Flat Sour in Canned Food: Causes, Prevention, and Tips
If you’ve ever opened a home-canned jar, expecting a delicious meal but instead got a whiff of something tangy (in the worst way), you might have met a little canning nuisance called flat sour.
Let’s talk about what causes flat sour, how to keep it from happening, and why—thankfully—it’s more annoying than dangerous.
@peeliesnpetals Flat sour can happen at anytime but some foods are more prone than others. Don’t fall asleep before removing those jars! #peeliesnpetals #canning #foodsecurity ♬ Sour Candy - Lady Gaga & BLACKPINK
What Is Flat Sour in Canned Food?
Flat sour is exactly what it sounds like: your canned food tastes sour, but without any visible signs of spoilage. No bubbling, no broken seals, no weird colors—just a disappointing flavor.
This isn’t a case of bad seasoning. It’s actually caused by a type of bacteria that loves heat—thermophilic bacteria, to be exact. These little microbes can survive the canning process if the conditions are right, and they thrive when jars are held at warm temperatures for long periods after processing.
How Flat Sour Happens
The biggest culprit? Letting your jars sit inside a warm canner for too long after processing—especially overnight.
Here’s what happens: you’ve run a full batch of jars through the pressure canner, you turn off the heat, go to bed, and leave them in there. But the canner stays warm for hours. That cozy, steamy environment is perfect for those heat-loving bacteria to get to work. By morning, you might have jars of food that look fine but taste disappointingly sour.
How to Prevent It
The fix is simple and easy to remember:
- Always remove your jars from the canner after the pressure has returned to zero and it’s safe to open.
- Let them cool at room temperature on your countertop.
That one small habit can make all the difference. Cooling jars quickly in a clean, room-temperature environment keeps those pesky bacteria from settling in and spoiling your hard work.
Is Flat Sour Dangerous?
Nope. While the bacteria that cause flat sour can ruin the taste and quality of your food, they aren’t harmful to your health. Still, it’s a bummer to waste a jar of food you worked hard to preserve—so it’s worth the effort to avoid it.
The Bottom Line
Flat sour is one of those things that’s easy to prevent once you know it exists. Just remember: don’t leave your jars in a warm canner for hours. Let them cool on the counter, and you’ll be one step closer to a shelf full of home-canned food that tastes just as good as the day you made it.
