Techniques to extract bioactive compounds from food by-products of plant origin
Hilde Wijngaard, Mohammad B. Hossain
, Dilip K. Rai, Nigel Brunton
Teagasc Food Research Centre Ashtown, Ashtown, Dublin 15, Ireland
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 17 April 2011
Accepted 27 September 2011
Keywords:
By-products
Carotenoids
Polyphenols
Pomace
Pressurised liquid extraction
Supercritical CO
2
extraction
Pulsed electric
fields
By-products of plant origin represent an abundant source of bioactive compounds. However, to exploit these
resources commercially relevant strategies for their extraction must be developed. This review focuses on the
extraction of bioactive compounds from food by-products of plant origin by a number of novel methods, in-
cluding pressurised liquid extraction and supercritical CO
2
extraction. In general supercritical CO
2
extraction
is most effective for apolar compounds such as carotenoids, while pressurised liquid extraction can be used to
extract more polar compounds such as polyphenols. Both techniques are sustainable and green techniques. In
addition, pre-treatment of plant by-products by novel non-thermal processing techniques in order to en-
hance extraction will be highlighted. In general the selection of an appropriate extraction strategy is depen-
dent on the type of compound to be extracted as well as the potential up scaling of the technique.
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Processing of foods of plant origin generates vast quantities of by-
products. Disposal of these by-products represents both a cost to the
food processor and a potential negative impact on the environment. Re-
search over the past 20 years has revealed that many of these by-
products could serve as a source of potentially valuable bio-active com-
pounds. Despite this the vast majority of by-products are currently not
exploited as sources of these compounds. This is in part due to the lack
of appropriate techniques for extraction of these compounds. In recent
times a number of novel extraction techniques have been used to opti-
mise extraction of bioactive compounds from by-products. In fact con-
siderable advances in this area have been made recently and
while some reviews exist (
Herrero, Cifuentes, & Ibañez, 2006; Schieber,
) no update has been made of the research carried
out in the last 5 years despite the considerable advances in extraction
techniques. Pressurised liquid extraction (PLE) and supercritical CO
2
ex-
traction (SC-CO
2
) are extraction techniques that are gaining popularity
due to their ability to increase target molecule speci
ficity and reduce
waste solvent production. Therefore the primary focus of the present re-
view will be critically evaluating the use of PLE and SC-CO
2
on the recov-
ery of carotenoids and polyphenols in food by-products of plant origin.
Both PLE and SC-CO
2
usually employ heat in combination with other
parameters to enhance extractions of target molecules. A number of
other non-thermal approaches are now available such as pulsed electric
field (PEF) extraction and ultrasound, which may be particularly effec-
tive for thermally-labile compounds. Therefore a secondary focus will
be on the use of non-thermal techniques to enhance extraction of
these compounds. The use of microwave assisted extraction (MAE) to
enhance the extraction of bio-active compounds of plant origin is also
reviewed and critically evaluated. For all extraction techniques a princi-
ple objective of the present review is to put forward a consensus on op-
timal conditions required to gain maximum recovery of target
compounds such that reader can bene
fit from research already carried
out.
2. Pressurised solvents
Traditionally, the nature and amounts of target compounds in nat-
ural products are determined after exhaustive extraction of the sam-
ple using solid
–liquid extraction techniques. Pressurised solvents use
elevated pressures and sometimes temperatures which drastically
improve the speed of the extraction process. In ideal solid
–liquid ex-
tractions, the desired compound should have high solubility in the
solvent employed while other compounds from the solid matrix
should not be solubilised during extraction (
).
In reality however, this is rarely achieved, and therefore much re-
search has been carried out on optimising conditions, such as
solvent-to-feed ratio, particle size, modi
fier concentration extraction
temperature, pressure and time and
flow rate, to enhance the recov-
ery of bioactive compounds from food by-products of plant origin
(
Kaur, Wani, Oberoi, & Sogi, 2008; Ku & Mun, 2008; Spigno, Tramelli,
& De Faveri, 2007; Wijngaard & Brunton, 2010
). However,
Food Research International 46 (2012) 505
Abbreviations: GAE, Gallic Acid Equivalent; MAE, Microwave Assisted Extraction;
PEF, Pulsed Electric Field; PLE, Pressurised Liquid Extraction; RSM, Response Surface
Methodology; SC-CO
2
, Supercritical CO
2
Extraction; SLE, Solid Liquid Extraction; SWE,
Subcritical Water Extraction; UAE, Ultrasound Assisted Extraction.
⁎ Corresponding author. Tel.: +353 1 805 9500; fax: +353 1 805 9550.
