The science of leftovers


What happens in the fridge

Think about what's in your fridge right now. How long have those containers filled with remnants of various dishes been sitting in there? Chances are, some of them are turning into science experiments while others still look edible—but they may not be truly safe to eat.

Once food has been cooked, it doesn't take long for potentially harmful bacteria to begin accumulating—usually within a couple of hours. But this can all depend on how food products are handled before being stashed in the fridge.

“We can contaminate cooked food with the organisms we might have either on our hands or on the kitchen counter, and they will grow,” explains Lynn McMullen, a professor of food microbiology at the University of Alberta. “It depends on the organism, how fast they will grow, but if you put things in the refrigerator that slows them down if not stops them.”

Freezing food is an alternative option, which completely halts bacteria growth. But even then, this doesn't mean food is invincible. Frozen food still has a shelf life of anywhere from two to six months, depending on the product.

Clostridium perfringens is a strain of bacteria that can be especially troublesome and McMullen says the Public Health Agency of Canada has ranked it as the No 1 cause of foodborne illness.
The cooking process itself can aid in making leftovers safer. McMullen notes once a food product is brought to a temperature between 70 and 75 C, a great deal of organisms are killed off, but there still may be dormant bacteria spores that won't begin to surface until food is cooled. Once food is cooked and ready to be cooled, McMullen says it is important to get it through what's known as the “danger zone” as quickly as possible—which means getting the product below 4 C.

“Traditionally people would leave things on the counter to let it cool, but the best thing is to put it in containers and put them in the fridge and let your fridge do the job because it will do the job faster and you won't run the risk of forgetting it on the counter,” McMullen adds, noting it is important to store food in smaller, shallow containers—ideally no more than two inches deep—in order to allow food to cool more efficiently than it would in a large container, which prevents dormant bacteria spores from germinating. “I'm going to use Clostridium perfringens as an example. It likes to grow between our body temperature and 20 degrees. It's going to grow really, really quickly so when we heat food and we are cooling it, the faster we get it through the danger zone as far as the temperatures for bacterial growth is a good thing.”

When it comes to digging into leftovers again, some items such as pizza that were great hot are just as good cold, but reheating can be a beneficial step.

“Something that you would typically eat hot, the best way is to heat the whole thing, and microwaves are great, but they do have hot spots and you can get cold spots,” McMullen explains. “One of the things about reheating food is it will actually help reduce the risk of foodborne illness because if something has contaminated your food, reheating may reduce the numbers and may provide some protection.”

Just how long an item will survive in the fridge can be something of a guessing game. In a fridge that is at less than 4 C, cooked meat or poultry lasts three to four days while fresh, uncooked poultry should be used within one to two days (fresh, uncooked beef, veal lamb and pork last longer at three to five days). Soups are usable for three to four days while sandwiches should only be kept overnight. There are resources online to assist with leftover storage times such as
During its time in the fridge, a food product continues to undergo chemical reactions that affect flavours and textures—for better or worse. Items such as tomato-based sauces, chili and curries often become more flavourful as they age. This is the result of cellular structures breaking down, which can release new flavour components. Individual flavours become less pronounced than they were during first tastes as they have had time to blend together, creating a much more developed flavour profile. Oxidation of fats is another component that affects flavour, McMullen notes.

“When fat is exposed to oxygen, the oxygen will attack some of the bonds in the fats and that's where you get some of the interesting flavour components produced,” she says, adding the “warmed-over” flavour leftover roast beef can take on is an example of this.

What about texture? Items like french fries or others that originally possessed a crispy texture become soggy the next day, and McMullen explains this can be a product of bacterial growth, but this would also push the limits of acceptable food quality. In other instances, such as leftover meat, reheating can break down gelatinous material that binds protein cells together, a process that continues each time the food is cooled and reheated, eventually resulting in a stringy texture.
“When you start seeing things actually breaking down or becoming slimy, sometimes things will become softer with time, then usually it's microbial changes that will do that,” she adds. “The bacteria or yeasts and moulds will produce enzymes that break down tissues in foods. As far as spoilage is concerned and texture, if you think about how if you leave fresh meat in your fridge too long it goes green and slimy, same with processed meats: they become milky and slimy. With processed meats it's usually what we call lactic acid bacteria. They're not harmful but they can produce some slime from the carbohydrate that might be present in the product.”

With so many variables and elements that can affect leftover food quality from initial handling to cooking to storing, McMullen has a simple rule of thumb to keep in mind.

“Traditionally we used to say keep it cold, keep it covered, keep it clean, which for leftovers is probably still the best advice.”


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