Acid keeps vegetables and legumes tougher, longer. Anything containing cellulose or pectin, including legumes, fruits, and vegetables, will cook much more slowly in the presence of acid. While ten to fifteen minutes of simmering in water is enough to soften carrots into baby food, they’ll still be somewhat firm after an hour of stewing in red wine. The acid in tomatoes explains why those pesky onions float to the top of a pot of sauce or soup and stay there, never getting soft, even after hours of cooking. To prevent this crunchy mishap, cook onions until they’re tender before adding any tomatoes, wine, or vinegar to the pot.
When cooking beans or any legumes, including the chickpeas for hummus, a pinch of baking soda will gently nudge the bean water away from acidity toward alkalinity , ensuring tenderness. And, just like those onions, cook legumes until they are completely tender before adding anything acidic. A great Mexican chef once told me that dousing cooked beans with vinegar or vinaigrette sort of
“uncooks” them, tightening and toughening the skins a bit. Account for that tightening when preparing beans for a salad, and cook the beans just a touch longer than you might otherwise.
Use this chemistry to your advantage when deciding how to cook vegetables.
Boiling dilutes the relatively acidic liquid contained in vegetable cells, so it will generally yield more tender vegetables than roasting will. Roast big, beautiful slices of cauliflower or Romanesco broccoli to ensure they retain their shape.
Boil potatoes or parsnips so that they melt into a puddle of tenderness, perfect for puréeing or mashing.
Acid also encourages bonds between pectin groups—the gelling agent in fruit
—so that they can trap water to help set jam or jelly. Some fruits, such as apples and blueberries, don’t contain enough acid to bond the pectin on their own, so we help them along by squeezing some fresh lemon juice into the jam pot and into fruit fillings for pies and cobblers to encourage them to set.
Acid is required when using chemical leavenings such as baking soda or baking powder. Visualize the baking soda and vinegar volcanoes of your elementary school science projects. Just like that, but on a much smaller scale, acid reacts with baking soda to release carbon dioxide bubbles to leaven baked goods. Doughs and batters leavened by baking soda should also have an acidic ingredient such as natural cocoa powder, brown sugar, honey, or buttermilk.
Baking powder, on the other hand, already contains powdered tartaric acid and doesn’t need an external source of acid to react.
Acid encourages the proteins in an egg white to assemble, or coagulate , more quickly but less densely than they otherwise would. Under normal conditions, strands of egg proteins unravel and tighten when heated. As they do, the strands squeeze out water, causing eggs to toughen and dry out. Acid draws egg proteins together before they can unravel, which inhibits them from joining too closely. A few secret drops of lemon juice will produce creamier, more tender scrambled eggs. For perfect poached eggs, add a capful of vinegar into boiling water to help speed up coagulation of the white and strengthen the outer texture, while preserving the runny yolk.
Acid aids in stabilizing whipped egg whites by encouraging more, finer air pockets, helping to increase the volume of the egg white foam. Though cream of tartar—a by-product of wine-making—is the form of acid traditionally added to egg whites as they’re whipped for meringues, cakes, and soufflés, a few drops of vinegar or lemon juice per egg white will yield a similar result.
Dairy proteins called casein will coagulate, or curdle, with the addition of acid. With the exception of butter and heavy cream, which are very low in protein, dairy should only be added to acidic dishes at the last minute. While curdled fresh dairy, when unintended, is usually inedible, this same reaction makes cultured dairy—from yogurt to crème fraîche to cheese—possible, offering us a delicious, entirely new category of acidic ingredients to incorporate
into food. Try making your own crème fraîche —it couldn’t be easier. Just combine 2 tablespoons of crème fraîche or cultured buttermilk with 2 cups of heavy cream. Pour into a clean glass jar, cover loosely or leave uncovered, and leave out at warm room temperature for 2 days, or until it thickens. That’s it. Use it in Blue Cheese Dressing , Chicken with Vinegar , or Tangy Whipped Cream . Cover and store it in the fridge for up to two weeks. Use the last few spoonfuls to start the next batch in the same way.
When acid is incorporated into doughs and batters, it will tenderize them, much as fat does. Whether it comes in the form of cultured dairy, natural (nonalkalized) cocoa powder, or vinegar, acid in a dough or a batter will disrupt the gluten network, resulting in a more tender product. If it’s chewiness you’re after, wait as long as possible into the process of dough-making to add acidic ingredients.
Acid tenderizes, then toughens, meat and fish proteins. Imagine protein as coiled strands folded up into bundles. When acid comes into contact with the coils, they unfold and unwind. This process is called denaturation . These denatured proteins then begin to bump into each other and coagulate , reconnecting into an intimate network. The same thing happens when proteins are heated, which is why acid is sometimes said to cook meat or fish.
At first, the intimate network traps water that was previously bound up in muscle fibers, leading to moist, tender food. But if the denaturation conditions persist—that is, if food continues to sit in acid—the protein network will continue to tighten, squeezing water out of the protein altogether, resulting in tough, dry food, much like an overcooked steak.
To understand this progression, consider the way the texture of a piece of sashimi will become tender, bright-tasting fish tartare with the addition of acid, and then turn into a chewy ceviche over time. Fish meant for cooking shouldn’t marinate in acid for more than a few minutes, but dip any flaky, white-fleshed fish into buttermilk and flour before frying, or toss sea bass with lemon juice and curry powder just before skewering and grilling, and you’ll get the benefit of moist texture along with the pleasant hit of tartness.
Acid also helps break down collagen , the main structural protein found in tough cuts of meat. Add wine or tomatoes as you begin to cook braises and stews, since the more quickly the collagen melts, the sooner the meat will grow juicy and succulent.