International Journal of Food Nutrition and Safety, 2017, 8(1): 45-72 International Journal of Food Nutrition and Safety Drying and the Different Techniques

Abstract

Drying is removing a large portion of the water contained in a product in order to considerably reduce the reactions which leads to deterioration of the products. In less developed countries where industry is not very important there is a general feeling that drying is an easy operation and not too much input is needed and anybody can do it. Drying of foods is a complex business and a mere translation from other fields is not often advisable. Drying plays an important role in food and agricultural industries and is the oldest method of preservation. The main feature of this process consists on lowering the water content in order to avoid or slow down food spoilage by microorganism. This review focuses upon conventional and new drying techniques and their advantages, limitations and applications.

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Copyright © 2017 by Modern Scientific Press Company, Florida, USA
International Journal of Food Nutrition and Safety, 2017, 8(1): 45-72
International Journal of Food Nutrition and Safety
Journal homepage: www.ModernScientificPress.com/Journals/IJFNS.aspx
ISSN: 2165-896X
Florida, USA
Review
Drying and the Different Techniques
Uwem Inyang 1,*, Innoccent Oboh 1, Benjamin Etuk 1
1Department of Chemical and Petroleum Engineering, University of Uyo, Uyo, Akwa Ibom State,
Nigeria
* Author to whom correspondence should be addressed; E-Mail:uweninyang7@yahoo.com
Article history: Received 30 May 2017, Revised 12 July 2017, Accepted 20 November 2017,
Published 20 December 2017.
Abstract: Drying is removing a large portion of the water contained in a product in order to
considerably reduce the reactions which leads to deterioration of the products. In less
developed countries where industry is not very important there is a general feeling that drying
is an easy operation and not too much input is needed and anybody can do it. Drying of foods
is a complex business and a mere translation from other fields is not often advisable. Drying
plays an important role in food and agricultural industries and is the oldest method of
preservation. The main feature of this process consists on lowering the water content in order
to avoid or slow down food spoilage by microorganism. This review focuses upon
conventional and new drying techniques and their advantages, limitations and applications.
Keywords: drying; moisture; preservation; food spoilage; deterioration of products
1. Introduction
Drying is probably the oldest and the most important method of food and vegetables preservation
practiced by humans (Doymaz, 2005a). It removes moisture and preserve products. Drying process
involves heat and mass transfer (Adeboye and Oputa, 1996; Bamire and Oke, 2003). Drying which is
the removal of moisture prevents the growth and production of microorganisms that causes decay; it
minimizes the moisture-mediated deteriorative reactions. It also can cause substantial reduction in
weight and volume, minimizing packaging, storage and transportation costs and enables storability of
the product under ambient temperature (Owolarefe et al., 2007). During drying a lot of changes occur

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46
which can be structural and physico-chemical modifications that can affect the final product quality
(Doymaz, 2005; Okos et. al, 1992).
Thermal damage incurred by a product during drying is directly proportional to the temperature
and time involved (Lin et al.1998), which means that decreasing drying time can improve the quality of
products.
However, drying has always been of great importance for the preservation of food, thus, the
major reason of drying food products is for the reduction of moisture content. Improvement of product
quality and reduction of post-harvest losses can only be achieved by the introduction of suitable drying
technologies (Bala and Janjai, 2009).
Drying processes can be classified into four categories namely solar drying, atmospheric drying,
sub atmospheric drying and novel drying technologies (Jayaraman and Gupta, 1995). Solar drying
includes sun or natural dryers, solar dryers-direct, solar dryers indirect and hybrid or mixed systems.
Atmospheric drying is either continuous or batch. Continuous drying utilizes dryers such as spray dryer,
fluidized bed dryer, belt dryer, rotary dryer, tunnel dryer and drum dryer whereas batch drying requires
dryers such as kiln dryer, cabinet or compartmental dryer and tower dryer. Sub-atmospheric drying
includes vacuum shelf dryer, continuous vacuum dryer and freeze dryer. Novel drying technologies are
microwave drying, infra-red radiation drying, electric or magnetic field drying, superheated steam
drying, explosion puffing, foam mat drying, acoustic drying and osmotic dehydration (Jayaraman and
Gupta, 1995).
In this paper, drying methods will be studies and their principles of operation, uses and the
advantages and disadvantages of these methods in terms of their drying effectiveness and efficiency will
be addressed, with the future prospects of drying and dryers discussed.
2. Methods of Drying
The essence of drying in the industries, especially processing and food industry cannot be
overestimated. To achieve the expected drying of materials, several methods of drying have been
developed and further researches are still been carried out. Some of these include but not restricted to
the following listed.
2.1. Air Drying
Air-drying is considered to be one of the simplest and most economical ways of commercially
processing fruit and vegetables (Brennan et al, 1990). This process takes place when materials are dried
with unheated forced air, taking advantage of its natural drying potential. The process is slow and
weather dependent (Roy, 2015).

