We all know that food is a basic need for our living and now a day we often hear and observe famine all around the world. The word famine in the above sentence means low production of hygienically produced foods and vitamin foods .we all know that it is useful to build and maintain our body and provide them useful vitamin and carbohydrates etcÂ¦.
Retail presentation of preserved foodstuffs is an ancient art. Freeze drying is a modern method, which has come in to the role for preserving food. Freeze drying in general is a process of freezing the foodstuff and then drying it to get rid off the moisture. In this process drying means changing of ice in to vapor with out being melted and further that atmospheric pressure must be vaccumised in special machines, all of which makes the process slow, elaborate and some what expensive, but the resultant quality, the water is restored perhaps many years later, makes it all worth while.
Freeze drying is an application of physics and chemistry. The quality then is a measure of the care we have taken and the cost rests on the engineering skill and the benefits of scale. Now a day many industries have come forward in this field to meet the living needs.
WHY FREEZE DRY
WHAT IS FREEZE DRYING
HISTORICAL BACKGROUND OF FREEZE DRYING
DIFFERENT PHASES OF FREEZE DRYING
PART OF A SIMPLE FREEZE DRYING MACHINE
FREEZE DRYING PROCESS
IMPORTANT FUNCTIONARY PARTS
WORKING OF FREEZE DRYING MACHINE
ADVANTAGES AND DISADVANTAGES
LIST OF FIGURES
"ASTRONAUT ICE CREAM," THE CLASSIC FREEZE-DRIED
A FREEZE-DRIED MEAL
DIFFERENT PHASE CHANGES OF WATER
A SIMPLIFIED FREEZE-DRYING MACHINE
"Astronaut ice cream," the classic freeze-dried treat for kids
Freeze-drying, or lyophilization, is like "suspended animation" for food. You can store a freeze-dried meal for years and years, and then, when you're finally ready to eat it, you can completely revitalize it with a little hot water. Even after all those years, the taste and texture will be pretty much the same. That's some trick!
In this article, we'll explore the basic idea behind freeze-drying, and we'll look at the different steps involved in the process. We'll also see how freeze-drying is different from ordinary dehydration, and we'll find out about some of its important applications.
The basic idea of freeze-drying is to completely remove water from some material, such as food, while leaving the basic structure and composition of the material intact. There are two reasons someone might want to do this with food:
Removing water keeps food from spoiling for a long period of time. Food spoils when microorganisms, such as bacteria, feed on the matter and decompose it. Bacteria may release chemicals that cause disease, or they may just release chemicals that make food taste bad. Additionally, naturally occurring enzymes in food can react with oxygen to cause spoiling and ripening.
Like people, microorganisms need water to survive, so if you remove water from food, it won't spoil. Enzymes also need water to react with food, so dehydrating food will also stop ripening.
Freeze-drying significantly reduces the total weight of the food. Most food is largely made up of water (many fruits are more than 80 to 90 percent water, in fact). Removing this water makes the food a lot lighter, which means it's easier to transport. The military and camping supply companies freeze-dry foods to make them easier for one person to carry. NASA has also freeze-dried foods for the cramped quarters onboard spacecraft.
A freeze-dried meal of spaghetti and meatballs, designed for campers: On the left is the dried version; on the right is the dehydrated version.
People also use freeze-drying to preserve other sorts of material, such as pharmaceuticals. Many pharmaceuticals will degrade pretty quickly when exposed to water and air, for the same basic reason that food degrades. Chemists can greatly extend pharmaceutical shelf life by freeze-drying the material and storing it in a container free of oxygen and water. Similarly, research scientists may use freeze-drying to preserve biological samples for long periods of time. Freeze-dried biological samples are also big in the florist world, oddly enough. Freeze-dried roses are growing in popularity as wedding decorations. The freeze-drying process has also been used to restore water-damaged materials, such as rare and valuable manuscripts.
Photo courtesy NASA
Freeze-dried foods have been a staple onboard many of NASA's space missions.
