Different Drying Methods: Their Applications and Recent Advances

Abstract

Drying is one of the oldest preservation processes available to the mankind, on that we can track since prehistoric times. In today food market dried foods play an important role in the food supply chain. As for fruits and vegetables it can be estimated that they constitute about 1% of the total drying in the food industry, by large being the grains the most important. The main feature of this process consists on lowering the water content in order to avoid or slow down food spoilage by microorganism. At this point some understanding can arise derived from the vocabulary employed; common words found are " drying " or " dehydration " , or even " dewatering ". There are various aspects that must be considered when drying small fruits and vegetables, whether for the food or nutraceutical and functional food industries. A system which minimizes exposure to light, oxidation and heat, (i.e. high heat 70 o C and shorter time duration) may help conserve critical bioactive compounds. This review focuses upon conventional and new drying technologies and pre-treatment methods based upon drying efficiency, quality preservation, and cost effectiveness.

Full text

Copyright © 2013 by Modern Scientific Press Company, Florida, USA
International Journal of Food Nutrition and Safety, 2013, 4(1): 34-42
International Journal of Food Nutrition and Safety
Journal homepage: www.ModernScientificPress.com/Journals/IJFNS.aspx
ISSN: 2165-896X
Florida, USA
Review
Different Drying Methods: Their Applications and Recent
Advances
Naseer Ahmed ٭, Jagmohan Singh, Harmeet Chauhan, Prerna Gupta Anisa Anjum, Harleen
Kour
Division of Post Harvest Technology, Sher-e-Kashmir University of Agricultural Science and
Technology, Jammu, India
* Author to whom correspondence should be addressed; E-Mail: ewsjammu@gmail.com.
Article history: Received 1 June 2013, Received in revised form 9 July 2013, Accepted 11 July 2013,
Published 16 July 2013.
Abstract: Drying is one of the oldest preservation processes available to the mankind, on
that we can track since prehistoric times. In today food market dried foods play an
important role in the food supply chain. As for fruits and vegetables it can be estimated that
they constitute about 1% of the total drying in the food industry, by large being the grains
the most important. The main feature of this process consists on lowering the water content
in order to avoid or slow down food spoilage by microorganism. At this point some
understanding can arise derived from the vocabulary employed; common words found are
“drying” or “dehydration”, or even “dewatering”. There are various aspects that must be
considered when drying small fruits and vegetables, whether for the food or nutraceutical
and functional food industries. A system which minimizes exposure to light, oxidation and
heat, (i.e. high heat 70 oC and shorter time duration) may help conserve critical bioactive
compounds. This review focuses upon conventional and new drying technologies and pre-
treatment methods based upon drying efficiency, quality preservation, and cost
effectiveness.
Keywords: drying method; food; fruit; vegetable; sun drying; solar drying; osmotic
dehydration; freeze drying.

Int. J. Food Nutr. Saf. 2013, 4(1): 34-42
Copyright © 2013 by Modern Scientific Press Company, Florida, USA
35
1. Introduction
Drying is the oldest method of preserving food. Throughout history, the sun, the wind, and a
smoky fire were used to remove water from fruits, meats, grains, and herbs. By definition, food
dehydration is the process of removing water from food by circulating hot air through it, which
prohibits the growth of enzymes and bacteria. Dried foods are tasty, nutritious, lightweight, easy-to-
prepare, and easy-to-store and use. The energy input is less than what is needed to freeze or can, and
the storage space is minimal compared with that needed for canning jars and freeze containers. The
nutritional value of food is only minimally affected by drying. Vitamin A is retained during drying;
however, because vitamin A is light sensitive, food containing it should be stored in dark places.
Yellow and dark green vegetables, such as peppers, carrots, winter squash, and sweet potatoes, have
high vitamin A content. Vitamin C is destroyed by exposure to heat, although pre-treating foods with
lemon, orange, or pineapple juice increases vitamin C content. Dried fruits and vegetables are high in
fiber and carbohydrates and low in fat, making them healthy food choices.
2. Drying Technologies
2.1. Sun Drying
The high sugar and acid content of fruits make them safe to dry in the sun (Plate 1). Vegetables
and meats are not recommended for sun drying. Vegetables are low in sugar and acid. This increases
the risks for food spoilage. Meats are high in protein making them ideal for microbial growth when
heat and humidity cannot be controlled. To dry in the sun, hot, dry, breezy days are best. A minimum
temperature of 86 oF is needed with higher temperatures being better. It takes several days to dry foods
out-of-doors. Because the weather is uncontrollable, sun drying can be risky. Also, the high humidity
in the South is a problem. Humidity below 60 percent is best for sun drying. Often these ideal
conditions are not available when fruit ripens. Fruits dried in the sun are placed on trays made of
screen or wooden dowels. Screens need to be safe for contact with food. The best screens are stainless
steel, teflon coated fiberglass or plastic. Avoid screens made from “hardware cloth”. This is galvanized
metal cloth that is coated with cadmium or zinc. These materials can oxidize, leaving harmful residues
on the food. Also avoid copper and aluminum screening. Copper destroys vitamin C and increases
oxidation. Aluminum tends to discolor and corrode. Outdoor drying rack most woods are fine for
making trays. However, do not use green wood, pine, cedar, oak or redwood. These woods warp, stain
the food or cause off-flavors in the food. Place trays on blocks to allow for better air movement around
the food. Because the ground may be moist, it is best to place the racks or screens on a concrete
driveway or if possible over a sheet of aluminum or tin. The reflection of the sun on the metal

