![US mechanochemistry releases camptothecin (CPT) herbal monoterpene. (a) Fluorescent micrograph of Nothapodytes nimmoniana stem sample showing autofluorescence of CPT in the epidermal layer. (b) The US mechanochemical cleavage of polymers disulfide bond that releases CPT from its β-carbonate linker presented in 2D and 3D structures (License Number: 5022331159441) [71,72].](https://www.researchgate.net/profile/Mostafa-Gouda/publication/350383479/figure/fig2/AS:1008186320510998@1617381758698/US-mechanochemistry-releases-camptothecin-CPT-herbal-monoterpene-a-Fluorescent_Q320.jpg)
![Schematic of the US-nebulization extraction device used to extract volatile compounds (left) and to evaluate aroma (right). SPME, Solid Phase Microextraction; PMP: Polymethylpentene membrane [9] (License Number: 4958661463662).](https://www.researchgate.net/profile/Mostafa-Gouda/publication/350383479/figure/fig3/AS:1008186320494606@1617381758801/Schematic-of-the-US-nebulization-extraction-device-used-to-extract-volatile-compounds_Q320.jpg)
![Scanning electron microscope image of Kenaf: (a) Before ultrasonic irradiation (b) After ultrasonic irradiation (20 kHz, 200 W, 10 min) [45] (License Number: 4958670625442).](https://www.researchgate.net/profile/Mostafa-Gouda/publication/350383479/figure/fig4/AS:1008186320502787@1617381758838/Scanning-electron-microscope-image-of-Kenaf-a-Before-ultrasonic-irradiation-b-After_Q320.jpg)
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Ultrasonics Sonochemistry 73 (2021) 105538
Available online 25 March 2021
1350-4177/© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
Review
Recent innovations of ultrasound green technology in herbal
phytochemistry: A review
Mostafa Gouda
a
,
b
,
1
, Alaa El-Din Bekhit
c
, Yu Tang
d
, Yifeng Huang
e
, Lingxia Huang
f
,
Yong He
a
,
*
, Xiaoli Li
a
,
*
a
College of Biosystems Engineering and Food Science, Zhejiang University, 866 Yuhangtang Road, Hangzhou 310058, China
b
Department of Nutrition & Food Science, National Research Centre, Dokki, Giza, Egypt
c
Department of Food Science, Otago University, New Zealand
d
College of Automation, Guangdong Polytechnic Normal University, Guangzhou 510665, China
e
College of Civil Engineering and Architecture, East China Jiaotong University, Nanchang 330013, China
f
College of Animal Sciences, Zhejiang University, Hangzhou 310058, China
ARTICLE INFO
Keywords:
Ultrasound phytochemistry
Sonochemistry
Herbal nanotechnology
Acoustic-based biosensors
Green technology
ABSTRACT
Ultrasound (US) has become one of the most important techniques in green chemistry and emerging technolo-
gies. Many research investigations documented the usefulness of US in a wide range of applications in food
science, nanotechnology, and complementary medicine, where effective extraction of natural products is
important. However, as with all novel technologies, US has advantages and limitations that require clarication
for full adaptation at an industrial scale. The present review discusses recent applications of US in herbal
phytochemistry with the emphasis on US effects on chemical structures of bioactive compounds extracted from
herbs and their bioactivities. The impact of different US processing conditions such as frequency, intensity,
duration, temperature, and pressure on the effectiveness of the extraction process and the properties of the
extracted materials are also discussed. Different frequencies and intensities of US have demonstrated its potential
applications in modifying, determining, and predicting the physicochemical properties of herbs and their ex-
tracts. US has important applications in nanotechnology where it supports the fabrication of inexpensive and eco-
friendly herbal nanostructures, as well as acoustic-based biosensors for chemical imaging of the herbal tissues.
The application of US enhances the rates of chemical processes such as hydrolysis of herbal bers, which reduces
the time and energy consumed without affecting the quality of the nal products. Overall, the use of US in herbal
science has great potential to create novel chemical constructions and to be used as an innovative diagnostic
system in various biomedical, food, and analytical applications.
1. Introduction
Integrating physical and chemical technologies for the character-
ization and modication of natural plants, like herbs and spices, has
been used for several decades to improve their potency and quality
[1,2]. Herbs have a long history of medicinal uses that have been
documented by their traditional uses and recent scientic evidence, in
particular in China. Chinese herbs have been traditionally used for the
treatment of different disorders, such as respiratory and heart diseases
and mental disorders. During the SARS epidemics, traditional Chinese
herbal medicine treatments were reported to have positive therapeutic
effects on SARS [3,4]. Scientists have been exploring the scientic basis
and mechanism of action of herbal medicine with great interest in their
chemical constituents [5]. Special attention has been paid to investigate
the different methods for the preparation and processing of herbs to
maximize the efcacy of their bioactivities and to understand the
changes in their chemical compositions [6].
Ultrasound treatment is an acoustic technology that can be used for
non-invasive detection and/or modication of herbal bioactive com-
pounds. As a physical treatment, it can modify the chemical and physical
* Corresponding authors.
E-mail addresses: mostafa-gouda@zju.edu.cn, goudarowing@yahoo.com (M. Gouda), aladin.bekhit@otago.ac.nz (A. El-Din Bekhit), lxhuang@zju.edu.cn
(L. Huang), yhe@zju.edu.cn (Y. He), xiaolili@zju.edu.cn (X. Li).
1
ORCID: 0000-0002-8174-4145.
Contents lists available at ScienceDirect
Ultrasonics Sonochemistry
journal homepage: www.elsevier.com/locate/ultson
https://doi.org/10.1016/j.ultsonch.2021.105538
Received 8 December 2020; Received in revised form 16 March 2021; Accepted 20 March 2021
Ultrasonics Sonochemistry 73 (2021) 105538
2
properties of biological systems at varying levels depending on the
processing conditions (e.g. frequency, intensity, and duration) and the
herb’s structure and composition [7]. The technology is a green tech-
nology that offers opportunities to create new functional products or
extract more powerful functional bioactive compounds from herbs. Also,
it is a low energy and maintenance cost technology that has several
economic benets [8]. However, US is not a largescale standardized
technology that could be adapted at a commercial scale for modication
of herbs and other foods. A better understanding of the complex re-
lationships among the processing conditions (frequency, power, and
processing duration) and herbs structures and chemical compounds can
support future applications in the herbal industry [9]. On the other
hand, phytochemical changes can occur by free radicals that are
generated by cavitation caused by the US shock wave energy. These
changes in phytochemicals are considered a huge challenge for US
technology. Collectively, recent research suggested that US can be
considered as a viable technology in quality assurance and food safety
applications [10], which adds greater benet for the optimization of the
latest extraction technologies [2].
This review aims to provide a comprehensive review of mechanisms
of action of ultrasound and their effects on the quality of herbal prod-
ucts. A special focus has been placed on the relationships among the
tested materials, mechanisms of action, techniques, and processing
conditions.
