Pandan and jasmine rice share one aroma, 2AP: how to grow and keep it
Research finds 2-acetyl-1-pyrroline (2AP) in pandan leaves and Khao Dawk Mali 105 rice. Your nose catches tiny amounts, but it fades. How to grow pandan and store rice.
- The main aroma of pandan leaves and Khao Dawk Mali 105 rice is the same compound, 2-acetyl-1-pyrroline (2AP). It was not found in non-aromatic rice.
- People can smell 2AP below 0.1 parts per billion in water, which is why a few pandan leaves scent a whole pot of rice.
- 2AP fades easily. Aromatic rice kept a long time smells less, and storing it at 15 °C slowed the loss better than room temperature over the first 4 months.
- Pandan needs steady water; short of water, leaves yellow and the scent weakens. Fresh, intact leaves smell of little until they are bruised, wilted or heated.
Pandan and jasmine rice smell alike because they share one aroma compound, 2-acetyl-1-pyrroline, or 2AP. Research going back to 1993 confirmed it in both pandan leaves and Khao Dawk Mali 105 rice, and found none in non-aromatic rice. The compound is so potent that the nose picks up the faintest trace, yet it also disappears easily. Knowing that helps you grow pandan and store rice so they stay fragrant longer.
2AP is a smell you already know
Most Thai kitchens smell 2AP every day without naming it. Laksanalamai and Ilangantileke, writing in Cereal Chemistry in 1993, used steam distillation and GC-MS and found:
- The aroma compound in pandan leaf oil has the same structure as 2AP.
- Fresh Khao Dawk Mali 105 rice has a clear 2AP signal.
- Non-aromatic rice showed no 2AP.
- Khao Dawk Mali 105 that had been stored a long time had little 2AP left.
The authors concluded that 2AP is a key compound behind the “pandan-like” aroma of jasmine rice, and stressed the need for storage methods that keep the aroma.
Two plants that make it in a similar way
A 2008 thesis at Chulalongkorn University fed the amino acid L-proline, labelled with a heavy form of nitrogen, to callus and leaves of pandan and of Khao Dawk Mali 105 rice, then checked whether that nitrogen ended up in 2AP. It did: the nitrogen in pandan’s 2AP comes from proline, as in rice. The author suggested pandan may make 2AP by the same route as Khao Dawk Mali 105.
JIRCAS work on jasmine rice points the same way. Adding proline to seedlings raised 2AP more than threefold over the control, and water stress at the milky ripening stage raised 2AP in the grain, while stress after the yellow ripening stage did not. That was a pot experiment explaining the mechanism, not advice to let paddies dry out.
Why a few pandan leaves scent a whole pot
JIRCAS notes that 2AP has an odour threshold below 0.1 ppb (parts per billion) in water, lower than the other volatile compounds in aromatic rice.
Example in numbers
- 0.1 ppb in water is 0.1 micrograms per litre.
- Say a rice cooker uses 1.5 litres of water. The 2AP needed to start being smelled = 1.5 × 0.1 = 0.15 micrograms.
- 0.15 micrograms is 0.00000015 grams.
That figure is only the point where the smell becomes detectable, not the amount that makes rice clearly fragrant. Still, it shows why one knotted, bruised bundle of pandan leaves can perfume a whole pot.
A compound the nose can catch in the tiniest amount can also slip away in the tiniest amount. 2AP needs care in growing, storing and cooking.
Growing pandan for scent
FAO records fragrant pandan (Pandanus amaryllifolius) as a plant kept in cultivation and not found in the wild. It spreads by suckers from the base: suckers about 30-40 cm tall are separated and planted out. A mature plant reaches about 1-1.5 m, with leaves 80-110 cm long and 6-8 cm wide.
What matters for aroma:
- Regular but light watering. FAO notes that without enough water, leaves turn yellow and the aroma is weak.
- Fresh, intact leaves smell of little. The scent develops when leaves wilt, bruise or heat up, so crush or knot them before they go in the pot.
- Cut mature leaves and leave the young ones in the centre so the clump keeps growing.
Pot sizes and light for pandan are in Growing Thai kitchen herbs in pots.
Storing aromatic rice so it stays fragrant
A 2003 study in the Thai Journal of Agricultural Science stored Khao Dawk Mali 105 paddy at 10 and 15 °C against room temperature, 28 °C, for 6 months, and found:
| Factor | Effect on 2AP and cooking |
|---|---|
| Brown rice vs white rice | Brown rice had more 2AP on average |
| Longer storage | 2AP tended to fall |
| 15 °C vs 28 °C | 15 °C slowed the loss of 2AP better over the first 4 months |
| Longer storage | Rice absorbed more water and took longer to cook |
| Low-temperature storage | Cooked rice was less hard and stickier |
What you can do at home, applying those results rather than following a recipe from the study:
- Buy aromatic rice in amounts you will finish fairly soon, instead of stockpiling large sacks for months.
- Keep it in a sealed container in the coolest, driest place in the house, away from the stove and sun-heated walls.
- To lift the scent of older rice or plain rice, crush some pandan leaves and add them when cooking.
Common mistakes
- Adding whole, unbruised leaves. Intact leaves release scent slowly; crush or knot them first.
- Planting pandan where the soil often dries out. Leaves yellow and lose scent.
- Buying lots of aromatic rice and keeping it by the stove. Heat speeds the loss of aroma.
- Using the same water for old rice as for new. Rice stored longer absorbs more water and may need more water and time.
- Taking a fragrant smell as proof of genuine jasmine rice. 2AP can come from pandan, so smell alone cannot identify the variety.
To keep pandan leaves longer, see Storing herbs so they last.
Sources
- Comparison of Aroma Compound (2-Acetyl-1-Pyrroline) in Leaves from Pandan and Thai Fragrant Rice (Khao Dawk Mali-105) — Cereal Chemistry, AACC International
- 2-Acetyl-1-pyrroline biosynthesis of pandan and rice Khao Dawk Mali 105 through amino acid metabolism — Chula ETD, Chulalongkorn University
- Formation of flavor compound in aromatic rice and its fluctuations with drought stress — JIRCAS
- Effect of storage temperature and time on texture of Khao Dawk Mali 105 rice (Thai) — PHTnet, Postharvest Technology Innovation Center
- Pandanus trees and shrubs: Pandanus amaryllifolius — FAO
FAQ
If I cook plain white rice with pandan, will it smell like jasmine rice?
It will smell similar, because the main aroma compound is the same. It will not be identical: jasmine rice has its own texture and other aroma compounds too, and pandan adds only the scent.
Is old jasmine rice still safe to eat?
Yes, if it was stored well: dry, with no weevils or mould. Research found that what drops over time is the aroma, and rice stored longer also absorbs more water and takes longer to cook. If it smells musty or you see mould, throw it out.
How do I keep cut pandan leaves?
Pandan is best fresh, cut when you need it. To keep it, see chilling, freezing and drying in the lesson on storing herbs. Dried leaves usually smell weaker than fresh.
General information, not medical advice. If you are pregnant, take regular medication, or have a liver or kidney condition, ask a doctor or pharmacist before taking herbs beyond normal food amounts.