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: Gα and Gβ, involving respectively the glucose-α and glucose-β anomers. We show in particular that the spontaneous melting below their reputed melting point T_m (∼151 °C for Gα and ∼156 °C for Gβ) corresponds to a horizontal displacement of the system in the eutectic phase diagram of the anomeric mixture glucose-α / glucose-β. 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 °C). Just above T_e, the liquefaction process is followed by a recrystallization leading to the crystalline form Gβ. On the other hand, just below T_e, the spontaneous liquefaction process stops as no melting is expected whatever the anomeric composition.