![Freeze-Dried-Ice-Cream](https://upload.wikimedia.org/wikipedia/commons/1/15/Freeze-Dried-Ice-Cream.jpg)
Freeze drying, also known as lyophilization or cryodesiccation, is a low temperature
dehydration
In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...
process
that involves freezing the product and lowering pressure, removing the ice by
sublimation.
This is in contrast to dehydration by most conventional methods that evaporate water using heat.
Because of the low temperature used in processing,
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.
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
excipient
An excipient is a substance formulated alongside the active ingredient of a medication, included for the purpose of long-term stabilization, bulking up solid formulations that contain potent active ingredients in small amounts (thus often referred ...
s (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.
History
The
Inca
The Inca Empire (also known as the Incan Empire and the Inka Empire), called ''Tawantinsuyu'' by its subjects, (Quechua for the "Realm of the Four Parts", "four parts together" ) was the largest empire in pre-Columbian America. The admin ...
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.
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. In 1909, L. F. Shackell independently created the vacuum chamber by using an electrical pump.
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.
A significant turning point for freeze drying occurred during
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
when
blood plasma
Blood plasma is a light amber-colored liquid component of blood in which blood cells are absent, but contains proteins and other constituents of whole blood in suspension. It makes up about 55% of the body's total blood volume. It is the intr ...
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.
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.
In the 1950s–1960s, freeze drying began to be viewed as a multi-purpose tool for both pharmaceuticals and food processing.
Early uses in food
Freeze-dried foods became a major component of
astronaut and
military
A military, also known collectively as armed forces, is a heavily armed, highly organized force primarily intended for warfare. It is typically authorized and maintained by a sovereign state, with its members identifiable by their distinct ...
rations. What began for astronaut crews as tubed meals and freeze-dried snacks that were difficult to rehydrate,
were transformed into hot meals in space by improving the process of rehydrating freeze-dried meals with water.
As technology and food processing improved,
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...
looked for ways to provide a complete nutrient profile while reducing crumbs, disease-producing bacteria, and toxins.
The complete nutrient profile was improved with the addition of an algae-based vegetable-like oil to add polyunsaturated fatty acids.
Polyunsaturated fatty acids
Polyunsaturated fatty acids (PUFAs) are fatty acids that contain more than one double bond in their backbone. This class includes many important compounds, such as essential fatty acids and those that give drying oils their characteristic proper ...
are beneficial in mental and vision development and, as they remain stable during space travel, can provide astronauts with added benefits.
The crumb problem was solved with the addition of a gelatin coating on the foods to lock in and prevent crumbs.
Disease-producing bacteria and toxins were reduced by quality control and the development of the
Hazard Analysis and Critical Control Points
Hazard analysis and critical control points, or HACCP (), is a systematic preventive approach to food safety from biological, chemical, and physical hazards in production processes that can cause the finished product to be unsafe and designs mea ...
(HACCP) plan, which is widely used today to evaluate food material before, during and after processing.
With the combination of these three innovations, NASA could provide safe and wholesome foods to their crews from freeze-dried meals.
Military rations
Military rations are food intended to feed military personnel. Types of military rations include garrison rations and field rations. They may be used where fresh meals are not available.
Australia
* Combat Ration One Man (CR1M)
Canada
* Indi ...
have also come a long way, from being served cured pork and corn meal to beefsteaks with mushroom gravy. How rations are chosen and developed are based on acceptance, nutrition, wholesomeness, producibility, cost, and sanitation.
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.
The new tray rations (
T Rations
T, or t, is the twentieth letter in the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is ''tee'' (pronounced ), plural ''tees''. It is deri ...
) which has 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
A Meal, Ready-to-Eat (MRE) is a self-contained, individual field ration in lightweight packaging purchased by the United States Department of Defense for its service members for use in combat or field conditions where other food is not availa ...
category.
Stages of freeze drying
![Drying](https://upload.wikimedia.org/wikipedia/commons/a/a6/Drying.svg)
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,
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
Individual Quick Freezing, usually abbreviated IQF, is a descriptive term for freezing methods used in the food processing industry. The food is in individual pieces, and is frozen quickly. Products commonly frozen with IQF technologies are typical ...
treated to make them free flowing prior to freeze drying. Freeze dried pharmaceutical products are in most cases
parenteral
A route of administration in pharmacology and toxicology is the way by which a drug, fluid, poison, or other substance is taken into the body.