2. Herbs and spices denition
Herbs and spices are botanical raw materials that have active com-
ponents that could be used in pharmaceutics, cosmetics, food additives,
and health supplements [11]. Herbs are used by complementary medi-
cine therapists to treat various diseases. Many plant-based poly or mono
herbal formulas are used for various non-communicable diseases like
cardiovascular diseases and cancers [12]. The words “herbs” and
“spices” have many denitions but the most common are those which
consider herbs to be obtained from the green parts of a plant, for
example stems and leaves such as tea, mints, and thymus. On the other
hand, spices are produced from other structures such as seeds, owers,
fruits, barks, or roots, for example garlic, turmeric, cinnamon, and black
pepper [13–15]. These plant materials have high contents of phyto-
chemicals; including phenolic, carotenoid, avonoid, and volatile
components that exhibit antimicrobial, antioxidant, and other biological
activities [16–18]. The long historical use of herbs and spices document
their safe use, thus they are generally recognized as safe (GRAS) mate-
rials. This is a great advantage to use herbs and spices as natural alter-
natives to chemical additives [19].
3. Ultrasound technology
Ultrasound is dened as any mechanical sound waves at frequencies
>20 kHz, which is beyond the human hearing threshold. Compression
and expansion waves generated by US lead to positive and negative
pressures that can create cavities in the treated materials [20]. The
formed cavities release high energy and generate high-pressure and
localized extremely high temperatures [21]. The produced shock wave
energy hydrolyzes water into free radicals (H⋅ and ⋅OH) and forms
hydrogen peroxide (H
2
O
2
) as a byproduct. The main parameters that
control the effectiveness of the treatment are the US velocity, attenua-
tion, and acoustic impedance [22].
3.1. Ultrasound generation and treatments
The US waves are formed from the conversion of mechanical oscil-
lation of a high-frequency electrical eld, which has an elastic effect
(stretching and compressing) on the deformation of ferroelectric mate-
rials [23]. For materials’ physicochemical measurement, the US trans-
ducer emits pulses against one side of the material being analyzed and
these waves are transmitted into the material. The time that the pulses
take to pass through the material and return to a detector is measured
and the echo velocity in the material is calculated [24]. This phenom-
enon is affected by several factors, such as the chemical composition and
the structural conguration of the material.
Ultrasound can be categorized according to the used frequency (kHz)
and the generated energy intensity power (W). Also, it could be cate-
gorized based on sound intensity (W/m
2
) or sound energy density (Ws/
m
3
) [23]. According to the used frequency, it can be categorized into
high and low frequencies. High-frequency and intensity US are those
using frequency higher than 100 kHz, and intensity from 10 to 1000 W/
cm
2
, whereas the low-frequency and intensity US uses frequency lower
than 100 kHz, and intensity lower than 3 W/cm
2
[25,26].
Different combinations of frequency and intensity power are used for
different US applications. High-frequency and intensity ultrasound
combination is generally used for non-food applications such as soft
tissue surgery, diagnostic imaging, or drug delivery [27]. Similarly,
high-frequency and low-intensity US is used in the simulation of tissue
regeneration [26]. Low-frequency US at low or high-intensity combi-
nations is generally used in food applications. The intensity plays a key
role in forming stable cavities and imploding them (Fig. 1). The
low-intensity US is used as a non-destructive technique for providing
information about the materials’ physicochemical properties, like
structure, composition, and physical state. On the other hand,
high-intensity US is used to change the chemical or physical properties
of materials such as generating emulsions, promote certain chemical
reactions, or change the functionality of proteins and carbohydrates
[24]. Low frequency and high-intensity US has been used in research to
study the composition of vegetable, fruit, milk, meats, gels geopolymers,
molecular interactions, and protein structures [22]. The high-intensity
US has been used to reduce microorganisms and to facilitate meat
tenderization.
There are three commonly used US transducers: liquid-driven,
magnetostrictive, and piezoelectric transducers. Modern instruments
based on piezoceramic US transducers are considered very important
because of their high acoustic properties like producing high sound
pressure amplitudes by simultaneous small power consumption [28].
3.2. Advantages and disadvantages of different US treatments
Different US processing conditions can exert several benets in the
eld of herbal science. For example, high-US penetrating power allows
the detection of aws deep in the plant parts. US increases the efcacy of
the herbal extraction methods at lower temperatures and improves the
rates of heat and mass transfer, cell disruption, and the penetration of
solvents to the herbal tissues [29,30]. Also, it speeds up the ltration
process, increasing the life of the lter, results in a faster drying process,
and thawing operations [31]. Bellumori, Innocenti, Binello, Boffa,
Mulinacci and Cravotto [32] demonstrated that high-intensity US (ti-
tanium horn, 19.5 kHz,140 W) is a rapid, efcient, and selective tech-
nique for rosemary (Rosmarinus ofcinalis) leaf extraction and provide
extracts with high bioactive compounds such as rosmarinic and carnosic
acids. The application of US in herbal extraction showed a signicant
reduction in processing time compared to the conventional methods and
the obtained extracts had slightly higher antimicrobial activity against
some pathogenic species [33]. O’Donnell, Tiwari, Bourke and Cullen
[34] stated that the low-intensity US has a high ability to monitor the
properties during processing as well as being non-destructive, rapid, and
precise for characterizing food and plant complexes.
On the other hand, high-US intensities (>400 W) generate heat and
increase the treatment medium temperature, which may cause adverse
physical and chemical effects on some herbs’ phytochemicals. Thus, the
intensity and energy of US should be optimized before application on
different plant tissues [31]. Further, free radicals generated due to
cavitation may result in several negative changes such as lipid oxidation
accompanied by off-odor compounds, protein denaturation/ oxidation,
M. Gouda et al.
Ultrasonics Sonochemistry 73 (2021) 105538
3
and reduction in total phenolic contents [35].
4. Application of US in herbal science
The application of US in herbal science is increasing due to its sig-
nicant effect on the bioactive compounds in the herbs. Table 1 presents
recent applications and the impact of different ultrasonic frequencies,
intensities, and duration as well as their major effects on herbs and
spices.
4.1. Ultrasound applications targeting the chemical composition of herbs
and spices
Ultrasound is useful for food composition measurement because it is
non-destructive, rapid, and could be adapted for optically opaque sys-
tems. For instance, low intensity (<1 W/cm
2
) or high-frequency (>100
kHz) US were used to obtain detailed information about the structure,
dimensions, and composition of the plant products during the storage
process [66]. In which, differences in the chemical composition produce
different responses to US properties (velocity, attenuation, frequency,
and power). In measuring particle size, US uses the same principles of
light scattering in emulsions or suspensions. In which, US velocity and
attenuation coefcient depends on the size and concentration of parti-
cles (Fig. 2).
Despite the benecial effects of the US, it has been observed that US
promotes several oxidation reactions and enzyme inhibitions of many
food enzymes, including peroxidases, and glucosidases [67]. This is
probably because of the intense pressures, temperatures, and shear
forces generated by the ultrasonic waves that denature proteins [10].
Furthermore, the extreme agitation created by microstreaming could
change Van der Waals interactions and hydrogen bonds in the poly-
peptides resulting in protein denaturation such as that observed in
Dolichos lablab [68]. Free radical-mediated deactivation mechanisms
and cleavage of the functional groups from enzymes could occur during
US treatment and is the likely reason for the modications in proteins
and enzymes. For example, the inactivation of peroxidase as a result of
haem group dissociation and the loss of iron was facilitated by hydroxyl
radicals from US cavitation [69].
