Freeze-Drying 101

History

Freeze drying, scientifically known as lyophilization, has its origins as early as 1890 when Richard Altman discovered how to freeze-dry tissue. Due to technological restrictions however, the method was not often used and was difficult to replicate. During World War II, there was a shortage of blood and medical supplies in Europe. This imposed the difficult challenge of transportation of biological cargo, as it could not be transported easily without spoiling. Earl W. Flosdorf discovered that freeze-drying blood serum kept it chemically stable without the need to refrigerate, making blood easily transportable from the United States to Europe. It was his applications that lead to freeze drying innovations in fields such as food, pharmaceuticals, and other science fields such as carbon dating and the preservation and restoration of historical documents.

Realizing the power of this technology, the founder of Eclectic Institute, Ed Alstat, set out to apply freeze-drying to a whole new industry – herbs and medicinal plants.  This had not previously been tried on a commercial scale, yet the result was a great success.  Freeze-drying yielded potent, shelf-stable medicines with all the active components of fresh plants intact.  Subsequent analytical testing confirmed the superiority of freeze-dried herbs versus their air-dried counterparts.  By the mid 1980s, Eclectic Institute had ushered in a new era of fresh, freeze-dried products that captured the benefits of fresh herbs that were easily available and transportable with long shelf-lives.  Today it is not uncommon to find freeze-dried fruits and vegetables like strawberries, mangos and kale on stores shelves, yet Eclectic remains one of the few producers of medicinal herbs in freeze-dried form.

Process

Freeze dryers are machines that consist of two main components, a vacuum pump and a condenser. Temperature control may also be incorporated into the drying shelves.  Each component plays a role in each stage of freeze drying. There are three main stages in freeze-drying: freezing, primary drying and a secondary drying.

During the freezing stage, fresh material may be placed in trays and loaded onto the cooling racks of the freeze-dryer. Alternatively, the material may be pre-frozen in a conventional freezer.  It is crucial that the temperature remain very low and the material doesn’t begin to thaw, this will ensure that sublimation will occur during the drying phase instead of melting. The prevention of liquid-water formation is critical to the drying process and distinguishes freeze-drying from other methods. The wondrous, life-producing qualities of liquid-water are essential for chemical reactions that take place in the normal aging process. If there is no liquid-water, the degradative reactions cannot take place or are greatly reduced.


During the primary drying stage, vacuum is introduced to the vessel which lowers the pressure to below the triple point of water. The cold temperatures and low pressures inside the freeze-dryer result in removal of water by sublimation, the transition of a solid directly to a gas without going through a liquid phase. This is the gentlest way to remove water from the product. The freeze-dryer shelves may provide gentle heat to the product to accelerate the drying process.

Secondary drying is not always performed, however to ensure the driest, best preserved product, it is an essential step. During primary drying it is not always possible to remove every water molecule; some water remains physically bound to the material’s surface. In order to release this bound water, the temperature is raised for a short time to about 40 C. The result is a final product with a water content of 1-4%.  


Properties

Freeze-drying can preserve the product for up to 25 years, remaining fresh as the day it was dried [1]. Along with the preservation time, it also retains the nutritional value, flavor and texture [2]. In many cases, the simple addition of water will transform the freeze-dried product back to its original, fresh form.

Freeze-drying is by far the best way to dry and preserve, essentially remaining fresh and natural, the same as it was before drying. What distinguishes freeze-drying from traditional drying is the quality, benefits and properties displayed by the finished product, obtainable only through freeze-drying. The difference is heat from traditional methods degrades essential and structural proteins, whereas freezing gives the product rigidity and minimizes degradative reactions [3]. This is why freeze-drying has become such an important method in many industries and will continue to grow.

 Fresh hops (left) and freeze-dried hops (right).

Fresh hops (left) and freeze-dried hops (right).

References

  1. How It Works. 2015. https://harvestright.com/how-it-works/
  2. Miclaus, A. “Freeze-Drying Kinetics for Different Types of Food Products.” Studia UBB Chemia. 2015. 1:147–156.
  3. Liapis, I. A, Bruttini, R. “Freeze-Drying.” Handbook of Industrial Drying, Fourth Edition. July, 2014. 259 -282.