Flat Sour (2024)

Flat sour is an unappealing off-flavour that canned goods, home or commercial, can develop.

It’s caused by keeping jars of just-processed food warm too long after processing, either by keeping them in the canner, or by covering them with a towel, etc.

There are thus two extremes to avoid with cooling jars:

(a) setting them in a cold draft that will cause them to cool too fast, which will affect the seal;

(b) doing something to cause them to hover in a warm zone after processing, which could cause flat sour.

What is flat sour?

Flat sour is an unappealing off-flavour that canned goods, home or commerical, can develop. It was studied as early as 1937 by a researcher named Barlow: “[Barlow] also described several causes of flat sours and recognized that the flat sour may be caused by more than one organism.” ^[1] Bitting, Arvill Wayne. 1937. “Appertizing or the art of canning; its history and development.” The Trade Pressroom, San Francisco, CA.

It is caused by some microorganisms that survive the canning process. They are referred to as thermophilic (aka “heat loving”). The actual bacteria that cause it can include (depending on the food product) strains of Bacillus stearothermophilus, Bacillus licheniformis, Bacillus coagulans, Bacillus macerans, and Bacillus subtilis. ^[2] Kalogridou-Vassiliadou, D. Biochemical activities of Bacillus species isolated from flat sour evaporated milk. J Dairy Sci. 1992 Oct;75(10):2681-6. https://www.ncbi.nlm.nih.gov/pubmed/1430475

They are harmless with regard to food safety, but they cause the food to ferment in a way that produces a sour off-taste and smell without producing any gas. Metal cans will not bulge, nor will lids pop off of jars as there is no gas to cause that.

It is described as sour because of the taste and smell, and flat because it doesn’t produce gas that could bulge or blow the lid .

It can lower the pH (0.3 – 0.5) of the product a small amount.

“Dextrose Tryptone Agar is used for isolation of mesophilic or thermophilic spoilage microorganisms from food and is not intended for use in the diagnosis of disease or other conditions in humans.Dextrose Tryptone Agar evolved from research by Williams, while studying the cultivation and enumeration of thermophilic bacteria caused by “flat-sour” spoilage of canned foods. In the 1930’s, the National Canners Association specified the use of Dextrose Tryptone Agar for isolating “flat sour” organisms from food products. “Flat sour” spoilage of canned foods is caused byBacillus coagulans(Bacillus thermoacidurans). Bacterial growth results in a 0.3 – 0.5 drop in pH, while ends of the can remain flat.B. coagulansis a soil microorganism, found in canned tomato products and dairy products. Conditions favorable for organism growth can result in spoilage of food products. Dextrose Tryptone Agar can be used to isolate other food spoilage bacteria including mesophilic, aerobic spore-formers and thermophilic “flat sour” spore-formers such asB. stearothermophilus.”^[3] Acumedia Manufacturers, Inc. Dextrose Tryptone Agar (7340). Accessed March 2015 at https://www.neogen.com/Acumedia/pdf/ProdInfo/7340_PI.pdf

It occurs particularly with peas, corn and tomatoes (either whole, crushed or juiced tomatoes.)

What causes it?

It occurs when processed jars are kept in a warm state for too long after processing. “Flat Sour develops at two ranges in temperature, the lower from 70 to 90 Fahrenheit (21 to 32 C) and the higher 140 to 159 Fahrenheit (60 to 70 C.)” ^[4] Cameron, Janet L. and Mary L. Thompson. Home Bulletin No. 128. Blacksburg, Virginia: Virginia Agricultural Extension Service. Revised June 1944. Page 5. Access March 2015 at https://scholar.lib.vt.edu/faculty_archives/cameron/canning_for_the_home.pdf

Barbara Lund in Microbiological Safety and Quality of Food says,

Bacillus stearothermophilus and Bacillus coagulans are the chief flat-sour thermophiles found in canned foods. While both cause flat-sour spoilage, they have different characteristics. B. coagulans is a problem particularly in canned foods exposed to temperatures in the range from 40 to about 60 C and at pH values of 4.3 to 4.8…. B. coagulans is not a major flat-sour spoilage organism of low-acid canned foods…. B. stearothermophilus is the primary cause of thermophilic flat-sour spoilage in low-acid canned foods because it will grow in thepH range from 5.4 to above 7…. B. stearothermophilus-type organisms sometimes may grow at temperatures as low as 37 C, but never below 30 C.” ^[5] Lund, Barbara, ed. Microbiological Safety and Quality of Food, Vol 1. Gaithersburg, Maryland: Aspen Publishers. 2000. Page 49.

In A Complete Course in Canning and Related Processes Susan Featherstone says,

Geobacillus stearothermophilus (used to be Bacillus stearothermophilus) … is a thermophile [Ed: heat lover] and is widely distributed in soil, hot springs, and ocean sediment and is a cause of spoilage of in food products by producing various organic acids. No gas is formed and it products ‘flat sour’ spoilage in canned foods. It will grow within a temperature range of 30-75 C but optimally at 55 C. Spores of G. stearothermophilus may enter a cannery in soil, on raw foods, and in ingredients such as spices, sugar, soya meal, flour and starches…Their presence in some processed containers of commercially sterile low-acid foods may be considered normal and not of particular concern because, although G. stearothermophilus can grow at temperatures as low as 30-45 C, the spores will not develop if the product is stored at temperatures below 43 C.

