Dehydration of Fruit and Vegetables in Tropical Regions

Abstract

The introductory part of this chapter contains information on forms of water and its role in foods. Also merits and demerits of sun drying over dehydration will be discussed. Abundant sunshine in tropical areas can efficiently be utilized for sun drying of fruits and vegetables. In sun drying, moisture is removed by solar energy. The discussion about sun drying covers the areas of equipment, preparatory treatments, drying procedure, precautions, and post-drying care of the products. Moisture is also removed by artificial means with different dehydrators. In dehydration, drying atmosphere and factors affecting evaporation of water from food surface will be discussed. This section will include descriptions of various methods such as dehydration, freeze drying and dehydrofreezing. Types of dehydrators, including natural-draft, forced-draft, solar drier, tunnel driers, and conveyer driers, are discussed at length. Information on fluidized-bed driers for particulate or diced materials will be given. Spray driers, drum driers, special drying techniques (e.g., foam-mat drying) and use of latest techniques such as drying by application of energy from radiating microwave/ dielectric sources will also be covered. Evaporating moisture, thereby concentrating the soluble solids in foods, will be dealt with under evaporation/concentration. Binding of available moisture to produce intermediate-moisture foods could be a useful technique for tropical countries. At the end of the chapter the effect of drying on the nutritional value of dried foods will be discussed.

Full text

9 Dehydration of Fruit and Vegetables
in Tropical Regions
Salim-ur-Rehman and Javaid Aziz Awan
Abstract: The introductory part of this chapter contains information on forms of water and its role in
foods. Also merits and demerits of sun drying over dehydration will be discussed. Abundant sunshine
in tropical areas can efficiently be utilized for sun drying of fruits and vegetables. In sun drying,
moisture is removed by solar energy. The discussion about sun drying cover the areas of equipment,
preparatory treatments, drying procedure, precautions, and post-drying care of the products.
Moisture is also removed by artificial means with different dehydrators. In dehydration, drying
atmosphere and factors affecting evaporation of water from food surface will be discussed. This
section will include description of various methods such as dehydration, freeze drying and dehydro-
freezing. Types of dehydrator, including natural-draft, forced-draft, solar drier, tunnel driers, and
conveyer driers, are discussed at length. Information on fluidized-bed driers for particulate or diced
materials will be given. Spray driers, drum driers, special drying techniques (e.g., foam-mat drying)
and use of latest techniques such as drying by application of energy from radiating microwave/
dielectric sources will also be covered. Evaporating moisture, thereby concentrating the soluble
solids in foods, will be dealt under evaporation/concentration. Binding of available moisture to
produce intermediate-moisture foods could be a useful technique for tropical countries. At the end of
the chapter the effect of drying on the nutritional value of dried foods will be discussed.
Keywords: dehydration; dehydro-freezing; drum drier; freeze drying; forced draft; natural draft; sun
drying; solar drier; tunnel drier
9.1 INTRODUCTION
Fruits and vegetables have been dried since prehistoric times. Drying has been practiced using
the sun’s energy and air. With the development of dehydration equipment at the beginning of
19th century, it moved into the factories. The terms drying and dehydration are used
synonymously for the removal of moisture from foods. However, drying is often restricted
to the process under natural conditions using solar energy. It is especially useful in tropical
regions where the sun shines for most of the year. Sun drying of fruits is still practiced in these
areas for drying of prunes, figs, grapes, dates, and apricots (Girdhari et al., 1998).
Dehydration is the term used for the process of removal of water under controlled
conditions of temperature, relative humidity, and air flow. Dehydration processes are applied
to apples, prunes, and several vegetables like spinach, potatoes, okra, bitter gourd, carrot,
green peas, and cauliflower. Continuous processes such as tunnel, belt-trough, fluidized-bed,
Progress in Food Preservation, First Edition. Edited by Rajeev Bhat, Abd Karim Alias and Gopinadhan Paliyath.
Ó2012 John Wiley & Sons Ltd. Published 2012 by John Wiley & Sons Ltd.
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and foam-mat drying are mostly used for drying vegetables. Dehydration equipment with
vacuum-enabled removal of moisture from delicate foods without loss of quality. Freeze
drying has been a very helpful discovery for the preservation of delicate and heat-sensitive
foods such as instant tea, coffee, and fruit juices. It involves sublimation of ice from frozen
food. Spray drying is suited for fruit-juice concentrates and vacuum dehydration is helpful for
low-moisture and high-sugar fruits including apricots, pears, and peaches.
9.2 FORMS OF WATER
Water is essential for chemical and biochemical reactions as well as microbial activity in
foods. The microorganisms need ample free water for their survival and multiplication.
Without biologically active moisture, spoilage agents would be completely inactive. It is
present in varying quantities and different forms practically in all foods (Rahman, 2007).
Fresh fruits and vegetables that are high in moisture spoil readily. Dry beans and cereal grains
contain relatively less moisture and are safely stored for a long period at ambient temperature
without a reduction in quality (Ramaswamy and Marcotte, 2006).
Water exists in foods in five different forms:
(i) free water (e.g., in tomato juice);
(ii) droplets of emulsified water (e.g., in butter);
(iii) water tied into colloidal gels (e.g., in jellies);
(iv) thin layer of adsorbed water (e.g., in powdered milk);
(v) chemically bound water of hydration (e.g., in sugar).
Moisture may be removed from the foods by sun drying and dehydration, evaporation/
concentration, freeze drying, and dehydro-freezing techniques. Chemically bound water is
extremely difficult to remove during drying, while free and adsorbed water is easy to
manipulate in drying and dehydration. Most free and absorbed moisture is removed by
suitable means. In this way, the remaining water becomes unavailable for chemical and
biochemical reactions, as well as activities of microorganisms.
