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Plant Extraction in Water: Towards Highly Efficient Industrial Applications

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Since the beginning of this century, the world has experienced a growing need for enabling techniques and more environmentally friendly protocols that can facilitate more rational industrial production. Scientists are faced with the major challenges of global warming and safeguarding water and food quality. Organic solvents are still widely used and seem to be hard to replace, despite their enormous environmental and toxicological impact. The development of water-based strategies for the extraction of primary and secondary metabolites from plants on a laboratory scale is well documented, with several intensified processes being able to maximize the extraction power of water. Technologies, such as ultrasound, hydrodynamic cavitation, microwaves and pressurized reactors that achieve subcritical water conditions can dramatically increase extraction rates and yields. In addition, significant synergistic effects have been observed when using combined techniques. Due to the limited penetration depth of microwaves and ultrasonic waves, scaling up entails changes to reactor design. Nevertheless, the rich academic literature from laboratory-scale investigations may contribute to the engineering work involved in maximizing mass/energy transfer. In this article, we provide an overview of current and innovative techniques for solid-liquid extraction in water for industrial applications, where continuous and semi-continuous processes can meet the high demands for productivity, profitability and quality.
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Citation: Gallina, L.; Cravotto, C.;
Capaldi, G.; Grillo, G.; Cravotto, G.
Plant Extraction in Water: Towards
Highly Efficient Industrial
Applications. Processes 2022,10, 2233.
https://doi.org/10.3390/pr10112233
Academic Editor: Carla Silva
Received: 2 October 2022
Accepted: 28 October 2022
Published: 31 October 2022
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processes
Review
Plant Extraction in Water: Towards Highly Efficient
Industrial Applications
Lorenzo Gallina 1, Christian Cravotto 2, Giorgio Capaldi 1, Giorgio Grillo 1and Giancarlo Cravotto 1, *
1Dipartimento di Scienza e Tecnologia del Farmaco, University of Turin, Via P. Giuria 9, 10125 Turin, Italy
2GREEN Extraction Team, INRAE, UMR 408, Avignon University, 84000 Avignon, France
*Correspondence: giancarlo.cravotto@unito.it; Tel.: +39-011-670-7183
Abstract:
Since the beginning of this century, the world has experienced a growing need for enabling
techniques and more environmentally friendly protocols that can facilitate more rational industrial
production. Scientists are faced with the major challenges of global warming and safeguarding water
and food quality. Organic solvents are still widely used and seem to be hard to replace, despite
their enormous environmental and toxicological impact. The development of water-based strategies
for the extraction of primary and secondary metabolites from plants on a laboratory scale is well
documented, with several intensified processes being able to maximize the extraction power of water.
Technologies, such as ultrasound, hydrodynamic cavitation, microwaves and pressurized reactors
that achieve subcritical water conditions can dramatically increase extraction rates and yields. In
addition, significant synergistic effects have been observed when using combined techniques. Due
to the limited penetration depth of microwaves and ultrasonic waves, scaling up entails changes to
reactor design. Nevertheless, the rich academic literature from laboratory-scale investigations may
contribute to the engineering work involved in maximizing mass/energy transfer. In this article, we
provide an overview of current and innovative techniques for solid-liquid extraction in water for
industrial applications, where continuous and semi-continuous processes can meet the high demands
for productivity, profitability and quality.
Keywords:
extraction in water; enabling technologies; ultrasound; microwaves; hydrodynamic
cavitation; subcritical water
1. Introduction
Conventional hydroalcoholic extraction systems based on maceration and percolation
are usually time- and energy-consuming. In addition, the rising price of ethanol has
led to an increase in production costs. Other organic solvents are commonly used in
the chemical and pharmaceutical industries for extraction processes but do not meet
the requirements for the environmental and economic sustainability of the ecological
transition. The need to improve process safety and product quality and the growing interest
in preserving the environment have led R&D groups to explore more environmentally
friendly methods [
1
]. While the literature is replete with excellent results achieved with new
technologies on a laboratory scale [
2
,
3
], industrial implementation remains a complicated
task [
4
]. The possibilities of saving energy (e.g., reducing extraction time) and avoiding
toxic and flammable solvents have led researchers to develop new technological solutions.
In addition, cold extraction using techniques such as cavitation reactors and pulsed electric
fields (PEF) allows heat-sensitive compounds to be preserved from degradation. Our
consolidated experience in the application of extraction technologies, such as microwaves
(MW), ultrasound (US), hydrodynamic cavitation (HC), pressurized reactors for subcritical
water extractions (SWE), PEF and enzymatic treatments, have paved the way for the
expansion of studies and the development of new pilot and semi-industrial reactors.
Processes 2022,10, 2233. https://doi.org/10.3390/pr10112233 https://www.mdpi.com/journal/processes
Processes 2022,10, 2233 2 of 14
2. Plant Material Pretreatment
Due to the influence of particle size, suitable milling and controlled size screening
processes are the first steps toward efficient extraction [
5
]. The parts of a plant differ in
hardness, fiber structure and chemical composition. The particle fractions isolated by siev-
ing and cyclone treatment have different physicochemical properties in addition to their
size [
6
]. In the extraction of vegetal matrices, a partially-damaged cell wall significantly
improves solvent accessibility for both intracellular solutes and cell-wall constituents [
7
].
