The double leaf of the kaffir lime (Citrus hystrix) — technically a petiole and leaf attached in a distinctive 'figure-8' form — is one of the most distinctive aromatics in the Thai kitchen and the source of the immediate, recognisable Thai fragrance that sets the cuisine apart from other Southeast Asian traditions. The compound primarily responsible for kaffir lime leaf's aroma is limonene (shared with most citrus) combined with beta-pinene and sabinene — producing a piney, floral, lime aromatic that is more complex and more persistent than lime juice's citric acid sourness.
**Tearing vs. cutting the midrib:** The midrib of the kaffir lime leaf is tough and fibrous — it is always removed before eating, either by folding the leaf and pulling off the midrib (for whole leaves in preparations) or by slicing it out before chiffonading. Eating the midrib is a textural problem; removing it is part of the preparation. **Fresh vs. frozen vs. dried:** Fresh: maximum aromatic impact. Frozen: 70–80% of the aromatic — the freezing partially breaks down the cell walls and volatilises some limonene. Dried: a shadow of the fresh leaf — the volatile limonene compounds are almost completely gone; what remains is the dried leaf's structural compounds, which are bitter and papery. Thompson uses fresh or frozen; dried kaffir lime leaf is not an equivalent in his preparations.
David Thompson, *Thai Food* (2002); *Thai Street Food* (2010)