That doesn't mean dumping vinegar after vinegar into the cooking pot, of course. Instead, use a quarter-cup measure to ladle out some of the soup or stew and add a few drops of vinegar to it. Try a couple of different ones and see which you like best.
You'll probably be surprised at how much difference there is. The other night I made a butternut-squash soup with ginger that needed a final lift of acidic seasoning.
Intuitively, I thought a squeeze of orange would probably be the right answer, but I decided to try several alternatives as well. Good thing. Orange juice was just fine, but the flavor of the fruit was too forward -- it tasted like squash-and-ginger soup with orange. I didn't want a distinctive flavor, I wanted a subtle hint.
Sherry vinegar worked well too, but again, its definite wine-like character stood out too much. Balsamic vinegar was not good: too soft and sweet. Neither was red wine vinegar. When I added enough to sharpen the flavor, the wine character was jarring.
Finally, I grabbed a bottle of apple cider vinegar and tried that. It was just the ticket. Though the vinegar by itself had a pleasant, identifiably apple flavor, when added to the soup it disappeared, leaving behind what seemed like just a more profound squash flavor.
Once I'd decided which acid to use, the question was how much. Just add it a little at a time until you find the right amount. For an eight-cup batch of soup, I added it in half-teaspoon doses until I got just the right effect with a little more than a tablespoon. Remember to go slowly -- you can always add more, but you can't take away.
Acidity can also work in surprising ways. Adding a little sour can help smooth out bitter flavors. The other night I made a soup from greens I'd harvested from my garden. Because dandelions predominated the mix (go figure), the soup had a sharp, pointed bitterness. Adding a little sherry vinegar rounded out the flavors, adding a quality that was almost sweet.
You need to be careful when adding acidity for more reasons than just its pungent effect on taste. Because acids are not just flavors, they're chemicals.
The most obvious potential negative effect of acidity is that it discolors green vegetables, turning them olive drab (it changes the chemical structure of the chlorophyll pigment). "But wait!" you say, "I thought that was because of overcooking." Well, that's right too -- the overcooking releases natural acidity from the plant itself, which causes the color change.
Acidity will also affect the texture of any kind of protein, "cooking" it without heat. But if left to marinate too long, it will break down the structure and create a mealy texture. Along the same lines, it will curdle cream if a sauce is too acidic. Also, it will delay the softening of dried beans if added too early in the cooking process.
At the same time, there are occasions when acids are used strictly for their chemical properties, with no flavor effect at all. Probably the most notable is when you use sour ingredients in pastries, such as pie crusts, cakes or even pancakes. You usually don't add enough to change the flavor, just enough to weaken the flour's gluten, creating a more tender texture.
Would you ever have guessed that just a little squeeze of something sour could accomplish so much?