Why Does Soup Taste Better The Next Day? Ask Paul 

Every cook has noticed the phenomenon: You painstakingly prepare a soup, stew, or sauce, sautéing your aromatics, steeping your broth, adding ingredients in stages to maximize the flavor, and it turns out pretty good, with enough for leftovers. Then, a day or two later, you take it out of the fridge, gently reheat it, and somehow it seems much more flavorful than that first day. What’s going on?

Theories abound. For one, it could just be subjective: After you’ve been cooking for a few hours, steadily sniffing and sampling the soup, your palate is fatigued and your senses saturated with the particular flavors you’ve been wrangling. At that point, a bowl of the soup is old news to your taste buds, good but not exciting. Another day, when the air in the kitchen is clear and your appetite sharp, the same soup seems to offer much more contrast and novelty.

That cook’s-fatigue factor is indeed significant—our senses very quickly build up a tolerance to smells and tastes with prolonged exposure—but it’s not the only factor. This can be shown by serving the fresh and the leftover soup to tasters who weren’t there during the cooking. They tend to agree that a night of fridge-aging improves the flavor. The soup itself changes. But how?

Many of the chemical transformations that make soup delicious—Maillard browning, conversion of collagen to gelatin, dissolving of pectin from plant cell walls—happen only under the influence of heat, during direct cooking. Others, such as the enzyme-mediated reactions that cleave flavorful amino acids from proteins and sugars from starches, do continue at cooler temperatures, but slow to a crawl, making it unlikely that they’re responsible for very much overnight change in the fridge.

What does happen readily in liquidy mixtures at refrigerator temperatures—as anyone knows who’s brined meat or vegetables—is the gentle flow of soluble flavor compounds from where they’re more concentrated to where they’re less concentrated. An array of salty, umami, tangy elements move in and out of the components in the soup or stew, balancing and integrating the overall flavor.

Cooking beans in broth allows them to absorb some flavor from their surrounding liquid; but letting them sit for hours in the broth does a much more complete job. As meat and tomatoes sit together in Sunday gravy over the course of the week, they exchange juice and savor, turning a young-tasting meat sauce into a rich and fully realized meal.