Lyophilization Article

JIHOESCK UNIVERZITA V ESKCH BUDJOVICCH

CZECH REPUBLIC

POLITEHNICA UNIVERSITY OF TIMISOARA

ROMANIA

ERASMUS programme

LYOPHILIZATION

Univ.Prof. Eng. Dumitru MNERIE, PhD POLITEHNICA University of Timisoara

ROMANIA

APRIL - MAY 2008 ESK BUDJOVICE

LYOPHLIZATION D. Mnerie

CONTENS

1. Introduction

1.1. Historical review

1.2. Evolution of process and equipment

1.3. Definition of lyophilization

1.4. General description of the process

2. Freezing Process

2.1. Primary Drying Process

2.2. Secondary Drying Process

3. Applications

3.1. Healthcare Industry

3.2. Veterinary

3.3. Food

3.4. Cereals

3.4. Other Applications

4. Traditional Lyophilization Technology

5. Lyophilization Equipment

6. Installation of laboratory

7. Conclusions

References

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1. INTRODUCTION

Freeze-drying (lyophilisation) is a widely used method for dehydrating a vast range of materials, including foodstuffs, pharmaceuticals, biotechnology products, vaccines, diagnostics and biological materials. It may be carried out on a range of scales, from bench top through pilot-scale to a full-scale manufacturing process and offers a number of advantages over conventional drying and many other processing methods. Despite its wide use, however, it is still apparent that many regard freeze-drying as somewhat of an art. This is perhaps not surprising, given the lack of available texts devoted to the process itself, with many published articles tending to describe specific applications of the process. Specialised training courses are now arguably the most effective means of learning about the various aspects of freeze-drying technology. This article seeks to provide a brief overview of the process: The foods conservation engineering - important part in program of human nutrition. Interdisciplinary research, for:

- to improve human nutrition - promote good health through new and traditional foods.

For all the people - the choices - the nutritional status of the World. One of the best methods for foods conservation engineering is the lyophilization, (freeze-

drying) That is a complex process that requires a careful balancing of product, equipment, and

processing techniques. Freeze-drying (lyophilization) is a widely used method for dehydrating a vast range of

materials, including foodstuffs, pharmaceuticals, biotechnology products, vaccines, diagnostics and biological materials.

The lyophilization process, also known as freeze-drying or sublimation, has many advantages over other processing methods. Since freeze-drying is achieved at lower pressures and temperatures than other methods, it is an inherently gentle process.

Lyophilization - drying, achieved by freezing the wet substance and causing the ice to sublime directly to vapor by exposing it to a low partial pressure of water vapor.

In practice the substance may not be completely frozen, especially if no aqueous solutions are present, and most lyophilization processes are completed by a period of desorption drying.

The conventional drying (with hot air) has for results dehydrated products that can have a warranty period of a year at most. Unfortunately, the quality of products dried by the conventional method is inferior to that of the initial product. Freeze-drying is based upon the dehydration of the frozen product through sublimation. Because this process does not require liquid water, and the product is at a low temperature, most of the microbiological reactions and deterioration are stopped, obtaining a final product of excellent quality. The water, being present in the freeze-drying process as a solid entity, it protects the primary structure and the shape of the products with minimum decrease volume. Despite the advantages, freeze-drying has been recognized as being the most expensive method to make a dehydrated product. These processes can affect (partially or totally) the quality of a product. Ended, a lot of changes can occur in the physical, chemical and/or biological characteristics of the products during the process, storage and distribution.

For demonstrated outstanding qualities, the food products, preserved by drying are in great demand on the market. Depending on the technology applied, they can finally have different characteristics, differently appreciated qualities, and cost and sale prices also. For the producer, the methodical concept integrated upon the production and the product can assure a technological management suited to the requests of the consumer. To grant the product with more quality, there can be used more expensive technologies. The dried food can be produced as the requests of the consumer, with a decisive effect on the technical level necessary to be obtained by the producer.

The more we learn about freeze-drying, the more it reveals itself to be an exact (yet complex) science, rather than the art (or even scientific curiosity) it was historically thought to be. However,

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there is clearly much more to be done in order for the process to be understood more fully and Biopharma Technology Limited strives to build upon its current position at the forefront of freeze-drying technology to rationalize the process further using a scientific approach.

Freeze-drying (lyophilisation) is a unit operation in which a solvent, usually water, is frozen and then sublimed in a vacuum. It is commonly used in the pharmaceutical industry when there are stability issues with the active ingredient in solution, as is often true for proteins1. In order to prevent processing defects during freeze-drying, active ingredients are formulated with excipients, which may serve specific functions, such as providing bulk properties, thermal stability and activity preservation to the product. Many groups of molecules have been shown to perform these functions, including disaccharides, amino acids, polymers and non-ionic surfactants. The aim of this study was to evaluate the correlation between the different analysis methods based on pre- and post-lyophilisation properties of a range of protein-excipient mixtures. Success of these mixtures was based on retained protein activity.

1.1. Historical review

The basic process of freeze-drying food is known to the ancient Peruvian Incas of the Andes.

They knew the basic process of freeze-drying food. Lyophilization, is the sublimation/removal of water content from frozen food.The Incas were drying their frozen meat in the sun under the reduced atmospheric pressure of high altitude. The dehydration occurs under a vacuum, with the plant/animal product solidly frozen during the process. Shrinkage is eliminated or minimized, and a near-perfect preservation results. Freeze -dried food lasts longer than other preserved food and is very light, which makes it perfect for space travel. The Incas stored their potatoes and other food crops on the mountain heights above Machu Picchu. The cold mountain temperatures froze the food and the water inside slowly vaporized under the low air pressure of the high altitudes.

Freeze - drying methods have been used for over 100 years for various technical purposes. During World War II, equipment and techniques were developed to supply blood plasma and penicillin to the armed forces. In the late 70's freeze - drying was commonly used for taxidermy, food preservation, museum conservation, and pharmaceutical production. It wasn't until the late 80's the freeze - drying industry discovered the allurement and longevity of freeze -dried flowers.

There is no real invention of a freeze -dryer. It appears to have evolved with time from a laboratory instrument that was referred to by Benedict and Manning (1905) as a "chemical pump". Shackell took the basic design of Benedict and Manning and used an electrically driven vacuum pump instead of the displacement of the air with ethyl ether to produce the necessary vacuum. It was Shackell who first realized that the material had to be frozen before commencing the drying process - hence freeze - drying. The literature does not readily reveal the person who first called the equipment used to conduct this form of drying a "freeze -dryer". For more information on freeze - drying or lyophilization, is referred to the book "Lyophilization - Introduction and Basic Principles". Dr. Jennings' company has developed a number of instruments that are directly applicable to the lyophilization process, including their patented D2 and DTA thermal analysis instrument.

Everybody knows that, in the winter time, snow might disappear from the land, without melting, by direct sublimation under radiant heat. It is claimed also, that, in the upper reaches of their mountainous kingdom, the Incas were drying their frozen meat in the sun under the reduced atmospheric pressure of high altitude. Freezing and subsequent drying from frozen state are thus well known phenomena since time immemorial. However, only in the XXth Century this technique has industrial applications.

During World War II, the freeze -dried process was developed commercially when it was used to preserve blood plasma and penicillin. Freeze - drying requires the use of a special machine called a freeze -dryer, which has a large chamber for freezing and a vacuum pump for removing moisture. Over 400 different types of freeze -dried foods have been commercially produced since the 1960s.

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Two bad candidates for freeze - drying are lettuce and watermelon because they have too high a water content and freeze -dry poorly. Freeze -dried coffee is the best-known freeze -dried product.

Drying is very old process used to preserve food. The term drying usually is to relegate to the elimination of moisture from the substance.

1.2. Evolution of process and equipment

Freeze-drying has a great impact upon the production of dehydrated foods because of the

superior quality of the product obtained and promise continued expansion of the number of applications.

Substances that are not damaged by freezing can usually be lyophilized so that refrigerated storage is unnecessary.

