Green Extraction Methods for Polyphenols from Plant Matrices and Their Byproducts A Review

The current review aims to provide updated technical information about extraction anisms, their advantages and disadvantages, and factors affecting efficiencies, and also presents a comp

Green Extraction Methods for Polyphenols from Plant Matrices and Their Byproducts: A Review

Kashif Ameer, Hafiz Muhammad Shahbaz, and Joong-Ho Kwon

Abstract: Polyphenols as phytochemicals have gained significant importance owing to several associated health benefitswith regard to lifestyle diseases and oxidative stress To date, the development of a single standard method for efficientand rapid extraction of polyphenols from plant matrices has remained a challenge due to the inherent limitations ofvarious conventional extraction methods The exploitation of polyphenols as bioactive compounds at various commerciallevels has motivated scientists to explore more eco-friendly, efficient, and cost-effective extraction techniques, based on

a green extraction approach The current review aims to provide updated technical information about extraction anisms, their advantages and disadvantages, and factors affecting efficiencies, and also presents a comparative overview

mech-of applications mech-of the following modern green extraction techniques—supercritical fluid extraction, ultrasound-assistedextraction, microwave-assisted extraction, pressurized liquid extraction, and pressurized hot water extraction—as alter-natives to conventional extraction methods for polyphenol extraction These techniques are proving to be promising forthe extraction of thermolabile phenolic compounds due to their advantages over conventional, time-consuming, andlaborious extraction techniques, such as reduced solvent use and time and energy consumption and higher recovery rateswith lower operational costs The growing interest in plant-derived polyphenols prompts continual search for green andeconomically feasible modern extraction techniques Modern green extraction techniques represent promising approaches

by virtue of overcoming current limitations to the exploitation of polyphenols as bioactive compounds to explore theirwide-reaching applications on an industrial scale and in emerging global markets Future research is needed in order toremove the technical barriers to scale-up the processes for industrial needs by increasing our understanding and improvingthe design of modern extraction operations

Keywords: microwave-assisted extraction (MAE), polyphenols, pressurized liquid extraction (PLE), pressurized hot waterextraction (PHWE), supercritical fluid extraction (SFE), ultrasound-assisted extraction (UAE)

Introduction

Plant phenolic compounds have considerable significance as

bioactive compounds with substantial health benefits There are

several key factors responsible for the shifting trend toward herbal

products among the consumer markets of polyphenols globally

Prominent factors include the rising proportion of aged people in

the populations of Japan and Europe, increased health awareness

of consumers, and the onset of various metabolic disorders due to

aging (Cherniack 2011; Visioli and others 2011)

A recent study conducted by Transparency Market Research,

a global market intelligence group, has predicted a boom in the

polyphenol market owing to increasing demands and market size

This study indicates that the global demand for polyphenols in

2018 will be expected to reach USD 873.7 million The estimated

demand is based on revenue (USD million) and volumes (tons)

CRF3-2016-1533 Submitted 9/20/2016, Accepted 12/6/2016 Authors Ameer

and Kwon are with School of Food Science & Biotechnology, Kyungpook Natl Univ.,

Daegu 41566, South Korea Author Shahbaz is with the Dept of Biotechnology,

Yon-sei Univ., 50 YonYon-sei-ro, Seodaemun-gu, Seoul 03722, South Korea Direct inquiries to

author Joong-Ho Kwon (E-mail:jhkwon@knu.ac.kr,knujhkwon@hanmail.net).

for the period 2012 to 2018, with an annual growth rate of 6.1%(Transparency Market Research 2016)

Implementation strategy for green extraction techniquesThe implementation process of green extraction on an indus-trial scale consists of 3 tiers related to the optimization of processvariables: raw materials, energy, and solvent consumption The3-tier implementation process is comprised of the following level-based innovation and modification of existing protocols and tech-nologies: (i) bringing about improvement by innovative designingand ensuring compliance with optimization strategies (Chematand others 2012; Buckley and others 2013), (ii) exploitation ofinherently undedicated equipment, and (iii) exploration of newalternatives to conventional solvents during the design of inno-vative processes (Cravotto and others 2011; Mustafa and Turner2011)

The current review aims to provide updated technical tion about extraction mechanisms, their advantages and disadvan-tages, and also presents a comparative overview of the applications

informa-of modern green extraction techniques—supercritical fluid traction (SFE), ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), pressurized liquid extraction (PLE), and

ex-pressurized hot water extraction (PHWE)—as alternatives to

con-ventional extraction methods (for example, Soxhlet, percolation,

and maceration)