4.2. Recent mechanistic insights of US process on herbal chemicals
composition
The effects of US depend on the changes in the chemical structure of
the material being treated. Heterogeneous reactions that involve
unfragmentable substrates inside the US bubbles have been shown to
occur by the mechanical effects of the US and lead to increasing
chemical reactivity [70], a relationship that is known as mechano-
chemistry. Huo, Zhao, Shi, Zou, Yang, Warszawik, Loznik, Gostl and
Herrmann [71] reported that US mechanochemistry can be exploited to
control transformations at the molecular level by rearranging or
cleaving bonds at predetermined breaking sites. A recent study inves-
tigated the impact of US on camptothecin (CPT) (plant alkaloid mono-
terpene produced by Camptotheca acuminata herb) and the authors
found that US transformed the disulde bonds to thiol bonds of the
molecules. As a result, US modied the molecules from covalently
attached linear polymer chains in the β-position to a disulde moiety
(Fig. 3b) [71].
The position of the disulde motif in the center of the macromolec-
ular framework (e.g. Hydrocarbon) enabled the efcient mechano-
chemical scission and the release of this monoterpene molecule [69]. A
US mechanophore breakage (mechanochemically breakage of the
polymer reactive units like cyclic rings) can occur in extracts solutions
via the shear stress caused by the collapse of US-induced cavitation
bubbles [73,74]. These modications in the structure of extracted
compounds caused US can facilitate better bioavailability of the herbal
bioactive molecules [75]. Patil and Akamanchi [72] used US (20 kHz,
150 W, 30 ◦C) for CPT extraction from the stem part of Nothapodytes
nimmoniana herb. The application of the US increased the camptothecin
yield (1.7-fold) and decreased the extraction time from 6 h to 18 min.
The authors suggested that the mechanochemical effects of the US dis-
rupted both the outer and interior parts of stem cell wall structure
(Fig. 3a). The bubble collapse creates high-speed solvent jets that dis-
associated the disulde and covalent bonds on the surface and interior
parts of the cells which release the CPT [72] (Fig. 3b).
Fig. 1. Principle effects of low-frequency and high-intensity ultrasound on herbs and spices cells.
M. Gouda et al.
Ultrasonics Sonochemistry 73 (2021) 105538
4
Table 1
Summarization of recent herbal plant treatments by using ultrasound in different application elds.
Samples
name
Scientic
Name
Process US method Compounds of
interest
Freq.
(kHz)
Int. (W) Time
(Min)
T. (◦C) Ultrasound effects Ref.
Alfalfa Medicago
sativa
Ultrasonic
bath
Thermosonication,
structural study
Saponins – 50–150 1–3 h 50–80 Ultrasound treated
samples yield rate
increased almost two
times compared to the
heat-reux method.
Also, it has greater
efciency for the
saponins yield and
bioaccessibility.
[36]
Arrowhead Sagittaria
sagittifolia
Sonication
chamber with
ultrasonic
generator
Thermosonication
treatment
Bioactive
proteins
28–40 60 W/L 10–60 30–50 The results showed
that ultrasound
treatment had a
considerable impact
on the protein
structure and it could
increase the protein
susceptibility
digestive enzymes like
pepsin and trypsin. In
which, us is providing
a powerful
endorsement for
increasing the protein
proteolysis.
[37]
Clove
Tarragon
Syzygium
aromaticum
Artemisia
dracunculus
Ultrasonic
horn and
probe
sonotrode
system
Thermosonication,
extraction and
application in
nanober
technology
Essential oils 53 500 20–40 32–52 Thermosonication
showed the most
important inuence
on the extraction
yield. Also, for
tarragon, the
antioxidant activity is
increased at 250 W
however it decreased
with increasing the
power until 500 W
because of the
destructive effect of
ultrasonic treatment
at high intensity.
[38–40]
Chinese
ginseng
Panax
Notoginseng
Ultrasound
bath
Sonication
extraction
Saponins – – 20 – Compared with
traditional extraction
method, ultrasound
has enhanced the four
saponins yields. With
potential advantages
involving shorter
extraction time, it
decreased the
consumed solvent.
[41,42]
Chinese
liquorice
Gancao
Kenfa
Glycyrrhiza
uralensis
Turpiniae
folium
Hibiscus
cannabinus
Ultrasonic
probe
sonotrode
system
Polysaccharide
thermosonication
extraction and
modication
Polysaccharide,
Cellulose
25 0–600 10–60 25–100 For
glycyrrhizauralensis
as thermal stable
polysaccharide, the
optimal extraction
parameters of
glycyrrhiza
polysaccharide is 600
W for 60 min at 70 ◦C.
And for turpiniae
folium is 200 W for 30
min at 30 ◦C
[43–45]
Citrus limon
leaves
Citrus
aurantium
Ultrasonic
probe
sonotrode
system
Sonication for
nanocubes and
nanosphere
nanocomposite
Phytochemical
mixture
20 250 29 In this work,
ultrasound
synthesized an
efcient photocatalyst
nano-cubes and
nanospheres by using
C. limon LE and Ag:
CdO. In which, C.
limon
phytocomponents
played an effective
[46]
(continued on next page)
M. Gouda et al.
Ultrasonics Sonochemistry 73 (2021) 105538
5
Table 1 (continued )
Samples
name
Scientic
Name
Process US method Compounds of
interest
Freq.
(kHz)
Int. (W) Time
(Min)
T. (◦C) Ultrasound effects Ref.
role as reducing and
stabilizing agent.
Coriander
leaves
Coriandrum
sativum
Cylindrical
jacket with in-
built
piezoelectric
US transducer
rode
Synthesis of iron
oxide nanoparticle
Phytochemical
mixture
33 – – 20–60 US assisted green
synthesis of iron oxide
nanoparticles using
coriander extract as a
reducing agent. In
which, it showed
higher antioxidant
and antimicrobial
activity compared to
the conventional
method due to the
combined effect of US
and the plant
molecules attached
with iron oxide
nanoparticles.
[47]
Cumin seeds Syzygium
cumini
Ultrasound
bath
Thermosonication,
extraction
Catechin, Gallic
acid
22 ,
40
44–215 0–20 25–65 Catechin and gallic
acid extracted yields
were increased by 3.7
and 2.1 times with
increasing US power
from 44 to 125 W.
However, the yields
were not signicantly
increased over 125w
power and 35◦C.
[48]
Erodiumherb
Lettuce
herb
Erodium
laucophyllum
Lactuca
sativa
Ultrasonic
probe
sonotrode
system
Sonication
extraction for
antimicrobial
evaluation
Phytochemical
mixtures
26 200,
400
5,10 40,45 Ultrasound showed
the highest level of
desirable phenolic
compounds. In which,
exhibit the highest
level of antimicrobial
and antiviral activities
especially against
hepatitis a and murine
norovirus. Also, it
increased the
decontamination of
lettuce herb.
[49,50]
Garlic peels Allium
sativum
Ultrasonic
disperser
Thermosonication
treatment
Polysaccharide 40 500 0–30 65 Ultrasound increased
C═C bond and c/o
ratio, which is
benecial to improve
the electrochemical
function of materials
[51]
Green tea Camellia
sinensis
Piezoelectric
ceramic
ultrasound
bath, horn,
probe
sonotrode
Sono-solid-phase
microextraction ,
hydrolysis, and
physicochemical
and functional
properties studies
Volatile
compounds
20
kHz,
1.7
MHz
100 15–120 20–70 Ultrasound increased
the polyphenols,
avonoids, and
volatiles
concentrations of
medium-to-low
volatility fractions
which may function as
an assisted tool to
volatiles extract from
tea herb. Also,
hydrolysis efciency
for tea reducing
sugars production is
increased by US.
