Reference point
Peel a green ambarella and smell the flesh as soon as it is exposed.
Do this before you read on. An entry only works when you have just smelled the thing.
Sour, green, with a clear resinous note that star fruit lacks. It is denser than star fruit and stays a little longer.
That resinous note is the interesting part. It recalls a freshly snapped leaf stalk more than a fruit, and it is exactly what sets ambarella apart from every other sour fruit in the family.
Ripe ambarella loses that note completely and turns sweet and floral. Two stages of the same fruit give two unrelated smells.
How to smell it
Peel it and smell the white flesh at once. Then break a fruit in two and smell near the stone — the resinous note is much stronger there, because the tissue around the seed is where the tree concentrates its defensive compounds.
Why this one is hard
Because the resinous note is easily mislabelled as “harsh” or “raw”. Those are judgements, not descriptions. Only once you call it resin can you recognise it in other fruits of the same family.
Scientific note
Ambarella belongs to the cashew family, Anacardiaceae, along with mango and cashew. The whole family exudes a sap containing long-chain phenolic compounds, and that is the source of the resinous note. The same group of compounds at high concentration is what makes mango sap or cashew shells itch on contact.
In Vietnamese life
Green ambarella dipped in chilli salt is a schoolchildren’s snack all over Vietnam. It also turns up in salads and juices — both uses exploit the sharp sourness rather than the sweetness.