The melting of glucose

Several sugars are known to undergo a spontaneous liquefaction below their reputed melting point (T-m), but the origin of this apparent melting is not yet clearly understood. In this paper we address this puzzling behavior in the particular case of the crystalline forms of glucose: G alpha and G beta, involving respectively the glucose-alpha and glucose-beta anomers. We show in particular that the spontaneous melting below their reputed melting point T-m (similar to 151 degrees C for G alpha and similar to 156 degrees C for G beta) corresponds to a horizontal displacement of the system in the eutectic phase diagram of the anomeric mixture glucose-alpha/glucose-beta. This displacement is associated with mutarotation in the liquid which, in turn, induces additional liquefaction of the remaining crystal. This feedback loop creates a vicious circle which stops when the mixture reaches the liquidus branch, i.e. when the liquefaction is total. It is also shown that this behavior becomes more complex on approaching the eutectic temperature T-e (120 degrees C). Just above T-e, the liquefaction process is followed by a recrystallization leading to the crystalline form G beta. On the other hand, just below T-e, the spontaneous liquefaction process stops as no melting is expected whatever the anomeric composition.