For some people, Leftover foods have no space in their kitchens, and they simply discard them. Which is bad because there are some foods that, in fact, improve in flavour the days following preparation. They can even help us save time and money, and that's before we even discuss that.
Some individuals dislike leftovers because they stigmatise them and link them to those of lower status in society. Others follow the rule that food that has been freshly prepared is always better in flavour and texture than food that has been chilled and reheated. Individuals who belong to the first group may work through their preconceptions. But perhaps a little knowledge will help us convince the latter group.
It's true that the textural variation cannot be disregarded. The busy home cook may find it difficult to return once crisp and crunchy foods to their former state. (However, we would attribute the majority of this issue to the microwave; you'll have more success reheating the same types of food in the oven, air fryer or on the stove).
If we focus on soups, stews, and other saucy foods instead. All are excellent choices for leftovers because they won't dry out and their texture won't probably be affected. What happens to these foods' flavours when they sit around? The short answer is that it can change, but how much it changes and how we see it depends on many other things.
Just because you take a dish from the burner or oven doesn't imply the flavours stop transforming. Food simply remains in that static form when we stop actively cooking but it is constantly changing. There are just two sites where changes can take place that affect how food tastes the next day—and even occasionally for the better. One of them takes place in the food itself, and the other significant portion occurs inside the person tasting the food. People frequently point out that leftovers differ from fresh food in that the spices have mellowed. Even if you cook a curry well, it still tastes a little bit harsh when you taste it that day. The flavor, though, sort of blends and matures and is not so strong the next day. Similar to the marinating procedure, flavour transference can also occur when items are floating in a liquid or sauce.
One thing that occurs is that, over time, the flavour molecules of many spices are able to locate the fats in a dish, making the flavour molecules more approachable to our senses. Fat can bring flavours and aromas to our palates that we might not otherwise perceive. Our experience of different flavours is intensified and prolonged when fat coats the tongue, allowing different aromatic components to stay in contact with our taste buds for longer periods of time.
When these phenomena come together, flavour develops from a variety of distinct elements to a united chorus, becoming more well-rounded and harmonic. However, depending on the dish and one's preferred flavour profile, the taste composition of leftovers can also change, becoming sweeter, losing bitterness, or developing more umami.
The proteins in the meat break down more and it becomes softer. This breakdown results in the release of amino acids that can improve the umami of food. Foods may release sugars after a further breakdown. Starchy foods in particular will release more glucose and sucrose. This extra sweetness might have the unintended consequence of muting the spice and acidity. Citrus juice loses some of its brightness with time (in terms of acidity).
However, not everyone is convinced that these modifications have a significant impact. Yes, soups and stews that have been allowed to rest for an overnight or more do have a little different flavor, although the differences are small and hard to distinguish. The main exception is chilli and other hot, sour foods: Their flavour fades and loses brightness over time.