Freeze drying

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Lyophilized
)

Freeze-dried ice cream

Freeze drying, also known as lyophilization or cryodesiccation, is a low temperature dehydration process[1] that involves freezing the product and lowering pressure, thereby removing the ice by sublimation.[2] This is in contrast to dehydration by most conventional methods that evaporate water using heat.[3]

Because of the low temperature used in processing,[1] the rehydrated product retains much of its original qualities. When solid objects like strawberries are freeze dried the original shape of the product is maintained.[4] If the product to be dried is a liquid, as often seen in pharmaceutical applications, the properties of the final product are optimized by the combination of excipients (i.e., inactive ingredients). Primary applications of freeze drying include biological (e.g., bacteria and yeasts), biomedical (e.g., surgical transplants), food processing (e.g., coffee), and preservation.[1]

History

The

Inca were freeze drying potatoes into chuño from the 13th century. The process involved multiple cycles of exposing potatoes to below freezing temperatures on mountain peaks in the Andes during the evening, and squeezing water out and drying them in the sunlight during the day.[5] The Inca people also used the unique climate of the Altiplano to freeze dry meat.[6]

Modern freeze drying began as early as 1890 by Richard Altmann who devised a method to freeze dry tissues (either plant or animal), but went virtually unnoticed until the 1930s.[7] In 1909, L. F. Shackell independently created the vacuum chamber by using an electrical pump.[8] No further freeze drying information was documented until Tival in 1927 and Elser in 1934 had patented freeze drying systems with improvements to freezing and condenser steps.[8]

A significant turning point for freeze drying occurred during World War II when blood plasma and penicillin were needed to treat the wounded in the field. Because of the lack of refrigerated transport, many serum supplies spoiled before reaching their recipients.[8] The freeze-drying process was developed as a commercial technique that enabled blood plasma and penicillin to be rendered chemically stable and viable without refrigeration.[8] In the 1950s–1960s, freeze drying began to be viewed as a multi-purpose tool for both pharmaceuticals and food processing.[8]

Early uses in food

Freeze-dried foods became a major component of astronaut and military rations. What began for astronaut crews as tubed meals and freeze-dried snacks that were difficult to rehydrate,[9] were transformed into hot meals in space by improving the process of rehydrating freeze-dried meals with water.[9] As technology and food processing improved, NASA looked for ways to provide a complete nutrient profile while reducing crumbs, disease-producing bacteria, and toxins.[10] The complete nutrient profile was improved with the addition of an algae-based vegetable-like oil to add polyunsaturated fatty acids.[10] Polyunsaturated fatty acids are beneficial in mental and vision development and, as they remain stable during space travel, can provide astronauts with added benefits.[10] The crumb problem was solved with the addition of a gelatin coating on the foods to lock in and prevent crumbs.[9] Disease-producing bacteria and toxins were reduced by quality control and the development of the Hazard Analysis and Critical Control Points (HACCP) plan, which is widely used today to evaluate food material before, during, and after processing.[10] With the combination of these three innovations, NASA could provide safe and wholesome foods to their crews from freeze-dried meals.[10]

meal, ready-to-eat category.[12]

Stages of freeze drying

In a typical phase diagram, the boundary between gas and liquid runs from the triple point to the critical point. Freeze-drying (blue arrow) brings the system around the triple point, avoiding the direct liquid–gas transition seen in ordinary drying (green arrow).

There are four stages in the complete freeze drying process: pretreatment, freezing, primary drying, and secondary drying.

Pretreatment

Pretreatment includes any method of treating the product prior to freezing. This may include concentrating the product,

parenterals
administered after reconstitution by injection which need to be sterile as well as free of impurity particles. Pre-treatment in these cases consists of solution preparation followed by a multi-step filtration. Afterwards the liquid is filled under sterile conditions into the final containers which in production scale freeze dryers are loaded automatically to the shelves.

In many instances the decision to pretreat a product is based on theoretical knowledge of freeze-drying and its requirements, or is demanded by cycle time or product quality considerations.[13]

Freezing and annealing

During the freezing stage, the material is cooled below its

annealing. The freezing phase is the most critical in the whole freeze-drying process, as the freezing method can impact the speed of reconstitution, duration of freeze-drying cycle, product stability, and appropriate crystallization.[14]

Amorphous materials do not have a eutectic point, but they do have a critical point, below which the product must be maintained to prevent melt-back[further explanation needed
] or collapse during primary and secondary drying.

Structurally sensitive goods

In the case of goods where preservation of structure is required, like food or objects with formerly-living cells, large ice crystals break the cell walls, resulting in increasingly poor texture and loss of nutritive content. In this case, rapidly freezing the material to below its

eutectic point avoids the formation of large ice crystals.[2]
Usually, the freezing temperatures are between −50 °C (−58 °F) and −80 °C (−112 °F).

Primary drying

During the primary drying phase, the pressure is lowered (to the range of a few

sublimate. The amount of heat necessary can be calculated using the sublimating molecules' latent heat of sublimation
. In this initial drying phase, about 95% of the water in the material is sublimated. This phase may be slow (can be several days in the industry), because, if too much heat is added, the material's structure could be altered.

In this phase, pressure is controlled through the application of partial vacuum. The vacuum speeds up the sublimation, making it useful as a deliberate drying process. Furthermore, a cold condenser chamber and/or condenser plates provide a surface(s) for the water vapor to re-liquify and solidify on.

It is important to note that, in this range of pressure, the heat is brought mainly by conduction or radiation; the convection effect is negligible, due to the low air density.

Secondary drying

A benchtop manifold freeze-drier

The secondary drying phase aims to remove unfrozen water

molecules and the frozen material. Usually the pressure is also lowered in this stage to encourage desorption (typically in the range of microbars, or fractions of a pascal
). However, there are products that benefit from increased pressure as well.

After the freeze-drying process is complete, the vacuum is usually broken with an inert gas, such as nitrogen, before the material is sealed.

At the end of the operation, the final residual water content in the product is extremely low, around 1–4%.

Applications of freeze drying