Yacon syrup is a dark, treacle-brown liquid sweetener boiled down from the juice of the yacon, a sweet Andean tuber. Thick and pourable like a light molasses, with a mild sweetness and gentle notes of caramel, fig and raisin, it is prized less for sweetening power than for what it is made of: a syrup unusually rich in fructooligosaccharides, prebiotic sugars that the body barely digests, giving it a low calorie count and a low glycaemic impact.