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Air-drying could be considered as appropriate to developing countries as the product, suitably
packaged, can be stored for several months without the risk of spoilage (Marriott and Lancester, 1983).
2.2. Hot- air Drying
In hot-air drying, enough water must be removed to lower the water activity to a level which
inhibits the growth of microorganisms and reduces the rate at which enzymatic and non-enzymatic
reactions occur. Thus, one of the primary requirements in using hot-air drying is to understand the drying
process, to be able to predict drying times, establish the distribution of moisture throughout the solid
pieces during drying and the influence of the processing variables such as air temperature and velocity,
pretreatment and the size of the pieces on drying behaviour (Johnson, et al, 1998). Hot-air drying is an
alternative drying method. Using hot air dryers leads to a more uniform, hygienic and attractive products
that can be produced rapidly (Karathanos and Belessiotis, 1999).
2.3. Sun Drying
Sun drying is one of the oldest and accessible processes used by farmers to preserve different
foodstuffs. It is an efficient way to reduce wastes (Touré, 2012). Sun drying is the traditional method of
drying in developing countries and it denotes the spreading of foodstuff in the sun (direct sunlight) on a
suitable surface such as mat, roof, or drying floors (Bindu et al, 2016).
Sun-drying method is cheaply executed, takes a longer time and may be prone to contaminations
from microorganisms due to unhygienic exposures (Yarkwa and Uvir, 2015). Moreover, the direct
exposure to sunlight, or more precisely ultra-violet radiation, can greatly reduce the level of nutrients
such as vitamins in the dried product (Tunde and Akitunde, 2011).
Folaranmi (2008) asserted that sun drying is plagued with in – built problems, since the product
during drying is unprotected from theft, rain, storm, windborne dirt, dust and infestation by insects,
rodents, and other animals, encourages mould growth and may result in relatively high final moisture
content. Consequently, the quality of dried products may be adversely affected, falling to meet the
required local and international standards (Ivanova and Andonov, 2001; Abdelhaq and Labuza, 1987).
Moreover, since sun drying depends on uncontrolled factors, production of uniform and standard
products is not expected. The quality of sun dried foods can be improved by reducing the size of pieces
to achieve faster drying and by drying on raised platforms, covered with cloth or netting to protect against
insects and animals (Bolea et al, 2012; Papu et al, 2014).
2.4. Solar Drying
Solar drying technology seems to be one of the most promising alternatives to reduce the post-
harvest losses (Wiriya et al, 2009). Solar drying technology is one of the renewable energy resources

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48
particularly for low temperature heating and is a very attractive option for the small scale and resource
poor enterprise (Chavda and Kumar, 2009). The solar dried products have much better colour and texture
as compared to open sun dried products (Mulokozi and Svanberg, 2003). The attractiveness of solar
dryers is further enhanced by its ability to dry the product rapidly, uniformly and hygienically to meet
national and international standards with zero energy costs (Condori et al, 2001). The food is dried using
solar thermal energy in a cleaner and healthier way (TaTEDO, 2007). However, it has been noted that,
drying at higher temperatures may cause damage to the flavour, colour and nutrients of the dehydrated
products (Praveenkumar et al, 2006). Due to the current trends towards higher cost of fossil fuels and
uncertainty regarding future cost and availability, use of solar energy in food processing will probably
increase and become more economically feasible in the near future.
They can be constructed from locally available materials at a relatively low capital cost and there
are no fuel costs. Thus, they can be useful in areas where fuel or electricity are expensive, land for sun
drying is in short supply or expensive, sunshine is plentiful but the air humidity is high
(Velayudham et al, 2015; Bindu et al, 2016). They give faster drying rates by heating the air to
10 - 300C above ambient, which causes the air to move faster through the dryer, reduces its humidity
and deters insects. The faster drying reduces the risk of spoilage, improves quality of the product and
gives a higher throughput, so reducing the drying area that is needed. However, care is needed when
drying fruits to prevent too rapid drying, which will prevent complete drying and would result in case
hardening and subsequent mould growth (Papu et al, 2014; Gavhale et al, 2015). Solar dryers also
protect foods from dust, insects, birds and animals (Sacilik, et al, 2006). They allow for less spoilage
and less microbiological infestation, thus leads to improved and more consistent product quality (Tunde
and Akitunde, 2011).
Solar drying technology produces better quality products and is considered to be an alternative
for drying agricultural products in developing countries (Gürlek, et al, 2009). However, dependency on
weather for drying operation is one of the setbacks in solar drying technology. (Prakash and Kumar,
2013; Sontakke and Salve, 2016).
2.4.1. Mechanism of solar dryer
A solar dryer has three main components and these are drying chamber, solar collector and some
type of airflow system. A drying chamber is an enclosed, insulated structure inside which both solar
collection and drying takes place. It is often insulated to increase efficiency (Chavda and Kumar, 2009).
The solar collector (or absorber) is often a dark coloured box with a transparent cover and Glass is
recommended for the absorber cover. The solar collector can be of any size and should be tilted toward
the sun to optimize collection. The size of solar collector required for a certain size of dryer depends on

