The main difference between them is the water remove theory: Vacuum drying based on water boiling point, and vacuum freeze drying based on water melting point.
Vacuum freeze drying also known as lyophilization or freeze drying. Dehydration by freezing raw product and then move to a drying chamber, reduce the chamber pressure, enable solid ice (inside raw product) sublimate to vapor (removed from raw product). Water remove based on sublimation (Solid to Vapor).
Vacuum drying without freezing stage. By reduce environment pressure enable water boiling point reduce, when reach a certain range, the water start boiling and go out of raw product. So, vacuum drying water remove based on water boiling.
As Water Triple Point Graphic Showed, Water have 3 phases: Solid phase (Ice), Liquid phase(Water) and Vapor phase(Gas). In a certain range of Temperature and Pressure, they can transition between.
Solid phase go through the border and convert to vapor phase called freeze drying(sublimation).
Liquid phase go through the border and convert to vapor phase called vacuum drying.
That also means, they will have different application.
Vacuum Freeze Drying
+80 ~ +40C
-35 ~ -5C
Dried Product Structure
Heat sensitive product
Freeze drying advantages mainly inflected in the microelement, vitamin, color and taste aspects. Is much better than vacuum drying, but the drying cost also much higher.
However, for some product that use freeze drying is not cost-effective, such as beef. The dried quality is very similar with vacuum drying.