[9,52–55]
Hibiscus Hibiscus
sabdariffa
Hibiscus
tiliaceus
Ultrasonic
cleaner bath
Sonication
nanostructures
Chlorophyll and
other
phytochemical
mixtures
500
Hz,
40
kHz
130 30–60 – Ultrasound can be
used for fabricating of
inexpensive, simple,
eco-friendly hibiscus
with ZnO and other
nanostructures to
extend their utility in
different areas of
nanotechnology.
[21,56]
Mulberry
leaves
Morus alba L. Ultrasonic
probe
sonotrode
system
Ultrasonic drying
process
Phytochemical
mixture
20 130 5–15 20 US pretreatment
enhances mulberry
drying kinetics and
reduces total energy
[57]
(continued on next page)
M. Gouda et al.
Ultrasonics Sonochemistry 73 (2021) 105538
6
Table 1 (continued )
Samples
name
Scientic
Name
Process US method Compounds of
interest
Freq.
(kHz)
Int. (W) Time
(Min)
T. (◦C) Ultrasound effects Ref.
consumption without
affecting product
quality.
Parsley
leaves
Petroselinum
crispum
Ultrasound
bath
Ultrasonic drying
preprocess
Phytochemical
mixture
21 100–300 20 30 US pretreatment
signicantly reduced
the drying time and
consumed energy up
to 29.8% and 33.6%.
Also, it increased
parsley chlorophyll a
and b resistance to the
drying process.
[58]
Peppermint
leaves
Mentha
piperita
Drying
chamber with
piezoelectric
US transducer
(1500 W)
Ultrasonic drying
process
Phytochemical
mixture
20 90–360 40–400 40–70 US improved the
convective drying of
peppermint. In which,
The results showed
that it decreased
energy consumption
and increased energy
efciency up to
3.69%.
[59]
Rosemary Rosmarinus
ofcinalis
Ultrasonic
titanium horn
Sonication
extraction
Rosmarinic
Carnosic
19.5,
35
140, 320 15 25 Increased efciency
and decreased
processing time
[60,61]
Scarlet Sage Salvia
coccinea
Ultrasonic
homogenizer
bath
Sonication
extraction
Polyphenols 50 – 15–450 – Ultrasonic assistant
extraction shortened
the processing time
and provided lower
solvent consumption
[62]
Thyme Thymus
serpyllum
Ultrasonic
horn
Ultrasonic drying
and extraction
processed
Polyphenols 20 750 W,
6.2–18.5
kWm
−3
– 25–80 Enhanced product
quality and shorted
time and decreased
processing cost of
thyme herb. Also, US
increased total
polyphenols and
avonoids yields and
extraction efciency
from Serpylliherba or
wild thyme.
[63–65]
Fig. 2. Principle effects of high-frequency and low-intensity ultrasound in measuring particle size of molecules.
M. Gouda et al.
Ultrasonics Sonochemistry 73 (2021) 105538
7
4.3. Ultrasound effects on herbal phenolics and polyphenols
Ultrasound has a signicant inuence on plant phytochemicals,
especially polyphenols [30]. For example, using US for extraction of
gingerols from ginger powder at 50 ◦C enhanced the extraction of gin-
gerol yield. However, a degradation of the active compound occurred at
higher temperatures [76]. Boulatov [75] reported that the mechano-
chemical effects of the US results in overstretching of macromolecules
polymers (like carbohydrate and protein chains) that lead to their
fragmentation. This stretching process helps in releasing small mole-
cules that are bound in the polymer chain. A comparison study of US-
assist and conventional extraction methods on herbal tea bioactive
compounds showed that total phenols and
α
-tocopherol were increased
by 44% and 20%, respectively. The breakdown of cells cytoarchitecture
has been found to release these bioactive compounds [77]. According to
Ranalli, Malfatti, Lucera, Contento and Sotiriou [78], the increase in
bioactive components in oil extracted by US could be due to the
breakage of cross-links between these compounds and other macro-
molecules like polysaccharides and protein. Also, Rashed, Tong,
Abdelhai, Gasmalla, Ndayishimiye, Chen and Ren [79] reported that
higher total phenolic content was extracted from Lavandula pubescent
herb using US compared to a maceration extraction. For most herbs, the
US acoustic cavitation can facilitate the ow of solvent into the plant
cells, and enhance desorption of bioactives from the matrix of solid
samples. Thus, US enhances the efciency of phenolic extraction. In
another study that investigated Elsholtzia ciliata herb, Pudziuvelyte,
Jakˇ
stas, Ivanauskas, Laukeviˇ
cien˙
e, Ibe, Kursvietiene and Bernatoniene
[80] reported that US signicantly increased the extracted apigenin
phenolic yield (855.54
μ
g/g) compared to the maceration method
(141.06
μ
g/g). The authors reported that US-assisted extraction for 11
min increased the mass fraction of total phenols by 20% compared to
water bath shaker for 30 min with the same solvent. Also, US treatment
increased chlorogenic acid content up to 2174.70
μ
g/g after 30 min
compared to the percolation extraction method that resulted in 683.40
μ
g/g [80].
4.4. Ultrasound effects on herbal carotenoids
Using of US has its signicant effect on the carotenoids content in oil
extracted from herbs. For example, Hu, Li, Qin, Zhang, Liu, Zhang, Liu,
Jia, Yin, Han, Zhu, Luo and Liu [77] reported that application of US (25
kHz, 550 W, 70 ◦C for 38 min) in the extraction of tea (Camellia sinensis)
oil increased β-carotene by 38% compared to conventional extraction
methods. The authors suggested the increase was due to the breakage of
cross-links between these compounds and other macromolecules like
polysaccharides and proteins, which release the carotene in the extrac-
tion medium. However, high-intensity US application (25 kHz and 600
W at 4 ◦C for 6 min) signicantly degraded (all-E)-astaxanthin carot-
enoid [81]. Additionally, after optimization of US parameters to extract
antioxidants from thyme (Thymus vulgaris) and rosemary (Rosmarinus
ofcinalis), Munekata, Alcantara, Zugcic, Abdelkebir, Collado, Garcia-
Perez, Jambrak, Gavahian, Barba and Lorenzo [82] reported that US-
assisted extraction at 400 W and 40 ◦C for 10 min increased the
extraction yields of carotenoids compared to conventional extraction
(heating under magnetic stirrer method). Also, the authors mentioned
that US improved the aqueous extraction of antimicrobial compounds
from thyme.
4.5. Ultrasound effect on herbal avonoids
While the formation of free-radicals during US processing is
considered a main disadvantage of the technology as it affects the
bioactivity of components such as phenols [83], chemical modication
such as increasing the extent of hydroxylation of avonoids [84] can
enhance the antioxidant activity or at least counteract some of the
negative effects of free radicals. This led to the fact that optimum US
processing conditions where maximum yield and bioactivity could be
optimized for various materials. In which, US inhibits the hydrolyzing
enzymes (such as
α
-amylase and
α
-glucosidase) which affects the total
avonoids yields and their antioxidant functionality [67]. For instance,
the extraction efciency of dihydromyricetin yield, which is the main
Fig. 3. US mechanochemistry releases camptothecin (CPT) herbal monoterpene. (a) Fluorescent micrograph of Nothapodytes nimmoniana stem sample showing
autouorescence of CPT in the epidermal layer. (b) The US mechanochemical cleavage of polymers disulde bond that releases CPT from its β-carbonate linker
presented in 2D and 3D structures (License Number: 5022331159441) [71,72].
