When you cook a soup uncovered, some of the liquid cooks away over time, making the broth thicker. Thickening soup by cooking it uncovered also intensifies flavors because seasoning does not cook away in the same way as broth. Although cooking soup uncovered is one method for thickening, other techniques work as well: using a roux or a thickener such as corn starch will also add body to your soup, although it won't make your soup more flavorful.
Soups Made With Legumes
Soups made with legumes, or members of the bean family, thicken as you cook them uncovered, evaporating the moisture. The beans also continue to break down, adding additional texture. Lentil and split pea soups, in particular, use legumes that are small enough to break down in under an hour so they will thicken quickly after this initial stage. Stir legume-based soups often while thickening them: The beans may settle, leaving the thinner broth at the top of the pan and increasing the risk of burning on the bottom.
After pureeing a soup thickened with vegetables such as squash, potatoes or carrots, you may find that you have not achieved the desired thickness from the pureed vegetables alone. Cooking the soup uncovered will increase the ratio of pureed solids to liquid broth, making the soup heartier and more satisfying. By the time you have pureed your soup, you should have already cooked the ingredients long enough to bring out their full flavors, so cook the pureed liquid just long enough to achieve the texture you desire.
Soups such as onion and chicken soup with clear broths don't exactly thicken when you cook them uncovered but the liquids do reduce, increasing the ratio of solids to liquids. The result is a more substantial soup which is satisfying in much the same way as a thickened pea soup or pureed soup. The flavors intensify as well as liquid evaporates, leaving a denser concentration of herbs, spices and infused stock.
The amount of time necessary to thicken an uncovered soup depends on the height of the flame: Hotter heat thickens soup faster, although you have to pay more attention to keep them from burning. The shape of the pot affects cooking time as well because greater surface area allows more liquid to evaporate. Cooking time also depends on how thick or thin your soup was when you started reducing it. The thinner the original soup, the more time it will take to thicken.
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