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I made some eggs in a cast iron skillet yesterday and didn't wash the skillet. Today I want to make eggs again.

Is it really necessary for me to wash the skillet before making more eggs? It would seem to me that heating it up would kill germs that had accumulated in the day since I used it.

Is it just breaking the social norm that is the problem or is there really danger here?

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migrated from skeptics.stackexchange.com Aug 5 '11 at 19:28

This question came from our site for scientific skepticism.

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I'm not sure if this is a real issue in this specific case, but certain bacteria produce endotoxins which might survive the high temperatures. Just killing the bacteria might not be enough in that case. –  Mad Scientist Aug 5 '11 at 20:10
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@Fabian: More commonly they would be exotoxins (Stx, Verotoxin, perfringens, botulism, etc.) Endotoxins are the ones produced in the body. –  Aaronut Aug 5 '11 at 20:47
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Ignoring for a moment any safety issues, there's the problem that oils will have gone rancid, food spoiled, etc.—these will taste bad. Not to mention that if you leave a cast-iron skillet wet, you will rust it. –  derobert Aug 6 '11 at 5:33

5 Answers 5

up vote 23 down vote accepted

Seems almost every question about bacteria seems to bring about both the bacteriophobes and the cavalier. In this case, the cavalier are probably closer to being right, but I'll try to present the facts so that you can decide for yourself.

The most common microbe groups responsible for household food poisoning are:

  1. Salmonella, found in poultry, produces the CdtB toxin. CDT can be denatured (technically, "heat-labile") but is one of the most resistant toxins, taking 30 minutes at 70° C to destroy. This type of bacteria isn't normally associated with eggs, but since chicken eggs and other poultry products may be handled in the same area, it's not unthinkable that the shells could become contaminated - and depending on how you handle the eggs, so could the frying pan.

  2. E.Coli is normally associated with beef but is also known to be found on egg shells. It's improbable, but not impossible for it to be found in yolks as well. This is a bit of a weird one because egg yolks also have immunization properties against e.coli, but not in the yolk itself or the frying pan. The strains of e.coli associated with food poisoning encode Shiga-Like Toxin (SLT) and also Enterotoxins, the latter of which are often heat stable. I'm not sure about SLT, but the STs can withstand boiling for 30 minutes.

  3. Campylobacter (causing Campylobacteriosis) produces certain enterotoxins and also CDT. As above, these tend to be heat-resistant.

  4. C.botulinum is responsible for for botulism and is ubiquitous. The spores are incredibly difficult to kill but they are also only harmful to infants and the immunodepressed. More importantly, the bacteria are killed by heating to 85° C for 5 minutes and are also anaerobic - meaning they can only grow when they are not exposed to air. In other words, these may be present, but won't grow and produce toxins on a reasonably clean pan.

  5. C.perfringens is one of the most common sources of food poisoning, and I believe is actually what drives a lot of the specific food safety/food inspection rules. The spores are very heat resistant (can survive boiling for a full hour), and even though it is anaerobic, it is also aerotolerant. It can grow on a pan - just more slowly than it would grow in an enclosed container or in raw/fresh food.

  6. S.Aureus causes staph infections but also produces heat-resistant enterotoxins. It doesn't generally come from food, but more than 20% of humans are carriers, and also many pets (cats and dogs), so it is very likely in your household somewhere and can easily get on your frying pan and multiply on leftover food bits.

  7. B.cereus is less common but very resilient (they're known for "fried rice syndrome" because they can survive steaming) and afterward produce spores and heat-resistant enterotoxins. Important to note that many strains of this are harmless - but not all.

  8. V.cholerae causes (as you may have guessed) cholera symptoms which come from CTX and TCP toxins. Both of these are heat-labile although I wasn't able to find a time/temperature data point.

Chances are, you have at least one these friends in your kitchen. Chances are, you have several. The question is, did enough of them manage to get onto your frying pan after your last meal to produce dangerous levels of toxins?

Probably not. It's really very unlikely. But it is possible, and you're taking a chance if you don't wash your dishes.

In fact, it's probably more dangerous with eggs, because typically you'd cook eggs only up to a temperature of around 70° C, at most (63° is the temperature at which they set). So day after day you're bringing all those lovely bacteria right into the danger zone for several minutes and not staying above it for very long at all!

