Freeze drying is the removal of ice or other frozen solvents from a material through the process of sublimation and the removal of bound water molecules through the process of desorption.
Lyophilization and freeze drying are terms that are used interchangeably depending on the industry and location where the drying is taking place. Controlled freeze drying keeps the product temperature low enough during the process to avoid changes in the dried product appearance and characteristics. It is an excellent method for preserving a wide variety of heat-sensitive materials such as proteins, microbes, pharmaceuticals, tissues & plasma.
Sublimation is when a solid (ice) changes directly to a vapor without first going through a liquid (water) phase. Thoroughly understanding the concept of sublimation is a key building block to gaining knowledge of freeze drying.As shown below on the phase diagram for water, low pressures are required for sublimation to take place.Sublimation is a phase change and heat energy must be added to the frozen product for it to occur.
Sublimation in the freeze drying process can be described simply as:
- FREEZE – The product is completely frozen, usually in a vial, flask or tray.
- VACUUM – The product is then placed under a deep vacuum, well below the triple point of water.
- DRY – Heat energy is then added to the product causing the ice to sublime.
Sublimation in freeze-drying
One of the lesser-known qualities of water is that it doesn’t exist as a liquid in the presence of a vacuum. In a vacuum, ice “sublimates” directly from solid to vapor at room temperature without melting. This process is hugely beneficial in freeze-drying because it allows frozen foods to be dried directly by vaporizing all water without ever having the water melt. It also avoids a common problem with melting frozen fruits and vegetables — the breakdown of cell structure into a mushy product. Most people have defrosted a bowl of strawberries and been greeted with this. Freeze-drying skips the melting phase and removes frozen water from frozen food in gaseous form, leaving behind perfectly preserved food.
Mercer Foods makes this process of sublimation happen with our custom-built freeze-drying chambers, which create a vacuum environment that forces the phase change of water from ice to vapor, leaving a perfectly dried, perfectly intact fruit or vegetable piece behind. It takes a lot of skill and experience to perfect the process of freeze drying in a vacuum chamber, and requires everything to be just right to avoid burning or otherwise ruining the food. But the end result is worth the effort — perfectly dried fruits and vegetables, with excellent shape, color, flavor, vitamin retention and long shelf life, without the need for refrigeration.