Hydrolytic enzymes, often used as a mixture, can selectively depolymerize and degrade
certain cell wall components [
8
,
9
]. Enzymatic pretreatment induces hydrolysis of cell-wall
biopolymers (pectin, cellulose and hemicellulose), resulting in structural disruption of
the cell wall that facilitates mass transfer and matrix dissolution. This means that several
enzymes (cellulases, hemicellulose, pectinases, etc.) are currently used in industrial extrac-
tions. In water-based extraction processes, the incubation with the enzymes can take place
simultaneously with the mixing phase. The mild operating conditions favor the recovery
of thermolabile products, and further processing of the solid fraction can be proposed as
part of a biorefinery concept for the integral use of biomass. The enzymatic pre-treatment
leads to a reduction in solvent consumption and extraction time. The effectiveness of
enzymatic pretreatment and enzyme-assisted extraction has been extensively documented
in the literature [
10
,
11
]. Pretreatment under intense cavitation using US (Figure 1) and
hydrodynamic rotor/stator units or with high cutting power in high-shear homogenizers
strongly promotes rehydration and solvation of the plants [12], (Figure 2).
Processes 2022, 10, x FOR PEER REVIEW 2 of 15
subcritical water extractions (SWE), PEF and enzymatic treatments, have paved the way
for the expansion of studies and the development of new pilot and semi-industrial reac-
tors.
2. Plant Material Pretreatment
Due to the influence of particle size, suitable milling and controlled size screening pro-
cesses are the first steps toward efficient extraction [5]. The parts of a plant differ in hardness,
fiber structure and chemical composition. The particle fractions isolated by sieving and cy-
clone treatment have different physicochemical properties in addition to their size [6]. In the
extraction of vegetal matrices, a partially-damaged cell wall significantly improves solvent
accessibility for both intracellular solutes and cell-wall constituents [7]. Hydrolytic enzymes,
often used as a mixture, can selectively depolymerize and degrade certain cell wall compo-
nents [8,9]. Enzymatic pretreatment induces hydrolysis of cell-wall biopolymers (pectin, cel-
lulose and hemicellulose), resulting in structural disruption of the cell wall that facilitates
mass transfer and matrix dissolution. This means that several enzymes (cellulases, hemicel-
lulose, pectinases, etc.) are currently used in industrial extractions. In water-based extraction
processes, the incubation with the enzymes can take place simultaneously with the mixing
phase. The mild operating conditions favor the recovery of thermolabile products, and fur-
ther processing of the solid fraction can be proposed as part of a biorefinery concept for the
integral use of biomass. The enzymatic pre-treatment leads to a reduction in solvent con-
sumption and extraction time. The effectiveness of enzymatic pretreatment and enzyme-
assisted extraction has been extensively documented in the literature [10,11]. Pretreatment
under intense cavitation using US (Figure 1) and hydrodynamic rotor/stator units or with
high cutting power in high-shear homogenizers strongly promotes rehydration and solva-
tion of the plants [12], (Figure 2).
Figure 1. Semi-industrial US flow reactor (Weber Ultrasonics AG) for biomass pretreatment.
Figure 1. Semi-industrial US flow reactor (Weber Ultrasonics AG) for biomass pretreatment.
This effect has been exploited in biomass conversion, where the delignification process
is promoted by cavitation pretreatment, leading to higher conversion rates in the subsequent
fermentation and extraction processes [
13
,
14
]. Cavitation pretreatments also affect the
matrix surface and increase the surface area of the material [
15
]. PEF can be used both
as pretreatment and directly for extraction as it can cause electroporation, which opens
pores in cell membranes, and cell rupture, both of which mechanisms greatly enhance the
recovery of intracellular compounds [
16
,
17
]. PEF extraction and pretreatment processes
are mainly studied on a laboratory scale [
18
,
19
]. Their application on the pilot and semi-
industrial scales is documented in Figure 3, with a sequential process in which PEF follows
US treatment.
Processes 2022,10, 2233 3 of 14
Processes 2022, 10, x FOR PEER REVIEW 3 of 15
Figure 2. Rotor/stator HC reactor (EPIC Srl) at DSTFUniversity of Turin.
This effect has been exploited in biomass conversion, where the delignification process
is promoted by cavitation pretreatment, leading to higher conversion rates in the subse-
quent fermentation and extraction processes [13,14]. Cavitation pretreatments also affect the
matrix surface and increase the surface area of the material [15]. PEF can be used both as
pretreatment and directly for extraction as it can cause electroporation, which opens pores
in cell membranes, and cell rupture, both of which mechanisms greatly enhance the recov-
ery of intracellular compounds [16,17]. PEF extraction and pretreatment processes are
mainly studied on a laboratory scale [18,19]. Their application on the pilot and semi-indus-
trial scales is documented in Figure 3, with a sequential process in which PEF follows US
treatment.
Figure 3. Continuous-flow extraction under combined US (C2FUT Srl) and PEF (Energy Pulse Sys-
tems), DSTFUniversity of Turin.
3. Ultrasound-Assisted Extraction
Cavitation is the formation, growth and decay of gaseous bubbles in a liquid. This
process can be triggered by acoustic or mechanical waves that create a cycle of compres-
sion and expansion. When the negative pressure created in the expansion phase is suffi-
cient to overcome the intramolecular forces of the liquid, a cavity, or bubble, forms.
Through the cycles of expansion and compression, a bubble can reach its critical size and
then collapse, releasing a large amount of energy and creating a microenvironment that
reaches up to 5000 K and 1000 bar [20,21].
Figure 2. Rotor/stator HC reactor (EPIC Srl) at DSTF—University of Turin.
Processes 2022, 10, x FOR PEER REVIEW 3 of 15
Figure 2. Rotor/stator HC reactor (EPIC Srl) at DSTFUniversity of Turin.
This effect has been exploited in biomass conversion, where the delignification process
is promoted by cavitation pretreatment, leading to higher conversion rates in the subse-
quent fermentation and extraction processes [13,14]. Cavitation pretreatments also affect the
matrix surface and increase the surface area of the material [15]. PEF can be used both as
pretreatment and directly for extraction as it can cause electroporation, which opens pores
in cell membranes, and cell rupture, both of which mechanisms greatly enhance the recov-
ery of intracellular compounds [16,17]. PEF extraction and pretreatment processes are
mainly studied on a laboratory scale [18,19]. Their application on the pilot and semi-indus-
trial scales is documented in Figure 3, with a sequential process in which PEF follows US
treatment.
