Predictive model for the surface melting and puffing of freeze-dried amorphous materials

It is thought that surface melting and puffing of freeze-dried amorphous materials are related to the difference between the surface temperature (T_sur) and freeze-concentrated glass transition temperature (T_g') of the materials. Although T_g' is a material-specific parameter, T_sur is affected by the type and amount of solute and freeze-drying conditions. Therefore, it will be practically useful for preventing surface melting and puffing if T_sur can be calculated using only the minimum necessary parameters. This study aimed to establish a predictive model for the surface melting and puffing of freeze-dried amorphous materials according to the calculated T_sur. First, a T_sur-predictive model was proposed under the thermodynamic equilibrium assumptions. Second, solutions with various solute mass fractions of sucrose, maltodextrin, and sucrose-maltodextrin mixture were prepared, and three material-specific parameters (T_g', unfrozen water content, and true density) were experimentally determined. According to the proposed model with the parameters, the T_sur of the samples was calculated at chamber pressures of 13, 38, and 103 Pa. The samples were freeze-dried at the chamber pressures, and their appearance was observed. As expected, surface melting and puffing occurred at calculated T_sur > T_g' with some exceptions. The water activity (a_w) of the freeze-dried samples increased as the T_sur - T_g' increased. This will have resulted from surface melting and puffing, which created a covering film, thereby preventing subsequent dehydration. The observations suggest that the proposed model is also useful for predetermining the drying efficiency and storage stability of freeze-dried amorphous materials.