Routes of administration are generally classified by the location at which the substance is applied. ...
s 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.
[J. Jeff Schwegman (2009]
"Basic Cycle Development Techniques for Lyophilized Products"
Freezing and annealing
During the freezing stage, the material is cooled below its
triple point
In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.. It is that temperature and pressure at which the sub ...
, 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.
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.
Amorphous materials do not have a
eutectic point
A eutectic system or eutectic mixture ( ) is a homogeneous mixture that has a melting point lower than those of the constituents. The lowest possible melting point over all of the mixing ratios of the constituents is called the ''eutectic tempe ...
, but they do have a
critical point, below which the product must be maintained to prevent melt-back 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 will break the cell walls which can result in increasingly poor texture and loss of nutritive content. In this case, the freezing is done rapidly, in order to lower the material to below its
eutectic point
A eutectic system or eutectic mixture ( ) is a homogeneous mixture that has a melting point lower than those of the constituents. The lowest possible melting point over all of the mixing ratios of the constituents is called the ''eutectic tempe ...
quickly, thus avoiding the formation of large ice crystals.
Usually, the freezing temperatures are between and .
Primary drying
During the primary drying phase, the pressure is lowered (to the range of a few
millibars
The bar is a metric unit of pressure, but not part of the International System of Units (SI). It is defined as exactly equal to 100,000 Pa (100 kPa), or slightly less than the current average atmospheric pressure on Earth at sea lev ...
), 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 vapour 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
![Benchtop freeze dryer](https://upload.wikimedia.org/wikipedia/commons/b/bc/Benchtop_freeze_dryer.JPG)
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 , 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.
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% to 4%.
Applications of freeze drying
Freeze-drying causes less damage to the substance than other
dehydration
In physiology, dehydration is a lack of total body water, with an accompanying disruption of metabolic processes. It occurs when free water loss exceeds free water intake, usually due to exercise, disease, or high environmental temperature. Mil ...
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
pharmaceutical
A medication (also called medicament, medicine, pharmaceutical drug, medicinal drug or simply drug) is a drug used to diagnose, cure, treat, or prevent disease. Drug therapy (pharmacotherapy) is an important part of the medical field an ...
s for many years.
Pharmaceuticals and biotechnology
Pharmaceutical companies often use freeze-drying to increase the shelf life of the products, such as live virus vaccines, biologics and other injectables. By removing the water from the material and sealing the material in a glass
vial
A vial (also known as a phial or flacon) is a small glass or plastic vessel or bottle, often used to store medication as liquids, powders or capsules. They can also be used as scientific sample vessels; for instance, in autosampler devices i ...
, 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
An excipient is a substance formulated alongside the active ingredient of a medication, included for the purpose of long-term stabilization, bulking up solid formulations that contain potent active ingredients in small amounts (thus often referred ...
as well as a rapidly absorbed and easily administered dosage form.
Freeze-dried pharmaceutical products are produced as lyophilized powders for reconstitution in vials and more recently in prefilled syringes for self-administration by a patient.
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
Factor VIII (FVIII) is an essential blood-clotting protein, also known as anti-hemophilic factor (AHF). In humans, factor VIII is encoded by the ''F8'' gene. Defects in this gene result in hemophilia A, a recessive X-linked coagulation disorder ...
,
interferon alfa
Interferon alfa (INN) or HuIFN-alpha-Le, trade name Multiferon, is a pharmaceutical drug composed of natural interferon alpha (IFN-α), obtained from the leukocyte fraction of human blood following induction with Sendai virus. Interferon alfa con ...
, anti-blood clot medicine
streptokinase
Streptokinase (SK) is a thrombolytic medication activating plasminogen by nonenzymatic mechanism. As a medication it is used to break down clots in some cases of myocardial infarction (heart attack), pulmonary embolism, and arterial thromboembol ...
, and wasp venom allergenic extract.
