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If I left food out of the refrigerator for some period of time, is it still safe? If I left it out too long, can I salvage it by cooking it more?

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Pro tips: star this so you can find it when you want to close duplicates, and since the answer here is very general, consider adding a comment with specific advice when you vote to close! –  Jefromi Jun 14 '13 at 6:04
    

2 Answers 2

When in Doubt, Throw it Out!

You cannot see or smell bacterial contamination. Mold that appears to be growing only on the surface may grow invisible roots into softer foods. Do not rely on a visual inspection or "smell test" to tell you whether or not a food is safe. It's not worth the risk - food poisoning can be much, much worse than an upset stomach.

The Danger Zone

  • Potentially hazardous food that stays in the temperature "danger zone", 40-140 °F (4-60 °C), for more than 2 hours should be discarded.*

  • Potentially hazardous foods are those foods that spoil most easily, such as unshelled eggs, raw meats, fish, shell fish, dairy products, almost all cooked foods.

  • This time is cumulative, so it includes time bringing the food home from the grocery store, time before cooking, time after cooking, and so on. The reason is that while cooking may destroy bacteria or other pathogens, it doesn't always destroy the toxins that they have produced.

  • So in general, regarding perishable foods like meat, most dairy, unshelled eggs and shell eggs (in the US), cooked casseroles, and so on: if the food (or its perishable components) have been at room temperature for more than two hours, you should discard that food.

  • To avoid the danger zone, keep cooked food hot until ready to eat, then refrigerate immediately. Separate large items into smaller containers to help them to cool more quickly. If you’re defrosting something, do it in the fridge or under cold running water.

Why does cooking not completely "reset the clock"?

  • Some bacteria leave behind harmful protein toxins that cannot be "killed" (denatured) by cooking. Cooking food is only effective against live organisms, not their toxic waste products. Spoiled food cannot be cooked back to safety and must be discarded.

  • Cooking is pasteurization, not sterilization. Pasteurization means killing most microbes, so as to render the food safe for human consumption. Sterilization methods (e.g. pressure-canning and irradiation) are the only safe methods for longer-term room-temperature storage. Otherwise, the danger zone rules always apply.

  • Even sterilized food can only remain sterile under an airtight seal, e.g. when properly canned or vacuum-sealed. Once it is opened, it is no longer sterile. Air contains countless bacteria and molds, and their spores, which will readily re-colonize any suitable environment they encounter. Cooked food tends to be an ideal medium for growth.

Regulation and Risks

Follow the guidelines set out by reputable regulatory agencies, especially when serving others. Local organizations include:

Other regulatory sources apply in other parts of the word, but major food safety organizations usually agree in essence (if not in complete detail) on most issues.

Failure to follow reputable guidelines is irresponsible if you are serving guests, and failure to follow your specific local codes is likely to be illegal if you are serving customers.

Health codes tend to be very conservative, to fully protect the community. You have the right to take risks on yourself by ignoring their recommendations, but please do not risk the safety of others.

Again, When in Doubt...

Once again, if you suspect spoilage or contamination, please, throw it out.

* Note: this is the FDA's rule. Other agencies may have variations on it. Additionally, government agencies generally make very conservative recommendations - they're trying to make sure that no one who follows the rules gets sick. Breaking the rules means maybe taking on some risk. That's up to you - just remember, eventually someone gets unlucky, and food poisoning is not fun.

Helpful Resources

Further Reading/Frequently Asked

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It's also a good idea to keep a thermometer in your fridge. They typically don't have built-in temperature readings, and you may be inadvertently endangering yourself and your guests. The appropriate temperatures are discussed here: cooking.stackexchange.com/questions/12013/… –  Steve Jun 14 '13 at 4:35
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I strongly disagree on dairy. In many European countries cheeses are sold at room temperature and are absolutely fine to eat, and you can definitely keep it out of the fridge for more than two hours with no issue whatsoever. Preserved meat (salami, ham and in general Charcuterie) can be kept in a cool place outside of the fridge for months with no problem. In summary, you cannot generalize. –  nico Jun 14 '13 at 6:24
    
@Steve absolutely! My fridge went on the blink last month, but it didn't stop working it just sneakily got warmer. Our first clue was milk kept spoiling. I now own two fridge thermometers, for fridge and freezer. ~$5 each, well worth it. –  Mark Allen Oct 15 '13 at 20:27
    
I am wondering how meat aging works then, how is it safe to age meat for days? –  Andrey Feb 6 at 9:36
    
Aging of meat is done inside a temperature and humidity controlled refrigerator when it is done correctly; and there certainly is some loss to spoilage. –  SAJ14SAJ Feb 6 at 12:49

The question was: "If I left food out of the refrigerator for some period of time, is it still safe? If I left it out too long, can I salvage it by cooking it more?"

Answer: It depends ...

  • How long did you leave it outside the fridge?
  • What kind of food are we talking about?
  • What is the moisture content of the food and the humidity of the air in the room?
  • Is the air reasonably clean. Did you put it on a clean or contaminated surface?
  • Is it raw or cooked food?

These are just a couple of questions so you get the general idea that it really is impossible to give a generalized answer to the question.

Considering that only a surprisingly small percentage of the worlds population actually has access to refrigerators and people still eat I think it might be obvious that refrigeration is really not the only way to store food for later consumption.

For a more in depth study of the topic I recommend:
Food Safety: The Science of Keeping Food Safe By Ian C. Shaw

As a more general tip I would stick to common sense. e.g. any food containing raw eggs (Mayonnaise) or food that must by its nature be considered contaminated (store bought raw chicken / raw meat) needs to be handled in a reasonable matter (you should be able to find the correct advise about handling these kinds of foods in any good cookbook) to avoid colonisation to an unhealthy (when eaten - even after cooking) point. On the other end of the spectrum leafy vegetables or fruit bought fresh from a farmers market actually comes with its own protection in the form of beneficial bacteria on the skin and is therefore less likely to be easily (meaning in a short time) colonized by pathogenetic organisms. Learn about your food and learn how to cook and you probably will be safe.

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