Some other less common applications of lyophilization are recovery of water-damaged books and manuscripts and preservation of archaeological specimens, tissue for spare-parts surgery, and museum specimens for display such as plants and animals, and vegetable matter for research programs.

Ratti and Crapiste (1992) developed a lumped parameter model for hygroscopic shrinking food systems. This model is represented by the following equation: ( )

+

= md

wwswgw

BiX

ppakn

0

1 (1.1)

where Bimd is the Biot number for mass transfer defined as

Bimd=(kgLo)/(P1Drs) (1.2)

and P1 is the equilibrium relationship at the solidgas interface, which can be obtained from

P1=(X/pw)T. (1.3)

The parameter was shown theoretically and experimentally to be independent of drying

conditions and particle geometry, and only a function of moisture content

079,1

0

31032,5

=

XX

(1.4)

The freeze-drying is analyzed under the viewpoint of the relationship between process conditions, transport properties and drying rate.

The freezing process has a strong influence on the entire freeze-drying procedure.Since a change in heat transfer has an effect on mass transfer and a change in pressure has

an effect on heat transfer, the relationship between the two processes is quite complicated. Thus, differentiating between the two processes in order to determine which one is the limiting factor can be difficult.

The transport properties conductivity and permeability mainly depend upon the structural nature of the dried layer (porosity) and secondary on the operating factors such as pressure or temperature.

Process improvements can be obtained if an accurate interpretation of the phenomena involved is available.

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Figure 1.1. Basic Industrial Freeze Dryer

The most important are listed below:

Separated drying chamber and ice condenser to reduce cross-contamination Provision of an isolation valve between chamber and ice condenser to allow for end-point

determination and simultaneous loading and defrosting Construction of the chamber and ice condenser as pressure valves to allow for steam

sterilization at 121 C or higher Cooling and heating of the product -support shelves by a circulating intermediate heat-

exchange fluid to give even and accurate temperature Additional instruments to control, monitor, and record process variables Movable product-support shelves to close the slotted bungs used in vials and to facilitate

cleaning and loading Automatic control system with safety interlocks and alarms, duplicated vacuum pumps,

refrigeration systems, and other moving parts to enable drying to proceed without endangering the product in the event of mechanical breakdown

1.3. Definition of lyophilization

From dictionary: r.v., -dried, -drying, -dries. To preserve (food, for example) is by rapid

freezing and drying in a high vacuum. Freeze-drying, (lyophilization) means the sublimation of water content from frozen food.

Lyophilization is a process in which liquid materials are slowly frozen in a vacuum, thereby removing water by maintaining a very low water vapor pressure. In practice the substance may not be completely frozen, lyophilization processes are completed by a period of desorption drying.

The technology of lyophilization appear as a relatively simple process but as the practitioner soon learns the process is deceptively complex and, as a result, is often treated as an art rather than a science. Lyophilization - process, which extracts the water from foods and other products so that the foods or products remain stable and are easier to store at room temperature (ambient air temperature).

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- Drying, evaporation, desiccation, dehydration mean the processes of extracting moisture. - Freeze, freezing are the withdrawal of heat to change something from a liquid to a solid.

Lyophilisation can be a method of drying food or blood plasma or pharmaceuticals or tissue without destroying their physical structure; material is frozen and then warmed in a vacuum so that the ice sublimes

Lyophilization can be a method of drying food or blood plasma or pharmaceuticals or tissue without destroying their physical structure; material is frozen and then warmed in a vacuum so that the ice sublimes

The fundamental principle in freeze - drying is sublimation, the shift from a solid directly into a gas. Just like evaporation, sublimation occurs when a molecule gains enough energy to break free from the molecules around it.

1.4. General description of the process

The technology of lyophilization appear as a relatively simple process but as the practitioner

soon learns the process is deceptively complex and, as a result, is often treated as an art rather than a science.

Lyophilization is a process, which extracts the water from foods and other products so that the foods or products remain stable and are easier to store at room temperature (ambient air temperature).

The literature does not readily reveal the person who first called the equipment used to conduct this form of drying a freeze-dryer.

The lyophilization (freeze drying): a process in which water is removed from a product after it is frozen and placed

under a vacuum, allowing the ice to change directly from solid to vapor without passing through a liquid phase.

The process consists of three separate, unique, and interdependent processes: freezing primary drying (sublimation), secondary drying (desorption).

The theoretical principle of freeze drying is clearly defined in the diagram "Pressure

Temperature". In order to avoid the liquid phase, it is absolutely essential to lower the partial pressure of water, below the triple point pressure. A freeze drying cycle is shown in this diagram, which has been designed to conform to a typical example (described below):

Freezing of a product from 20 C to -20 C at atmospheric pressure. Sublimation of the product at -20 C. Transfer of evolved vapor to the condenser at low temperature. Vacuum release. Defrost.

Freeze Drying is a complex operation, and all facets cannot be addressed in this type of explanation. Instead, certain aspects will be highlighted which play a part in the development of a freeze drying operation:

Freezing. Drying. Vacuum influence. The liquid shelf on which the product is placed. Essential control aspects during freeze drying.

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Freezing. Upon completion of product freezing, the product will have acquired a frozen structure, which cannot be changed during freeze drying. Sublimation, and the qualities of the finished product are greatly dependent on this crystal structure. In fact, it is considered the most crucial stage of the freeze drying process.

Speed of Freezing. On the pilot level, fast or very fast freezing is relatively easy to achieve. However, for industrial production settings, freezing at the same rates is unrealistic because of the problems of product preparation (filling, loading time) and larger systems costs will dictate compromises in the same process. As soon as the product reaches 0 C, some of the particles transform to ice. This is the nucleation process. Generally, biological products contain between 80% and 95% water. The temperature of the product stabilizes after time period at about 0 C. At Point B, the ice crystals previously formed have expanded, and consist practically of pure water. At Point C, the crystals have grown larger, and now occupy 80% to 90% of the initial volume of the solution. The crystallization of the free water is nearly complete. These crystals seem to be contained in an interstitial state, still liquid, but which constitutes the principal active element of the solution. At Point D, the interstitial component itself has reached freezing temperature, and the amorphous appearance is even more apparent, and a barely visible skin has formed on the surface. This structure is ideal for sublimation.

It has a paradoxical situation: a slow cooling which can lead to a rapid coagulation of the constituent water. In many cases, freezing induced by these conditions may be necessary to achieve successful freeze drying of a sensitive product.

Freezing Temperature. In these examples, Point D on the curve, represent the temperature of the complete freezing of the product. The establishment of this eutectic zone is very important. Between, the concentration and consistency of the liquid phase is increased, and in the case of biological products, may produce a change in the bacteria as a result of the hyper concentration of the active ingredient, and the mechanical effect of the ice crystals.

During the sublimation phase, signs of melting appear in the product, which induces a temperature higher than that corresponding to the eutectic point. Temperature analysis permits determination of the onset of the fusion temperature. This acts as an indicator to help prevent melt back, or other such accidents during the course of freeze drying.

Generally, a structure of large crystals presents more difficulties with regard to freeze drying, in that a thick crust forms at the surface. The appearance of the freeze dried product is heterogeneous. This very often makes dissolution difficult.

A product structure of fine crystals freezes more easily. The freeze dried product has an amorphous appearance, and re-dissolves more quickly. Obtaining the desired crystalline structure is not always easy, as the formation of this type of crystal depends on several factors:

The nature of the product. The processing. Freezing speed. Type of freezing.

Given the diverse range of products, which may be treated in the dryer, the desired freezing rate, and the type of final packaging that may used, the manufacturer must consider:

The quantity of product per container. The form of the container. The type of freezing. And, plan for a freeze dryer that is flexible to these different demands.

Freeze Drying. Once the product is properly frozen, it must be sublimated (evaporated) at a low temperature under reduced pressure. The ideal curve of lyophilization is depicted on Curve, with temperature displayed on the ordained line, and time displayed on the abscissa.

