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Love them or hate them, leftovers are a near-inevitability of the holiday season. That’s especially true if you’re gathering with a lot of loved ones who are all bringing their own dishes to the table.
The art of cooking harnesses all kinds of chemistry to give food the tastes, colors and textures we want. But those reactions don’t always come to a halt when we pop what’s left of a meal in the fridge, and the chilling process can have its own effects. That’s why it’s often impossible to recapture the exact same dining experience from a dish that’s been heated up again on day two or three.
Food scientists have figured out a lot about what gives foods the qualities we love and loathe, but theirs is an ever-evolving field.
“We don’t even know what all of the molecules are that are present in these foods. We don’t even know all of the ones that cause it to taste or smell a different way. We know a lot, but it’d be really difficult to know absolutely all of them,” said Cordelia Running, an associate professor of nutrition science at Purdue University.
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That’s because there’s a lot of complexity built into cooking. Many of the ingredients that comprise a dish, whether animal, plant or fungus, were once living things, Running noted. Pair them with the cornerstones of cooking — like heat, water, acid, salt and others — and you’ve got a symphony of interactions going on within your boiling pot or sizzling pan.
But there are plenty of specific, well-studied chemical reactions and physical processes that play a huge role in how we experience food, whether it’s eaten immediately or as a convenient snack after a celebration. Here’s some science you can keep in mind when you’re finishing off the leftovers in your fridge, plus a few tips for safer cooking and eating.
Illustration by Megan McGrew/PBS NewsHour
Some starchy side dishes can be disappointing when they’ve spent a day or two in the fridge. You might see a thin, unappetizing layer of water when popping the lid off a container of lentils or mashed potatoes, or lose the battle to resurrect brittle rice to its original fluffiness. That’s due to a phenomenon dubbed retrogradation, which is what happens when starches that were gelatinized during the cooking process cool down.
The potatoes or uncooked grains of rice you started with contain starch granules. That’s because the living plants they came from made glucose via photosynthesis and packed it into those granules, said Paige Luck, a food science lecturer at North Carolina State University. When those starch granules are heated with water — say, while boiling — they “blow up like a balloon” as they take in some of that water, which causes them to swell, she said.
But as a starchy dish cools, the retrogradation process happens as granules start to revert to their original crystalline structure. In the case of day-old mashed potatoes or lentils, that process causes them to release some of the water they absorbed during cooking, which can cause a water layer to appear.
“You’re seeing the actual water that you originally would have cooked in this and would have been dispersed at the molecular level in between all these big starch chains and parts of its structure,” Running said.
There’s nothing unsafe about the water, and it can usually be mixed back into the dish, but it’s impossible for the starch granules to reabsorb the water molecules and return to their original state. That’s why mashed potatoes can never truly be as soft and creamy as they were on day one.
That said, fats like butter and cream play a key role beyond augmenting texture and flavor in freshly prepared mashed potatoes. They also tenderize the network that starch molecules form once they cool, Luck noted. That means the more fat is present in a dish like that, the creamier it should stay upon being reheated.
There’s less to be optimistic about when it comes to leftover rice, particularly rice with a longer grain. Those starch granules form an even tighter network, which means the water molecules they absorb while boiling behave completely differently, according to Brenda Kelly, associate professor of biology and chemistry and provost and dean of the college at Gustavus Adolphus College.
“They’re so trapped, they can’t actually be released, nor do they have water-like characteristics, which is what makes that rice feel very, very hard in terms of structure,” Kelly explained. That’s why brittle, day-old rice is often better enjoyed when you repurpose it into a dish like fried rice.
Oxygen can do some funky things to our food. That’s due to a chain reaction called oxidation that fiddles with the flavors, smells and colors of foods, Caitlin Karolenko, scientific program manager at the Institute for the Advancement of Food and Nutrition Sciences, told the PBS NewsHour in an email. She said it can happen “during the cooking or storage of foods like oils, meats and fishes.”
