Why Can't You Eat Food That's Been Left Out Overnight?

If you're anything like me, you may sometimes find yourself eyeballing a questionable food item, wondering whether it's safe to eat.  The idea of throwing out what might be perfectly good food is a hard one to swallow, especially if your grocery budget is tight.  On the other hand, the idea of being holed up with food poisoning isn't very appealing either. 

I've been deeply curious about food safety, and I wanted to lay some myths to rest once and for all.  So without further ado, here are some real answers about just how safe it really is to eat that pizza that was sitting on the counter all night.

What is Food Poisoning?

Food poisoning is actually a catch-all term for the illness caused by eating any number of microorganisms dwelling in food.  The most common of these are e. coli, Salmonella, Staphylococcus aureus and Clostridium perfringen.  These pathogens spread in different ways and in different foods.
Symptoms of food poisoning will vary.  Depending on your immune system, you may not feel any effects at all, or you may get diarrhea, stomach cramps, vomiting or fever.  Severe cases can lead to kidney failure, liver damage, and even some neurological effects like meningitis.  So food poisoning runs the gamut, and people with reduced immunity -- the very young, the elderly, the chronically ill -- are at the highest risk.  

I personally have had food poisoning only once, and it was the product of some questionable chili mac at my campus eatery my freshman year of college.  Since then, I've eaten all sorts of foods in "the danger zone" without any ill effects.  It's quite possible that years of questionable eating habits have built up my natural immunity -- a real "chicken and egg" situation, really.

However, that doesn't mean that the foods I'm eating are safe.  Someone else could eat them and get sick.  And if I were in a position of feeding others, I'd have to be a whole lot more careful about what I was eating. 

Officially, any "high risk" foods need to be eaten or refrigerated within two hours.  If not, they'll need to be kept at a high enough temperature to inhibit bacteria growth (think the steam tables in a buffet). 

What foods are at the highest risk?  Mostly, moist foods -- things like soup, creamy salads and cooked vegetables or grains.  Basically anything that is damp or mushy invites a playground for bacteria, which like to live in damp, mushy, warm places.  Dry foods -- like jerky or bread -- don't invite bacterial growth because there's just nowhere for the bacteria to live; they need moisture.  That's why drying is such an effective food preservation tactic, by the way. 

Why Cooking or Freezing Food Doesn't Save Questionable Food

Here's the million dollar question: If cooking a food to a safe internal temperature or keeping it refrigerated will prevent foodborne illness, then why can't you just re-heat the food to kill off any bacteria that might be on it after being left out?

Say you made a pot of chicken noodle soup last night.  Everybody ate some, you put the lid back on the pot and then forgot to put it away until morning.  Is it safe to eat it?  What if you put it in the freezer?  What if you heated it back to a boil?  Surely that would kill the bacteria, right?

Here's the issue: While extreme heat or cold will indeed kill bacteria, it won't kill the toxins created by those bacteria.  And in the case of many foodborne pathogens like e. coli, it's the toxins emitted by the bacteria (not the bacteria themselves) that are making you sick.

Think about it this way.  Imagine that you went away for the weekend.  While you were gone, a bunch of rowdy teenagers used your house for a party, and made a huge mess.  You kick them out when you get home, but the mess is still all over your floor.  It's the same with that pot of chicken soup.  Bacteria are living organisms, and they produce waste just like we do.  It's their waste that no amount of cooking can get rid of.  

And just like your teenage kids that invited their friends over, the dangerous bacteria already live in your food.  They create spores (sort of like seeds) that linger around in food.  Unlike live bacteria, these spores do not get killed by freezing or boiling.  When the opportunity arises to grow and multiply (due to perfect temperature conditions), these spores will "hatch" bacteria.  And then the party begins. 

So Should You Eat the Food Left on the Counter? 

Eating food that's been left out is really a calculated risk.  Most of the microorganisms that cause food poisoning don't have any discernible effect on the food, so you won't notice a funky smell or odd taste.  And symptoms can take hours or days to kick in, so you may not realize right away that you've eaten something contaminated.

When assessing your contamination risk, you really have to look at a few factors:

• Are you in generally good health with a healthy immune system? 
• How many contaminants could the food have been subjected to? (the more ingredients in it, or the more people who have handled it, the more likely that it could have bacteria in it)
• What is the makeup of the food?  (a sandwich made with a loaf of french bread and salami will probably last longer than one made of eggs and mayonnaise)
• How was it stored out on the counter?  (was it covered? is your house clean?) 

From there, you'll have to decide for yourself how safe you feel eating the food left in the danger zone.  Everybody will draw their line somewhere.  Just realize that anything over two hours at room temperature is not considered safe from an official perspective.  

Further reading:
Food Safety Myths from the Department of Health
Your Food Safety Handbook from the USDA
Bending the Rules on Bacteria and Food Safety from the New York Times
I Forgot to Refrigerate Food.... from Still Tasty (an invaluable resource I use quite often)