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Dehydration (Drying)
Drying of foods is an important food processing operation
used to preserve foods.
The distinguishing features between drying and
concentration are the final level of water and nature of theproduct.
Concentration leaves a liquid food
Whereas drying typically produces product with water
content sufficiently low to give solid food.
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Reasonsfor drying foods
Drying is used to remove water from foods for two reasons:
to prevent (or inhibit) micro-organisms and hence preserve the
food.
to reduce the weight and bulk of food for cheaper transport and
storage.
Historically, there was a need to preserve foods for longer times
so that food was available during times of limited food production or
availability.
Hunters needed a technique to preserve meat for more than a few
days to ensure a continuous food supply. In the same manner, we
have techniques that allow us to preserve foods as they are harvested,
so that we can enjoy them at later times.
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One of the easiest ways to preserve foods is to remove water,
since
microorganisms need water to survive and grow. many chemical reactions require water to proceed.
Early hunters dried their meat to help maintain a more continuous
food supply.
Nowadays, we dry foods for the same reason: to provide a
continuous supply of foods that we can enjoy at any time.
Other reasons for drying foods
Removal of water leaves a product reduced in weight and oftenin bulk. This reduces shipping costs and makes the food supply
more economical. Dried foods also provide convenience. Dried
convenience foods may be used for special expedition--type
(military) foods where weight is a major concern.
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There are many methods and technologies by which we can
dehydrate foods.
We must first understand the nature of water in food products to
appreciate (realize fully) the difficulties in producing high-
quality dried products.
Removal of water from foods is not a difficult task. However,removing the water in such a way that the product regains its
initial form when rehydrated is not so easy.
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EFFECTSOF DRYINGON PRODUCT
QUALITY
After dehydrating the food cannot reach the original quality.
There is always some change that gives a loss of quality in the
product.
The goal is to minimize these changes, while optimizing process
efficiency and minimizing costs.
Several types of changes can occur during drying.
Two main problems are
loss and change of flavors, and
change in physical qualities of dried products.
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Effect on flavor
One problem with dried foods is that the flavor of therehydrated product is not the same as that of the original.
During drying, flavor compounds that are typically more
volatile than water are removed in the drying process.
The physical forces that cause water molecules to be removed
from the food during drying also cause volatile compounds
(alcohols, aldehydes, ketones, etc.) to be removed.
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burnt flavor
Dried products have less of these volatile flavoring compounds
than the original starting material.
In addition, the rates of chemical reactions are enhanced at the
elevated temp., and many of these reactions generate undesired
flavor compounds.
For example, the browning reaction (between reducing sugars
and proteins) is enhanced and generates a burnt flavor.
(reconstituted milk from a dried powder)
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Browning
Other chemical reactions may also take place during drying.
Browning occurs in many foods which results in color changes.
Protein denaturation can occur during drying, which causes
increased viscosity
Thermal degradation of vitamins and proteins may also
influence the nutritional status of dried products.
The extent of these changes depends on the nature of the drying
process. Some types of dryers produce products having superior
properties on reconstitution. The instant coffee spray-dried andfreeze-dried is different. Since freeze-drying does not involve a
vapor-liquid interface, the volatile flavor and aroma compounds
are not lost during drying, and freeze-dried products have higher
quality.
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Dehydrating FruitsAnd Vegetables
Dehydration of fruit and vegetables is one of the oldest forms of
food preservation techniques and consists primarily of
establishments engaged in sun drying or artificially dehydrating
fruits and vegetables.
By reducing both the weight and volume of the final productdehydration of fruits and vegetables lowers:
o the cost of packaging
o storing, and
o transportation
Process Description
Dried or dehydrated fruits and vegetables can be produced by a
variety of processes.
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These processes differ primarily
by the type of drying method used, which depends on the type of
food.
by the type of characteristics of the final product.
In general, dried or dehydrated fruits and vegetables undergo the
following process steps:
o Predrying treatments, such as size selection, peeling, and colorpreservation
o Drying or dehydration, using natural or artificial methods; and
o Postdehydration treatments, such as sweating, inspection, and
packaging. I. Predrying Treatments
Predrying treatments prepare the raw product for drying or
dehydration and include raw product (a) preparation and (b) color
preservation.
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(a) Raw product preparation includes
o selection and sorting
ow
ashing
o peeling (some fruits and vegetables)
o cutting into the appropriate form, and
o blanching (for some fruits and most vegetables)
Fruits and vegetables are selected; sorted according to size,maturity, and soundness; and then washed to remove dust, dirt, insect
matter, mold spores, plant parts, and other material that might
contaminate or affect the color, aroma, or flavor of the fruit or
vegetable. Peeling or removal of any undesirable parts follows washing. The
raw product can be peeled
o by hand (generally not used in the United States due to high labor
costs)
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o with lye or alkali solution
o with dry caustic and mild abrasion
o with steam pressureo with high-pressure washers, or
o with flame peelers
For fruits, only apples, pears, bananas, and pineapples are usually
peeled before dehydration.
