LOS ANGELES - When cooks read "season to taste," they reach for the salt shaker. That's not a bad start: A judicious sprinkling with salt will awaken many a dull dish. But just as a little salt unlocks flavor, so can a few drops of acidity.

Add a shot of vinegar to a stew of white beans and shrimp and notice how the earthy flavor of the beans gains definition and complexity.

Do the same with pureed winter squash and a dish dominated by rich and sweet now has a round, full-fruit character.

Though the results may be similar, salt and acidity work differently. Salt is a flavor potentiator - it works chemically to make other flavors taste like themselves. Acidity as a seasoning gives a dish backbone or structure, which allows other flavors to stand out.

It doesn't take much. As with salt, you don't want to taste the seasoning itself; you just want the effect it has on other flavors. Sometimes only a couple of drops of lemon juice will be all that it takes.

Many cooks understand this. After all, what would a salad taste like dressed only with oil? It's the vinegar that makes vinaigrette. And think of the way a squirt of lemon elevates the flavor of broiled or grilled fish.

How many times have you deglazed a roasting pan with red wine? It's not just the fruit flavor you're after, but the acidity. Cooking down tomatoes in a sauce or soup has the same effect.

If you've heard about a "gastrique" served with a dish, it's basically a syrup of boiled vinegar and sugar. Traditionally, it is used to season dishes of meat combined with fruit. Used clumsily, it's a fancy version of sweet-and-sour sauce.

I wouldn't think of cooking vegetables without acidity - a squirt of lemon for sauteed broccoli, a hit of red wine vinegar for summertime ratatouille. And almost every time I cook fruit, there's bound to be a jolt of citrus to balance the sweetness.

But all acids are not created alike. Any well-stocked pantry should have several to choose from.

Start with citrus fruit: Lemons are the most common and the most useful because they are harvested so early that they don't have much flavor besides their tartness. Meyer lemons, which are harvested fully ripe, are an exception. Oranges have a sweeter sourness than lemons, and limes are tart but have a bracing herbaceous quality.

Then there is vinegar or, more appropriately, vinegars. Every pantry ought to have several. It's funny how cooks who brag about $30-a-bottle extra-virgin olive oil will make do with cheap vinegar.

Good vinegars offer more variety and typically can be found for less than $15 a bottle - rarities such as true aceto balsamico and aged Vinaigre de Banyuls aside.

Although there are a lot of fancy flavored vinegars, concentrate on using the basics before exploring things such as fig-balsamic.

The mainstay acid should be a good quality red wine vinegar, one that tastes like an extremely tart but otherwise well-made red wine. These are hard to find commercially - many taste acrid.

Fortunately, they are incredibly easy to make at home. Here's the outline: Buy a couple bottles of decent fruity wine, such as a Zinfandel or a Syrah. Put them in a jar with a bottle of unpasteurized commercial red wine vinegar. Cover the jar with a cloth napkin secured with a rubber band to let the air in and keep the fruit flies out. Leave in a cool, dark corner of the kitchen for a month or a month and a half. That's it.

Good white-wine vinegars are harder to make at home because they oxidize quickly. But the standard restaurant supply model made by Vilux is perfectly acceptable. Or use Champagne vinegar, which is sharp with a subtle sweetness. Rice vinegar is also sweet, but with a rounder texture.

Balsamic vinegar is known for its sweetness, but in a burnt-sugar-caramel way that doesn't fit most culinary purposes (and certainly not salads). It is useful in marinades, though, or brushed on a piece of meat before grilling. Sherry vinegar has a distinctive nutty, winelike quality and great depth of flavor.

Finally, don't overlook that old American standby, apple cider vinegar. Good ones have terrific fruit character. Look for unfiltered or unpasteurized.

How do you decide which one to use? Until you get an instinctive feeling for different vinegars, the best solution is to try them all.

That doesn't mean dumping vinegar after vinegar into the cooking pot, of course. Instead, use a one-quarter-cup measure to ladle out some of the soup or stew and add a few drops of vinegar to it. Try different ones and see which you like best.

You'll probably be surprised at the difference. The other night I made a butternut-squash soup with ginger that needed a final lift of acidic seasoning.

I thought a squeeze of orange would be the right answer, but I decided to try several alternatives. Good thing. Orange juice was 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 its definite winelike character stood out. 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. It was just the ticket. The vinegar by itself had an apple flavor, but when added to the soup it disappeared, leaving 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 one-half-teaspoon doses until I got the right effect with a tablespoon. Go slowly - you can always add more, but you can't take away.

Acidity also works in surprising ways. Adding a little sour can smooth out bitter flavors. The other night, I made a soup from greens I'd harvested from my garden. Because dandelions predominated in the mix, the soup had a bitterness. Adding a little sherry vinegar rounded out the flavors, adding a quality that was almost sweet.

Be careful when adding acidity, because acids are not just flavors, they're chemicals.

The most obvious 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 protein, "cooking'' it without heat. If left to marinate too long, it will break down the structure and create a mealy texture.

Along the same lines, if a sauce is too acidic it will curdle cream. Also, acids 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 for their chemical properties, with no flavor effect at all. The most notable is using 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 have guessed that a little squeeze of something sour could accomplish so much?

White Bean and Shrimp Stew With Dandelion Greens

Pimenton de la vera, or smoked Spanish paprika, is available at select gourmet markets and cooking stores.

3 tablespoons olive oil, divided

1/4 pound diced pancetta or Spanish chorizo

1 onion, diced (about 1 cup)

4 cloves garlic, minced, divided

1 pound dried Great Northern beans

7 cups water

• Salt

1 pound peeled medium shrimp

1 teaspoon pimenton de la vera

1/4 teaspoon ground cumin

1 bunch dandelion greens, chopped (or Swiss chard, mustard greens or kale)

• Sherry vinegar

Heat the oven to 350 degrees.

Heat 2 tablespoons olive oil in a Dutch oven over medium heat. Add the pancetta and cook, stirring frequently, until it begins to brown and render its fat, about 3 minutes. Add the onion and cook until soft, about 5 minutes. Add 3 cloves of minced garlic (about 2 tablespoons) and cook until fragrant, about 3 minutes.

Add the dried beans and water, cover tightly and bring to a simmer. When the water is simmering, stir and place in the oven. Bake for 1 hour before stirring in 1 teaspoon salt. Cover again and return to the oven to cook, stirring occasionally, until the beans are tender, about 1 1/2 to 2 hours total.

Meanwhile, season the shrimp with the pimenton, ground cumin, the remaining 1 clove minced garlic and 1/2 teaspoon salt. Heat a medium nonstick skillet over medium-high heat, add 1 tablespoon olive oil and, when it is very hot, add the shrimp. Cook, stirring constantly, until the shrimp are bright red on the outside and firm, about 3 minutes. Remove from heat and reserve.

When the beans are tender, stir the shrimp and the chopped dandelion greens into the stew. Cook just long enough to wilt the greens, about 5 minutes. Season to taste, adding first salt and then the sherry vinegar, adding it about 1/4 teaspoon at a time. It will take only about 1 teaspoon total. The stew shouldn't taste noticeably sour, just more complex.

Total time: 45 minutes, plus 11/2 to 2 hours cooking time for the beans.

Servings: 6 to 8.

Each of 8 servings: 353 calories; 28 grams protein; 35 grams carbohydrates; 11 grams fiber; 12 grams fat; 3 grams saturated fat; 123 milligrams cholesterol; 763 milligrams sodium.