Extraction techniques for plant-derived polyphenols

Plant materials have been increasingly exploited to isolate and

purify bioactive compounds, and recent studies have reported on

the antioxidant potential of byproducts of fruits (Shahbaz and

others 2016a, 2016b) Traditional techniques involve application

of solid–liquid extraction (SLE) simply by means of solvent

ap-plication and leaching A domestic apap-plication of conventional

solvent extraction (CSE) is quite familiar to everybody in daily

life from the making of coffee or tea at home SLE encompasses

conventional methods: Soxhlet extraction (SE), percolation, and

maceration extraction (ME) These techniques have been utilized

for more than a century for the isolation of polyphenols However,

certain disadvantages pertaining to CSE render its application quite

uneconomical due to excessive consumption of time, energy, and

polluting solvents (Cravotto and others 2011) These underlying

drawbacks have triggered research that explores more cost-effective

and greener techniques for the extraction of polyphenols from a

wide range of plant matrices and their byproducts (Azmir and

others 2013), as illustrated in detail in Figure 1

SFE

In a broad sense, SFE has established itself as a prominent green

method, particularly in the case of solid matrices, owing to the

advantages described in Table 1 SFE has been widely used for

value-addition to plant byproducts generated during processing

Such products apparently have no commercial significance, and

extraction allows effective exploitation of waste components in

order to extract the targeted phenolic compounds (Herrero and

others 2010)

Extraction principle and mechanism of SFE

Supercritical (SC) fluid can be defined as fluid existing in a

phase which possesses features of both liquids and gases above its

characteristic critical temperature and pressure Critical pressure

(Pc) is regarded as the minimum quantity of pressure required to

liquefy a gas at its unique critical temperature The critical

tem-perature (Tc) of a gas is the temtem-perature at which the gas does not

become liquid until application of extra pressure SC fluids have

become solvents of choice due to the combined properties of 2

individual phases: gaseous and liquid simultaneously This results

in an improved mass transfer rate of solutes during extraction

Hence, SC fluid provides unique properties of viscosity, density,

and solvation at a phase between liquids and gases These

proper-ties can be modified by varying temperature/pressure (Lang and

Wai 2001; Pereira and Meireles 2009) The SFE system is depicted

in detail in Figure 2 During the extraction process, plant raw

material is fed into an extraction vessel The desired extraction

conditions are maintained by the operation of a pressure release

valve and temperature controllers attached to the extraction vessel

European Food Safety Authority and the United States Food and

Drug Administration have assigned CO2 a generally recognized

as safe (GRAS) status CO2 is the most widely used SC solvent

because of its particular features, notably its economic

inexpen-siveness and GRAS status (Pereira and Meireles 2009; Herrero

and others 2010; Khosravi-Darani 2010) CO2 is pumped as a

fluid at critical conditions (Tc < 278.15 K and Pc= 5.7 MPa) to

the extraction vessel Heat exchangers are used to cool the CO2

located in the inlet and outlet of the CO2pump (Figure 2) In

the CO2-extract separator, the solvation power of the SC fluid islowered by combined manipulation of temperature/pressure Thephenolic compounds dissolved in the fluid are separated from thefluid in the CO2-extract separator by means of an outlet valvelocated on the lower side of the separator The cyclic process ofSFE continues until maximum recovery rates of polyphenols areachieved from the targeted plant sample matrices (Wang and Weller2006)

Factors Influencing SFE EfficiencyCorrect SC fluid selection

This is one of the crucial factors governing SFE efficiency forpolyphenol extraction Due to the thermolabile nature of pheno-lic compounds, SC water is not a good choice in spite of highextraction yields in the case of polar compounds Polyphenolspossess a low degree of solubility in SC-CO2, making CO2usagealone unfavorable (King 2014) To overcome this limitation, mod-ifiers have been added to SC-CO2in order to improve solubilityand recovery rates of phenolic compounds Acetonitrile, acetone,methanol, ethyl ether, ethanol, and water are the most commonlyused effective polar modifiers (Pereira and Meireles 2009; Azmirand others 2013) Ethanol has been reported to be the better op-tion as a modifier compared to others by virtue of its lower toxicityand enhanced extraction of polyphenols with a lower degree of se-lectivity Water has very low solubility in SC-CO2and is normallynot used as a modifier alone In order to increase water solubility,binary mixtures of ethanol and water are used (Wang and Weller2006; King 2014)