Figure 3. Continuous-flow extraction under combined US (C2FUT Srl) and PEF (Energy Pulse Sys-
tems), DSTFUniversity of Turin.
3. Ultrasound-Assisted Extraction
Cavitation is the formation, growth and decay of gaseous bubbles in a liquid. This
process can be triggered by acoustic or mechanical waves that create a cycle of compres-
sion and expansion. When the negative pressure created in the expansion phase is suffi-
cient to overcome the intramolecular forces of the liquid, a cavity, or bubble, forms.
Through the cycles of expansion and compression, a bubble can reach its critical size and
then collapse, releasing a large amount of energy and creating a microenvironment that
reaches up to 5000 K and 1000 bar [20,21].
Figure 3.
Continuous-flow extraction under combined US (C2FUT Srl) and PEF (Energy Pulse
Systems), DSTF—University of Turin.
3. Ultrasound-Assisted Extraction
Cavitation is the formation, growth and decay of gaseous bubbles in a liquid. This
process can be triggered by acoustic or mechanical waves that create a cycle of compression
and expansion. When the negative pressure created in the expansion phase is sufficient to
overcome the intramolecular forces of the liquid, a cavity, or bubble, forms. Through the
cycles of expansion and compression, a bubble can reach its critical size and then collapse,
releasing a large amount of energy and creating a microenvironment that reaches up to
5000 K and 1000 bar [20,21].
The parameters affecting the cavitation process and its mechanisms of action have been
extensively discussed over the years. Cavitation affects the matrix and improves extraction
yield and recovery of metabolites. The mechanisms of action are erosion, fragmentation,
sonoporation and the so-called ultrasonic capillary effect. The erosion and fragmentation
of the matrix are phenomena resulting from the mechanical effect of cavitation, as the
jet streams and shear forces generated by the bubble collapse have a destructive effect
on the matrix. These effects greatly increase the contact area with the solvent, which
enables better mass transfer. Also worth mentioning is the effect on oil glands and similar
structures, which can be ruptured so that their contents pass directly into the extraction
medium. As shown in the literature, these changes can be detected by SEM analysis [
22
,
23
].
Reduction in particle size is another effect that cavitation has on a matrix, and this effect
can be observed both for larger fragments visible to the eye and for smaller, micrometric
Processes 2022,10, 2233 4 of 14
fragments. Again, the main benefit of reducing particle size is to increase the surface
area of the matrix and its exposure to the extraction solvent [
24
,
25
]. Sonoporation is the
formation of pores in cell membranes and can be reversible or irreversible [
26
28
]. This
process has been studied using various methods, such as fluorescence imaging, the voltage
clamp technique and optical observations of cells exposed to sonoporation [
29
31
]. The
pores formed by sonoporation allow solutes to penetrate the membrane and enhance
mass transfer by removing a physical barrier. This phenomenon also has applications
in various other fields, such as gene transfer and drug delivery [
32
36
]. The mechanism
behind the ultrasonic-capillary effect is still unclear. It consists in increasing the penetration
of fluids into the channels and pores of a matrix, both in-depth and at high speed [
37
].
This mechanism promotes the swelling of the matrix and improves mass transfer through
enhanced diffusion [
38
]. The choice of solvent is influenced by its physical properties,
e.g., viscosity and vapor pressure, and its suitability for the particular extraction process
based on the target molecules and their solubility in the chosen medium. To trigger
cavitation, the intramolecular forces in the liquid must be overcome by the negative pressure
generated during the expansion phase [
39
]. Water is the preferred solvent for cavitation
because it generates a high acoustic pressure, which leads to a stronger collapse of the
bubbles [
40
]. Vapour pressure affects the cavitation process by changing the composition of
the bubbles, where an increase in vapor pressure means that more solvent vapor enters the
bubble cavities, which are more stable and collapse less violently [
37
]. The vapor pressure
in a given solvent is mainly influenced by temperature, and an increase in temperature
is reflected in an increase in vapor pressure; when using water, optimal cavitation can be
achieved in a temperature range between 20 and 35
C [
41
,
42
]. Cavitation can be generated
by ultrasonic (US) or hydrodynamic (HC) reactors. US reactors can be either bath or
probe systems (horn and cup horn). Probe systems (also called horn or sonotrode systems)
usually have higher power because they emit US intensity only over the small surface
area of their tips, while bath systems emit less powerful and less uniform acoustic waves
because of their design (since their transducers are embedded in the bath walls) [
23
,
43
].
HC devices produce cavitation either by forcing a fluid through a constricted channel
(orifices or Venturi tubes) or by exerting a force on the fluid using fast-moving parts, such
as rotor-stator cavitators [44].
While acoustic cavitation has a localized cavitation effect due to the poor penetration
of the pressure wave through solid-liquid mixtures, HC cavitation affects the entire mixture.
Both technologies are already partly used on a pilot and industrial scale. The up-scaling
of these two technologies is associated with different difficulties and technical consider-
ations [
44
,
45
]. The main issues in HC scaling-up are related to the distance between the
rotor and stator, their surface shapes, dimension and optimal rotation speed aiming to
limit maintenance stops. US scaling-up efforts are focused on the uniform distribution
and intensity of cavitation bubbles. The main strategy is moving from batch to flow and
deploying more transducers or wide areas of cavitation to ensure optimal mass transfer
and uniform bubble generation [46] (Figure 4).
