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If cooking meat kills bacteria, and bacteria are responsible for problems with eating meat which has been left out, then why is it dangerous to eat meat which has been left out at room temperature and then thoroughly cooked?

A related question mentions that "Even if the bacteria is dead, toxins can remain if the food was out too long, causing problems". However, there's no further detail given. Are these toxins as dangerous as the bacteria themselves? How long does meat have to be left out to accumulate a dangerous level of toxins and thus be dangerous even if thoroughly cooked? Are these toxins the reason for the usual guideline of keeping meat unrefrigerated for a maximum of 2 hours?

update

The revelation, courtesy of Aaronut, that e. coli is actually dangerous because of its toxins — which cannot be denatured at temperatures which will leave meat in an edible state — has pretty much answered this question. And also given me further incentive to stop eating meat altogether :)

Our discussion (see the comments on hobodave's answer) has progressed into the realm of microbiology. Some highlights from my ongoing research:

Detail on heat-shock proteins. These seem to be the reason for the importance of keeping meat at a high temperature for a period of time.

Some background on heat resistance in bacteria. This also provides fascinating insight into how bacteria evolved immunity to antibiotics. Fungi.

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I suggest also taking a look at our questions tagged food-safety. We've amassed a great number of quality Q&A on this topic. –  hobodave Mar 10 '11 at 1:10

4 Answers 4

up vote 35 down vote accepted

hobodave's answer is most of the way there but I think it understates the importance of protein toxins. With the vast majority of foodborne illnesses, the bacteria aren't particularly harmful at all; what you need to worry about is the protein toxins they produce.

E.Coli - probably the most well-known form of food poisoning along with Salmonella - is actually a harmless bacteria that already lives in your lower intestine. But there is a particular strain of E.Coli, notably O157:H7, that is primarily associated with food poisoning. The reason? It produces what's called a Shiga-like Toxin.

E.Coli contamination is actually dangerous on two fronts. Because the bacteria are so well-adapted to surviving in the human digestive system (as I pointed out earlier, that's their primary habitat), ingesting even a relatively small number of the bacteria will result in them multiplying and producing those toxins in your gut (and the rest of the way down). This is why it normally takes several days for you to feel the effects of this type of food poisoning; that's how long it takes for them to produce the toxins in sufficient quantity for your body to notice.

But they don't need to be in your gut to produce those toxins; a piece of meat at room temperature provides good enough conditions and more than enough raw material for them reproduce and emit those same toxins. So if you leave it sitting out too long, then it really doesn't matter how many bacteria you kill, you are going to end up with E.Coli poisoning fast, because you don't even need to wait for them to produce the toxins; they're already there.

The problem is that you can't "kill" a protein toxin with a brief burst of heat because a protein isn't alive. It's just a protein. The temperatures and times needed to destroy that toxin would be similar to the temperatures and times needed to destroy all of the protein in the food, draining all the nutrition value and quite possibly turning it into a lump of charcoal.

Salmonella seems to be a fountain of misinformation with all sorts of people saying that it doesn't produce toxins. This simply isn't true. Inside the host it produces what's called an AvrA toxin (which isn't "toxic" per se, but allows the bacteria to grow to larger numbers), and some strains can also produce a CdtB toxin, which is highly toxic. (Apparently there's also a similar toxin produced by other strains.) I'll be honest, a lot of the medical mumbo-jumbo is way beyond my ability to comprehend, but it seems that a lot of the public confusion comes from the fact that salmonella can do some nasty things even without the toxins - but that doesn't mean that the toxins themselves can't do plenty of damage even if you manage to kill the bacteria.

The same applies to many other types of dangerous bacteria; C.diptheriae produce the diphtheria toxin, C.botulinum produce the botulinum toxin (botulism); even the infamous mad cow disease was, as far as we know, caused by a protein, not a bacteria, which is why it was able to be transmitted to humans even through cooked beef.

Are protein toxins the only reason why the USDA insists on a maximum 4-hour cumulative danger zone? Probably not. As hobodave says, the more the bacteria multiply, the harder is to kill all of them, even at high temperatures. The figure of 74° C / 165° F that the food agencies give us for poultry is not going to kill exactly 100% of all the bacteria, and if it only kills - I'm just throwing out a number here - 99.999% of them, that may be good enough for relatively fresh poultry but won't be enough if you've got a whole bacterial colony to worry about.

We can only speculate as to exactly what's entailed by the "danger zone" but my guess is that it's actually a combination of statistics, probabilities, and safety margins, which include, but are not limited to, the effects of protein toxins.

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Very informative, thank you. So I guess that while the cow is still alive, its antibodies would be limiting the numbers of e. coli and cleaning out the toxins that it is immune to? Otherwise, there wouldn't be any difference between leaving the meat out at room temperature and leaving the cow out at room temperature before slaughtering it. —————— Also, see my comments on hobodave's answer for some more questions about the 99.9*% per amount of time kill ratio at a given temperature. –  intuited Mar 10 '11 at 18:28
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@intuited: I really don't know what you're getting at. Most bacterial contamination does not come from the animal itself, it comes from the processing. If you're running your own farm and slaughtering your own animals then you don't really have to worry about a lot of this. –  Aaronut Mar 10 '11 at 18:56
    
