Most vegetable cooking water should not be discarded. Some of the vegetables’ flavor and color dissolve into it, and the water can be used in soup, rice, sauces, or dough. But not all cooking water should be reused: the water from sulfurous vegetables such as cauliflower and cabbage has a strong smell, water from green potatoes should not be used, and overly salty water can throw off a recipe’s balance.
When vegetables are boiled, flavor, color, and water-soluble compounds pass into the cooking water. The color makes this clear: carrot water turns orange, beet water deep red, and pea water green. This water is essentially a light vegetable broth and is an ingredient in its own right. Pouring it out means wasting a ready-made source of flavor, even though most kitchens have an immediate use for it. The water carries the vegetable’s aroma too, so consider in advance which dish it will go into. For this reason, cooking water can be considered part of the recipe, just like the vegetable itself.
The most direct use is soup. Vegetable cooking water can replace plain water added to soup and enrich the base. A second common use is rice: rice cooked in vegetable broth instead of water has a noticeably fuller flavor. It can also be used in sauces and stews. It is possible to use it in dough as well, but the water must be completely cooled first. Pasta and bulgur can also be cooked in this water; the grains absorb the liquid, so its flavor goes directly into them.
Not all cooking water should be reused, and it is important to know the difference. The water from cauliflower, cabbage, and broccoli contains sulfur compounds; it has a strong smell and transfers that smell to the dish, so it should be discarded. Do not use water from green or sprouted potatoes. Discard the water from any bitter-tasting vegetable as well, because the bitterness collects in the water and carries over into the dish. The water’s color is also a clue: cloudy, foamy water is often too pungent to use.
Salt is another limitation. Vegetable cooking water is often salted, and adding it to a recipe changes the recipe’s salt balance from the outset. For this reason, leave the salt out of a dish that will use cooking water until the end, then taste and adjust it. If the water is very salty, dilute it with plain water. Cooking liquid from dried legumes is a separate case: chickpea cooking liquid can be used as a binder in some recipes. For this reason, if you plan to save cooking water, use salt sparingly while boiling.
It is easy to store too. Strain the water, cool it quickly, and refrigerate it in a covered container; use it within a few days. For longer storage, pour it into ice cube trays and freeze it, then transfer the cubes to a bag so you can take out only what you need. Write which vegetable the water came from on the container, because it is harder to tell after freezing. Frozen cubes can go directly into a pot; there is no need to thaw them first. Write the date on the bag too, because the flavor weakens after a few months in the freezer.