Int. J. Food Nutr. Saf. 2013, 4(1): 34-42
Copyright © 2013 by Modern Scientific Press Company, Florida, USA
36
increases the drying temperature. Cover the trays with cheesecloth to help protect the fruit from birds
or insects. Fruits dried in the sun must be covered or brought under shelter at night. The cool night air
condenses and could add moisture back to the food, thus slowing down the drying process.
Plate 1. Sun drying
2.2. Solar Drying
Recent efforts to improve on sun drying have led to solar drying (Plate 2). Solar drying also
uses the sun as the heat source. A foil surface inside the dehydrator helps to increase the temperature.
Ventilation speeds up the drying time. Shorter drying times reduce the risks of food spoilage or mold
growth.
Plate 2. Solar dryer
2.3. Freeze Drying
Freeze-drying, also known as lyophilisation, or cryodesiccation, is a dehydration process
typically used to preserve a perishable material or make the material more convenient for transport

Int. J. Food Nutr. Saf. 2013, 4(1): 34-42
Copyright © 2013 by Modern Scientific Press Company, Florida, USA
37
(Plate 3). Freeze-drying works by freezing the material and then reducing the surrounding pressure to
allow the frozen water in the material to sublimate directly from the solid phase to the gas phase.
Plate 3. Freeze dryer
2.4. Oven Drying
Everyone who has an oven has a dehydrator. By combining the factors of heat, low humidity
and air flow, an oven can be used as a dehydrator (Plate 4). An oven is ideal for occasional drying of
fruit leathers, banana chips or for preserving excess produce like celery or mushrooms. Because the
oven is needed for every day cooking, it may not be satisfactory for preserving abundant garden
produce. Oven drying is slower than dehydrators because it does not have a built-in fan for the air
movement. (However, some convection ovens do have a fan). It takes about two times longer to dry
food in an oven than it does in a dehydrator. Thus, the oven is not as efficient as a dehydrator and uses
more energy.
Plate 4. Oven dryer

Int. J. Food Nutr. Saf. 2013, 4(1): 34-42
Copyright © 2013 by Modern Scientific Press Company, Florida, USA
38
3. Recent Advances in Drying Technology
Osmotic dehydration is the process of water removal by immersion of water-containing cellular
solid in a concentrated aqueous solution (Ponting, 1973). The fundamental purpose of food
dehydration is to lower the water content in order to minimize rates of chemical reactions and to
facilitate distribution and storage. In osmotic dehydration, foods are immersed or soaked in a saline or
sugar solution. This results in three types of counter mass transfer phenomenon (Ponting, 1973). First,
water outflow from the food tissue to the osmotic solution, second, a solute transfer from the osmotic
solution to the food tissue, third, a leaching out of the food tissue’s own solutes (sugars, organic acids,
minerals, vitamins) into the osmotic solution. The third transfer is quantitatively negligible compared
with the first two types of transfer, but essential with regard to the composition of the product. Its
driving force is the difference in the osmotic pressure of solutions on both sides of the semi-permeable
cell membranes. Dabhake and Khedkar (1980) observed that drying of raw mango pieces in a cabinet
drier is much faster as compared to sun drying. Mehta and Tomar (1980) reported that air circulation
drier yield good quality of products by drying guava and papaya slices. Kalra and Bhardwaj (1981)
conducted an experiment using solar dehydration for fruit and vegetable products and concluded that
dehydration model-II with high temperature 70-75 oC is more efficient as compared to dehydration
model-I with low temperature 50-55 oC. Bhutani and Sharma (1988) found that drying of apricot
processed at a faster rate in a cross flow dehydrator than in open sun drying. Treatments of fruits with
alkali reduced the drying time. Selective and low-molecular cell sap components such as sugars and
organic acids diffuse into the surrounding solution of higher osmotic pressure. Other cell components,
only to a small extent, pass outside of the membrane. The diffusion of water and low-molecular weight
substances from the tissue structure during the osmotic dehydration is accompanied by the counter-
current diffusion of osmo-active substances. For this reason, osmotic dehydration as opposed to
conventional drying is characterized by the complex movement of water, substances dissolved in cell
sap and osmo-active substances. This significantly influences the process itself and its final effect with
respect to preservation, nutrition and organoleptic properties (Lenart, 1992). The process of water
removal and increase in osmo-active substances lowers the water activity in the cell (Lewicki and
Lenart, 1992). Food tissues are normally immersed in concentrated solution of osmo-active substances
such as sucrose, fructose, glucose, glycerol, starch syrup and sodium chloride at moderate temperatures
thereby reducing heat damage to texture, colour and flavour of food (Torreggiani, 1993). Thus,
through the process, de-watering and direct formulation of a product is possible by introducing the
desired amount of an active principle, a preservative agent, any solute of nutritional interest, or a
sensory quality improver into the food tissue (Ponting, 1973; Raoult-Wack, 1994). Osmotic
dehydration is also one of the methods for food preservation that enables to obtain organoleptically

