A powerful foe exists in kitchens the world over—a force so strong it can render even a famous chef's roast chicken cardboard-y, stale, and faintly rancid. It's called warmed-over flavor, or WOF for short, and we most recently met when I reheated some chicken I'd braised for a dinner party the night before. One bite in, I panicked—had I really served my guests a bird tainted with that much funk? But I distinctly recalled that dinner had been delightful, the chicken perfectly cooked. The truth was, warmed-over flavor had struck again.
If you've ever wrinkled your nose at brand-new leftovers, or tossed them because they tasted uncharacteristically ripe, then you, too, have experienced this phenomenon firsthand. Perhaps you, too, would like to prevent future leftovers from succumbing to the clutches of WOF. After my latest debacle, that's precisely what I set out to do.
I wasn't alone in my endeavor. Food scientists have devoted years of research to determining precisely what alchemy occurs in leftover food to give it WOF, and how to prevent it from happening in mass-produced meat products, like deli meats. These scientists have teased apart the chemistry at play, which should, in theory, allow the crafty cook to keep WOF at bay in their home kitchen.
The Science of Warmed-Over Flavor
Stopping WOF starts with understanding precisely where it comes from. Scientists and observant eaters alike agree that the flavor is most noticeable in cooked meats that have been refrigerated for 24 hours or more, then reheated. Though it's especially obvious in leftover fish and poultry, discerning connoisseurs can pick out the WOF bouquet in most reheated meats. These flavors are the result of a series of chemical reactions that begins with the deterioration of specific kinds of fats known as polyunsaturated fatty acids, or PUFAs. (Fatty acids are the precursors to the fats that build up in an animal's body, like the stuff you trim off a chicken thigh or hope to get rid of at the gym.) PUFAs, in particular, are found in the membranes of cells.
The muscles that we consume as meat are made up of millions of microscopic cells, each of which is surrounded by a membrane of tightly organized fat molecules that behave like an oil drop in water.* That membrane serves as a barrier to enclose all the machinery that makes the cell tick. The amount of PUFAs in cell membranes differs from animal to animal; chicken and fish have a much higher concentration of PUFAs in their cells than lamb, pork, or beef, hence their increased tendency toward WOF.
* Unsaturated fats tend to behave like oils; "unsaturated" refers to the fact that the carbon chains that make up their molecular structure aren't all paired with hydrogen atoms. The presence of free carbon in these chains gives fats the ability to flow. In contrast, because the carbon chains in saturated fats have bonded with as many hydrogen atoms as they can, they're stiff and waxy, like a candle—this is why foods high in saturated fats, like butter or lard, have a solid consistency. PUFAs are called "polyunsaturated" because they're missing hydrogen atoms at many positions along the fatty-acid chain.
Eric Decker, a professor of food science at the University of Massachusetts in Amherst, has spent his career trying to thwart WOF. He explains that part of the challenge is that the chemistry behind WOF is so speedy. "The reaction is really fast—it's probably the fastest lipid oxidation in any food," Decker says. "It's occurring as soon as you take the meat out of the oven...it's probably starting in the oven itself."
The process goes something like this: When you're cooking a chicken breast, the heat starts to break down the cells that make up the muscle. Each cell membrane deforms, like a stick of butter melting, and the proteins within the cells begin to lose their shape, or denature. This is bad news if you're a cell, but good news if you're about to eat a couple million of them in the form of a chicken breast—all that breakdown allows melted fat to permeate the meat and loosens up gristly connective tissues, resulting in juicier, more tender chicken.
Right out of the skillet, that chicken is delicious, but that very same tenderizing breakdown process creates the potential for WOF to develop. When certain proteins denature, they loosen their hold on iron molecules. Free iron roams around cells and catalyzes a chemical reaction between PUFAs and oxygen. That reaction in turn creates free radicals, the cell-destroying agents that antioxidant foods and juices supposedly keep in check. Those free radicals start a chain reaction that transforms the normally inoffensive PUFAs into by-products with the tastes and aromas of warmed-over flavor. They're not harmful to eat, but they stink. And, unfortunately, once the reaction starts, there's nothing you can do to stop its malodorous spread.