9.2.1 Role of water in food
Water plays one or more roles in a food system. It may serve as a solvent and participate in
chemical and biochemical reactions as a prime reactant in processes involving hydrolysis. It
may be a product of chemical reactions involving condensation. It may serve as a modifier of
the catalytic activity of other substances in food. Reducing the water content lowers or even
inhibits the chemical and biochemical reactions and activities of microorganisms. It also
retards the activities of metallic catalysts associated with lipid oxidation.
9.3 ADVANTAGES OF DRIED FOODS
Dried foods are less expensive than most foods preserved by other methods. These are
concentrated sources of nutrients and are high in total solids. Dehydrated foods, because of
reduced weight, are less costly to transport. Due to a reduction in bulk of the product they also
require less storage space. The color of dehydrated fruits and vegetables is more uniform due
to controlled drying conditions. The process of dehydration should be applied in such a
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manner that the nutritional value, flavor, and cooking properties of the fresh fruits and
vegetables are retained.
9.4 DRYING PROCESSES
There are three fundamental types of drying process:
(i) sun drying/solar drying;
(ii) controlled atmospheric batch-type drying with tower, cabinet, and kiln dehydrators and
continuous drying with belt, belt-trough, tunnel, fluidized-bed, foam-mat, spray,
explosion puff, microwave, and drum dehydrators;
(iii) sub-atmospheric drying such as vacuum shelf/belt and freeze dehydrators.
9.4.1 Sun drying/solar drying of fruit and vegetables
Sun drying refers to elimination of moisture from foods by exposing them to the direct energy
from the sun’s rays. Many food grains such as cereals and pulses become naturally dried in the
field by solar energy and are protected from autolysis and microbial attack. Their moisture
content usually varies from 8 to 16% and water activity is normally 0.75 or below. In addition
to these naturally preserved foods, other perishable commodities are sun-dried in order to
extend their shelf life. In tropical and sub-tropical regions with an abundance of sunshine,
several fruits and vegetables are sun-dried. Grapes are sun-dried in Afghanistan, Greece, and
the USA to produce raisins and dates are dried in Saudi Arabia, Iraq, Tunisia, Egypt, and
Algeria. Guava, tomato, pears, and peaches are also sun-dried in several parts of the world. In
Pakistan figs, plums, grapes, dates, and apricots are sun-dried since the preservation is simple
and inexpensive. These are more often dried by the farmers at the farms and in producing
areas when there is surplus produce and it is often difficult to transport fresh commodities to
market. This method is applicable on a commercial level or at the village level subject to a hot
and dry climate with no rainfall during and after the harvesting period. The crop should be
fresh and of good quality. Various lots of different stages of maturity must not be mixed
together; this would result in a poor dried product. Insect-, rodent-, and disease-affected parts
and parts which are discolored or have a bad appearance must be removed before drying.
9.4.1.1 Equipment requirements
Equipment required for sun drying varies according to the product to be dried, local
conditions, and the scale of production. However, anyone engaged in sun drying of fruits
and vegetables would need wooden boxes of appropriate size for collection of raw material
from the field. In the preparation shed, cutting tables, cutting knives, peeling knives, aprons,
etc., are required. The prepared raw material needs to be placed in suitable-sized trays that are
usually constructed of wood. Blanchers or sulfuring equipment is also needed. After drying,
the raw material is brought to the sweating chamber. Packaging equipment is required to pack
the dried product in suitable-sized moisture-proof containers.
9.4.1.2 Method of production
The raw material for sun drying needs preparatory treatments similar to those required for
preservation by canning, freezing, or other techniques (Awan and Rehman, 2009). Before
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Dehydration of Fruit and Vegetables in Tropical Regions 193

trimming and cutting, most fruits and vegetables should be washed with potable water to
remove contaminants, spray residues, and soil particles. Onions are washed after they have
been peeled. After washing, trimming, and peeling, bigger-size fruits may be cut into even
slices of about 3–7 mm thickness or in halves/quarters and then blanched or sulfured. It is very
essential to have all slices of uniform thickness in one drying batch. Different sizes dry at
different rates and result in a poor-quality product. Onions and root vegetables are sliced with
a hand or vegetable slicer; apples, bananas, tomatoes, and potatoes are sliced with stainless
steel knives. After peeling and slicing, apples, bananas, and potatoes turn brown very quickly
when exposed to air due to an active enzyme, phenoloxidase. So, these must be kept in water
until drying can be initiated. Salt or sulfite solution gives better protection.
Broadly speaking, plums, grapes, figs, and datesare dried as whole fruits without cutting and
slicing. Slices are left in the open sun in trays and turned occasionally until dried to the desired
degree. Aftersulfuring at the rate of 1.8–3.6 kg of sulfur per tonneof fruit, the trays are placed in
the sun with 8560 cm trays and sides about 5 cm high: the approximate load for a tray is
3.5 kg and the material shouldbe spread in even layers. During theinitial stage of the drying, the
material should be turned over at least once every hour by shaking. It will facilitate quick
removal of moisture, prevent sticking together and improve the quality of end product. At night,
the trays should bestacked in a ventilated roomor covered with canvas. The dried fruits are then
sorted and stacked in boxes or bins for moisture equilibration, a process known as sweating.
After drying, the products are subjected to attack by insects and rodents, so that product should
now be packed or stored in insect- and rodent-proof storage rooms. Occasionally this room may
be fumigated with suitable gas to kill insect pests (Saravacos and Kostaropoulos, 2002). In view
of the fact that dried foods usually deteriorate at room temperature and lose both color and
flavor, it is preferable to store them at lower temperatures, especially if stored in bulk bins. Most
processors prefer to keep their products in cold stores at 0–4C, which prevents damage caused
by insects and rodents in addition to protecting product quality.
Some varieties of vegetables and fruits are better for sun drying; they must be able to
endure natural drying without their texture becoming tough so that they are not difficult to
reconstitute.