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49
the ambient temperature, amount of sun, and humidity (Green and Shwartz, 2001; Chavda and kumar,
2009)
Tilting the collectors is more effective than placing them horizontally, for two reasons. First,
more solar energy can be collected when the collector surface is more nearly perpendicular to the sun's
rays. Second, by tilting the collectors, the warmer, less dense air rises naturally into the drying chamber
(Chua and Chou, 2003; Prakash and Kumar, 2013).
Solar dryers use one of two types of airflow systems which are natural (passive) and forced
(active) convection. The natural convection utilizes the principle that hot air rises, and forced convection
dryers force air through the drying chamber with fans. The effects of natural convection may be enhanced
by the addition of a chimney in which exiting air is heated even more (Eltawil et al, 2012). Additionally,
prevailing winds may be taken advantage of (Green and Shwartz, 2001; Zomorodian, et al, 2007).
The Solar Dryers may be classified into several categories, depending upon the mode of heating
or the mode of their operations and airflow systems. Depending on how heat is provided for drying, solar
dryers can be broadly divided into four categories namely; direct, indirect, mixed and hybrid types
(Fudholi et al., 2010; El-Sebaii and Shalaby, 2012).
2.5. Infra-red Drying
Infra-red is usually the best choice where a process requires high temperature and a lower capital
expense. It is used for heat setting as well as drying and curing thinner coatings, such as paints, ink paper,
textiles, adhesives in making films for appliances and electronics (Pawar and Pratape, 2017).
Infra-red drying involves heat transfer by radiation between a hot element and a material at lower
temperature. The peak wavelength of the radiation is dependent on the temperature of the heated element.
Thermal radiation is considered to be infrared in the electromagnetic spectrum between the end of the
visible 0.78µm and 1000 µm (Sulistiyanti et al, 2009).
In this method heating the product is performed without undesirable changes in structure, so the
structural quality of the product is improved, its biological yield is increased and costs of operation are
decreased (Strumillo and Kudra, 1987). Another advantage of drying using infrared radiation is the
minimization of product losses such as color change and shrinkage. Also IR radiation causes rapid and
direct heat concentration on the material compared to the convective dryers in which part of the heat is
absorbed by the inlet air and wasted. IR drying method in fact developed for high drying rate without
the risk of burning the material (Nonhebel, 1973).
In infrared method of drying, effective diffusion coefficient of moisture showed an increasing
trend with increasing radiation intensity and decreasing airflow rate. Increasing the intensity of radiation,
elevated the temperature gradient of the surface and underlying layers of the product. Also, decreasing

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50
the air velocity by reducing the cooling effect of air flow in this method increased the diffusion
coefficient inside the product (Saeid et al, 2011).
2.6. Microwave Drying
Microwave drying is an alternative method that has been used in the food industry. Microwaves
are electromagnetic waves which range between 0.3 GHz and 300 GHz. The most commonly used
frequencies are 915 MHz and 2450 MHz. The microwave heating method dehydrates food by
interactions between the electromagnetic energy and polar molecules within the material. Polar
molecules rotate in response to the applied oscillating electromagnetic waves. The reorientation of these
molecules in a high frequency electric field occurs frequently and rapidly, resulting in molecular friction
that generates heat (McGurk et al, 2017). Microwave heating is a desirable alternative drying method
since it enhances the energy efficiency and has less negative impact on the quality of the dehydrated
products (Huang, 2013).
Microwave drying of products has become common because microwave drying prevents a
decline in the quality of the product and; ensures the rapid and efficient distribution of heat within the
material (Li et al., 2009; Dong et al 2011; Silva et al., 2007). Moreover, microwave drying reduces
drying time and saves energy and also producing high quality dry products (Balbay et al., 2011; Li et al.,
2010).
Applying microwave energy under vacuum combines advantages of both vacuum drying and
microwave drying as far as improved energy efficiency and product quality are concerned. Vacuum dried
materials are characterized by higher porosity, depending on level of vacuum and less deterioration of
colour and volatile aroma ( Larrosa et al, 2017; shiby et al, 2015).
2.7. Drum Drying
In a drying operation, liquid, slurry, or puree material is applied as a thin layer onto the outer
surface of revolving drums that are internally heated by steam (Kasiri et al, 2004; Tang et al, 2010).
In drum drying, a large amount of thermal energy is released by the condensing steam in the
drum and conducted through drum wall to the product. During drying, a product may go through three
general periods. The first is the initial heating period where wet materials are applied onto the drum
surface in a thin layer. Intensive heat transfer occurs due to a great temperature difference between the
drum surface and the wet product and product temperature increases rapidly to reach the boiling point
of free water. The second period is the constant product temperature period where after reaching the
boiling temperature, a large amount of free water evaporates and product temperature remains constant.
The drum surface temperature, however, decreases due to an intense evaporative cooling. The third is
the rising product temperature period, where after removing most of the free water, the amount of

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51
moisture for evaporation is dramatically reduced. The heat transferred from the steam gradually exceeds
the energy used for evaporation. As a result, drum surface temperature increases. The bound water starts
to play a major role in controlling the rate of evaporation. As bound water has a higher boiling
temperature, product temperature gradually increases as drying proceeds (Rodriguez, et al, 1996).
In the operation of a drum dryer, a delicate balance needs to be established among feed rate,
steam pressure, roll speed, and thickness of the product film. It is desirable to maintain a uniform film
on the drum surface to ensure maximized throughput and consistent final moisture content. Problems,
however, are often encountered due to fluctuations in the moisture content and thickness of the feed.
Means have been developed to automatically detect the moisture content and temperature, integrated
with automated feedback control to minimize the fluctuations. (Rodriguez, et al, 1996)
Products containing high sugar contents, such as tomato puree, may be difficult to remove from
the drums at high temperatures due to the thermo plasticity of those materials. A cooling mechanism
(e.g., a jet of cold air) may be used at the location just before the product reaches the scraper. The purpose
of the cooling is to bring the product from a rubbery state into a glassy state to facilitate separation of
the product from the drum surface (Tang et al, 2010).
Drum dryers are used in the food industry for drying a variety of products, such as, milk products,
baby foods, breakfast cereals, fruit and vegetable pulp, mashed potatoes, cooked starch, and spent yeast
(Rodriguez et al, 1996).
2.8. Spray Drying
This technique enables the transformation of feed from a fluid state into dried particulate form
by spraying the feed into a hot drying medium. It is a continuous particle processing drying operation.
The feed can be a solution, suspension, dispersion or emulsion. The dried product can be in the form of
powders, granules or agglomerates depending upon the physical and chemical properties of the feed, the
dryer design and final powder properties desired (Michael, 1993).
Spray drying is presently one of the most exciting technologies for the pharmaceutical industry,
being an ideal process where the end-product must comply with precise quality standards regarding
particle size distribution, residual moisture content, bulk density and morphology. The production of
particles from the process of spraying has gained much attention in recent years (Suthur et al, 2009).
2.9. Oven Drying
In conventional oven heating, the heat is transferred from the surface to the interior of the
material. Thus a pressure is generated between the surface and interior due to evaporation, such that the
interior moisture is driven out and evaporation continues at the surface. However, conventional oven
heating has low energy efficiency with negative quality effects (Huang, 2013).

