Basics of drying

1. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Drying
Drying: Drying may be defined as the simultaneous heat and mass transfer process where the
water content becomes reduced that the products can store for a long time and the seed remains its
viability.
In Other words, Drying is the simple process of dehydrating foods until there is not enough
moisture to support microbial activity. Drying removes the water needed by bacteria, yeasts, and
molds need to grow. If adequately dried and properly stored, dehydrated foods are shelf-stable
(safe for storage at room temperature).
o The separation operation of drying converts a solid, semi-solid, or suspension into a solid
product by evaporation of the liquid into a vapor phase via the application of heat.
o This definition excludes conversion of a liquid phase into a concentrated liquid phase
(evaporation), mechanical dewatering operations such as filtration, centrifugation,
sedimentation, supercritical extraction of water from gels to produce extremely high
porosity aerogels (extraction), or so-called drying of liquids and gases by use of molecular
sieves (adsorption).
o Phase change and production of a solid phase as an end product are essential features of
the drying process.
o Heat may be supplied by convection (direct dryers), by conduction (contact or indirect
dryers), radiation, or volumetrically by placing the wet material in a microwave or
radiofrequency electromagnetic field.
o All modes except the dielectric (microwave and radiofrequency) supply heat at the
boundaries of the drying object so that the heat must diffuse into the solid primarily by
conduction.
o The liquid must travel to the boundary of the material before it is transported away by the
carrier gas (or by application of vacuum for non-convective dryers).

2. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
So, in drying there two events happened
1. Heat transfers the materials to being dry, Mainly the heat energy pass to the
products from hot air. (Freeze drying is exceptional).
2. Moisture transfer from the products to the atmosphere or the surrounding.
Importance of Drying:
1-To avoid or eliminate moisture which may lead to corrosion and decrease the product stability.
2-To improve or keep the good properties of a material, e.g., flowability, compressibility.
3-To reduce the cost of transportation of large volume materials (liquids)
4-To make the material easy or more suitable for handling.
5- To increase shelf life
6- Stop chemical and enzymatic activity
7- Stop biological activity
8- Increase storage quality
9. Enhances long time storage of foods without deterioration
10. Enhances continuous supply of product throughout the year

3. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
11. Reduces the post-harvest losses and increases the utilization of food materials even in off
seasons.
12. Reconstitution of dried material by additional water is possible.
13. Enhances the value addition of food materials and hence the quality of the food product is
improved.
Driving Force of Drying
Diving Force means the factor which drives the rate of the drying process. For example, if we want
to get a good CGPA it should depend upon some of our characteristics such as How much time I
spend my time in sturdy. How attentive I am in class. Performance of in the examination. That
means our attentiveness in the study would drive our result. In Drying, there are also some Driving
force
➢ Differences in Temperature (The higher the temperature, the faster the process)
➢ Differences in Moisture Content. (The higher the moisture content lowers the drying
Process)
➢ Differences in Concentration. (Lower the concentration, Faster the process)
➢ Differences in Total Pressure
➢ Differences in Partial Pressure
The total pressure of a mixture of gases can be defined as the sum of the pressures of each gas:
P total=P1+P2+…+Pn
The partial pressure of an individual gas is equal to the total pressure multiplied by the mole
fraction of that gas.
Boyle’s Law and the Ideal Gas Law tell us the total pressure of a mixture depends solely on
the number of moles of gas, and not the kinds of molecules; Dalton’s Law allows us to calculate
the total pressure in a system from each gas’ contribution.

4. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Parameter influencing the drying process
Some parameters impact on drying process such as
✓ Temperature
✓ Air Velocity
✓ Moisture Content
✓ Time
✓ Pressure
✓ Specific heat in constant pressure (Cp)
✓ Concentration
For Example,
If temperature increases then the heat would go through the products and more mass transfer will
occur that will increase the rate drying process.
Again, if the velocity of air increases it will help to transfer more vapor of moisture which will
increase the rate of the Drying Process.
To remove more moisture content, it should take more time.
Drying for a long time will increase the dryness of the products and decrease more moisture
content.
Pressure also impacts on dry process. Such in low pressure the rate of drying will increase than the
rate of drying in the general atmosphere.
The Specific heat in constant pressure (Cp): Specific heat at constant pressure, Cp, is the amount
of heat required to raise the temperature at constant pressure. If the pressure remains constant it
should take more drying time than the remaining in a constant volume. Because the product will
need more heat energy or the moisture of the products which want to dry would take the higher
heat energy to be vapor.

5. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Types of drying process:
From the traditional to the most innovative, there are several drying solutions for your industrial
process. Depending on our needs, the processing time, the product’s final quality, or the efficiency
of the process can vary considerably. Here is an overview of the different drying methods used
depending on the applications and products.
1) Heated air drying
a. Sun Drying
b. Solar Drying
c. Shed Drying
d. Recirculating mixed flow drying
e. Fluid Based Drying
f. Spray drying
g. Two stages drying
2. Osmotic Drying
3. Vacuum Drying
4. Absorption Drying
5. Drum Drying
6. Freeze Drying
Heated air drying
Heated air drying is the most common drying method as of today. It is simple and effective for
sturdy industrial and food products and an inexpensive solution.
However, this process can completely dehydrate the product surface, leading to cracks or
heterogeneous results. It can also be a slow process depending on the product and the
permissible drying temperature.

6. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Sun drying
Sun drying is a traditional drying method for reducing the moisture content of paddy by
spreading the grains under the sun. The solar radiation heats up the grains as well as the
surrounding air and thus increases the rate of water evaporating from the grains.
Advantages Of Sun Drying
1) It is the most common drying method in Asia because of its low cost compared to
mechanical drying.
2) It requires little investment and is environmentally friendly since it uses the sun as the
heat source and therefore produces no CO2.
Disadvantage Of Sun Drying
1) Sun-drying tends to be labor-intensive
2) limited capacity.
3) Temperature control is also difficult in this method
4) Grains can easily be overheated causing cracked grains which leads to low milling
quality.
5) It is also not possible to sun dry at night or during rain.

7. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Solar Drying
Solar drying may be defined as the drying process in which the object’s products get in a chamber
in which the solar light passes and the products gained heat and the using special characteristics
the heat doesn’t release from there in the result of a higher drying rate.
There is various solar dryer
a) Active solar Dryer
b) Passive Solar Dryer

8. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Advantages of solar drying
1. More drying rate
2. Easier contact ruction
3. Low cost.
The disadvantage of solar drying
1. limited capacity.
2. Temperature control is also difficult in this method
3. Grains can easily be overheated causing cracked grains which leads to low milling quality.
4. It is also not possible to sun dry at night or during rain.
Shed Drying
Shed Drying may be defined as the drying process in which the object’s products do not dry in
direct contact of the sun. There are some products which we cannot dry in the direct contact of air.
Such as If we want to dry the onion then we need shed drying methods. Though initially, we can
dry them in the direct contact of the sunlight we cannot run the process for a long time. If we do
that then the onion becomes a boiled feature and results in a bad smell. Thus, we need the shed
drying.

9. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Advantage of shed drying
1. Protect from direct heat.
2. Decrease the damage rate
3. Required structure gained.
Disadvantage of shed drying
1. Time-consuming
2. Higher cost than sun drying
3. Need shed which is costly
Mixed Flow Dryer
Mixed flow dryers are column dryers that have airflow in both counter and concurrent directions.
These dryers often have multiple zones and can use higher air temperatures without crop damage
than cross-flow dryers because like the concurrent flow dryers, all-grain kernels are exposed to the
same air temperatures. Example LSU dryer (Louisiana State University Dryer)

10. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Advantage of mixed-flow dryer
1. The dryers are usually self-cleaning
2. and have energy efficiency similar to a counter-flow dryer,
3. using about 40% less energy than a cross-flow dryer without heat recovery.
4. One major advantage compared to a dryer with a screen column like a cross-flow dryer is
the ability to use the dryer for a wide variety of different grains; from small rapeseed to
corn or soybeans.
Disadvantage of mixed-flow dryer
1. This type of dryer is popular outside of the US but has not been adapted as readily in the
corn-growing regions possibly due to the higher capital cost.
Fuild Based Dryer
A fluidized bed dryer (also called a fluid bed dryer) is a kind of equipment used extensively in the
pharmaceutical industries to reduce the moisture content of pharmaceutical powder and granules.
The equipment works on a principle of fluidization of the feed materials.
In the fluidization process, hot air is introduced at high pressure through a perforated bed of moist
solid particulate. The wet solids are lifted from the bottom and suspended in a stream of air
(fluidized state). Heat transfer is accomplished by direct contact between the wet solid and hot
gases. The vaporized liquid is carried away by the drying gasses. Sometimes to save energy, the
exit gas is partially recycled

11. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Advantage of Fluid based dryer
1. High rates of moisture removal due to an excellent gas-particle constant which results in
high heat and mass transfer rates.
2. High thermal efficiency is usually achieved if part of the thermal energy for drying is
supplied by the internal heat exchanger
3. Lower capital and maintenance cost
4. Reduced contact time for drying.
5. Ease of control.
Disadvantage of Fluid based dryer
1. High pressure drops results as a result of the need to suspend the entire bed in gas which
equally leads to high energy consumption.
2. Requires increased gas handling due to extensive recirculation of exhaust gas for high
thermal efficiency operation.
3. Poor fluidization and low flexibility especially if the feed is too wet.

12. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
4. Not the best choice of equipment when organic solvents need to be removed during drying.
5. Non-uniform product quality for certain types of fluidized bed dryers.
6. Entertainment of fine particles.
7. High potential for attrition; and in some cases, agglomeration of fine particles.
8. The conventional hot air fluidized bed dryers are not a good choice of dryer when handling
toxic or flammable solids since there is danger of fire or explosion of flammability limits
are exceeded
Spray drying
Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with
a hot gas. This is the preferred method of drying many thermally-sensitive materials such as foods
and pharmaceuticals, or materials which may require extremely consistent, fine, particle size. Air
is the heated drying medium; however, if the liquid is a flammable solvent such as ethanol or the
product is oxygen-sensitive then nitrogen is used.
Principle of spray drying method
There are many types of drying methods, but the spray drying method is the most widely used one.
It refers to atomizing the liquid to be dried into tiny droplets in the drying chamber and let the
droplets directly contact and mix with the hot gas of the drying medium to evaporate the water,
and then collecting it by gas-solid separation to obtain a powder or granular product. The raw
material liquid may be a solution, an emulsion, or a suspension, or maybe a molten liquid or a
paste. The drying medium can be air, nitrogen, or superheated steam.

13. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Advantage of spray drying
1. Rapid drying process.
2. Materials can be directly dried into powder.
3. It is easy to change the drying conditions and adjust product quality standards.
4. There is a certain negative pressure in the drying room, which guarantees the hygienic conditions
in the production, avoids the dust flying in the workshop, and improves the purity of the product.
5. High production efficiency and fewer operators.
6. Large production capacity and high product quality. The spray volume can reach several
hundred tons per hour
Disadvantage of spray drying
1. The equipment is complex, covers a large area, and requires a large investment.
2. The price of a spray dryer and powder recovery device is relatively high.
3. The thermal efficiency is not high and the heat consumption is large.

14. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Two-stage drying
Two-stage drying is also referred to as combination drying.
A typical first-stage dryer takes advantage of the different drying rates during the three drying
periods and surface moisture can be removed rapidly from very wet paddy without causing damage
to the grains by using very high temperatures for a short period of time. Drying air temperatures
in first stage dryers can reach over 100ºC in fluidized bed dryers where the grain is exposed to the
drying air only for a few minutes. After this rapid pre-drying to a MC of 18%, the grain is
considered safe for up to two weeks of storage. The grain is then transferred to a storage bin with
aeration facilities where it is slowly dried to the desired moisture content of 14% or lower with
only slightly pre-heated air or even ambient air if the climatic conditions are feasible.
Although two-stage drying has many advantages since it uses two different drying principles well
suited to the different drying phases of paddy grains at different MC ranges the introduction of two
stage drying in Southeast Asia has so far failed. This strategy is used only in Thailand by the
commercial sector.

15. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Advantages of two-stage drying
1. Decentralized drying to safe MC levels can be done close to the production in relatively
small mobile pre-dryers extending the allowable time for handling. Final drying can be
centralized in storage bins utilizing energy saving aeration.
2. Low specific energy requirement since the two different drying technologies are optimized
with respect to maximizing the drying potential of the drying air in the respective drying
phases.
3. The system can produce excellent quality since the last critical drying stage at low moisture
contents is done with low temperature which prevents the kernel from cracking through
heat stress or moisture adsorption.
Disadvantage of two-stage drying
1. Users who want to dry from harvest to safe storage MC need two machines to complete
the job.
2. In-store drying only makes sense if the grain remains in the storage container for storage
after drying. In most Southeast Asian countries, bulk handling and storage is not yet
practiced. If, in addition, there is a need to sell the paddy as quickly as possible after drying,
as it is the case in most cases in SE-Asia, a dryer with a shorter drying time is more
appropriate.
3. Economics of scale require a certain size of the in-store dryers. In farming environment
with small scale farms where different varieties are grown often the necessary amount of
the same variety for filling an in-store dryer cannot be collected.

16. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
4. In countries where electricity cuts are part of daily life extended drying translates to
increased risk. In this case, a dryer with a short drying time and ideally with a self-propelled
fan and conveyors using a combustion engine reduces the risk of spoilage.
Osmotic Drying
Osmotic dehydration is accomplished by placing foods such as fruits and vegetables into
concentrated soluble solid solutions having higher osmotic pressure and lower water activity.
The difference in the chemical potential of water between the food and the osmotic medium is
the driving force for dehydration. Compared to other conventional methods, osmotic
dehydration treatment is a simple procedure that requires no mechanical aid and involves
decreased cost of energy. It is easy to perform at room temperature, which ensures the
retention of color, texture, and nutrients with limited loss of volatile compounds and less
oxidative changes.
.
The process of mass transfer and tissue shrinkage extends from the surface to the center of the
material with the passage of operation time. At last, the cells in the center of the material lose
water and the mass transfer flux likely to equilibrate after an extended period of liquid-solid
contact. The shrinkage of the tissues and mass transfer takes place concurrently during the osmotic
dehydration process

17. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Advantage of osmotic drying
1. Minimized heat damage to fruits and vegetables while drying
2. Least discoloration of fruits by enzymatic browning.
3. Increase retention of volatile, flavors and aroma.
4. Improved textural quality and lower energy consumption than air drying.
Disadvantages of osmotic drying
The main disadvantage of the osmotic process is that it may increase the saltiness or sweetness or
decrease the acidity of the product thus reduces the characteristic taste of some products
Vacuum Drying
Vacuum drying is the mass transfer operation in which the moisture present in a substance, usually
a wet solid, is removed by means of creating a vacuum. In chemical processing industries like food
processing, pharmacology, agriculture, and textiles, drying is an essential unit operation to remove
moisture.[1]
Vacuum drying is generally used for the drying of substances which
are hygroscopic and heat sensitive, and is based on the principle of creating a vacuum to decrease
the chamber pressure below the vapor pressure of the water, causing it to boil. With the help of
vacuum pumps, the pressure is reduced around the substance to be dried. This decreases the boiling
point of water inside that product and thereby increases the rate of evaporation significantly. The
result is a significantly increased drying rate of the product.[2]
The pressure maintained in vacuum

18. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
drying is generally 0.03–0.06 atm and the boiling point of water is 25-30 °C. The vacuum drying
process is a batch operation performed at reduced pressures and lower relative humidity compared
to ambient pressure, enabling faster drying
Advantage of vacuum drying
Vacuum drying is conceptually the ideal method for drying thermal and/or oxygen sensitive
materials (such as fruits and vegetables) due to the advantage of removing moisture at low
temperatures and minimizing the possibility of oxidation reactions.
Disadvantages of vacuum drying
• Vacuum Drying process is batch type drying process (#dryingprocess) It has low
efficiency.
• Vacuum dryers are expensive.
• Vacuum dryers require skilled labor to operate.
• Cost of maintenance are high.

19. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Absorption Drying
Absorption drying is a chemical process in which water vapor is bound to absorption material. The
absorption material can either be a solid or liquid. Sodium chloride and sulfuric acid are frequently
used, which means that the possibility of corrosion must be taken into consideration.
Advantage of Absorption Drying
• Low capital cost compared to other dryers at the time
• Although this type of dryer will reduce the dew point, it cannot produce a minus pressure dew point
• It has a tendency to contaminate downstream systems.
• The energy consumption is very little, just be 6%, and the switching period is long, so the parts
wear little to improve the reliability of the equipment.
• The final dryness of outlet air is quite better than the heatless regenerative gas.
Disadvantage of Absorption Drying
• Low Efficiency
• Not appropriate for a huge amount of product.

20. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Drum Drying
The drum dryer is commonly used to dry viscous, concentrated solutions, slurries or pastes on
rotating steam-heated drums.1,2 It can also be used to dry concentrated solutions or slurries that
become more viscous or pasty because of flashing or boiling off of moisture or of irreversible
thermochemical transformations of their content that occur on their first contact with the hot drum
surface.3–5 The viscous slurry or paste is mechanically spread by the spreading action of two
counter-rotating drums into a thin sheet that adheres on the hotter drum in single drum dryers or
split sheets on both hot cylinders in double drum dryers. The adhering thin sheet of paste is then
rapidly dried conductively by the high heat flux of the condensing steam inside the drum. For very
wet slurries that produce wet sheets, the drying of the wet thin sheet can be further enhanced by
blowing hot dry air on the sheet surface. The thin sheet containing heat-sensitive materials, such
as vitamins, can also be dried at a lower temperature in a vacuum. The irreversible thermochemical
transformations during the slurry’s first contact with the hot drum can also be used to
simultaneously impart certain required quality of the dried product.6 Starch slurries can be
gelatinized or ‘‘cooked’’ before the sheet is dried to produce pregelatinized or ‘‘precooked’’ starch
for instant food formulations. Exposure of the thin sheet to the high heat flux and high temperature
for a short period of time can also impart a porous structure to the dried sheet because of the rapid
formation of vapor bubbles within the sheet during ‘‘boiling like’’ drying. Porous products are
excellent in instant food formulations because they are more readily wetted and can be easily
rehydrated. It is for these reasons that the drum dryer is widely used around the world in the
production of pregelatinized starch for instant food formulations.

21. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Advantage of Drum drying
1.The products have good porosity and, hence, good rehydration due to boiling evaporation.
2. Drum dryers can dry very viscous foods, such as pastes and gelatinized or cooked starch, which
cannot be easily dried with other methods.
3. Drum dryers normally have high energy efficiency.
4. Drum drying can be clean and hygienic.
5. Drum dryers are easy to operate and maintain.
6. The dryers are flexible and suitable for multiple but small quantity production.
Disadvantage of Drum drying
1. Some products may not form a good film on the drum surface and are not suitable for drum
drying.
2. Some products, especially those with high sugar content, may not be easily scraped off from the
drum.
3. There is relatively low throughput compared to spray drying.
4. Because of the precision machining that is required, there is a high cost of changing the drum
surface.

22. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
5. There is a possible scorching of the product to impart cooked flavor and off-color due to direct
contact with the high-temperature drum surface.
6. It is not possible to process salty or other corrosive materials due to the potential pitting of the
drum surface.
Freeze Drying
Freeze Drying is a process in which a completely frozen sample is placed under a vacuum in order
to remove water or other solvents from the sample, allowing the ice to change directly from a solid
to a vapor without passing through a liquid phase. This process, called sublimation, along with the
minimal heat input that is required, is ideal because of the long-term preservation properties it
provides to the integrity of the sample’s biological and chemical structure. Lyophilization can be
achieved in various volumes, from small at-home freeze dryers all the way up to large, production-
scale equipment.
The first major utilization of freeze drying was during World War II, for the transportation of
blood, serum and penicillin. Since the development of these modern freeze-drying techniques,
lyophilization technology has continued to advance and has been used for a broad spectrum of
applications in a variety of fields. Laboratory freeze dryers are commonly used for research in
biological and environmental disciplines, as well as in the research and development of many
modern medicines.
Stages of Freeze Drying
There are three stages in the lyophilization process: Pre-freezing, primary drying, and secondary
drying.
Pre-freezing
The pre-freezing stage is the most important stage of the freeze-drying process. In this stage,
sample material will need to be cooled to at least the temperature of the melting point for that

23. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
sample. This ensures the sample will be completely frozen and can then undergo sublimation. If
the sample isn’t frozen solid, evaporation will occur and the sample will not achieve the same
preservation properties that occur with sublimation.
The rate at which your sample freezes will affect the size of the ice crystals that form. If not done
properly, it can impact the speed of reconstitution, length of the freeze-drying process and integrity
and stability of your sample.
Larger ice crystals facilitate faster and more efficient lyophilization because water molecules are
able to move more freely out of the sample during sublimation. For samples like food or tissues,
large crystals can break the cell walls and damage your sample. In these situations, it is best for
freezing to be done quickly through flash freezing, creating smaller ice crystals.
Primary Drying
Primary drying begins when you start your freeze dryer and vacuum pump. With the low-pressure
environment, evaporative cooling of the sample begins, allowing for energy in the form of heat to
speed the freeze-drying process. At the end of primary drying, roughly 93% of the water in the
sample is sublimated out. This stage can take several days, depending on the sample type and heat
input. For laboratories that are using their freeze-drying equipment for sample preparation and
resuspension, primary drying is where the run would end. For long term preservation of the sample,
the run would continue on to secondary drying.
Secondary Drying
In the secondary drying phase, water molecules that are bound to the sample are released.
Additional heat is added in this stage to drive off excess moisture, leaving behind a moisture
content of about 2%. Secondary drying is typically used in samples that are being prepared for
long term preservation and storage.

24. Md Shahjahan Kabir
B.Sc. in Food and Process Engineering
Level 3 Semester I
Hajee Mohammad Danesh Science and technology University, Dinajpur, Bangladesh.
Advantage of Freeze Drying
▪ Once the water is removed from foods, they become very light.
▪ It’s easier and cheaper to transport and delivery a large amount of freeze-dried food.
▪ Freeze-dried foods tend to retain most of their nutritional quality, taste, shape and size.
▪ Freeze-dried foods don’t require refrigeration.
▪ Freeze-dried foods can last for months or years.
▪ Freeze-dried foods can also be rehydrated very quickly, unlike dehydrated foods.
▪ It doesn’t contain bacteria because there’s no water
Disadvantage of Freeze Drying
▪ The equipment needed for this process is quite expensive
▪ It’s not so common to know how to use freeze-dried food to prepare meals
▪ Freeze-dried foods also take up almost as much space as fresh foods