9.4.1.3 Precautionary measures
As the raw material for sun drying is directly exposed to the atmosphere, there is danger of
contamination from natural sources. Microorganisms in the atmosphere will settle on the
product. Birds and animals in and around the vicinity of the drying yard should be kept away.
Cleanliness in all operations is absolutely essential. Great care must be taken during the
drying process to prevent contamination. All equipment should properly be disinfected.
Waste fruit should not be dumped near the drying area and, where this is not possible, it should
be treated in a pit with calcium chloride to prevent its decay and multiplication of
microorganisms and insects, especially flies.
The main problems of sun drying are dust, rain, and cloudy weather. The drying area should
be free from flies and dust. In order to produce dust-free and hygienically clean products,
materials should be dried well above ground level and covered with nylon mosquito net.
9.4.2 Solar driers
For commercial-scale operation, due consideration should be given to the cost of production,
particularly energy consumption. The large-scale driers are more promising than small-scale
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ones. However, small-scale driers should not be overlooked. The drier should be designed to
maximize capital investment; i.e., multi-product and multi-use. In a broader sense, an
auxiliary heat source should be provided to ensure reliability to handle peak loads and also
to ensure continuous drying during periods of no sunshine. Forced-convection indirect solar
driers are preferred because they offer better control and more uniform drying, and because of
their high heat collection efficiency results in a smaller collector area. However, energy
wastage should be minimized.
Two drier systems are identified: (i) a cabinet-type drier with natural convection for
internal air circulation for the processing of fruit such as bananas, pineapples, mangoes,
apricots, apples, and pears, and also for potato chips; and (ii) a greenhouse-type drier with
forced-air circulation.
Barriers to commercial development include the following:
linitial investment is high, especially for poor farmers;
lmisuse is possible due to lack of training and technical skills;
ldurability is doubtful if low-cost building materials are used;
ldependability and reliability are reduced during the rainy season, when drying is critical,
due to non-availability of solar energy;
lthe lack of national policies directed to promoting the drying of produce at the production
site, in order to reduce losses, improve quality, and increase farmers’ earnings.
The benefits of solar driers include:
lthe cost of conventional energy is high;
lit helps to trim down fuel costs;
lwhere the land is short supply and expensive;
lthe quality of existing sun-dried products can be improved;
llabor is in short supply;
lthere is a plenty of sunshine, but high humidity.
9.4.3 Drying under shade
Drying in the shade is applied in those vegetables for which there is a problem of natural
color loss or browning under direct sunlight. Herbs, coriander leaves, green and red sweet
peppers, chillies, green beans, okra, and fenugreek have an attractive color, and hence
are dried under shade. The principles for shade drying are similar to those for sun drying.
Shade drying is carried out in a shed which has open sides. Air circulation is required for
efficient drying. Shade drying takes little more time than is normally required for drying under
the sun.
9.4.4 Osmotic drying
In this process, prepared fruits and vegetables are immersed in a strong syrup and brine,
respectively, and these are then sun-dried. During immersion the material loses some of its
moisture. The salt/syrup serves as a protective coating on the surface of slices. The protective
effect on color, flavor, and texture remains during the drying process and results in a high-
quality product.
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Dehydration of Fruit and Vegetables in Tropical Regions 195

9.5 DEHYDRATION
Dehydration is an operation in which the moisture content of food is substantially lowered
under controlled conditions of temperature, relative humidity, and airflow in a chamber.
Under these conditions, high-quality products are obtained that retain their natural char-
acteristics upon reconstitution. Foods that are successfully preserved by dehydration include
fruits, vegetables, milk, eggs, fish, cake mixes, soup mixes, and others. The raw material
determines the handling techniques and type of equipment needed. Savings may be made in
shipping costs from reduced weight, but not in terms of container size. Moreover, sometimes
shipping costs are not based on weight but on volume. Another reason for dehydration is the
production of convenience food items. A good example is instant mashed potato. In this case,
cooking steps are completed before the product is dried. The consumer only needs to add
water and mix.
9.5.1 Drying conditions
In the removal of moisture from natural foodstuffs, heat energy is applied to the food. Water,
which evaporates, is removed from the vicinity of the raw material. Foods may be dried (i) in
hot air, (ii) in superheated steam, (iii) in a vacuum, (iv) in heated inert gas, or (v) by direct
application of energy from a radiant microwave or dielectric source. However, air is the most
common medium for elimination of moisture from foods for several reasons. Dehydrators
using air are less expensive to construct and are convenient to install and operate. By using air
as a drying medium, overheating, discoloration, and scorching of the product is greatly
avoided. Air also permits gradual drying of foods, thereby avoiding loss of juice by dripping
(Lee and Pyun, 1993; Brennan, 2006).
Primarily, there are three major roles of air in dehydration. It conveys heat energy to food,
vaporizes moisture from the commodity and transfers the liberated moisture to the outside
atmosphere. A larger volume of air is required to vaporize moisture from the food than to
transport it out of the drying atmosphere. The volume of air required for drying is calculated
from the initial temperature of air entering the drier, volume of moisture required to be
evaporated, time in which the operation has to be completed, and temperature of air leaving
the dehydrator. Other factors connected with this calculation are the heat losses through
leakage and heat required to raise the temperature of trays and walls of the dehydrator.
9.5.2 Factors affecting evaporation of water from food surfaces
The rate of moisture evaporation from the free surface of a food material depends upon the
nature of food material, particle size, bed depth (in case of pieces placed on a surface), relative
humidity, temperature, and velocity of the air. Not all food raw materials react in the same
manner to changes in atmospheric conditions. As moisture has to travels out of the food from
inside, larger pieces take a longer time to dry than smaller ones. Thus, while it may take a few
hours for okra pieces to dry in a tunnel drier, more time would be required in the case of whole
okra dried under the same conditions. When tissue foods are dried in cabinet or tunnel driers,
the depth of raw material on the tray surface will also determine the length of the process.
