Love them or hate them, leftovers are almost inevitable during the holiday season. That is especially true if you are gathering with many loved ones who all bring their own dishes to the table.
The art of cooking harnesses all kinds of chemistry to give the desired flavours, colors and textures to food. But those reactions don’t always stop when we pop what’s left of a meal in the fridge, and the cooling process can have its own effects. That is why it is often impossible to recapture the exact same eating experience from a reheated dish on the second or third day.
Food scientists have discovered a lot about what gives foods the qualities we love and hate, but theirs is an ever-evolving field.
“We don’t even know what all the molecules are that are present in these foods. We don’t even know all the ones that cause it to taste or smell differently. We know a lot, but it would be very difficult to know all of them,” said Cordelia Running, associate professor of nutritional science at Purdue University.
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That’s because there is a lot of complexity involved in cooking. Many of the ingredients that make up a dish, whether animal, plant or fungus, were once living things, Running pointed out. Pair them with the cornerstones of cooking – such as heat, water, acid, salt and others – and you have a symphony of interactions happening in your boiling pot or sizzling pan.
But there are plenty of specific, well-studied chemical reactions and physical processes that play a huge part in how we experience food, whether it’s eaten straight away or as a convenient snack after a celebration. Here’s some science to keep in mind when you’re finishing off the leftovers in your fridge, along with some tips for safer cooking and eating.
Why starchy textures shapeshift
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Some starchy side dishes can be disappointing when they have spent a day or two in the fridge. You may see a thin, annoying layer of water when you pop the lid off a container of lentils or mashed potatoes, or lose the battle to resurrect brittle rice to its original fluffiness. That’s because of a phenomenon called retrogradation, which is what happens when starch that was gelatinized during the cooking process cools.
The uncooked potato or grain of rice you started with contains starch particles. That’s because the living plants they came from made glucose through photosynthesis and packed it into those particles, said Paige Luck, a food science lecturer at North Carolina State University. When those starch particles are heated with water – say, by boiling – they “blow up like a balloon” as they take in some of that water, which causes them to swell, he said.
But as a dish with starch cools, the retrograde process occurs as particles begin to return to their original crystalline structure. In the case of day-old mashed potatoes or chickpeas, that process causes them to release some of the water they absorb during cooking, which can cause a water layer to appear.
“You’re seeing the actual water that you would have cooked in this originally and that would have been dispersed at the molecular level between all these big starch chains and parts of its structure,” Running said.
There is nothing unsafe about the water, and it can usually be mixed back into the dish, but it is impossible for the starch particles to reabsorb the water molecules and return to their original state. That’s why mashed potatoes can never be as soft and creamy as they were on the first day.
That said, fats such as butter and cream play a key role beyond adding to the texture and flavor of freshly prepared mashed potatoes. They also tenderize the network that starch molecules form once they cool, Luck noted. That means the more fat present in such a dish, the creamier it should remain after being reheated.
There is less to be optimistic about leftover rice, especially rice with longer grains. Those starch particles form an even tighter network, meaning that the water molecules they absorb when boiled behave completely differently, according to Brenda Kelly, associate professor of biology and chemistry and the college’s provost and dean at Gustavus Adolphus College.
“They’re so trapped, they can’t really be released, and they don’t have water-like properties, and that’s what makes that rice feel very, very hard structurally,” Kelly explained. That’s why day-old, brittle rice is often better enjoyed when you reuse it in a dish like fried rice.
How oxidation can influence taste
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Oxygen can do some funky things to our food. That’s because of a chain reaction called oxidation that accompanies the flavors, aromas 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. He said it can happen “during the cooking or storage of foods such as oils, meats and fish.”
“When exposed to air, the lipids or fats in the food products react with oxygen to form lipid by-products (lipid hydroperoxides) which are further degraded into small volatile molecules (aldehydes and ketones) that produce flavors and aromas off the skin,” added Karolenko.
One particularly notorious oxidation reaction is called “reheating flavor,” an unpleasant flavor that some people encounter when eating reheated meats such as pork or chicken, Luck said. This phenomenon tends to occur with fattier cuts, he added, because it is linked to fat oxidation.
The taste is hard to put into words, but it’s different from when the meat was cooked fresh (although it’s not necessarily a sign that the meat has gone off or is no longer safe to eat) eat). If you want to try to keep it from happening, antioxidants can help.
“There are plenty of ingredients that are antioxidants that we can add to recipes and we add them to recipes that work against that oxidation reaction,” Luck noted. “Many herbs have antioxidants in them, rosemary and thyme – [which] are commonly used when cooking chicken and pork.”
The more a dish incorporates an herb like rosemary — say, if it’s ground into the meat itself rather than a single sprig placed next to a roast chicken — the better it will be able to help prevent oxidation, he said, noting that these herbs are commonly used in making sausages.
Why some dishes taste better on day two (or three)
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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 that they didn’t when it was cooked. fresh
Countless molecular interactions go into establishing the flavors of any meal at every point on its journey from the stove to your plate. But in a well-seasoned soup, fatty molecules and their admirers have a key role to play in achieving peak palatability.
The flavor molecules in many spices, including black pepper, cloves and nutmeg, are fat-soluble, says Running, meaning they seek out lipids by nature. The more time they have to disperse out of their original plant tissues and into the fatty elements of a dish – such as coconut milk, heavy cream or oil – the more accessible they are to our taste buds and smell receptors in our noses , she said.
Whether you’re 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, especially if the those fatty components are well mixed.
“Everything wants to balance,” Running added. “So over time, when you take a spicy chili pepper and you put it in something that has any kind of fat present in it, over time, some of that spicy, fat-soluble compound will slowly penetrate the fat itself. .”
He noted that a similar process explains why foods heavy in tomatoes or paprika tend to turn plastic storage containers red. Like spice molecules, the red pigment in those dishes is fat soluble, meaning it would rather spend its time in fat than water. Plastic is more fat soluble than water soluble, says Running, so pigment will seep into the surrounding plastic as it sits in the container, causing a reddish stain.
The role of reheating
Heating food properly is key to ensuring leftovers are safe to eat and enjoyable. The best route is to reheat only the amount you’re ready to eat—rather than the entire remaining dish—and make sure it reaches 165 degrees Fahrenheit or at least “steaming hot,” said Ghaida Batarseh Havern, extension educator for food security with the Michigan State University Extension Health and Nutrition Institute.
Running said she prefers to reheat more slowly, by lowering the microwave power to about 50 percent, to help avoid the unpleasant result of a dish that ends up scalding hot around the edges while still being ice cold. in the middle of it.
Leftovers can stay safely in the refrigerator 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 safely there indefinitely – but they tend to lose moisture and flavor over time, the agency said. 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 majority of the experience should be nearly as pleasant as it was when you put down your first plate one.
