How long do leftovers really last? And other holiday food safety questions, answered

For many, the holiday season means tucking into big meals with friends and family who gather to celebrate.

But once the dinner ends, attention turns to packing away leftovers.

Every year, one in eight Canadians — about four million — are affected by a food-borne illness. Of these, there are 11,600 hospitalizations and more than 230 deaths, the Public Health Agency of Canada estimated.

The prospect of leftovers can stir debate: How long does cooked food last for? Will old pasta really kill you? And do you really need to throw out hard cheese that's started to grow mould, or is it fine to just cut those bits off and eat the rest?

To save you the hassle — and the fight — CBC News put those questions and more to Canadian food scientists. Here's what they said.

Can I leave cooked food out on the counter for a week, as long as I heat it through every time I eat it?

Lawrence Goodridge, a food safety professor at the University of Guelph, said it's not a good idea to leave food out at room temperature.

"There's what's called the danger zone, which is between 4 C and 60 C," Goodridge said in an interview. "This is where bacteria will grow very happily."

The kitchen advice to "keep hot food hot and cold foods cold" is based on this temperature range.

Food safety experts recommend portioning soup into smaller containers for storage rather than a large pot. (Shutterstock)

As for why, microbiologists point to a bacterial species called Bacillus cereus, which can produce spores that survive being cooked.

Jennifer Ronholm, an associate professor at McGill University, said that bacteria produces a heat-stable toxin called cereulide that can cause serious illness.

"If you ingest small amounts of cereulide, you can get vomiting, liver failure, respiratory distress, or even damaged muscle tissues releasing proteins and electrolytes into your blood and these can damage your heart and kidneys," said Ronholm, who also holds the Canada Research Chair in Agricultural Microbiology.

Deaths from cereulide are extremely rare, Ronholm said. However, she pointed to a 2011 lethal case report of a 20-year-old man in Brussels, Belgium, associated with eating pasta contaminated with Bacillus cereus.

The pasta had been prepared five days before and left in the kitchen at room temperature that whole time, researchers said. Before his death, four fatal cases attributed to cereulide had been reported, they said.

Should I cool down a hot pot of food before putting it in the fridge?

Anything you can do to get a pot of food out of that danger zone as soon as you safely can is important, Ronholm said.

Ronholm suggested keeping the pot on the countertop until you no longer see steam, and then putting it in the fridge.

Balance is the goal, said microbiologist Keith Warriner, a food science professor at the University of Guelph. If a pot of food is too hot when placed in the fridge, it can cause the refrigerator temperature to rise into that danger zone and produce condensation, which can allow microbial growth by pathogens such as Listeria.

Food safety professor Lawrence Goodridge says the danger zone for bacteria is between 4 C and 60 C. (Mark Bochsler/CBC)

"However, modern fridges are more efficient at keeping temperature and reducing condensation," he said.

Does the depth of the container matter when I'm storing cooked food?

Yes, it matters, said Kaidi Wang, an assistant professor at the University of Saskatchewan who also holds the Agriculture and Agri-Food Innovation Chair in Applied Microbiology.

"Deep containers usually retain heat for longer, increasing the risk of bacterial growth," Wang said.

Shallow containers allow food to cool more quickly, reducing the time in the danger zone.

And smaller batches of food cool faster than larger ones, Warriner said.

Fridges should be at 4 C or a little lower, scientists suggest. (Michael M. Santiago/Getty Images)

Time can be an important factor for some dangerous bacteria, said Warriner. Clostridium perfringes, which can cause an infection with symptoms such as diarrhea and cramps, one of the fastest-growing known bacteria. The bacteria can double in number every nine minutes.

"It doesn't take long to reach dangerous levels."

What's the deal with eating refrigerated fried rice or pasta?

Spores of Bacillus cereus can germinate and grow on food that's kept at conditions including between 8 C and 55 C for a sufficient time, according to the B.C. Centre for Disease Control.

The bacteria are "more of an issue with high-starch foods (rice and pasta) stored at room temperature and has been responsible for deaths," Warriner said.

"Again, it comes down to how long the rice is held in the danger temperature zone."

How long do leftovers really last?

You should always throw away food that has been left at room temperature for more than two hours, Ronholm said.

That's why it's better to portion things out, said Goodridge, such as putting leftovers in the fridge or the freezer.

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A general rule is that leftovers should be consumed within five days of cooking. Week-old food may look and smell OK, but some pathogens, like Listeria monocytogenes and Clostridium botulinum, can grow under refrigeration temperatures and become a hazard, Warriner said.

Beyond food safety, the University of British Columbia is offering tips on reducing food waste during the holidays.

I have some hard cheese that's just started to mould. Can I just cut that off and eat the rest?

The texture of the cheese matters, food scientists say.

Goodridge said if the cheese is a hard cheese then at least 2.5 centimeters or an inch of  the cheese around and below the mould should be removed.

"All other cheeses soft, semi-hard, should be discarded if mould is present," Goodridge said. "Same with other foods like bread."

Ronholm said for hard cheese, mould is more of a quality issue than it is a safety issue.

"If we're talking about something like cottage cheese or ricotta or cream cheese, that's not true," Ronholm said. With soft cheeses, the mould and toxins can penetrate more.

Spores of Bacillus cereus bacteria, seen here, can produce a heat-stable toxin that can cause serious illness when ingested. (Dr. William A. Clark/CDC)

Do I need to warm my leftovers to a certain temperature?

Ronholm, who said she owns four food thermometers, said yes.

Health Canada advises: Reheat leftovers to an internal temperature of 74 C and check with a digital food thermometer. Soups, stews and gravies should be reheated to a rolling boil.