Atomization of fluid foods into small particles, as observed in spray drying of fruit juices and
instant beverages, cuts the drying time to a matter of seconds.
The relative humidity and velocity of air are important factors that must carefully be
controlled in dehydration processes. Air, high in relative humidity, will not accommodate the
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196 Progress in Food Preservation

extra moisture from the food. At low temperature, air is easily saturated with moisture and so
will not be efficient for the purpose. The rate of moisture evaporation from the free surface of
food material is directly proportional to the velocity of air, provided other factors remain
constant. It has been found that with an air velocity of about 70 m/min, drying is twice as rapid
as in still air and at 140 m.min it is three times as fast.
Food materials are living units and moisture has to travel out from the inside to the outside
of food pieces. At high temperature, low relative humidity, and high velocity of the drying air,
the food surface can get dried, while the moisture inside the food is unable to travel towards
the outer surface. This condition is known as case hardening, and results in the food being dry
on the outside but wet on the inside. Therefore, air velocity is usually regulated in commercial
dehydrators to between 91 and 304 m/min. Increasing the air velocity beyond this range is
uneconomical. Part of the spent air may be re-circulated to avoid case hardening and to cut
down on heating costs.
9.5.3 Types of dehydrator
Depending upon the mechanism of transferring heat energy from the heat exchanger to the
food, all dehydrators can be grouped into one of the following categories:
lhot-air driers:
Tnatural-draft driers: kiln, cabinet, tower, and Oregon tunnel driers,
Tforced-draft driers: tunnel driers (concurrent, counter-current, centre-exhaust, and
cross-flow tunnel driers), conveyer driers, fluidized-bed driers, and spray driers;
ldrying by contact with a heated surface: drum driers, vacuum shelf driers;
ldrying by application of energy from a radiating microwave or dielectric source: radiant
heating drier, continuous infrared drier, microwave and dielectric heating driers;
lspecial drying techniques: puff drying, foam-mat drying.
9.5.3.1 Hot-air driers
Natural-draft driers
Natural-draft driers, in general, consist of a furnace room or other heating arrangement (steam
pipes, electric heaters), surmounted by a drying chamber. The air is heated by contact with
radiating pipes and enters the drying chamber by natural convection currents. These driers are
inexpensive to build and have low fuel efficiency. Most natural-draft driers are now equipped
with fans which have considerably increased their efficiency.
The kiln drier is one of the oldest types. The drier consists of an upper storey that houses the
drying floor, and a ground floor that accommodates the heating system. At the top of the upper
floor is an exit for escape of the gases. It is employed in the drying of hops (flowers of hop plant
used in brewing to impart a bitter taste), cacao, and apples. It is unsuitable for drying soft fruits.
In the tower drier, the drying chamber can accommodate several drying trays stacked one
over another in the form of a tower. The heating system is like the kiln drier. The cabinet drier
is similar to a tower drier except that steam coils are placed below the trays to furnish heat. In
this type, the temperature of the air can conveniently be regulated and drying made more
rapid. The modern types of cabinet drier consist of compartments that hold several trays over
or through which hot air is blown by special fans.
The Oregon tunnel drier consists of a series of parallel, sloping narrow chambers above the
furnace room. The hot air enters at the lower end of each tunnel through an opening or throat,
while trays of food material (normally fruits) enter the drier at the upper or cool end and move
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towards the lower or warm end. The dry fruit is removed from the lower end of each tunnel.
This type is very suitable for drying plums to produce prunes.
Forced-draft dehydrators
In principle, forced-draft dehydrators consist of a heating chamber, which may be heated by
any conventional means, the drying chamber where food is placed, and a system to circulate
air through the foodstuff. Normally these driers have an arrangement for re-circulation of air,
hence they have increased fuel efficiency and prevent case hardening, which makes them
more efficient and allows control over the product.
Tunnel driers consist of a chamber in the form of a tunnel, which is longer than its width.
Depending on the direction of the movement of air over the product, a tunnel drier may be
concurrent, where air and raw material move in the same direction, or counter-current
(Figure 9.1), where they move in opposite directions to each other. In some driers, air enters
from both sides of the tunnel and leaves from the middle. Such a drier is called center-exhaust
tunnel drier (Figure 9.2) and has the advantage of producing a better-quality product than the
other driers. In another version, called cross-flow drier, the air flows across the food.
In tunnel driers, food is normally placed in trays that are stacked on trolleys or trucks which
move through the tunnel. Heating of air is done either by steam coils, electrically heated grids
or hot-air furnace. The air supply is maintained by using disc, multivane, airplane propeller,
axial, or paddle wheel types of fan.
The conveyer drier such as continuous draper or belt drier is similar to the tunnel drier in
construction, except that the raw material moves on an endless conveyer rather than trucks or
trolleys. This type is suitable for drying vegetables, starch, etc. Air is usually blown across the
product in a concurrent or counter-current design.
Fluidized-bed driers are used for dehydration of particulate or diced materials, e.g., grains,
cacao, coffee, onions, and diced vegetables, as well as for drying granulated materials such as
sugar and salt (Potter and Hotshkiss, 1995). Hot air passes from the bed through a porous plate
Figure 9.1 A cabinet drier (source: Awan, 2004).
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198 Progress in Food Preservation

at a pressure that is enough to float the food particles in suspension. The supply of air is so
regulated at different points in the bed that, in addition to removing moisture and transferring
it to the outside atmosphere, it also conveys the food particles and discharges them at the exit
end of the drier. The pressure of hot air through the food is just enough to suspend the particles
giving an impression of a gentle boiling motion.
Spray driers are employed for liquid and other foods of low viscosity that can be atomized
(Bhandari et al., 1993). In a typical spray drier, liquid food is atomized into a drying chamber
through a fine nozzle. Hot air is passed into the drying chamber and the food material dries
instantly. As some powdered material may be lost with exhaust air, a recovery system
(cyclone system) is usually attached to the air exhaust in which the suspended powdered
particles are recovered and spent air is allowed to escape or re-circulate.