Several studies have been published on laboratory-scale extractions with acoustic and
hydrodynamic cavitations [
37
,
38
,
48
,
49
]. However, there are currently few papers reporting
on the application of these technologies on a pilot or semi-industrial scale. Only a few
studies address the up-scaling of these techniques, which are listed in Table 1below.
As can be seen from Table 1, there are few published studies on cavitation-assisted
extraction on a pilot or semi-industrial scale. Moreover, few of the published studies have
thoroughly analyzed the effects of various parameters on the extraction process. Further
research on cavitation processes is therefore essential if this promising technology is to be
effectively applied in real industrial processes.
Processes 2022,10, 2233 5 of 14
Processes 2022, 10, x FOR PEER REVIEW 5 of 15
Figure 4. Hydrodynamic cavitation phenomenon in a fluid: Venturi effect and rotor-stator system
[47].
Several studies have been published on laboratory-scale extractions with acoustic
and hydrodynamic cavitations [37,38,48,49]. However, there are currently few papers re-
porting on the application of these technologies on a pilot or semi-industrial scale. Only a
few studies address the up-scaling of these techniques, which are listed in Table 1 below.
Table 1. Pilot and semi-industrial scale US extraction studies, from 2017 to 2022.
Matrix
Technology
Scale
Results
Ref.
Grape stalks
US; recirculating flow
2 kg/60 L
Efficient process scaling-up in
terms of extraction yield and
antioxidant power of the ex-
tract
[50]
Olives
US; continuous flow
2 tons/h
22.7% increase in oil yield and
10.1% increase in oil phenolic
compounds
[51]
Spirulina
US; recirculating flow
1.5 kg/30 L
127% increase in protein yield
compared to the conventional
method
[26]
Undaria pinnati-
fida
US; recirculating flow
200 g/20 L
111% increase in extraction
yield compared to the conven-
tional method
[52]
Sesame oil cake
US; recirculating flow
33 kg/1000 L
193% increase in extraction
yield compared to conven-
tional method
[53]
Soybean
HC; high-pressure ho-
mogenizer
9 L/h
Protein yield increase of 82%; a
single step was optimal
[54]
Orange peel
HC (reactor with Ven-
turi cross-section); re-
circulating flow
42 kg/120 L (1°
test)
6.38 kg/147 L (2°
test)
No optimal extraction parame-
ters are given. Efficient and
rapid extraction of flavones
and monoterpenes; isolation of
high-quality pectin.
[55]
Figure 4.
Hydrodynamic cavitation phenomenon in a fluid: Venturi effect and rotor-stator system [
47
].
Table 1. Pilot and semi-industrial scale US extraction studies, from 2017 to 2022.
Matrix Technology Scale Other Parameters Results Ref.
Grape stalks US; recirculating flow 2 kg/60 L Water; 1 h; S/L 1:30; 30 L/min;
RT; 29 kHz; 2 kW
Efficient process scaling-up in terms
of extraction yield and antioxidant
power of the extract [50]
Olives US; continuous flow 2 tons/h RT; 20 kHz; 2.8 kW 22.7% increase in oil yield and 10.1%
increase in oil phenolic compounds [51]
Spirulina US; recirculating flow 1.5 kg/30 L Water (Phosphate buffer);
40 min; S/L 1:20; 20 kHz 127% increase in protein yield
compared to the conventional method [26]
Undaria pinnatifida US; recirculating flow 200 g/20 L Water; 3 h; S/L 1:100; 30 C;
20 kHz; 960 W 111% increase in extraction yield
compared to the conventional method [52]
Sesame oil cake US; recirculating flow 33 kg/1000 L Water; 4 h; S/L 1:30; 25 C;
20 kHz; 900 W 193% increase in extraction yield
compared to conventional method [53]
Soybean HC; high-pressure
homogenizer 9 L/h Water; S/L 1:7; 100 MPa Protein yield increase of 82%; a single
step was optimal [54]
Orange peel HC (reactor with
Venturi cross-section);
recirculating flow
42 kg/120 L (1test)
6.38 kg/147 L (2test)
Water; S/L 1:2.85; 330 L/min;
0.62 kWh/kg matrix (1test)
Water; S/L 1:23.04; 330 L/min;
2.20 kWh/kg matrix (2test)
No optimal extraction parameters are
given. Efficient and rapid extraction
of flavones and monoterpenes;
isolation of high-quality pectin.
[55]
Silver fir needles HC (reactor with
Venturi cross-section);
recirculating flow 0.529 kg/120 L Water; S/L 1:227; 330 L/min;
4.8 kWh
No optimal extraction parameters are
given. The ORAC/ TPC ratio
increased as a function of cavitation
time, reaching a maximum at 60 min
of extraction.
[56]
The studies listed in Table 1show how US can be effectively applied in flow pro-
cesses such as olive oil extraction, resulting in higher recovery of oil richer in phenolic
compounds [
51
]. Grillo et al. [
50
] developed a method to intensify the water extraction of
grape stems using a US system with recirculating flow (Figure 5). The extract was subjected
to nanofiltration after centrifugation to obtain a concentrated final product with strong
antioxidant activity.
Similar systems were used for the extraction of several other matrices, such as Spir-
ulina [
26
], Undaria pinnatifida [
52
], and sesame oil cake [
53
]. Extraction of Spirulina with US
resulted in increased protein yield. In addition, microscopic observations showed the effects
of acoustic cavitation on Spirulina filaments caused by fragmentation, sonoporation and
deconstruction mechanisms. This effect facilitates the extraction, release and solubilization
of bioactive Spirulina compounds [26].
Processes 2022,10, 2233 6 of 14
Processes 2022, 10, x FOR PEER REVIEW 6 of 15
Silver fir nee-
dles
HC (reactor with Ven-
turi cross-section); re-
circulating flow
0.529 kg/120 L
No optimal extraction parame-
ters are given. The ORAC/ TPC
ratio increased as a function of
cavitation time, reaching a
maximum at 60 min of extrac-
tion.