M. Gouda et al.
Ultrasonics Sonochemistry 73 (2021) 105538
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avonoid in Chinese vine herbal tea (Ampelopsis grossedentata), was
increased up to 40% (3% yield) with the increase in US power and time
until a certain extent (5.5 min at 240 W), after which, the yield was
decreased due to hydrolysis of the compound [85]. Similarly, total a-
vonoids extracted from Syzygium cumini seeds using US (22–24 kHz and
44–215 W at 35 ◦C for 12 min) were increased with the increase in
processing time [48]. Maximum total avonoid content was obtained
after 12 min of processing at US power of 125 W at 35 ◦C for 12 min. The
yields of gallic acid and catechin exhibited similar trends with 54.5 and
2.2 mg/g after 12 min [48]. Collectively, the above information suggests
a scope for optimization where maximum bioactives could be obtained
before negative effects are caused by heating or free radicals.
Optimum US extraction conditions for total avonoids from sour
jujube seeds (Caenorhabditis elegans) were obtained at 404 W and 60 ◦C
for 60.03 min. The use of US increased the avonoids’ yield by 17.11%
compared to heat reux extraction. Moreover, the US extracted avo-
noids showed higher antioxidant capacity against DPPH, superoxide,
and hydroxyl radicals from the signicant differences in their chemical
construction due to using US-assisted extraction (UAE) or heat reux
extraction (HRE) (Fig. 4) [86].
4.6. Ultrasound effect on essential oils and other volatiles
The effects of ultrasound on plant essential oils have been recently
studied [87]. Essential oils are unstable to heat, thus, the use of US at low
intensities could prevent the degradation of these thermally sensitive
compounds. Furthermore, excellent diffusion rates were afforded by the
use of US, which can facilitate successful extraction with minimum
solvent use and subsequently lower solvent residues in the extracted
compounds. The composition of the extracted essential oil could be
modied and more selective extraction of desired compounds could be
obtained [30]. For example, by using 20 kHz and 90 ◦C for 70 min of US,
red pepper seeds’ alcohol and aldehyde contents were decreased, while
other volatile components, such as pyrazine derivatives, esters, and
olen components were increased [88]. These changes in the composi-
tion of the extract could eventually have subsequent effects on the ef-
cacy of the extracts and potentially on their sensory attributes.
For Lu’an Guapian tea herb (Camellia Sinensis), Meng and Zhengquan
[9] established a method for extracting volatiles by US nebulization
extraction (UNE) combined with solid-phase microextraction method
(SPME) (Fig. 4). In that study, the authors reported that the extracted
total volatiles percent was increased signicantly (p <0.05) to 42.23%
by using 1.7 MHz and 50 ◦C for 20 min of US. A signicant increase in
some aldehydic volatiles such as pentanal, heptanal, octanal, and
dodecanal was also found in the obtained extracts compared to controls
[9].
It is worth mentioning that the extraction conditions play an
important role in determining the composition of the volatiles. For
example, while no signicant differences were observed in alcoholic
volatiles such as hexanol at 20 ◦C, increasing the extraction temperature
to 50 ◦C resulted in a signicant increase in alcoholic volatiles, such as
α
-terpineol. Therefore, US can increase the volatile extraction efciency
of medium-to-low volatiles compounds in herbs [9]. The combined use
of US and heat treatment using a thermosonication method improved
the extracted essential oil efciency and extracted yield from clove (53
kHz and 52 ◦C) [38]. Also, time savings have been reported for the US
processing where Gavahian, Farhoosh, Javidnia, Shahidi, Golmakani
and Farahnaky [89] reported of reducing essential oil extraction time by
17% for peppermint herb.
4.6.1. Ultrasound effect on terpenes
It has been shown that thermosonication can signicantly increase
the percentage of some extracted terpenes from herbal sources [9]. For
example, a signicant increase in β-ocimene, and D-limonene contents
of tea (Camellia Sinensis) were reported as a result of using 50 ◦C of US
(1.7 MHz, 20 min) compared to 20 ◦C. On the other hand, alcoholic
terpenes such as
α
-terpineol were not affected by US treatments [9]. As
mentioned earlier, CPT extraction from the Camptotheca acuminata herb
by US transformed the disulde bonds to thiol bonds of these molecules,
which resulted in the disulde motif to be in the center of the macro-
molecular framework (e.g. Hydrocarbon) and enabled efcient mecha-
nochemical scission and the release of this monoterpene molecule [69].
High-intensity US using a titanium horn at frequency 19.5 kHz and in-
tensity 140 W increased the extraction efciency of carnosic (13%) and
rosmarinic (6.8% of dry extract) acids from rosemary leaves [60,61].
Similarly, it has been found that US increased the efciency of terpenes
extraction from oregano herb, especially carvacrol (a monoterpene),
ursolic acid (a triterpenoid), and oleanolic (a triterpenoid saponin)
compared to the maceration conventional method [90].
4.7. Ultrasound effect on herbal proteins
Ultrasound treatment affects the protein structure in herbs due to its
modifying effect on some secondary bonds of β-sheets and β-turns,
which affects the protein’s hydrophilic groups and the hydrophobic core
[91]. A study on arrowhead (Sagittaria sagittifolia) herbal protein showed
Fig. 4. Schematic of the US-nebulization extraction device used to extract volatile compounds (left) and to evaluate aroma (right). SPME, Solid Phase Micro-
extraction; PMP: Polymethylpentene membrane [9] (License Number: 4958661463662).
M. Gouda et al.
Ultrasonics Sonochemistry 73 (2021) 105538
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that US treatment (28–40 kHz, 30–50 ◦C) caused unfolding of the pro-
tein structure, decreased
α
-helix and β-turn contents, and increased
β-sheet and random coil contents. In which, the content of free sulfhy-
dryl (SH) increased by 38.87% at 40 kHz and 40 ◦C over the control
samples [59]. Also, it can enhance the formation of peptides subunits
which formed as a result of plant proteins enzymolysis [37]. The over-
stretching of folded macromolecule disrupts the non-covalent in-
teractions responsible for its three-dimensional conguration and affects
its ability to be involved in interactions, e,g, change in the active sites of
enzymes [75]. For instance, Ayim, Ma, Alenyorege, Ali and Donkor [92]
investigated the effect of US (20 kHz, 20–50 ◦C, 13 min) pretreatment on
the enzymolysis of tea residue protein extracted with sodium hydroxide.
The authors stated that Michaelis constant (substrate concentration
required to half saturate the enzyme) in US pretreated enzymolysis was
decreased by 32.7% over the traditional enzymolysis, which just uses
sodium hydroxide. An increase in the protein susceptibility to the pro-
teolysis enzymes like pepsin and trypsin was also suggested [37].
Hadidi, Khaksar, Pagan and Ibarz [93] reported that US enhanced alfalfa
leaves (Medicago sativa) protein extraction based on the pH, tempera-
ture, and duration. A high-intensity US (20 kHz, 100 W, 30–50 ◦C for
120 min) changed alfalfa leaves protein physicochemical properties and
caused an increase in the protein surface hydrophobicity through
disruption of proteins’ hydrogen bonds, which affects the proteins sur-
face charge. That US treatment increased the proteins’ solubility due to
forming hydrophilic soluble fractions. Also, US (20 kHz, 60 ◦C for 78.1
min) promoted the interaction between polycysteine and xylose sugar.
This eventually increased the antioxidant properties and decreased the
formation of sulfur-containing volatiles from the Maillard reaction that
is commonly encountered in conventional extractions [94].
4.7.1. Ultrasound effect on enzymes activity
In general, most studies reported that the high-intensity US can
chemically and physically inactivate many kinds of enzymes [37].