E-mail address:
(M.B. Hossain).
0963-9969/$
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conventional solid
–liquid extraction techniques such as Soxhlet ex-
traction and maceration are time consuming and use high amounts
of solvents (
) emphasising the need for more
sustainable techniques, including techniques based on pressurised
fluids.
The use of pressurised solvent techniques also offers the advantage
of enhanced target molecule speci
ficity and speed due to physicochem-
ical properties of the solvent, including density, diffusivity, viscosity and
dielectric constant, which can be controlled by varying pressure and
temperature of the extraction system.
Two pressurised
fluid extraction methods are very popular:
a. Supercritical
fluid extraction (SFE); called supercritical CO
2
extraction
(SC-CO
2
) when CO
2
is used.
b. Pressurised liquid extraction (PLE); where 100% water is used, this
technique is frequently called subcritical water extraction (SWE)
(
).
3. Supercritical
fluid extraction
Supercritical extraction technologies have been used extensively
in the past to extract target compounds from a variety of matrices
at laboratory and commercial scale. In some cases laboratory scale
studies have been carried out with view to recovering 100% of the tar-
get analyte for quanti
fication purposes. Whilst in other cases the ob-
jective is to provide optimal conditions for recovery of bio-active
compounds with a view to up-scaling to commercial extraction.
There are a number of examples where SFE has been up-scaled per-
haps the most well-known examples are the decaffeination of coffee
beans (
) and the extraction of
α-acids from hops to pro-
duce hop resins (
Bath, Ennis, Laws, & Wheldon, 1980
). For reasons
outlined below carbon dioxide is the main solvent used in this tech-
nique especially when the target molecule is apolar. However, super-
critical water systems have also been used to extract polar
compounds (
). The critical point of water is
very high (374 °C, 22.064 MPa) therefore superheated water cannot
be used to extract thermally-labile compounds (
).
CO
2
on the other hand, has a relatively low critical temperature of
31.1 °C and a low critical pressure of 7.4 MPa (
). In addition
to its physical characteristics, CO
2
is safe, foodgrade and widely avail-
able at a relatively low cost and high purity (
). Solvents other than CO
2
are not neces-
sarily required and the expense of removing organic solvents can be
eliminated (
). SC-CO
2
makes use of the
supercritical region of CO
2
, which is above its critical temperature
and above its critical pressure. In the critical region, there is only one
phase, which possesses properties of both a gas and a liquid (
). Solvation power is high due to a high, liquid-like, density. In ad-
dition, mass transfer rates are increased because of high, more gas-like,
diffusion coef
ficients and low viscosity values (
). Surface tensions of supercritical
fluids are
relatively low, which means more delicate compounds remain intact
(
). A disadvantage of using CO
2
as a solvent is
that it is relatively apolar thus for polar analytes a co-solvent such as
ethanol is often added to CO
2
). The main pa-
rameters that can be changed in SC-CO
2
are solvent-to-feed ratio, particle
size, modi
fier concentration extraction temperature, pressure and time
and
flow rate (
). In by-products from food
processing, SC-CO
2
has been mainly used for the recovery of apolar bio-
active compounds and a selection of these are outlined below.
3.1. Carotenoids
Rising consumer awareness of the health bene
fits of carotenoid con-
sumption has served to fuel global markets for these compounds with
the result that in 2010 the market was an estimated $1.07 billion and is
projected to top $1.2 billion by 2015 (
Global Industry Analysts, 2010
).
Tomatoes are a rich source of carotenoids particularly lycopene
(
a) and are an important crop worldwide. Tomatoes are often pro-
cessed into other forms before use which generates signi
ficant amounts
of waste. It is not surprising therefore given the known advantages of SC-
CO
2
for hydrophobic compounds such as carotenoids that the majority of
studies have concentrated on tomato waste as a source of carotenoids
(
). A number of other rest raw materials also represent promising
targets for the recovery of carotenoids particularly
β-carotene.
