Salt is doing four separate jobs and only one of them is taste

The most-used ingredient and the least examined

Salt goes into nearly everything and gets almost no thought beyond how much. But it is not a single-purpose ingredient, and the question of when to add it has a different answer depending on which of its effects you are after. That is why cooks give contradictory advice about timing and are all, in their own context, right.

There are roughly four things salt does in a pan. Only the first is about flavour in the direct sense, and the other three explain most of the timing arguments.

It moves water, in whichever direction it can

Salt draws water out of cells through osmosis. Put salt on sliced onion, cucumber or grated vegetables and within minutes there is liquid in the bowl that was previously inside the plant. That is the single most useful physical effect salt has, and it is exploited constantly — in pickling, in drawing bitterness and water out of certain vegetables, in getting moisture out of a mixture before frying.

The same mechanism works against you when you do not want it. Salt added early to a vegetable you intend to brown pulls water out into the pan, and that water then has to evaporate before browning can resume. Whether that is helpful or not depends entirely on whether you wanted the vegetable dry and collapsed or firm and separate.

And over longer periods the movement reverses. Given enough time, salt penetrates the food and water follows it back in, which is the basis of brining and the reason a salted mixture left for an hour behaves quite differently from one salted five minutes ago.

It changes what proteins do

Salt alters the electrical charges on protein molecules, and that changes how they interact with water and with each other. At moderate concentrations it helps some proteins dissolve and hold water; at higher concentrations it does the opposite and encourages them to aggregate. This is not a subtle laboratory effect — it is why a well-salted kebab mixture binds and a poorly salted one falls apart.

It is also why salt destabilises yoghurt in a hot pan, since it screens the charges that were keeping the milk protein particles apart and makes curdling easier. Holding the salt back until a curd-based dish has finished cooking removes one of the factors pushing it towards separation.

In pulses, salt appears to have a modest softening effect by displacing the calcium and magnesium that bridge the cell walls, which runs contrary to the traditional advice. That one is genuinely debated and the effect is small, so it is worth reporting as contested rather than settled.

It suppresses bitterness and lets other flavours through

Salt does more to the perception of other tastes than it does to its own. It suppresses bitterness noticeably, which is why a pinch of salt improves things that have nothing salty about them, and it makes sweetness and savoury notes read as more pronounced. Much of what people describe as a dish tasting flat is a dish where bitterness has not been suppressed and everything else is competing with it.

It also increases the perception of aroma to some degree, though the mechanism there is more complicated and involves how volatile compounds behave in a salted solution. The practical upshot is familiar: the same dish, correctly salted, smells and tastes of more things, not merely of salt.

This is why under-salting is diagnosed wrongly so often. A cook tastes a flat curry and adds more spice, which adds more bitterness, which makes it flatter. The correction was usually salt.

It travels slowly, and that is the timing argument

Salt diffuses through food at a rate that is slow by kitchen standards. Sprinkled on the surface at the end, it sits on the surface; the first impression is salty and the interior is not. Added early to a liquid, it has hours to distribute evenly through everything in the pot.

That is the real case for salting in layers rather than once at the end. Each component gets seasoned while it can still absorb, and the final adjustment is small. It is also the case against salting only at the start, since a dish that reduces concentrates whatever salt is in it, and a pot judged at the beginning will be judged against a volume that will not exist later.

The honest summary is that both pieces of advice are partial. Season lightly as you go so that the salt gets inside the food, hold back enough that you can adjust once the dish has reached its final volume, and taste at the point where correction is still easy. Over-salted food is one of the very few kitchen errors with no good fix.