It's pretty simple to dry food, drugs and just about any other biological material. Set it out in a hot, arid area, and the liquid water inside will evaporate: The heat gives the water molecules enough energy to "break free" of the liquid and become gas particles. Then you seal it in a container, and it stays dry. This is how manufacturers make dehydrated meals like powdered soup and baking mixes.
There are two big problems with this approach. First, it's difficult to remove water completely using evaporation because most of the water isn't directly exposed to air. Generally, dehydrating food in this way only removes 90 to 95 percent of the water, which will certainly slow down bacteria and enzyme activity, but won't stop it completely.
Secondly, the heat involved in the evaporation process significantly changes the shape, texture and composition of the material, in the same way that heat in an oven changes food. Heat energy facilitates chemical reactions in the food that change its overall form, taste, smell or appearance. This is the fundamental purpose of cooking. These changes can be good, if they make the food taste better (or taste good in a different way), but if you're drying something so you can revitalize it later, the process compromises quality somewhat.
The basic idea of freeze-drying is to "lock in" the composition and structure of the material by drying it without applying the heat necessary for the evaporation process. Instead, the freeze-drying process converts solid water -- ice -- directly into water vapor, skipping the liquid phase entirely. In the next section, we'll find out how freeze-drying machines pull this off.
What is freeze Drying
Lyophilization, commonly referred to as freeze drying, is the process of removing water from a product by sublimation and desorption. This process is performed in lyophilization equipment which consists of a drying chamber with temperature controlled shelves, a condenser to trap water removed from the product, a cooling system to supply refrigerant to the shelves and condenser, and a vacuum system to reduce the pressure in the chamber and condenser to facilitate the drying process.
Lyophilizers can be supplied in a wide variety of sizes and configurations and can be equipped with options that allow system controls to range from fully manual to completely automated. For pharmaceutical compounds that undergo hydraulic degradation, lyophilization offers a means of improving their stability and shelf life. Many parenteral medications such as vaccines, proteins, peptides, and antibiotics have been successfully lyophilized. New biotechnology products will also increase the demand for freeze drying equipment and processes.
Early attempts at lyophilization were largely empirical in nature because the process variables were not thoroughly understood. However, much of the "black magic" of freeze drying has been replaced through basic research over the last twenty years. Lyophilization equipment and control mechanisms continue to evolve, based on scientific evaluation of thermal, physical and chemical data derived from freeze drying cycles and products.
Lyophilization cycles consist of three phases: Freezing, primary drying, and secondary drying. Conditions in the dryer are varied through the cycle to insure that the resulting product has the desired physical and chemical properties, and that the required stability is achieved.
During the freezing phase, the goal is to freeze the mobile water of the product. Significant super cooling may be encountered, so the product temperature may have to be much lower than the actual freezing point of the solution before freezing occurs. The rate of cooling will influence the structure of the frozen matrix. If the water freezes quickly, the ice crystals will be small. This may cause a finer pore structure in the product with higher resistance to flow of water vapor and longer primary drying time. If freezing is slower, ice crystals will grow from the cooling surface and may be larger. The resultant product may have coarser pore structure and perhaps a shorter primary drying time.
The method of cooling will also effect the structure and appearance of the matrix and final product. If the solution is frozen in vials on the cooled shelf, ice will grow from the bottom of the vial toward the top, while immersion in a cooling fluid will cause crystal growth from the bottom and sides of the vial. Because some materials form glassy layers, cooling conditions must be controlled to avoid the formation of the dense "skin" on the surface of the frozen product that may impede the escape of water vapor during subsequent drying phases.
A term that is frequently encountered' in discussions about freeze drying is eutectic point. On a phase diagram, this is the temperature and composition coordinate below which only the solid phase exists.
It should be understood that, depending on the composition of the solution, there may be more than one eutectic point for a product or none at all. During the freezing phase, the product must be cooled to a temperature below its lowest eutectic point. This temperature may then be maintained throughout the primary drying phase.
It should be noted that products will not necessarily have a eutectic point. For products with components that do not crystallize during freezing, drying should be performed at temperatures below the glass transition temperature of amorphous phase (multicomponent mixture). The glass transition temperature will be determined by the composition of the amorphous phase in the frozen product, which, in turn, is dictated by the product formulation and the freezing procedure employed. Mannitol and some other compounds can exist as an amorphous phase or exhibit a crystalline phase depending upon its thermal history.
