Freeze Dry Technology was developed in the first quarter of the 1900s. The first lyophilization device was derived from a chemical pump developed by Benedict & Manning in 1905.
Since 1930s, lyophilization has been improved by pharmacy and biotechnology companies, on the other hand, coffee, the first freeze-dried food sample, was first developed by Nestlé in Brazil in 1938, upon demands that it finds solution to Brazil’s excess coffee stocks.
What is Lyophilization?
Lyophilization and freeze drying are synonymous. Lyophilization is a water removal process typically used to preserve perishable materials, to extend shelf life or make the material more convenient for transport. Lyophilization works by freezing the material, then reducing the pressure and adding heat to allow the frozen water in the material to sublimate.
Lyophilization is a process more commonly known as freeze-drying. The word is derived from Greek, and means “made solvent-loving”. This process is a way of drying something that minimizes damage to its internal structure. Because it is a relatively complex and expensive form of drying, it is limited to those materials which are sensitive to heat and have delicate structures and substantial value. One of the only substances which cannot be preserved effectively by freeze-drying is mammalian cells, which are too fragile.
The preferred method of preservation in the biotechnology industry, lyophilization is regularly used to preserve vaccines, pharmaceuticals, and other proteins. Freeze-drying is also used to preserve special food products, eliminating the need for refrigeration. Freeze-dried food is eaten by mountain climbers and astronauts. Lyophilization is used by botanists to preserve flower samples indefinitely. Because the process of freeze-drying removes most of the water from the sample, freeze-dried materials become highly absorbent, and merely adding water can restore the sample to something close to its original state.
Lyophilization’s 3 Primary Stages
Lyophilization occurs in three phases, with the first and most critical being the freezing phase. Proper lyophilization can reduce drying times by 30%.
There are various methods to freezing the product. Freezing can be done in a freezer, a chilled bath (shell freezer) or on a shelf in the freeze dryer. Cooling the material below its triple point ensures that sublimation, rather than melting, will occur. This preserves its physical form.
Lyophilization is easiest to accomplish using large ice crystals, which can be produced by slow freezing or annealing. However, with biological materials, when crystals are too large they may break the cell walls, and that leads to less-than-ideal freeze drying results. To prevent this, the freezing is done rapidly. For materials that tend to precipitate, annealing can be used. This process involves fast freezing, then raising the product temperature to allow the crystals to grow.
Primary Drying (Sublimation) Phase
Lyophilization’s second phase is primary drying (sublimation), in which the pressure is lowered and heat is added to the material in order for the water to sublimate. The vacuum speeds sublimation. The cold condenser provides a surface for the water vapor to adhere and solidify. The condenser also protects the vacuum pump from the water vapor. About 95% of the water in the material is removed in this phase. Primary drying can be a slow process. Too much heat can alter the structure of the material.
Secondary Drying (Adsorption) Phase
Lyophilization’s final phase is secondary drying (adsorption), during which the ionically-bound water molecules are removed. By raising the temperature higher than in the primary drying phase, the bonds are broken between the material and the water molecules. Freeze dried materials retain a porous structure. After the lyophilization process is complete, the vacuum can be broken with an inert gas before the material is sealed. Most materials can be dried to 1-5% residual moisture.
Advantages of lyophilization
The energy and equipment costs of lyophilization are around 2-3 times higher than those of other drying methods. The drying cycle is also longer, about 24 hours. First, the temperature of the sample is lowered to near freezing point. Then, the sample is inserted into a vacuum chamber. The more energetic molecules escape, lowering the temperature further, while the extremely low pressure causes water molecules to be drawn out of the sample. Attached to the vacuum chamber is a condenser, which converts the airborne moisture into liquid and siphons it away.
Great care is taken throughout the process to ensure that the structure of the sample remains constant. For instance, the sample could merely be frozen by the vacuum rather than being frozen under atmospheric pressures, but that would cause shrinkage in the sample, damaging its structure irreversibly.
The primary mechanism that allows for freeze-drying is sublimation, whereby ice is directly converted to water vapor, without passing through the intermediary stage of a liquid. Rather than through heating, this is done by removal of pressure so that the ice boils without melting. The result is a sample whose structure is largely preserved, which can be stored at room temperatures and pressures.