Hot legumes that are drained come into contact with air and dry quickly on the surface; the beans shrink, their skins crack, and their texture turns floury. Legumes cooled in their own cooking liquid retain moisture, continue absorbing salt and flavor, and stay plump. That is why legumes for a cooked dish are left in their liquid. The order is different for legumes cooked for a salad, where cooking needs to be stopped.
A cooked legume is full of water, and some of that water sits near the surface. When a hot bean is transferred to a strainer, this surface water evaporates quickly; the outer layer dries and shrinks, and the skin stretches and cracks. The result is a bean with a wrinkled exterior and a floury interior. If the same legumes are left in their cooking liquid, the surface does not dry out at all and the texture stays as it is. Cooked legumes are not an ingredient to forget in a strainer while hot. So when cooking is done, take the pot off the heat but leave the beans in the pot.
Another benefit is flavor. The cooking liquid carries salt, spices, and substances released from the legumes. As the legumes cool, they shrink slightly and draw in some of the surrounding liquid. In other words, cooling is another stage in which the beans take in flavor. Legumes cooled after being separated from their liquid miss this stage and remain blander; adding salt afterward does not produce the same result. For the same reason, do not leave the cooking liquid unsalted; salt seasons both the liquid and the beans.
Storage follows the same principle. If cooked legumes are going into the refrigerator, do not separate them from their liquid; put the beans in a container with enough cooking liquid to cover them. This keeps them plump for days, and they can be drained when needed. Legumes stored without liquid dry out and become firm in the refrigerator. The same rule applies to freezing; add some liquid to the container. Write the cooking date on the container so you do not have to guess how long it has been stored. A glass container does not retain odors, and its liquid level can be seen from the outside.
The approach is different for legumes cooked for a salad. The goal is for the beans to stay whole and firm; legumes left in their liquid continue cooking and become soft and fall apart. So when salad legumes reach the desired texture, drain them and cool them immediately. To prevent them from drying out, drizzle a little olive oil or dressing over them while they are hot. To cool them, spread the beans in a single layer on a wide tray so they cool quickly and do not stick together.
Cooling speed also matters for food safety. Do not leave a potful of hot legumes on the counter for hours; transferring them to a wide, shallow container and spreading them in a thin layer helps them cool much faster. Setting the container in an ice bath shortens the time further. As soon as the legumes are no longer warm, and no later than 2 hours after cooking, move the container to the refrigerator and eat them within 3 to 4 days. Make sure the food has cooled before closing the container; steam condenses in a container sealed while hot. Place it on the middle refrigerator shelf, where the temperature is stable, rather than on the coldest shelf.