2.1 Polymeric/Biopolymeric Cryogels Formed by
Freeze–Thaw Aging of Colloid Sols
Perhaps the earliest example of a technologically realized process in which freeze–
thaw-induced gel formation was employed is the manufacture of the food product
named “kori-tofu,” which has been known for centuries and is still very popular in
Japan [36]. The starting material for the fabrication of this soybean-protein foodstuff is the soya curd “tofu,” a colloid-type dispersion of the salting-out coagulate of
11S globulins. Tofu is subjected to freezing, during which the SH groups of
cysteine residues of the neighboring macromolecules are coupled into
intermolecular disulfide bridges, thus resulting in the formation of a 3D supramolecular network of protein particles [37–40]. Another example of cryotropic gel
formation of colloidal dispersions is the freeze–thaw-caused structuration of
minced meat and pastes of myofibrillar protein isolates, e.g., those extracted from
various kinds of fish, shrimp or Antarctic krill [41–48]. Since the major myofibrillar
proteins, actin and myosin, are rich in cysteine, chemical crosslinking of proteinaceous colloid particles in non-deeply frozen systems is accompanied by the
formation of interparticle covalent SS-bonds with the participation of air oxygen
dissolved in the unfrozen liquid microphase. This gelation mechanism was
elucidated using studies on the cryogel formation of model thiol-bearing polymers
[49–52].
Cryotropic crosslinking of discrete particles of colloidal dispersions in the
presence of a crosslinking agent to produce 3D macroporous polymeric materials
has also been reported. The preparation of cryostructured collagen sponges [53–55]
or the process of fabrication of leather-like materials from milled tanned leather
wastes [56–58] are examples of such type of cryogel formation. In these cases, the
respective dispersion was mixed with a crosslinking agent (e.g., glutaraldehyde)
and then this heterophase reaction mass was frozen and kept in the frozen state for a
necessary period of time and finally defrosted, resulting in spongy matter built of a
supramolecular framework of chemically bound polymer particles. The same
approach was also employed in subsequent years for the creation of macroporous
materials based on covalently linked particulate matters like latexes [59, 60],
microbial cells [27, 61], or small gel particles [27, 62, 63].
As well as covalent freeze–thaw gels derived from colloid dispersions,
noncovalent cryogels with aligned macroporous morphology have also been prepared
from particulate precursors. These include protein-containing systems [64–69],
cryogels fabricated from the colloid solutions of gelatinized starch [5, 70–78],
and cryostructured polymer matrices formed as a result of the freeze–thaw treatment of frost-sensitive latexes [79–84]. The nature of the interparticle links in the
first case is a combination of hydrogen bonding, ionic interactions, and hydrophobic
interactions. Multiple H-bonds are responsible for gelation of the starch-based
systems, while the hydrophobic associations are the basis of particles “glueing”
in the latex examples. The macroporous morphology of the resulting polymer
materials and their physicochemical properties are determined by the initial
6
V.I. Lozinsky
Freeze–Thaw Aging of Colloid Sols
Perhaps the earliest example of a technologically realized process in which freeze–
thaw-induced gel formation was employed is the manufacture of the food product
named “kori-tofu,” which has been known for centuries and is still very popular in
Japan [36]. The starting material for the fabrication of this soybean-protein foodstuff is the soya curd “tofu,” a colloid-type dispersion of the salting-out coagulate of
11S globulins. Tofu is subjected to freezing, during which the SH groups of
cysteine residues of the neighboring macromolecules are coupled into
intermolecular disulfide bridges, thus resulting in the formation of a 3D supramolecular network of protein particles [37–40]. Another example of cryotropic gel
formation of colloidal dispersions is the freeze–thaw-caused structuration of
minced meat and pastes of myofibrillar protein isolates, e.g., those extracted from
various kinds of fish, shrimp or Antarctic krill [41–48]. Since the major myofibrillar
proteins, actin and myosin, are rich in cysteine, chemical crosslinking of proteinaceous colloid particles in non-deeply frozen systems is accompanied by the
formation of interparticle covalent SS-bonds with the participation of air oxygen
dissolved in the unfrozen liquid microphase. This gelation mechanism was
elucidated using studies on the cryogel formation of model thiol-bearing polymers
[49–52].
Cryotropic crosslinking of discrete particles of colloidal dispersions in the
presence of a crosslinking agent to produce 3D macroporous polymeric materials
has also been reported. The preparation of cryostructured collagen sponges [53–55]
or the process of fabrication of leather-like materials from milled tanned leather
wastes [56–58] are examples of such type of cryogel formation. In these cases, the
respective dispersion was mixed with a crosslinking agent (e.g., glutaraldehyde)
and then this heterophase reaction mass was frozen and kept in the frozen state for a
necessary period of time and finally defrosted, resulting in spongy matter built of a
supramolecular framework of chemically bound polymer particles. The same
approach was also employed in subsequent years for the creation of macroporous
materials based on covalently linked particulate matters like latexes [59, 60],
microbial cells [27, 61], or small gel particles [27, 62, 63].
As well as covalent freeze–thaw gels derived from colloid dispersions,
noncovalent cryogels with aligned macroporous morphology have also been prepared
from particulate precursors. These include protein-containing systems [64–69],
cryogels fabricated from the colloid solutions of gelatinized starch [5, 70–78],
and cryostructured polymer matrices formed as a result of the freeze–thaw treatment of frost-sensitive latexes [79–84]. The nature of the interparticle links in the
first case is a combination of hydrogen bonding, ionic interactions, and hydrophobic
interactions. Multiple H-bonds are responsible for gelation of the starch-based
systems, while the hydrophobic associations are the basis of particles “glueing”
in the latex examples. The macroporous morphology of the resulting polymer
materials and their physicochemical properties are determined by the initial
6
V.I. Lozinsky