dehydration class lecture

8/6/2019 Dehydration Class Lecture

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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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dehydration class lecture

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