Keep hot food at or above 140 F through the use of slow cookers, warming trays or steam tables. Make sure cold food is on ice and store it at or below 40 F.




Is this an emergency? If you are experiencing serious medical symptoms, please see the National Library of Medicine’s list of signs you need emergency medical attention or call 911.

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