When you think of soup and soup ingredients, vinegar might not rank highly on your mental list. Yet, many home cooks and professional chefs treat it as a sort of "secret weapon" in their soup-making arsenal. A splash of good-quality vinegar is strikingly effective at brightening and enhancing the flavors of broth, pureed or even cream soups. Of course, like any other powerful ingredient, it's easy to use too much vinegar. In that case, you'll need to take steps to rescue your meal.
A simple and logical fix is to increase the quantity of soup until its flavors are balanced. Add more broth or water, and top up the other ingredients as needed, until the vinegar's tang is adequately checked. Adding a touch of sugar or another sweetener to counter the acidity is also an option, especially if your soup is based on a sweet vegetable such as carrots or pumpkin. Chili heat plays well with acidity, as evidenced by hot-and-sour soup, so adding hot sauce, cayenne or chili powder -- if appropriate -- is another potential strategy. Aside from its own compatibility with vinegar, the hot and earthy seasoning can also help mask any added sugar.
The Chemical Solution
The simplest solution is often to negate the vinegar's acidity through chemical means. That's not as intimidating as it sounds, because the "chemical" you'd use is nothing more than baking soda. Add your soda in small increments -- from a pinch to a teaspoon at a time, depending on the size of your batch of soup -- and let it fizz and bubble until it subsides. The bubbles provide visible evidence of the soda's impact, reacting chemically with the vinegar to produce carbon dioxide. When it's done, stir your soup and taste it; then repeat the process if needed.