With the product maintained at a constant temperature, it will be necessary to supply the energy of sublimation, a combination of the latent heat of fusion (which supplies the transformation of the liquid to the ice state) and the sublimation energy (about 700 calories per gram of ice

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evaporated). The ability of vapor release from the matrix is a function of molecular agitation inside the matrix.

After the disappearance of the final ice crystals, the temperature of the product rapidly increases, and must be maintained at the most maximum permissible temperature to liberate the lowest residual moisture embedded in the matrix (secondary drying). The liquid shelves on which the product is loaded transfer the required energy of sublimation. (Curves B, C, or D represent the heating rate)

A chilled surface known as the ice condenser collects the vapor from the evolving product. During lyophilization, the pressure in the drying chamber follows the fluctuations identified.

Importance of Vacuum in the Freeze Drying Process. Freeze Drying can only take place if the partial pressure of the vapor in the drying chamber is lower than the water vapor pressure above the product. By strict definition, the vacuum in the chamber is not essential, as sublimation can take place at atmospheric pressure by passing dehydrated air above the product. By artificially lowering the pressure in the drying chamber the time can be reduced.

For example, a product sublimated at 20 C at the given vapor pressure shown as 0.8 torr. As soon as the value in the chamber reaches below 0.8 torr, the ice begins to sublime.

Flosdorff has shown that as soon as the pressure in the chamber is reduced, evaporation increases, but the rate of evaporation is not without limit, and reaches a maximum when the pressure in the chamber has a value equal to about 50% of the vapor pressure above the product.

Contrary to widely held opinion, it is not necessary to have a very low vacuum during the sublimation period, because below the limit defined, the evaporation rate is not improved, and that too low a pressure acts as a barrier to effective heat transfer.

Freeze Drying. Once the product is properly frozen, it must be sublimated (evaporated) at a low temperature under reduced pressure.

The ideal curve of lyophilization is depicted, with temperature displayed on the ordinand line, and time displayed on the abscissa.

With the product maintained at a constant temperature, it will be necessary to supply the energy of sublimation, a combination of the latent heat of fusion (which supplies the transformation of the liquid to the ice state) and the sublimation energy (about 700 calories per gram of ice evaporated). The ability of vapor release from the matrix is a function of molecular agitation inside the matrix.

Ideally, the temperature of the frozen product should be brought to the highest temperature compatible with the frozen condition, without exceeding it, which will lead to the irreparable production of foam (commonly known as melt back) and product deterioration.

On the other hand, if the heat energy is insufficient, the product will sublime at too low a temperature, and the length of the freeze drying cycle will become abnormally long.

After the disappearance of the final ice crystals, the temperature of the product rapidly increases, and must be maintained at the most maximum permissible temperature to liberate the lowest residual moisture embedded in the matrix (secondary drying). The liquid shelves on which the product is loaded transfer the required energy of sublimation.

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2. FREEZING PROCESS

The primary goal of research on freeze-drying is to improve process economics by reducing processing time. An important step in this process is determining the rate limiting factors.

Heat transfer and mass transfer are the two processes that are rate controlling processes. Understanding the mechanisms of heat and mass transfer and their dependence on product temperature and pressure in the drying chamber is very important for determining the rate limiting process. If the outer surface temperature limit is encountered first as surface temperature of the dried layer is raised, the process is considered to be heat transfer controlled; to further increase the drying rate, in the food drying case, the thermal conductivity of the dried layer must be raised. If the melting point temperature is encountered first, then the process is considered to be mass transfer limited, and in order to increase the drying rate, the effective diffusivity of water vapor in the pores of the dried layer and total mass flux must be raised. This means an increase of the convective velocity of the vapor in the pores of the dried layer. The values of effective diffusivity and total mass flux could be raised by decreasing the pressure in the drying chamber.

To extract water from foods, the process of lyophilization consists of: Freezing the food so that the water in the food become ice; Under a vacuum, sublimating the ice directly into water vapour; Drawing off the water vapour; Once the ice is sublimated, the foods are freeze-dried and can be removed from the machine.

Figure 2.1. Diagram pressure temperature.

There are two major factors that determine what phase (solid, liquid or gas) a substance will

take: heat and atmospheric pressure. For a substance to take any particular phase, the temperature and pressure must be within a

certain range. Without these conditions, that phase of the substance can't exist. Lyophilization is carried out using a simple principle of physics called sublimation.

Sublimation is the transition of a substance from the solid to the vapor state, without first passing through an intermediate liquid phase.

Water will sublime from a solid (ice) to a gas (vapor) when the molecules have enough energy to break free but the conditions aren't right for a liquid to form. The chart below shows the necessary pressure and temperature values of different phases of water. The water can take a liquid form at sea level (where pressure is equal to 1 ATM) if the temperature is in between the sea level freezing point (32 degrees Fahrenheit or 0 degrees Celsius) and the sea level boiling point (212 F or 100 C). But it increase the temperature above 32 F while keeping the atmospheric pressure below 06 atmospheres (ATM), the water is warm enough to thaw, but there isn't enough pressure for a liquid to form. It becomes a gas.

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Figure 2.2. - The transformation of phases for lyophilization process.

Freeze-drying is a multiple operation which involves the following three stages:

1. the freezing stage, 2. the primary drying stage (sublimation), and 3. the secondary drying stage (desorption).

The first step in the lyophilization of a product is to convert it into a frozen state. The material is cooled down to a temperature that is always below its freezing point, which depends on the nature of the product. It is important that during the freezing process that solvent (water) is crystallized. The shape of the pores, the pore size distribution, and pore connectivity of the porous network of the dried layer formed by sublimation of frozen water during the primary stage depend on the ice crystals that formed during freezing stage [7-10]. This dependence is very important because the parameters that characterize the mass and heat transfer rates in the dried layer are influenced by the porous structure of the dried layer and the performance of the overall drying process depends significantly on this stage.

2.1. Primary Drying Process

After freezing stage, the chamber pressure is reduced to a value that would allow the sublimation of the ice. For sublimation, the applied heat may come from warmed fluid flowing through the shelves or walls of the chamber, or from microwave energy, or any conceivable source. The maximum allowable temperature that the frozen layer could tolerate, without loss of product property or stability, is denoted by convention the melting temperature of the sublimation interface of the frozen layer.

There have been many experimental studies of sublimation rates and mechanisms of mass transfer through the dried layer during freeze-drying.

In a study of sublimation rates for aqueous solutions, Pikal et al took measurements of weight loss on a cylindrical micro-sample suspended from a vacuum micro balance kept at a constant temperature. The variables in the experiments were freezing rate, product thickness, temperature, pressure and solute concentration. An important result of this work was the conclusion that at higher temperatures hydrodynamic surface flow is an important flow mechanism in the dried layer.

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Livesey and Rowe, in their discussion on the effect of chamber pressure, noted that the rate limiting process in freeze-drying changes as sublimation proceeds. Initially, the process is limited by heat transfer when the dried layer is small and an unrealistic heat flux is required to push the sublimation rate to its maximum. After the dried layer thickens, the process becomes limited by mass transfer since the required heat flux is easily maintained for the decreasing sublimation rate.

Wolff and Gibert determined the rate limiting factor in vial freeze-drying of milk. By fitting their model to measured data, they were able to determine three transport parameters in their model. These were the water vapor diffusivity in the dried layer, the external mass transfer coefficient and the resistance to the heat transfer from the heated shelf to the ice. Since the contact resistance between the vial and the shelf is the most significant barrier to heat transfer, it was determined to be the overall rate controlling parameter and thus controlled the drying kinetics.

The water vapor produced by sublimation travels by diffusion and convection the porous structure of dried layer and enters in the drying chamber of the freeze dryer. The water vapor must be continuously removed from drying chamber in order to maintain non-equilibrium conditions for the drying process in the system. The time at which there is no more frozen layer is taken to represent the end of the primary stage

During the primary drying phase the pressure is lowered and enough heat is supplied to the material for the water to sublimate. The amount of heat necessary can be calculated using the sublimating molecules latent heat of sublimation. In this initial drying phase about 98% of the water in the material is sublimated. This phase may be slow, because if too much heat is added the materials structure could be altered.