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The oven takes two or three times longer to dry food when compared with other dryers. Thus,
the oven is not as efficient and uses more energy. There are two types of ovens namely batch and
conveyor ovens. The oven uses convective process (force air convective and gravity convective). It is
used in various industrial applications for drying, curing (rubber), baking, etc. Lapses in oven drying are
due to induced evaporation. Explosion can occur since the product when dried reaches its combustion
level. It is difficult to get humidity of the oven due to lack of flow velocity for easy circulation (Brennand,
1994).
2.10. Vacuum Drying
Vacuum drying is a process that allows for materials to be dried in a reduced pressure
environment, which lowers the heat needed for rapid drying. This drying method is recommended to
counter the undesirable effects of Infrared drying method and to improve the product quality, as well as
nutritional value. The method allows effective moisture elimination under low pressure (Jaya and Das,
2003). Vacuum drying enhances the mass transfer because of an increased vapour pressure gradient
between the inside and outside of the product (Pere and Rodier, 2002).
Vacuum dryers offer low-temperature drying of thermolabile materials and are suitable for
solvent recovery from solid products containing solvents (Parikh, 2015) and also less energy usage
and hence greater energy efficiency, improved drying rates, and in some cases, less shrinkage of the
product (Alibas, 2007; Alibas, 2009).
2.11. Freeze Drying
Lyophilization or freeze drying is a process in which water is frozen, followed by its removal
from the sample, initially by sublimation (primary drying) and then by desorption (secondary drying).
Freeze drying is a process of drying in which water is sublimed from the product after it is
frozen.(Akers et al,1987; Chien and Yiew, 1981). The term “lyophilization” describes a process that
produce a product that “loves the dry state” (Remington, 2000a).
Intact cake, sufficient strength, uniform color, sufficiently dry, sufficiently porous, sterile, free
of pyrogens, free of particulates, chemically stable are the desired characteristics freeze dried products
(Khairnar et al, 2013; Pandhare et al, 2015; Rajeevini et al, 2015).
Three methods of freeze drying are commonly used namely; manifold drying, batch drying, and
bulk drying. Each method has a specific purpose, and the method used depends on the product
and the final configuration desired. Since freeze drying is a change in state from the solid phase to the
gaseous phase, material to be freeze dried must first be adequately prefrozen (Khairnar et al,2013;
Gaidhani et al, 2015; Rajeevani et al, 2015).

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It is a drying process applicable to manufacture of certain pharmaceuticals and biologicals that
are thermo labile or otherwise unstable in aqueous solutions for prolonged storage periods, but that are
stable in the dry state. The knowledge of how to control, or at least manipulate, the freezing step will
help to develop more efficient lyophilization cycles and biopharmaceutical products with an improved
stability (Kasper and Friess, 2011; Esfandiary et al, 2016).
2.12. Osmotic Drying
Osmotic dehydration is the phenomenon of removal of water from lower concentration of solute
to higher concentration through semi permeable membrane results in the equilibrium condition in both
sides of membrane (Tiwari 2005). Osmotic dehydration found wide application in the preservation of
food-materials since it lowers the water activity of fruits and vegetables. Osmotic dehydration is
preferred over other methods due to their color, aroma, nutritional constituents and flavor compound
retention value (Yadav and Singh, 2014; Alakali et al., 2006; Torres et al., 2006; Pokharkar and Prasad,
1998). Osmotic dehydration results in increased shelf-life, little bit loss of aroma in dried and semidried
food stuffs, lessening the load of freezing and to freeze the food without causing unnecessary changes
in texture (Petrotos and Lazarides, 2001).
Osmotic dehydration involves the immersion of foods (fish, vegetables, fruits and meat) in
osmotic solution such as salts, alcohols, starch solutions and concentrated sugars, which some extent to
dehydrates the food (Erle and Schubert, 2001). Different types of solutes such as fructose, corn syrup,
glucose, sodium chloride and sucrose are used as osmotic agent for OD (Azuara and Beristain, 2002).
It involves dehydration of fruit slices in two stages, removal of water using as an osmotic agent
(osmotic concentration) and subsequent dehydration in a dryer where moisture content is further reduced
to make the product shelf stable (Ponting, 1973).
Therefore, the characteristics of the product can be varied by controlling temperature, sugar syrup
concentration, concentration of osmosis solution, time of osmosis etc., to make osmotic concentration
process faster (Chavan and Amarowicz, 2012).
2.13. Foam Mat Drying
The foam mat drying is a process in which the trans-formation of products from liquid to stable
foam is followed by air drying (Franco et al, 2015; Sangamithra et al, 2015; Affandi et al, 2017).
Stable gas-liquid foam is the primary condition for successful foam drying. Proteins, gums and
various emulsifiers such as glycerol monostearate, propyleneglycerol monostearate, carboxymethyl
cellulose [CMC], trichlorophosphate are used as foaming agents. Mixtures are whipped to form stable
foams using blender or specially designed device. The foam is then spread as a thin sheet or mat and
exposed to a stream of hot air until it is dried to desired moisture content (Rajkumar et al, 2007). Drying

