Reference point
Break a freshly roasted sweet potato in two and smell the flesh.
Do this before you read on. An entry only works when you have just smelled the thing.
Deeply sweet, with a honeyed note and a faint layer of earth left from the skin. Denser than grilled corn and less smoky.
A roasted sweet potato is clearly sweeter than a boiled one, and that is no illusion — slow roasting really does create more sugar.
The earthy layer underneath is the mark of the root. It links this entry to the earthy family and sets it apart from every other sweet thing in the kitchen.
How to smell it
Break it in two rather than cutting it, and smell right at the hot flesh. Then smell the charred skin on its own — the two are different enough to count as two smells.
Why this one is hard
Because people merge smell and taste. A roasted sweet potato is so clearly sweet on the tongue that the nose gets ignored. Hold your nose, take a bite, then let go, and you will see at once which part belongs where.
Scientific note
Slow roasting holds the sweet potato at about 60 to 75 degrees long enough for the amylase enzymes to break starch down into maltose. That is why a roasted sweet potato is sweeter than a boiled one — not from losing water but from making more sugar. The maltose then caramelises on the outside, giving the honey note.
In Vietnamese life
Sweet potatoes roasted in oil-drum ovens and sold from carts are an image of winter in the north. In the countryside, sweet potatoes buried in the ashes of the kitchen fire are the oldest way of roasting them and give the sweetest result, because the ashes hold a low, even heat — exactly the conditions the enzymes need.