The 3 major parts of a freeze drying recipe include freezing, primary drying (sublimation), and secondary drying (desorption). Each step is critical in producing the required end product is critical in producing the required end product.
1- Freezing may be the most under-rated step. Proper freezing will ensure a good cake, while improper freezing may result in a product that cannot be successfully freeze dried. The typical objective of freezing is to produce a frozen matrix with sufficient crystal structure to allow the sublimating material to escape. The larger the crystal the better. Slow freezing produces larger crystals. Some products form a glassy material and annealing may be required during the freezing process. Annealing, first lowering the temperature then raising the temperature and then lowering it again, locks the constituents in place and then allows the crystals to grow. Freezing can range from 1 hour to 24 hours, depending on the application.
2- Primary drying (sublimation) drives the unbound moisture out of the product. Sublimation occurs under vacuum with the product temperature below its critical temperature. This is typically the longest process. At the end of the primary drying cycle, the product will have 3 to 5% moisture content.
3- Secondary drying (desorption) drives the ionically bound water from the material. This is done by heating the product. Secondary drying can result in moisture levels of 0.5%.