9.5.3.2 Drying by contact with a heated surface
Some foods – such as milk, soup mixes, baby foods, mashed potato, and others – that are either
liquid or in slurry form are dried by exposing them directly to a hot surface. The drum drier is a
good example in this group. It essentially consists of one or more hollow metallic cylinders or
drums that are heated internally by steam, or other heating medium. Usually drum driers are
classified according to the number of drums in one unit, e.g., single- or double-drum systems.
The cylinder revolves on a horizontal axis. As the drum revolves, it collects a thin film of the
liquid or slurry at a certain point. By the time it goes around by about 270, the film is dried and
a special scraping assembly removes the dried film and drops it onto a conveyor. Another film
Figure 9.2 Schematic representation of a typical counter-current (top) and center-exhaust (bottom) tunnel
driers (source: Awan, 2004).
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Dehydration of Fruit and Vegetables in Tropical Regions 199

of wet material then coats the drum for the next round of drying. Conventional drum driers are
economical to use, but heat-sensitive products may be adversely scorched (Valentas et al.,
1977; Heldman and Hartel, 1997).
In the vacuum shelf drier, food is placed between two plates in a system where a vacuum
can be created. The foods are dehydrated under vacuum at low temperature. The procedure is
comparatively expensive, hence is used for highly priced and heat-sensitive foods.
9.5.3.3 Drying by application of energy from radiating microwave or dielectric sources
Several types of radiation in the electromagnetic spectrum are capable of producing heat in
food materials and thus evaporating or boiling water from them. Infrared radiation is absorbed
to a great amount by foods, resulting in heat generation. The radiation commonly employed
has a wavelength of 0.8–400 nm. These are used primarily for surface heating; hence they
have application in the dehydration of fruits, vegetables, grains, and tea, as well as in baking,
freeze drying, and roasting of cacao beans and nut kernels (Jayaraman and Das-Gupta, 1992;
Vega-Mercado et al., 2001).
Some waves, especially at frequencies of 915 MHz (wavelength 3.28 108 nm, dielectric
heating) and 2450 MHz (wavelength 1.22 108 nm, microwave heating), are used in the food
industry for drying and other purposes. When food materials absorb waves at these
frequencies, heat energy is generated. In dielectric heating, food material is heated by
keeping it between parallel electrodes, while in microwave heating it is placed in a resonance
cavity. In microwave heating, microwaves produced by a magnetron tube, when absorbed by
the food, generate considerable heat through a series of rapidly alternating currents. The polar
molecules of water are excited by the microwaves, thus generating heat. Dielectric and
microwave heating have found applications in the food industry such as in concentration and
drying, besides baking, cooking, blanching, pasteurization, and sterilization. These methods
are also becoming popular for preliminary or finishing steps in drying potato chips, cooking
chicken pieces, and baking crackers.
9.5.3.4 Special drying techniques
Special techniques are applied to produce certain effects in foods that are not possible with the
normal methods of dehydration. Puff drying is employed to increase the porosity of food
particles to give them a spongy look. Such products are easier to reconstitute and have a better
texture than those dried by the usual techniques. Production of potato puffs represents a good
example of this technique in which the escaping steam tends to puff the product.
Foam-mat drying has a similar objective, i.e., producing a product with the same or more
volume than the fresh food, but having spongy texture. In this case the raw material may be
whipped to trap the air, e.g., egg white. In concentrated citrus juices, fruit purees, and tomato
paste an edible whipping agent is added prior to whipping. Stable foams are then cast in thin
layers onto trays or belts and are dried by any appropriate system (Smith and Hui, 2004).
9.6 EVAPORATION AND CONCENTRATION
Evaporating some moisture, thereby concentrating the soluble solids, lowers the water
activity of a food material. Evaporation helps to reduce the weight of food materials, thereby
lowering packaging and transportation costs (Potter and Hotchkiss, 1995). Tomato paste is a
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common example where evaporation is a part of the preservation technique. Initially tomato
juice contains about 6% solids that are concentrated by evaporation to about 32%. Normally
water activity of most evaporated foods is still high enough to encourage growth of spoilage
microorganisms and, therefore, such foods require further processing. Most are given heat
treatment milder than would be the case for their un-evaporated counterparts or may be
concentrated by the addition of soluble constituents to further decrease their water activity.
Common salt is obtained by concentration from seawater through evaporation in artificial
lagoons. In the sugar industry, sugar cane or sugar beet juice is concentrated by evaporation
from approximately 15% sugar content to saturation. Sugar syrups are prepared to 70Brix. In
the production of jams, jellies and other such foods, sugar is added to fruit pulp and part of the
moisture is removed by evaporation in open type or vacuum pans. Microorganisms normally
do not thrive in foods that are high in sugar, especially above 65%. Hence, jams and similar
products packed in hermetically sealed containers must contain at least 65% dissolved solids.
If the product is packed in non-hermetically sealed containers, a minimum of 68% dissolved
solids is usually desirable.
Concentration of fruit juices that must retain their very delicate chemical, textural, and
sensory characteristics is carried out in special types of evaporator. These could be the natural
circulation evaporators that include open-pan type, horizontal, or vertical short-tube type,
natural circulation type, forced-circulation evaporator, long-tube evaporators (climbing film,
falling film, and the climbing-falling film models), plate type, expanding-flow evaporator,
centrifugal evaporator, low-temperature evaporator, or bubbling-type evaporator. Each of
these has its own advantages and disadvantages and is suitable for concentration of specific
products.
