Salt reduces the tongue’s perception of bitterness; when the bitterness recedes, the same food’s sweet and aromatic notes stand out. That is why a small amount of salt added to grapefruit, bitter greens, mildly bitter eggplant, or over-brewed coffee makes them seem milder and sweeter. Salt does not eliminate bitterness, only reduces its perception; it cannot rescue bitterness caused by spoilage, such as a burnt flavor.
The effect comes from receptors on the tongue. Tastes are not perceived independently; one taste changes how strongly another is felt. Salt is one of the tastes that noticeably suppresses the perception of bitterness. When bitterness recedes, the flavors that were in its shadow, especially sweetness and fruity aroma, are easier to perceive. Part of why salty food can taste sweet and lively comes from this: salt does not only add saltiness; by reducing bitter notes, it makes room for other flavors.
In cooking, this effect is most apparent with bitter greens. When greens such as radicchio, chicory, and dandelion greens are boiled in salted water, some of their bitterness passes into the water, and salt suppresses what remains; the same greens boiled in unsalted water taste noticeably more bitter. A very small pinch of salt sprinkled over grapefruit slices reduces the fruit’s astringency and brings out its sweetness. Mildly bitter eggplant or zucchini tastes more balanced after it is salted and releases its liquid.
The same mechanism works in drinks. A very small pinch of salt added to over-brewed, astringent drip coffee or tea that has steeped too long and turned bitter noticeably reduces the bitterness. The amount is critical: if you can taste the salt, you have added too much. Salt is used for the same reason in desserts with dark chocolate and cocoa; when the cocoa’s bitterness recedes, the chocolate’s aroma stands out. In these cases, salt works as an adjustment, not a flavor. To test it, add salt to one cup first, then decide.
This effect has limits. Salt reduces the perception of bitterness but does not remove the bitter substance; with very bitter food, you will run out of room to add salt before the food becomes too salty. The bitterness of burnt spices, burnt onions, or blackened butter comes from spoilage and cannot be rescued with salt; in that case, remove the burnt part or start over. Intense bitterness near the stem end of cucumbers and some zucchini also does not go away with salt; cut off and discard that section.
Account for this effect of salt when balancing a dish. In a recipe using bitter greens, radishes, coffee, or cocoa, salt is there not only for saltiness but also to balance bitterness; if you skip it, the dish will taste harsher than expected. When there is not enough salt, adding sugar to sweeten the dish is usually not the right solution; check the salt first. If the salt is right but the bitterness still dominates, an acidic or fatty accompaniment, such as lemon or olive oil, completes the balance.