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POSSIBILITIES OF ULTRASOUND APPLICATION IN CONVECTIVE
DRYING OF FOOD PRODUCTS
Najafli M.R.
1
Safarov J.E.
2
Sultanova Sh.A.
2,3
Samandarov D.I.
2
1
Alov Inshaat LLC, Azerbaijan
2
Tashkent state technical university, Uzbekistan
3
Deputy Mayor Tashkent city, Uzbekistan
https://doi.org/10.5281/zenodo.15171975
Convection drying of foodstuffs still has some limitations that make it
difficult to apply in certain areas. These include slow drying rate and loss of
product quality. Some of these limitations can be overcome by introducing new
technologies used as additional energy sources. Among others, ultrasound
stands out, which is able to influence the drying rate without causing a
significant increase in the temperature of the material. This favours its
application in the drying of various temperature-sensitive materials or in drying
processes carried out at low temperatures, such as convection drying at
atmospheric pressure [1].
Acoustic energy is one of the fundamental forms of energy found in nature.
Sound waves are elastic mechanical vibrations of medium particles propagating
in time and space. Unlike electromagnetic waves such as microwaves, sound
waves require a material medium (solid, liquid or gaseous) for their
propagation, as their transmission is due to elastic deformations of matter. Thus,
in a vacuum where there are no material particles, sound propagation is
impossible, while electromagnetic waves can be freely transmitted in such a
medium due to their nature, which does not depend on the presence of matter
[2].
Ultrasound is acoustic waves with a frequency exceeding the upper
threshold of audibility of the human ear - about 20 kHz. This classification is
conditional and is based solely on the physiological peculiarities of human
perception of sound, not on physical differences. From the physical point of
view, ultrasonic waves obey the same basic laws of generation, propagation and
interaction with materials as sounds in the audible range, since the principles of
acoustics remain unchanged across the entire frequency spectrum [3].
The presence of ultrasound in nature is quite widespread. Some animals
have a hearing limit in the ultrasonic frequency range. For example, the upper
limit for dogs is 50 kHz and up to 100 kHz for bats and dolphins. Communication
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of these animals with the environment is largely based on the use of ultrasound
[4].
In 1880, the Curie couple discovered the piezoelectric effect, a phenomenon
on which some modern ultrasound generation systems are based, so this date
can be considered the date of the beginning of ultrasound [5].
Galton in 1883 developed the first ultrasonic transducer, which was a
whistle used to explore the limits of human hearing. The first commercial
application of ultrasound was the "Echo sounder" [6].
Ultrasonic parameters.
Ultrasonic waves, like all types of mechanical waves, are described by a set
of physical parameters that determine their behaviour and interaction with the
medium. The main characteristics include frequency, wavelength, amplitude,
propagation velocity, intensity, and acoustic impedance. These parameters
determine the properties of ultrasound during its generation, propagation and
interaction with different materials, and play a key role in the choice of modes in
practical applications such as medical diagnostics, defectoscopy or material
processing (Fig. 1) [2].
Fig. 1.
Ultrasonic wave propagation
Frequency (f, Hz).
The frequency of a wave is defined as the number of
oscillations or cycles made by the wave per unit of time. The International
System of Units uses hertz (Hz), which is defined as the number of cycles a wave
makes in one second. The inverse of frequency is called the period and is defined
as the time it takes for a wave to complete one cycle.
Wavelength (λ, m).
The wavelength is the distance between two planes in
which the particles are in the same state of oscillation, i.e. the distance between
two consecutive points vibrating in phase. It is determined from the velocity and
frequency of the wave (1).
𝜆 =
𝐶
𝑓
(1)
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Velocity (c, m/s).
Acoustic velocity is the speed of propagation of waves. The
propagation velocity of longitudinal and transverse waves is characteristic of the
material and can generally be considered constant for a given material, although
it can be affected by environmental variables such as temperature and pressure.
Amplitude (A, m)
is the maximum displacement of a particle of the medium
from its equilibrium position under the influence of an ultrasonic wave, which
determines the intensity and energy of the oscillatory process.
Sound pressure (PA, N/m
2
).
Sound pressure is the pressure in different
zones of a material. This pressure will be higher than normal in zones of particle
compression and lower in zones of particle expansion, hence acoustic pressure
is variable. The maximum deviation from normal is called the amplitude of the
acoustic pressure and is related to the amplitude of the oscillation of the wave.
Noise intensity (I, W/m
2
).
The intensity of an acoustic wave is defined as the
average energy transmitted through a unit area perpendicular to the direction of
wave propagation per unit time. The intensity of an acoustic wave is
proportional to the square of the maximum sound pressure (2).
𝐼 =
𝑃
𝐴
2
2𝜌𝐶
(2)
Energy density (E, J/m
3
).
During wave propagation, energy is transferred
and can be dissipated as heat due to the work expended to move particles in a
medium subject to forces that counteract the movement of the particles. Energy
density can be expressed as the ratio of the intensity of acoustic radiation to the
speed of wave propagation (3).
𝐸 =
𝐼
𝐶
(3)
Energy density is measured in units of J/m
3
. If energy is expressed in
Newtons per metre (Nm), energy density units are converted to N/m
2
, i.e.
pressure units. In other words, energy density is equivalent to sound pressure
level.
