How do I know if food left at room temperature is still safe to eat?
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?
When in Doubt, Throw it Out!
You cannot always 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
Per the USDA guidelines, potentially hazardous food that stays in the temperature "danger zone", 40-140 °F (4-60 °C), for more than 2 hours should be discarded. For temperatures above 90°F (32°C), the limit is 1 hour.*
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.
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.
If you can be certain that the food was not in the danger zone, then yes, it is safe. For example, if you left a large chunk of frozen meat out and it is still frozen solid (including the surface) when you come back to it, it was not in the danger zone.
Why is it so strict? / Why didn't I get sick?
These guidelines are about making sure you don't get a foodborne illness, i.e. reducing the risk to where it's so small as to not be an issue. So if you break the rules, e.g. eating food that's been left at room temperature for 8 hours, that doesn't mean you will get sick, just that you're taking a risk.
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.
What can I leave out longer?
For foods that aren't potentially hazardous as described above, there's no solid rule, but things are generally safe for much longer than the 2 hours given above. For example, things which are sold at room temperature (e.g. fresh produce, bread, or cookies) are most likely safe at least all day or overnight if not for days or even months. You can find guidelines for common things at StillTasty.
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 USDA'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.
- Wikipedia: Food Safety
- USDA Fact Sheet
- Still Tasty - has information on storage methods/times for almost every food.
Further Reading/Frequently Asked
- Why is it dangerous to eat meat which has been left out and then cooked?
- How long can I store a food in the pantry, refrigerator, or freezer?
- What Do I Need To Know About Temperature and Food Safety?
- Is there a problem with defrosting meat on the counter?
- How long can cooked food be safely stored at room/warm temperature?
- Is it safe to cook a steak that was left out (raw) for 7 hours?
- How long can eggs be unrefrigerated before becoming unsafe to eat?
- Should I refrigerate eggs?
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: http://cooking.stackexchange.com/questions/12013/what-is-the-ideal-fridge-temperature
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.
@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.
Perhaps this answer should be clarified to point out that the "danger zone" applies to the _food's_ temperature and not to the _ambient_ temp around the food. For instance, if you set some frozen meat out to defrost at room temperature, it hasn't gone bad after sitting for two hours. It's probably still frozen solid at that point. The "danger zone" timer is only ticking when the food's internal temperature is between 40 and 140°F. Similarly, putting warm food in a fridge/freezer doesn't immediately stop the clock, as it takes some time for the food's internal temperature to drop.
Rules like this are devoted to big distribution, food industry, public restaurants etc. That a bowl soup is dangerous after a couple of hours in my kitchen is ridiculous. They are kinda laws and have regulatory purpose. It is not a kitchen book. Still the answer is good as for it provides guidelines. But it should be taken with one, even two, grains of salt.