Add a little seasoning to taste

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 flavour, 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 flavour 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 flavour potentiator - it works chemically to make other flavours taste like themselves. Acidity as a seasoning gives a dish backbone or structure, which allows other flavours 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 flavours. Sometimes a couple of drops of lemon juice will be all 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 flavour of broiled or grilled fish.

I wouldn't think of cooking vegetables without acidity - a squirt of lemon for sauteed broccoli, a hit of red wine vinegar for 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. 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 expensive extra-virgin olive oil will make do with cheap vinegar. Good vinegars offer more variety.

Though there are a lot of fancy flavoured vinegars, concentrate on using the basics before exploring the others.

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

Fortunately, they are incredibly easy to make at home. Buy a couple of bottles of decent fruity wine, such as a merlot or shiraz. Put them in a jar with a bottle of unpasteurised commercial red wine vinegar. Cover the jar with a cloth secured with a rubber band to let air in and keep 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 oxidise quickly.

Champagne vinegar 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, or brushed on a piece of meat before grilling. Sherry vinegar has a distinctive nutty, wine-like quality and great depth of flavour.

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

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 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 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 flavour of the fruit was too forward - it tasted like squash-and-ginger soup with orange.

Sherry vinegar worked well too, but its definite wine-like character stood out. Balsamic vinegar was not good: too soft and sweet. Neither was red wine vinegar. When I added enough to sharpen the flavour, the wine character was jarring.

Finally, I tried cider vinegar. It was just the ticket. The vinegar by itself had an apple flavour, but when added to the soup it disappeared, leaving a more profound squash flavour.

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. 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 flavours. The other night, I made a soup from greens from my garden. Because dandelions predominated in the mix, the soup had a bitterness. Adding a little sherry vinegar rounded out the flavours, adding a quality that was almost sweet.

Be careful when adding acidity, because acids are not just flavours, they're chemicals. The most obvious negative effect of acidity is that it discolours 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, which causes the colour 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 flavour 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 flavour, 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?

- Russ Parsons

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