In this phase, pressure is controlled through the application of partial vacuum. The vacuum speeds 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. This condenser plays no role in keeping the material frozen; rather, it prevents water vapor from reaching the vacuum pump, which could degrade the pump's performance. Condenser temperatures are typically below 50C (-58F).

2.2. Secondary Drying Process

At the end of the sublimation phase (primary drying), all the ice will have disappeared. The

product will begin to rise in temperature, and will tend to approach the control temperature of the shelf. However, at this stage the product is not sufficiently dry for long term storage. For most products, the residual moisture is in the region of 5% to 7%.

The product now enters the desorption phase, during which the last traces of water vapor are removed, along with traces of the bound water within the product matrix. This phase is identified as secondary drying. The aim of this final phase is to reduce the product to the acceptable moisture levels needed for long term storage (3% to 1%).

The reasons for drying the product to these levels are desirable for several benefits: When the water content is higher than these levels, the product will denature. When the residual moisture is forced lower than these levels, many products may

undergo chemical or enzymatic changes. Residual moisture in the product is generally dependent on four factors:

The product matrix (both in frozen and sublimation mode.) The vacuum in the drying chamber.

The secondary drying stage starts when all the ice has been removed by sublimation. In the

secondary stage the water that did not freeze (bound water) is removed. The bound water is due to physical adsorption, chemical adsorption and water of crystallization. Since the amount of the non-frozen water is about 10 35% of the total moisture contents, its effect on the drying rate and overall drying time is very significant.

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The bound water is removed by desorption which takes place by heating the product under vacuum. Moisture may move by vapor diffusion through the solid under a vapor pressure gradient.

Usually the pressure is also lowered in this stage to encourage sublimation. 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.

Secondary drying is the process of removing the unfrozen or bound water contained in the amorphous material. The mechanism for removal of the unfrozen moisture is primarily desorption. This means that release of the bound moisture can be predicted by desorption isotherms. A desorption isotherm gives the moisture content as a function of humidity for a given temperature. Using these data one can determine a temperature-pressure protocol for secondary drying that will bring the residual moisture content to the required level. Water will follow the same path as in primary drying, from the material to the condenser. However, in secondary drying, diffusion of unfrozen water from within the material to its surface and subsequent desorption of that water become the most important mechanisms.

The primary goal of research on freeze-drying is to improve process economics by reducing processing time. An important step in this process is determining the rate limiting factors. Since the freeze-drying rate is limited by the rates of heat and mass transfer across a dried layer, the values of its thermal conductivity and permeability are indispensable to determine the drying rate. These values are mainly governed by the structure of the dried layer and operating factors such as temperature and pressure. Since the structural nature of the dried layer is also affected by freezing operations, the freezing condition, the structure and transport properties of the dried layer are fundamental information to design the optimum drying cycle and to control the quality of final products.

Although the basic concepts of freeze-drying are known, the details are not well understood. Fundamental knowledge in some areas concerning the physical processes of freeze-drying is not complete. Optimum parameter settings are usually found by trial and error, a lengthy and expensive procedure. The understanding of the fundamental phenomena involved in this complex process allows process optimization. However, fundamental knowledge of the physical and chemical processes of freeze-drying needs to be put into a form that is useful to equipment designers and users. Combining this fundamental knowledge with process development research can result in an improved freeze-drying process for food industry and not only.

The freezing stage represents the first separation step in the freeze-drying process, and the performance of the overall freeze-drying process depends significantly on this stage. The structural nature of the dried layer is mainly affected by freezing operations. The fast freezing before freeze-drying leads to smaller pore spacing after freeze-drying. It follows that faster freezing should lead to lower thermal conductivities at a given pressure. If a freeze-drying process is rate limited by internal heat transfer, the rate of freeze-drying for fast-frozen material should then be less than that of a slowly frozen material.

The relationships among the freezing condition, structure and transport properties (thermal conductivity and permeability) of the dried layer are fundamental information to design the optimum drying cycle and control the quality of final products. As long as no collapse occurs,23 during the primary and secondary drying, the space previously occupied by the ice crystals becomes the main passageway for vapour transport.24 Different morphologies of solidification texture may results depending on the chosen solidification parameters. These different textures then lead to different resistances against vapor transport during drying. In fact, the porous structure of the dried product determines the values of the heat and mass transfer coefficients, and thus the drying rate.

The quality of the final freeze-dried product is greatly influenced by the freezing process. For instance, the freezing process controls the color and the flavour of freeze-dried coffee extract or retention of volatile compounds. Proteins cells, subjected to freeze-drying, can be damaged during the freezing process already.

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One important goal of the freezing step is to produce a uniform product batch, which is difficult because of the stochastic nature of nucleation. The degree of super-cooling, defined as the difference between the equilibrium freezing point and the temperature at which ice crystals first form, is both a statistical or random event as well as one that depends on the solution properties and process conditions. The degree of super-cooling is important because it determines the number of nuclei at any time, and thus determines the number of ice crystals formed. More ice crystals from the same amount of water means smaller crystals, which means smaller pore size and thus longer primary drying time. Nucleation rates increases rapidly with the degree of super-cooling, yielding ice crystal sizes that are inversely proportional to the degree of super-cooling. Thus, as super-cooling is increased, the resulting increases in nucleation rate cause the ice crystal particle size distribution to shift to smaller sizes. In turn, this produces pores with lower volume/surface area rations, which results in decreased diffusive flux and slower sublimation.

Theoretically, solidification may result in different patterns: the interface may be planar, i.e., smooth and flat, cellular (columnar) or dendritic, i.e. needle, tonque or finger shaped, or tree-like with side branches.

A large portion of most freezing processes used in freeze-drying may be regard as directional solidification, i.e. the growth direction of the solidification interface is opposite to the main heat flow direction. The solidification pattern is governed by two parameters: velocity of solidification interface general case and mean temperature gradient at the interface ice-liquid interface in the direction of ice finger growth. A complete description of the geometric form of dendrites or cells in freezing aqueous solutions is difficult to achieve because of complexity of the theoretical models.

Y. Sagara studied the influence of the freezing stage on the transport properties in the foods freeze drying. He found a linear relationship between permeability and ice crystallization time for mashed sample. The freezing rate of the group A was relatively larger than the group B. This behavior is attributed to the larger ice crystals formed in the sample as the freezing rate decreased. The effects of the freezing rate on the permeability were found to be critical for the mashed cellular food materials. In liquid food systems the porosity of the dried layer is governed by the solid content or concentration, and thus the porosity mainly affects the thermal conductivity of the dried layer as shown in Fig.2.3.

Fig. 2.3 - Permeability vs. ice crystallization time. Fig. 2.4 - Thermal conductivity vs. porosity

14


In addition to concentration, the permeability is influenced significantly by freezing manner, because the capillary type ice columns is formed by ice crystals along with the direction of heat flow during freezing. Consequently, the morphology of capillary type void as well as grain orientation and pore size of the dried layer are arranged during freezing process and thus the transport properties of the dried layer is decisively governed by the structure fixed during freezing.

From these results, it would be suggested for liquid materials that under the constant solid concentration the material should be frozen in a manner to form straight ice columns whose orientation is parallel to that of heat transfer during drying and also make larger ice crystals by employing the slower freezing rate.

If the higher permeability coefficient could be obtained by controlling freezing method, the drying rate would be limited by heat transfer rate across the dried layer. Under this condition the surface temperature of the dried layer is allowed to increase within a certain range that is decided from viewpoint of quality control for final product.

The freezing process thus has a strong influence on the entire freeze-drying procedure. The main processing factors in freeze-drying are product temperature and chamber pressure.

The freeze-drying process should be conducted within certain temperature and pressure limits in order to avoid product damage during drying process.

