The root and lower stem of the coriander plant (Coriandrum sativum) — an aromatic that Thompson identifies as the most overlooked and underutilised Thai ingredient in cooking outside Thailand. The coriander root has a completely different aromatic profile from the coriander leaf: deeper, more earthy, more resinous, with a complex combination of citrus and camphor notes produced by its linalool, geraniol, and borneol content. The root is used in virtually all Thai curry pastes (Entry TH-01's pounding sequence includes it), in the marinade for gai yang (Entry TH-26), in the poaching liquid for khao man gai (Entry TH-24), and in many soups.
**Availability:** Coriander with roots attached is common in Asian grocery stores (where the plants are sold whole) and from specialty greengrocers. In most Western supermarkets, coriander is sold pre-trimmed with no root. The root can be found where plants are sold with full roots intact; it can also be replaced by the lower 3–4cm of the coriander stem (not the leaf) — which shares some of the root's aromatic compounds in lower concentration. **Preparation:** Scrape the root with a paring knife to remove the outer skin and any soil residue. The cleaned root should smell of earthy, deep, slightly camphor-citrus complexity. For curry paste use: simply slice and add to the pounding sequence. For marinade use: pound with the garlic. For infusion in soups and poaching liquid: add whole, bruised. **Aroma comparison:** - Coriander leaf: bright, citrus-soapy (from linalool and terpinolene — intensely volatile, destroyed by heat). - Coriander root: earthy, complex, resinous (from geraniol, borneol, and less volatile linalool compounds) — withstands heat and contributes depth to long-cooked preparations. **Substitution:** There is no precise substitute. The combination of a small amount of coriander seed (slightly similar structural compound) and a larger amount of coriander leaf in curry pastes is the closest approximation — but it understates the root's contribution.
David Thompson, *Thai Food* (2002); *Thai Street Food* (2010)