Freeze drying removes water or solvents from a frozen product. The process of prefreezing, primary drying, and secondary drying is all done with freeze dryers.
Typically used in the pharmaceutical industry, freeze drying helps to stabilize microbial cultures, preserve animal specimens, and concentrate and recover reaction products. Other applications include freeze drying fish samples and fungi as well as for monitoring soil samples and anti-cancer drugs.
Once freeze dried properly, products do not need refrigeration and items can be stored at ambient temperatures.
The Components of a Freeze Drying System
The main components of freeze drying equipment are:
- Refrigeration System
- Vacuum System
- Control System
- Product Chamber or Manifold
The refrigeration system cools the (ice) condenser located inside the freeze dryer. The refrigeration system can also be employed to cool shelves in the product chamber for the freezing of the product.
The vacuum system consists of a separate vacuum pump connected to an airtight condenser and attached product chamber.
Control systems vary in complexity and usually include temperature and pressure sensing ability. Advanced controllers will allow the programming of a complete “recipe” for freeze drying and will include options to monitor how the freeze drying process is progressing. Choosing a control system for the freeze dryer depends on the application and use (i.e. lab vs. production).
Product chambers are typically either a manifold with attached flasks, or, a larger chamber with a system of shelves on which to place the product.
The purpose of the condenser is to attract the vapors being sublimed off of the product. Because the condenser is maintained at a lower energy level relative to the product ice, the vapors condense and turn back into solid form (ice) in the condenser. The sublimated ice accumulates in the condenser and is manually removed at the end of the freeze drying cycle (defrost step). The condenser temperature required is dictated by the freezing point and collapse temperature of the product. The refrigeration system must be able to maintain the temperature of the condenser substantially below the temperature of the product.
In shelf freeze dryers, the condenser can be located inside the product chamber (internal condenser) or in a separate chamber (external condenser) connected to the product chamber by a vapor port.
Manifold freeze dryers rely on ambient conditions to provide the heat of sublimation to the product. This heat input does not melt the product because an equivalent amount of heat is removed by vaporization of the solvent. Advanced shelf freeze dryers can provide a heat source to control/expedite the drying process and they can also employ the refrigeration system to allow freezing of product inside the unit.
Types of Freeze dryers
Freeze dryers can be informally classified by the type of product chamber: (1) Manifold dryers where the product is typically pre-frozen & in flasks (2) Shelf dryers where the product is placed in a tray or directly on a shelf (3) Combination units with both drying options.
Freeze-dryers can also be grouped by size & use: (1) laboratory bench-top units for R&D (2) pilot units for process development and scale-up, and (3) larger production-sized units. It should be noted that in addition to process scale-up work, pilot-sized freeze dryers are often used for product R&D as well as small volume production applications.
Advantages of freeze drying
Freeze-drying causes less damage to the substance than other dehydration methods using higher temperatures. Nutrient factors that are sensitive to heat are lost less in the process as compared to the processes incorporating heat treatment for drying purposes.Freeze-drying does not usually cause shrinkage or toughening of the material being dried. In addition, flavours, smells, and nutritional content generally remain unchanged, making the process popular for preserving food. However, water is not the only chemical capable of sublimation, and the loss of other volatile compounds such as acetic acid (vinegar) and alcohols can yield undesirable results.
Freeze-dried products can be rehydrated (reconstituted) much more quickly and easily because the process leaves microscopic pores. The pores are created by the ice crystals that sublimate, leaving gaps or pores in their place. This is especially important when it comes to pharmaceutical uses. Freeze-drying can also be used to increase the shelf life of some pharmaceuticals for many years.