During the primary drying stage there are two temperature constraints (limits). First, the surface temperature of the dried layer must not become too high because of risk of thermal damage loss of bioactivity, color change, degradative chemical and biochemical reactions, structural deformation in the dried layer; the second, the temperature at the interface must be kept below the melting temperature, which may be 10C or more below the melting point of ice. The melting at the sublimation interface, or any melting that occur in the frozen layer, can give rise to gross material faults such as puffing, shrinking, and structural topologies filled with liquid solution. When melting has occurred in the frozen layer, then the solvent at that point cannot be removed by sublimation. If the material has a glass form and the minimum freezing temperature is exceeded during primary drying stage then the phenomenon of collapse can occur. The product collapse causes a loss of rigidity in the solid matrix. The maximum allowable temperature in the frozen layer is determined by both structural stability and product stability factors.

During secondary drying the moisture concentration and temperature in the sample could vary widely with location and time and this implies a potential for product alteration. Since very many products are temperature sensitive, it is usual to control product stability by limiting the value of the temperature during secondary drying process and then the final moisture content.

The pressure constraints (limits) are more complex to define because of the pressure effect on both the heat transfer (the pressure has effect on the thermal conductivity of the dried layer) and the mass transfer (the pressure has effect on the effective diffusivity of water vapor in the dried layer and the total mass flux). In the dried material, the effective diffusivity varies significantly with the total pressure and with the type of gas present. At very low pressures, independent of the surrounding gas, the thermal conductivity reaches a lower asymptotic value. This reflects the geometric structure of the solid matrix with no contribution from the gas in the voids of the material because the pressure is very low. At high pressures, the thermal conductivity reaches a higher asymptotic value and is characteristic of the heterogeneous matrix composed of solid material and gas voids. The thermal conductivity is dependent of the gas composition and its thermal conductivity. There is a transition between asymptotes corresponding to 0.1 and 100 mmHg pressure range. The pressure effect on the mass transfer is explained by the fact that the effective diffusivity of water vapor in the dried layer and the total mass flux are function of the total pressure in the drying chamber. The magnitude of the effective diffusivity decreases if the total pressure is increased. Furthermore, when the total pressure is increased, the gradient of total pressure in the dried layer is reduced, and this decreases the convective velocity and total mass flux.

Above a certain pressure, the freeze drying process becomes internal mass transfer controlled. The highest rate mass transfer control will occur at the pressure of the transition from heat transfer control to mass transfer control (diffusivity and mass flux decrease with increasing pressure and the

15


thermal conductivity increases with pressure), and the attainable drying rate will decrease at higher pressures.

There has been considerable interest in investigating the factors affecting the batch time of freeze-dryers because these are most amenable to control, and efforts have been made to minimize the batch time.

Nail studied a typical pharmaceutical freeze-drying situation: small glass vials of a frozen product sitting on a metal tray which is placed on a heated shelf in the freeze-dryer. He states that in some situations mass transfer may be the rate limiting factor, but that heat transfer from the heat source to the sublimation front is usually the rate limiting process. Nail established that resistance to heat transfer caused by air gaps between the vial and the tray and between the tray and the heated shelf becomes significant in the low pressure environment of freeze-drying. This is explained by the reduction of the thermal conductivity of a gas at pressures in the range where free molecular flow occurs. Nail's theory shows that the drying rate decreases with decreasing pressure.

Pikal et al determined that the resistance of the dried layer decreases with decreasing pressure and that a lower pressure increases the sublimation rate.

Livesey and Rowe considered a situation in which the temperature of the product was at its optimum and pressure was increased. This would result in the decrease of the sublimation rate, because the heat flow to the product had to be reduced to maintain the temperature at its optimum value without overheating. Another situation considered was one in which the shelf temperature was kept at a constant value. In this situation increasing the pressure resulted in an increased heat flow, increased product temperature and an associated increase in sublimation rate.

In a study that focused on the secondary drying stage, Pikal et al determined that pressure variation in the range of 0 - 0.2 Torr had no effect on drying rate during secondary drying for the materials used. However, it was determined that temperature has a significant effect on the drying rate and that the rate limiting process was either desorption from the solid-vapor interface or diffusion in the solid.

The transport properties, such as the thermal conductivity and the permeability, have a great influence on the processing factors, and finally on the drying rate. Various methods, both transient and steady state, have been proposed to determine the transport properties of freeze dried food, the latter gives better accuracy.

To analyze the effects of the structural parameters on transport properties and drying rate, one physical and mathematical model is necessary.

To this end, let consider a material to be dried on a tray (Fig. 2.5) The thickness of the sides and bottom of the tray is small and thermal conductivity of the material of tray is high, such that the resistance of the tray to the heat transfer could be considered negligible. As a result, the temperature of the tray can be considered equal with the temperature of the heating plate.

Fig. 2.5 - Drying of the material in a tray

16


Heat qI could be supplied to the material by conduction, convection and /or radiation. The heat qII is supplied by a heated plate and is conducted through the bottom of the tray to

the material. The heat qIII represents the amount of heat transferred between the environment in the drying chamber and the vertical sides of the tray. The contribution of the qIII is negligible when compared to the contribution of qI and qII.

The study of the freeze-drying can be done in the following cases (Fig 2.5): 1. qI 0 and qII = 0, 2. qI = 0 and qII 0 and qI 0 and qII 0. The transport properties of the dried layer were determined by applying the drying data to a

model based on heat and mass transport in a sample, and then the effects of processing parameters on transport properties and drying rate in connection with the freezing operations. The theoretical model used by Y. Sagara to determine the thermal conductivity and permeability is shown in Fig. 2.6.

Fig. 2.6 - Freeze-drying model for transport properties analysis

The sample is assumed to have the geometry of a semi-infinite slab dried layer is separated

from the frozen layer by an infinitesimal sublimation interface retreating uniformly from the sample surface. The bottom of the sample is insulated while the surface is exposed to an evacuated space at the temperature s and pressure ps. Since the thermal conductivity of the frozen layer is 20 - 50 times greater than that of the dried layer, the temperature of the frozen layer can be supposed to be uniform and same as that of sublimation front.

An approximate method was developed for predicting the structural parameters of the dried layer by assuming this layer to be a bundle of capillary tubes with the pore space having an equivalent pore radius, porosity and tortuosity factor.

The expression of the rate of heat transfer across the dried layer according to Massey and Sunderland, is:

= sf

dcmtm

q pfs

)(

)( (2.1)

The mass flux may be expressed as

pgradRT

KMm w .= (2.2) Equations representing the thermal conductivity k and permeability K are presented by Sagara

and expressed as:

17


)(2 += sf

dcHlk pww

(2.3)

wfw MRTlK /2= (2.4)

where:

)/)((1

dtdmm

fs = (2.5)

)/)((1

dtdmppm

fs = (2.6)

The thermal conductivity was found to increase with increasing the solute concentration and is markedly affected by the porosity of the dried layer and the permeability was found to increase with pressure and temperature of the dried layer. This behavior is in good agreement with Mellor and Lovetts theoretical investigations based on the collision theory, and also with their experimental results obtained for several kinds of solutions. The expressions for the permeability coefficient given by Mellor and Lovett are:

= KDK (2.7)

where:

/211

))/21(/2

4643

rrrr

++++= (2.8) indicates the contributions due to Poiseule`s flow, slip flow and Knudsen`s flow. The mean free path of water vapor molecule is given by:

pT

w22

= (2.9)

and Knudsen diffusivity is defined in terms of the pore radius r and the average molecular velocity as follows

vrDK 32=

(2.10)

where: 2/1

8

=

wMRTv (2.11)

In the Sagara`s the experimental data, the permeability was found to depend mainly on the porosity (Fig. 2.7) of the dried layer and then other factors such as temperature or pressure of it.

One can conclude that the transport properties mainly depend upon the structural nature of the dried layer and secondary on the operating factors such as pressure or temperature.

