The Importance of Understanding the Freezing Step and Its Impact on Freeze-Drying Process Performance

The freeze-drying process is a combination of 2 equally important processes, freezing, and drying. In the past, the effort was mainly focused on optimizing the drying process without considering the possible effects of the freezing step. During freezing, a solution undergoes several physical changes, including a supercooling state. The degree of supercooling of a solution dictates the ice habit (size, number, and morphology) during freezing, which impacts the subsequent drying process, such as the resistance to water vapor flow. Therefore, heterogeneous degree of supercooling leads to heterogeneous ice habits and, in turn, to heterogeneous drying behavior. This poses significant challenges during freeze-drying process development, optimization, and scale up. Hence, controlling the degree of supercooling significantly improves freeze-drying process design. The aim of the current review is to gather existing information on the physicochemical phenomena involved in the freezing process and how these phenomena impact the subsequent drying step of the freeze-drying process. In addition, modification of the freezing process and different techniques used to actively control the degree of supercooling during freezing will be reviewed and discussed. Their impact on freeze-drying process performance will be also addressed.