The Three Stages
Freeze drying (lyophilization) is a dehydration process that operates in three stages:
- Freezing: the product is frozen to below its eutectic point, typically -30ยฐC to -50ยฐC. The freezing rate affects ice crystal size, which impacts product quality.
- Primary Drying (Sublimation): under deep vacuum (โค30 Pa), heat is applied to the product to drive sublimation. Ice transitions directly to vapor without passing through the liquid phase. This stage removes 90-95% of the water.
- Secondary Drying (Desorption): the temperature is raised to remove bound water molecules through desorption. This stage brings the final moisture content to 1-4%.
The stages are sequential by necessity. Sublimation can proceed only while the ice structure remains frozen, so the product must stay below its eutectic point throughout primary drying; if it thaws, the structure collapses and the porous matrix that allowed vapour to escape is lost.
Why Deep Vacuum Needs a Roots Pump Stage
Primary drying depends on a pressure gradient. Water vapour leaves the ice surface, travels through the already-dried layer, and must be removed by the pump system fast enough to keep the chamber pressure down. At an operating pressure of โค30 Pa the volume of water vapour to be handled is large, and a single oil-sealed pump cannot sustain the pumping speed required, so a Roots pump stage working ahead of the rotary vane pump is necessary.
That requirement belongs to the deep-vacuum range. Vacuum cooling operates at โค600 Pa, and at that target pressure a Roots stage brings no gain: produce pre-cooling works with a rotary vane pump, and hot-food cooling (80โ90ยฐC initial temperature) with a water ring pump backing a rotary vane pump.
Cycle Duration and Product Variables
The total process can take 12-48 hours depending on product thickness, composition, and equipment specifications. Thin slices and small pieces dry faster because the vapour path is short. Sugar-rich and dense products dry more slowly and are more sensitive to collapse. Loading pattern matters too: the same mass spread in a thin layer dries faster than the same mass packed into a deep tray. Where these conditions differ from the reference case, cycle time is established by trial.