Flat-Sour in Home Canning - Causes and Prevention

9.6.25
by: Jill@321Oak
Pennsylvania, USA

Warmth-Loving Bacillus and Flat-Sour Spoilage

The members of the genus Bacillus are the culprits that make “flat sour” an issue in home canning. They are spore-forming, and some strains are exceptionally heat resistant and can survive the canning process (yes, even pressure canning which inactivates many other spore-forming bacteria. Some strains can grow in anaerobic environments, and they grow fastest at warm temperatures. So, when you leave your jars in hot water after processing longer than recommended, you create an environment that is ideal for Bacillus growth. The flat-sour taste is actually caused by the bacteria’s byproducts, not the bacteria themselves.

So, if Bacillus can be even more heat tolerant than other spore formers, and they can grow in anaerobic environments like C. botulinum, why isn’t it a huge issue? Well, it doesn’t typically cause foodborne illness. Bacillus makes food unpleasant, but it won’t make you sick.

So why do some canners say they always leave their last batch in the canner water until the next morning, and they’ve never had jars go sour?

• Because the jars didn’t have Bacillus in them in the first place.
• Or because the that were present Bacillus spores were killed in the canning process (they can survive, but that doesn’t mean they always do).
• Or because the food was too high in acid to allow Bacillus to flourish.
• Or because the bacterial load was too small to produce a noticeable change in flavor (low initial contamination).
• Or (most likely) some combination of all of the above.

How to Avoid Flat-Sour in Home Canning

Goldilocks cooling! The trick here is not to cool your jars quickly; that can lead to siphoning and jar breakage. See Home Canning Risks - Spoilage and Botulism Basics for more on siphoning and jar handling. The trick is to cool them just right. That means aiming to get them out of the canner 10–15 minutes after the natural release of pressure. Aim to never leave jars in for more than 30 minutes.

Wait until the pressure naturally releases; crack the lid but leave it on the canner for about 5 minutes; take the lid off but leave your jars in for about 5 minutes. Then remove the jars. Cool your jars at room temperature, on a towel, with a couple of inches of space between them. Don’t try to slow this by covering them with a towel, a sheet, or any other means. Don’t try to rush it, either. It’s unnecessary and risks jar breakage during jar cooling.

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