Enzyme molecular weight is very important for the sensitivity of en-
zymes to the US. For example, polymeric enzymes are fragmented into
monomeric subunits, and these monomeric enzymes could be subse-
quently fragmented further or form aggregates upon extended US
treatment. Enzyme protein denaturation that inactivates enzymes is
promoted by free radicals and shear forces caused by cavitation [95].
For instance, US inactivated pectin methylesterase, which hydrolysis
pectin and results in a product with low stability [96]. A combination of
US, pressure, and heat treatment (manothermosonication) showed the
highest inactivation of this enzyme compared to sonication or thermo-
sonication alone at the comparable intensity levels. The high inactiva-
tion rates by temperature and pressure could be due to their effects on
pectin and other molecules that interact with the enzyme and their
absence lead to reduced enzyme resistance against temperature and
pressure effects [97].
For polyphenol oxidase which causes enzymatic browning of natural
plants, Cheng, Soh, Liew and Teh [98] found that US (35 kHz, 20 ◦C for
15 min) treatment increased the enzyme activity, however, longer time
(35 kHz, 20 ◦C for 30 min) have inactivated the enzyme (20% inacti-
vation) due to its denaturation [99]. Moreover, peroxidases which are
categorized as high thermal stable enzymes are associated with unde-
sirable avors and pigments loss was reported to be inactivated using
appropriate processing conditions. In watercress herb (Nasturtium
ofcinale), thermosonication (20 kHz, 40–92.5 ◦C) reduced its inacti-
vation time from 70 to 5 s [100]. This change was related to changes in
enzyme tertiary structure, which affected the enzyme prosthetic group
[100].
Manosonication can be dened as a combination of US and high
pressure, which can make additional effects with US. This method can
signicantly change the protein and carbohydrates’ conformations and
conguration at pressure (100–300 kPa) and low temperatures. Thermal
processing with 20 kHz of US is reported to be effective against many
enzymes, like dehydrogenase and catalase. Manothermosonication is a
combination of US and both heat and pressure that lead to synergistic
effects on the extraction of compounds, inactivation of enzymes, and
microorganisms. Almost complete enzyme inactivation of heat tolerated
enzymes can be at 70 ◦C, 300 kPa for 2 min [101,102]. Moreover, it has
been reported that manothermosonication has the potential to inacti-
vate many enzymes that tolerate thermosonication. The inclusion of
pressure enhances the action of US and heat in cleaving prosthetic
groups of enzymes or denaturize protein subunits. For example, the
splitting of the prosthetic heme group of peroxidase, which is the
mechanism of heat inactivation, is reported for manothermosonication
[103]. Also, manothermosonication was found to be more effective than
heat treatment alone in the inactivation of heat-resistant protease and
lipase secreted by Pseudomonas uorescens [104].
4.8. Ultrasound effect on herbal lipids and phytosterols
Chemat, Grondin, Shum Cheong Sing and Smadja [105] studied the
effect of various US frequencies (20 and 47 kHz) treatment during the
processing of sunower oil. The authors found signicant negative
changes in the oil composition (degradation of linoleic acid and sterols
and increasing of aldehydic volatiles like hexanal and hept-2-enal), due
to free-radicals oxidation during the treatment. On the other hand, the
oil of red pepper (Capsicum annuum) seed was not affected by the US
treatment (20 kHz, 30–90 ◦C for 70 min) [88]. The authors did not nd
changes in acid and saponication values after using US which indicates
that US did not affect the oil molecular weight. However, the peroxide
value was slightly increased due to slight oxidation of the oil [88]. Also,
relatively high amounts of phytosterols (stigmasterol, sitosterol, and
sitostanol) were observed after US treatment that was paralleled by a
decrease in campesterol stability (38% loss) [88]. Hu, Li, Qin, Zhang,
Liu, Zhang, Liu, Jia, Yin, Han, Zhu, Luo and Liu [77] showed that US (25
kHz, 550 W, 70 ◦C for 38 min) treatment for oil extraction from tea
(Camelia sinensis) increased total phytosterols, β-sitosterol, stigmasterol,
campesterol and other phytosterols by 20, 25, 16, 37 and 26%, respec-
tively. Furthermore, Panadare, Gondaliya and Rathod [106] found that
US pre-treatment (150 s at 30 W) increased oil yield by 11% from
Annona squamosal seeds compared to conventional methods. The oil
characteristics from US and conventional methods were not different in
their acid value or free fatty acids. The Oleic / Linoleic ratio was 2.21
which is similar to the range reported in the literature [68].
4.9. Ultrasound effect on herbal carbohydrates
Herbal polysaccharides, as natural macromolecules, have been
demonstrated to have signicant bioactivities, such as anti-
inammatory, antimicrobial, and antioxidant activities [107]. The
application of US can enhance the medicinal properties of poly-
saccharides but the extent of improvement is dependent on the treated
herbs. For instance, Zhao, Xia, Lin, Xiong, Tang and Liao [44] showed
that US has a signicant effect on polysaccharides extracted from dried
leaves of Chinese herb Turpiniae Folium and optimum extraction condi-
tions for high yield were 200 W at 30 ◦C for 35 min.
US (45 kHz) at different power levels (40–100 W) were used
extraction of polysaccharides from Acantho panaxsenticosus herb and
their antioxidant activity have been reported by Zhao, Xu, Ye and Dong
[108]. An extraction time of 75 min at 80 ◦C, and 100 W US power
resulted in the greatest yield (10.9 mg/g). The obtained polysaccharide
by US treatment possessed considerable antioxidant activity against
DPPH, hydroxyl, and superoxide free radicals [108]. Similarly, the
highest yield of polysaccharides from hibiscus leaves was obtained using
US power of 93.59 W for 25.71 min at 93.18 ◦C. Under these conditions,
the extracted polysaccharide content was increased by 10% [109]. A
study used US (25 kHz, 50–70 W, 50–70 ◦C, and time range of 10–30
min) for extraction of bioactive polysaccharides from mulberry (Morus
Alba) leaves proved higher yields and lower water/raw material ratio
compared to micro-wave-assisted extraction method [110].
M. Gouda et al.
Ultrasonics Sonochemistry 73 (2021) 105538
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For cellulose carbohydrate molecule, Nakayama and Imai [45] found
that US pretreatment (20 kHz, 200 W, 10 min) can enhance the enzy-
matic hydrolysis of kenaf herb (Hibiscus cannabinus) cellulose to produce
glucose sugar. In which, the authors reported that US induced a higher
association of cellulase and cellulose by removing the covering materials
on the kenaf cellulose, as could be seen in Fig. 5.
Joshi and Gogate [52] reported that US horn (20 kHz, 100 W, and 60
◦C) enhanced tea acid hydrolysis for the production of reducing sugars.
In which, it reduced the reaction time from 120 to 60 min with a high
yield of reducing sugars (24.75 g/L). Also, they reported that US com-
bined with oxidants such as H
2
O
2
effectively decreased the acid hy-
drolysis time of the tea polysaccharides through facilitating the
breakage of lignin, which increased the rate of reducing sugars pro-
duction from tea powder [52].
4.10. Effect of US on herbal extract activity against pathogenic bacteria
Intense US treatment and long contact times are required to inacti-
vate microorganisms. For instance, to inhibit Staphylococcus aureus, a US
treatment of 187 min and 150 W, 20 kHz is required [111]. Also,
Kazibwe, Kim, Chun and Gopal [112] studied the US (20, 60 kHz; 200,
300 W; 2 min) assisted extraction effects of Tagetes erecta herb on their
antimicrobial activity. They subjected two different bacterial strains to
these extracts (Streptococcus mutans and Pseudomonas aeruginosa).
