In general, a freeze-drying process comprises three stages: freezing (material solidification), primary drying (ice sublimation), and secondary drying (bound water desorption) [ 9].
The chamber pressure and the 5 model formulations had no significant impact on sublimation rate and total drying time, respectively.
There could be two possible explanation for this behavior, firstly, the birefringence arises due to the inherent semi-crystalline nature of cellulose derivatives and secondly, this could be a result of structural birefringence induced due to the alignment of polymer molecules at the air water interface under freezing and subsequent drying through sublimation [ 32].
The impact of several process and formulation variables on the mass of sublimed ice after 15 min of primary drying (i.e., sublimation rate) and the total drying time was examined.
For CO2 treatment, leaves or protoplasts were placed in a plastic box filled with CO2 that was generated by sublimation of dry ice in water, contained in a small beaker in the box.
The presence of porous particles is expected after the sublimation of ice crystals during freeze drying process.
This was due to the better settlement of oil in the cyclodextrin cavities after the initial sublimation from the cotton surface during drying.
Stability of the unfrozen fraction is important during the drying process: if the sublimation temperature is above the Tg of the sample, damage will be incurred due to recrystallization [51].
The theoretical model was capable of predicting and describing the observed motion of the ice sublimation front, the ice temperature profile, the drying time and the influence of the total pressure.
The resulting materials may be then subjected to treatment, such as heat-treating by sintering in an oven or drying with the use of the sublimation method.
For example, it was proved that drying the silver-hydroxyapatite using the sublimation method leads to the preparation of a material with finer crystals, thereby also enhancing the activity of antibacterial Ag-HAp [ 57].
