Page 26 - Medicinal Plants_PharmD general

(e) Freeze drying (lyophilization)
A drying method where the solvent is frozen prior to drying and is
then sublimed, i.e., passed to the gas phase directly from the solid
phase, below the melting point of the solvent. Vacuum is introduced
to the vessel which lowers the pressure to below the triple point of
water. The cold temperatures and low pressures inside the freeze-
dryer result in removal of water by sublimation, the transition of a
solid directly to a gas without going through a liquid phase. This is
the gentlest way to remove water from the product. In absence of
liquid-water, degradative reactions cannot take place or are greatly
reduced. Freeze-drying can preserve the product for up to 25 years,
remaining fresh as the day it was dried. Along with the preservation
time, it also retains the medicinal value, flavour and texture.

(a) (b)

(c) (d)

Figure 3. Artificial drying. (a) Oven drying. (b) Vacuum drying.
(c) Microwave vacuum drying. (d) Spray drying

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