intention should be to induce supercooling in the suspension to
encourage uniform cooling and freezing throughout the sample
contents.
Freezing may be defined as the abrupt conversion of the solution or suspension into a mixture of ice and solute concentrate. It is
a two-step process during which water initially nucleates, followed
by the growth of the ice crystals that pervade the solute phase,
resulting in a mixture of ice and solute concentrate. Under typical
processing conditions, ice nucleates heterogeneously around
microscopic particles within the suspension and is encouraged by
reducing temperature and agitating the supercooled suspension to
increase the probability of contact between nucleating foci and
water clusters. Nucleation depends on the number and physical
nature of particulate impurities within the suspension or solution.
Ice is a particularly effective nucleation focus, and cryobiologists
may deliberately seed samples with ice to induce nucleation. Other
effective ice nucleators include glass shards and specifically formulated nucleation promoters. Whereas nucleation aids can be added
to experimental systems, deliberate attempts to add ice inducers to
pharmaceutical materials would be at variance with good pharmaceutical manufacturing practice.
In contrast to nucleation, ice growth (proliferation) is encouraged by raising the temperature, thereby decreasing the suspension
viscosity. Ice nucleation and proliferation are inhibited at temperatures below the glass transition temperature (T g
0 ), whereas above
the melting temperature (T m ), the suspension or solution will melt.
The consequences and measurements of these parameters are
important elements in the formulation exercise [11]. To facilitate
the sublimation of water vapor from the drying mass, the ice
crystals should be large, wide, and contiguous, extending from
the product base toward its surface, thereby providing an optimized
structure for vapor migration. Crystal structures commonly
observed during freeze-drying when solutions are frozen in trays
or vials include dendritic structuring, where the ice crystal branches
continuously from the nucleating focus, and the spherulite form,
where sub-branching is discouraged because the solution viscosity
is high, or fast rates of cooling are used.
2.2.1 Controlled
Nucleation
The use of controlled (or induced) ice nucleation in freeze-drying is
receiving increased attention due to the advantages it can offer in
terms of increasing sublimation efficiency through control of the
ice nucleation temperature, which affects product porosity and
uniformity [12, 13] and may also reduce activity losses in cases
where ice formation or freeze-concentration effects are known to
induce protein denaturation [14–16]. Any method that allows for a
control of the nucleation temperature results in several degrees of
freedom for the freeze-drying process. In principle, there are three
parameters which can directly be controlled: the nucleation temperature, the isothermal hold time post-nucleation, and the cooling
Principles of Freeze-Drying
105
encourage uniform cooling and freezing throughout the sample
contents.
Freezing may be defined as the abrupt conversion of the solution or suspension into a mixture of ice and solute concentrate. It is
a two-step process during which water initially nucleates, followed
by the growth of the ice crystals that pervade the solute phase,
resulting in a mixture of ice and solute concentrate. Under typical
processing conditions, ice nucleates heterogeneously around
microscopic particles within the suspension and is encouraged by
reducing temperature and agitating the supercooled suspension to
increase the probability of contact between nucleating foci and
water clusters. Nucleation depends on the number and physical
nature of particulate impurities within the suspension or solution.
Ice is a particularly effective nucleation focus, and cryobiologists
may deliberately seed samples with ice to induce nucleation. Other
effective ice nucleators include glass shards and specifically formulated nucleation promoters. Whereas nucleation aids can be added
to experimental systems, deliberate attempts to add ice inducers to
pharmaceutical materials would be at variance with good pharmaceutical manufacturing practice.
In contrast to nucleation, ice growth (proliferation) is encouraged by raising the temperature, thereby decreasing the suspension
viscosity. Ice nucleation and proliferation are inhibited at temperatures below the glass transition temperature (T g
0 ), whereas above
the melting temperature (T m ), the suspension or solution will melt.
The consequences and measurements of these parameters are
important elements in the formulation exercise [11]. To facilitate
the sublimation of water vapor from the drying mass, the ice
crystals should be large, wide, and contiguous, extending from
the product base toward its surface, thereby providing an optimized
structure for vapor migration. Crystal structures commonly
observed during freeze-drying when solutions are frozen in trays
or vials include dendritic structuring, where the ice crystal branches
continuously from the nucleating focus, and the spherulite form,
where sub-branching is discouraged because the solution viscosity
is high, or fast rates of cooling are used.
2.2.1 Controlled
Nucleation
The use of controlled (or induced) ice nucleation in freeze-drying is
receiving increased attention due to the advantages it can offer in
terms of increasing sublimation efficiency through control of the
ice nucleation temperature, which affects product porosity and
uniformity [12, 13] and may also reduce activity losses in cases
where ice formation or freeze-concentration effects are known to
induce protein denaturation [14–16]. Any method that allows for a
control of the nucleation temperature results in several degrees of
freedom for the freeze-drying process. In principle, there are three
parameters which can directly be controlled: the nucleation temperature, the isothermal hold time post-nucleation, and the cooling
Principles of Freeze-Drying
105