On Food and Cooking

Though caramel is most often made with
table sugar, its sucrose molecules actually
break apart into their glucose and fructose
components before they begin to fragment and
recombine into new molecules. Glucose and
fructose are “reducing sugars,” meaning that
they have reactive atoms that perform the
opposite of oxidation (they donate electrons to
other molecules). A sucrose molecule is made
from one glucose and one fructose joined by
their reducing atoms, so sucrose has no
reducing atoms free to react with other
molecules, and is therefore less reactive than
glucose and fructose. This is why sucrose
requires a higher temperature for
caramelization (340ºF/170ºC) than glucose
(300ºF/150ºC) and especially fructose
(220ºF/105ºC).