Tamarind is the sticky, sweet-and-sour pulp inside the brown pods of Tamarindus indica, a tropical tree of the legume family. Jammy and date-brown, it is intensely tart from tartaric acid yet deeply sweet, with a raisiny, almost caramelised undertone — the world's great natural souring agent. It threads through pad thai, sambar and rasam, chutneys and Mexican agua de tamarindo and candies, and is the secret sour backbone of Worcestershire and HP sauce. It is sold as whole pods, pressed seedless blocks, or a dark, ready-to-use concentrate.