[56]
As can be seen from Table 1, there are few published studies on cavitation-assisted
extraction on a pilot or semi-industrial scale. Moreover, few of the published studies have
thoroughly analyzed the effects of various parameters on the extraction process. Further
research on cavitation processes is therefore essential if this promising technology is to be
effectively applied in real industrial processes.
The studies listed in Table 1 show how US can be effectively applied in flow processes
such as olive oil extraction, resulting in higher recovery of oil richer in phenolic compounds
[51]. Grillo et al. [50] developed a method to intensify the water extraction of grape stems
using a US system with recirculating flow (Figure 5). The extract was subjected to nanofil-
tration after centrifugation to obtain a concentrated final product with strong antioxidant
activity.
Figure 5. Recirculating flow system used in Grillo et al. [50]. Left: set-up scheme (1: mixed tank; 2:
US flow-through cell); and right: facility picture.
Similar systems were used for the extraction of several other matrices, such as Spir-
ulina [26], Undaria pinnatifida [52], and sesame oil cake [53]. Extraction of Spirulina with
US resulted in increased protein yield. In addition, microscopic observations showed the
effects of acoustic cavitation on Spirulina filaments caused by fragmentation, sono-
poration and deconstruction mechanisms. This effect facilitates the extraction, release and
solubilization of bioactive Spirulina compounds [26].
Extraction with US of Undaria pinnatifida and sesame oil cake showed an increase in
extraction yield of 111% and 193% compared to the corresponding conventional methods.
Moreover, high-intensity US can produce a fine emulsion of lipids in water, as reported
in the case of solvent-free extraction of fatty alcohols from rice bran [57].
Encouraging results have been obtained by using pilot-scale HC cavitation systems
with recirculating flow, for example for orange peels [55] and silver fir needles [56].
The truly new paradigm in extraction is the shift from batch to continuous flow pro-
cesses. Suspensions of plant material in water or aqueous-alcoholic mixtures can be rap-
idly processed in high-intensity multi-transducer ultrasonic units in flow-through treat-
ments and/or rotor/stator hydrodynamic cavitation reactors. These processes are at the
forefront of extraction systems due to a significant increase in extraction efficiency and the
potential application on an industrial scale.
Figure 5.
Recirculating flow system used in Grillo et al. [
50
]. Left: set-up scheme (1: mixed tank;
2: US flow-through cell); and right: facility picture.
Extraction with US of Undaria pinnatifida and sesame oil cake showed an increase in
extraction yield of 111% and 193% compared to the corresponding conventional methods.
Moreover, high-intensity US can produce a fine emulsion of lipids in water, as reported in
the case of solvent-free extraction of fatty alcohols from rice bran [57].
Encouraging results have been obtained by using pilot-scale HC cavitation systems
with recirculating flow, for example for orange peels [55] and silver fir needles [56].
The truly new paradigm in extraction is the shift from batch to continuous flow
processes. Suspensions of plant material in water or aqueous-alcoholic mixtures can
be rapidly processed in high-intensity multi-transducer ultrasonic units in flow-through
treatments and/or rotor/stator hydrodynamic cavitation reactors. These processes are at
the forefront of extraction systems due to a significant increase in extraction efficiency and
the potential application on an industrial scale.
4. Microwave-Assisted Extraction
Microwaves (MWs) are electromagnetic waves ranging from 300 to 300,000 MHz. The
energy associated with MWs is not enough to break chemical bonds (from 0.004 to 0.4 meV,
while bond strengths are usually a few eV), classifying them as non-ionizing [58].
MWs are mainly used for selective and rapid heating and provide reduced energy
consumption compared to conventional technologies [
59
61
]. Based on the interactions
with MW, materials can be classified as reflective, transparent, or absorptive [62].
Reflective materials, such as metals, interact with the MWs by reflecting them. Trans-
parent materials transmit MWs, which pass through them with little to no interaction:
examples of transparent materials include but are not limited to quartz, PTFE and PFA. Ab-
sorptive materials interact with the MWs and absorb them partially or entirely, depending
on the material characteristics; polar liquids are an example of such materials.
Two main processes determine the absorption of MWs in a liquid medium: ionic
conduction and dipole rotation. The ionic conduction mechanism is based on the interaction
between ions and the oscillating electric field generated by MWs, resulting in an ion flow.
Localized heating is then produced by the resistance opposing ion movement inside the
liquid. Dipole rotation is a phenomenon generated by the oscillating electric field and
affects dipolar molecules. These molecules will align themselves with the electric field and
then return to a disordered state billions of times per second (ca. 5 billion times per second
for an instrument working at 2450 MHz).
While there is no lack of studies on microwave-assisted extraction (MAE) on a lab-
oratory scale [
63
67
], process scalability is limited by a need for technological advance-
ment. Nevertheless, MW reactors are used in various industrial processes [
66
68
], despite
extraction-system scalability still being a major challenge; the problems in up-scaling are
not only technical but are also related to the nature of the biomass to be processed. MWs
have limited penetration depth, which limits the volume that can be processed in batch
reactors [
68
]. Switching to continuous systems can overcome the disadvantages of batch
Processes 2022,10, 2233 7 of 14
processes, but the design of MW flow reactors raises other issues that need to be addressed.
Moreover, the addition of carbonaceous material (such as biochar) can directly improve
MW energy transmission in batch reactors, while moving from single-mode to multimode
systems can provide an indirect way to overcome this challenge [4,69].
An efficient process is microwave-assisted hydrodistillation (MAHD), which is used
for the extraction of essential oils (EO). The principles of this process are similar to those
of conventional hydrodistillation, but with improved heating and distillation efficiency.
