Freeze-dry microscopy of protein/sugar mixtures: drying behavior, interpretation of collapse temperatures and a comparison to corresponding glass transition data

The purpose of this study is to investigate the change in collapse appearance and temperature of protein/sugar mixtures as a function of nucleation temperature (T(n)), sublimation velocity (V(sub)) and the sugar/protein mole ratio when performing freeze-dry microscopy experiments. BSA and HSA were used as sample proteins and mixed with either sucrose or trehalose. Differential scanning calorimetry was used to determine the corresponding glass transition temperatures (T'g). To allow a more representative comparison between these analytical methods, a collapse midpoint temperature (T(c-50)) was introduced. While there was no distinct correlation between T(n) and the onset of collapse (T(oc)) for either mixture, V(sub) was found to correlate with the measured collapse temperature which is important for comparability of experiments. Furthermore, V(sub) could be used to qualitatively investigate the product resistance to water vapor flow. A dramatic change in the appearance of collapse was found for high sugar/protein mole ratios (> or =362:1) which needs to be considered to avoid a misinterpretation of T(oc) data. At low protein concentrations midpoint T'g data showed good agreement with T(oc) values but were found significantly lower at higher protein concentrations. Application of the Gordon-Taylor equation failed to predict the critical temperature for any of the protein/sugar mixtures studied.