Freeze-concentrated glass-like transition temperature of carbohydrate–phosphate buffered saline systems and impact on collapse of freeze-dried solids

Abstract

Phosphate buffered saline (PBS) and carbohydrates are used to freeze-dried biomaterials. It has been suggested that PBS reduces the freeze-concentrated glass-like transition temperature ( $$T_{\text{g}}^{\prime }$$ ) of carbohydrate solutions and that freeze-dried solids collapse readily during freeze-drying. In order to obtain more insight into the subject, the effect of PBS on the $$T_{\text{g}}^{\prime }$$ of trehalose solution was systematically investigated. The $$T_{\text{g}}^{\prime }$$ of trehalose solution was found to be independent of the trehalose content. The $$T_{\text{g}}^{\prime }$$ of trehalose–PBS solution, on the other hand, decreased with decreasing trehalose content. The behavior was described as a $$T_{\text{g}}^{\prime }$$ curve for a trehalose–PBS pseudobinary system. The $$T_{\text{g}}^{\prime }$$ curve was analyzed using the modified Gordon–Taylor equation, and the $$T_{\text{g}}^{\prime }$$ of PBS was extrapolated to be 195.0 K. In order to compare the plasticizing effect of PBS, the $$T_{\text{g}}^{\prime }$$ values of other carbohydrate materials including sugar alcohols (glucose, maltitol, sucrose, inulin, and maltodextrin) were also investigated. It was found that freeze-concentrated trehalose was more sensitive to the plasticizing effect of PBS than the other sugars. Finally, trehalose and trehalose–PBS solutions with varying trehalose content were freeze-dried, and their water activity (aw) and apparent volume ratio against pre-dried (frozen) solids (VFD VF−1 ) were evaluated. Freeze-dried trehalose samples had low aw values and high VFD VF−1 values. Freeze-dried trehalose–PBS samples, on the other hand, had significantly higher aw values and lower VFD VF−1 values than freeze-dried trehalose samples depending on the trehalose content. These results corresponded to the $$T_{\text{g}}^{\prime }$$ and initial solute content.