These aspects enable a reduction in energy consumption and operating time (Figure 6).
In addition, microwave hydrodiffusion and gravity (MHG) has shown promise as an
extraction technique for the simultaneous recovery of EO and other compounds such as
pec-tins, pigments and polyphenols [68].
Processes 2022, 10, x FOR PEER REVIEW 8 of 15
Orange peel
MAE
3 kg
S/L 1:5; 1 h 30 min
64% increase in pectin yield
[74]
Figure 6. Ethos XL and Ethos X (Milestone Srl), DSTFUniversity of Turin.
Table 2 shows that MAE requires further investigation on a pilot scale but has already
shown promising results. Garcia-Garcia et al. [74] analyzed the life cycle assessment
(LCA) of the pectin production process using MW and compared it to a conventional pro-
cess. Their results showed a reduction in the environmental impact of about 75%, mainly
due to the higher energy efficiency of the MW process. Research in this area requires fur-
ther investigation for effective industrial implementation.
5. Subcritical Water Extraction
Water is the most environmentally friendly solvent as it is non-flammable, non-toxic
and readily available. Although the dielectric properties of water mean that only polar prod-
ucts can be extracted from plant matrices, limiting its use, water can be used in a subcritical
state to overcome these limitations. In the temperature range between 100 °C and its critical
temperature (374 °C), the properties of water can be modified as long as it is kept in the
liquid state by a suitable pressure. As the temperature rises, the dielectric constant of water
decreases, reaching values similar to ethanol at 250 °C, allowing the extraction of less polar
compounds. The viscosity also decreases, allowing better penetration into matrices, while
the ionic product increases above 1011, favoring the depolymerization of complex struc-
tures.
By varying the temperature, these properties can be adjusted to optimize product yield
while avoiding thermal degradation. Thermal degradation can also be avoided due to the
shorter extraction times required under subcritical conditions compared to conventional ex-
tractions. Both temperature and water flow rate play an important role in subcritical water
extraction (SWE) [7578]. SWE of plants varies greatly depending on the technology and
scale of use. Common laboratory-scale SWE uses high-pressure pumps for water supply
and a heated cell loaded with biomass [2]. SWE can also be combined with MW heating to
reduce operating times [66,79,80].
So far SWE scaling up is focusing on reactor dimensions and unit replication besides
a higher water flow. Due to higher costs and technical complexity flow, SWE reactors still
present technical issues that need to be addressed.
Figure 6. Ethos XL and Ethos X (Milestone Srl), DSTF—University of Turin.
As shown in Table 2, most of the few available results on MW-assisted extraction on a
pilot scale are hydrodistillation processes.
Table 2. Studies on pilot scale MAE, from 2017 to 2022.
Matrix Technology Scale Other Parameters Results Ref.
Lemon peel MHG 20 kg S/L 1:1.8; no fixed time 0.025% w/wEO [70]
Cannabis MAHD 2.6 kg S/L 1:1; 1 h 50 min 0.35% w/wEO [71]
Opuntia ficus-indica peel MAHD; MHG 1–2 kg S/L 1:0.067; 1 h; 1.5 kW (MAHD)
S/L 1:0.121; 70 C; 40 min; 1.2 kW (MHG) About 128 mL extract/kg matrix (MAHD)
About 342 mL extract/kg matrix (MHG) [72]
Hops MAHD 2–8 kg S/L 2:1; 1 h 50 min Increased extraction yield: 4 times higher
(pellets), 2 times higher (dry matrix) than
on a laboratory scale [73]
Orange peel MAE 3 kg S/L 1:5; 1 h 30 min 64% increase in pectin yield [74]
Table 2shows that MAE requires further investigation on a pilot scale but has already
shown promising results. Garcia-Garcia et al. [
74
] analyzed the life cycle assessment (LCA)
of the pectin production process using MW and compared it to a conventional process.
Their results showed a reduction in the environmental impact of about 75%, mainly due
to the higher energy efficiency of the MW process. Research in this area requires further
investigation for effective industrial implementation.
Processes 2022,10, 2233 8 of 14
5. Subcritical Water Extraction
Water is the most environmentally friendly solvent as it is non-flammable, non-toxic and
readily available. Although the dielectric properties of water mean that only polar products
can be extracted from plant matrices, limiting its use, water can be used in a subcritical
state to overcome these limitations. In the temperature range between 100
C and its critical
temperature (374
C), the properties of water can be modified as long as it is kept in the
liquid state by a suitable pressure. As the temperature rises, the dielectric constant of water
decreases, reaching values similar to ethanol at 250
C, allowing the extraction of less polar
compounds. The viscosity also decreases, allowing better penetration into matrices, while the
ionic product increases above 1011, favoring the depolymerization of complex structures.
By varying the temperature, these properties can be adjusted to optimize product yield
while avoiding thermal degradation. Thermal degradation can also be avoided due to the
shorter extraction times required under subcritical conditions compared to conventional
extractions. Both temperature and water flow rate play an important role in subcritical
water extraction (SWE) [
75
78
]. SWE of plants varies greatly depending on the technology
and scale of use. Common laboratory-scale SWE uses high-pressure pumps for water
supply and a heated cell loaded with biomass [
2
]. SWE can also be combined with MW
heating to reduce operating times [66,79,80].
So far SWE scaling up is focusing on reactor dimensions and unit replication besides a
higher water flow. Due to higher costs and technical complexity flow, SWE reactors still
present technical issues that need to be addressed.
Only a few pilot-scale studies on SWE have been published, which are listed in Table 3.
Table 3. Studies on pilot scale SWE, from 2017 to 2022.
Matrix Scale Other Parameters Yield % Ref.
