During cooking, starch passes from the beans into the liquid. While the dish is hot, this starch is fluid and goes unnoticed; as it cools, the molecules link together and the liquid thickens noticeably. Water that evaporates as it cools adds to this effect. That is why a dish left at the right consistency in the evening looks solid in the morning. Stir in a little water or broth while reheating to restore the consistency.
Beans do not just soften as they cook; some of their starch also passes into the water. That is why the liquid in a pot of dried beans is noticeably thicker at the end of cooking than it was at the start. While the dish is hot, this starch is fluid and does not affect the consistency much. As the temperature drops, the molecules link together and the liquid starts to hold like a network; that is what causes it to thicken. That is why the same recipe, even when made with the same measurements, looks different on the first and second day. For the same reason, recipe consistency is described while the dish is hot.
The second factor is simpler: evaporation. Water continues to evaporate from a dish as it cools with the pot uncovered, reducing its volume. If the lid is closed, condensed water drips back into the food. That is why, while the dish cools, the lid is left ajar, and once the food is lukewarm, it is transferred to a sealed container. When these two factors combine, a dish that had the right consistency in the evening can be firm enough to cut with a spoon by morning. Do not put the pot directly in the refrigerator; a hot pot raises the temperature inside the refrigerator. Instead, divide the food into shallow containers and refrigerate it within 2 hours.
This is not a flaw and often works in the dish’s favor. In legume dishes that sit, the flavors meld and the result is usually richer than on the first day. The only problem is the consistency, and it is easy to fix. While reheating, add a little hot water or broth to the pot and loosen the dish by stirring over low heat. Add the water a little at a time; adding it all at once will make the dish watery. Using hot water also makes this easier; cold water makes the dish seize up all at once.
The reheating method also determines the texture. Legumes heated over high heat and stirred vigorously fall apart; the spoon crushes the beans and turns the dish into purée. Low heat and an occasional gentle stir are enough. Keep an eye on the bottom of the pot, too, because thickened starch sticks very easily. If reheating in the microwave, stir the food from time to time and cover it. Before reheating, check the bottom once with a spoon; stir in the starch that has settled there first.
The final adjustment is salt, which is often overlooked. When the dish is thinned, its salt concentration drops, so taste and adjust the salt after adding water. The acidity can also be readjusted; a few drops of lemon can noticeably brighten a dish that has been sitting. The dish is reheated once; to avoid reheating it repeatedly, set aside and heat only the amount needed for that meal, not the entire pot. If leftovers are cooled again, apply the same rapid-cooling rule a second time. Finish the reheated portion; do not pour it back into the pot to cool again.