Whey is the clear liquid separated during yogurt straining or cheese making. It contains some milk proteins, milk sugar, and minerals, and has a mildly tart taste. When used in place of water in dough, it gives a softer interior and a darker crust. It can also be used in soups, sauces, and grain cooking. It keeps in the refrigerator for a few days and can be frozen.
The clear, greenish-yellow liquid that separates from the curds when yogurt is strained or cheese is cultured is called whey. Although it looks like plain water, its contents are different: it carries some milk proteins, milk sugar, and minerals. It tastes mildly tart. In the kitchen, it therefore behaves like an acidic liquid, and that property determines where to use it. Making use of the liquid is a natural second step after straining yogurt. Liquid from cheese may be a little cloudier and saltier than liquid from yogurt.
Dough is the most effective use. When substituted for water in bread, savory bun, and pancake dough, three things change at once: its protein and sugar produce a darker crust in the oven, its acidity softens the gluten network, and the interior becomes softer. In recipes that use baking soda, it also helps the dough rise. Use a one-to-one measure: replace one measure of water with one measure of whey. If using it in dough, bring the liquid to room temperature; cold liquid straight from the refrigerator slows the yeast.
The second area of use is soups and sauces. It adds a little depth of flavor to vegetable soup and is already a good fit in yogurt-based soups. It is also used in smoothies and drinks. When cooking bulgur and rice, it can replace some of the water. By contrast, do not use it for dried legumes, because the acid hardens their skins and extends the cooking time. When cooking grains, do not replace all of the water; substitute only part of it. For the same reason, do not use it without measuring in pickling and fermented recipes.
There are a few more uses. Added to a marinade for meat and chicken, its acidity softens the surface. It also works well with yeast in bread dough. Suggestions for uses outside the kitchen, such as watering plants, are set aside here; the subject is cooking. Do not boil the liquid: at high heat, its proteins coagulate and leave particles as it loses its clarity. Marinate the meat covered in the refrigerator, and not for a long time, because the acid softens the surface more than needed. So think of the liquid as an acidic ingredient, not just as a liquid.
Storage is easy, but it is not indefinite. Transfer the liquid to a clean, sealed container; it keeps in the refrigerator for a few days. For longer storage, pour it into ice cube trays, freeze it, and transfer the cubes to a bag; that way, you can take out only what you need each time. Do not use liquid that has developed a sharp smell, become cloudy, or started to foam. Write the date on the container so you do not have to guess how long it has been stored. Do not fill the container to the top; the liquid expands as it freezes and may lift the lid.