They observed that the extracts by US showed signicantly higher in-
hibition of the two bacteria compared to the hot water extract. Also, they
used eld emission scanning electron microscope imaging to indicate
why the antibacterial increase was happened by US. In which they
observed high membrane damage, as observed with Streptococcus
mutans (Fig. 6). The bacterial cell sensitivity may be due to the changes
in cell surface peptidoglucans adherent. In general, most micro-
organisms showed greater sensitivity to US at temperature over 50 ◦C
[113].
Extracted polyphenols from Erodium glaucophyllum, which is a
Mediterranean herb, exhibited higher antimicrobial activity against
Salmonella enterica, Staphylococcus aureus, and Listeria as well as antiviral
activities especially against hepatitis A and murine norovirus compared
to conventional extraction methods [49].
5. Critical analysis of US and other emerging technologies
Compared to other technologies, US technologies show high poten-
tial in the eld of herbal science due to several unique advantages found
in the US. From a nancial point of view, it is easier and less expensive to
scale-up the US technology compared to other techniques like
microwave (MW), pulsed electric elds (PEF), high voltage electric eld
(HVEF), and high-pressure processing (HPP) methods [32]. Further, the
technology generates better yield and thus improves the economics of
the extraction process. For instance, Tsaltaki, Katsouli, Kekes, Chanioti
and Tzia [42] compared the recovery of bioactive compounds from
Damiana leaves (Turnera diffusa) by using US, MW, heat reux (CON),
and soxhlet (SOX) extraction methods using 50% ethanol. In that study,
US (20 kHz 40 ◦C, and 15 min) achieved the highest total phenolic yield
203.96 mg GAL/ dry leaves (DL), compared to MW (300 W, 50 ◦C, and
15 min) and SOX (2.4 ×10
6
Pa, 100 ◦C, and 6 h) with 191.36 and
161.62 mg GAL/DL. Similarly, the US increased (p <0.05) the Ginsen-
side recovery from ginseng (Panax ginseng) herb by 31.1%, 19.5%, and
12.1% compared to CONV, MW, and SOX methods, respectively. Addi-
tionally, Panadare, Gondaliya and Rathod [106] found that US pre-
treatment (150 s at 30 W) increased oil yield by 11% from Annona
squamosal herb compared to CON methods.
Carbone, Macchioni, Petrella and Cicero [114] compared the use of
US and MW for the extraction efciency of bioactive compounds from
hop herb (Humulus lupulus). They mentioned that MW (2.45 GHz, 2.4 ×
10
6
Pa, 210 ◦C, and 1 min) resulted in a higher extraction power for its
phenolic contents (95 mg GAL/g) than US (40 kHz, 25 ◦C, and 30 min)
with 25 mg GAL/g. This study suggested that in terms of yield, the ef-
cacy of the US will depend on the plant/ material used, and thus
optimization reported in the literature cannot be extrapolated to other
materials. In the study of Carbone and colleagues [112], it is important
to highlight that US treatment did not use high power like MW (2.45
GHz, 210 ◦C, 2.4 ×10
6
Pa), which could have a negative effect on the
phytochemicals structure and function. In terms of selectivity, treating
rosemary leaves with US (19.5 kHz,140 W) showed remarkably high (p
<0.05) recovery of carnosic acid and rosmarinic acid contents (13%,
6.8% of the dried extract) compared to MW (100 ◦C, 20 bar) [32].
However, MW showed tendency of higher (p >0.1) total terpenoid (28
mg/gDL) compared to US with 27.5 mg/g DL [32].
In a recent study, Nguyen, Gavahian and Tsai [67] compared the
effects of US (150 W, 20 min) with conventional (CON), high voltage
electric eld (HVEF, 4000 kV m
−1
min), HPP (300 MPa, 3 min) and their
combinations treatments on Gac (Momordica cochinchinensis) leaves. The
authors found that US resulted in the highest (p <0.05) total chlorophyll
content (TCC) recovery with 13.67 mg/g DL followed by HPP (12.65
mg/g DL), and then CON (12.58 mg/g DL). Similar results were
observed for Stevia (Stevia rebaudiana) leaves, in which, the highest TCC
was obtained using US and HVEF treatments (20.7 and 20.4 mg/g,
respectively) [115].
On the other hand, combined emerging technologies (e.g., US and
HPP) are considered superior to individual methods alone. For example,
Fig. 5. Scanning electron microscope image of Kenaf: (a) Before ultrasonic irradiation (b) After ultrasonic irradiation (20 kHz, 200 W, 10 min) [45] (License Number:
4958670625442).
M. Gouda et al.
Ultrasonics Sonochemistry 73 (2021) 105538
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the combination of US and HPP increased (p <0.05) the total avonoids
extraction from Gac leaves to 623 mg QE/100 g DL compared to US and
HPP with 592 and 582 mg QE/100 g DL. Furthermore, the inhibitory
effect of
α
-amylase of the combined technique was increased up to 37%
compared to US and HPP with 34% and 29% [67]. The better outcome
achieved by the combined technologies can be explained by the more
efcient disruption of plant cell walls by US and the improved diffusion
caused by HPP that enhances the release of bioactive compounds
compared to their individual potentials [83,84,116]. A method com-
bined US with MW conrmed that they could enhance the oil extraction
from tea (Camelia sinensis) seeds. In that study, an oil yield of 31.52%
was obtained under optimum extraction conditions of MW (440 W), US
(550 W), at 70 ◦C for 38 min compared to MW and US methods (27.45%
and 25.13%, respectively) [77]. Similarly, Tzima, Brunton, Lyng,
Frontuto and Rai [117] found that PEF (5 kW, 1.1 kV cm
−1
, 30
μ
s), as a
pre-treatment step, enhanced (p <0.05) the phenolic extraction and
their antioxidant potential of fresh rosemary and thyme herbs by US
(200 W, 13 min). The reason could be due to the rearrangement of the
phenolic molecules electric ion by PEF and PEF ability to electroporate
the cell envelopes, which facilitate the improved recoveries of the US
extracted bioactive compounds [117].
6. New technologies combined US with herbs and their
compounds
6.1. Ultrasound with herbal phytochemicals in nanotechnology
The application of US to incorporate herbal phytochemicals in
nanotechnology showed promising potential to stabilize bioactives,
improve functionality and bioavailability, and delivery. High-intensity
US and clove essential oil were used for producing nanober hydrogel
using cellulose nanober. In this study, US (20 kHz, 4 ◦C) strongly
increased the clove essential oil entrapment efciency by 34%, cell
viability rates by 74–101% to human gingival broblast cells, water
retention, and color characteristics of the prepared hydrogel [40]. Citrus
limon leaves extract played effectively their role as reducing, capping,
and stabilizing agent for forming silver and cadmium oxide Ag:CdO
photocatalyst nanotubes and nanospheres [46]. The phytocomponents
of Citrus limon played an important role against radicals formed by US
during nanocomposite formation [46]. Meanwhile, Sathya, Sar-
avanathamizhan and Baskar [47] demonstrated that coriander leaves
extract can be used as a reducing agent in iron oxide nanoparticle for-
mation by US assisted technique. The authors reported that the formed
nanocomposite had substantial antimicrobial activity against patho-
genic bacteria such as Staphylococcus aureus and Micrococcus luteus that
was higher than iron oxide nanoparticle synthesized by the traditional
method.