Doing this one time... really not a big deal. Bacteria don't get to multiply much in the 5 minutes (or less) it takes to fry an egg, and most eggs aren't even contaminated to begin with, although there have been small outbreaks.

But doing this several times, day after day - you're increasing your risk each day because the pan may never get hot enough, long enough, to kill all of the new bacteria from the day before.

I would strongly recommend that you wash your pans after use. You don't need to use soap if they're cast-iron; a good scrub with salt and hot water is fine. But please - especially if you're going to be cooking for guests - don't just leave the dirty pans sitting there and use them again the next day without washing them first. It's not safe.


P.S. I'd like to draw attention to the fact that of all of the items in the list above, botulism, despite having some of the worst symptoms, is generally the least of your worries. Both the bacteria and the toxin are easy to kill, and the spores are harmless to healthy adults. So if you hear people on this site or any site talk about food safety entirely in terms of botulism and the temperatures required to kill/inactivate the bacteria/toxin, ignore them, because they haven't done their homework.

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Your link for C.botulinum is dead. –  Carey Gregory Dec 12 '12 at 20:14

If you don't wash your skillet, and your home is between 8 and 45 C (45-113F), conditions are good for salmonella to grow.

Salmonella bacteria can survive a long time under adverse conditions and temperatures. Ph can range from 4 (roughly tomato juice) to 8 (baking soda)source

Depending on how well you cook your eggs, you may not heat them up enough to kill the bacteria

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Salmonella bacteria die at ~145F, and while I like my eggs cooked lower than that, I usually let my skillet get much hotter. –  Satanicpuppy Aug 5 '11 at 20:26

While microbes may be killed by heat, the toxins they produce are not.

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With cast iron it is not uncommon to simply wipe it out and put it back on the shelf. Alton Brown, who is so phobic about bacteria that he washes fresh mushrooms, just scrubs his cast iron with a little salt. Since cast iron users don't tend to fall over dead with any more frequency than users of regular pans, I'd say it's fine. You don't wash the cook surface of your grill every time you use it, do you? (Some people recommend that you do, which baffles me. The antibacterial properties of my dishwasher are somewhat less than the antibacterial properties of raging fire.)

Now, understand, this is not to say that a pan full of yesterdays food is anything but disgusting, but if you wipe it out, and you heat the pan to kill anything that might be alive you're unlikely to have much trouble. Yes, bacteria leave toxins, but if you wipe out the pan the concentrations should be too low to do you any harm.

As long as you heat the skillet a bit before you add any food, you won't have anything to worry about. 150F kills damn near everything in 30 seconds or less...If you're in the US, your car is probably hitting that temperature right now: in cooking terms, it's not very hot at all. If you had some C. botulinum you might want to bump it to 180F, but you won't have C. botulinum living on your skillet, so don't worry about it.

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There's an important point you're glossing over, which is that most cooks rinse and wipe cast iron (or non-stick) shortly after use, they don't leave it lying around at room temperature with bits of food on it. I wouldn't be satisfied with just a wipe after waiting a full day, I'd do a quick burn-off afterward just to make sure everything's dead or inactivated. –  Aaronut Aug 5 '11 at 20:44
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@aaronut: Well, I'm in the "Don't need no damn water on my cast iron" camp, but I don't leave food sitting in it either. I wouldn't be worried about bacteria the next day (as long as food wasn't being added to a cold pan), but I would be concerned about lingering tastes, because remaining oils will pick up flavours. –  Satanicpuppy Aug 5 '11 at 20:53

Wow. Do you wash your barbecue grill between uses, or just scrape it clean and cleanse by fire? The idea of washing cast iron pans strikes me as funny. I have three that I've used for decades, as have my mother and grandmother. The smallest and largest I don't use so often, so I after use I wipe with table salt to get rid of excess grease and anything stuck. The medium size pan is near-daily use and NEVER gets washed. Could I look up a list of rare bacteria and imagine how they might survive frying and hang out in my salty bacon grease? Sure, but in daily activity we accept so many higher risks that this is a pretty silly one to worry about.

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