9.6.1 Freeze drying
In response to demand by consumers for preserved foods which show minimal quality
differences from the fresh product, food technologists have provided various innovative
solutions. Freeze drying is one processing technique that does minimal harm to a product and
produces food of exceptional quality. Foods that are freeze-dried are light, porous in structure,
and, when reconstituted, exhibit characteristics of the fresh commodity (Donsı
`et al., 1998).
Additionally, freeze-dried products retain shape and size of the original raw material. Freeze
drying is considered superior to conventional drying. The only drawback is the high cost of
production (Table 9.1).
Freeze drying involves freezing of food material followed by removal of moisture by
sublimation, i.e., evaporation of moisture from the solid state to vapor state without first
changing into liquid. The sublimation process is carried out in a low vacuum pressure (usually
at 0.1–2 mmHg). The product is often finished in an ordinary drier since some moisture may
remain in the food. Some disadvantages of freeze drying include the possible damage to raw-
material cell structure when freezing is poorly executed. Moreover, the product is brittle and
therefore susceptible to mechanical damage. Also, the freeze-drying process is expensive
compared with other conventional drying techniques. This method is suitable for coffee, fruit
juices, whole shrimps, diced chicken, etc.
9.6.2 Dehydro-freezing
Dehydro-freezing is a less commonly used method of food preservation. Fundamentally, it
involves partial moisture reduction by any suitable dehydration technique followed by
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freezing rest of the moisture present in the food. The freezing ensures an extra reduction in
water activity. In practice, the process consists of removal of about 50% moisture from the
foodstuff and then subjecting this partly dehydrated material to normal freezing operations. In
this way, the bulk of the food is reduced, thereby lowering storage, transportation, and
handling costs. The product is superior in quality to a purely dehydrated one, as it better
retains the flavor and texture.
9.6.3 Intermediate-moisture food technology
The available moisture in food can be bound, making it unavailable for spoilage agents. This
technique is employed in the production of intermediate-moisture foods that have moisture
content usually varying from 20 to 50%. Examples of such foods are honey, jams, jellies,
confectionery products, and sweetened condensed milk. The water activity of intermediate-
moisture foods generally ranges from 0.70 to 0.85, which is low enough to prevent the growth
of many spoilage bacteria and yeasts (Erickson, 1982). However, this water activity is
relatively high for several spoilage molds and osmophilic yeasts. The moisture content in
these foods is lower than in fresh commodities but higher than in conventionally dehydrated
foods. The available moisture left after binding with chemicals is not high enough to support
enzyme and microbial activity. Thus, the food remains semi-moist and can be kept on kitchen
or retail shelves for long periods. The greatest preservation effect in these products is on
account of the high concentration of solutes resulting in high osmotic pressure. Additionally,
salts, acids, and other chemical substances may be used to extend storage life further. These
foods also have simpler packaging requirements and can be packed in inexpensive protective
wrappers.
The principle underlying the technology of intermediate-moisture foods is that water
activity of the material is lowered by partly removing the moisture. This is followed by the
Table 9.1 Advantages of freeze drying over conventional drying.
Freeze drying Conventional drying
Suitable for cooked and raw animal products Unsuitable for meat and meat products
Adequately low temperature is applied to prevent
thawing.
Normal temperatures range between 37 and 93 C
are applied.
Moisture loss takes place by sublimation of ice
without entering the intermediate liquid stage.
Evaporation of water takes place from surface of the
food.
Highly porous dried and hygroscopic particles
produced which are readily reconstituted.
Produces solid dried particles
Yields lower-density dried products than the original
food
Yields higher-density dried products than the orig-
inal food
Product has natural odor Product often has an abnormal odour
Rapid and complete rehydration is possible Product takes more time for rehydration, which is
partially complete
Color of product is almost nearly natural Color of product is typically darker than the fresh
one
Product has normally natural flavor Product has normal flavor
Carried out at pressures below 4 mmHg. Commonly carried out at atmospheric pressure
Product has excellent storage stability The product has tendency to darken during storage
Dehydration may be completed within 12–24 h Dehydration may be completed within a short time,
commonly less than 12 h
Data from various sources.
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addition of such chemicals as common salt, glycerol, sorbitol, sucrose, glucose, or others that
bind part of the remaining moisture. In these products, microbial growth is further retarded by
employing antimicrobial agents, especially antimycotics such as propylene glycol and/or
sorbic acid or its potassium salt. The technology of producing intermediate-moisture foods
could particularly be useful and practicable for developing countries that lack the normal low-
temperature storage facilities.
9.7 SPOILAGE OF DRIED FRUITS AND VEGETABLES
During storage, dried fruits and vegetables are attacked by insects which can be controlled by
fumigation. Before storing, rooms should be sanitized to minimize the chances of infestation.
Fumigation with ethylene oxide or phostoxin inside storage rooms can reduce the invasion of
insects. Under damp conditions, dried fruits become musty or moldy and dried vegetables
become soft and slimy. Proper packaging of dried products can minimize the chances of
attack of these spoilage agents. Table 9.2 shows the possible defects, causes, and remedial
measures.
9.8 MERITS OF DEHYDRATION OVER SUN DRYING
During the drying process, the relationship between the temperature of the air and moisture in
food is referred to as the psychrometric relation. There are a number of factors affecting the
drying process:
lsize, shape, and arrangement of stacking of product;
lcomposition of fruits and vegetables;
ltemperature, humidity, and velocity of air;
lheat transfer to the surface of raw material;
Table 9.2 Causes of spoilage of dried products during storage (Dauthy, 1995).
Defect Cause Remedies
Mold growth Due to high moisture content of a
product, above equilibrium relative
humidity corresponding to water
activity a
w
¼0.70.