Taha, Modwi, Elamin, Arasheed, Al-Fahad, Albutairi, Arasheed,
Alfaify, Anojaidi, Algethami and Bagabas [21] used US for synthesizing
zinc oxide (ZnO) nanostructures by using Hibiscus sabdariffa extract as a
reducing and a stabilizing agent. In that study, the structure
morphology, and photocatalytic activity were tested. X-ray diffraction
(XRD) conrmed that hibiscus phytochemicals reduced ZnO crystallite
size from 40 to 31 nm. Also, as a consequence of band gap reduction and
surface area increase, the nanostructures showed better photocatalytic
degradation performance with the hibiscus extract. In another study that
investigated the use of Hibiscus tiliaceus chlorophyll with US to synthesis
multi-walled carbon nanotubes [56]. The authors observed an increase
in the interaction potential and covalent bonds formation with
increasing of US time which led to an increase in the thickness of the
nanotube by 160%. Thus, hibiscus phytochemicals are one of the
promising approaches in the fabrication of nanoparticles [56].
6.2. Ultrasound with acoustic-based sensors and biosensors for the
chemical composition of herbs
The use of US for herbal chemical composition measurement and to
draw chemical images of plant tissues and visualizing their biomolecules
has become one of the hot scientic research areas. An acoustic wave
sensor typically consists of a piezoelectric substrate (eg. quartz crystal),
coated with sensing material (polymeric lm), and two interdigital
transducers (one input and one output) are commonly used for chemical
composition purposes [118]. There are three different types of these
kinds of sensors (Surface acoustic wave sensors (SAW), Bulk acoustic
wave sensors (BAW), and Micro/nano-acoustic biosensors) [119]. The
acoustic wave propagates on the surface of the substrate is called SAW,
while the wave propagates through the substrate is called BAW (Fig. 7)
[120]. For example, Sharma, Ghosh, Tudu, Sabhapondit, Baruah,
Tamuly, Bhattacharyya and Bandyopadhyay [121] used BAW sensors
based on quartz crystal microbalance (QCM) to detect tea aroma (e.g.,
linalool, geraniol, linalool oxide, Methyl salicylate, and Trans-2-
hexenal) during its fermentation process. Zheng, Gao, Zhang, Li, Yu
and Hui [122] used SAW as a rapid determination method to study
Chinese quince (Cydonia oblonga Miller) freshness. They mentioned that
using of SAW validated a high predicting accuracy (R
2
=0.987). On the
other hand, there are still some limitations to these kinds of sensors. For
example, QCM sensors have complex circuitry, poor signal-to-noise
ratio, and can be inuenced by humidity [118].
Micro/nano-Acoustic Biosensors are frequently used to enhance the
activity of specic biomolecules such as enzymes for increasing the
detection sensitivity. These biosensors are based on a unique class of air-
lled protein nanostructures called gas vesicles that vibrate in response
to US waves [123]. The principle of using acoustic-based biosensors is
based on coupling the measurement nature (like analyte adsorption) as a
modulation in the physical properties of the acoustic wave (like US
frequency and velocity) that could be correlated with the analyte
Fig. 6. Field Emission Scanning Electron Microscopy images of S. mutans following interaction with US of Tagetes erecta ower extracts showing (a) stripping of
membrane (b) showing leakage of cell contents (indicated using pointers) [112] (License Number: 4958671204565).
M. Gouda et al.
Ultrasonics Sonochemistry 73 (2021) 105538
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concentration [119]. Existing molecular biosensors, based on uores-
cent emission, have limited utility due to the scattering of light and the
interference with their phytochemicals’ uorescents. The use of US can
easily image deep tissue with high spatiotemporal resolution. Jiang, Jin
and Gui [124] used US-assisted solvothermal reaction for bio-imaging of
plant zinc-ion by using quantum dots technology. The authors suggested
that the viability of the technique could be used for in-vitro cell imaging
and in-vivo imaging of natural plants.
6.3. Ultrasound and new chemical assisted extraction of herbal elements
and heavy metals
Ultrasound is commonly used in phytochemicals and
macromolecules (e.g. protein, and polysaccharide) extractions [25].
However, its benecial applications in the eld of elemental or metal
analysis are not well established [125]. A study on heavy metals from
herbal medicines such as Hoodia, Shirafza, and Dineh herbs used US-
assisted emulsication microextraction (USAEME) to extract Lead,
chromium, and cadmium [126]. In that work, the authors conrmed
that USAEME is an efcient, rapid, inexpensive, and eco-friendly
method for the extraction of macro-elements from herbal medicinal
plants [126]. A recent US-assisted method (20 kHz, 80 ◦C, 50 min) for
extraction and determination of trace and ultra-trace impurities (Pb, Cd,
Cr, Mn, Fe, Cu, Zn) from 7 plant edible oils including Mustard (Mustum
ardens) [127]. They mentioned that US-assisted extraction of trace ele-
ments efciency was increased by increasing pH.
Fig. 7. Graphic depicting in general terms the processes for the generation of surface and bulk acoustic waves [118,119] (Copyright permissions: 1093837,
201130–008292).
Fig. 8. Principle applications between ultrasonic and herbal science and technology.
M. Gouda et al.
Ultrasonics Sonochemistry 73 (2021) 105538
13
Fig. 8 summarizes the different common applications that use US in
herbal science and technology. In which all these technologies could be
improved through the combination of both two elds.
7. Conclusion and future directions
Several studies have documented the efcacy of US for the replace-
ment, enhancement, and improvement of various conventional pro-
cessing techniques in the herbal eld. Most of the reports showed that
US (25–50 kHz) increases the yields of polyphenols, carotenoids, a-
vonoids, and essential oils depending based on the used temperature,
pressure, and duration parameters. The cavitation’s effects on different
granules induced by ultrasonication (20–40 kHz, <300 W) further
facilitate the chemical, physical, and enzymatic reaction efciency for
herbal bioactive polysaccharide and protein extractions. However,
higher power intensity (400–600 W) signicantly oxidized and
degraded some phytochemicals like (all-E)-astaxanthin carotenoid. The
high-frequency US (>100 kHz) is used to obtain chemical information
about herbal products. The future perspective will be to combine US and
herbal phytochemicals with other technologies like nanosensors and
biosensors for forming advanced materials that have unique characters.
In which, the optimization of the US parameters for further application
development on a large scale level is a very important key role in the
herbal industry. Also, US transducers’ energy should be further stan-
dardizing to present a better green way compared to the commercial
methods.
Author contributions
Mostafa Gouda studied the literature, drafted, and edited this
manuscript; Xiaoli Li, Bekhit, Yu Tang, Yifeng Huang, Lingxia Huang,
Yong He commented and directed the manuscript preparation; Xiaoli Li
and Yong He supervised the framework and content. The nal edited
manuscript was proofread and accepted by all the authors.
Declaration of Competing Interest
The authors declare that they have no known competing nancial
interests or personal relationships that could have appeared to inuence
the work reported in this paper.
Acknowledgments
This research was funded by the National Key Research and Devel-
opment Program of China (2018YFD0700500), National Natural Sci-
ence Foundation of China (No: 31771676), and Zhejiang Province
Public Technology Research Program (Project No: 2017C02027). Also,
the authors would like to thank Prof. Donghong Liu, Zhejiang University
and Dr. Amany Bayoumi, Cairo University for their help regarding the
manuscript revision.
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