Freeze-dried

[Coleman John]

Freezedrying, 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 many 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]

The Inca were freeze drying potatoes into chuo since 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]

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,[12] were transformed into hot meals in space by improving the process of rehydrating freeze-dried meals with water.[12] As technology and food processing improved, NASA looked for ways to provide a complete nutrient profile while reducing crumbs, disease-producing bacteria, and toxins.[13] The complete nutrient profile was improved with the addition of an algae-based vegetable-like oil to add polyunsaturated fatty acids.[13] Polyunsaturated fatty acids are beneficial in mental and vision development and, as they remain stable during space travel, can provide astronauts with added benefits.[13] The crumb problem was solved with the addition of a gelatin coating on the foods to lock in and prevent crumbs.[12] 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.[13] With the combination of these three innovations, NASA could provide safe and wholesome foods to their crews from freeze-dried meals.[13]

Military rations have also come a long way, from being served cured pork and corn meal to beefsteaks with mushroom gravy.[14] How rations are chosen and developed are based on acceptance, nutrition, wholesomeness, producibility, cost, and sanitation.[15] Additional requirements for rations include a minimum shelf life of three years, be deliverable by air, consumable in worldwide environments, and provide a complete nutritional profile.[15] The new T-rations have been improved upon by increasing acceptable items and provide high quality meals while in the field. Freeze-dried coffee was also incorporated by replacing spray-dried coffee in the meal, ready-to-eat category.[15]

Pretreatment includes any method of treating the product prior to freezing. This may include concentrating the product, formulation revision (i.e., addition of components to increase stability, preserve appearance, and/or improve processing), decreasing a high-vapor-pressure solvent, or increasing the surface area. Food pieces are often IQF treated to make them free flowing prior to freeze drying. Freeze dried pharmaceutical products are in most cases 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.

During the freezing stage, the material is cooled below its triple point, the temperature at which the solid, liquid, and gas phases of the material can coexist. This ensures that sublimation rather than melting will occur in the following steps. To facilitate faster and more efficient freeze drying, larger ice crystals are preferable. The large ice crystals form a network within the product which promotes faster removal of water vapor during sublimation.[2] To produce larger crystals, the product should be frozen slowly or can be cycled up and down in temperature in a process called 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.[17]

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.

During the primary drying phase, the pressure is lowered (to the range of a few millibars), and enough heat is supplied to the material for the ice to 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.

The secondary drying phase aims to remove unfrozen water molecules, since the ice was removed in the primary drying phase. This part of the freeze-drying process is governed by the material's adsorption isotherms. In this phase, the temperature is raised higher than in the primary drying phase, and can even be above 0 C (32 F), to break any physico-chemical interactions that have formed between the 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.

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.[2] Freeze-drying does not usually cause shrinkage or toughening of the material being dried. In addition, flavors, 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.

Pharmaceutical companies often use freeze-drying to increase the shelf life of the products, such as live virus vaccines,[18] biologics,[19] and other injectables. By removing the water from the material and sealing the material in a glass vial, the material can be easily stored, shipped, and later reconstituted to its original form for injection. Another example from the pharmaceutical industry is the use of freeze drying to produce tablets or wafers, the advantage of which is less excipient as well as a rapidly absorbed and easily administered dosage form.

Examples of lyophilized biological products include many vaccines such as live measles virus vaccine, typhoid vaccine, and meningococcal polysaccharide vaccine groups A and C combined. Other freeze-dried biological products include antihemophilic factor VIII, interferon alfa, anti-blood clot medicine streptokinase, and wasp venom allergenic extract.[20]

Many bio-pharmaceutical products based on therapeutic proteins such as monoclonal antibodies require lyophilization for stability. Examples of lyophilized biopharmaceuticals include blockbuster drugs such as etanercept (Enbrel by Amgen), infliximab (Remicade by Janssen Biotech), rituximab, and trastuzumab (Herceptin by Genentech).

Cell extracts that support cell-free biotechnology applications such as point-of-care diagnostics and biomanufacturing are also freeze-dried to improve stability under room temperature storage.[21][22]

The primary purpose of freeze drying within the food industry is to extend the shelf-life of the food while maintaining the quality.[1] Freeze-drying is known to result in the highest quality of foods of all drying techniques because structural integrity is maintained along with preservation of flavors.[1] Because freeze drying is expensive, it is used mainly with high-value products.[4] Examples of high-value freeze-dried products are seasonal fruits and vegetables because of their limited availability, coffee; and foods used for military rations, astronauts/cosmonauts, and/or hikers.[4]

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