As previously discussed, freeze-drying is an expensive and lengthy process, but one that can be improved. The characteristics and physical properties of the material, the mechanism of the drying that occurs (depending on the structure of the ice crystals) and operating conditions, in particular the way heat is supplied, determine which factor becomes rate controlling. One way to improve the process is to initially look at the physical mechanisms of drying in the primary and secondary drying stages, then determine the rate limiting mechanisms and subsequently alter the characteristics of the material and/or process which are responsible for the rate limitations.

Modification of operative conditions for enhancing the transfer fluxes is used. The injection of inert gas is sometimes used as a control variable in lyophilization process.

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Fig. 2.7 - Permeability vs. porosity of the dried layer

In the case of the processes limited by heat transfer through dried material, which is

determined by the thermal conductivity of the porous solid, it was proposed to use nitrogen, hydrogen or helium pulsed atmospheres. The diffusion coefficient of these gases is sufficiently high to allow them to diffuse into the solid countercurrent to the water vapor, increasing two to threefold the heat transfer as effect of their high thermal conductivity. The optimal pressure cycle must be determined, depending not only on the characteristics of the product but also on the size of the chamber and on the performances of condenser and pump, and it must be avoided that the temperature oscillation at the interface be of such an amplitude to cause melting.

Mellor evidenced that a cyclic-pressure process can significantly reduce the time as not only the effective thermal conductivity of the porous solid increases during the part of the cycle in which the pressure increases, enhancing the heat transfer, but also the vapor transfer mechanism is altered from bulk diffusive flow to a much more effective combination of viscous and free flow.

Liapis et al pointed out that an increase in pressure increases the thermal conductivity of the solid but decreases the temperature driving force, as a consequence of the increased resistance to mass transfer that causes the ice temperature to rise. It is important to find an optimal pressure which maximizes the heat transfer. However, in practice was shown that an optimal pressure policy brings only minor improvements in drying time when compared to a policy at constant pressure, especially if the additional complexity and cost of the apparatus are considered.

Mellor discussed the effect of pressure on heat and mass transfer in freeze-drying. He noted that low pressure increases the driving force for mass transfer, but reduces heat flow in the dried layer because of the dependence of heat transfer coefficients on pressure. In an attempt to overcome this problem, Mellor developed a cyclic pressure freeze-drying process. In this process, pressure is momentarily increased to increase heat flow and then momentarily decreased to increase vapor flow.

Litchfield et al. studied a typical vial freeze-drying case using a numerical simulation that includes a resistance to heat transfer between the bottom of the vial and the heated surface. In this study, Litchfield et al. found that even the best of the cyclical pressure policies was not as good as an optimal constant pressure policy previously determined by Liapis and Litchfield.

The heat required for ice sublimation is usually supplied by conduction or radiation. Different techniques were studied for improving the efficiency of energy transfer, but technical difficulties or poor quality control have limited their use.

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The main driving force for the movement of water is a pressure difference. The high pressure is at the sublimation interface where the water vapor is generated and the low pressure is on the surface of the condenser where the water vapor condenses to form ice again. Most of the low pressure range typically encountered in freeze-drying, this resistance is at least partially dependent on pressure: decreasing pressure increases the resistance. Thus, when the chamber pressure is decreased, the heat flow to the sublimation interface decreases and subsequently the drying rate decrease.

A better design for a freeze-dryer would be one in which changes in pressure do not affect the ability to provide heat to the sublimation interface. If heat is supplied volumetrically, as with microwave energy, the working chamber pressure could be lowered and the drying rate increased.

Previous attempts at using microwave energy to accelerate the freeze-drying process have focused on food applications. It is evident that incorporating microwave energy into the pharmaceutical freeze-drying process has the potential to reduce the overall processing cost by reducing the processing time. The most significant advantage to volumetric heating is the ability to decrease chamber pressure without a detrimental effect on heating. Other advantages are the ability to heat the dried layer during primary drying and improved control of heating. The benefit of heating the dried layer during primary drying is that secondary drying will start at lower residual moisture content, thereby significantly reducing the secondary drying time. Improved control of heating results from the heating rate being reduced as the volume of ice is reduced and also power can be quickly adjusted allowing for an optimized power schedule.

Process improvements can be obtained if an accurate interpretation of the phenomena involved is available. Indeed, chemical engineering research can give a sound contribution to freeze-drying processes by studying kinetics, mass and heat transfer.

By controlling chamber pressure, you can reduce the heat transfer coefficient between the warm shelves and the product. This simple control will greatly improve the energy transfer and reduce primary drying times. The curve of shelf temperature is shown in schematic form in B of and the vacuum curve in P.

The control of vacuum in the lyophilization process can become a useful means of controlling heat transfer, and the means of getting energy to the product. A laboratory example illustrates the influence of pressure in heat transfer:

A specially fitted freeze dryer, equipped with the necessary measurement and control equipment was used to simulate these sublimation phenomena. Freeze drying of the product was carried out at 20 C, with a controlled pressure in the chamber in the range of 0.4 torr, and a shelf temperature of 30 C.

When the injection of non-condensable gas was terminated, the product cooled rapidly, and the vapor removal rate slowed. To regain the sublimation temperature of 20C, it was necessary to bring the shelf temperature to 125 C, a difference of 95 C in the heating source to produce the same sublimation temperature of the product, and virtually the same evaporation rate!

The chamber pressure acts as a thermal regulator, which can, in the space of a few moments can produce the same effect as raising the temperature 95 C.

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3. APPLICATIONS

As a leading multi-product freeze drying company, European Freeze Dry works with manufacturing companies both large and small to develop appropriate drying processes for a diverse range of products. Over the years, our company has acquired a wealth of expertise in freeze drying in a wide range of applications:

Specialty Chemicals Natural Enzymes Biological Products Pigments Resins Active Pharmaceutical Intermediates Food Ingredients Flavorings Flood damage/Document Recovery The freeze drying process is particularly suitable for products which are sensitive to heat,

oxidation, or are shear sensitive. The key benefits of freeze drying are: Retention of structure and surface area Retention of morphological, biochemical and immunological properties High viability/activity levels maintained through the drying cycle Lower temperature, oxygen, and shear conditions compared to other drying methods High recovery of volatiles High yield Long shelf life of dried product Reduced weight for shipping, storage and handling

3.1. Healthcare Industry

Freeze-drying (lyophilisation) is commonly used in the pharmaceutical industry when there are stability issues with the active ingredient in solution, as is often true for proteins.

In order to prevent processing defects during freeze-drying, active ingredients are formulated with excipients, which may serve specific functions, such as providing bulk properties, thermal stability and activity preservation to the product.

Many groups of molecules have been shown to perform these functions, including disaccharides, amino acids, polymers and non-ionic surfactants. The aim of this study was to evaluate the correlation between the different analysis methods based on pre- and postlyophilisation properties of a range of proteinexcipient mixtures. Success of these mixtures was based on retained protein activity.

Pharmaceutical companies often use freeze drying to increase the shelf life of products, such as vaccines and other injectables. By removing the water from the material and sealing the material in a vial, the material can be easily stored, shipped and later reconstituted to its original form for injection.

Supercritical during. Freeze drying is a method of drying food or blood plasma or pharmaceuticals or tissue without destroying their physical structure; material is frozen and then warmed in a vacuum so that the ice sublimes lyophilisation, lyophilization

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3.2. Veterinary

Taxidermy is a general term describing the many methods of reproducing a life-like, three-dimensional representation of an animal for permanent display. The word "taxidermy" is derived from two Greek words; taxis; meaning movement; and derma; meaning skin. The modern practice of taxidermy incorporates many crafts, such as carpentry, woodworking, tanning, molding and casting; but it also requires artistic talent, including the art of sculpture, painting and drawing.

Because of the diversity of specimens being freeze-dried for taxidermy purposes (from Brahma bull head mounts to minnows used for fishing bait), trial and error has often been the method used to determine how different taxidermy specimens should be pre-frozen and freeze dried. The success or failure often directly relates to the proper pre- freezing of the taxidermy specimen, and the temperature at which the specimen chamber is held during the drying stage.

