freeze drying machine

Freeze Drying Seminar: 2013

ABSTRACT

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 etc….

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 without

being melted and further that atmospheric pressure must be vaccumised in special

machines, all of which makes the process slow, elaborate and somewhat expensive, but

the resultant quality, the water is restored perhaps many years later, makes it all

worthwhile.

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.

Dept: Of Mechanical Eng 1 PKCET, Kandala


CONTENTS

INTRODUCTION……………………………………………………………….4

WHY FREEZE DRY……………………………………………………………5

WHAT IS FREEZE DRYING…………………………………………………10

HISTORICAL BACKGROUND OF FREEZE DRYING……………………..15

DIFFERENT PHASES OF FREEZE DRYING……………………………….17

PART OF A SIMPLE FREEZE DRYING MACHINE………………………..20

FREEZE DRYING PROCESS………………………………………………...26

IMPORTANT FUNCTIONARY PARTS……………………………………..28

WORKING OF FREEZE DRYING MACHINE………………………………29

ADVANTAGES………………………………………………………………..30

DISADVANTAGES…………………………………………………………...31

CONCLUSION………………………………………………………………...33

REFERENCE…………………………………………………………………..34



LIST OF FIGURES

"ASTRONAUT ICE CREAM," THE CLASSIC FREEZE-DRIED

A FREEZE-DRIED MEAL

DIFFERENT PHASE CHANGES OF WATER

A SIMPLIFIED FREEZE-DRYING MACHINE

LABORATORY SCALE FREEZE-DRYER



INTRODUCTION

"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.



Why Freeze-Dry?

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.


http://home.howstuffworks.com/question168.htm


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.

Freeze-dried coffee, a form of instant coffee


http://en.wikipedia.org/wiki/Instant_coffee


Freeze dried bacon bars

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.


http://en.wikipedia.org/wiki/Bacon


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

(multi component 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.



HISTRORICAL BACKGROUND

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 flavor and mouth feel, 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 comeback 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. Favorable 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!



There are essentially three categories of freeze-dryers: the manifold freeze-dryer, the

rotary freeze-dryer and the tray style freeze-dryer. Two components are common to all

types of freeze-dryers: a vacuum pump to reduce the ambient gas pressure in a vessel

containing the substance to be dried and a condenser to remove the moisture by

condensation on a surface cooled to −40 to −80 °C (−40 to −112 °F). The manifold,

rotary and tray type freeze-dryers differ in the method by which the dried substance is

interfaced with a condenser. In manifold freeze-dryers a short usually circular tube is

used to connect multiple containers with the dried product to a condenser. The rotary

and tray freeze-dryers have a single large reservoir for the dried substance.

Rotary freeze-dryers are usually used for drying pellets, cubes and other pourable

substances. The rotary dryers have a cylindrical reservoir that is rotated during drying to

achieve a more uniform drying throughout the substance. Tray style freeze-dryers

usually have rectangular reservoir with shelves on which products, such as

pharmaceutical solutions and tissue extracts, can be placed in trays, vials and other

containers.

Manifold freeze-dryers are usually used in a laboratory setting when drying liquid

substances in small containers and when the product will be used in a short period of

time. A manifold dryer will dry the product to less than 5% moisture content. Without

heat, only primary drying (removal of the unbound water) can be achieved. A heater

must be added for secondary drying, which will remove the bound water and will

produce a lower moisture content.

Tray style freeze-dryers are typically larger than the manifold dryers and are more

sophisticated. Tray style freeze-dryers are used to dry a variety of materials. A tray

freeze-dryer is used to produce the driest product for long-term storage. A tray freeze-


http://en.wikipedia.org/wiki/Biological_tissue


dryer allows the product to be frozen in place and performs both primary (unbound

water removal) and secondary (bound water removal) freeze-drying, thus producing the

driest possible end-product. Tray freeze-dryers can dry products in bulk or in vials or

other containers. When drying in vials, the freeze-dryer is supplied with a stoppering

mechanism that allows a stopper to be pressed into place, sealing the vial before it is

exposed to the atmosphere. This is used for long-term storage, such as vaccines.

Improved freeze drying techniques are being developed to extend the range of products

that can be freeze dried, to improve the quality of the product, and to produce the

product faster with less labor.



LABORATORY SCALE FREEZE-DRYER (Lyo Star 2)



Standard LyoStar IITM Features at a Glance

• Powerful reliable Scroll compressors with 5.5 total

horsepower for fast cooling rates

• 4" Vapor port between product chamber and condenser

ensures no vapor restriction

• Corrosion resistant, 316L chamber construction increases

system longevity

• Stainless steel coil condenser design improves

lyophilization efficiency

• Additional chamber ports standard for accessories/

options, facilitate field retrofits

• Front panel access to vacuum pump and electrical

components increases ease of maintenance



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 vacuum release with N2 gave half an hour

comfortably to pack the product.

Freezing Process

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 -5°C to -40°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.

Drying process



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 0°C with the help of

thermostat. The vacuum 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.

Heating systems

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.

Refrigeration system

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 ° 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 vapor in the chamber

increases the vacuum pump is arranged in such a way that it functions and it maintains

vacuum 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 vacuum 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 doesn’t require freezers

3. Last long

4. Packing is easy

5. Provide high quality product



Disadvantages

1. Very costlier process

2. Fire hazards if the chamber is not in proper vacuum, while heater is on.



USES

1. In pharmaceutical companies

2. Food industries

3. In flower industries

4. For protecting old books



CONCLUSION

This process is much cheaper for pharmaceuticals. Cost mainly depends on

engineering skills and techniques. The quality of the products is very high.



REFERENCES

FREEZE DRYING FOR FOOD PRODUCTS

By J.M C N. DALGLEISH

WWW.HOWSTUFFSWORKS.COM

WWW.FREEZE -DRYERS.COM

WWW.LOUGHBOROUGH UNIVERSITY.COM

WWW.PUBMED.COM


http://WWW.PUBMED.COM/

http://WWW.LOUGHBOROUGH/

http://WWW.HOWSTUFFSWORKS.COM/

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