Miscanthus 150 kg/1200 L 160 C; 2 h; S/L 1:8 33.05% [81]
Zingiber zerumbet 0.25 kg/5 L 170 C; 20 min; S/L 1:20 20.70% [82]
Wheat bran 0.1 kg/1 L 160 C; 60 min; S/L 1:10 15.82% [83]
Chestnut peel 60 kg/180 L 150 C; 30 min; S/L 1:3 39.42% [84]
In the studies listed in Table 3, SWE was compared at laboratory and pilot scales,
with promising results. Different reactor types were used for the extraction of bioactive
compounds and all tests were performed in batch mode. Rudjito et al. [
83
] used a PEG-filled
autoclave for the extraction of arabinoxylans from wheat bran pretreated by deamidation
processes. Cravotto et al. [
84
] performed polyphenol extraction from chestnut hulls by
moving from MAE-SWE on a laboratory scale to SWE on a pilot scale using a double
vessel reactor with a total maximum capacity of 60 kg of plant material, and a maximum
water flow rate of 1000 L/h (Figure 7). The extract was then rapidly concentrated by flash
evaporation under a mild vacuum and steam condenser (Figure 8). Both dry extract and
polyphenol yields were comparable to those obtained in the laboratory, demonstrating
efficient up-scaling of the process.
The research group of Amir et al. [
82
] used a batch reactor for direct extraction of
Zingiber zerumbet on a pilot scale and investigated the different parameters and their
influence on the yield. Wang et al. [
81
] studied Miscanthus hot water extraction (at
160
C) at three different scales: laboratory, intermediate and pilot scale. On a pilot
scale, the authors extracted about 150 kg of biomass in a fermenter, with a yield loss of only
17% compared to laboratory-scale experiments.
The results described are promising and indicate the great potential and multiple
applications of SWE on an industrial scale. However, much work is still needed to translate
the numerous laboratory-scale studies into viable industrial applications.
Processes 2022,10, 2233 9 of 14
Processes 2022, 10, x FOR PEER REVIEW 9 of 15
Only a few pilot-scale studies on SWE have been published, which are listed in Table
3.
Table 3. Studies on pilot scale SWE, from 2017 to 2022.
Matrix
Scale
Other Parameters
Yield %
Ref.
Miscanthus
150 kg/1200 L
160 °C; 2 h; S/L 1:8
33.05%
[81]
Zingiber zerumbet
0.25 kg/5 L
170 °C; 20 min; S/L 1:20
20.70%
[82]
Wheat bran
0.1 kg/1 L
160 °C; 60 min; S/L 1:10
15.82%
[83]
Chestnut peel
60 kg/180 L
150 °C; 30 min; S/L 1:3
39.42%
[84]
In the studies listed in Table 3, SWE was compared at laboratory and pilot scales,
with promising results. Different reactor types were used for the extraction of bioactive
compounds and all tests were performed in batch mode. Rudjito et al. [83] used a PEG-
filled autoclave for the extraction of arabinoxylans from wheat bran pretreated by deami-
dation processes. Cravotto et al. [84] performed polyphenol extraction from chestnut hulls
by moving from MAE-SWE on a laboratory scale to SWE on a pilot scale using a double
vessel reactor with a total maximum capacity of 60 kg of plant material, and a maximum
water flow rate of 1000 L/h (Figure 7). The extract was then rapidly concentrated by flash
evaporation under a mild vacuum and steam condenser (Figure 8). Both dry extract and
polyphenol yields were comparable to those obtained in the laboratory, demonstrating
efficient up-scaling of the process.
Figure 7. Pressure-resistant reactors for SWE (C2FUT Srl.). Left: plant photo; right: reactor tank and
loading vessel scheme, DSTFUniversity of Turin.
Figure 8. Flash evaporation unit (C2FUT Srl.) DSTFUniversity of Turin.
Figure 7.
Pressure-resistant reactors for SWE (C2FUT Srl.).
Left
: plant photo;
right
: reactor tank and
loading vessel scheme, DSTF—University of Turin.
Processes 2022, 10, x FOR PEER REVIEW 9 of 15
Only a few pilot-scale studies on SWE have been published, which are listed in Table
3.
Table 3. Studies on pilot scale SWE, from 2017 to 2022.
Matrix
Scale
Other Parameters
Yield %
Ref.
Miscanthus
150 kg/1200 L
160 °C; 2 h; S/L 1:8
33.05%
[81]
Zingiber zerumbet
0.25 kg/5 L
170 °C; 20 min; S/L 1:20
20.70%
[82]
Wheat bran
0.1 kg/1 L
160 °C; 60 min; S/L 1:10
15.82%
[83]
Chestnut peel
60 kg/180 L
150 °C; 30 min; S/L 1:3
39.42%
[84]
In the studies listed in Table 3, SWE was compared at laboratory and pilot scales,
with promising results. Different reactor types were used for the extraction of bioactive
compounds and all tests were performed in batch mode. Rudjito et al. [83] used a PEG-
filled autoclave for the extraction of arabinoxylans from wheat bran pretreated by deami-
dation processes. Cravotto et al. [84] performed polyphenol extraction from chestnut hulls
by moving from MAE-SWE on a laboratory scale to SWE on a pilot scale using a double
vessel reactor with a total maximum capacity of 60 kg of plant material, and a maximum
water flow rate of 1000 L/h (Figure 7). The extract was then rapidly concentrated by flash
evaporation under a mild vacuum and steam condenser (Figure 8). Both dry extract and
polyphenol yields were comparable to those obtained in the laboratory, demonstrating
efficient up-scaling of the process.
Figure 7. Pressure-resistant reactors for SWE (C2FUT Srl.). Left: plant photo; right: reactor tank and
loading vessel scheme, DSTFUniversity of Turin.
Figure 8. Flash evaporation unit (C2FUT Srl.) DSTFUniversity of Turin.
