"Leftovers" is a dirty word to some. Which is unfortunate, because there are some things that actually taste better in the days after they're made. And that's before we even talk about their ability to save us time and money.
Some people are averse to leftovers because they attach a stigma to them, associating them with a lower socioeconomic status. Others are guided by the notion that freshly prepared is always superior in taste and texture to anything refrigerated and reheated. For the first group, it's up to the individual - and maybe their therapist - to work through their preconceptions. But perhaps I can persuade the latter segment with a bit of information.
It's true, the textural difference can't be ignored. It's easy for the busy home cook to struggle with restoring dishes that were once crispy and crunchy to their original state. (However, I'd blame the microwave for most of this problem; you'll get better results reheating those kinds of foods in the oven, in the air fryer or on the stovetop.)
But let's focus on soups, stews and other saucy dishes. All are prime candidates for leftovers because they'll never dry out and the texture isn't likely to be compromised. What happens to the flavor of such dishes as they rest? The simple answer is that it can change, but the magnitude of that difference and our perception of it rely on a variety of factors.
"There are really two places where changes can occur that make food taste different, and sometimes better, the next day," said Pamela Dalton, of the Monell Chemical Senses Center in Philadelphia. "One of them is happening in the food itself, and the other big piece is happening within the individual that's tasting the food."
As for the food, just because you take something out of the oven or off the stove doesn't mean the flavors of that dish stop evolving. "We think that when we stop actively cooking food, it just stays in that static state," Dalton said. "But, of course, there are changes happening all the time."
One of the main differences people cite with leftovers is a mellowing of spices. Chef Vikram Sunderam of Rasika in Washington points to curries as an example. "When you taste it that very day, even though you cook it well, it still feels a bit harsh on the palate," he said. "But when you have it the next day, it sort of blends and matures and is not so strong."
There's also a transference of flavor that can happen with ingredients suspended in a liquid or sauce, Sunderam said, similar to the process of marinating.
One thing that happens is the flavor molecules of many spices are able to seek out the fats in a dish over time, which results in the flavor molecules being more accessible to our senses. As cookbook author Samin Nosrat wrote in "Salt, Fat, Acid, Heat": "Fat can convey aromas - and enhance flavors - to our palates that would otherwise go unnoticed. Fat coats the tongue, allowing various aromatic compounds to stay in contact with our taste buds for longer periods of time, intensifying and prolonging our experience of various flavors."
When these phenomena combine, flavor becomes more rounded and harmonious, evolving from a collection of individual ingredients to a unified chorus.
But the composition of leftovers' tastes can also shift - becoming sweeter, decreasing in bitterness or developing more umami - which can be a positive or negative depending on the dish and one's preferred flavor profile.
"Sugars can break down further and be released from foods," Dalton said. "Particularly, starchy foods will liberate more glucose and sucrose." This additional sweetness can have the potentially adverse effect of dampening spice and acidity. (Regarding acidity, the brightness of citrus juice also diminishes over time.)
Oxidation is primarily associated with the browning of cut apples or potatoes, but it can affect flavor, too. With produce high in sulfur, such as alliums and cruciferous vegetables, exposure to oxygen will help them to become less bitter over time.
When it comes to meat, "the proteins in meats will break down more and it'll become softer," Dalton said. As a result of this breakdown, amino acids are released that can enhance the umami of a dish.
But not everyone believes that these changes make much of a difference. "Yes, there are some minor differences in flavor with soups and stews that have been allowed to rest overnight or longer, though the differences are subtle and difficult to tease apart," cookbook author J. Kenji López-Alt wrote in Serious Eats. "Chili and other spicy, acidic dishes are the big exception: With time, their flavor becomes muted, losing brightness."
I know there are others - perhaps you - who would vehemently disagree. So perhaps it comes down more to the individual: their taste buds, experiences and emotions.
"If you're the person who was cooking, you're probably smelling and tasting as you go along and you have essentially adapted your sensory systems, your smell and taste, to that food," Dalton said. "It's often why when you finally sit down to eat, the cook is really not all that hungry. But next day, those leftovers are incredibly enticing, especially if you saw everyone else enjoying the food."