I’d seen references to flat-sour here and there over the past 30 or so years that I’ve been canning. But I had no idea why it occurred (or if it was even real), how to prevent it, and whether it was hazardous. I decided it was time to do some sleuthing.
I pulled out all three canning references. Only the Ball Blue Book contained any mention of it. That right there suggests that flat-sour is not hazardous. Here’s what it had to say on the matter:
Flat-Sour: A heat-resistant organism common in canned vegetables, caused by bacteria, which gives food an unpleasant, sourish flavor. Flat-sour is easily avoided by the use of correct and sanitary methods of selecting, handling, preparing, packaging, processing, and cooling foods.[1]
But is that answer at all helpful? Does it tell you exactly why it happens, what foods are most likely to be affected, and how to avoid it? Does it tell you that while flat-sour is definitely unpleasant, there’s nothing at all toxic about it?
Flat-sour is caused by bacteria that survive the canning process. These thermophilic (heat-loving) bacteria include strains of Geobacillus stearothermophilus (formerly Bacillus stearothermophilus) and Bacillus coagulans, the most common culprits in flat-sour. And they can survive water bath and pressure canning.
The foods most likely to be affected are low-acid foods like corn, peas, and lima beans, as well as medium-acid foods like spinach, green beans, asparagus, and beets. The offending bacteria multiply when the veggies are allowed to sit at a hot (50°C or higher) temperature for hours. The bacteria multiply and are not killed by the canning process. The offending practice, one that yours truly has been guilty of on numerous occasions (though I’ve never experienced flat-sour), involves letting the veggies cool down overnight in the pressure canner. Sure, all the guides say to remove the jars when the pressure is fully released, but they never indicate the consequences for failing to do so, only that you should never try to hasten the cooling and release of pressure after a load completes.
G. stearothermophilus, the most common offending bacteria implicated in flat-sour, is a widely-distributed soil-borne bacteria, which explains in part why home-canned garden vegetables are so often afflicted with flat-sour. The bacteria in the canning jars remain so comfortably warm (for them, hot for us) for hours, and they’re in the dark where they can reproduce with reckless abandon. Take them out of the canner (and space them well for optimal cooling) and they suddenly become much more modest, in fact, unable to reproduce at all. A few sources indicate that the bacteria are killed at temperatures above 249.8°F for 12 minutes. Most pressure canning occurs at 240°F.
Because these thermophilic bacteria produce acid but no gas, the seal isn’t compromised. The liquid in canned goods may become a little cloudy, but otherwise there are no visible changes to the canned product. However, once you open it, you can sure smell it.[2] A few reckless souls proceed to taste the product after smelling it and immediately regret their life choices. However, there is nothing toxic about flat-sour. Unfortunately, there is no fixing it, either.
And that is why we are supposed to remove canned goods from the pressure canner as soon as it cools down.
References:
https://www.simplycanning.com/what-is-flat-sour-in-home-canning-causes-and-prevention/
https://www.sciencedirect.com/topics/medicine-and-dentistry/bacillus-stearothermophilus
https://journals.asm.org/doi/pdf/10.1128/am.16.1.45-47.1968
https://www.healthycanning.com/flat-sour/
I have never experienced flat-sour. I’m OCD about my canning and don’t let jars stay in the canner overnight EVER.
ReplyDeleteI have had one experience with flat sour when I canned yellow string beans. I purchased half a bushel of them from a "fruitstand" in an area well known for selling fresh produce. The "fresh" beans were refrigerated. I don't know when they were picked. I put them in an ice chest and then drove the 350 miles home. They were not canned until at least the next day. I don't remember how I determined they had flat soured, but of course they were all dumped out. I do know they were not left in the canner to cool down. I researched trying to find out what caused this and what I recall was that it was most likely already in the beans before I canned them. If my memory is correct, the research said that storing them or waiting too long to can them increases the liklihood of getting flat sour. I appreciate your research on the subject to help educate us.
ReplyDeleteYou would need over 30psi of steam pressure to reach 260F. That is way beyond safe pressure for a canner. You would need a commercial unit
ReplyDeleteExile1981