In the primary drying phase, the chamber pressure is reduced, and heat is applied to the product to cause the frozen mobile water to sublime. The water vapor is collected on the surface of a condenser. The condenser must have sufficient surface area and cooling capacity to hold all of the sublimed water from the batch at a temperature lower than the product temperature. If the temperature of the ice on the condenser is warmer than the product, water vapor will tend to move toward the product, and drying will stop.
It is important to control the drying rate and the heating rate during this phase. If the drying proceeds to rapidly, the dried product can be blown out of the container by escaping water vapor. If the product is heated too rapidly, it will melt or collapse. This may cause degradation of the product, and will certainly change the physical characteristics of the dried material, making it visually unappealing and harder to reconstitute. While frozen mobile water is present, the product must be held below the eutectic temperature or glass transition temperature.
As water sublimes, the product cools. Therefore, throughout this phase, the product will remain colder than the shelf temperature which is supplying the heat of sublimation. At the end of primary drying, the product temperature will rise asymptotically toward the shelf temperature. This and several other methods may also be used to detect the endpoint of primary drying. Many modern drying cycles use chamber pressure control to control drying rate. At very low pressures, the main form of heat transfer is conduction from the shelf through the bottom of the product container. Since glass is an insulator, this process is not very efficient, and drying can be slow. To improve the heat transfer mechanism, inert gas such as nitrogen may be introduced into the drying chamber at a controlled rate. The presence of these gas molecules facilitates heating of the walls of the container in addition to conduction through the bottom of the container, thereby increasing the amount of heat being supplied to the product per unit time. This will enhance the drying rate, reduce the cycle time, and reduce energy and labor costs associated with a lengthy process.
However, if the pressure in the chamber exceeds the ice vapor pressure of the product, water may not be able to sublime. All of the energy from the heat source will be used to increase the product temperature until melting occurs. Therefore, the accuracy and precision of the pressure control system are critical to successful lyophilization.
Since there is no mobile water in the product at the end of primary drying, the shelf temperature may be increased without causing melting. Therefore, temperature is increased to desorb bound water such as water of crystallization until the residual water content falls to the range required for optimum product stability. This phase is referred to as secondary drying, and is usually performed at the maximum vacuum the dryer can achieve, although there are products that benefit from increased pressures, too. Be careful not to increase product temperatures too fast so as not to exceed the glass transition of some products. Products containing 10% or less water can still collapse if the tg1 is exceeded.
tg1: glass transition temperature
The length of the secondary drying phase will be determined by the product. Many products such as proteins and peptides require water to maintain secondary and tertiary structure. If this water is removed, the material may be denatured and lose some or all of its activity. In this case, water content must be carefully controlled. In addition, excessive heat may cause the dry cake to char or shrink.
Lyophilization equipment has improved over the years, and, with the advent of automated, sophisticated control mechanisms, has become much easier to use. The complexity of the controls, however, has made validation efforts more complicated and usually quite time consuming.
In addition to cooling, heating, and vacuum control functions already discussed, many freeze dryers will also incorporate clean in place (CIP), sterilize in place (SIP), computerized cycle control, and cycle monitoring functions. The reliable reproducible performance of all these functions must be validated rigorously to insure consistent product quality.