Also @intuited, do keep in mind that bacteria grow exponentially (at least until they run out of food), so you would need to exponentially increase your cooking times as well - assuming that were actually enough (which it likely isn't). –  Aaronut Mar 10 '11 at 18:59
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@Aaronut: regarding the percentage of bacteria killed. You had mentioned that you were "just throwing out a number" of 99.999%. The USDA tables I've seen have indicated 6–7 decimal reduction, meaning 99.9999% – 99.99999% reduction in pathogens. Typically 6D is used for listeria and E. Coli, 7D for Salmonella –  Ray Apr 14 '11 at 15:23
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Just double-checked on this: "The 1999 FSIS final rule, Performance Standards for the Production of Certain Meat and Poultry Products, requires a 6.5 log10 relative reduction (6.5 log10 lethality) of Salmonella for cooked beef, roast beef and corned beef (9 CFR318.17)." See edocket.access.gpo.gov/cfr_2008/janqtr/pdf/9cfr318.18.pdf –  Ray Apr 14 '11 at 15:36

One important thing to know is that heat doesn't instantly kill bacteria. At least not at temperatures that leave edible material behind.

Bacteria take both time and temperature to destroy. The higher the temperature, the less time required. Take Salmonella senftenberg for example, it takes 60 minutes at 140 F (60 C) to kill 99.9999% of the population. But at 160 F (70 C) it takes less than two minutes. I go into a lot more detail in my answer to, "Is it safe to eat a cooked steak that briefly touched the plate holding raw meat?."

The other important thing to realize is that at these temperatures some of the bacteria population survives. It takes as few as 100,000 Salmonella cells to make you sick, and much fewer for E. Coli. So if you've left your meat in the danger zone you could easily have a starting population in the billions. I provide more detail in my response to, "Can chicken not completely cooked then cooled be fully cooked later?."

To address your concerns regarding bacterial waste products: yes, they are dangerous. In some cases they can be more dangerous than the bacteria themselves. Botulism is actually caused by the botulinum toxin produced by the C. botulinum bacteria. Even worse, the waste products are not easily destroyed by heat. They typically require much higher temperatures and much longer periods of time to destroy them.

As noted in the answer to "How dangerous is it to refreeze meat that has been thawed?", if meat has spent more than 4 hours in the danger zone over the course of its entire "life" it cannot be trusted. Since you cannot know exactly how well your meat has been handled throughout its lifetime I generally do not recommend letting it sit out for more than 60 - 90 minutes, at least when I'm serving it to people. I also don't ever work with cuts of meat that would require 2 or more hours to reach room temperature.

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So.. the USDA guidelines state that 2 seconds at 74°C will kill 99.9999% of salmonella or e. coli. So 4 seconds will kill an additional 99.9999% of the remaining 0.0001% of the original billion, leaving 0.0000001%. After 10 seconds, there will remain only 0.0000000000000001% of the original numbers. Even if there were a trillion bacteria to begin with, that's only leaving a ten-thousandth of a bacteria, which seems pretty safe. … –  intuited Mar 10 '11 at 5:10
    
… (continued) So assuming that botulism is not a possibility, which I understand is the case unless there's been some anaerobic fermentation happening, is there really a risk in eating thoroughly cooked meat? Assuming that "thoroughly cooked" means "cooked for long enough at a high enough temperature to kill enough bacteria to reduce their numbers below the danger threshold". I guess this is really a question of how high the numbers of bacteria can really rise: if there can potentially be a googol of salmonella on a chicken breast, it would take a while to kill them all off. –  intuited Mar 10 '11 at 5:12
    
* Note that the math used in these comments is not to scale. I'm erring on the side of safety though, unless I've got things mixed up. –  intuited Mar 10 '11 at 5:16
    
Whoops, just looked at the charts on that link again. So for the heat-resistant varieties, it's 2 minutes (at 70°C) rather than 2 seconds (at 74°C). Still, 10 minutes is not an unreasonable amount of time, at least not for something that's being slow cooked and won't have its outside charred while the internal temperature is being maintained. –  intuited Mar 10 '11 at 5:33
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@intuited you can use heat to kill off like you are talking about, but there are a lot of different bacteria and botulism isn't the only one that produces toxins while it's growing. Most importantly, bacteria breaks down the meat, which can be good (dry aging), but if you do it wrong you end up with rotted terrible meat. That said, if you make sure that it is cooked to 250 F (121 C) for at least fifteen minutes, you will have killed all possible bacteria. However any meat cooked to that temp is going to be shoe leather, you can eat it, but you aren't going to like it very much. –  sarge_smith Mar 10 '11 at 5:55

As well as ecoli. Staphylococcus aureus (lives on you skin as well as the environment) and b.cereus can generate a heat proof toxin.

B.cereus is commonly associated with cooling rice at room temperature - it's usually not a problem with a small bowl which cools slowly but a large batch may remain at the optimum temperature long enough for b.cereus to spew out a load of toxins. If this rice is then put into the fridge an warmed or even boiled later then enough of the toxin may remain to cause nausea and vomiting (though seldom anything more serious).

So you have to think about the temperature at all stages of buying, preparation and storing.

Harold McGee has a pragmatic approach here: http://www.curiouscook.com/site/2011/08/bending-the-rules-on-bacteria-new-york-times.html

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Agreeing to hobodave, I can recommend a joint thermometer. You stick it into your roast, and it reports the temperatur from inside.

In the oven you can have 180°C, which should be the same at the surface of the roast, but in the center it is only 60° or 70°C after an hour, depending, of course, on the sort of meat, on the size, form and bones.

There are small pictograms on the thermometer where you see recommendations for beef, pork, chicken and so on - I guess partially for the taste, but maybe partially to the risks of eating not well done meat.

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