Modifier effectClean extracts with higher recovery rates are achieved by ex-ploiting a temperature range just above the critical point at whichthe targeted phenolic compounds have solubility in a fluid Thisalso helps to minimize the extraction of unwanted compoundsfrom matrices Modifiers can also influence extraction efficiencyduring the SFE operation (Wang and Weller 2006) Higher extrac-tion rates of a glycosylated flavonoid (naringin) have been reported

from Citrus paradise when ethanol (15%) was used as modifier with

SC-CO2at 331.83 K (Tc) and 5.9 to 9.5 MPa (Pc), as compared

to the yield obtained using SC-CO2alone SFE results in a higheryield (14.4 g/kg in 45 min) in comparison with the yield ratesobtained from ME (11.1 g/kg in 24 h) and heat reflux extraction(HRE) (13.5 g/kg in 3 h) (Giannuzzo and others 2003)

Particle size effectParticle size is another factor that has an influential effect on SFEefficiency during phenolic compound extraction Particle size isreported to influence mass transfer of solutes during the extractionprocess, thereby affecting polyphenol recovery rates (Pereira andMeireles 2009) In another study, the effect of particle size on SFEefficiency was investigated for the extraction of isoflavones fromsoybean meal It was concluded that an optimum particle size inthe range of 20 to 30 mesh provides a greater surface area and, con-sequently, results in increased mass transfer Improved isoflavoneyield was obtained because of enhanced extractant penetration inthe matrix Any deviation of particle size—less than 20 mesh orgreater than 30 mesh—leads to a marked decline in isoflavonerecovery from soybean meal (Zuo and others 2008)

SFE applications for the green extraction of polyphenolsRecently published studies have reported on the extensive ap-plication of SFE in the food sector, due to the distinctive merits

Figure 1–Conventional and modern extraction methods for plant-derived polyphenols.

Figure 2–Operational schematic mechanism of supercritical fluid extraction (SFE) system.

of SFE Promising features include its potential as a sustainable

and environmentally friendly technique that limits the use of toxic

organic pollutants, improved selectivity for isolating targeted

com-pounds, faster extraction rate, comparable yields, and utilization

of food-grade organic modifiers (Pereira and Meireles 2009; De

Melo and others 2014) Zuo and others (2008) extracted the

soy-bean isoflavones (predominantly daidzein, genistein, and daidzin)

from soybean meal, and the effects of various factors, such as

mod-ifier (aqueous methanol) concentration and operating conditions

(Pc, Tc, and SC-CO2flow rate), were investigated CSE was

com-pared with SFE in terms of isoflavone recoveries After studying

the effect of various factors, a maximum isoflavone yield of 87.3%

was obtained with the following conditions: 80% aqueous

modi-fier, 50 MPa (Pc) at 313.15 K (Tc), 20 to 30 mesh as the optimum

particle size range, with SC-CO2 flow rate of 9.80 kg/h SFE

of isoflavones was reported to be more efficient, with a 10.2%

increase in yield and rapid extraction time (the optimum is only

150 min), in comparison with CSE, which takes up to 4 h at

333.15 K, and may lead to degradation of thermolabile

soybean-meal isoflavones (Zuo and others 2008)

Cajaninstilbene acid (CSA) and pinostrobin (PI) are, tively, a stilbene and flavonone from pigeon pea leaves and Kongand others (2009) compared the antioxidant activities of SFE ex-tracts of these compounds with those recovered by HRE Af-ter optimizing the SFE process by employing response surfacemethodology (RSM), a statistical process optimization technique,SFE resulted in enhanced recovery of CSA and PI under the fol-

respec-lowing operating conditions: a Pcof 30 MPa with ethanol (80%)

as a modifier and a solid/liquid ratio of 10:1, a CO2flow rate of

12 kg/h at Tcof 333.15 K, and extraction time of 2 h Higheryield rates of CSA (11.17 mg/g) and PI (2.73 mg/g) were achieved

in the case of SFE extracts with relatively higher antioxidant pacities compared to the HRE yield rates (8.31 and 1.99 mg/gfor CSA and PI, respectively) A higher SFE extract yield wasachieved due to an increased mass transfer rate Table 3 shows acomparative overview of SFE for polyphenols in comparison withother green methods

ca-SFE process was optimized by RSM for effective and

effi-cient extraction of flavonoids from kudzu (Pueraria lobata) owhi

plant roots The highest flavonoid contents were obtained at the

Table 1–Advantages, disadvantages, precautions, and applications of 1 SFE, 2 UAE, 3 PLE, and 4 PHWE.