The role of the Aberdeen Experimental Factory
To say about the historical background, the second half of this century has been considerable emphasis placed on foods that are quick and easy to prepare, whether the feeding pattern was in famine relief, on in combat zones, or in highly urbanized societies. Tastiness, whole-someness, variety of diet, reasonable cost, with no waste and with ready availability, outline the consumer demand to which industry has responded by increasing preparation within the factory of top quality ingredients and materials and stressing subsequent convenience at the point of consumption. As with all forms of food presentation, assurance had to be inherent of freed from bacterial spoilage and off putting enzymatic changes while flavour and mouthfeel, original or acquired had to be acceptable and enticing. But at this time freeze drying came near to being dismissed out of hand as inordinately expensive by the food industry, not the general public. To the entrepreneur processor, unaided capital outlay appeared too high for this process, which for a start had a long production time. Freeze-drying of foodstuffs, in general much cheaper than for pharmaceutical products and therefore a less able to bear a high value, added cost, was thoroughly researched and developed. The advantages made it very attractive, technically. The whole process is conducted at relatively low temperature; there is no shrinkage as with hot - air drying, so the light and porous pieces retain their shapes and dimensions. It was soluble solids of infused beverages which brought freeze drying lack out of the cold. In bringing freeze drying lack on role Aberdeen experimental factory helped a lot. The factory helped many of the food industries by providing them knowledge of new freeze drying techniques and gave many practical and teory classes on this by sending their scientists to many industries all over world.
The Spread of Activity
Freeze drying came into existence in the year 1960. It was extremely hard for freeze drying to come back on role. The simple message that accelerated freeze-drying brought, that a process previously thought of in terms of 48 hours or so and now capable of drying 15mm thickness in less than an 8 hour shift made the come back easy. Many of the industries like Atlas freeze drying MAFF, SACAF etc., developed freeze drying techniques and they even helped many countries to start freeze drying process. Atlas in 1961 built six production AFD cabinets for Nestle group in Germany, Holland. The MAFF experimental research centre made samples of freeze dried cod steaks were stacked, neatly without much movements in suitable round tin canisters and distributed to partially interested processors by the ministry. Amongst these were pet foods, which ran cat acceptability tests in March 1959. Favourable reports came from owners who expressed a preference.
Some owners did not cook the samples, merely reconstituting them in cold water to the intended 50 ounces 42g, a matter of a moment or two yet to the cat the temperature or rawness seemed to make no difference.
DIFFERENT PHASES OF FREEZE DRYING
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. 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.
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. The chart below shows the necessary pressure and temperature values of different phases of water.
DIFFERENT PHASE CHANGES OF WATER
You can see from the chart that 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 if you 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.
PARTS OF A SIMPLE FREEZE DRYING MACHINE
This is exactly what a freeze-drying machine does. A typical machine consists of a freeze-drying chamber with several shelves attached to heating units, a freezing coil connected to a refrigerator compressor, and a vacuum pump.
A simplified freeze-drying machine
With most machines, you place the material to be preserved onto the shelves when it is still unfrozen. When you seal the chamber and begin the process, the machine runs the compressors to lower the temperature in the chamber. The material is frozen solid, which separates the water from everything around it, on a molecular level, even though the water is still present.
Next, the machine turns on the vacuum pump to force air out of the chamber, lowering the atmospheric pressure below .06 ATM. The heating units apply a small amount of heat to the shelves, causing the ice to change phase. Since the pressure is so low, the ice turns directly into water vapor. The water vapor flows out of the freeze-drying chamber, past the freezing coil. The water vapor condenses onto the freezing coil in solid ice form, in the same way water condenses as frost on a cold day.
This continues for many hours (even days) while the material gradually dries out. The process takes so long because overheating the material can significantly change the composition and structure. Additionally, accelerating the sublimation process could produce more water vapor in a period of time then the pumping system can remove from the chamber. This could rehydrate the material somewhat, degrading its quality.
Once the material is dried sufficiently, it's sealed in a moisture-free package, often with an oxygen-absorbing material. As long as the package is secure, the material can sit on a shelf for years and years without degrading, until it's restored to its original form with a bit of water (a very small amount of moisture remains, so the material will eventually spoil). If everything works correctly, the material will go through the entire process almost completely unscathed!
THE FREEZE DRYING PROCESS
Aspects of Food Stuffs
The freezing and heating depends on the aspects of food stuff i.e. it depends on the water contents in its cell wall structure. When a substance was freeze dried it was easy to pack the substance in a double coated polythene pack and more quantity product can be packed in that pack and preserved. The packing is done with the help of nitrogen gas. E.g. Chicken and prawns were freezed to 20 'c and dried to O'c for 21 hr. cabinet pressure ranged between 0.8 and 0.5 mm Hg vaccum release with N2 gave half an hour comfortably to pack the product.
