tamarind pulp
The sour, fruity flesh from the pods of the tamarind tree, usually sold as a pressed block or a ready-made paste.
Tamarind pulp is sold as a pressed block with seeds and fibres in Asian, Indian or Turkish grocers and by mail order, and also as a ready-made paste. The block is soaked in warm water, squeezed and strained; the paste is more convenient but often thinner and somewhat flatter in flavour.
Substitute:
Lime juice with a little brown sugar replaces the sourness but not the dark, fruity depth reminiscent of dates or prunes, so the dish tastes brighter and sharper.