Attempts to freeze-dry cells in water or a simple salt solution
typically result in poor survival. A wide range of protective media
has been developed for preserving freeze-dried vaccines, including
augmented growth media or sugar solutions. Carbohydrates are
widely used as freeze-drying protectants either individually or in
combination with other solutes. They should be chosen on the
basis of experimentally determining their freeze-drying characteristics rather than on a pragmatic basis. Monosaccharides, such as
glucose, provide good bioprotection during freezing and freezedrying but exhibit a low glass transition temperature (T g
0 ) and
collapse temperatures (T collapse ) and tend to dry with collapse
when orthodox freeze-drying cycles are used. Disaccharides can
be effective lyoprotectants, and because they display higher glass
transition temperatures than monosaccharides, typically freeze-dry
successfully when conventional drying cycles are used. Reducing
sugars such as lactose may induce damaging Maillard reactions,
thereby compromising stability, and for this reason, nonreducing
disaccharides such as sucrose or trehalose are preferred to reducing
sugars [19]. The addition of salts to formulations containing sugars
will markedly depress T g
0 or T collapse [2, 20]. Morgan et al. provide a
helpful review article discussing microorganism preservation by
various drying technologies highlighting factors such as growth
phase and growth medium, as well as cell concentration and bacteria type [21].
Although presenting technical difficulties such as product collapse, during freeze-drying the amorphous phase may be an essential prerequisite for stabilizing biomaterials, such as vaccines and
live cells, by providing an integration of the protective additive and
biomolecule, thereby minimizing damage during freezing and
drying.
Collapsed cakes are not only cosmetically unacceptable but may
be poorly soluble, exhibit reduced activity, or compromise shelf
stability. Collapse may be exacerbated by the formation of a surface
skin, which impedes vapor migration from the drying structure. To
avoid collapse, it is necessary to maintain the sublimation interface
below T g
0 or T collapse throughout primary drying and to include
excipients in the formulation that reduce the likelihood of collapse.
It is therefore essential to characterize formulations during the
process development program. Although collapse may cause operational difficulties during freeze-drying, the induction and maintenance of the amorphous state may be essential for protecting labile
biomolecules during freezing, drying, and storage [22].
A further issue that can affect the stability of a product despite
the lack of obvious loss of structure is a phenomenon known as
microcollapse. This may occur when separate amorphous and crystalline phases form in the frozen structure, with each displaying
independent behavior and having its own critical temperature,
which will be T g
0 or T collapse for the amorphous phase and T eu for
110
Kevin R. Ward and Paul Matejtschuk
typically result in poor survival. A wide range of protective media
has been developed for preserving freeze-dried vaccines, including
augmented growth media or sugar solutions. Carbohydrates are
widely used as freeze-drying protectants either individually or in
combination with other solutes. They should be chosen on the
basis of experimentally determining their freeze-drying characteristics rather than on a pragmatic basis. Monosaccharides, such as
glucose, provide good bioprotection during freezing and freezedrying but exhibit a low glass transition temperature (T g
0 ) and
collapse temperatures (T collapse ) and tend to dry with collapse
when orthodox freeze-drying cycles are used. Disaccharides can
be effective lyoprotectants, and because they display higher glass
transition temperatures than monosaccharides, typically freeze-dry
successfully when conventional drying cycles are used. Reducing
sugars such as lactose may induce damaging Maillard reactions,
thereby compromising stability, and for this reason, nonreducing
disaccharides such as sucrose or trehalose are preferred to reducing
sugars [19]. The addition of salts to formulations containing sugars
will markedly depress T g
0 or T collapse [2, 20]. Morgan et al. provide a
helpful review article discussing microorganism preservation by
various drying technologies highlighting factors such as growth
phase and growth medium, as well as cell concentration and bacteria type [21].
Although presenting technical difficulties such as product collapse, during freeze-drying the amorphous phase may be an essential prerequisite for stabilizing biomaterials, such as vaccines and
live cells, by providing an integration of the protective additive and
biomolecule, thereby minimizing damage during freezing and
drying.
Collapsed cakes are not only cosmetically unacceptable but may
be poorly soluble, exhibit reduced activity, or compromise shelf
stability. Collapse may be exacerbated by the formation of a surface
skin, which impedes vapor migration from the drying structure. To
avoid collapse, it is necessary to maintain the sublimation interface
below T g
0 or T collapse throughout primary drying and to include
excipients in the formulation that reduce the likelihood of collapse.
It is therefore essential to characterize formulations during the
process development program. Although collapse may cause operational difficulties during freeze-drying, the induction and maintenance of the amorphous state may be essential for protecting labile
biomolecules during freezing, drying, and storage [22].
A further issue that can affect the stability of a product despite
the lack of obvious loss of structure is a phenomenon known as
microcollapse. This may occur when separate amorphous and crystalline phases form in the frozen structure, with each displaying
independent behavior and having its own critical temperature,
which will be T g
0 or T collapse for the amorphous phase and T eu for
110
Kevin R. Ward and Paul Matejtschuk