Reference point
Open a tin of condensed milk and smell from a hand’s span away.
Do this before you read on. An entry only works when you have just smelled the thing.
Thickly sweet, with a buttery note and a faint caramel layer. Heavy, hardly evaporating.
Fresh and condensed milk are quite different. Concentration under heat sets off a reaction between sugar and protein, producing caramel compounds that fresh milk lacks. Here again heat creates a new smell rather than releasing an old one.
The fatty part comes from lactones, the same group found in toasted coconut and osmanthus. The three sound alike towards the end.
How to smell it
Put a drop of condensed milk on a white plate and smell it. Then smell a little fresh milk. The distance between the two is the work of concentration.
Why this one is hard
Because it is too familiar. Condensed milk is in coffee and in dozens of Vietnamese desserts, so the brain has already labelled it.
Scientific note
Condensed milk is concentrated at high temperature and held there long, enough for the Maillard reaction to take place between lactose and milk proteins. The products include lactones, which give a creamy feel, and furanones, which give a caramel note. Fresh milk has none of these — they are created in processing, not present beforehand.
In Vietnamese life
Sweetened condensed milk came to Vietnam in the colonial period as a way of keeping milk without refrigeration, and became a fixed part of Vietnamese coffee. In very few countries has a preserved product shaped a whole way of drinking like this.