A grapefruit sprinkled with salt tastes less bitter within a minute, and the salt itself barely registers. That's not masking. Sodium blunts the actual signal bitter compounds send to the tongue, so the fruit reads as rounder instead of saltier — which is the real job salt does most of the time, turning down a flavor rather than adding one.
How salt quiets bitter compounds
Sodium interferes with how strongly bitter compounds signal to the brain once both are present on the tongue at the same time. The bitter compound itself is unchanged; the tongue just reports less of it. This has been measured directly with quinine, with caffeine, and with the naringin that makes grapefruit pith bitter — the suppression only shows up when the two are tasted together, not one after the other.
A pinch of salt in dark-roast coffee works the same way, quieting the bitterness enough that some coffee drinkers add it without knowing why it helps. Bitter greens need a heavier hand: a salted kale salad, massaged and rested for a few minutes, tastes markedly less sharp than the same leaves dressed straight from the bag. Salted caramel leans on the effect from the other direction, since the caramelization that gives it a burnt edge gets muted by the salt stirred in at the end. The same sodium that mutes bitterness also nudges up how savory food tastes, which is part of why salt and umami-rich ingredients so often get reached for together.
Why food that's short on salt tastes flat, not unsalted
Most food carries some bitterness already — char on a seared steak, the outer leaves of a lettuce, the skin of a roasted tomato — and salt normally keeps it low enough to go unnoticed. Cut the salt back and that bitterness doesn't get added. It just stops being suppressed. A taster reads that as dullness rather than as "not salty enough," which is why a plain pinch added at the table afterward doesn't always fix it: the bitterness has already had the whole cooking time to build with nothing holding it down.
Why salting early tastes different from salting at the table
Salt stirred into a dish early has time to dissolve completely and spread evenly before anyone takes a bite, so every mouthful carries roughly the same low, steady concentration, and the bitterness-suppressing effect above works on the whole plate at once. A flake of finishing salt added at the table never gets that chance. It sits on the surface until saliva dissolves it right as the bite lands, so the tongue gets one short, concentrated spike of sodium exactly where the crystal was. Both deliver salt. They don't deliver the same experience — cooked-in salt reads as balance, and salt added at the table reads as a distinct, salty crunch on top of whatever's already there.
Why a teaspoon of salt isn't always the same amount of salt
Salt crystals aren't uniform, and a teaspoon measures volume, not weight. Diamond Crystal kosher salt's flaky, hollow crystals pack loosely, so a teaspoon holds close to 3 grams. Morton's kosher crystals are denser, closer to 5 grams a teaspoon. Fine table salt packs tightest of all, around 6 grams a teaspoon — roughly double Diamond Crystal for the same spoon. A recipe written against one and measured with another isn't off by a little. It's off by a factor large enough to decide whether the bitterness in that dish gets suppressed at all.