Vegetables normally peeled include beets, carrots, parsnips,
potatoes, onions, and garlic.
Prunes and grapes are dipped in an alkali solution to remove the
natural waxy surface coating which enhances the drying process.
Next, the product is cut into the appropriate shape or form (i. e.,
halves, wedges, slices, cubes, nuggets, etc.), although some items,
such as cherries and corn, may by-pass this operation.
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Flame peeling: A flame peeler utilizes a conveyor belt to transport
and rotate the material through a furnace heated to temperature
above 1000 C. The skin (root hairs) is burned off and then removed
by high-pressure water sprays. Flame peeling is used, for example,
for peeling onions.
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Some fruits and vegetables are blanched by immersion in hot water
(95 to 100 C [203 to 212 F]) or exposure to steam.
(b) Color preservation The final step in the predehydration treatment is color preservation,
also known as sulfuring.
The majority of fruits are treated with sulfur dioxide (SO2) for its
antioxidant and preservative effects.
The presence of SO2 is very effective in retarding the browning of
fruits, which occurs when the enzymes are not inactivated by the
sufficiently high heat normally used in drying.
In addition to preventing browning, SO2 treatment reduces the
destruction ofcarotene and ascorbic acid, which are the important
nutrients for fruits.
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Sulfuring dried fruits must be closely controlled so that enough
sulfur is present to maintain the physical and nutritional properties of
the product throughout its expected shelf life, but not so large that itadversely affects flavor.
Some fruits, such as apples, are treated with solutions of sulfite
(sodium sulfite and sodium bisulfite in approximately equal
proportions) before dehydration.Although dried fruits commonly use SO2 gas to prevent browning,
this treatment is not practical for vegetables.
Most vegetables (potatoes, cabbage, and carrots) are treated with
sulfite solutions to retard enzymatic browning.
In addition to color preservation, the presence of a small amount of
sulfite in blanched, cut vegetables improves storage stability and
makes it possible to increase the drying temperature during
dehydration,
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thus decreasing drying time and increasing the drier capacity without
exceeding the tolerance for heat damage.
II. Drying or dehydration
Drying or dehydration is the removal of the majority of water
contained in the fruit or vegetable and is the primary stage in the
production of dehydrated fruits and vegetables.
Several drying methods are commercially available and the
selection of the optimal method is determined by:
o Quality requirements
o Raw material characteristics, ando Economic factors
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There are three types of drying processes:
Sun and solar drying
Atmospheric dehydration including:
Stationary or batch processes (kiln, tower, and cabinet driers) and
Continuous processes (tunnel, continuous belt, belt-trough,
fluidized-bed, explosion puffing*, foam-mat*, spray, drum, and
microwave heated driers) and
Subatmospheric dehydration (vacuum shelf, vacuum belt,
vacuum drum, and freeze driers)
Sun and solar drying
Sun drying (used almost exclusively for fruit) and solar drying
(used for fruit and vegetables) of foods use the power of the sun to
remove the moisture from the product.
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In explosion puffing the partially dried (15 to 35 wt.-% moisture)
vegetable pieces are conveyed into a puffing chamber (gun: the
chamber has a quick opening lid) which is brought to a predetermined
pressure. When the water contained within the pieces is heated above
its atmospheric boiling point in the gun, pressure builds up and when
the pieces are instantly discharged, the flushing of water vapors from
within each piece creates a porous structure. The porous structure
permits much faster dehydration and rapid rehydration of the dried
product.
Carrot and potato cubes made by this method rehydrate in 5-6 min.
by simmering in water as against 30-60 min. for conventionally air
dried product.
(Used for rice and wheat breakfast cereals)
Explosion puffing*
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Foam-mat drying*
Concentrated liquids have be dried by heated air on conveyor driers,
using a technique known as foam mate drying. The liquid is made into
a foam, by the addition of a small amount, 1% or less, of a foaming
agent, such as soya protein, albumin, fatty acid ester of sucrose and
glycerol monostearate, and the incorporation of air or other gases by
injection or mixing. The foam is spread in thin layers or strips on a
wire mesh belt and conveyed through the drier.
relatively rapid drying can be achieved, of the order of 1 hour in air
at 100 C, yielding a porous dry product with good rehydration
properties.
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Microwave heating occurs between 300 MHz and 300 GHz
(wavelength 1mm to 1m).
Microwave heating of a substance by electromagnetic energy
operating in that frequency range.
Microwaves themselves do not represent heat but absorbed energy
is converted into heat inside the product.
The heating of foods by microwave energy is accomplished both
by the absorption of microwave energy by dipole water molecules
and ionic components (often salt) of food.
