When cooks talk about “acidity”, what they mean is simply sourness. Scientifically, they are the same thing. The sour taste buds fire when an acid meets water and releases hydrogen ions in your mouth. End of chemistry class.
At least in America, acidity is a quality frequently missing from home-cooked meals, especially in what we think of as comfort food.
I’m not speaking here of the cases where acid is needed for a technical purpose, like preventing the browning of peeled artichokes, or reacting with baking soda in a pancake batter. I’m talking about sour flavors, which have a place in almost any dish. Why?
First of all, acid stimulates the palate. Specifically, the production of saliva. Just imagine sucking on a lemon for a second and I bet your mouth literally waters. That is the very definition of appetizing: you are physically preparing your body to eat.
Also, acid gives the sensation of cutting through fat. A sauce that might seem too rich is magically enlivened. The classic heavy French sauces, like hollandaise, mayonnaise and bearnaise all include a crucial acidic component. You probably wouldn’t like to dress a bowl of lettuce with just oil, but oil and vinegar or lemon juice is a staple.
Finally, sourness balances out sweet or salty flavors. Think of a crepe dusted with powdered sugar and a squeeze of lemon, or salty fries dipped in sweet and tart ketchup. Less obviously, even a basic vegetable or lentil soup delivers mostly sweet, earthy flavors. That same squeeze of lemon makes it much more interesting.
For the best effect, consider the balance of flavors for the whole meal, not just a single dish. If dessert is going to be a tart cherry pie, maybe lemony white beans aren’t the ideal appetizer.
Things that can add sourness: any citrus juice, any of the hundreds of wonderful vinegars on the market, wine, tomatoes, tamarind, fruits (especially when underripe), anything pickled, amchoor powder (an Indian spice made from dried mango), and carbonation. Yep. CO2 dissolved in water produces a little bit of carbonic acid, which is part of why seltzer is so refreshing.
Speaking of citrus, don’t forget to look beyond standard lemons and limes. Grapefruits, mandarins, tangerines, pomelo, Meyer lemons, yuzu, and even Buddha’s Hand offer amazing variations.
There are also times when you don’t want or need any more acidity in a dish, but you still want some of the effects. In that case, consider citrus zest or kaffir lime leaves. Although they aren’t really significantly acidic, they have such a strong association with the fruits that you will experience a similar effect. The zest of a lemon added to a simple tomato sauce is amazing.
So remember to taste your food frequently, throughout the cooking process. And once you are satisfied that the salt level is correct, ask yourself, “would a little splash of acid wake this puppy up?”.