Effects of freezing methods on size distribution of ice crystals during pre-freezing mannitol

The distribution of ice crystal grain size has a significant effect on the drying process and the quality of freeze-dried products. Three freezing methods, i.e., keepingg the shelf at a constant temperature, cooling from room temperature at a constant cooling rate, and cooling at a constant cooling rate containing crystal growing process, were proposed. Effects of three different freezing methods on the size distribution of ice crystal were investigated. The results show that when products are cooled at the shelf with constant temperature or cooled at a constant cooling rate, large ice crystals are found in the middle of the vial while small at the extremity. Moreover, when temperature changes, the lower the temperature of the shelf is, the tinier and more uniform the ice crystal is, and larger cooling rate leads to tinier ice crystals with more uniform distribution. Furthermore, when the final temperature of cooling at constant rate is the same as the temperature of shelf, the size of crystals generated by the former methods is larger than that produced by the latter methods while it is with the more uniform distribution. If the crystal growing process is added into the cooling at constant cooling rate, the mean size of ice crystal would increase, and the uniformity of the distribution can be regulated by controlling the time of crystal growing. © 2019, Central South University Press. All right reserved.