success and reduce the length of the freezing stage and increase
the efficiency of the primary drying phase, much as with annealing as discussed earlier. Additionally, since it occurs from the
surface to the base, it can also help minimize the possibility of
surface skin (crust) formation.
2.3 Primary
and Secondary Drying
For clarity it is usual to separate the drying cycle into primary
drying (the sublimation stage) and secondary drying (primarily
desorption). The first step in the drying cycle is defined as primary
drying and represents the stage where ice, which typically constitutes between approximately 70% and 95% of the product’s water
content, is converted into water vapor. Sublimation is a relatively
efficient process although the precise length of primary drying will
vary depending on the product formulation, solute concentration,
fill depth, and the heat transfer characteristics of the container,
among other factors. During primary drying, the product dries at
a discrete boundary (the sublimation interface), which recedes
through the product from surface to base as drying progresses.
2.3.1 Primary Drying
(Sublimation)
Under atmospheric conditions, liquid water is converted into vapor
by warming, a process defined as evaporation. However, the three
states of water—ice, liquid, and vapor—coexist at the triple point
and illustrate that at sub-atmospheric pressures, ice can convert
directly to vapor by sublimation. Ice sublimation from a frozen
aqueous solution or suspension results in an open, porous, dry
structure where solutes are spatially arranged as in the original
solution or suspension. In contrast to evaporation, where components are concentrated as drying progresses, sublimation under
vacuum minimizes concentration effects providing a dry product
that is active and readily soluble. Having frozen the liquid formulation, the next step is to dry it by subliming ice directly into water
vapor. In order to initiate the sublimation process, it is essential to
lower the chamber pressure so that the partial pressure of water is
maintained below its corresponding vapor pressure to ensure the
direct conversion of ice into water vapor.
Sublimation Rate
and Chamber Pressure
Conditions
Decreasing the chamber pressure will increase the rate of sublimation by reducing the gas/vapor concentration above the sample to
provide minimal resistance to water molecules migrating from the
product. However, reducing the system pressure beyond a certain
point will be counterproductive, and indeed, contrary to expectations, at very low system pressures the sublimation rate will decrease
due to the reduction in the number of gas or vapor molecules in the
chamber to conduct heat energy from the shelf into the product.
Essentially, under high-vacuum conditions a “thermos flask” effect
is induced in the chamber, which inhibits heat transfer from the
shelf. Under high-pressure (poor vacuum) conditions, heat transfer
from the shelf to the product is primarily via gas/vapor conduction
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Kevin R. Ward and Paul Matejtschuk
the efficiency of the primary drying phase, much as with annealing as discussed earlier. Additionally, since it occurs from the
surface to the base, it can also help minimize the possibility of
surface skin (crust) formation.
2.3 Primary
and Secondary Drying
For clarity it is usual to separate the drying cycle into primary
drying (the sublimation stage) and secondary drying (primarily
desorption). The first step in the drying cycle is defined as primary
drying and represents the stage where ice, which typically constitutes between approximately 70% and 95% of the product’s water
content, is converted into water vapor. Sublimation is a relatively
efficient process although the precise length of primary drying will
vary depending on the product formulation, solute concentration,
fill depth, and the heat transfer characteristics of the container,
among other factors. During primary drying, the product dries at
a discrete boundary (the sublimation interface), which recedes
through the product from surface to base as drying progresses.
2.3.1 Primary Drying
(Sublimation)
Under atmospheric conditions, liquid water is converted into vapor
by warming, a process defined as evaporation. However, the three
states of water—ice, liquid, and vapor—coexist at the triple point
and illustrate that at sub-atmospheric pressures, ice can convert
directly to vapor by sublimation. Ice sublimation from a frozen
aqueous solution or suspension results in an open, porous, dry
structure where solutes are spatially arranged as in the original
solution or suspension. In contrast to evaporation, where components are concentrated as drying progresses, sublimation under
vacuum minimizes concentration effects providing a dry product
that is active and readily soluble. Having frozen the liquid formulation, the next step is to dry it by subliming ice directly into water
vapor. In order to initiate the sublimation process, it is essential to
lower the chamber pressure so that the partial pressure of water is
maintained below its corresponding vapor pressure to ensure the
direct conversion of ice into water vapor.
Sublimation Rate
and Chamber Pressure
Conditions
Decreasing the chamber pressure will increase the rate of sublimation by reducing the gas/vapor concentration above the sample to
provide minimal resistance to water molecules migrating from the
product. However, reducing the system pressure beyond a certain
point will be counterproductive, and indeed, contrary to expectations, at very low system pressures the sublimation rate will decrease
due to the reduction in the number of gas or vapor molecules in the
chamber to conduct heat energy from the shelf into the product.
Essentially, under high-vacuum conditions a “thermos flask” effect
is induced in the chamber, which inhibits heat transfer from the
shelf. Under high-pressure (poor vacuum) conditions, heat transfer
from the shelf to the product is primarily via gas/vapor conduction
112
Kevin R. Ward and Paul Matejtschuk