The Grammar of Sour

Ask most cooks to make something more sour and they will reach for a lemon. Ask an Assamese kitchen and the first question back is which sour — because the acid you choose changes not just the intensity but the shape and the timing of the sourness.

Sour is dominated by hydrogen ion concentration, but the experience of it is shaped by which acid is present, what else the fruit brings with it, and how fast it hits and clears.

Six souring agents, and what each one does

• Ou tenga (elephant apple) — sharp, resinous, with a slightly astringent tail. Lands mid-palate and stays. The signature sour of a tenga.
• Thekera (dried Garcinia) — rounder and slower, with a smoky note from the drying. Forgiving, because it releases gradually rather than all at once.
• Tomato — mild acid plus sugar, glutamate and body. As much a thickener as a souring agent.
• Kaji nemu (Assam lemon) — bright, immediate, and gone quickly. Aromatic oils in the peel do half the work. Always at the end.
• Roselle (tenga mora) — clean and very direct, with a red colour that will announce itself in a pale broth.
• Fermented sours (khorisa, poita water) — sour plus glutamate. These add savour as well as acid, which is a different job entirely.

Three rules that hold

1. Slow sours go in early; fast sours go in at the end. Thekera has to steep. Lemon added early tastes stewed and dull.
2. Sourness needs salt to read properly. An under-salted sour dish tastes thin and sharp rather than sour.
3. Build sourness in the liquid before the protein returns. Acid added at the end sits on top of a dish instead of running through it.

The question is never "is this sour enough". It is "is this the right sour, arriving at the right moment".

The dish that demonstrates all of this is Masor Tenga, where there is nothing else in the bowl to hide a bad decision.