“When exposed to air, the lipids or fats in the food products reacts with oxygen to form lipid byproducts (lipid hydroperoxides) which are further degraded into small volatile molecules (aldehydes and ketones) which produce off-flavors and aromas,” Karolenko added.
One particularly notorious oxidation reaction is known as the “warmed-over flavor,” an unpleasant taste that some people encounter when eating reheated meats like pork or chicken, Luck said. This phenomenon tends to occur with fattier cuts, she added, because it’s tied to the oxidation of fat.
The flavor is tough to put into words, but it is different from when the meat was freshly cooked (though it’s not necessarily a sign that the meat has gone off or is no longer safe to consume). If you want to try to keep it from happening, antioxidants can lend a hand.
“There’s plenty of ingredients that are antioxidants that we can add to recipes and that we do add to recipes that work against that oxidation reaction,” Luck noted. “A lot of herbs have antioxidants in them, rosemary and thyme — [which are] commonly used in cooking chicken and pork.”
The more a dish incorporates an herb such as rosemary — say, if it is ground up in the meat itself as opposed to a single sprig laid beside a roasting chicken — the better it’ll be able to help curb oxidation, she said, noting that these herbs are commonly used in making sausage.
Sometimes dishes like soups, curries or chilis pleasantly surprise us as leftovers, when the flavors we infused in them seem to harmonize in a way they didn’t when it was freshly cooked.
Countless molecular interactions go into establishing the flavors of any dish at every point on its journey from the stovetop to your plate. But in a well-seasoned soup, fatty molecules and their admirers have a key role to play in achieving peak tastiness.
The flavor molecules in many spices, including black pepper, cloves and nutmeg, are fat-soluble, Running said, which means they seek out lipids by nature. The more time they have to diffuse out of their original plant tissues and into the fatty elements of a dish — like coconut milk, heavy cream or oil — the more accessible they’ll be to our taste buds and odor receptors in our noses, she said.
Whether cooking on a hot stove or sitting in the fridge, the flavors from those spices have more of a chance to make their way around the dish, particularly if those fatty components are well-mixed.
“Everything wants to balance out,” Running added. “So over time, when you take a spicy chili pepper and you set it in something that has any sort of fat present in it, over time, some of that spicy, fat soluble compound is going to just slowly seep out into the fat itself.”
She noted that a similar process explains why tomato- or paprika-heavy foods tend to turn plastic storage containers red. Like spice molecules, the red pigment in those dishes is fat-soluble, which means it would rather spend its time in fat as opposed to water. Plastic is more fat-soluble than water-soluble, Running said, so that pigment will make its way into the surrounding plastic as it sits in the container, causing a reddish stain.
Properly heating food up is key to ensuring that leftovers are both safe to eat and enjoyable. The best route is to only reheat the amount you’re prepared to eat — as opposed to the entire remaining dish — and ensure it reaches 165 degrees Fahrenheit or is at the very least “steaming hot,” said Ghaida Batarseh Havern, extension educator for food safety with Michigan State University Extension’s Health and Nutrition Institute.
Running said she prefers a slower reheat, by lowering the microwave power to 50 percent or so, to help avoid the unpleasant outcome of a dish that ends up scalding hot at its edges while still being ice-cold at its center.
Leftovers can safely stay in the fridge for three to four days before it’s time to throw them out, according to the United States Department of Agriculture. They can be stored for three to four months in the freezer — and can technically stay safe there indefinitely — but they tend to lose moisture and flavor over time, the agency says. For more information, Havern recommends checking out the USDA’s FoodKeeper app, which offers more specific guidelines depending on the food item in question.
As long as you follow food safety guidelines — and your palate — when enjoying leftovers this holiday season, the bulk of the experience should be nearly as pleasant as it was when you dished out your very first plate.
Bella Isaacs-Thomas is a digital reporter on the PBS NewsHour's science desk.
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