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

For many, the holiday season is a time for large meals shared with family and friends. But after the feasting ends, the focus shifts to storing leftovers.

Every year, about one in eight Canadians — approximately four million people — are affected by foodborne illnesses, resulting in an estimated 11,600 hospitalizations and more than 230 deaths, according to the Public Health Agency of Canada.

The question of how long cooked food lasts often sparks debate: How long can you safely keep leftovers? Is old pasta really harmful? And when hard cheese grows mould, is it safe to simply cut off the mouldy bits and eat the rest?

To provide clarity, Canadian food scientists were consulted. Here’s what they recommend:

Can I leave cooked food out on the counter for a week, heating it up each time? Lawrence Goodridge, a food safety professor at the University of Guelph, advises against leaving food at room temperature for long periods.

"There’s a temperature range called the 'danger zone' between 4°C and 60°C," Goodridge explains. "This is where bacteria thrive."

The common guideline to "keep hot food hot and cold foods cold" is based on this principle.

Food safety experts often cite Bacillus cereus, a bacteria that can survive cooking. Jennifer Ronholm, an associate professor at McGill University, highlights that Bacillus cereus produces a heat-stable toxin, cereulide, which can cause severe illness.

“If ingested in small amounts, cereulide can cause vomiting, liver failure, respiratory issues, and even damage to muscle tissues, affecting the heart and kidneys,” says Ronholm.

Although fatalities from cereulide are rare, Ronholm refers to a 2011 case in Brussels where a young man died after eating pasta that had been left at room temperature for five days.

Should I cool down hot food before refrigerating it? It’s crucial to cool food as quickly as possible to prevent bacterial growth, says Ronholm.

She suggests leaving the pot on the counter until it stops steaming, then placing it in the fridge.

Keith Warriner, a food science professor at the University of Guelph, emphasizes balance. If food is too hot when refrigerated, it can raise the fridge temperature, allowing harmful bacteria like Listeria to grow. However, modern fridges are better equipped to manage these temperature changes.

Does container depth matter when storing food? Yes, it does. Kaidi Wang, an assistant professor at the University of Saskatchewan, explains that deep containers retain heat longer, increasing the risk of bacterial growth.

Shallow containers, on the other hand, allow food to cool faster and reduce the time it spends in the danger zone. Smaller portions cool more quickly than larger amounts, adds Warriner.

What about refrigerated fried rice or pasta? Bacillus cereus spores can grow in high-starch foods like rice and pasta if stored at unsafe temperatures. According to the B.C. Centre for Disease Control, these foods can cause serious illness if left in the danger zone for too long.

How long do leftovers really last? Foods left at room temperature for more than two hours should be discarded, says Ronholm.

A good rule of thumb is that leftovers should be eaten within five days of cooking. Even if food looks and smells fine after a week, pathogens like Listeria monocytogenes or Clostridium botulinum can grow under refrigeration and pose a risk, warns Warriner.

Can I eat hard cheese with mould? For hard cheeses, it’s usually safe to cut away at least 2.5 cm (1 inch) around the mould, according to Goodridge. For soft or semi-soft cheeses, mould should prompt disposal.

Ronholm adds that for soft cheeses like cottage cheese or ricotta, mould and toxins can penetrate deeper, making them unsafe to eat.

Do I need to reheat leftovers to a specific temperature? Yes, you should reheat leftovers to an internal temperature of 74°C, as recommended by Health Canada. Soups, stews, and gravies should be brought to a rolling boil.

Following these guidelines can help ensure food safety and prevent foodborne illnesses during the holiday season and beyond.