Figure 3.1.

When frozen, various taxidermy specimens will totally solidify at different temperatures. The temperature of complete solidification is the taxidermy specimens eutectic temperature. The American Heritage Dictionary, 1969, defines eutectic as, "the lowest possible temperature of solidification for any mixture of specified constituents"

An example of the variance found in freezing temperatures is that of water - not all water freezes at 32 degrees F. When water is contaminated with other elements, such as salt, nitrogen, mercury, industrial waste, etc., it's temperature of complete solidification will change. Since one of the basic premises if freeze dry taxidermy is animals are made up of about 70% water by weight, the importance of eutectic temperature is readily evident.

When determining eutectic temperatures, taxidermy specimens may be divided into two general groups: those that contain large amounts of body fats and oils (most fish, certain species of ducks, amphibians, etc.) and those that do not (the majority of mammals, reptiles, game birds, delicate flowers, etc.) Because of the chemical makeup, taxidermy specimens in the group (greasy specimens) require lower eutectic temperatures than the second group (non-greasy specimens).

Because natural salts are usually found in combination with water in taxidermy specimens, pre-freezing should be done in the shortest time possible. Slow freezing times lead to salt concentration in the specimen resulting in lower eutectic temperatures and increased chance for shrinkage. Rapid freezing reduces salt concentrations and also results in smaller ice crystals, which create less tissue distortion.

Rapid freezing may be obtained by freezing taxidermy specimens in commercial freezers with temperatures of less than -13 degrees F. (-25 degrees C.) it is generally more economical to pre-freeze the specimens in a commercial deep freezer, than use the specimen chamber as a freezer. Freeze dry machines are most efficient when "drying" frozen material, rather than initially freezing the material, then drying it. The important factor in pre-freezing any type of specimen is that the specimen is completely frozen throughout.

Once the specimen has been prepared for preservation, mounted, posed, and completely frozen, it is ready for the drying stage. the important factor in the drying stage is that the eutectic

22


temperature is maintained. Because of the lower eutectic temperature of greasy specimens, a low processing temperature in the specimen chamber is needed to prevent shrinkage. Generally this temperature should be as 0 degrees F. and then raised even higher after a period of time. Using these figures as guidelines, the taxidermist using the freeze dry machine should determine at what temperature his work is most successful. Some factors to consider when analyzing proper processing temperatures include:

1. Working in batch loads (all the same type specimen) may be the most economical way to work, depending on your own situation. Batch loads allow easy raising of temperatures as the process progresses.

2. When working with mixed loads (greasy & non-greasy specimens) one initial temperature should be decided upon (e.g. -5 degrees F. or 0 Degrees F).

3. Some shrinkage may be traded off for speed. Mammals may be run at slightly higher temperatures allowing slightly faster drying times. The animals fur will cover most of the shrinking that takes place.

4. Not only does a variety of chemical make-ups exist between different species, it may also exist between members of the same species due to different diets, and physical environments.

5. Because if the lack of air to conduct heat within the specimen chamber, some differences of temperature will exist inside the specimen chamber itself. This will require "rotating of the stock".

6. The greatest water loss will occur early in the drying stage when the dried shell is the thinnest and offers the least resistance to water vapor movement.

The rate of freeze drying is directly related to vapor pressure. the higher the specimen chamber temperature is, the higher the vapor pressure is at a given vacuum, and the faster the drying is achieved. The rate of moisture migration from the specimen chamber, to the ice bank, is a also related to vapor pressure. The greater the temperature difference between the specimen chamber, and the ice bank. for example, a unit with the specimen chamber set at 5 deg. F. and the Ice bank running at -55 deg. F., resulting in a temperature difference of 60 deg. Will move moisture faster than the same machine with the specimen chamber set a -5 deg. F., resulting in a temperature difference of 50 deg. F.

Certain steps may be taken to raise the low eutectic temperatures of greasy specimens. This will allow the taxidermist to operate the freeze dry unit more economically at higher temperatures. The first step is to eliminate the extreme amounts of fats and oils that cause the low eutectic temperatures. In the preparation stage, a 20 minute bath in a mixture of one cup of bicarbonate of soda per gallon of water will assist in neutralizing fats and oils. Also, injections of grease tallowfiers, such as antioxidant will help in preparing greasy and oily specimens for more efficient freeze drying.

Some of the questions that should be addressed when considering the right temperatures to process a particular specimen are:

What is the Physiology of the specimen? What is the Chemistry of the specimen? What solid mass of the specimen can be removed to speed up the process, and not effect

shrinkage? If you analyze specimens with these questions in the initial planning stages, problems can be

eliminated. There is nothing, to our knowledge, that can not be successfully freeze dried without shrinkage, as long as it is freeze dried at the proper temperature.

23


Duck Wildcat

Bobwhite quail Fisher

Wolf Longhorn Figure 3.2.

24


3.3. Food

Lyophilization is recommended for most of the domains in food industry, especially in meat, milk, canned goods industry and groups of food that cannot be preserved through other methods (dairy products, eggs whites, enzimes, lactic bacterias) or food products destined to small children.

Bean curd

Green bean

Garlic

Scallion

Red pepper

Beef

Crab meat

Chicken meat

Pork

Corn

Dried shrimp

Ham

Carrot

Mushroom

Goloden needle

mushroom

Figure 3.4 Lyophilization products

The process is used for drying and preserving a number of food products, including meats,

vegetables, fruits, and instant coffee products. The dried product will be the same size and shape as the original frozen material and will be found to have excellent stability and convenient reconstitution when placed in water. Freeze-dried products will maintain nutrients, colour, flavour, and texture often indistinguishable from the original product.

25


Prunes lyophilization (freeze-drying) consists of freezing below -30C and later removing the ice formed by sublimation in a chamber with controlled temperature under vacuum. The water vapor produced is removed by condensation at temperatures around -60C. Different lyophilised prunes derived from many kind of these have been developed and can be used to improve existing method, create new ones, or to extract compounds of interest in a fruits matrix almost free from water. The know-how on lyophilization and the problems during the process have been acquired by using pilot equipment. The optimization of the process can be also addressed by calculating the curves of weight loss during drying and the electrical consumption. Quality and safety parameters in fresh and lyophilised prunes can be also determined compared with the fresh prunes. The main fruits are freeze-dried samples obtained from a near isogenic line collection. This collection rendered products with different taste, flavor, color, etc.

It made some comparisons between the prunes resulted through heating using the roaster (shown in figure 1) with those after lyophilization.

Lyophilised (freeze-dried) prunes are with long shelf life under storage temperatures above the regular used for freezing. The manufacture of lyophilised prunes requires knowledge about the optimum conditions of processing in each fruit including the right format for any of them. Usually, lyophilization, a non-thermal treatment, preserves most of the nutrients and aroma compounds from the original prunes, particularly when compared with other treatments. Vacuum is a critical step of the process in order to avoid melting ice. Optimization of freeze-drying requires a pilot plant and knowledge on the curves of water loss from the product, though this information is not easy to find in fruits. In the laboratory the research group has experience in many fruits. Also there is a pilot freeze-dryer with heated trays able to condense up to 18 kg ice. With this equipment the weight loss of the products can be modeled in order to select the optimum conditions of the process and to know the electrical consumption of the operation. It is interested in contacting companies able to optimized lyophilization conditions to be used later for developing new fruits. Quality parameters from all the dried prunes produced can be determined compared with the quality of fresh product in the laboratories.

a. b.

Figure 35. - Dried prunes: a.- lyophilizated; b. by heating (roaster).

It can realize o lot of researches about the prunes behavior in different conditions. For example X-, K- and Q-band EPR (Electron paramagnetic resonance) studies on lyophilized whole pulp parts of prunes before and after Xray irradiation are reported. Before irradiation show in X band a weak singlet EPR line with g=2.00300.0005. Immediately after irradiation all samples exhibit complex fruit-depending spectra, which decay with time and change to give, in ca. 50 days, an asymmetric singlet EPR line with g=2.00410.0005. Singlet EPR lines recorded after irradiation in X -band are K- and Q-band resolved as typical anisotropic EPR spectra with g =2.00230.0003|| and g =2.00410.0005. In addition, K- and Q-band EPR spectra of all samples show a superposition with the six EPR lines of Mn2+ naturally present in the prunes. The differences in g factors of samples before and after X-ray irradiation might be used for the identification of radiation processing of fruits in the case of prunes pulp .