As the energy enters the foods, dipolar water molecules try to
align in the electric field orientation. They oscillate around their
axis, generating heat within the food, resulting in dehydration.
Microwave Drying
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When ionized compounds are subjected to a microwave field, they
randomly collide with non-ionized groups in an electric field. The
kinetic energy of these ions is transmitted into heat during thecollisions.
The depth of the material and type of material being exposed also
affects penetration.
The waves bounce from wall to wall, until the product absorbs
eventually all of the energy. In this manner, the drying rate is
increased greatly. This type of heating is highly efficient; and power
utilization efficiencies are generally greater than 70 %.
Important commercial aspects include the ability to maintain
color and quality of the natural food. This has been found prevalent
in potato chips.
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Sun drying of fruit crops is limited to climates with hot sun and dry
atmosphere, and to certain fruits, such as prunes, grapes, dates, figs,
apricots, and pears.
These crops are processed in substantial quantities without much
technical aid by simply spreading the fruit on the ground, racks,
trays, or roofs and exposing them to the sun until dry.
Advantages of this process are its simplicity and its small capital
investment.
Disadvantages include complete dependence on the elements and
moisture levels no lower than 15 to 20 percent (corresponding to a
limited shelf life).
Solar drying utilizes black-painted trays, solar trays, collectors,
and mirrors to increase solar energy and accelerate drying.
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Atmospheric dehydration
Atmospheric forced-air driers artificially dry fruits and vegetables
o by passing heated air with controlled relative humidity over thefood to be dried, or
o by passing the food to be dried through the heated air, and is the
most widely used method of fruit and vegetable dehydration.
Various devices are used to control air circulation and
recirculation.
o Stationary or batch processes include kiln, tower (or stack), and
cabinet driers.
o Continuous processes are used mainly for vegetable dehydration
and include tunnel, continuous belt, belt trough, fluidized-bed,
explosion puffing, foam-mat, spray, drum, and microwave-heated
driers.
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Tunnel driers are the most flexible, efficient, and widely used
dehydration system available commercially.
Subatmospheric dehydrationSubatmospheric (or vacuum) dehydration occurs at low air
pressures and includes vacuum shelf, vacuum drum, vacuum belt, and
freeze driers.
The main purpose of vacuum drying is to enable the removal of
moisture at less than the boiling point under ambient conditions.
Because of the high installation and operating costs of vacuum
driers, this process is used for drying raw material that may
deteriorate as a result of oxidation or may be modified chemically as
a result of exposure to air at elevated temperatures.
There are two categories of vacuum driers:
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In the first category, moisture in the food is evaporated from the
liquid to the vapor stage, and includes vacuum shelf, vacuum drum,
and vacuum belt driers.
In the second category of vacuum driers, the moisture of the food
is removed from the product by sublimination, which is converting
ice directly into water vapor.
The advantages of freeze drying are
o High flavor retention
o Maximum retention of nutritional value
o Minimal damage to the product texture and structure
o Little change in product shape and color, and
o A finished product with an open structure that allows fast and
complete rehydration
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Disadvantages include
o High capital investment,
o High processing costs, and
o Need for special packing to avoid oxidation and moisture gain in
the finished product.
III. Postdehydration treatments
Treatments of the dehydrated product vary according to the type of
fruit or vegetable and the intended use of the product.
These treatments may include
Sweating
Screening
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Inspection,
Instantization treatments, and
Packaging.
Sweating involves holding the dehydrated product in bins or
boxes to equalize the moisture content.
Screening removes dehydrated pieces of unwanted size, usually
called "fines".
Inspection: The dried product is inspected to remove foreign
materials, discolored pieces, or other imperfections such as skin,
carpel, or stem particles.
Instantization treatments are used to improve the rehydration
rate of the low-moisture product.
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Packaging is common to most all dehydrated products and has a
great deal of influence on the shelf life of the dried product.
Packaging of dehydrated fruits and vegetables must Protect the product against moisture, light, air, dust, microflora,
foreign odor, insects, and rodents
Provide strength and stability to maintain original product size,
shape, and appearance throughout storage, handling, and marketing Consist of materials that are approved for contact with food. Cost
is also an important factor in packaging.
Package types include cans, plastic bags, drums, bins, and cartons,
and depend on the end-use of the product.
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Water activity
Water activity (aw
) is defined as the ratio of the vapor pressure
of water measured at the food surface (Pw) to the saturation
vapor pressure of pure water at the same temperature (Pw)
oP
P
a!
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Water activity
For a cup of water, the vapor pressureover the surface is measured as thesaturation vapor pressure, and aw is 1.
When there are solutes in the water suchas sugars, salts, etc. the vapor pressureover the water surface is lower than thesaturation vapor pressure, and aw is
reduced to some value less than 1. Thereduction in water activity depends on thetype of solutes present and their levels.
