User:Chronley/sandbox

Introduction
Freeze drying, also known as lyophilisation, is a low temperature dehydration process which involves freezing the product, lowering pressure, then removing the ice by sublimation. This is in contrast to dehydration by most conventional methods that evaporate water using heat.

Freeze drying results in a high quality product because of the low temperature used in processing. The original shape of the product is maintained and quality of the rehydrated product is excellent. Primary applications of freeze drying include biological (e.g. bacteria and yeasts), biomedical (e.g. surgical transplants), and food processing (e.g. coffee).

History
Freeze drying began in 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, Shackell independently created the vacuum chamber by using an electrical pump. However, no further data on freeze drying 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 because blood plasma and penicillin were needed to treat the wounded within the field because of the lack of simultaneous refrigeration and transport, many serum supplies were spoiling before reaching their intended 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 having to be refrigerated. In the 1950s-60s, freeze drying began to be viewed for its multi-purpose application to both pharmaceuticals and food processing.

Early uses in food
Freeze-drying products became a major commodity for astronauts and military food rations. What began for astronaut crews as tubed meals and freeze-dried snacks that were difficult to rehydrate, they are now able to enjoy warm hot meals while in space by improving the easability fo rehydrating free-dried meals with water. As technology and food processing improved NASA looked for ways to provide a providing a complete nutrient profile, reduce crumbs and 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 are beneficial in mental and vision development, and as it remains stable after space travel can provide astronauts with its 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 Critical Control Point (HACCP) plan, which is widely used today to evaluate food material before, during and after processing. With the combination of these 3 things, NASA could provide safe and wholesome foods to their crews while in space in a freeze-dried meal source.

Military rations have also come a long way from being served spoiled pork and corn meal to beefsteak with mushroom gravy. How rations are chosen and developed are based on acceptance, nutrition, wholesomeness, producibility, cost and sanitation. Also, additional requirements which the rations must have include a minimum shelf life of 3 years, be deliverable by air, consumable in worldwide environments, and provide a complete nutritional profile. The new tray rations (T Ration) 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 spay-dried coffee within the meal, ready-to-eat category.

Stages of freeze drying
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 treated to make it free flowing prior to freeze drying. 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.

Freezing
During the freezing stage, it is important to cool the material below its triple point, the lowest 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 forms 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, 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 quickly, thus avoiding the formation of ice crystals. Usually, the freezing temperatures are between −50 °C and −80 °C (-58 °F and -112 °F).

Primary 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 sublime. 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-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
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, 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 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.

Freeze drying of food
The primary purpose of freeze drying within the food industry 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 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, as military rations, or astronaut meals.[1] A greater amount of dried food can be carried compared the same weight of wet food. In replacement of wet food, freeze dried food can be 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 personal or astronauts. The development of freeze drying increased meal and snack variety to include items like shrimp cocktail, chicken and vegetables, butterscotch pudding, and apple sauce.

Coffee
Coffee contains flavor and aroma qualities that are created due to Maillard reaction 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 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. This is of crucial importance because amino acids, fats and protein notably contribute to the development of flavor and aroma profiles in coffee.

Fruits
Freeze drying can be beneficial for fruit as it can make them available year-round and can preserve their taste and color, making them more more similar to the fresh fruit than other dehydration methods. 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, flavor and ability to be re-hydrated Additionally freeze-dried fruit can be powdered to make it more soluble and act as bases for food products, as well as being a natural source of antioxidants and coloring.

Tofu
Freeze dried tofu is a popular foodstuff in Japan ("Koya-dofu" or "shimi-dofu" in Japanese). The Japanese first used frozen tofu, but then needed something that would last longer from winter to the next spring. Developed on Mt. Koya (hence the derivation of the name) by a monk, freeze dried tofu was produced outside in the mountains taking frozen tofu then rehydrated and pressed to expel the ice and dried in shed to obtain the final product. Today, freeze dried tofu can be comparable to other tofu products and is often found in frozen or instant soups because when it's rehydrated its ability absorb additional flavors makes it appealing.