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
Etanercept, sold under the brand name Enbrel among others, is a biologic medical product that is used to treat autoimmune diseases by interfering with tumor necrosis factor (TNF), a soluble inflammatory cytokine, by acting as a TNF inhibitor. It ...
(
Enbrel
Etanercept, sold under the brand name Enbrel among others, is a biologic medical product that is used to treat autoimmune diseases by interfering with tumor necrosis factor (TNF), a soluble inflammatory cytokine, by acting as a TNF inhibitor. It ...
by
Amgen
Amgen Inc. (formerly Applied Molecular Genetics Inc.) is an American multinational biopharmaceutical company headquartered in Thousand Oaks, California. One of the world's largest independent biotechnology companies, Amgen was established in T ...
),
infliximab
Infliximab, a chimeric monoclonal antibody, sold under the brand name Remicade among others, is a medication used to treat a number of autoimmune diseases. This includes Crohn's disease, ulcerative colitis, rheumatoid arthritis, ankylosing spon ...
(Remicade by
Janssen Biotech
Janssen Pharmaceuticals is a pharmaceutical company headquartered in Beerse, Belgium, and wholly-owned by Johnson & Johnson. It was founded in 1953 by Paul Janssen.
In 1961, Janssen Pharmaceuticals was purchased by New Jersey-based American c ...
),
rituximab
Rituximab, sold under the brand name Rituxan among others, is a monoclonal antibody medication used to treat certain autoimmune diseases and types of cancer. It is used for non-Hodgkin lymphoma, chronic lymphocytic leukemia (in non-geriatric pa ...
, and
trastuzumab
Trastuzumab, sold under the brand name Herceptin among others, is a monoclonal antibody used to treat breast cancer and stomach cancer. It is specifically used for cancer that is HER2 receptor positive. It may be used by itself or together wi ...
(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.
Dry powders of probiotics are often produced by bulk freeze-drying of live microorganisms such as
lactic acid bacteria
Lactobacillales are an order of gram-positive, low-GC, acid-tolerant, generally nonsporulating, nonrespiring, either rod-shaped ( bacilli) or spherical ( cocci) bacteria that share common metabolic and physiological characteristics. These bact ...
and
bifidobacteria.
Freeze drying of food
![Bulgarian space food](https://upload.wikimedia.org/wikipedia/commons/4/45/Bulgarian_space_food.jpg)
The primary purpose of freeze drying within the food industry is to extend the shelf-life of the food while maintaining the quality.
Freeze-drying is known to result in the highest quality of foods amongst all drying techniques because structural integrity is maintained along with preservation of flavors.
Because freeze drying is expensive, it is used mainly with
high-value products.
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.
NASA and military rations
Because of its light weight per volume of reconstituted food, freeze-dried products are popular and convenient for
hikers
Hiking is a long, vigorous walk, usually on trails or footpaths in the countryside. Walking for pleasure developed in Europe during the eighteenth century.AMATO, JOSEPH A. "Mind over Foot: Romantic Walking and Rambling." In ''On Foot: A Hist ...
, as military rations, or astronaut meals.
A greater amount of dried food can be carried compared to the same weight of wet food. In replacement of wet food, freeze dried food can easily be rehydrated with water if desired and shelf-life of the dried product is longer than fresh/wet product making it ideal for long trips taken by hikers,
military personnel
Military personnel are members of the state's armed forces. Their roles, pay, and obligations differ according to their military branch (army, navy, marines, air force, space force, and coast guard), rank (officer, non-commissioned officer, or ...
, or
astronauts. The development of freeze drying increased meal and snack variety to include items like
shrimp cocktail
Prawn cocktail, also known as shrimp cocktail, is a seafood dish consisting of shelled, cooked prawns in a Marie Rose sauce or cocktail sauce, served in a glass. It was the most popular hors d'œuvre in Great Britain, as well as in the United ...
, chicken and vegetables,
butterscotch pudding, and
apple sauce
Apple sauce or applesauce is a purée (not necessarily served as a true sauce) made of apples. It can be made with peeled or unpeeled apples and may be spiced or sweetened. Apple sauce is inexpensive and is widely consumed in North America and s ...
.
