A good chaat is sour in three different ways at once

Sour is not one flavour

It is easy to treat sourness as a single dial, something you turn up with whatever acidic thing is nearest. Chaat makes the argument against that better than almost any other food, because a plate of it will usually contain two or three distinct sour elements, and swapping one for another produces something noticeably worse.

The reason is that the acids involved are chemically different, they taste different, and they arrive at different points in the mouthful. A vendor who has been making the same plate for years has arranged all of that deliberately, even if nobody would describe it in those terms.

Different acids, different behaviour

Tamarind is dominated by tartaric acid, which is unusually sharp and lingers. It also brings colour, a large amount of dissolved sugar, and a slightly fruity, almost fermented depth that has nothing to do with the acidity itself. That combination is why tamarind chutney can be simultaneously very sweet and unmistakably sour without either one cancelling the other.

Lime and lemon carry citric acid along with a great deal of volatile aromatic oil from the peel. The acid hits early and fades quickly; the aroma is what you actually notice, and it is fragile. Squeeze lime over something and leave it for twenty minutes and you keep most of the sourness while losing much of what made it feel fresh.

Raw mango and amchur bring malic acid, which reads as rounder and greener than either of the others, and dried amchur brings it without any liquid at all. That last point is not trivial — it lets you make something sour without making it wet, which is the entire reason amchur exists in a dry masala.

Yoghurt is doing something the others cannot

The curd on a plate of chaat is contributing lactic acid, which is mild and soft compared with tartaric or citric. But acidity is only part of its job. It is also cold, it is thick, and it coats the mouth in a way a thin liquid does not, which slows down every other flavour it comes into contact with.

That coating effect is why yoghurt tames chilli heat. Capsaicin is not water-soluble but it does dissolve into fat, and dairy fat and protein together carry it away from the receptors it was irritating. A dish that would be punishing without curd becomes comfortable with it, and the chilli level a vendor uses assumes the curd is there.

It also supplies the temperature contrast the dish is built around. Warm potatoes, cold curd, room-temperature chutneys, crisp fried elements — the plate is deliberately inconsistent, and consistency would be a loss rather than a refinement.

Salt and sugar are part of the same system

Sourness is not perceived in isolation. Sugar suppresses the perception of acid substantially, which is why tamarind chutney with jaggery in it tastes balanced at an acidity that would be unpleasant on its own. Salt does something similar and also sharpens the sweetness, so all three are being adjusted against one another rather than set independently.

Chaat masala is the compressed version of this idea. It carries dried sour agents, salt, and black salt with its sulphurous note, so a pinch of it changes acidity, salinity and aroma in one move. The sulphur compounds are pungent enough that a small quantity registers immediately, which is exactly what you want in something sprinkled at the last second.

This is also why chaat is difficult to correct once assembled. Adding acid to a finished plate lands on the top layer only and never reaches the potato underneath, so the first mouthful is sharp and the rest is flat. The balance has to be built in as the layers go down.

Why the order of assembly is not arbitrary

Watch closely and the sequence is consistent: the dense, absorbent things go in first, the chutneys next, the curd after that, and the dry, crisp, aromatic elements go on last, in the final couple of seconds before the plate leaves the counter. Everything about that order is designed to delay contact between water and anything crisp.

A fried element loses its texture through moisture migration, and moisture always moves from the wet thing to the dry thing. The clock starts the moment they touch. Putting sev on before the chutney rather than after is not a stylistic choice; it is the difference between a plate that works and one that has already begun failing before it reaches you.