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is carried out at relatively low temperatures to form a thin porous honeycomb sheet or mat, which is
disintegrated to yield a free-flowing powder. The larger surface area exposed to the drying air is the main
cause of moisture removal acceleration (Brygidyr et al. 1977). However, capillary diffusion is also the
main reason for the moisture movement within the product during foam mat drying (Sankat and
Castaigne 2004).The dried product obtained from foam mat drying is of better quality, porous and can
be easily reconstituted.
This process can be used for large-scale production of fruit powders because of its suitability for
all types of juices, rapid drying at lower temperature, retention of nutritional quality, easy reconstitution
and is cost-effective for producing easily re-constitutable juice powders. Fruit juice powders obtained
through this process have high economic potentials over their liquid counterparts such as reduced volume
or weight, reduced storage space, simpler handling and transportation, and much longer shelf life
(Sangamithra et al, 2015; Sharifi et al, 2015; Chandrasekar et al, 2015; Singh and Dixit, 2014;). The
fruit powders obtained through this method can find applications in snacks, beverages, ice creams,
bakery products, as a starter for the preparation of instant foods, pastes, etc. (Sangamithra et al, 2015)
A proper understanding of selection of suitable foaming agents, foaming properties such as foam
density, foam expansion, foam stability, method of drying, drying temperature, are required for the
process optimization, in order to obtain products with better nutritional characteristics and process yield
(Kandasamy et al, 2014; Kumar et al, 2015; Ismaila et al, 2016).
2.14. Impingement Drying
Air impingement drying is not a new technology but it is a complicated process and it is used for
food and agricultural product processing. Impingement drying can be affected by many factors such as
drying temperature, air velocity and relative humidity which can influence its drying characteristics and
quality (Xiao et al, 2010).It is an efficient drying process and has been used successfully in paper and
textile industries and also with the purpose of drying, cooling, or heating different artefacts and metals.
It has only recently been applied to food products (Moreira 2001). One of the obvious advantages of
impingement drying techniques is rapid drying. In air impingement processing, the air impinges on
the product surface at high velocity, removes thermal boundary layers of moisture and cold air
and increases the rate of heat transfer (Anderson and Singh 2006). The high-velocity air from the nozzles
creates a bed of hot air that suspends the products; thus, temperature at the center of the product rises
rapidly to the drying air temperature. So, air impingement drying can greatly accelerate heat transfer and
reduce process time (Mujumdar 1986). Uniform, hygienic and attractively colored product can be
produced rapidly using air impingement drying (Xiao et al, 2010).

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Energy consumption and environmental issues are naturally important factors in drying processes
and impingement jet drying are no exception (Ljung et al, 2017).
2.15. Acoustic Drying
This process can cause the removal of moisture from a material under high intensity sound field
(audible and ultrasound frequency range). There are two phenomenas involve in acoustic drying. In
primary phenomena, the factors involve in acoustic drying are sound frequency, intensity, sound pressure
and particle velocity while the change of hydrodynamic condition, barometric pressure and turbulent
agitation of air at the boundary layer close to the material is the secondary phenomena(Seya and Otsuka,
1980; Kouchakzadeh, 2013).
2.16. Explosion Puffing Drying
This is a new drying technology that uses high temperature and pressure to dry products. It has
the combine advantages of a convective hot air and a vacuum freeze drying. The puffed fruits and
vegetables are natural, healthy foods and contain plentiful nutrition, also it has excellent qualities and
wide application prospects (Sullivan and Craig Jr, 1994; Kozempel et al, 2008).
2.17. Hybrid Dryers
There are different types of hybrid dryers available with different operational techniques and
designs. Their major advantages are increased drying rates, better, safer, efficient and effective and good
quality of products. Solar dryers may be useful as hybrid dryer when added as a means of heating air for
artificial dryers to reduce fuel costs (Fellows, 1997). The combined process of osmotic dehydration (OD)
and freezing is called osmodehydrofreezing which is used to get better texture properties of fruits and
vegetables as well as lessen the structural collapse and drip loss. Giannakourou and Taoukis (2003)
studied that change in quality of osmodehydrofrozen of green peas treated with maltitol and trehalose
combined with CaCl2 and NaCl and they observed that osmotic treatment lowered the quality changes
in term of texture, color and retention of ascorbic acid for frozen samples. The dehydrofreezing process
also concerned with improving of quality (Khan, 2012).
Infrared radiation has been used in conjunction with several drying methods because it has
advantages of increasing the drying efficiency (Ratti and Mujumdar, 1995). Examples are the Infrared –
vacuum drying (Swasdiasevi et al, 2009; Saetan et al, 2013; Yunhong et al, 2015; Alaei and Chayjan,
2015), Foam mat – cabinet dryer (Thirupathi et al, 2008; Rajkumar et al, 2007), microwave – vacuum
dryer (Berteli et al, 2009); Yongsawatdigul and Gunasekaran,1996); Clary et al, 2007), vacuum – spray
dryer (Wisniewski, 2015), oven vacuum dryer (Amellai and Benamara ,2008); Kumaravel et al, 2012),
low – pressure superheated steam drying and far infrared radiation (LPSSD – FIR),Nimmol et al, 2007;

