depends on the nature, concentration, cooling rate, and interactions between individual solutes present in the medium and forms
the basis for experimental review during a formulation development exercise [9].
Overall, four patterns of solute response are observed during
freeze-drying:
1. Solute crystallizes readily, regardless of cooling rate or freezing
conditions, to form a mixture of ice and solute crystals (this
behavior is termed eutectic freezing).
2. Solute crystallizes but only when the solution is subjected to a
slow rate of cooling.
3. Solute crystallizes only after the solution has been annealed.
4. Solute fails to crystallize regardless of cooling rate or regime
adopted, and solute remains associated with unfrozen water as
a metastable amorphous mass or glass.
For a crystallizing solute, the eutectic point is the lowest temperature in a system in which a residual liquid phase and solid phase
are in equilibrium. Above the eutectic point, ice and solute concentrate persists, whereas below the eutectic point, a mixture of ice and
solute crystals is formed. Eutectic temperatures for aqueous solutions containing crystallizing salts are characteristic for each solute
and are significantly below the freezing point of water (e.g., eutectic
temperature for sodium chloride (À21.1
C). Exposing cells or
proteins for prolonged periods to a eutectic solution comprising
hypertonic salt concentrations can cause damage by plasmolysis or
precipitation by “salting out.”
The eutectic zone is the range of temperatures encompassing
all the eutectic temperatures within the system. For a two-part
water/solute system, the eutectic temperature is a discrete, quantifiable temperature in contrast to multi-solute systems where a
eutectic zone may be observed that represents a range of temperatures where the minimum eutectic temperature is lower than that of
any individual eutectic temperatures in the medium.
Typical freeze-dried vaccine formulations fail to crystallize
completely when cooled, and a proportion of the solutes in the
sample persists as an amorphous, noncrystalline, glass. When
exposed to temperatures above their glass transition (T g
0 ) or collapse temperature (T collapse ), these samples may warm during sublimation causing the amorphous mass to soften, so that the freezedrying progresses with collapse to form a sticky structureless residue within the vial. Less severe collapse will result in the formation
of a shrunken, distorted, or split cake, although a small degree of
shrinkage while maintaining the aspect ratio of the original fill
volume is typical for most amorphous materials [18].
Principles of Freeze-Drying
109
the basis for experimental review during a formulation development exercise [9].
Overall, four patterns of solute response are observed during
freeze-drying:
1. Solute crystallizes readily, regardless of cooling rate or freezing
conditions, to form a mixture of ice and solute crystals (this
behavior is termed eutectic freezing).
2. Solute crystallizes but only when the solution is subjected to a
slow rate of cooling.
3. Solute crystallizes only after the solution has been annealed.
4. Solute fails to crystallize regardless of cooling rate or regime
adopted, and solute remains associated with unfrozen water as
a metastable amorphous mass or glass.
For a crystallizing solute, the eutectic point is the lowest temperature in a system in which a residual liquid phase and solid phase
are in equilibrium. Above the eutectic point, ice and solute concentrate persists, whereas below the eutectic point, a mixture of ice and
solute crystals is formed. Eutectic temperatures for aqueous solutions containing crystallizing salts are characteristic for each solute
and are significantly below the freezing point of water (e.g., eutectic
temperature for sodium chloride (À21.1
C). Exposing cells or
proteins for prolonged periods to a eutectic solution comprising
hypertonic salt concentrations can cause damage by plasmolysis or
precipitation by “salting out.”
The eutectic zone is the range of temperatures encompassing
all the eutectic temperatures within the system. For a two-part
water/solute system, the eutectic temperature is a discrete, quantifiable temperature in contrast to multi-solute systems where a
eutectic zone may be observed that represents a range of temperatures where the minimum eutectic temperature is lower than that of
any individual eutectic temperatures in the medium.
Typical freeze-dried vaccine formulations fail to crystallize
completely when cooled, and a proportion of the solutes in the
sample persists as an amorphous, noncrystalline, glass. When
exposed to temperatures above their glass transition (T g
0 ) or collapse temperature (T collapse ), these samples may warm during sublimation causing the amorphous mass to soften, so that the freezedrying progresses with collapse to form a sticky structureless residue within the vial. Less severe collapse will result in the formation
of a shrunken, distorted, or split cake, although a small degree of
shrinkage while maintaining the aspect ratio of the original fill
volume is typical for most amorphous materials [18].
Principles of Freeze-Drying
109