According to Decker, because the reaction involves cell membranes rather than the visible white fat that marbles meat, buying lean cuts doesn't help reduce WOF, nor does trimming excess fat from your chicken. Dark meat, like a chicken thigh, is dark because of high concentrations of iron in its cells, making it particularly susceptible to WOF. Decker also says it probably doesn't matter how the chicken is raised—whether it's organic, free-range, or raised in feedlots. "The only thing that would help would be to feed the chickens vitamin E," he says. Vitamin E is an antioxidant that makes its way into cell membranes and protects them from degradation, but, while Decker notes that some vitamin E is generally fed to all livestock, putting an entire barnyard on a high-antioxidant diet just to control WOF wouldn't be cost-effective.
On the industrial scale, commercially produced meats, like cold cuts and precooked chicken, are processed with phosphates and vacuum-packed while still hot to minimize WOF. Vacuum-packing sucks out all the air, limiting the oxygen that's available to react with iron. Phosphate, on the other hand, pairs up with all the free iron and holds on to it, preventing it from catalyzing chemical reactions. In a vacuum with little free iron, WOF will develop more slowly.
Without the amenities of a meat-processing facility, home cooks have a more limited range of options to slow down WOF-inducing reactions. The best way, according to Decker, is to take a page from the industrial playbook and limit cooked meats' exposure to oxygen as soon as feasibly possible. You don't have to take your dinner guests' plates while they're still eating, but you might pack the leftovers tightly in heat-safe containers after everyone is served. If you're especially sensitive to warmed-over flavor, you may even consider investing in a vacuum sealer of your own. "The faster you vacuum-pack it, the more effective it's going to be," Decker says.
Flavorful sauces are another potential solution, since they create a barrier to air, which will slow down WOF-forming processes—especially in soups, stews, or curries in which smaller morsels of meat are fully submerged. These may even be doubly effective if flavored with ground herbs or spices that are known to quash free radicals. "Rosemary and oregano are good antioxidants, so they could have some protection," Decker says. As an added benefit, a punchy sauce will help mask any WOF when you reheat the leftovers the next day. Unfortunately, no matter how powerful the antioxidants in a sauce, there's no way they can suffuse an entire, intact piece of meat, like a chicken thigh. "There's not a lot you can do," Decker admits.
Though WOF seems like an insurmountable obstacle, I was invited by the editors of Serious Eats to try to devise workable strategies for circumventing these oxidation reactions in a home kitchen, using our understanding of the chemistry behind them. We tested out a number of different approaches.
The goal in all our testing was to determine whether different cooking and storing methods could produce a discernible impact on WOF. To test cooking methods, we started with bone-in, skin-on chicken breasts and thighs, all seasoned with 1.5% kosher salt by weight. Twenty-four hours before the taste-testing commenced, we cooked a whole mess of chicken breasts sous vide at 160°F (71°C) for 1.5 hours, then rapidly chilled them in ice baths. We also cooked chicken thighs dressed in a number of different ways—marinated, herbed, coated in a variety of oils—which we roasted until an instant-read thermometer registered 160°F, then let cool naturally.
For the storage testing, we stored individual bone-in, skin-on breasts, either tightly wrapped in plastic or placed in oversize Tupperware containers; we did the same with breasts that we had deboned and skinned after cooking and cooling.
We also tested whether the method of reheating had an impact on WOF, comparing chicken breasts reheated in a microwave, in an oven, and sous vide against freshly cooked sous vide chicken breast. We then tasted rewarmed plain chicken thighs and thighs coated with different fats—peanut oil, olive oil, butter—against freshly cooked thighs, and we also tested rewarmed chicken thighs sprinkled with herbs (rosemary and tarragon, separately), as well as chicken thighs marinated in lemon juice, once more against plain, freshly cooked chicken thighs.