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Pawar and Pratape (2017), impingement and superheated steam drying (Moreira, 2001), impingement
and hot air drying (Xiao et al, 2012; Jambunathan et al, 1992; Xiao et al, 2014), etc. Efforts are been
made to design more of these hybrid dryers which can perform better under adverse weather conditions.
3. Advantages, Limitations and Applications of Drying Methods
Table 1 gives a summary of all the drying techniques discussed with their advantages,
limitation(s) and application(s).
Table 1: Advantages, limitations and applications of drying methods
S/N
METHOD
ADVANTAGE(S)
LIMITATION(S)
APPLICATION
REFERENCES
1.
Natural(Open)
air Drying
 Simplest and
most economical
way of drying.
 Use unheated
heat(free air)
 Natural drying
potential
 Can be stored for
some months
without spoilage
 No equipment
 Kept in open
space
 Has problem of
contamination,
infestation, microbial
attack and the drying
time is long.
 Cannot handle large
quantities and to
achieve consistent
quality standards.
 Problem of predators
 Process is slow and
weather dependent.
Fruits and
Agricultural
products e.g
corn, rice, millet,
bean, pepper,
okro, groundnut,
yam, sorghum,
plantain chips,
etc Practiced in
tropical
countries.
Itodo et al, 2002
Togrul and
Pehlivan, 2004
Bolaji, 2005
2.
Hot air Drying
 Rate of drying is
fast
 Low capital and
maintenance
cost
 Flexible in
operation
 Water activity
will be reduced
to a level that
inhibits growth
of
microorganism
 Influence the
processing
variables e.g
temperature, air
velocity,
humidity
 Low energy
efficiency
 Quality loss and long
drying time during
falling rate period
 Relatively poor
quality/control as
food dries rapidly if
close to heat source.
Food and
agricultural
products
Boudrioua et al,
2003
Johnson et al , 1998
3.
Fluidized Bed
drying
 Thorough
mixing of solids
which results in
efficient mass
and heat
transfer.
 Rapid and
economic drying
 Ease of control
 Loss of product
qualities such as
color, texture,flavour
and nutrients
Food and
Agricultural
products
Sagar and Kumar,
2010
Elkhodiry et al,
2015

Int. J. Food Nutr. Saf. 2017, 8(1): 45-72
Copyright © 2017 by Modern Scientific Press Company, Florida, USA
57
 Temperature
uniformity
 Easy handling
and transport of
material
4.
Sun Drying
 Natural heat
from sun
 Cheap to use
and economical
 Spreading of
foodstuff under
direct sunlight
on a suitable
surface
 Require little
expertise
 Efficient(drying)
way to reduce
waste
 Reducing the
thickness of the
product for
faster drying.
 Poor quality due to
vulnerability to
contamination by
insects, birds and dust
 Direct exposure to
sunlight (or ultra –
violet radiation) can
reduce the level of
nutrients.
 Unprotected from
theft, rain and
infestation
 May result in
relatively high final
moisture content
which encourages
mould growth
 Takes longer time to
dry
 Due to uncontrollable
factors, production of
uniform and standard
products is not
expected.
Food and
agricultural
products
Ivanova and
Andonov, 2001
Yarkwa and Uvir,
2015
Abdelhaq and
Labuza, 1987
Karathanos and
Belessiotis, 1997
Bolea et al, 2012
Papu et al, 2014
5.
Solar Drying
 Require little
maintenance
 Low
temperature
heating
 Most cost
effective: uses
energy from
the sun to heat
a stream of air
to provide air.
 Uses natural or
forced
convective air
 Less
contamination
because it is in
a closet
 Less
susceptible to
adverse
weather
conditions
 Depending on
the size and
sophistication
of the dryer.
 Reduces land
required when
compare to air
drying.
 Requires adequate
solar radiation
 UV radiation can
damage food
nutrients
 Expensive than direct
sun drying
 Need skilful fellow to
construct.
 Each location have
different
configuration/type
Food and
agricultural
products
Akbulut and
Durmus, 2010
Bolaji and olalusi,
2008
Chavda and Kumar,
2009
Wiriyi et al, 2009
Mulokozi and
Svanberg, 2003
Praveenkumar et al,
2006
Fellows, 1997
Gurlek et al, 2009
Sontakkre and
Salve, 2016
Condori et al, 2001

Int. J. Food Nutr. Saf. 2017, 8(1): 45-72
Copyright © 2017 by Modern Scientific Press Company, Florida, USA
58
 Attractive to
small scale
entrepreneur
 It is hygiene,
prevent
contamination
and infestation
6.
Infra-red
Drying
 Requires
heating at high
temperature
 Cost of
operation is
reduced
 minimization of
product losses
 Rapid and
direct heat
concentration
on the material.
 Gives high
drying rate
without
burning the
material.
 Has an increasing
radiation intensity
 Decreases airflow
rate
 Diffusion coefficient
inside the product is
increased.
 Requires electricity
 Dryer is a
sophisticated one and
maybe difficult to
repair.
It is used for heat
setting as well as
drying and
curing thinner
coatings such as
paints, ink paper,
textiles,
adhesives in
making films for
appliances and
electronics.
Sulistiyanti et al,
2009
Pawar and Pratape,
2017
Ratti and Mujumdar,
1995
Nonhebel, 1973
Saeid et al, 2011
7.
Microwave
Drying
 Short or
reduces drying
time
 Temperature
and moisture
gradients are in
the same
direction
 Enhances
energy
efficiency and
has distribute
more thermal
efficiency
 Improve
quality and
flavour of the
product.
 Non uniform heating
(uneven distribution
of microwave field
can occur)
 Overheating may
take place
 Quality deterioration
can take place.
Food,
agricultural and
dairy products
Huang, 2013
Balbay et al, 2011
Li et al, 2010
Larrosa et al, 2017
Shiby et al, 2015
Silva et al, 2007
Dong et al, 2011
8.
Drum Drying
 Have high
drying rate
 High energy
efficiencies
 Suitable for
slurries drying
 High capital cost of
machined drum
 High damage to
sensitive foods
 Electricity failure
could affect the
operation
 Fluctuations in the
moisture content and
thickness of the feed
 Product like puree
may be difficult to
remove at high
temperature due to
the thermal
plasticity.
Food industry
e.g Potato flakes,
cereals, fruits
purees, baby
foods, etc
Rodriguez et al,
1996
Kasiri et al, 2004
Tang et al, 2010

