Sound assisted fruit drying

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Applications of Ultrasound in Fruit Drying
Article ID: 33057
Pooja Yaddanapudi1
1
Ph. D. Scholar, Department of Fruit Science.
Introduction
The human population is continuously increasing, whereas the world’s food resources are declining. Efficient
evaluation of food and energy resources and reduction of losses have therefore become important global
issues. Reductions in food losses require the establishment of safe food production methods, processing with
the most appropriate technologies, and use of appropriate preservation methods. Food dehydration has been
used as a basic preservation method since ancient times. The basic aim of the dehydration process is to remove
water from the food to prolong its shelf life and delay microbial and biochemical deterioration. Although the
main objective of the dehydration processes is food preservation, it has many other purposes.
Need of Ultrasound in Fruit Drying
1. The ultrasonic process is cost effective for several fruits because of the lower energy consumption during the
air-drying step.
2. Drying processes can be destructive to the quality of the fruits due to the long drying times and high
temperatures in some cases.
3. Ultrasound is known for its varying effects on different fruits; however, it has been proved to greatly increase
the drying rates and hence reduce the overall processing time.
Fruits Suitable to Drying
Dried foods are tasty, nutritious, lightweight, and easy to store and use. The energy input is less than what is
needed to freeze or can, and the storage space is less than that needed for canning jars and freezer containers.
Some foods such as apples, pears, peaches, and apricots dry better when pre-treated. Pre-treatment reduces
oxidation, giving a better colour, reducing vitamin loss, and lengthening shelf life.
Examples: Apple, Apricot, Banana, Cherry, Date, Fig, Grape, Nectarine, Peach, Pear, Pineapple, Plum,
Strawberry.
Principle of Ultrasound Drying
The underlying principle of food dehydration is mainly simultaneous heat and mass transfer.
Cavitation: Cavitation is the main ultrasound mechanism in a liquid system. The moisture transfer rate from a
solid food to the surrounding environment can be increased if cavitation is generated in the inner liquid phase
of the food.
Sponge effect: Ultrasound generates a series of rapid compressions and expansions of the material in the solid.
This is called the “sponge effect” and facilitates the flow of liquid out of the foods by creating micro channels
that are suitable for fluid movement. It is widely used in dehydration processes, as well as in preservation,
crystallization, filtration, extraction, and cutting technologies. Ultrasound technology can show different effects
in different systems because many factors, such as ultrasonic intensity, frequency, application form,
pressure/vacuum, and temperature, affect its efficiency. However, the underlying mechanism of ultrasound
involves acoustic cavitation, which can be divided into transient and stable forms.
Methods of Ultrasound Drying
Ultrasound-assisted (US-assisted) dehydration methods are based on a combination of the ultrasonic process
with appropriate dehydration techniques. The ultrasound wave strongly accelerates mass transfer, so the

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ultrasound technique assists in dehydration by providing a high drying rate, thereby maintaining food quality.
US-assisted dehydration has been combined with other forms of dehydration, including convective, osmotic,
vacuum and freeze-drying in order to improve the overall effectiveness of the drying. In all these processes,
ultrasound is employed to increase the drying rate and/or decrease the drying temperature.
Ultrasound-Assisted Convective Dehydration
1. Convective drying is one of the most widely used and oldest preservation methods in the food industry
because of its ease of operation. A considerable amount (12–25 %) of the overall industrial energy consumption
is associated with the drying industry. This process gives food products a longer shelf life by reducing water
activity. In this process, hot air with low humidity is used to remove moisture from the food material. The
resulting reduction in the water activity provides some advantages to the food, such as preventing microbial
growth and enzymatic activity and enabling easy packaging and transportation (Goyal et al., 2006. and Villa et
al., 2010.).
2. Convective drying involves two types of water transfer resistances: the internal resistance to water
movement inside the material and external resistance between the solid surface and the air (Rossello et al.,
1997; Simal et al., 2001).
3. The convective drying process also has several disadvantages, such as high energy consumption, low
dehydration rate, low product quality, high drying temperature, and long drying time (Bantle et al., 2014).
Ultrasound-Assisted Osmotic Dehydration
1. The basis of osmotic dehydration is osmosis, which is based on removal of a fluid across a semipermeable
membrane. When two solutions having different solute concentrations are separated by a semipermeable
membrane, some ions or molecules are removed from the denser medium to less heavy medium until
equilibrate.
2. Osmotic dehydration (OD) is mainly used as a pre-drying treatment prior to air-drying, freezing, freeze-drying,
or vacuum-drying, in order to reduce energy consumption or the load of the following operations, to improve
the quality of preserved food product, or to increase the shelf life of a product with high moisture content
without changing its integrity.
3. Ultrasound-assisted osmotic dehydration is a dehydration process in which an osmotic solution is exposed to
ultrasonic waves. The complex cellular surface of the material acts as an effective semipermeable membrane
in the osmotic dehydration process. Water moves from the tissue into the hypertonic solution and the solutes
from the osmotic solution moves into the treated material in the reverse direction (Rastogi et al., 2002).
Ultrasound-Assisted Vacuum Dehydration
1. Vacuum dehydration is the dehydration of humid foods under pressure lower than zero. Lowering the
pressure allows water removal at lower temperature so that the quality of the dehydrated food is improved
over that prepared with traditional methods.
2. The vacuum dehydration method has specific properties, such as prevention of oxidation, which separates it
from conventional atmospheric dehydration methods because the sample does not come into contact with air
during the vacuum dehydration process.
3. The vacuum dehydration can also decrease the dehydration time more than the traditional dehydration
methods, making it a more energy-efficient dehydration process. The short dehydration time and/or low drying
temperature also help to retain the organoleptic and nutritional properties of food substances (Wu et al., 2007).
Ultrasound-Assisted Freeze Dehydration
1. Freeze-drying, also known as lyophilization, is a widely used drying process that is especially performed for
the prevention of loss of bioactive or nutritive compounds during drying. In this process, the temperature of
the product is reduced below its freezing point and water vapour is formed from ice by sublimation at pressures

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lower than the triple point of water. The shape, colour, and flavour of the product can be maintained as the
sample is freeze-dried at low temperature and pressure and the resulting sponge-like structure favours faster
water penetration and recovery of the original characteristics during a rehydration process.
2. Freeze-drying enables preservation of the activity of nutraceuticals and pharmaceuticals, and flavour and
aroma of food products (Nakagawa and Ochiai 2015). These advantages of freeze-drying have led to its
acceptance as the best drying method for keeping the fresh-like quality characteristics of the dried product.
However, despite its many advantages, its high cost due to operation and the long drying period, which ranges
from several hours and to days, restricts its industrial application.
Conclusion
The use of US in processing industries has steadily increased over the past several years, resulting in permanent
changes in food materials in liquid systems through cavitation. Ultrasound processes activate microorganisms
and enzymes to preserve or decontaminate foods, particularly when US is combined with heat and high-
pressure techniques.
Future Line of Work
Ultrasonic processes are still under development and more studies are required to fully comprehend the effects
of ultrasound on the fruit tissue and on the sensory characteristics of the fruit, such as texture, adhesiveness
and other. Recent studies have shown a good potential of ultrasonic treatments.
References
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4. Rastogi NK, Raghavarao KSMS, Niranjan K. and Knorr D., 2002. Recent developments in osmotic dehydration: methods to enhance mass transfer.
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