Finally, we tested whether some degree of Maillard browning could mitigate WOF, comparing a leftover browned chicken thigh and a leftover unbrowned but fully cooked chicken thigh against a freshly cooked (and browned) thigh.
In the storage portion of the testing, we tasters nearly unanimously agreed that freshly cooked chicken was free of WOF. With the leftovers, however, there wasn't much that we did that could stave off the rubbery, lunch meat–y flavor indicative of WOF. That was about the only thing we agreed on, though there was mild consensus that storing breasts in Tupperware resulted in the least offensive funk, regardless of whether the chicken was deboned before refrigeration.
With our palates primed and our stomachs still relatively empty, we moved on to the tests of flavorings and Maillard browning. Across the board, we generally detected WOF in the leftovers regardless of whether the thighs were browned or not. Adding flavorings to the meat, through either fats or spices, did decrease the intensity of WOF, though, for the most part, we could still pick the leftover thighs out of the lineup. Probably because of their milder flavors, the different oils produced more mixed results.
Perhaps unsurprisingly, the least offensive leftover in the entire experiment was the chicken initially marinated with lemon, followed closely by the chicken seasoned with rosemary. Whether this is attributable to flavor-masking or to the antioxidant effects of these seasonings couldn't be determined (and it's worth noting that one out of four testers still picked up some WOF notes from these samples). Decker suggests that seasonings work best to prevent oxidation reactions when they're mixed with ground meat, where they can coat a larger surface area than just the outermost layer.
From the reheating-methods portion of the testing, the clearest result was that microwaving does gross things to chicken and should be avoided at all costs. Compared with the other reheating methods, the microwave gave the chicken an unappealingly spongy texture that, combined with WOF undertones, is no way to win over the leftover-leery. Reheating leftover breasts sous vide resulted in the lowest degree of WOF detected, followed by reheating in the oven, which may be more practical for the home cook.
As our guts processed the massive quantity of chicken we had just consumed, we processed our results. The most universal finding from our taste-testing was more philosophical than anything else: When you put a bite of food in your mouth and critically scrutinize it for any funky flavor, more often than not, you're going to find it. Perhaps this is the root of the problem with WOF: If you're the type who tends to give leftovers the stink eye in the first place, you're definitely going to pick out the WOF when you reheat them the next day.
Less WOF, Less Waste
An aversion to off flavors like WOF can have more profound consequences than an unpleasant leftover-eating experience. According to Jonathan Bloom, journalist and author of the book American Wasteland: How America Throws Away Nearly Half of Its Food, food waste has increased by 50% since the 1970s. "A significant amount of that happens at home," he says, and much of it comes from a preference for avoiding leftovers. "Sixty-three million tons of food is squandered in the US, and that has a direct financial cost of $280 billion," Bloom adds. These are daunting statistics, but, on the upside, as Bloom puts it, "That means we're wasteful, but we have the potential to have a major impact on the issue."
Bloom acknowledges that it's a challenge to eat leftovers with WOF. He employs a technique called "planned-overs," a one-two punch that incorporates both careful planning of meals and thinking ahead to how their leftovers can be applied to tasty new dishes. Because flavors tend to be less pronounced in cold food, try second-day meals that avoid the microwave to keep WOF under the radar. "Have grilled chicken one night and a grilled-chicken Caesar salad the next," he says. (Or, turn your leftovers into a lighter, mayo-free chicken salad, punched up with kimchi, ginger, and scallions.)
While the traditional European-style practice of buying food for same-day preparation is impractical for the vast majority of us, Bloom says that becoming a smarter shopper is essential. He recommends you "shop with a purpose, for meals that you've planned ahead." Finally, if he knows that a particular meal won't be good the second or third time around, he makes an effort to adjust the recipe so that it feeds the number of people he's serving and no more. With no leftovers, there's no need to worry about WOF.
In the end, the best advice we can give you on leftovers is to know thyself, and what you're willing to eat, lest you waste part of a perfectly good meal. And, if all else fails, maybe just add hot sauce.
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