Water Activity Governs Ice Nanocrystal Evolution in Rapidly Frozen Sugar Solutions

Formation and evolution of ice nanocrystals in rapidly frozen sugar solutions were investigated using spin-contrast-variation small-angle neutron scattering (SCV-SANS). As the sugar concentration decreased, the SCV-SANS signal from nanocrystalline ice plates decreased relative to that from micrometer-sized ice crystals, while the diffraction peaks became sharper. These results indicate that the micrometer-sized crystals formed predominantly through Ostwald ripening, which depended primarily on sugar concentration and secondarily on sugar species. Regardless of the initial sugar concentration, ice crystal growth was arrested when the sugar concentration in the freeze-concentrated amorphous ice reached 66–71 wt % for trehalose and sucrose and 62–63 wt % for glucose. Despite its lower glass-transition temperature, glucose showed growth arrest at a lower degree of freeze concentration than trehalose and sucrose. However, the water activity at these growth-arrest concentrations converged to 0.80–0.82 under the present rapid-freezing conditions, indicating that water activity determines ice nanocrystal evolution until vitrification arrests further growth.