Formulating a product for freeze drying is a precise process.
Freeze drying is a potentially damaging process, especially to labile materials such as proteins or living organisms. Depending on the product type damage can occur at various stages of the process including freezing, drying and heating. Excipients must be selected and combined so as to protect the active ingredient from these stresses without damaging it in any other way, and without lowering the critical temperatures of the formulation overall to make freeze drying unviable. Every component used will have an impact on the final acceptability of the product.
The success of a formulation is measured on many criteria, including final moisture content, stability, activity and rehydration characteristics.