Rule 9 of 27 · Chapter II — Salt and Seasoning
Keep acid within reach and use it to finish
Why this rule exists
Salt makes food taste more like itself; acid makes it taste alive. A squeeze of lemon, a splash of vinegar, a spoon of yogurt at the end lifts a dish that tastes flat, heavy, or somehow finished-but-not-quite. Acid cuts through richness, brightens muddy flavors, and provides the contrast that keeps a bite interesting rather than one-note. It is the most underused tool in the home kitchen, and the reason restaurant food often tastes more vivid than the same dish made at home is frequently just a final hit of acid you did not think to add. When something tastes good but boring, the fix is usually not more salt but a little brightness.
In practice
Keep lemons, a few vinegars, and something tangy like yogurt or capers on hand as finishing tools, not just recipe ingredients. When a dish tastes flat despite being properly salted, add acid a little at a time and taste; the point is balance, not sourness. Add most acid at the very end, off the heat, because bright acidity fades when it is cooked. Match the acid to the dish: lemon for fish and greens, sherry or red wine vinegar for hearty stews, lime for anything with chili. Learn to recognize the specific dullness that acid, rather than salt or fat, is the answer to.
When it doesn't apply
Some acid belongs early, built into a braise or a marinade where it tenderizes and melds over time. Delicate or already-tangy dishes need a lighter hand or none. And too much acid is its own problem, thinning and sharpening a dish past pleasant; as with salt, add gradually and taste toward balance.