Sound power (P, W).
Acoustic power is the total energy emitted by the
ultrasound source per unit time. It can be calculated from the intensity of
acoustic radiation and the area of the radiating surface (4).
𝑃 = 𝐼 · 𝑆
(4)
Power can be expressed in decibels as the logarithmic ratio between the
existing acoustic intensity (or power) in the environment and the reference
acoustic intensity (or power) (5).
𝑃
𝑑𝐵
= 1𝑙𝑜𝑔
𝐼
𝐼
0
(5)
Like power, sound pressure can be denoted in decibels (6).
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𝑃
𝐴𝑑𝐵
= 10𝑙𝑜𝑔
𝑃
𝐴
2
𝑃
𝐴0
2
(6)
here
𝐼
0
is usually taken to be 1×10
-12
(W/m
2
);
𝑃
𝐴0
is usually taken to be
2×10
-5
(Pa).
Acoustic impedance (Z).
It is defined as the ratio of acoustic pressure to the
vibration velocity of the particle. It is a very important parameter that
determines the fraction of energy that is reflected and transmitted when an
acoustic wave changes medium. Like light, when an ultrasonic wave hits an
interface, some of it is reflected and some of it is transmitted. The fraction of
energy reflected depends largely on the difference in impedance between the
two media. The greater this difference, the more energy reflected and the less
energy transmitted. This characteristic is of great importance in ultrasonic
applications.
Attenuation.
When an ultrasonic wave propagates in a medium, the
intensity of the wave decreases with increasing distance from the source of the
wave. This phenomenon is known as attenuation. Equation (7) gives an
expression for calculating the intensity of an acoustic wave at a point located at a
certain distance (
𝑑
) from the source of radiation.
I = 𝐼
0
𝑒𝑥𝑝(−𝛼
𝑎
𝑑)
(7)
Attenuation can be the result of reflection, scattering or diffraction of a
wave during propagation, as well as the conversion of part of the wave's kinetic
energy into heat. Attenuation becomes more important as the frequency of the
wave increases due to the increase in the attenuation coefficient (
𝛼
𝑎
).
Classification of acoustic waves.
Acoustic waves are classified by frequency in the range from 20 Hz to 20
kHz. Waves with frequencies below 20 Hz are called subsonic and those above
20 kHz are called ultrasonic (Fig. 2).
Fig. 2.
Classification of sound by frequency
In terms of industrial applications, ultrasonic waves are classified based on
their frequency and energy characteristics [2].
There are two main types: power ultrasound (or high intensity ultrasound)
and signalling ultrasound (or low intensity ultrasound). Power ultrasound
covers a frequency range of 20 to 100 kHz at intensities greater than 1 W/cm².
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Due to their high acoustic energy, these waves are able to induce
physicochemical changes in the materials being processed or actively influence
technological processes. Signal ultrasound is characterised by frequencies from
100 kHz to 20 MHz and intensities of less than 1 W/cm².
Application of ultrasound in drying food products.
The use of ultrasound in convection drying of food products such as
beetroot, pumpkin and others significantly increases the efficiency of the
process. The results of our research show that the use of ultrasound with a
frequency of 25˂ kHz can reduce the time of convection drying by 35% by
facilitating the internal migration of moisture. In addition, better colour
retention and less structural shrinkage of plant tissue are observed. The
combination of ultrasound and hot air (at 50 °C) improves the porosity of the
product, allowing more efficient rehydration. These advantages make
ultrasound a promising tool for optimising beetroot drying [7].
References:
1. Safarov J.E. et al. Research of the effective diffusion coefficient and activation
energy for the purpose of energy saving during convection drying. Enеrgеtika.
Proс. СIS Higher Educ. Inst. аnd Power Eng. Assoc. V. 68, No 1 (2025), pр. 58–75.
2. Cárcel J., Benedito J., Rosselló C., Mulet A. Influence of ultrasound intensity on
mass transfer in apple immersed in a sucrose solution. Journal of Food
Engineering, 2007, 78: 472-479.
3. Raj B., Rajendran V., Palanichamy P. Science and technology of ultrasonics.
Alpha Science International, Oxford, Reino Unido, 2004.
4. Qingyang Li et al. Preparation high quality camellia oil by combining
ultrasound pre-treatment and microwave as drying method: Interactive effect
on drying kinetics, metabolite profile and antioxidant ability. Ultrasonics
Sonochemistry, 2025, 107338.
5. Sultanova S.A., Safarov J.E., Usenov A.B., Muminova D. Analysis of the design of
ultrasonic electronic generators. Journal of Physics: Conference Series, 2176 (1),
2022.
6. Analaura G.C. et al. Orange sweet potato flour production: Comparative effects
on ultrasound, drying, storage, and techno-economic assessment. Applied Food
Research, Vol. 5, Iss. 1, 2025, 100751.
7. Нажафли М.Р., Самандаров Д.И., Султанова Ш.А., Сафаров Ж.Э.
Исследование математических моделей кинетики сушки свеклы.
Universum: технические науки. –Москва, 2025. №3(132), часть 3. -С.54-58.
