Squeeze a little fresh lemon juice on sautéed mushrooms or splash a bit of wine vinegar into bean soup, and you’ll quickly taste how acids can brighten the flavor of food. But acidic ingredients do a lot more in cooking than enhance flavors. They have a major impact on the color and texture of fruits and vegetables.
Pinning down the precise ways that acids affect ingredients in recipes is a bit tricky, though, because acids can seem paradoxical. They contradict themselves time and again. Just when you think you understand how acids behave, they turn around and do the opposite.
For example, acids make cooked broccoli fade to a dull grayish green; and yet acids also prevent cooked red cabbage from turning blue. Acids help pickles stay crisp and delicious; but then again, vinaigrette seems to make lettuce go limp. So, do acids drain color or protect it? Keep food firm or turn it soft? Answer: all of the above.
Fortunately, acids aren’t fickle; they follow a simple set of rules. Learn them and you’ll be able to use acids to your advantage when cooking fruits and vegetables.
What’s an acid?
Most often when we talk about acids, we mean the tart ingredients that you add to foods. Some of these acids are fairly obvious, but others might surprise you. Even lightly acidic ingredients—and the naturally occurring acids in fruits and vegetables, as well—can cause a color or textural change during cooking. Here are some common acidic ingredients:
• lemons and limes
• soy sauce
How acids affect color
Fruits and vegetables get their colors from different natural chemical pigments. When a fruit or vegetable changes color during cooking, the pigments have undergone a chemical reaction—and more often than not, acids are involved. Here’s what’s going on:
Acids help red fruits and vegetables stay red. Red cabbage, red grapes and plums, cherries, cranberries, strawberries, and raspberries contain pigments called anthocyanins, which are responsible for most of the red, purple, and blue colors in plants. Anthocyanins turn red in acidic environments and blue in alkaline environments. So when cooking a red fruit or vegetable, adding an acidic ingredient such as vinegar, lemon juice, or tomato juice helps prevent the food from turning purple or blue.
Acids turn green vegetables muddy. Take broccoli, for instance. Steamed briefly, it turns a vibrant green; boiled for 15 minutes, it turns olive-drab. This also happens to other green vegetables, like spinach, kale, sweet peas, and green beans. The longer these vegetables cook, the more their cell walls break down. Natural acids escape the cells and react with the green pigment chlorophyll, turning it dull.
So how can you cook a green vegetable and keep its color bright? Cook it to a minimum, using a quick-cooking method such as steaming, boiling, or stir-frying. Less cooking time means less opportunity for acids to escape the cells and react with chlorophyll. If you don’t plan to eat steamed or boiled green vegetables as soon as they’re cooked, plunge them into icy water to bring the cooking (and the escape of acids) to a halt. And if you want to season a cooked green vegetable with lemon juice or vinegar, do it right before serving or the color will fade.
That said, greenness isn’t everything. With slow-cooking methods like braising or stewing, you sacrifice green vegetables’ color, but you gain deeper flavor.
Orange and yellow vegetables don’t mind acids. Carrots, squash, sweet potatoes, tomatoes, and red peppers get their color from pigments known as carotenoids, which are relatively immune to acids.
How acids affect texture
Textural changes in fruits and vegetables—as when they become tender during cooking— have to do with cell walls breaking down. Acids can prevent texture changes or encourage them, depending on the situation.
In acidic surroundings, fruits and vegetables stay firm. If you’ve ever tried to add raw onions or other vegetables to an already simmering tomato sauce, then you know that the vegetables won’t become tender. That’s because the cellulose in vegetables and fruits doesn’t dissolve in acidic conditions, even after hours of cooking. So cook fruits and vegetables to the desired texture before adding tomatoes, wine, or other acidic ingredients. And if it’s a firmer texture you’re after, as when poaching a pear, a strategic addition of acid to the cooking liquid should do the trick.
With dried legumes, the tactical use of acids can help you achieve a texture that’s neither too firm nor too soft but just right. For example, when making baked beans (which use molasses, an acidic ingredient) or tomato-based bean soups, soak and cook the beans in water until they’re soft. Then add your acidic sauce or soup ingredients, which will help firm the outside of the beans, so they won’t become too mushy as they cook further and absorb the flavors of the sauce or soup.
Acids (and oil) make lettuce go limp. When a dressed salad wilts, both the oil and the acid in the dressing are to blame. The oil in a vinaigrette briefly protects the surface of lettuce leaves from the acid, which will make the leaves turn dark. But before long, the oil breaks through the waxy coating on the leaves’ surface. When this happens, the leaves soak up the dressing, wilt, and darken. Cuts and tears on the lettuce also let vinaigrette seep into the leaves.
Of course, a green salad isn’t a salad without a zippy vinaigrette; so if you want yours to be tasty and have a pleasing texture, dress it right before serving.
Acids help jams and jellies set
Setting jams and jellies successfully depends on a balance between acid, pectin, and sugar. Acid encourages the natural pectin in fruit to gel. Fruits like sweet apples, peaches, pears, apricots, and sweet cherries don’t release enough natural pectin during simmering, so jams using these low-acid fruits usually include lemon juice or highly acidic fruit (sour cherries, tart apples, rhubarb) to increase the gelling power.