Advantages
Freeze-drying is viewed as the optimal method of choice for dehydration because of the preservation of quality, meaning characteristics of the food product such aroma, rehydration, bioactivity, are noticeably higher compared to foods dried from other techniques.

Shelf-life extension
Shelf-life extension is a result from low processing temperatures in conjunction with rapid transition of water through sublimation. With these processing conditions, deterioration reactions, including nonenzymatic browning, enzymatic browning, 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.

Rehydration
If a dried product can not be easily or fully rehydrated, it is considered to be of lower quality. Because the final freeze dried product is porous, complete rehydration can occur in the food. This signifies greater quality of the product and makes them 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 it original shape and has a characteristic soft, crispy texture.

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 in inhibited by the low moisture conditions, it can still survive in the food product. An example of this is a hepatitis A 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. In order to avoid this, adequate steps before freeze-drying (e.g. washing with disinfectants) should be taken.

Cost
The cost for freeze-drying are about 5 times more than conventional drying methods, making it not an ideal choice if the value of the product is not increased after processing. However, costs are also variable depending on the product, the packaging material, processing capacity, etc. Also, the cost in terms of energy is highest to perform the sublimation process in comparison to the freezing, vacuum and condensation steps.

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 in the silicone oil at weak areas that connect the shelf and hose. This can contaminate the product leading to major losses of food product. Hence, to avoid this issue, mass spectrometers are used to identify vapors released by silicone oil to immediately take corrective action and prevent contamination of the product.

Equipment/Types of Freeze-Dryers
There are many types of freeze-dryers available, however, they usually contain a few essential constituents. These are a vacuum chamber, shelves, process condenser, shelf-fluid system, refrigeration system, vacuum system and control system.

Function of 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 that comprises 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 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.

Rotary dryers are another form of contact freeze dryers but instead of a tray, rotary dryers have a cylindrical reservoir that is rotated during drying to achieve a more uniform drying throughout the substance.These are commonly used for drying pellets, cubes and other pourable substances.

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 setup 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.

Article Evaluation
Choose an article on Wikipedia related to your course to read and evaluate. As you read, consider the following questions (but don't feel limited to these):

Pascalization (HPP)
 * Is everything in the article relevant to the article topic? Is there anything that distracted you? Yes, everything is pertinent to HPP but the constant switching from pascalization to HPP to HHP can be confusing for readers.
 * Is the article neutral? Are there any claims, or frames, that appear heavily biased toward a particular position? It is fairly neutral but it does include a Criticism section where it details arguments against HPP as it may not totally disinfect. So that could be somewhat biased as it highlights the criticism of the process. Claims there is 'practically no debate' on microbial survival at high pressures.
 * Are there viewpoints that are overrepresented, or underrepresented? Criticism was slightly overrepresented, and the one sentence about consumer acceptance is underrrepresented
 * Check a few citations. Do the links work? Does the source support the claims in the article? Links mostly seemed to work, some of the books are inaccessible though, so hard to check the page number that was cited. Note 30 was a dead link however.
 * Is each fact referenced with an appropriate, reliable reference? Where does the information come from? Are these neutral sources? If biased, is that bias noted? Some facts were simply directed to a company website to demonstrate the practical applications. This is not reliable as companies can change and their products can change.
 * Is any information out of date? Is anything missing that could be added? Some information is from the 2nd edition of Food Processing Technology when the 4th edition is already out., there are also dead links as mentioned earlier. Latest book source was more than 10 years ago.
 * Check out the Talk page of the article. What kinds of conversations, if any, are going on behind the scenes about how to represent this topic? Only conversations about possible mergers and current industry produced foods in the UK.
 * How is the article rated? Is it a part of any WikiProjects? Start-Class for both Microbiology and Food and Drink Wikiprojects
 * How does the way Wikipedia discusses this topic differ from the way we've talked about it in class? Very detailed in the history of HPP, only a few short paragraphs on the theory with no variations on machinery of applications. Focus on the criticism as to why it may be somewhat unused currently.
 * Bibliography and notes might need to merged, very confusing to navigate.

Citation Addition
modified atmosphere

The atmosphere in an MA package consists mainly of adjusted amounts of N2, O2, and CO2......