Reduce water content down to optimum level
Pack in a hermetically sealed package
Insect infestation Presence of larvae or insects in dried
product
Disinfection of storage chamber with toxic
gases
Fumigation of packed products and of
packages
Disinfection by heating (60–65 C) of products
prior to packing
Browning Chemical reaction (Maillard, etc.) Reduce water content down to optimum level
Store in a cool place
Enzyme-catalyzed reactions Enzyme inactivation by heat treatment such as
blanching or steaming prior to drying
Low rehydration
ratio
Temperature too high in final stages of
drying
Maintain final temperatures as recommended
inside dehydrator
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Dehydration of Fruit and Vegetables in Tropical Regions 203

lpressure of atmosphere.
Since the dehydration is done under controlled conditions, it has many advantages over the
process of sun drying:
ldehydration is quicker than sun drying;
ldehydration needs much less floor area than sun drying;
ldehydration is done in a hygienic environment;
ldehydration is not at the mercy of weather; it is possible during the rainy season;
lthe color of dehydrated product is more attractive and uniform due to controlled drying
temperature.
9.9 EFFECTS OF DEHYDRATION ON NUTRITIVE VALUE
OF FRUITS AND VEGETABLES
Due to the loss of moisture during drying from fresh fruits and vegetables, the concentration of
nutrients is enhanced. The amount of nutrients such as proteins, fats, and carbohydrates
is greater in dried food than in fresh food per unit weight (Table 9.3). However, the quality
of dried food is not comparable to fresh food in terms of loss of vitamins (Awan, 2007).
The water-soluble vitamins are oxidized. They are also decreased during blanching, which
causes enzyme inactivation. Ascorbic acid and carotene are destroyed by oxidation. Riboflavin
is damaged by light. Thiamin is heat-sensitive and destroyed by sulfuring. More appreciable
ascorbic acid is lost in sun-dried fruits than when freeze-dried. The carotene content of
vegetables is decreased by 80% if processing is done without blanching. Rapid drying retains
more ascorbic acid than slow drying. The major losses of carbohydrates take place in fruits.
Discoloration may be due to the action of enzymes or caramelization-type reactions.
9.10 EFFECTS OF DRYING ON MICROORGANISMS
Microorganisms need moisture for their metabolic activities and multiplication. While yeasts
and bacteria require higher amounts of moisture, typically above 30%, molds need much less,
12% moisture or even lower. Some can grow on food substrates even having less than 5%
moisture. Fruits are dried to between 16 to 25% moisture. There are chances of mold growth if
these are exposed to air and stored under high-humidity conditions. Above 2% moisture and
under favorable conditions, mold growth is expected.
The most effective control is the use of high-quality vegetables with low contamination,
blanching prior to drying, processing in a hygienic environment, and storage under conditions
where the dried foods are protected from invasion of microbes, insects, rodents, and other
animals (Gallardo-Guerrero et al., 2010).
Table 9.3 Chemical composition of fresh and dried peas (Desrosier and Desrosier, 1987).
Nutrient Fresh Dried
Moisture (%) 74.0 5.0
Fats (%) 1.0 3.0
Carbohydrates (%) 17.0 65.0
Proteins (%) 7.0 25.0
Ash (%) 1.0 2.0
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9.11 EFFECT OF DRYING ON ENZYME ACTIVITY
Enzymes need moisture for their activities. Activity is nil at moisture levels below 1%.
Enzymes are more sensitive to moist heating conditions and are inactivated or destroyed near
the boiling point of water. When enzymes are exposed to dry heat at the same temperature,
such as applied in drying, they are markedly insensitive to the effect of the heat. Even,
exposure to a temperature near 204C for a short time has little effect on enzymes if heating
and the enzyme preparation is dry. The enzymes catalase and peroxidase are used as
indicators of enzyme activity.
9.12 INFLUENCE OF DRYING ON PIGMENTS
The color of fruits and vegetables are altered as a result of drying. Carotenoids are mostly
changed during drying. Anthocyanins are also destroyed by drying treatments. They are
bleached by sulfur treatment. On the other hand, browning and Maillard reactions occur
during conventional dehydration of fruits. If fruits are treated with sulfur, enzymatic
browning and Maillard reactions can be inhibited or minimized.
The natural green pigment of vegetables is a mixture of chlorophyll a and chlorophyll b.
The retention of the natural green color of chlorophyll is directly related to the retention of
magnesium in the pigment molecules. During moist heat treatment, it is converted to
pheophytin due to loss of magnesium. The color becomes an olive green instead of grass
green. However, this conversion of magnesium can be controlled by changing the medium to
slightly alkaline.
9.13 RECONSTITUTION TEST
(i) Weigh out a sample of 35 g from the bulked and packed final product of the previous
day’s production.
(ii) Put the sample into a small container (beaker) and add 275 mL of cold water and 3.5 g of
salt.
(iii) Cover the container with a watch-glass and bring the water to boil.
(iv) Boil gently for 30 min.
(v) Turn the sample out into a white dish.
(vi) At least three judges should then examine the sample for palatability, toughness, flavor,
and presence or absence of bad flavors. The testers should record their results
independently.
(vii) The liquid left in the container should be examined for traces of sand/soil and other
foreign matter.
(Srivastava and Sanjeev, 2002; Awan and Rehman, 2009)
The test can be used also to examine dried products after they have been stored for some
time. Evaluation of rehydration ratio may be performed according to the following
calculations.
Rehydration ratio: if the weight of the dehydrated sample (a) is used for the test is 5 g and
the drained weight of the rehydrated sample (b) is 35 g, then
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Dehydration of Fruit and Vegetables in Tropical Regions 205

Table 9.4 Schedule for drying fruit.
