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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.

 

 

 
 


Technical Discussion
To fully understand freeze drying, it is important to understand the water phase diagram shown to the right. This diagram is generated from the Clausius-Clapeyron equation.

The x-axis is temperature and y-axis is pressure. You can see that at a pressure of 1 atmosphere (760 Torr or 760 mmHg) water boils at 100° C (212 °F) like you’d expect. This is true at the beach (i.e., at sea level). If you go into the mountains on a backpacking trip, the pressure drops below 1 atm. And herein lies the key to understanding this diagram. As long as the pressure is above 0.006 atm (4.58 Torr) only liquid water and vapor exist and must follow the liquid-gas line.  If you know the pressure, then you know the boiling point. If you measure the boiling point, then you know the pressure on the water. This is demanded by the Gibb’s Phase Rule.

So, as you ascend a mountain, say K2, at the Apex (28,251 ft) the pressure is only about 226 Torr (0.3 atm). At this pressure your water must boil at a temperature of 154 °F. That’s not very hot water but it is boiling and can’t get any hotter at that pressure.

Let’s reverse this analogy and look at a pressure cooker. By placing the lid on the pot of water and sealing it, you effectively increase the pressure. As the pressure goes up, our phase diagram says the boiling point of water must go up. With a pressure cooker you can get water hotter than 212 °F.

These analogies to boiling water, which we have a good sense of, are easily translated into freeze drying. The only difference is that we are at different points on the phase diagram. When a material is placed under low pressure (below ~4.58 Torr) the water must freeze and we enter the part of the phase diagram below the triple point. Here we have two phases—ice and vapor—similar to above the triple point where we also had two phase in equilibrium—liquid and vapor.

Applying heat to the ice while maintaining low pressure causes the ice to sublimate and convert directly into vapor without first melting. This is freeze drying!  And it is a drying method that imparts special properties to many materials.

Freeze drying isn't the only way to dry. To the right is a table showing a comparison among different methods. In some cases your material may be better suited for drying by one of these other methods. Freeze drying isn't the cheapest method but always produces the highest quality dried material.

Apex Lyo can freeze dry a variety of materials. Find out if your material can benefit from freeze drying.
Contact us today.


Additional Information
Wikipedia - Freeze Drying
How Stuff Works - How Freeze Drying Works
Inventors - Freeze Drying & Freeze Dried Foods
Wise Geek - What Is Freeze Drying
 

 


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