1. Sesquiterpenes
Sesquiterpenes are C15 terpenoids and the major contributors to the characteristic scent of agarwood. They are primarily formed in the heartwood as a defense response to wounding or microbial infection.
Key features:
- Lipophilic, volatile compounds.
- Responsible for woody, balsamic, and earthy fragrance notes.
- Common sesquiterpene skeletons include guaiane, eudesmane, agarofuran, and vetispirane.
Examples of sesquiterpenes in agarwood resin:
| Compound | Aroma Profile | Notes |
|---|---|---|
| α-Guaiene | Woody, sweet | Major contributor to high-quality oud aroma |
| α-Eudesmol | Woody, floral | Enhances depth of scent |
| Agarofuran | Sweet, balsamic | Often associated with premium agarwood |
| Vetispirane | Smoky, earthy | Found in aged resin |
| δ-Guaiene | Woody, warm | Common in Southeast Asian agarwood |
Biosynthesis:
- Derived from farnesyl pyrophosphate (FPP) via the mevalonate (MVA) pathway in the cytoplasm.
- Induced in response to microbial infection (e.g., Fusarium spp.) or mechanical wounding.
2. 2-(2-Phenylethyl)chromones (PECs)
Chromones are oxygenated heterocyclic compounds and are considered chemotaxonomic markers of agarwood quality.
Key features:
- Non-volatile or semi-volatile compounds.
- Responsible for sweet, balsamic, and slightly floral undertones.
- Contribute to resin’s stability and aging potential.
Common chromones in agarwood:
| Compound | Notes |
|---|---|
| 6-Methoxy-2-(2-phenylethyl)chromone | Sweet, floral |
| 5,6-Dimethoxy-2-(2-phenylethyl)chromone | High-quality agarwood marker |
| 2-(2-Phenylethyl)chromone derivatives | Correlate with age and resin content |
Biosynthesis:
- Likely via polyketide pathway, starting from phenylalanine.
- Their accumulation is associated with stress responses (wounding or fungal inoculation).
3. Aromatic Compounds
These include volatile organic compounds (VOCs) that add top notes and freshness to the fragrance.
Common aromatic compounds:
| Compound | Aroma Profile | Function |
|---|---|---|
| Benzylacetone | Sweet, floral | Enhances aroma complexity |
| 2-Phenylethanol | Rose-like | Floral note |
| Vanillin | Sweet, creamy | Minor in agarwood, more common in aged resin |
| Cinnamyl alcohol | Spicy, balsamic | Adds warmth to scent |
Role in aroma:
- Provide lighter, volatile fragrance notes.
- Often modulate the olfactory perception of sesquiterpenes and chromones.
- Contribute to species-specific scent profile.
Summary of Contributions to Agarwood Aroma
| Compound Class | Volatility | Aroma Type | Contribution to Quality |
|---|---|---|---|
| Sesquiterpenes | High | Woody, earthy, balsamic | Core aroma; high-quality marker |
| Chromones | Low | Sweet, floral, balsamic | Enhances complexity and aging potential |
| Aromatics | High | Floral, spicy, sweet | Top notes; refine and brighten scent |
Additional Notes
- The resin composition varies depending on species, age, infection method, and environmental factors.
- Artificial inoculation with fungi like Fusarium oxysporum increases sesquiterpene and chromone accumulation.
- Chemical profiling (GC-MS, LC-MS) is often used to determine agarwood quality and authenticity.