Int. J. Food Nutr. Saf. 2017, 8(1): 45-72
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59
9.
Spray Drying
 Rapid drying
 Low labour
cost
 Relatively
simple
operation and
maintenance
 Reduces the
transport
weight of foods
 Compliance
with the
product
standards.
 High capital cost
 High feed moisture
content to ensure that
it is pump to the
atomiser
Food processing
and
Pharmaceutical
industries.
Products like
milk, ice cream ,
egg yoghurt,
juice, etc
Michal, 1993
Surthur et al, 2009
10.
Oven Drying
 Removes
moisture content
 Low labour cost
 Flexibility in
operation
 Less
contamination
from infestation
 Cost efficient
 Longer drying time
 Harder to control
than drying with
dehydrator
 Not efficient and uses
more energy
 Explosion might
occur due to induced
evaporation
 Case Harding of
product may take
place because of
much heat
Food and
agricultural
products
Bennand, 1994
Huang, 2013
11.
Vacuum Drying
 It allows effective
moisture
elimination under
low pressure
 Enhances the
mass transfer
because of an
increased vapour
pressure gradient
between the
inside and outside
of the product
 Improved drying
rates
 Offer low-
temperature
drying for
thermolabile
materials
 Suitable for
solvent recovery
from solid
products
containing
solvents
 Operating under low
pressure only
 Uses to counter infra
red effect
 Uses much energy
 Less shrinkage of the
product
Food and
agricultural
products
Parikh, 2015
Alibas,2007,2009
Alaei and Chayjan,
2015
Jaya and Das, 2003
Pere and Rodier,
2002
12.
Freeze Drying
 Uses sublimation
technique
 Oxidizable
substances are
well protected
under vacuum
conditions
 Volatile compounds
may be removed by
high vacuum
 Single most
expensive unit
operation
 Pharmaceutical
s and
biologicals that
are
thermolabile
 Pharmaceutical
and
Long et al, 2013
Rajeevini et al, 2015
Harish et al, 2017)
Gatin et al, 2008
Khairnar et al, 2013

Int. J. Food Nutr. Saf. 2017, 8(1): 45-72
Copyright © 2017 by Modern Scientific Press Company, Florida, USA
60
 Long
preservation
period owing
water removal
 Loading quantity
accurate and
content uniform
 Little
contamination
owing to aseptic
process
 Minimal loss in
volatile
chemicals and
heat-sensitive
nutrient and
fragrant
components
 Minimal changes
in the properties
because microbe
growth and
enzyme effect
can not be
exerted under low
temperature
 Transportation
and storage under
normal
temperature
 Rapid
reconstitution
time
 Constituents of
the dried material
remain
homogenously
dispersed
 Product is
process in the
liquid form
 Sterility of
product can be
achieved and
maintained
 Stability problems
associated with
individual drugs
 Some issues
associated with
sterilization and
sterility assurance of
the dryer chamber
and aseptic loading
of vials into the
chamber.
biotechnology
industry
 In Food
Industry
 In
Technological
Industry for
chemical
synthesis and
bioseparations
Pandhare et al,
2015
Akers et al,1987
Chien and Yiew,
1981
Sunderland, 1980
Shuka, 2011;
Ciurzynska and
Lenart,2011
Gaidhani et al, 2015
Sanjith and Gatin,
1993
13.
Osmotic Drying
 Removal of water
from lower
concentration of
solute to higher
concentration
through semi
permeable
membrane results
in the equilibrium
 It lowers the
water activity
 Retention value
of color, aroma,
nutritional
constituents and
flavor compound
 Some of the
osmotic agent may
be costly and not
readily available.
 The high viscosity
of the osmotic
solution
 low density
difference between
the solid and the
solution
 It is a time taking
process
 The reduction in
acidity level
reduces the
Preservation of
food-materials
Fruits and
vegetables such
as banana,
sapota,
pineapple,
mango, and leafy
vegetables, etc
Tiwari, 2005
Yadav and Singh,
2014
Alakali et al., 2006;
Torres et al., 2006
Petrotos and
Lazarides, 2001
Azuara and
Beristain, 2002
Giannakourou and
Taoukis, 2003