Coffee
Coffee contains flavor and aroma qualities that are created due to the
Maillard reaction
The Maillard reaction ( ; ) is a chemical reaction between Amino acid, amino acids and Reducing sugar, reducing sugars that gives browned food its distinctive flavor. Searing, Seared steaks, fried dumplings, cookies and other kinds of biscuits, b ...
during roasting
and can be preserved with freeze-drying.
Compared to other drying methods like room temperature drying, hot-air drying, and solar drying,
Robusta coffee
''Coffea canephora'' ( syn. ''Coffea robusta'', commonly known as ''robusta coffee'') is a species of coffee that has its origins in central and western sub-Saharan Africa. It is a species of flowering plant in the family Rubiaceae. Though wid ...
beans that were freeze-dried contained higher amounts of essential amino acids like leucine, lysine, and phenylalanine.
Also, few non-essential amino acids that significantly contributed to taste were preserved.
Fruits
With conventional dehydration, berries can degrade in quality as their structure is very delicate and contains high levels of moisture. Strawberries were found to have the highest quality when freeze dried; retaining color, flavour, and ability to be re-hydrated.
Insects
Freeze-drying is used extensively to preserve insects for the purposes of consumption. Whole freeze-dried insects are sold as
exotic pet
An exotic pet is a pet which is relatively rare or unusual to keep, or is generally thought of as a wild species rather than as a domesticated pet. The definition varies by culture, location, and over time—as animals become firmly enough est ...
food, bird feed, fish bait, and increasingly for
human consumption.
Powdered freeze-dried insects are used as a
protein base in animal feeds, and in some markets, as a
nutritional supplement
A dietary supplement is a manufactured product intended to supplement one's diet by taking a pill, capsule, tablet, powder, or liquid. A supplement can provide nutrients either extracted from food sources or that are synthetic in order ...
for human use.
Farmed insects are generally used for all of the aforementioned purposes versus harvesting wild insects, except in the case of grasshoppers which are often harvested out of field crops.
Technological industry
In chemical synthesis
As a topic of chemistry, chemical synthesis (or combination) is the artificial execution of chemical reactions to obtain one or several products. This occurs by physical and chemical manipulations usually involving one or more reactions. In mod ...
, products are often freeze-dried to make them more stable, or easier to dissolve in water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
for subsequent use.
In bioseparations, freeze-drying can be used also as a late-stage purification procedure, because it can effectively remove solvents. Furthermore, it is capable of concentrating substances with low molecular weights that are too small to be removed by a filtration membrane. Freeze-drying is a relatively expensive process. The equipment is about three times as expensive as the equipment used for other separation processes, and the high energy demands lead to high energy costs. Furthermore, freeze-drying also has a long process time, because the addition of too much heat to the material can cause melting or structural deformations. Therefore, freeze-drying is often reserved for materials that are heat-sensitive, such as protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...
s, enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...
s, microorganism
A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s, and blood plasma
Blood plasma is a light amber-colored liquid component of blood in which blood cells are absent, but contains proteins and other constituents of whole blood in suspension. It makes up about 55% of the body's total blood volume. It is the intr ...
. The low operating temperature of the process leads to minimal damage of these heat-sensitive products.
In nanotechnology, freeze-drying is used for nanotube purification to avoid aggregation due to capillary forces during regular thermal vaporization drying.
Taxidermy
Freeze-drying is among the methods used to preserve animals in the field of taxidermy
Taxidermy is the art of preserving an animal's body via mounting (over an armature) or stuffing, for the purpose of display or study. Animals are often, but not always, portrayed in a lifelike state. The word ''taxidermy'' describes the proc ...
. When animals are preserved in this manner they are called "freeze-dried taxidermy" or " freeze-dried mounts". Freeze-drying is commonly used to preserve crustacean
Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...
s, fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...
, amphibians, reptiles, insect
Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...
s, and smaller mammals. Freeze-drying is also used as a means to memorialize pets after death. Rather than opting for a traditional skin mount when choosing to preserve their pet via taxidermy, many owners opt for freeze-drying because it is less invasive upon the pet's body.