Int. J. Food Nutr. Saf. 2013, 4(1): 34-42
Copyright © 2013 by Modern Scientific Press Company, Florida, USA
39
attractive products (Sitkiewiez et al., 1996). Nejad et al. (2002) studied effect of various drying
methods viz. sun drying, bin drying, vertical continuous drying, vertical cylindrical drying and funnel
cylindrical drying on quality of pistachio nuts, and reported that bin drying is the commercial method
and produce nuts with good quality. Pragati et al. (2003) while conducted an experiment on effect of
different drying methods on nutritional composition of dehydrated aonla, and reported that osmo-air
drying method was found to be the best, because of the better retention of nutrients after 90 days of
storage. The removal of moisture arrests the growth and reproduction of microorganisms that would
cause decay and minimizes many of the moisture mediated deterioration reactions (Krokida and
Marinos-Kouris, 2003; Araujo et al., 2004). Sharma et al. (2006) studied the organoleptic and chemical
evaluation of osmotically processed apricot whole and halves, and reported that lye dipping + 70o Brix
syrup concentration resulted in better quality product. Sahari et al. (2006) studied physiochemical
properties of sliced peach during osmotic pretreatment and dehydration, and showed that after 6 h the
solutions of sucrose (50 and 60%), glucose syrup (60%) and their mixture with salt (40% sucrose, 20%
glucose syrup and 3% NaCl) caused higher removal of water. Pretreated black mulberry samples were
dehydrated by artificial air-drying, solar drying and open sun drying. It was found that pretreatment
combinations did not cause drastic changes in measured color values of mulberries during all drying
trails (Taser et al., 2007). 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 (Doymaz,
2008). Sharma et al. (2011) reported that fruits of apricot and plum are most suitable for the
manufacture of dehydrated fruit based powder, chutney mix, ready to serve dried mix and dried pickle.
Raquel et al. (2011) studied different methods of drying of pear namely solar stove, a solar dryer and a
tunnel dryer, and reported that the drying time for solar and solar dryer showed a reduction of more
than 40 per cent while in the drying tunnel this reduction increased to more than 60 per cent as
compared with traditional open-air sun drying.
4. Future Prospects and Needs
There is a large amount of research, and available knowledge, carried out by academia, but the
industry is not taking full advantages even in the more developed countries. A lack of interaction
between researchers and industry is observed worldwide, as a consequence industrial advancement is
slow because there is a mismatch between research and industrial needs. 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. A consequence of this attitude has been the failure of
many drying projects. Drying of foods is a complex business and a mere translation from other fields is
not often advisable.

Int. J. Food Nutr. Saf. 2013, 4(1): 34-42
Copyright © 2013 by Modern Scientific Press Company, Florida, USA
40
Energy efficiency linked to environmentally friendly processes and products also appear as a
growing trend mostly in some developed countries. There is a need to check existing data on drying
kinetics in order to establish their applicability regarding the prevailing resistance. Also there is a need
to study products (isotherms and drying kinetics) not common in more developed countries where the
main research in the field has been carried out. This will lead to discover new niches of activity. This is
also true for countries where drying research is an active field where the use of off-quality fresh market
products is often disregarded.
One can say that there is a general concern towards improving the final consumer products
quality. As a consequence, pretreatments (including enrichment), optimization and control of operating
conditions, combination of drying strategies according to the drying stage should be studied for a
better migration of water, storage and consumption.
5. Conclusions
Advances in dehydration techniques and development of novel drying methods have in recent
years enabled the preparation of a wide range of dehydrated products and convenience foods from
fruits and vegetables meeting the quality, stability and functional requirements coupled with economy.
This has been made possible by the sustained experimental studies over the years to understand the
theoretical and fundamental aspects of the process and optimization of the techniques to achieve a
favourable combination of cost and quality. This paper reviews and highlights the developments in the
dehydration of fruits and vegetables and their products during the last decade covering theoretical
aspects and practical applications with major emphasis on techniques that have received the maximum
attention.
References
Bhutani, V. P., and Sharma, V. P. (1988). Studies on the drying of apricots grown under temperate
conditions. Indian Food packer, 42(4): 83-88.
Dabhade, R. S., and Khedkar, D. M. (1980). Studies on drying and dehydration of raw mangoes for
preparation of mango powder (Amchur). Part IV-VI. Indian Food Packer, 34(3): 35-54.
Doymaz, I. (2008). Influence of blanching and slice thickness on drying characteristics of leek slices.
Chem. Eng. Process, 47: 41-47.
FAO. (2000). Production Year Book, Vol. 54. Food and Agriculture Organization, Rome.
George, S. D., Cenkowski, S., and Muir, W. E. (2004). A review of drying technologies for the
preservation of nutritional compounds in waxy skinned fruit. North Central ASAE/CSAE
Conference, Winnipeg, Manitoba, Canada, MB 04-104.

