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I remember that in my biology class in high school the teacher told showed us a puffed up can of pineapples which he claimed had botulism in it.

Is that remotely true? and if so, can I tell if my canned ketchup has botulism in it through the same "method"? and if so how long might that take?

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up vote 4 down vote accepted

One reason botulism is so scary is that you can's see, smell, or taste the bacteria growing in there. The only way to know for sure if botulism is growing in food is to have it tested by a laboratory.

Fortunately, though, you can see its evidence.

Yes, a puffed-up can or a jar w/ the lid popped up means something's growing in there. Discard the material; do not consume it.

Discard a metal can if there's any visible opening, no matter how small, if the ends are bulging, if the seam doesn't look intact, or if there's a leak.

Discard a jar if the jar appears cracked at all, if the pop-top doesn't pop when opened (meaning there's no longer a vacuum inside), or if the seal appears damaged.

As for how long it takes for a problem to arise, I'm not sure; I've had the same question.

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It can. C. botulinum (botulism is the disease you get from this bacteria) and other bacteria produce gas that will cause a can to bulge, given enough time. The pineapples he showed you either had a severe C. boulinum colony growing inside or some other nasty bacteria.

Regarding your ketchup, if your can is distended in any way, throw it out. However, the fact that it is not showing outward signs of infection does not mean that it's safe. You could still have unwanted visitors that just haven't produced enough gas to distend the can. How long is variable, it depends on the conditions inside the can. Under "ideal" conditions C. botulinum can double in population every few minutes.

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yes, botulism can occur if a jar has a popped up lid. To be safe, everytime you eat canned goods, make sure that you've heated it at 121 °C. It's one way to kill the botulism bacteria. But if a can is already puffed, it's better to just throw it away. Remember that botulism is really dangerous - it can cause permanent paralysis. To make sure that you're not promoting botulism when you're canning, read this do's and don'ts in home canning.

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Heating to 121ºC? – BaffledCook Aug 22 '11 at 21:35
@BaffledCook German wikipedia gives that temperature, but I didn't find it in the english one. The article link given above says 240F, which is about 116°C I think. – takrl Aug 23 '11 at 13:28
How can one heat foods with mainly water content over 100C / 212F ? – J.A.I.L. Nov 13 '12 at 23:16

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