Other uses
Organizations such as the Document Conservation Laboratory at the United States National Archives and Records Administration (NARA) have done studies on freeze-drying as a recovery method of water-damaged books and documents. While recovery is possible, restoration quality depends on the material of the documents. If a document is made of a variety of materials, which have different absorption properties, expansion will occur at a non-uniform rate, which could lead to deformations. Water can also cause mold to grow or make inks bleed. In these cases, freeze-drying may not be an effective restoration method.
In bacteriology Bacteriology is the branch and specialty of biology that studies the morphology, ecology, genetics and biochemistry of bacteria as well as many other aspects related to them. This subdivision of microbiology involves the identification, classificat ...
freeze-drying is used to conserve special strains.
Advanced ceramic
A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...
s processes sometimes use freeze-drying to create a formable powder from a sprayed slurry mist. Freeze-drying creates softer particles with a more homogeneous chemical composition than traditional hot spray drying
Spray drying is a method of changing a dry powder from a liquid or slurry by rapidly drying with a hot gas. This is the preferred method of drying of many thermally-sensitive materials such as foods and pharmaceuticals, or materials which may requ ...
, but it is also more expensive.
A new form of burial which previously freeze-dries the body with liquid nitrogen has been developed by the Swedish company Promessa Organic AB, which puts it forward as an environmentally friendly alternative to traditional casket and cremation
Cremation is a method of final disposition of a dead body through burning.
Cremation may serve as a funeral or post-funeral rite and as an alternative to burial. In some countries, including India and Nepal, cremation on an open-air pyre is ...
burials.
Advantages
Freeze-drying is viewed as the optimal method of choice for dehydration of food because of the preservation of quality, meaning characteristics of the food product such as aroma, rehydration, and bioactivity, are noticeably higher compared to foods dried using other techniques.
Shelf-life extension
Shelf-life extension results from low processing temperatures in conjunction with rapid transition of water through sublimation. With these processing conditions, deterioration reactions, including nonenzymic browning, enzymatic browning, and protein denaturation, are minimized. When the product is successfully dried, packaged properly, and placed in ideal storage conditions the foods have a shelf life of greater than 12 months.
Re-hydration
If a dried product cannot be easily or fully re-hydrated, it is considered to be of lower quality. Because the final freeze dried product is porous, complete re-hydration can occur in the food. This signifies greater quality of the product and makes it ideal for ready-to-eat instant meals.
Effect on nutrients and sensory quality
Due to the low processing temperatures and the minimization of deterioration reactions, nutrients are retained and color is maintained. Freeze-dried fruit maintains its original shape and has a characteristic soft crispy texture.
Disadvantages
Microbial growth
Since the main method of microbial decontamination for freeze drying is the low temperature dehydration process, spoilage organisms and pathogens resistant to these conditions can remain in the product. Although microbial growth is inhibited by the low moisture conditions, it can still survive in the food product. An example of this is a viral hepatitis A
Hepatitis A is an infectious disease of the liver caused by ''Hepatovirus A'' (HAV); it is a type of viral hepatitis. Many cases have few or no symptoms, especially in the young. The time between infection and symptoms, in those who develop them ...
outbreak that occurred in the United States in 2016, associated with frozen strawberries. If the product is not properly packaged and/or stored, the product can absorb moisture, allowing the once inhibited pathogens to begin reproducing as well.
Cost
Freeze-drying costs about five times as much as conventional drying, so it is most suitable for products which increase in value with processing. Costs are also variable depending on the product, the packaging material, processing capacity, etc. The most energy-intensive step is sublimation.
Silicone oil leakage
Silicone oil is the common fluid that is used to heat or cool shelves in the freeze-dryer. The continuous heat/cool cycle can lead to a leakage of silicone oil at weak areas that connect the shelf and hose. This can contaminate the product leading to major losses of pharmaceuticals and food products. Hence, to avoid this issue, mass spectrometer
Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is us ...
s are used to identify vapors released by silicone oil to immediately take corrective action and prevent contamination of the product.
Products
Mammalian cells generally don't survive freeze drying even though they still can be preserved.
Equipment and types of freeze dryers
There are many types of freeze-dryers available, however, they usually contain a few essential components. These are a vacuum chamber, shelves, process condenser, shelf-fluid system, refrigeration system, vacuum system, and control system.