Possible Editing Choices
Pascalization

- Could use more detail on process, going over differences in processing for HPP, HHP, and other ways it can be done. Also references and biblio need to be merged

-Could use some pictures/diagrams

Modified atmosphere

- No real citations in bulk of writing, just generalize sources. Many sections are redundant and it is not organized appropriately.

-Might be good to include more on mechanisms and diagrams

Vacuum cooling

-no inline citations

-Can go over limitations or add in more detail on advantages

-Detail the science and mechanism a little more

snap freezing

-No citations

- can go into further details on how the snap freezing is achieved with machinery

-Can clarify difference between scientific and culinary use

Mid importance Food and Drink

Flambé  Food extrusion  Industrial fermentation Sous-vide   Glaze (cooking technique)  parboiling  Blanching (cooking)

Group Choices for Article

 * 1) Aseptic processing
 * 2) * Start-Class, High-importance article
 * 3) * Doesn't go into process flow
 * 4) * Mostly talks about history
 * 5) * Can include comparison to conventional retort
 * 6) Freeze Drying
 * 7) * The article is rated as C-class and has High Importance
 * 8) ** Additional citations could be added throughout the majority of the article, especially within the "stages" and "applications" sections of the article. The sources appear to be neutral and mostly from journal articles
 * 9) ** There are some instances which references are outdated and could be updated (e.g. reference 3 and 13). Additional food applications and processing steps could be elaborated further


 * 1) Food extrusion
 * 2) * B-Class, Mid-importance article
 * 3) * Can talk about extrusion cooking
 * 4) * Can more in depth in types of extrusion and the machinery used.
 * 5) * Can talk about wet/dry and other advantages/disdvantages
 * 6) * Needs citations
 * 7) Food drying
 * 8) * Start-Class, High-importance article
 * 9) * Can mention different industrial methods
 * 10) * Can also mention different equipment used

Outline Of Additions to Freeze Drying-Good outline!

 * 1) Introduction
 * 2) History
 * 3) Stages
 * 4) Pretreatment
 * 5) sometimes Annealing is necessary
 * 6) Freezing
 * 7) Primary Drying
 * 8) Secondary Drying
 * 9) Silicone Oil Contamination Challenge maybe include this into one of the equipment sections
 * 10) Properties of Freeze-Dried Products
 * 11) Protectants
 * 12) Applications
 * 13) Pharmaceutical and Biotechnology
 * 14) Food and Agriculture-Based Industry
 * 15) * Primary application is to preserve aroma or textures in high-valued products (Fellows 2017)
 * 16) ** eg: coffee, mushrooms, herbs and spices
 * 17) * Also used for complete meal packages for hikers/campers, military rations, or space flights (Fellows 2017)
 * 18) * Can be used to prepare active cultures or enzymes for long term storage (Fellows 2017)
 * 19) * add Effect on Emulsifying systems ?
 * 20) * Process for Solid Food Products (Fellows 2017)
 * 21) ** Stage 1: Rapidly freeze small food particles for small ice crystal formation (damage to cell structure is reduced)
 * 22) ** Stage 2: Reduce pressure around food to below 610Pa and slowly apply heat until water vapor is produced and is released by the food into the drying chamber and removed by condensing on refrigeration coils
 * 23) * Process for Liquid Food Products (Fellows 2017)
 * 24) ** Difference between solid food products is slow freezing to create a network of large ice crystals which allow for faster removal of water vapor than solid products
 * 25) *** To prevent formation of glassy state when freezing because it prevents the release of water vapor, liquid products are either frozen as foam or if a juice with pulp, it is dried with the pulp
 * 26) Technological Industry
 * 27) Other Uses
 * 28) ADD: Advantages/Disadvantages Section
 * 29) Advantages
 * 30) * Reduced damage to nutrients susceptible to heat and sensory characteristics (Fellows 2017)
 * 31) * Is able to rehydrate quickly (Fellows 2017)
 * 32) Disadvantages
 * 33) * Costly (Fellows 2017)
 * 34) * Does not kill all spoilage pathogens and microorganisms (Fellows 2009)
 * 35) Equipment
 * 36) Important components of a freeze-dryer (GR. Nireesha and others 2013)
 * 37) * Process condenser
 * 38) * Shelf fluid system
 * 39) * Refrigeration system
 * 40) * Vacuum system
 * 41) * Control system
 * 42) In Popular Culture
 * 43) * Remove? probably not real point
 * 44) See Also:
 * 45) References
 * 46) ADD: Fellows, P.J. (2017). Food Processing and Technology: Principles and Practices. Part IV.23: freeze drying and freeze concentration. Woodhead. pp: 26488 - 26946
 * 47) ADD: GR. Nireesha and others (2013). Lyophilization/Freeze-Drying - An Review. INTERNATIONAL JOURNAL OF NOVEL TRENDS IN PHARMACEUTICAL SCIENCES. 3(4)
 * 48) could add https://doi.org/10.1016/j.tifs.2014.10.008 freeze spray drying
 * 49) https://doi.org/10.1016/j.profoo.2011.09.097 microstructure changes in foods

