Did you know that 1 in 6 Americans suffers from foodborne illness every year? Though you may not realize it, that stomachache you got after eating that 4-day-old pizza or even that headache or lazy feeling after eating those leftover noodles may be a sign of a serious food illness. 

A big cause of food-related illness is simply leaving leftovers in the fridge for too long before eating them. Food that is left in the fridge should be eaten within 2-3 days, but this is only a general rule. It's important to realize that different foods have different spoilage rates, and there are many factors that go into food spoilage.

The Science of Food Spoilage

What makes food "go bad" is actually the reproduction of bacteria on the food. Even when food is cooked at high temperatures, the bacteria is never completely eradicated. This small amount of bacteria that remains present in the food will continue to multiply as time goes on.

Food that has “gone bad” may smell bad because a number of bacteria in the food is very high. The bacteria is actually eating your food, and this decomposition process releases chemicals into the food that can smell pretty nasty.

The strains of bacteria that are the most harmful tend to grow more on protein-rich foods than on starchy, sugary foods like rice. So, carbohydrate-rich foods are a safer bet to refrigerate, while you may want to freeze your proteins if you can.

Bacteria also grows faster in moist environments, so drier food will last longer than moister food.

macaroni, pasta, spaghetti, cheese, cheddar, sauce, vegetable
Jocelyn Hsu

Another factor that goes into how fast food spoils is when and how it was cooled. In fact, one of the biggest causes of foodborne illness is improper cooling of cooked foods. Leftovers from cooked foods should be placed in the fridge no longer than two hours after cooking.

Bacteria grows fastest between the temperatures of 40 and 140 degrees Fahrenheit. Most refrigerators are kept at 40 degrees or lower, so keeping food in the fridge will slow (but not halt) bacteria growth. Freezers are kept under 32 degrees, so freezing food can actually stop bacteria growth.

Food that is kept outside the fridge at room temperature for more than two hours should be thrown out. Bacteria multiplies very fast at room temperature, which is around 72 degrees. So, you may want to think twice about taking food to go from your favorite restaurant if you’re not going to be home for several hours.

Note: I am not referring to individual ingredients that can be left at room temperature; I’m referring to cooked dishes that were either cooked at home or from a to-go box from a restaurant. It’s perfectly fine (and sometimes better) to leave foods like tomatoes, potatoes, fruits, bread, and uncooked grains at room temperature.

rice, vegetable, fried rice, risotto, meat, chicken
Abigail Wilkins

As another protection against food illness, leftovers should not be reheated more than once. Repeatedly reheating and cooling leftovers increases the time that the food spends growing bacteria at room temperature.

Now, I myself struggle to follow all of these food guidelines, and I sometimes take the risk of eating days-old food or food I hadn't refrigerated right away, but nothing happens to me. However, this doesn't mean something can't happen in the future–it's important to be mindful of how you feel after eating questionable food and possibly connect any slight physical symptoms with a serious food illness that your body is fighting off. 

Tips for food safety & storage:

1.  If you are cooking a big batch of something, like a large soup, for example, it is not a good idea to put the giant pot straight into the fridge to cool down. Trust me, I’ve tried it. It messed with the temperature of my fridge and the whole fridge got warm.

Also, it will take a while for a large pot of food to cool down, probably more than two hours. To keep bacteria at bay, I suggest separating the batch into several shallow containers and then putting them in the fridge/freezer. This way, your food will cool much faster.

2. Freeze your food when you can. I can say that I have mastered the art of not wasting any food, and the freezer has been my best friend. It’s just important to be aware of what food will still taste good after being frozen.

The kind of food you might take home from restaurants is probably not going to taste good frozen and thawed. I mean, you can try, but the texture and taste of restaurant-quality food probably won’t be the same.

When I cook food at home, I freeze lots of it, and it’s pretty successful. I don’t see much of a texture or flavor difference when I freeze beans, grains, tofu, bread, and some fruits and vegetables. 

3. Wrap food well. Whether you are storing it in the fridge or the freezer, wrapping food in airtight packaging or sealing it in storage containers helps keep out bacteria and air particles.