SFE 1) Cutting down usage of

Recovery of natural products and thermosensitive polyphenols from wide range of plant matrices

(Herrero and others 2010; Sairam and others 2012)

of SFE

Achievement of equilibrium (phase equilibrium of solvent and solutes)

Phenolic compounds and flavors

Byproducts recovery from fruits and vegetables

(Herrero and others 2010; Sairam and others 2012)

as CO 2 pumps and pressure vessels

(Ghude and others 2013; King 2014)

of extraction

(Sairam and others 2012; King 2014)

6) Continual process with

no intermittence COused as SC solvent2cannot always be

due to its nonpolar nature for polar solutes

(Herrero and others 2010; King 2014)

7) Cost-effective

handling Postextractionresidual solvents

disposal issues from plant matrices complying to EPA regulations

(Herrero and others 2010; Sairam and others 2012)

1) Ease of use due to

of bioactive compounds

Careful experimentation required to choose optimum solvents for high recovery

Extraction and recovery of natural products

(Vilkhu and others 2008; Tadeo and others 2010)

Nondestructive method for extraction of active principles from vegetable matrices

(Vilkhu and others 2008; Kentish and Feng 2014)

Nondestructive method for extraction of active principles from vegetable matrices

(Vilkhu and others 2008; Kentish and Feng 2014)

(Continued)

Table 1–Continued

4) Less time consuming

in active constituents in plants by free radical formation and result in undesirable changes in extracted components

Extraction of polyphenols from plant matrices/

byproducts or waste parts (bark, leaves, peel, and seeds)

(Vilkhu and others 2008; Kentish and Feng 2014)

of dynamic ultrasound-assisted extraction (DUAE)

Intensification of bioactive compounds (polyphenols) from natural plant sources

(Knorr and others 2011; Simsek and others 2012)

techniques such as,

PLE 1) Extraction of target

Care must be taken to select solvent mixtures from wide range of choices, especially modifier and surfactant assisted in terms of sustainability and safety

Polyphenols and nutraceuticals from wide range of plant matrices

(Mustafa and Turner 2011; Knorr and others 2011; Heng and others 2013)

Bioactive compounds from herbal sources

(Carabias-Mart´ınez and others 2005; Knorr and others 2011)

(Continued)

from plant matrix

Lower recovery rates

of potentially thermosensitive polyphenols at elevated temperatures

(Carabias-Mart´ınez and others 2005; Mustafa and Turner 2011)

solvent usage with

(Mustafa and Turner 2011; Santos and others 2012)

PHWE 1) Employment of water

of physicochemical properties of superheated water

Maintenance of constant pressure

to ensure subcritical water conditions

Extraction of numerous phenolic compounds (Flavonoids and nonflavonoids) from vast range of plant matrices

(Teo and others 2010; Plaza and Turner 2015)

3) Reduction of organic

solvents usage to

greater extent

(Vergara-Salinas and others 2015)

5) Low operational and

1 SFE (Supercritical fluid extraction).

2 UAE (Ultrasound-assisted Extraction).

3 PLE (Pressurized fluid extraction).

4 PHWE (Pressurized hot water extraction).

following set conditions: a Tc of 323.15 K, SC-CO2 flow

rate of 20 L/h, with approximately 181 mL ethanol

modi-fier at a Pc of 20 MPa (Wang and others 2008a) Ethanol

used as cosolvent caused an increased polarity of SC-CO2,

which resulted in improved extraction yields of flavonoids

from plant roots Moreover, the model-predicted values showed

a fair match with experimental values under optimum

conditions

Rice wine lees was investigated for antioxidant activity of

phe-nolic compounds from SFE extracts Ethanol (as the employed

modifier) in combination with SC-CO2 has been described as a

critical factor for enhanced SFE efficiency for polyphenols SFEhas been compared with SE in terms of extract yield and extrac-tion times SFE yielded 43% of total SE extract yield In contrast,the SFE process has been reported to consume considerably lessethanol (approximately one-tenth of the amount used in SE) andtime (only 1 h compared to 6 h in the case of SE) Polyphenolextraction was found to increase with a corresponding increase in95% ethanol volume during SFE A final yield rate of 11.9% was

reported using the following extraction conditions: Pc (35 MPa)

and Tc(313 K) with SC-CO2flow rate of 25 mL/min (Wu andothers 2009)