As we all know that when a substance is freezed some amount of the water content gets vaporized and sticks on to the chamber surface as we have seen in the ordinary freezers. Freezing is done in a temperature ranging from -5Ã‚Â°C to -40Ã‚Â°C for ordinary food stuffs. Freezing is done with the help of absorption cycle and instead of compressor generators are used. This process is the first step and this is similar to the ordinary refrigeration system.
In this process the heating is done electrically but heater is set on only when chamber is vaccumised. The heating electrodes are fixed within the shelves on which the material is placed. Heating temperature is kept just below 0Ã‚Â°C with the help of thermostat. The vaccum gauges and the temperature indicators are provided on the chamber. If 1 kg of a substance is freeze dried its weight reduces to less than 1/4 kg of finished product i.e. the water content in each piece will be approximately less than 3%.To find the percentage of moisture content in the product we use the formulae MC % = Mass of moisture capable of being driven of reversibly x 100 Mass of bone - dry sample
Reduced water activity arrests the forces of natural decay in many ways. Micro organisms are successively inhibited from growth. Bacteria effect the food stuff if the food is kept open in room having room temperature just above 0.6 AW of room temperature. Physical Considerations
As we all know that when an item of food is freezed The water molecules in gets freezed and these makes the food hard which makes the consumer feel unsatisfied with the taste and softness in eating. Similarly when dried the water molecules get vaporized and the food material changes even in its shape, smell and taste. When freeze dried their is no vast changes in its size, shape, smell, texture, aroma, colour, nutrition and freshness.
Important functionary parts
To say about the functional parts it consists of a heating system, refrigeration devices, Vaccum pumps, pressure gauges, thermostats, double layered insulated chamber, car loaded trays, hydraulic lids etc.
It is optional to use electricity for heating or steam ejectors and circulating system. Mainly electrically heating is used in general. Heating electrodes are attached with the shelves to be heated on which the product is kept. The heating electrodes are coated with thick polythene in order to prevent fire in the chamber.
It is similar to the ordinary refrigerators. It works with absorption cycle. In the cycle water condenser are mainly used, ice condenser can also be used. Refrigeration system can be placed outside and inside the chamber. The refrigerant mainly used is NH3.This is mainly used because it has boiling point 33.33 Ã‚Â° C. So we can cool the foods to a low temperature with out reducing pressure below atmospheric pressure. The vapour absorption system is used because it has increased efficiency at partial loads. It uses heat energy in order to change the state of refrigerant, which is the main difference from compression system. If there is any leakage of refrigerant occurs it can easily detected since NH3 is used.
WORKING OF FREEZE DRYING MACHINE
The sliced material is placed into the car loaded trays and then the material is freezed to the temperature required after closing the lid of the chamber and then the chamber is completely vaccumised and the heaters are on. As the vapour in the chamber increases the vaccum pump is arranged in such a way that it functions and it maintains vaccum in the chamber. Thermostats are provided for the heater and freeze so that their functioning is regulated. When the freezing temperature reduces the heater functions, the heater cuts of its function and the freezer starts functioning. This process is continued for 24hr and after this the vaccum is released and with the help of N2 gas the product is packed in a double coated polythene cover.
Advantages of Freeze Drying
1. Preserve its colour, Aroma, Taste, Shape
2. Transportation doesnâ„¢t require freezers
3. Last long
4. Packing is easy
5. Provide high quality product
1. Very costlier process
2. Fire hazards if the chamber is not in proper vacuum, while heater is on
1. In pharmaceutical companies
2. Food industries
3. In flower industries
4. For protecting old books
This process is much cheaper for pharmaceuticals. Cost mainly depends on engineering skills and techniques. The quality of the products is very high.
Â¢ FREEZE DRYING FOR FOOD PRODUCTS
By J.M C N. DALGLEISH
Â¢ WWW://FREEZE -DRYERS.COM
Â¢ WWW://LOUGHBOROUGH UNIVERSITY.COM