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Water Activity
For food products, the water activity is
generally less than 1. aw is related to the
moisture content of the food, the types and
concentrations of different solutes, and the
structure or physical characteristics of the food.
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DRYING METHODS
1.DirectContact Dryers
Sun Dryer/Bin Dryer/Kiln Dryer/ Tray or Cabinet Dryer/Tunnel Dryer/Belt or Conveyor Dryer/
Fluidized Bed Dryer/Rotary Air Dryer/Spray Dryer. 2.IndirectContact Dryers
Drum Dryer
3.Infrared or Dielectric Dryers
Infrared Dryers/Microwave Dryers.
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Tray or Cabinet Dryer
The food product placed in a pan is placed
inside a drying chamber with hot air blowing
across the product until drying is complete.
Some of the hot air used for drying may be
recirculated through the dryer to conserve
energy However, increased relative humidity
of the recirculated air decreases dryerefficiency.
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Tunnel Dryer
The food product is loaded onto trays that are
placed into carts. The carts are input at one end
of the tunnel dryer and move through to the
outlet. Air blowing within the tunnel causes
drying at a specified rate, so that the food
product reaches the exit on completion of
drying. (1) cocurrent; (2) countercurrent; or (3)mixed flow,
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Belt or Conveyor Dryer
Product may also be moved through a dryer by
placing it on a belt or conveyor. In order to
extend the time within a conveyor dryer, a
series of conveyors may be arranged one
above the other. In this case, product drops
from an upper conveyor to a lower conveyor.
air flow can be through the conveyor andthrough the bed of food product laying on the
conveyor.
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Fluidized Bed Dryer
Air flow through a bed of product is sufficient
to lift the product. Since there is intimate
contact between air and product drying rates in
this type of dryer are quite good. This type of
dryer is limited to granulated powders, or
small pieces of product. The air velocity
depends on particle size, density.
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Rotary Air Dryer
These dryers are typically arranged ashorizontal cylinders that rotate along theirmain axis. Wet product enters one end of the
dryer and moves towards the other end by acombination of gravity and the bafflearrangement within the cylinder. As thecylinder rotates, air forced into the cylinder
blows across the product as it tumbles, to provide effective contact between air andproduct.
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SPRAY DRYING
Atomization
Air Handling
Dryer Chamber Powder Separation
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Atomization
Atomization produces a cloud of droplets with very
large surface area for drying.
High pressure nozzle: droplet size is controlled by
pressure of the fluid food against the nozzle.
Centrifugal atomizer: Liquid food is pumped into a
spinning disk, where it is accelerated by centrifugal
force and expelled from the ends of the disk-shaped
atomizer, become a cloud of droplets .
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Air spray atomization
with high velocity air
Centrifugal atomization
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Air Handling
Ambient air is taken in through a vent and
heated prior to circulation into the drying
chamber. Heating can be accomplished in
several ways. Air can be passed either through
steam coils or an electric heater to attain
elevated temperatures, typically between l50
and 500.
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Dryer Chamber
In the residence time of droplets in the spray dryingchamber, the droplets go from a moisture content inthe range of about 40% to only about 5-10%.
The food droplets are sprayed at the top of thechamber and fall down to the bottom by gravity. Bothair and food droplets enter the chamber at the top andfall to the bottom of it, where air is separated fromdry powder and the product is removed from thedryer.
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Product quality of Freeze Drying
The original structure of the food is
maintained and flavor retention in is excellent.
The cost of freeze drying is very high.
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Steps in Freeze Drying
Freezing.
Primary Drying.
Secondary Drying. Heat and Mass Transfer in Freeze Drying
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Primary Drying
Sublimation of ice is accomplished by controlling the
vacuum level in the freeze dryer and through careful
heat input. A high vacuum is desired to enhance
sublimation rate. Introduction of heat is to supply energy to a plate on
which the food is sitting (conduction heating), while
also providing a radiation source above the product.
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Sublimation Front
The ice recedes into the food product as drying
occurs. This boundary between frozen and
dried product is called the sublimation front.
Heat must be transferred into the product to
this front to promote sublimation, and water
vapor must then be removed by mass transfer
through the dried product
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Secondary Drying
Once all the ice is sublimed out of the frozen
food, the secondary drying process begins.
Heat is continually added, but at a slower rate
since moisture loss occurs only by diffusion of
water molecules out of the freeze--dried matrix.
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Collapse Behavior
Rapid heat addition causes the temperature ofthe product to exceed its collapse temperature.product becomes sufficiently flowable that it
"collapses!' During collapse, the pockets whereice crystals have sublimed disappear as thefood slowly flows into these regions. Thiscauses product to have higher density and
reduces its ability to be rehydrate.
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VaporCondensor
A condenser collects the vapors as they exit
the freeze dryer to enhance efficiency and
prevent fouling of the vacuum pump.
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