Figure 8. Flash evaporation unit (C2FUT Srl.) DSTF—University of Turin.
6. Pulsed Electric Field Extraction
Pulsed electric field (PEF) treatment consists of the application of an external electric
field with very short pulses, favoring its permeabilization (Figure 9). Since the typical field
intensity ranges from 10 to 80 kV cm
1
with pulse durations of micro- or milliseconds, PEF
treatments have low energetic requirements that classify PEF as a green technology [85].
Processes 2022, 10, x FOR PEER REVIEW 10 of 15
The research group of Amir et al. [82] used a batch reactor for direct extraction of
Zingiber zerumbet on a pilot scale and investigated the different parameters and their
influence on the yield. Wang et al. [81] studied Miscanthus hot water extraction (at 160
°C) at three different scales: laboratory, intermediate and pilot scale. On a pilot scale, the
authors extracted about 150 kg of biomass in a fermenter, with a yield loss of only 17%
compared to laboratory-scale experiments.
The results described are promising and indicate the great potential and multiple
applications of SWE on an industrial scale. However, much work is still needed to trans-
late the numerous laboratory-scale studies into viable industrial applications.
6. Pulsed Electric Field Extraction
Pulsed electric field (PEF) treatment consists of the application of an external electric
field with very short pulses, favoring its permeabilization (Figure 9). Since the typical field
intensity ranges from 10 to 80 kV cm1 with pulse durations of micro- or milliseconds, PEF
treatments have low energetic requirements that classify PEF as a green technology [85].
Figure 9. PEF unit by EnergyPulse Systems, Lisbon, Portugal.
Permeabilization of the cell occurs by polarization and reorientation of membrane
components, inducing the formation of hydrophilic pores in the cellular membrane. De-
pending on the applied electric field intensity, the formation of pores can be reversible or
irreversible, potentially leading to cell disruption [86].
Although PEF extraction treatments are widely explored at the lab scale, their use in
scaled-up systems is limited, being more common in food processing. To the best of our
knowledge, only two research papers on scaled-up PEF-assisted extraction of plant mate-
rials in water have been published between 2017 and 2022, and are reported in Table 4.
Table 4. Studies on pilot scale PEF, from 2017 to 2022.
Matrix
Technology
Scale
Other Parameters
Results
Ref.
Grapes
PEF, flow
mode.
2500 kg/h
0.09 s residence time; 3.7
pulses of 4 kV cm1
Reduced maceration time, improved polyphe-
nols concentration (up to 35% increase).
86
Grapes
PEF, flow
mode.
200 L/h
0.09 s residence time;
pulse duration of 816 µs
Increased varietal aroma precursors extrac-
tion, limited impact on wine color.
87
Andres Maza et al. [86] described the application of PEF to grape mass to reduce the
maceration time in red wine making while improving polyphenols concentration under PEF
treatment. Comuzzo et al. [87] also processed grapes for winemaking. Focusing on white
wine production, authors could increase the number of varietal aroma precursors in wine,
while avoiding undesired changes in the final product, such as a darker color.
Figure 9. PEF unit by EnergyPulse Systems, Lisbon, Portugal.
Processes 2022,10, 2233 10 of 14
Permeabilization of the cell occurs by polarization and reorientation of membrane
components, inducing the formation of hydrophilic pores in the cellular membrane. De-
pending on the applied electric field intensity, the formation of pores can be reversible or
irreversible, potentially leading to cell disruption [86].
Although PEF extraction treatments are widely explored at the lab scale, their use
in scaled-up systems is limited, being more common in food processing. To the best of
our knowledge, only two research papers on scaled-up PEF-assisted extraction of plant
materials in water have been published between 2017 and 2022, and are reported in Table 4.
Table 4. Studies on pilot scale PEF, from 2017 to 2022.
Matrix Technology Scale Other Parameters Results Ref.
Grapes PEF, flow mode. 2500 kg/h 0.09 s residence time;
3.7 pulses of 4 kV cm1Reduced maceration time, improved
polyphenols concentration (up to 35% increase). 86
Grapes PEF, flow mode. 200 L/h 0.09 s residence time;
pulse duration of 8–16 µsIncreased varietal aroma precursors extraction,
limited impact on wine color. 87
Andres Maza et al. [
86
] described the application of PEF to grape mass to reduce the
maceration time in red wine making while improving polyphenols concentration under
PEF treatment. Comuzzo et al. [
87
] also processed grapes for winemaking. Focusing on
white wine production, authors could increase the number of varietal aroma precursors in
wine, while avoiding undesired changes in the final product, such as a darker color.
Both studies validate a specific industrial application of PEF but the results also show
the valuable effects on the extraction of natural products from plant materials.
7. Conclusions
This review highlighted the state of the art in the industrial scaling up of new tech-
nologies for water extraction of plant matrices. Despite the large gap between academia
and industry, recent advances in the development of unconventional industrial reactors
offer interesting new opportunities. Although extraction yields achieved in very small
units for analytical purposes are hardly reproducible, the solid/liquid ratio can be dramati-
cally improved on a larger scale, bringing remarkable benefits to the process downstream.
The general trend of extractions in ultrasonic and hydrodynamic reactors is to work in
continuous flow. Dielectric heating in microwave extraction and especially new reactors
for subcritical water extraction will expand water extraction capabilities like never before.
All of these technologies can greatly expand the use of water as a nearly universal solvent.
Author Contributions:
Conceptualization, C.C., L.G. and G.G.; writing—original draft preparation,
L.G., G.C. (Giorgio Capaldi) and C.C.; writing—review and editing, G.C. (Giancarlo Cravotto);
supervision, G.C. (Giancarlo Cravotto) and G.G. All authors have read and agreed to the published
version of the manuscript.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.
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