During primary drying the product must stay frozen and structurally stable. Two temperatures define that boundary for most materials: the collapse temperature and, for amorphous systems, the glass transition of the maximally freeze-concentrated phase.
Collapse temperature
Above the collapse temperature the dried matrix softens enough to lose its open porous structure. Collapse typically shows up as shrinkage, loss of the intended shape, a longer or less predictable rehydration, and in regulated products, a potential quality issue. The cycle keeps the product temperature below this limit for the entire sublimation stage.
Glass transition and the maximally freeze-concentrated state
Amorphous materials such as sugars, proteins and many formulations do not have a sharp eutectic; instead, as water freezes the remaining solution concentrates until it becomes highly viscous and stops crystallising. The resulting maximally freeze-concentrated solution has a characteristic glass transition temperature (Tg′). Drying above Tg′ risks collapse of the amorphous phase.
Annealing changes the ice, not the chemistry
Annealing holds the product above Tg′ but below collapse for a defined time, allowing ice crystals to grow and the matrix to reorganise. This can improve vial-to-vial or batch-to-batch uniformity and shorten primary drying, but it must be designed for each formulation rather than applied as a default step.
Measurement. Collapse and glass-transition data come from thermal analysis of the specific formulation. They are not read from a machine manual, and they are measured separately for each material.