The degradation of vitamin C [ascorbic acid] in prunes as affected by processing temperature and moisture content of samples (0.05, 0.11, 0.18, 0.38 or 1.40 g water/g dry

26


matter) was modeled using the Bigelow equation. Prunes were freeze-dried and conditioned in desiccators with various salt solutions at 4C. After reaching equilibrium, the temperature of the desiccators was adjusted to 40-80C. Vitamin C degradation was analyzed for 5 days by titration method using a solution of 0.01% 2,6-dichlorophenolindophenol. The results showed that the samples subjected to 40-60C exhibited slower degradation of vitamin C than those subjected to higher temperatures. The parameters z-value and DT showed a linear and quadratic dependence, respectively, with the moisture content of the samples. In general, vitamin C degradation was faster in samples with higher moisture content, due to the high solubility of vitamin C.

As a member of the largest freeze drying group in the world, European Freeze Dry offers an extensive range of high quality freeze dried ingredients. These are ideal for inclusion in dried ready meals, pot snacks, dried soups, breakfast cereals and bars, drinks, dairy items, confectionery and bakery products.

The freeze dried meat, seafood and vegetable products are perfect for dried savory applications such as instant pot and sachet products. With rapid rehydration within 3 to 5 minutes in boiling water they meet customer expectations of "instant" food. When the customer then tastes the freeze dried ingredients, they will recognize all the textural and flavor attributes of a freshly cooked product.

It can also offer an extensive range of freeze dried culinary herbs. Chives and culinary herbs have wide application possibilities. Examples of use are dressings and dips, soups and sauces, dairy product applications and dry meals based on rice, potatoes and pasta. For catering and retail purposes, herbs and herb mixes give both colour and taste in restaurants as well as in private households.

For manufacturers of breakfast cereals and bars, bakery ingredient suppliers, cake manufacturers and confectioners we present a wide range of freeze dried fruits. These products are natural, wholesome, and contain no artificial additives or added sugar. Our fruit range includes both berries and orchard fruits - please check our product list to see the range available. Alternatively, if you can't see what you require, contact us, and we will be happy to discuss your requirements.

Here at European Freeze Dry we take pride in the service that we offer to all our customers. Whether your requirement is for small quantities or for large scale manufacture, we have the flexibility to fulfil your order promptly ands efficiently.

A selection from our product list is shown below. Please remember to contact us if you can't see what you need.

Vegetables and Pulses Meats Bacon, diced various dimensions Beef, diced, strips, mince Chicken Powder. Pure chicken meat. Chicken Pressed, diced to various

dimensions Chicken, Whole muscle breast meat

(no additives) Ham, diced various dimensions

Seafood

Alaskan Pollock, diced (uncooked) Arctic Prawns cooked and peeled Clams / Vongole COD, diced (uncooked) Hoki, diced (uncooked) Langostino Mussels

Broccoli, beads Courgette Grilled Mixed Bell Pepper Kidney Beans, red Leek Lentils, brown Olives, sliced, wedges or granulated Peas, whole and crushed Red Bell Peppers, diced Spinach Sun Dried Tomato, granules Sweetcorn

Herbs

Basil Chervil Chives

27


Dill Green Peppercorns Juniper berries Lemon Grass Marjoram Mint Oregano Parsley Rosemary Sage Savory Tarragon Thyme

*Blends of the above herbs are available upon request

Fruits and Berries

Bananas, sliced or diced Bilberries Blackberries Blackcurrants, whole or powder Blueberries, whole or powder Cherries, whole, sliced or diced Cranberries, sliced Fig Pieces Freeze Dried Fruit Blends Grape slices Lingonberries, whole or powder Mango, diced Pineapple, diced Pomegranate Kernels Raspberries, whole, pieces or

crumble Redcurrants Strawberries, whole, sliced, powder

or diced *Kosher available upon request

Smoked Salmon Surimi (Crabstick) Tuna Warm water shrimps

*All of the above are also available in powdered form

Dairy and Egg Cheddar Cheese Mozzarella Cheese Omelette Red Leicester Cheese Ricotta Cheese Stilton Cheese

Foods for space: Mercury astronauts following John Glenn were forced to endure bite-sized

cubes, freeze dried foods, and semi-liquids in aluminum toothpaste-type tubes. They found the food unappetizing, had trouble rehydrating the freeze-dried foods, and disliked squeezing the tubes.

In the Gemini missions eating in space became more normal. In the Apollo program, food packages were similar to those used on Gemini missions but the

variety of food was considerably greater. Apollo astronauts had the added luxury of heated water for hot drinks and foods at a temperature of 67 degrees C (154 degrees F) and chilled water at 7 degrees C (45 degrees F). Water temperatures from the dispenser of the Gemini spacecraft hovered at the 21 degrees C (70 degrees F) ambient spacecraft temperature. With hot water available, food was easier to rehydrate and much improved in taste.

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In 1973 and 1974, the Skylab spacecraft was occupied by three teams of astronauts. Space food systems there were much improved over systems used in Apollo, Gemini, and Mercury. Unlike previous space vehicles for astronauts, Skylab featured a large interior volume and space was available for a dining room table. The table was a pedestal where food trays were mounted. When dining, the three-astronaut teams would "sit down" in the air by means of foot and thigh restraints and eat in an almost normal fashion. The food trays not only held the food in place but also served as warming devices. Underneath three of eight cavities in the trays were warmers that could raise temperatures of foods needing heating to 66 degrees C

In 1975, the last Apollo flights took place with the Apollo-Soyuz docking mission. The Apollo spacecraft did not have the freezer that Skylab featured but many of the food advances from Skylab and the earlier Apollo missions were incorporated. Because of the short duration of the flight (nine days), many short shelf-life items were added to the foods carried. Fresh breads and cheese were included as a part of 80 different varieties of food dined upon by the Apollo while others were placed in spoon-bowl packages or plastic drinking bags. To make eating easier, a food tray was carried on the mission. The tray did not warm the food as the Skylab tray did, but it held the food in place with springs and Velcro fasteners. The tray was secured to the crewmember's leg during meal time.

The process of lyophilization consists of: - freezing the food so that the water in the food becomes ice; - under a vacuum, sublimating the ice directly into water vapor; - drawing off the water vapor; - once the ice is sublimated, the foods are freeze-dried and can be removed from the machine; - lyophilization has many advantages compared to other drying and preserving techniques.

Lyophilization maintains food quality. The use of lyophilization is particularly important when processing lactic bacteria, because

these products are easily affected by heat. Foods, which are lyophilized, can usually be stored without refrigeration, which results in a

significant reduction of storage and transportation costs. Lyophilization greatly reduces weight, and this makes the products easier to transport. For

example, many foods contain as much as 90% water. These foods are 10 times lighter after lyophilization.

Because they are porous, most freeze-dried foods can be easily rehydrated. Lyophilization does not significantly reduce volume; therefore water quickly regains its place in the molecular structure of the food.

Ratti and Crapiste (1992) developed a lumped parameter model for hygroscopic shrinking food systems.

Lyophilization, is like suspended animation for food. It can store a freeze-dried meal for years and years, and then, when that is finally ready to eat it, it can completely revitalize it with a little hot water. Even after all those years, the taste and texture will be pretty much the same. It can to explore the basic idea behind freeze-drying, and that can look at the different steps involved in the process. Also it can see how freeze drying is different from ordinary dehydration, and it find out.

Figure 3.7. Ice cream after freeze dried and rehydratation process

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A package of freeze-dried ice cream, sold as a novelty item. The process has been popularized in the forms o

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