Function of essential components
Chamber
The chamber is highly polished and contains insulation, internally. It is manufactured with stainless steel and contains multiple shelves for holding the product. A hydraulic or electric motor is in place to ensure the door is vacuum-tight when closed.
Process condenser
The process condenser consists of refrigerated coils or plates that can be external or internal to the chamber. During the drying process, the condenser traps water. For increased efficiency, the condenser temperature should be 20 °C (36°F) less than the product during primary drying and have a defrosting mechanism to ensure that the maximum amount of water vapor in the air is condensed.
Shelf fluid
The amount of heat energy needed at times of the primary and secondary drying phase is regulated by an external heat exchanger. Usually, silicone oil is circulated around the system with a pump.
Refrigeration system
This system works to cool shelves and the process condenser by using compressors or liquid nitrogen, which will supply energy necessary for the product to freeze.
Vacuum system
During the drying process, a vacuum of 50-100 microbar is applied, by the vacuum system, to remove the solvent. A two-stage rotary vacuum pump is used, however, if the chamber is large then multiple pumps are needed. This system compresses non-condensable gases through the condenser.
Control system
Finally, the control system sets up controlled values for shelf temperature, pressure and time that are dependent on the product and/or the process. The freeze-dryer can run for a few hours or days depending on the product.
Contact freeze dryers
Contact freeze dryers use contact (conduction) of the food with the heating element to supply the sublimation energy. This type of freeze dryer is a basic model that is simple to set up for sample analysis. One of the major ways contact freeze dryers heat is with shelf-like platforms contacting the samples. The shelves play a major role as they behave like heat exchangers at different times of the freeze-drying process. They are connected to a silicone oil system that will remove heat energy during freezing and provide energy during drying times.
Additionally, the shelf-fluid system works to provide specific temperatures to the shelves during drying by pumping a fluid (usually silicone oil) at low pressure. The downside to this type of freeze dryer is that the heat is only transferred from the heating element to the side of the sample immediately touching the heater. This problem can be minimized by maximizing the surface area of the sample touching the heating element by using a ribbed tray, slightly compressing the sample between two solid heated plates above and below, or compressing with a heated mesh from above and below.
Radiant freeze dryers
Radiant freeze dryers use infrared radiation to heat the sample in the tray. This type of heating allows for simple flat trays to be used as an infrared source can be located above the flat trays to radiate downwards onto the product. Infrared radiation heating allows for a very uniform heating of the surface of the product, but has very little capacity for penetration so it is used mostly with very shallow trays and homogeneous sample matrices.
Microwave-assisted freeze dryers
Microwave-assisted freeze dryers utilize microwaves to allow for deeper penetration into the sample to expedite the sublimation and heating processes in freeze-drying. This method can be very complicated to set up and run as the microwaves can create an electrical field capable of causing gases in the sample chamber to become plasma. This plasma could potentially burn the sample, so maintaining a microwave strength appropriate for the vacuum levels is imperative. The rate of sublimation in a product can affect the microwave impedance, in which power of the microwave must be changed accordingly.
See also
* Chuño, ancient Incan freeze dried potatoes
* Cryofixation Cryofixation is a technique for fixation or stabilisation of biological materials as the first step in specimen preparation for electron microscopy and cryo-electron microscopy. Typical specimens for cryofixation include small samples of plant or an ...
* Frozen mummies
* Freeze-dried food and NASA
* List of dried foods
This is a list of dried foods. Food drying is a method of food preservation that works by removing water from the food, which inhibits the growth of bacteria and has been practiced worldwide since ancient times to preserve food. Where or when d ...
* Supercritical drying
Supercritical drying, also known as critical point drying, is a process to remove liquid in a precise and controlled way. It is useful in the production of microelectromechanical systems (MEMS), the drying of spices, the production of aerogel, t ...
References
External links
FDA Guide to Inspection of Lyophilization of Parenterals
Fruit proccesing
{{DEFAULTSORT:Freeze Drying
Dried foods
Food preservation
Preservation methods
Drying processes
NASA spin-off technologies