Intro
''Freeze-drying—technically known as lyophilisation, lyophilization, or cryodesiccation—is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport. Freeze-drying works by freezing the material and then reducing the surrounding pressure while precisely heating to allow the frozen water in the material to sublime directly from the solid phase to the gas phase.''

Food and Agriculture-Based Industry
(edited draft is incorporated with current published material on wiki page which is italicized)

The primary application of freeze-drying within the food industry is to preserve the aroma or textures of high-valued products. High-value products can be viewed based on the amount of processing, semi-processed (e.g. roasted coffee beans) or highly processed (e.g. cheese), or if they are high-value unprocessed products (e.g. fresh fruits). Common high-value products that are freeze-dried include coffee, mushrooms, and herbs and spices. Another application of freeze-drying is the preparation of active cultures or enzymes for long-term storage.

''Although freeze-drying is used to preserve food, its earliest use in agriculturally based industries was in processing of crops such as peanuts/groundnuts and tobacco in the early 1970s. Because heat, commonly used in crop and food processing, invariably alters the structure and chemistry of the product, the main objective of freeze-drying is to avoid heat and thus preserve the structural and chemical integrity/composition with little or no alteration.[13] Therefore, freeze-dried crops and foods are closest to the natural composition with respect to structure and chemistry. The process came to wide public attention when it was used to create freeze-dried ice cream, an example of astronaut food. It is also widely used to produce essences or flavourings to add to food.''

The freeze-drying process for solid food products requires 3 steps. The first step is to rapidly freeze the food particles in order to obtain small ice crystal formation within the particle. The second and third steps are reduce the pressure to below 610Pa then slowly add heat until sublimation occurs.

When freeze-drying liquid products there is a slight difference in the process in that the liquid is frozen slowly (rather than rapidly) to stimulate large ice crystal formation. The large ice crystals forms a network within the product which promotes faster removal of water vapor during sublimation. One area to keep in mind when freeze-drying liquid products is to prevent the product from entering into the glassy state because it prevents the release of water vapor. To avoid this, the liquid product is either frozen as a foam or if it a juice with pulp, it is dried with the pulp (increase solids count in the liquid).

Because of its light weight per volume of reconstituted food, freeze-dried products are popular and convenient for hikers, as military rations, or astronaut meals ''. More dried food can be carried per the same weight of wet food, and remains in good condition for longer than wet food, which tends to spoil quickly.'' When the food is ready to be consumed, it can easily be rehydrated with water.

''Instant coffee is sometimes freeze-dried, despite the high costs of the freeze-driers used. The coffee is often dried by vaporization in a hot air flow, or by projection onto hot metallic plates. Freeze-dried fruits are used in some breakfast cereal or sold as a snack, and are a popular snack choice, especially among toddlers, preschoolers, hikers and dieters, as well as being used by some pet owners as a treat for pet birds. Most commercial freezing is done either in cold air kept in motion by fans (blast freezing) or by placing the foodstuffs in packages or metal trays on refrigerated surfaces (contact freezing).''