Fruit Preparation and pretreatment Sulfuring time Drying temperature and time
Aonla Wash, grate, add salt (at 40 g/kg grated material) – Sun dry
Apples Sort, wash, peel, core trim and cut slices into 5 mm thick slices 30 min or immerse in 1–2% KMS
solution for 30 min and drain
Kiln dry at 60–71 C for 6–10 h or sun dry
Apricots Wash, halve, destone, steam blanch 15–25 min 57–68 C for 10–12 h or sun dry
Banana Wash, peel, halve cut lengthwise, or slice crosswise 12 mm thick 15–30min 55–91 C for 18–20 h or sun dry
Dates Wash, dip in boiling 0.5–2.5% lye solution, then rinse – 45–50 C for 20–24 h or sun dry
Figs Wash 1 h (only Adriatic variety) 55–60 C for 17–18 h or sun dry
Grape
(Muscat and
wine variety)
Dip in boiling 0.5% lye solution, then rinse 3–5 h 66–82 C for 20–30 h in a drier or sun dry
Mango Wash, peel, cut into 12 mm-thick slices 2 h 45–50 C for 24–30 h or sun dry
Papaya Wash, peel, halve, remove seeds, cut into 6 mm-thick slices 2 h 60–65 C for 24–26h or sun dry
Peach Wash, halve, remove pits, cut into halves, steam blanch 15–20min 60–63 C for 15–20 h or sun dry
Pear Wash, peel, halve, remove core, keep in 1–2% salt solution,
steam blanch
15–30 min or immerse in 1–2%
KMS solution for 30 min and
drain
60–65C for 24 to 30 h or sun dry
KMS, potassium metabisulfite; data from various sources.
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Table 9.5 Schedule for drying vegetables.
Vegetable Preparation Pretreatment Drying temperature and time
Beet Wash, peel, cut into 10 mm-thick slices Steam for 10 min 60–65 C for 12–15 h or sun dry
Bitter gourd Wash, remove both ends, scraped, cut into 10 mm-
thick slices
Blanch for 7–8 min 66–70 C for 7–9 h or sun dry
Brinjal Wash, cut lengthwise into 10 mm-thick slices Blanch 4–5 min, immerse for 1 h in 1% KMS solution,
drain
49–52 C for 9–11 h or sun dry
Cabbage Wash, remove outer leaves and core, cut into 10 mm-
thick shreds
Blanch 5–10 min, immerse for 10 min in 0.5% KMS
solution and drain or boil in sodium bicarbonate
solution for 3–4 min
60–66 C for 12–14 h or sun dry
Carrot Wash, remove stalks and tips, cut into 10mm-thick
slices
Blanch for 2–4 min in boiling 2–4% common salt
solution.
68–74 C for 14–16 h or sun dry
Cauliflower Wash, remove stalks covering leaves and stems,
break flowers apart into pieces of suitable size
Blanch 4–5 min, immerse for 1 h in 1% KMS solution,
drain
60–66 C for 10–12 h or sun dry
Chillies (red) String mature dark red pods and hang in sun No treatment 50–60 C for 16–18 h or sun dry
Garlic Peel clove, use as such or cut into 5 mm-thick slices Dip for 10 min in 5% salt solution, drain 60–65 C for 15–17 h or sun dry
Green beans Wash and remove strings, split pods lengthwise Blanch for 3–6 min 60–65 C for 16–20h or sun dry
Green peas Wash, remove shell Blanch or steam for 3–4 min, immerse in 0.5% KMS
solution and drain
60–65 C for 15–18 h or sun dry
Okra Wash, whole or cut into 10 mm disc or halve cut
lengthwise
Blanch in boiling water for 4–8 min or steam blanch
for 2–5 min, rinse with cold water
63–68 C for 6–8 h or sun dry
Onion Remove outer dry scales, cut into 5 mm-thick slices Dip for 10min in 5% salt solution, drain 60–65C for 11–13 h or sun dry
Potato Wash, peel, cut into 8–10 mm-thick slices Blanch in boiling water or steam for 3–5 min, immerse
in 0.5% KMS solution
60–66 C for 9–11 h or sun dry
Pumpkin Wash, peel, cut into 10 mm thick slices, dip in 2%
common salt solution
Blanch in 2% common salt solution for 3–4 h 65–71 C for 7–8 h or sun dry
Spinach, fenugreek,
other leafy green
vegetables
Sort, wash, trim off rough stems and stalks, shred Blanch for 2 min in boiling water or steam 60–65 C for 6–8 h or sun dry
Tomato Wash Blanch for 30–60 seconds, peel and slice 10 mm thick 60–65 C 18–20 h or sun dry
Turnip Wash, peel, cut into 5 mm-thick slices Blanch for 2–4 min in boiling water, immerse for
1–2 hours in 1% KMS solution
50–55 C 13–15 h or sun dry
KMS, potassium metabisulfite; data from various sources.
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Dehydration of Fruit and Vegetables in Tropical Regions 207

Rehydration ratio ¼b=a¼35=5¼7:1
Rehydration coefficient: if the drained weight of 12 g of dried sample containing 4% moisture
after rehydration is 65 g and the fresh sample before drying contained 90% moisture, then:
Rehydration coefficient ¼Að100BÞ=ðCDÞ100
Where Ais the drained weight of the dehydrated sample, Bis the moisture content of the
sample before drying, Cis the weight of the dried sample taken for rehydration, and Dis the
amount of moisture present in the dried sample taken for rehydration:
Rehydration coefficient ¼65 ð10090Þð120:4Þ100 ¼0:55
Water present in the rehydrated product ð%Þ¼ðab=aÞ100
Where ais the drained weight of the rehydrated product and bis the dry matter content in the
sample.
The moisture content of the rehydrated sample is (65 11.8/65) 100 ¼81.85%.
9.14 DRYING PARAMETERS
In drying of fruits and vegetables by conventional methods, certain operations are essential to
produce good quality products. These include preparatory operations such as washing,
peeling removal of inedible constituents and size reduction (Fellows, 2000). To prevent
discoloration and maintain color of the product, sulfuring by the use of burning sulfur or
sulfiting by dipping in potassium metabisulfite solution in appropriate concentration are
common practices prior to drying (Girdhari et al., 1998). Some essential requirements for
drying of fruits are given in Table 9.4 and for vegetables in Table 9.5.
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