In culinary terms, the definition of the word sweat means to cook something over low heat in a small amount of fat, usually in a covered pan or pot. The word sweating is often used to describe the way aromatic vegetables such as onions, carrots, and celery are cooked before adding other ingredients.
The objective in sweating vegetables is to soften them and release the moisture in them, not to brown them.
This release of moisture is how the term sweat gets its name.
Sweating is usually a preliminary step in preparing vegetables to add to a dish, ensuring they aren't still raw and they have the desired texture when a crunchy vegetable isn't wanted in the finished dish.
It may also be known as butter-steam, especially because butter is a commonly used fat for the sweating step of a recipe.
What Happens During Sweating in Cooking?
Sweating concentrates flavors and releases sugars. The vegetables become tender as tough cell walls are broken down, and in the case of onions, they may become translucent. This change in texture is often desirable in soups, stews, and sauces.
Sweating is similar to sautéeing, with the difference being that in the latter technique, higher heat is used, and the food will often be browned. Sweating is more about softening, not browning. It's used when you don't want the brown color and flavors that happen in the Maillard reaction of browning.
Often, the cook will keep stirring the vegetables during sweating to ensure they are cooking uniformly and they haven't started to brown. Cutting the vegetables into uniform pieces also helps ensure that they all cook at the same rate. Salt is also often added as it helps draw out the moisture.
How to Sweat Vegetables
A common step when making a dish where aromatic vegetables are used is to dice them and sweat them.
Usually, these are dishes, such as stews, where the vegetables aren't the featured item, but they add their background flavors and aromas to complete the dish. You'll see it in recipes for soups and braised meat dishes as well.
First, the vegetables are chopped uniformly. Diced into quarter inch pieces, they will sweat in less time, and there is more surface area for the process than with a larger chop. If garlic is called for, it should be minced as well, but often you will wait to add it to the other vegetables as it may cook too quickly if added at first.
The pan is heated over medium-low heat, and a small amount of butter or oil is added to coat the bottom of the pan. Once it is hot, the vegetables and salt can be added. Now the cook needs to ensure the pan isn't getting too hot and there is just gentle sizzling rather than any vigorous pops. Adjust the heat accordingly.
Stir the vegetables often and observe for any signs of unwanted browning. It will take five to 10 minutes for the vegetables to become softened. If onions are part of the mixture, you will know you are done when they are translucent.