There are four stages in the complete drying process:
- pretreatment
- freezing
- primary drying and
- secondary drying.
Pretreatment
Pretreatment includes any method of treating the product prior to
freezing. This may include concentrating the product, formulation revision
(i.e., addition of components to increase stability and/or improve processing),
decreasing a high vapor pressure solvent or increasing the surface area. In
many instances the decision to pretreat a product is based on theoretical knowledge
of freeze-drying and its requirements, or is demanded by cycle time or product
quality considerations. Methods of pretreatment include: Freeze concentration,
Solution phase concentration, Formulation to Preserve Product Appearance,
Formulation to Stabilize Reactive Products, Formulation to Increase the Surface
Area, and Decreasing High Vapor Pressure Solvents
Freezing
In a lab, this is often done by placing the material in a
freeze-drying flask and rotating the flask in a bath, called a shell freezer,
which is cooled by mechanical refrigeration, dryice and methanol, or liquid nitrogen. On a larger scale, freezing is usually done
using a freeze-drying machine. In this step, it is important to cool the
material below its triplepoint, the lowest temperature at which the solid and
liquid phases of the material can coexist. This ensures that sublimation rather
than melting will occur in the following steps. Larger crystals are easier to
freeze-dry. To produce larger crystals, the product should be frozen slowly or
can be cycled up and down in temperature. This cycling process is called annealing. However, in the case of food, or objects with formerly-living
cells, large ice crystals will break the cell walls (a problem discovered, and
solved, by ClarenceBirdseye), resulting in the destruction of more cells,
which can result in increasingly poor texture and nutritive content. In this
case, the freezing is done rapidly, in order to lower the material to below
its eutecticpoint quickly, thus avoiding the formation of
ice crystals. Usually, the freezing temperatures are between −50 °C and −80 °C.
The freezing phase is the most critical in the whole freeze-drying process,
because the product can be spoiled if badly done. Amorphous materials do not have a eutectic point,
but they do have a critical point, below which the product must be maintained
to prevent melt-back or collapse during primary and secondary
Primary drying
During the primary
drying phase, the pressure is lowered (to the range of a few millibars), and enough heat is supplied to the material
for the water to sublime. The amount of heat necessary can be calculated using the
sublimating molecules’ latent heat of
sublimation. In this initial drying
phase, about 95% of the water in the material is sublimated. This phase may be
slow (can be several days in the industry), because, if too much heat is added,
the material’s structure could be altered. In this phase, pressure is controlled through the application
of partial
vacuum. The vacuum speeds
sublimation, making it useful as a deliberate drying process. Furthermore, a
cold condenser chamber and/or condenser plates provide a surface(s) for the
water vapour to re-solidify on. This condenser plays no role in keeping the
material frozen; rather, it prevents water vapor from reaching the vacuum pump,
which could degrade the pump's performance. Condenser temperatures are
typically below −50 °C (−60 °F).
The secondary drying phase aims to remove unfrozen water
molecules, since the ice was removed in the primary drying phase. This part of
the freeze-drying process is governed by the material’s adsorption isotherms. In this phase, the temperature is raised higher than in the
primary drying phase, and can even be above 0 °C, to break any physico-chemical
interactions that have formed between the water molecules and the frozen
material. Usually the pressure is also lowered in this stage to encourage
desorption (typically in the range of microbars, or fractions of a pascal) .However, there are products that benefit from increased
pressure as well.
After the freeze-drying process is complete, the vacuum is usually
broken with an inert gas, such as nitrogen, before the material is sealed.
At the end of the operation, the final residual water content in
the product is extremely low, around 1% to 4%.
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