UAE has emerged as a promising technique that fulfills the

re-quired criteria as an inexpensive green extraction technique

No-table UAE features include versatility, simplicity, safety, rapidity,

eco-friendliness, and cost-effectiveness, due to the reduced

con-sumption of time, energy, and expensive organic solvents, which is

in contrast to traditional extraction techniques (Wang and Weller

2006; Azmir and others 2013) The advantages, disadvantages, and

limitations of UAE are presented in Table 1

Extraction principle and mechanism of UAE

Acoustic waves are classified as longitudinal waves that require a

certain medium for their propagation However, ultrasound waves

differ from normal sound waves by virtue of their relatively higher

frequencies than those of the normal audible range of humans

(ࣙ20 Hz to 20 kHz) (Priego-Capote and de Castro 2004; Azmir

and others 2013) As ultrasound has longer wavelengths, typically

in the millimeter (mm) range, which are longer than medium-sized

molecules, chemical changes are not brought about by the direct

interaction of waves with matrix molecules Instead, chemical

changes are brought about due to phenomenal changes, resulting

in implosion caused by massive energy production (de Castro and

Delgado-Povedano 2014; Kentish and Feng 2014) Generally, the

most widely used extractor device for sonication is an ultrasonic

bath, as shown in Figure 3

In order to conduct sonication for extraction purposes, the

op-erational frequencies of ultrasound waves generally range from 20

kHz or above When media are exposed to such high frequencies,

ultrasound waves produce their effects by transmittance of massive

energy and pressure, which are radiated by base transducers to the

targeted sample causing cavitation (Figure 3) Strong ultrasonic

waves result in the formation of bubbles in liquid media upon

interaction with a matrix The bubbles continue to absorb energy

up to their maximum limit, and further exposure to ultrasonic

waves causes the bubbles to grow and collapse; in sonochemistry,

this collapse is described as cavitation (Azmir and others 2013) As

a consequence of this collapse, a considerable amount of energy is

produced, which occurs in an uncontrollable manner in each

di-rection in the ultrasonic bath tank (Figure 3) This massive energy

release causes significantly extreme changes in temperature (up to

5000 K) and pressure (100 MPa) within bubbles during

extrac-tion, causing liquid solutes to leach at speeds of 280 m/s (Vilkhu

and others 2008; V´azquez and others 2014) Figure 3 provides a

detailed schematic overview of the UAE mechanism Upon

son-ication, ultrasonic waves break up solid particles (disruption) and

remove inert material layers, which may cause passivation,

result-ing in an increased surface area for mass transfer of solutes durresult-ing

extraction (Figure 3)

Factors Influencing UAE Efficiency

Operational conditions

Application of ultrasound during extraction influences other

process variables, namely, the extraction temperature and pressure

A reduction in temperature occurs due to the applied ultrasound

during operation, making UAE a preferred choice for the

extrac-tion of thermosensitive phenolic compounds from a wide range of

plant matrices However, it is noted that this temperature change

can be unfavorable in terms of extract recovery, due to changes

in extraction time Consequently, temperature should be regulated

carefully during optimization studies (Wang and Weller 2006) Ma

and others (2008) have observed a declining trend in hesperidin

yield from citrus peel due to thermal degradation of phenolic pounds at elevated temperatures during longer extraction times

com-Sonication parameters (power, temperature, and frequency)

Lower frequency has been reported to be correlated with creased cavitation during UAE Increased efficiency is obtaineddue to the positive influence on mass transfer of solutes from plantmatrices The porous nature of plant matrices is also found to beaffected by the applied frequency during sonication This necessi-tates optimization studies for enhanced extract yields of phenoliccompounds (Wang and Weller 2006) Hesperidin extract yield wasfound to be higher (1077.62 mg/g DW) from Satsuma mandarinpeel under an optimum frequency of 60 kHz instead of 100 kHz,303.15 K temperature for 20 min extraction time, and 8 W power,compared to ME for 8 h, which resulted in a lower recovery rate(601.2 mg/g DW) It was also noted that decreased recovery ofhesperidin was obtained with a gradual increase in extraction pro-cess variables Therefore, the authors stressed the need for carefullyoptimized application of UAE (Ma and others 2008)

in-UAE applications for the green extraction of polyphenols UAE

of isoflavones from Pueraria lobata (Wild.) ohwi stem was

car-ried out and extraction efficiency was compared with that ofCSE using aqueous solutions of n-butanol (95%) and ethanol of

2 concentrations: 95% (v/v) and 50% (v/v) (Huaneng and others2007) The authors found UAE to be more efficient for total ex-tract yield of isoflavones than CSE under the following extractionconditions: 50% aqueous ethanol (extractant), 650 W ultrasonicpower at 298.15 K with agitation at 300 rpm A positive cor-relation was also observed between extract yield and ultrasonicpower Similarly, UAE was used to carry out the extraction of

phenolic acids (caffeic acid, p-coumaric acid, ferulic acid, sinapic

acid, protocatechuic acid, and 4-hydroxybenzoic acid) from

Sat-suma mandarin (Citrus unshiu Marc.) peels and compared with

ME as a control The authors investigated the effects of differentoperational parameters on phenolic compound extraction frompeel matrix: sonication time (10 to 60 min), temperature levels(288.15, 303.15, and 313.15 K) at different ultrasonic power lev-els (3.2, 8, 30, and 56 W) Ultrasonic power resulted in higherextract yields of phenolic acids than those obtained by conven-tional ME for 8 h expressed inµg/g DW units as follows: caffeic

acid (UAE= 64.28, ME = 31.7), p-coumaric acid (UAE = 140,

ME= 63.1), ferulic acid (UAE = 1513, ME = 763), sinapic acid(UAE = 133, ME = 132), protocatechuic acid (UAE = 15.8,