Int. J. Food Nutr. Saf. 2017, 8(1): 45-72
Copyright © 2017 by Modern Scientific Press Company, Florida, USA
61
 Decrease the
energy costs
 Lessening the
load of freezing
 Inhibiting the
browning of
enzymes
 Conducted at low
or ambient
temperature.
characteristic taste
of some products
Khan, 2012
Pokharkar and
Prasad, 1998
Ponting, 1973
Chavan and
Amarowicz, 2012
14.
Foam mat
Drying
 Used for large-
scale production
of fruit powders
because of its
suitability for all
types of juices
 Rapid drying at
lower
temperature
 Retention of
nutritional
quality
 Easy
reconstitution of
the product
powders
 cost-effective
for producing
easily re-
constitutable
juice
 have high
economic
potentials over
their liquid
counterparts
 much longer
shelf life
 Low bulk density
 High cost of water
removal
 Energy demand
Reduces drying time
 Necessity to addition
of forming agent for
stability of the liquid
or semi – solid food
material which make
such products
unavailable in their
pure form.
In food
processing
industry:
Applications in
snacks,
beverages, ice
creams, bakery
products, as a
starter for the
preparation of
instant foods,
pastes, etc
Sangamithra et al,
2015
Chandrasekar et al,
2015
Franco et al, 2015
Affandi et al, 2017
Rajkumar et al,
2007
Brygidyr et al. 1977
Sankat and
Castaigne 2004
Kudra and Ratti
2006
Kandasamy et al,
2014
Kumar et al, 2015
Ismaila et al, 2016
Kadam et al. 2010a
15.
Impingement
Drying
 Most suitable
for products that
have a high
surface area
 Used to increase
the convection(
i.e convective
heat transfer)
 Rapid drying
 increases the
rate of heat
transfer
 Thermal damage
incurred by a
product during
drying is directly
proportional to the
temperature and
time involved
 Heat transfer at the
surface of the
product
 High capital cost
 Web tension issues
 Energy
consumption and
environmental
issues
Paper, textile and
metal industry.
Food and
agricultural
product
processing.
Ljung et al, 2017
Xiao et al, 2010
Mujumdar 1986
Anderson and Singh
2006
Moreira, 2001
Lin et al.1998
16.
Acoustic
Drying
 Effective at
lower
temperatures
 Uses high
intensity
 It is expensive
 Usefulness is
limited because of
sophistication
 Needs to be used in
combination with
Pharmaceuticals,
food and
chemical
industry
Seya and Otsuka,
1980
Kouchakzadeh,
2013

Int. J. Food Nutr. Saf. 2017, 8(1): 45-72
Copyright © 2017 by Modern Scientific Press Company, Florida, USA
62
 It improve
elasticity
 The rate of
water removal
increased when
acoustical
energy is
applied
 Enhancing
drying
other process(e.g
heating)
 Posible damage by
free radical
 Complex mode of
action
17.
Explosion Puff
Drying
 Faster
rehydration of
product
 Rapid drying
 Gives low
moisture content
 Retention of
texture and
flavour of
product
 Lower nit
processing cost
 Processing of
large pieces of
product feasible
 Labour intensive
 Loss of product
intergrity
 High level of heat
Food and
agricultural
products like
apple,
blueberries,
carrots, potatoes,
etc
Sullivan and Craig
Jr, 1984
Kozempel et al,
2008
4. Future Prospects
Drying researches and developments have seen exponential growth over the past three decades.
Initially driven by the need to conserve energy in this highly energy-intensive operations found in almost
all industrial sectors, and focuses on product quality, environmental impact, safety issues, new products,
and processes. Drying provides challenging areas for multi- and cross-disciplinary research of
fundamental as well as applied nature coupling transport phenomena with material science. New areas
of development in drying technologies are Hybrid drying, Superheated Steam drying, Pulse Combustion
drying, Intermittent drying, Spray drying, Impingement drying, etc. Further development in design of
dryers like the continuous foam dryer for example will help to achieve stable foam, which in turn results
in dried powder of high quality and also studies on the microstructure characterization of foams and
foam-dried powders, computer simulation techniques for the prediction of moisture and temperature
distribution in the product requires the attention of researchers for further up gradation of the process. It
is highly expected that the further improvement in foam mat drying process, as well as the use of other
drying method combined with foam mat drying, will intensify the adoption of this renewed method in
the food industry (Sangamithra et al, 2015).The need for industry-academia interaction and for a stake
of industry in academic research is noted as a key step towards successful transfer of innovative drying
technologies to industry (Mujumdar, 2004).

Int. J. Food Nutr. Saf. 2017, 8(1): 45-72
Copyright © 2017 by Modern Scientific Press Company, Florida, USA
63
5. Conclusion
Drying is an important process to preserve raw food materials and the drying process occurs
when water vapor is removed from its surface into the surrounding space, resulting in a dried material
with an extended shelf life and reduced water activity of food products. During drying, the moisture
content can be reduced to a level ranging 1 – 5%, which avoids microbial and undesirable enzymatic
reactions.
There is lots of drying methods and each of these has its own advantages and disadvantages and
could be applied effectively. We also have a combination of two or three methods as in the cases of
hybrid which can give better drying.
Traditional drying methods (sun, oven and solar) are simple to use, however they are not
economical. They have low energy efficiency and require longer drying time. As a result, it negatively
affects the flavour, nutrient content, and may lead to undesirable colours in the dried end product. In
addition, case-hardening is another problem in drying products with these traditional methods. Since the
evaporation of surface water is faster than the movement of water inside the food, case hardening occurs
which prevents the proper drying of the food to its optimal moisture level for storage. The formation of
the outer hard case not only affects the appearance but also the taste of the dried products. Compared to
conventional drying methods, microwave drying has more thermal efficiency and provides an end
product with improved quality and flavour (Huang, 2013).
Acknowledgments
The authors gratefully acknowledge the University of Uyo for superseding.
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