Why do leftovers smell bad?

In this short article, we will provide an answer to the question “why do leftovers smell bad?” and types of leftovers based on the intensity of risk of spoilage.

Why do leftovers smell bad?

The bad smell in leftovers is primarily caused by microbial or enzymatic spoilage (1,2).

If you leave your food leftovers at room temperature or stored improperly in the refrigerator, bacteria can grow and multiply on the surface and inside the food (2,4). 

These microorganisms can produce gasses and other undesirable compounds as part of their metabolic processes, which can result in a foul odor (3,5).

In addition, the compounds of some foods can break down into volatile chemicals that contribute to the unpleasant smell if your food is not stored properly (2).

Be aware that some spices and seasonings used in cooking can also affect the odor of your leftovers as they might interact with compounds in your food and produce undesirable flavors (6).  

How can you tell if leftovers are still safe to eat?

If you want to determine if your leftovers are still safe to eat you can look for the following sings:

  • Check the appearance: If your food looks slimy, discolored, or has mold growing on it, it should be thrown away- most probably it has gone bad!
  • Check the smell of the food: If you notice a sour or rancid odor in your food, this is a clear sign that bacteria have started to grow, and it may not be safe to eat.
  • Check the texture: If you feel your food slimy or mushy, this is a sign that it has started to break down and may not be safe to eat.
  • Check the storage time: If your leftovers have been in the refrigerator for more than four days, it’s best to throw them away to avoid any potential health risks. Be careful- you can experience a severe case of food poisoning (7)!

You should discard your leftovers immediately if you are not sure if they are still safe to eat. Some harmful microorganisms do not produce visible signs of spoilage (8). Remember that it is always best to err on the side of caution!

What are the health risks associated with eating spoiled leftovers?

Eating spoiled leftovers can pose several health risks, as the harmful bacteria that cause food spoilage can also cause foodborne illness (9-10).

When food is left at room temperature or stored improperly, such as in the “danger zone” between 40°F and 140°F (4.4°C and 60°C), bacteria can grow and multiply rapidly (11). 

Some of these bacteria could produce potent toxins that can cause food poisoning, which can lead to symptoms such as diarrhea, nausea, vomiting, abdominal cramps, and fever and in severe cases they can be fatal (12).

Eating spoiled food can also lead to more serious health problems, particularly in individuals with weakened immune systems, such as young children, pregnant women, older adults, and people with chronic illnesses and can even be life-threatening (8).

The most common types of bacteria that can cause foodborne illness from eating spoiled leftovers are:

  • Salmonella: A type of bacteria that can cause symptoms such as fever, diarrhea, and abdominal cramps. Salmonella can be found in a variety of foods, including poultry, meat, eggs, and dairy products (13).
  • Listeria: A type of bacteria that can cause a severe form of food poisoning called listeriosis. Symptoms may include fever, muscle aches, and gastrointestinal problems. Listeria can be found in a variety of foods, including deli meats, soft cheeses, and smoked seafood (14).
  • Campylobacter: A type of bacteria that can cause symptoms such as diarrhea, fever, and abdominal cramps. Campylobacter is commonly found in raw poultry and contaminated water (8).
  • E. coli: A type of bacteria that can cause symptoms such as severe stomach cramps, diarrhea, and vomiting. E. coli is commonly found in undercooked ground beef, raw vegetables, and unpasteurized dairy products (15).
  • Clostridium perfringens: A type of bacteria that can cause symptoms such as diarrhea and abdominal pain. Clostridium perfringens is commonly found in cooked meats that have been left at room temperature for too long (8, 16).

How can you prevent leftover food from smelling bad?

Here you can find five tips that could help you to prevent your leftovers from smelling bad:

Store leftovers properly: Make sure to store your leftovers in airtight containers in the refrigerator or freezer as soon as possible. This will help prevent the growth of bacteria that can cause bad odors 

Use odor-absorbing materials: Place odor-absorbing materials, such as baking soda or activated charcoal, in the container with your leftovers. These materials will help absorb any unpleasant odors and keep your food smelling fresh.

Keep the refrigerator clean: Regularly clean your refrigerator to prevent the buildup of food residue and bacteria. Use a mixture of water and vinegar or baking soda to clean the shelves and drawers.

Reheat properly: When reheating your leftovers, make sure to do so thoroughly. This will help kill any bacteria that may be present and prevent the development of bad odors (11).

Don’t keep leftovers for too long: Try not to keep leftovers for more than 3-4 days in the refrigerator (17). Discard any leftovers that have been sitting in the refrigerator for too long to avoid unpleasant smells and potential health risks.

Follow food safety guidelines: Be sure to follow food safety guidelines when cooking and handling food, such as washing your hands before and after handling food, and using separate cutting boards and utensils for raw and cooked foods.

By following these tips, you can help prevent your leftovers from spoiling and smelling bad, and reduce the risk of foodborne illness.

Remember that any leftovers that have been stored for more than four days should be discarded.


In this short article, we provided an answer to the question “why do leftovers smell bad?” and types of leftovers based on the intensity of risk of spoilage.


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