ME = 20.6), and 4-hydroxybenzoic acid (UAE = 34.1, ME =23.5) However, degradation of phenolic acids was reported totake place at high temperatures for longer periods of time underUAE Therefore, the UAE technique must be used carefully byoptimizing all parameters for extraction in order to avoid thermaldegradation of phenolic compounds Maximum yields of pheno-lic acids were reported using the following optimized operationalparameters: 20 min of extraction time at 303.15 K temperatureand 8 W ultrasonic power (Ma and others 2008)

The potential of UAE was exploited for a comparative studywith traditional mix-stirring (TMS) regarding the efficiency of ex-tracting isoflavones from freeze-dried soybeans The extract yield

of soy isoflavones was found to be higher in the case of UAEextracts Optimal UAE parameters for enhanced soy isoflavoneextraction at 333.15 K comprise a sonication power of 200 W for

20 min at 24 kHz frequency Under these optimized conditions,sonication led to an enhanced extract yield of isoflavones (1180

µg/g DW) from soybeans compared with that obtained using the

TMS method (1098 µg/g DW), which was used as a control

(Rostagno and others 2003)

Figure 3–Operational schematic principle of ultrasound-assisted extraction (UAE) system.

In another instance, UAE for anthocyanin, antioxidant, and

to-tal phenolic contents from grape seed was optimized by a central

composite rotatable design using RSM Extraction time as a

pro-cess variable was reported to play an influential role in enhancing

the total extract yield of anthocyanins (2.29 mg/mL), total

pheno-lic content (5.41 mg GAE/mL), and increased antioxidant activity

(12.28 mg/mL) The optimal extraction conditions for

antho-cyanins were found to be an ethanol concentration of 53% and

326.15 K temperature for 29 min extraction time, while keeping

the ultrasonic power and frequency constant: 250 W and 40 kHz,

respectively The authors reported a reasonable match between the

predicted and real values of response variables after optimization

(Ghafoor and others 2009)

In conclusion, UAE has a demonstrated edge over traditional

extraction techniques and has proven to be more suitable for

polyphenol recovery as a modern green extraction technique

Pe-culiar features differentiating UAE from conventional techniques

include decreased solvent consumption, easy residual recovery

from plant matrix in bound form, rapid and substantial

recov-ery rates of polyphenols, and suitability of the UAE method for

routine analysis due to cost-effectiveness of equipment and

in-frastructure (Wang and Weller 2006; Vilkhu and others 2008)

Table 3 presents a comparative overview of the UAE of polyphenol

from various plant matrices in comparison with other extraction

methods

MAE

Initially, all technologies collectively known as

“microwave-assisted processes (MAPs)” were developed and patented by the

Canadian Department of the Environment (Environmental

Tech-nology Centre) These methods were developed for the extraction

of bioactive compounds from various targeted plant matrices and

their byproducts A review of the published literature indicates that

this technique is also acknowledged as a clean process

technol-ogy with several environmental, economic, and social advantages

(Kwon and others 2003a, 2003b) The advantages, disadvantages,

and limitations of MAE are tabulated in Table 1

Principle and mechanism of MAE MAE is an efficient and

promising method involving derivation of natural compounds

from raw plants or their byproducts The MAE process allows

rapid and efficient extraction of polyphenols with similar or

bet-ter yields as compared to conventional techniques (Abdel-Aal andothers 2014) Two oscillating perpendicular fields, electric andmagnetic fields, act directly to heat materials having the ability toconvert part of the absorbed energy to thermal energy MAE of-fers several advantages over CSE, including reduction in extractiontime, improved yield, better accuracy, and suitability for thermo-labile chemical components (Delazar and others 2012; Azmir andothers 2013)