Bacillus coagulans is a termoduric and acid-tolerant bacterium that has been isolated from canned tomato products, where it causes a reduction in the pH of spoilt products by acid production without CO2 formation…. Optimal growth is between 37 and 45 C and at pH levels between 5.0 and 7.0. Bacillus coagulans is not as heat resistant as G. stearothermophilus but can grow at lower pH values (e.g. down to pH 4.2.) ^[6] Featherstone, Susan, ed. A Complete Course in Canning and Related Processes: Vol 2. Sawston, Cambridge, UK: Woodhead Publishing. 14th edition. 2015. Pp 17-18.

The Kerr canning manual had this information:

Do all types of spoilage cause the seal on the jar to release? No. Flat sour is an exception and ordinarily shows no indication of spoilage until the jar is opened…” ^[7] Answers to your canning questions. Kerr. Question 106, page 52. 1948. Accessed March 2015.

At home, you can create conditions for flat sour to develop by leaving your jars in the canner too long after processing (either water bath or pressure canning), or, do something deliberately to slow down the cooling, such as cramming the hot jars right next to each other on the counter, and / or covering them with a towel.

How can you avoid flat sour?

Do leave your jars in the canner for 5 to 10 minutes after water bath processing, or 10 to 20 minutes after the pressure has gone away in your pressure canner (opinions vary amongst the experts as to the exact rest time that is best), as per current recommendations to give the jars time to rest and calm down a bit after processing, but do not leave them in for hours after that if you can help it.

Processed food should cool quickly. Jars should be removed from cooker or water bath as soon as processing is complete. Avoid a draft but do not cover with a cloth.” ^[8] McClelland, Bessie E. Causes And Prevention of Spoilage In Home Canned Food. Bulletin (Montana Extension Service in Agriculture and Home Economics); 234. 1945-07-01. Page 5.

Utah State Extension says,

Cooling too slowly after removing from canner. (Example: stacked jars close together.) There is a group of harmless organisms called thermophiles that can survive canning. If bottles are held hot for long periods, they can produce acid (fermentation). This results in the defect known as “flat-sour.” This is harmless, but produces an undesirable flavor.” ^[9] Riggs, Kathleen. Avoiding Common (Major and Minor) Canning Mistakes. Utah State Cooperative Extension. March 2009. FN/Food Preservation/2009-01. Accessed July 2015

The Presto Pressure Canner manual says,

Flat sour, a type of food spoilage, is caused by canning overripe food or allowing precooked foods to stand in jar too long before processing. It may be prevented by using fresh products and properly processing, cooling, and storing. Flat sour shows no indication of spoilage until jar is opened.” ^[10] Presto Pressure Canner and Cooker, 23 quart model 01781, #72-719F. 2014. p 6

So Easy to Preserve says,

Do not leave the canner unopened to cool completely, or the food inside could begin to spoil…” ^[11]Andress, Elizabeth L. and Judy A. Harrison. So Easy to Preserve. University of Georgia Cooperative Extension. Bulletin 989. Sixth Edition. 2014. Page 32.

You can view a brief discussion of flat sour here:

It’s also discussed in a war-time book, “Causes And Prevention of Spoilage In Home Canned Food“: ^[12] McClelland, Bessie E. Causes And Prevention of Spoilage In Home Canned Food. Bulletin (Montana Extension Service in Agriculture and Home Economics); 234. 1945-07-01. pages 9 and 13

What is the maximum time you can leave something in a canner? A blogger, Susan, who writes under the name “imstillworkin” says three hours is the maximum time, but she doesn’t give a source for that information and didn’t respond to queries.

Previously, it was thought that allowing produce to heat up before processing or sitting in jars before processing was one of the causes of of flat sour. Now the pre-concern is less about that, or not at all it seems, but more about reducing the bacterial load going into the process:

The Putting Food By people say,

Canned tomato juice is noted for encouraging growth of the highly heat-resistant bacillus that causes flat-sour spoilage, a sneaky and nasty-tasting condition indeed. However, even though the organism is very hard to destroy, it can be avoided quite easily: just follow carefully all the requirements for handling food in a sanitary way.” ^[13] Hertzberg, Ruth; Greene, Janet; Vaughan, Beatrice (2010-05-25). Putting Food By: Fifth Edition (p. 128). Penguin Publishing Group. Kindle Edition.

Historical speculation about causes

In the past, there was speculation that flat sour could be caused by poor quality produce, taking too long to pack the jars, etc, but now the cause is actually known, and it occurs at the tail end of the processing: it can develop if your canned goods take too long to cool down.

The Kerr manual, for example, thought that it could be caused by pre-processing errors:

What causes flat sour? How can it be prevented? Flat sour is usually caused by canning overripe food or allowing precooked foods to stand in jars too long before processing or slow cooling of jars after processing. It may be prevented by using fresh products and properly processing, cooking and storing.” ^[14] Answers to your canning questions. Kerr. Question 105, page 52. 1948. Accessed March 2015.

Further Reading

Lin, Chau-Ching et al. Spoilage Bacteria in Canned Foods. In: Applied Microbiology. Washington, DC: American Society for Microbiology. Vol. 16, #1. January 1968, pp 45 to 47.
Willenburg, Barbara. What causes a flat sour taste in home canned tomato juice? MissouriFamilies.org web site. University of Missouri Extension. May 2009.