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Lyophilization is a technical term for freeze
drying. It means "solvent loving." Freeze dried materials have
had all the water removed and the resulting structure has a strong
affinity for water (the solvent). This is why freeze dried
materials must be packaged to protect them from moisture. Most
materials are dried to 1-5% moisture. At this level, the action of
microorganisms and enzymes is greatly inhibited. If done properly,
freeze dried materials are stable indefinitely.
Freeze drying begins with freezing the
material. This requirement preserves its physical form. Fruits and
vegetables are good examples. Once a strawberry is frozen it
maintains its original shape even after it is dried. Simple air
drying would yield a shriveled, or collapsed, strawberry that has
lost its form.
Freeze dried materials retain a porus
structure. The space occupied by water in the original wet
material is now hollow. A freeze dried strawberry acts like a
sponge and will easily rehydrate back to its original form as
water fills the voids created in the drying process. You may have
enjoyed freeze dried fruits in your breakfast cereal.
Once the material is frozen, it is placed in a
vacuum chamber where the pressure is reduced to less than 4.6 Torr
(0.006 atm). Below this pressure ice cannot melt. Its form is
maintained and now a new process can occur—sublimation. The ice
evaporates and bypasses the liquid phase—it never has a chance to
melt.
A familiar example of this is dry ice (frozen
carbon dioxide, CO2). It
never melts but simply evaporates. Dry ice is unique in that it
sublimates at normal atmospheric pressure. Frozen water (regular
ice) will not do this. It must be under very low pressure or it
melts as it warms up.
Some materials don’t have a form to
preserve. Solutions of extracts, biologicals, chemicals, etc. are
frozen solid and then lyophilized. This allows drying to occur at
very low temperature. This protects the chemical structure of
those materials. They are preserved and not degraded by heat. The
end product is a powder. Freeze dried strawberries taste good
because the drying process retains the essential oils and
flavinoids of the fruit. The same is true for solutions that are
freeze dried to powders. The flavinoids, polyphenols,
phytochemicals, tannins, etc. are retained and preserved in high
concentration. Freeze drying is a great way to preserve and
concentrate the
antioxidants found in biological materials.
Once the material is frozen and under low pressure,
heat is applied. In the freeze drying process this heat is used to
convert the ice to vapor. The temperature of the product stays low
and does not heat up.
Frozen material under vacuum will sublimate at a
fixed temperature depending on the pressure. As long as low
pressure can be maintained, the ice will sublimate without melting
and without heating up. This give freeze dried materials their
unique properties.
So where does the vapor go? It is removed from the
vacuum chamber and collected on a very cold condenser. Freeze
drying is a process of moving the ice from a material to a
condenser. Once all the ice is removed from the material, the
lyophilization process is complete and the pressure can be
restored to normal atmospheric levels.
Because of the pore structure created from the
freeze drying process, and because the material is “solvent
loving,” lyophilized materials rehydrate easily. Again, fruit is a
common example. Some pharmaceuticals act this way, too. They are
stabilized and preserved through lyophilization and stored in
sealed vials. When needed, water is added to rehydrate and the
medicine is ready for immediate use.
Bacteria cultures are another material that are
stabilized by freeze drying and then “reanimated” by adding
water. The acidophilus and bifidus in yogurt are examples of
cultures that may have been freeze dried at one time. Or you may
have purchased encapsulated acidophilus that is reanimated in your
stomach to populate your GI tract. The ability to stabilize
materials and then reconstitute them at a later time is one of the
main benefits of freeze drying.
When materials are properly freeze dried they are
friable. Friable means easily crushed and/or reduced to a
powder. In the powder form, materials can be used in a variety of
ways.
Powders can be encapsulated, tabletized, formed,
mixed, and generally handled in creative ways that are not
possible with liquid materials. Freeze dried powders are
easier to dissolve and dissolve faster than materials dried by other means. This allows
for convenience of delivery, storage, handling, and even
transportation. Freeze dried materials are often 90% lighter than
in the un-dried state. This makes freeze drying ideal for
backpacking food where you want to make your load as light as
possible. You can then use available water to rehydrate the food
instead of carrying the water as part of the food. |
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