Dried plants contain minute microscopic traces of moistureserving as target for microwave heating High temperature andpressure is generated inside the oven upon interaction of mi-crowave radiation with chemically bound water molecules Hightemperature causes the dehydration of cellulose leading to de-creased mechanical strength The MAE process consists of severalsteps in order to achieve efficient extraction of phytochemicals,particularly polyphenols from plants (Veggi and others 2013) TheMAP starts with the generation of electromagnetic waves from acavity magnetron (Figure 4) Tissues and cell walls of plants andtheir byproducts inside the plant matrix interact with the emit-ted radiation waves This interaction results in the heating up ofmoisture trapped inside the plant matrix due to absorption of thecharacteristic photonic energy of electromagnetic waves Electro-magnetic energy causes moisture evaporation from the plant ma-trix This causes considerable pressure to be exerted on plant cellwalls at the cellular and subcellular levels, resulting in the swelling

of plant cells during the MAE process This swelling eventuallybrings about structural changes in the plant matrix, thereby pro-moting an increased mass transfer of solutes due to the rupturing

of cells This, in turn, facilitates phytochemical leaching from theplant cellular matrix into the extractant during MAE (Delazar andothers 2012; Azmir and others 2013; Veggi and others 2013)

Commercial MAE system typesCommercially, 2 types of MAE systems are available for indus-trial and commercial applications for natural product extraction:(i) the closed-vessel system and (ii) the open-vessel system (Figure4) A comparison regarding the advantages and disadvantages ofcommercial MAE systems for extraction purposes is presented inTable 2

Figure 4–Operational schematic principle and mechanism of microwave-assisted extraction (MAE) system (a) Open-vessel MAE system (b)

Closed-vessel MAE system.

Closed-vessel system (multimode)

In this system, extraction is carried out under controlled

condi-tions of temperature and pressure This is generally employed for

extractions under extremely high temperature conditions (Wang

and Weller 2006) Diffused microwaves from a cavity magnetron

radiate in all directions to interact with plant samples placed in

extraction vessels in a closed-vessel chamber Owing to the even

dispersion of microwaves, this technique is also known as the

mul-timode system (Figure 4)

Open-vessel system (monomode)

In this system, also known as the monomode system, the

ex-traction vessel is partially exposed to microwave radiation (focused

radiation) (Mandal and others 2007) A circular metallic waveguide

directs the focused microwaves toward the extraction vessel inside

the microwave (monomode) cavity This interaction promotes the

initiation of mass transfer between the solute and extractant upon

solvation, as shown in Figure 4

Factors Influencing MAE Efficiency

The efficiency of the MAE process is subjected to changes

caused by various factors: extractant nature, microwave irradiation

power, extraction temperature and time, the peculiar

character-istics of individual plant matrices, and the solvent-to-feed (S/F)

ratio A detailed description of the factors influencing this

tech-nique is provided in the next section

Microwave power

During the process of phenolic compound extraction, high

mi-crowave power levels can result in poor recovery rates due to

degradation of thermolabile components It is noted that the

ex-traction rates of phenolic compounds from various plant matrices

have been shown to increase up to certain levels followed by a

declining trend in extraction yields Evidently, the sample matrix

becomes heated because of localized interaction with microwaves

This phenomenon leads to a higher diffusion of polyphenols out

of the plant matrix and into the extractant (Chan and others 2011)

In the case of Oolong tea, the total phenolic content increasedwith a rise in extraction temperature, and the optimum extractyield was achieved at 443.15 K Reduced yields were reportedbeyond the optimum extraction temperature, confirming the im-portance of an optimal combination of microwave power andtemperature for high extract yields of thermolabile phenolic com-pounds from targeted plant matrices (Tsubaki and others 2010)

Extraction time During MAE, the extraction time is an tant influential factor, along with microwave power and temper-ature As mentioned previously, exposure to microwave radiationfor longer time periods decreases overall extract yield due to dis-ruption of the structural integrity of chemically active principles(polyphenols) present in plant matrices To circumvent thermaldegradation of phenolic compounds, extraction time during theMAE process can be manipulated by controlling exposure, rang-ing from a few minutes to 30 min, excluding the solvent-freeMAE Moreover, in the case of longer extraction times, extrac-tion cycles can be employed in order to reduce the degradation ofphenolic compounds This can be accomplished by repetition ofthe extraction procedure until completion, with subsequent ad-dition of extractant during the extraction cycle (Chan and others2011)

impor-Pan and others (2003) have investigated the MAE process oftea polyphenols and tea caffeine from tea leaves under vari-ous experimental conditions They found that MAE proved to

be more advantageous in terms of fast extraction with higheryield rate For the extraction of phenolic compounds from tealeaves, MAE is completed in a shorter time interval (4 min) usingethanol compared with CSE (20 h) at room temperature and HRE(45 min)