REMOVE THIS PARAGRAPH? ''Culinary herbs, vegetables (such as vitamin-rich spinach and watercress), the temperature sensitive baker`s yeast suspension and the nutrient-rich pre-boiled rice can also be freeze-dried. During three hours of drying the spinach and watercress has lost over 98% of its water content, followed by the yeast suspension with 96% and the pre-boiled rice by 75%.[14] The air-dried herbs are far more common and less expensive. Freeze dried tofu is a popular foodstuff in Japan ("Koya-dofu" or "shimi-dofu" in Japanese).''

Advantages
With various processing and preservation methods for foods, freeze-drying has many advantages which makes it an appealing method for processing food. Freeze-drying is viewed as the optimal method of choice when removing water in high quality products because the quality and high-value are still maintained. When considering a dehydration method, the maintenance of texture is a major aspect when deciding a process. After freeze-drying, the texture of the food product is still preserved, along with retention of the food’s shape and volume. Another advantage to freeze-drying is shelf-life extension of the food product to more than 12 months if correctly packaged. Shelf-life extension occurs through the immobilization of liquid water prevents microbiological reactions and microbial growth since a non-ideal environment is created. Also, the amount of nutrients lost is minimal; however, vitamin C has a loss rate between 8%-30% and vitamin A has a loss rate between 0% - 24% in green vegetables. The volatile aroma compounds also have a retention rate of 80% - 100%, which allows for flavor retention for rehydrated foods. Additional sensory qualities, such as color is also maintained during freeze-drying and can be used as an indication of adequate treatment and preservation of nutrients.

Disadvantages
Despite the numerous advantages with freeze-drying, there are a few disadvantages. One of the main disadvantages is with rehydration, microbial growth can occur because spoilage organisms and pathogens are not destroyed (unless the food is blanched before drying). Microorganism growth may also occur if the product is not properly packaged and/or stored because rehydration may occur. Also, if not properly packaged and rehydration occurs, oxidation may occur and reduce quality of the product. Another major disadvantage to freeze-drying is the cost. The costs for freeze-drying are about 4-8 times more than other preservation processes, making it an unideal choice if the value of the product is not increased after processing.

Equipment/Types of Freeze-Dryers
There are many types of freeze-dryers available, however, they usually contain a few essential constituents. These are a vacuum chamber, shelves, process condenser, shelf-fluid system, refrigeration system, vacuum system and control system.

Function of 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 that comprises 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 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 changer. 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 [4]. 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 [1].

Rotary dryers are another form of contact freeze dryers but instead of a tray, rotary dryers have a cylindrical reservoir that is rotated during drying to achieve a more uniform drying throughout the substance. These are commonly used for drying pellets, cubes and other pourable substances.

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 [1].

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 setup 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. [1]. These requirements mean a high cost for the instrumentation and necessitate trained staff so use of this type of freeze dryer for commercial purposes has yet to be observed.

Contact Freeze Dryers
Contact freeze dryers use contact (conduction) of the food with the heating element to supply the sublimation energy. These are the one of the more basic style of freeze dryers and are very simple to setup and place samples into. 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.

Rotary dryers are another form of contact freeze dryers but instead of a tray, rotary dryers have a cylindrical reservoir that is rotated during drying to achieve a more uniform drying throughout the substance.These are commonly used for drying pellets, cubes and other pourable substances.

move Silicon oil Leakage problem to here
''The majority of commercial freeze-dryers use Silicone oil as a heat transfer fluid to cool-down and/or heat-up the freeze-dryer shelves. However, due to the repeated heat/cool cycle stress that the hose-to-shelf joints experience, a potential Silicone oil leak can occur within the weakened joints. Silicone oil leaks will compromise the quality and safety of the lyophilized pharmaceutical products and will result in a major economic loss of the spoiled pharmaceutical batch in question. Miniaturized mass spectrometers can detect the presence of minute amounts of the leaking Silicone oil vapor within the freeze-dryer product chamber and allow for action to be taken to save the pharmaceutical batch being processed.''

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 setup 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 sublimation of the product also changes the impedance of the microwaves so the power of the microwave must be constantly adjusted as well. These requirements mean a high cost for the instrumentation and necessitate trained staff so use of this type of freeze dryer for commercial purposes has yet to be observed.