Int. J. Food Nutr. Saf. 2013, 4(1): 34-42
Copyright © 2013 by Modern Scientific Press Company, Florida, USA
41
Kalra, S. K., and Bhardway, K. C. (1981). Use of simple solar dehydrator for drying fruits and
vegetable products. J. Food Sci. Technol., 8(1): 23-26.
Krokida, M. K., and Marinos-Kouris, D. (2003). Rehydration kinetics of dehydrated products. J. Food
Eng., 57: 1-7.
Lenart, A. (1992). Mathematical modelling of osmotic dehydration of apple and carrot. Polish J. Food
Nutr. Sci., 1: 1-33.
Lewicki, P. P., and Lenart, A. (1992). Energy consumption during osmoconvection drying of fruits and
vegetables. In: Drying of solids, Mujumdar, A. S. (Ed.). International Science Publisher, New York,
USA, pp. 344-366.
Mark, E. M., and Perry, R. L. (1948). Dehydrating free stone peaches. California Agricultural Export
Statistics Cire, No. 381, Berkeley, California.
Mehta, G. L., and Tomar, M. C. (1980). Studies on dehydration of tropical fruits in Uttar Pradesh. III-
Papaya (Carica papaya-L). Indian Food Packer, 34(4): 12-13.
Nejad, M. K., Tabil, L. G., Mortazavi, A., Kordi, A. S., Nakhaei, M., and Nikkho, M. (2002). Effect of
drying methods on quality of Pistachio nuts. Central Intersectional Meeting, Canada.
Nijjar, G. S. (1977). New peach introductions in Punjab. In: Fruit Breeding in India, pp. 70-74.
Ponting, J. D. (1973). Osmotic dehydration of fruits - recent modifications and application. Proc.
Biochem., 8: 18-20.
Pragati, G., Dahiya, S., and Dhawan, S. S. (2003). Effect of drying methods on nutritional composition
of dehydrated aonla fruit (Emblica officinalis Garten) during storage. Plants Food Human Nutr., 58:
1-9.
Raoult-Wack, A. L., Lenart, A., and Guilbert, S. (1992). Recent advances during dewatering through
immersion in concentrated solution. In: Drying of Solids, Majumdar, A. S. (Ed.). International
Science Publisher, New York, USA, pp. 21-51.
Raquel, P. F., Maria, J. B., and Lima, M. J. (2011). Comparative study of the drying of pears using
different drying systems. Int. J. Fruit Sci., 11: 55-73.
Sahari, M. A., Souti, M., and Jomeh, Z. E. (2006). Improving the dehydration of dried peach by
osmotic method. J. Food Tech., 4: 189-193.
Sethi, V. (1996). Appropriate postharvest technology of horticultural crops. Bever. Food World, 23(1):
41-42.
Sharma, H.R., Handa, P. and Verma, R. 2006. Organoleptic and chemical evaluation of osmotically
processed apricot wholes and halves. Natural Product Radiance, 5(5): 350-356.
Sitkiewiez, I., Lenart, A., and Lewicki, P. P. (1996). Mechanical properties of osmotic-convection
dried apples. J. Food Nutr. Sci., 5: 105-112.
Taser, F. O., Tarhan, S., and Ergunes, G. (2007). Effects of chemical pretreatments on air-drying

Int. J. Food Nutr. Saf. 2013, 4(1): 34-42
Copyright © 2013 by Modern Scientific Press Company, Florida, USA
42
process of black mulberry (Morus nigre L.). J. Sci. Indus. Res., 66: 477-482.
Tonini, G., and Tura, E. (1998). Influence of storage and shelf-life time on rots of peaches and
nectarines. Acta Horticul., 464: 364-367.
Torreggiani, D. (1993). Osmotic dehydration in fruit and vegetable processing. Food Res. Int., 26: 59-
68.