Features of the plant matrix Apart from other factors, the acteristic nature and features of individual plant samples also in-fluence efficiency of the MAE process Prior to MAE, optimumextraction of phenolic compounds demanded that the desired sam-ple should be in particular forms: finely ground powdered sample,dried, sieved, or preleached Very small-sized sample particles will

char-Table 2–Comparison of closed-vessel and open-vessel MAE systems according to intrinsic advantages and disadvantages

Enhanced and safer possibility of reagent addition

Comparatively exhibit less precision than

in close-vessel system

(Zhang and others 2011; Veggi and others 2013; Destandau and others 2013) 2) Avoidance of loss of

volatile substances High solutiontemperature are

not permissible by constituent material of vessel

Utilization of vessel manufactured from various materials that is, quartz or glass

Inability to process multiple samples simultaneously due

to low throughput

of equipment

(Mandal and others 2007; Alupului and others 2012; Afoakwah and Teye 2012)

Easy removal of excessive quantities

of employed solvents

Longer time spans are required than closed-vessel system

(Zhang and others 2011; Delazar and others 2012; Alupului and others 2012)

Easy processing of larger samples volumes

(Alupului and others 2012; Afoakwah and Teye 2012)

5) High yield by using

ionic liquids (IL’s) at

ambient

temperature

(70 °F/294.26 K)

Handling and processing of limited sample volumes

No need of operational cooling down or

depressurization

(Zhang and others 2011; Afoakwah and Teye 2012; Veggi and others 2013)

Cost-effective availability of sophisticated equipment for polyphenol extraction

(Delazar and others 2012; Veggi and others 2013; Destandau and others 2013) More effective for

extraction of thermosensitive phenolic compounds than closed-vessel

(Alupului and others 2012)

lead to difficult separation of the extract from plant residues upon

completion of the extraction process This will necessitate an

ad-ditional clean-up step after extraction of phenolic compounds

from plant matrices (Ruan and Li 2007; Chan and others 2014)

Furthermore, pretreatment of powdered samples with extractant

for 90 min prior to the extraction step has been reported to

en-hance MAE efficiency This resulted in improved kinetics, such

as improved mass transfer and increased diffusion rate of phenolic

compounds from plant sample residues (Kaufmann and Christen

2002) In the case of dried plant sample matrices, pretreatment

with water has been reported to promote the heating effect of

microwaves in a localized manner It was noted that heating up of

moisture present in the plant sample matrix continued to increase

as the extraction process progressed and resulted in higher

evap-oration rates By allowing the release of polyphenols from plant

molecules due to matrix rupturing, this caused a further rise in

the internal pressure within the extraction cell, producing higher

extraction yields of phenolic compounds A high moisture content

in plant matrices favors a high hydrolyzation tendency, which

im-proved the diffusion of solute molecules (Alfaro and others 2003;

Wang and Weller 2006; Mandal and others 2007; Azmir and others

2013)

Stirring effect With the introduction of stirring during the

MAE process, the negative effects of the S/F ratio upon extraction

recovery can be minimized (Chan and others 2011) Furthermore,

concentrated polyphenols as bound active principles in the plant

matrix lead to the creation of a barrier to mass transfer rate due

to a deficiency of the extractant Stirring action helps to reduce

this barrier, thereby enhancing the extraction yield by causing

ac-celerated agitation, which leads to increased extraction efficiency(Ruan and Li 2007)

Effect of additives and solvent choice Binary solutions of ganic solvents with water were reported to have a desirable impact

or-on extractior-on efficiency In this regard, it is also noted that thepresence of water in organic solvents leads to enhanced pene-tration of the extractant in matrix molecules This subsequentlypromotes microwave heating and imparts a positive impact onoverall efficiency and extraction time compared with MAE usingorganic solvents alone (Alfaro and others 2003; Wang and Weller2006) Solvent toxicity is another important factor that must beevaluated regarding the selection of a suitable extractant for MAE.For instance, a recent study was conducted by Makris and others(2015) to investigate the effects of solvent toxicity on the extrac-tion of phenolic compounds from red grape pomace by utilizing 2aqueous solutions of glycerol/tartaric acid Upon optimization byBox-Behken using the RSM approach, they concluded that glyc-erol was a more suitable choice for enhanced flavonoid recovery.Tartaric acid was found to exert a negative effect with regard tophenolic compound recovery from grape pomace

In the case of phenolic compound extraction from grape seedsand skin, methanol has shown a relatively better performance

as an extractant in terms of higher recovery rates in contrast toethanol, which resulted in lower extract recovery with higherantioxidant activity (Casazza and others 2010) MAE efficiencywas reported to be enhanced if the plant material is impregnatedwith alternative solvents, such as an ionic liquid (IL) at roomtemperature, in comparison with traditional organic solvents alone.The advantages associated with IL addition include higher heating