What is Flat Sour in Home Canning? Causes and Prevention.

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When it comes to home canning, what is ‘flat sour’?

Ever heard of flat sour? It’s that unappealing off-flavor – think a disagreeably sour or bitter taste of your canned goods.  It can actually happen in either home canned or commercially canned foods. Of course this article is going to deal with flat sour as it relates to home canning.

While it won’t harm you, flat sour can make your canned food very unappetizing. It is caused by a form of bacteria, that withstands the canning process and is triggered by extended warmth when you leave jars in a canner for too long.  

Nine half pints of tomato sauce cooling on a brown towel.

What Causes Flat Sour In Home Canned Food? 

Flat sour in home canning happens when jars of just-processed food cool down too slowly.

I’ve mentioned in my tutorials on pressure canning and water bath canning that you don’t want to speed up the cool down process, however that doesn’t mean go to the opposite extreme and slow it down too much. Letting your jars linger too long in the heat is not wise either.  Balance in everything. 

Leaving the jars in the heat for too long (like overnight or several hours) might create a good seal, and the jars may look fine. However, there are some harmless microorganisms called thermophiles that can survive the canning process. This leads to a harmless but not-so-great flavor and smell.

What Does Flat Sour Look Like?

As you may know there are several types of food spoilage.

Food with flat sour will not show any signs of spoilage until you open the jar and smell it. The reason is that the bacteria that is causing the issue does not produce any gas like other bacteria. There will be no bulges in the lids. Because there is no expansion of gasses the lids will not come unsealed. When you open the jar you’ll simply be surprised (and dismayed) by the odor!

How to Avoid Flat Sour.

While it is not harmful, it can significantly change the taste of your home-canned food. The easiest prevention is to use the correct canning methods, including the cooling down steps. Follow correct canning procedures and you won’t face any issues.

  • Don’t leave jars in the canner overnight or for several hours. Doing this leaves the jars in the heat for too long. 
  • Don’t stack jars too closely on the counter or covering them with a towel will cause the cool down process to be too long. 
  • Don’t can overripe fruit. Save the overripe fruit for projects like canning jam or jelly. (Presto is the only source I found that mentions this potential. See more below.)
Top down close up of canning jars in a water bath canner.

Is Flat Sour Unsafe?

The good news is that flat sour is actually not unsafe. However, It is very unappealing!  And while I’ve never personally experienced issues, based on my readings, the smell and flavor are very noticeable. If you notice a sour unpleasant odor you should dispose of it.

Does Flat Sour Occur In Both Pressure and Water Bath?

According to these journal articles, it primarily occurs in low-acid foods, although it isn’t limited to just low acid.

The flat-sour aerobes are two in number, one Bacillus stearothermophilus, typical of flat-sour spoilage of low acid foods and the other Bacillus coagulans, typical of flat-sour spoilage of acid foods.


Flat sour spoilage occurs chiefly in low-acid foods such as peas, corn, and lima beans (6), but can also occur in medium-acid foods, e.g., spinach, green beans, asparagus, beets (1).


Acid foods: pH 4.0-4-6 e.g. tomatoes, pears, etc. 1. Undergo flat-sour and gaseous spoilage.

Top down close up of canning jars in a pressure canner.

Expert Insights

Several extension agencies and the Presto canning company all mention cooling too slowly as a cause.  Presto has also mentions overripe fruit as a potential problem. 

According to Washington State University Extension. 

There are a group of harmless organisms called thermophiles which can survive canning. This results in the defect knows as “flat-sour,” a harmless, but very undesirable flavor.

The Tennessee Extension states 

Flat Sour: A common type of spoilage in canned vegetables caused by bacteria that give food an unpleasant fl avor. It can be prevented by following correct methods of preparing, packing, processing and cooling foods.  Tennessee State Extension Service 

According to the University of California Extension service

These particular bacteria produce no gas (unlike some others), so even if the jar originally sealed, there will be no bulges which later cause the lid to become unsealed. Food with flat sour show no signs of spoilage until the jar is opened. While not harmful, this food should be discarded.


In an article about canning overripe food Presto has this to say. I found it interesting that they are the only source to mention food being overripe as a potential cause.  

“Flat sour is a type of food spoilage that is caused by canning overripe food or allowing precooked foods to stand in the jar too long before processing. Use ripe or slightly underripe food and properly process, cool, and store. Flat sour shows no indication of spoilage until the jar is opened.”


Flat sours Bacteria such as Bacillus stearothermophilus render canned food sour by fermenting carbohydrates to lactic, formic, and acetic acids, without gas production. This means that the ends of the can are not swelled out but remain flat.


Some historical mentions of the spoilage flat sour.

Even though it is not mentioned often in current canning circles, the spoilage flat sour has been recognized for a very long time.  I love finding old publications and learning a bit of history as I research!

Bacillus stearothermophilus is the typical organism causing flat sour spoilage in low acid foods. It was first named by Donk. The spores are ubiquitous in nature, being isolated in areas from the arctic to the deserts as well as from foods.


Flat sour: Disagreeable sour or bitter flavor, a sour odor, and sometimes a brighter color. No gas forms. The spoilage is caused by a spore-forming bacteria. It ruins food for use but is not poisonous.


Flat Sour Spoilage: Flat sour spoilage is caused by thermophilic bacteria which produce acid without gas. It occurs most frequently in corn and peas, but may occur in other low acid vegetables.


Other canning issues you might be interested in.


  • https://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/flat-sours
  • https://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/flat-sours
  • https://ucanr.edu/sites/mfp_of_cs/files/382077.pdf
  • https://pickett.tennessee.edu/wp-content/uploads/sites/66/2020/02/FCS-Canning-Vocabulary-SP325-B.pdf
  • https://www.gopresto.com/content/s/canning-faq#:~:text=Flat%20sour%20is%20a%20type,process%2C%20cool%2C%20and%20store
  • https://s3.wp.wsu.edu/uploads/sites/2071/2013/12/Canning-Errors.pdf

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4 months ago

Hadn’t come across the term flat sour before. I’m glad I read about it here before experiencing it with my canned goods. Thanks, Sharon!