Apricot pomace is for example a promising source of
β-carotene (
Bayraktar, Mehmetoglu, & Çalimli, 2005
). Similarly carrot pulp, the resi-
due from carrot juice production, contains signi
ficant amounts of this
compound (
Vega, Balaban, Sims, O'Keefe, & Cornell, 1996
). In gener-
al only two compounds have been targeted lycopene (
a) and
β-carotene (
b).
details extraction conditions used to
extract carotenoids from various sources. A number of parameters can
be manipulated in a typical SC-CO
2
system to enhance extraction in-
cluding extraction temperature, pressure and time, CO
2
flow rate and
modi
fier percentage. Most studies compare the effectiveness of SC-
CO
2
with conventional solid liquid extraction. In most cases ef
ficiencies
of SC-CO
2
extractions are equal to or exceed those for conventional
solid
–liquid extraction. In some cases the authors have examined SC-
CO
2
as a tool for analytical extraction of carotenoids, however in most
cases the objective is recovery of potentially valuable components
from rest raw material. As outlined earlier in general by-products of to-
mato processing (skins, seeds and tomato paste waste) are targeted as
sources of carotenoids but other sources such as apricot by-products
and carrot press cakes have also been examined (
). Extraction
temperature is a critical factor affecting extraction ef
ficiency of SC-CO
2
systems. Whilst high temperatures may favour the extraction of some
carotenoids, this can be offset by thermal degradation of the com-
pounds during extraction. Fresh tomatoes contain a mixture of cis and
trans-lycopene isomers however the all-trans form predominates as
this is the most thermodynamically stable isomer (
), and so this isomer is more suitable to manipulate and incorpo-
rate in functional foods and nutraceuticals than the cis form. Most au-
thors recommend extraction temperature of up to 80 °C for this form.
On the other hand, some studies attribute to the cis isomer a higher bio-
availability (
). However, a lower temperature of
60 °C gives the highest yields for trans-lycopene (
2008; Nobre, Palavra, Pessoa, & Mendes, 2009
) which the authors pos-
tulate is due to promotion of isomerisation into the cis form at temper-
atures above 80 °C (
). Temperatures below 70 °C
seem to favour the extraction of
β-carotene with most authors report-
ing that temperatures between 55 and 59 °C are optimal (
Güvenç, Salgin, Mehmetoglu, & Çalimli, 2004; Vega et al., 1996
). Extrac-
tion pressure can also be manipulated to increase extraction yields of
carotenoids. Most authors recommend extraction pressures between
30 and 40 MPa for best recoveries of both lycopene and
β-carotene. Ma-
nipulation of extraction pressure will bring about changes in the solvent
density this in turn affects the solvating power of SC-CO
2
. As the pres-
sure is raised at a constant temperature, the CO
2
density also increases,
thereby decreasing the intermolecular space between the CO
2
-
Carotene (b)
Lycopene (a)
Fig. 1. Structures of common carotenoids extracted from food by-products.
506
H. Wijngaard et al. / Food Research International 46 (2012) 505
–513
molecules, and hence increasing the interactions between the target
compound and CO
2
molecules (
Topal, Sasaki, Goto, & Hayakawa,
). Therefore, solubility increases with pressure, especially at pres-
sures from 10 to 20 MPa as there is a sharp increase in the density of
SC-CO
2
in this range (
). At pressures above
20 MPa only small increases in solvent density occur and since most au-
thors investigate pressures from 20 to 40 MPa pressure has less of an in-
fluence on extraction efficiency than temperature in this range.
After temperature and pressure, modi
fier percentage appears to be
the most critical factor affecting carotenoid yields in SC-CO
2
extractions.
Ethanol is the most widely used modi
fier solvent in SC-CO
2
and most au-
thors recommend relatively low amounts (10
–15%) of ethanol be added
to enhance SC-CO
2
extraction of carotenoids. In general carotenoids are
polyene hydrocarbon chains which sometimes undergo ring closure at
one or both ends meaning they are largely hydrophobic in nature. It is
therefore surprising that the addition of a polar aprotic solvent such as
ethanol (dielectric constant 33) enhances the extraction of carotenoids.
However it is clear that in all cases the use of modi
fier increases extrac-
tion yields over and above pure CO
2
. The reason for this in not entirely
clear, however some authors have postulated that the addition of a mod-
i
fier enhances extraction by distorting the analyte–matrix bonds, allow-
ing the supercritical
fluid better access to the analyte (
). Ethanol is of course an attractive solvent for SC-CO
2
extractions
because of its food friendly nature. The use of vegetable and other oils
as modi
fiers has many advantages over ethanol since they are food
friendly and deliver all the bene
fits of the use of modifiers in general
but are also cheap and more hydrophobic thus enhancing extraction
yields. This has been clearly illustrated in the study of
where the use of olive oil as modi
fier gave better recoveries of ly-
copene than ethanol or water from tomato skins at 75 °C.
Table 1
Ef
ficiency of and conditions used to extract bioactive compounds from by-products of plant origin using supercritical fluid extraction.
Source
Target
compounds
Extraction conditions
Yield
Reference
Tomato paste waste
Carotenoids
30 MPa, 5% ethanol
Lycopene (54%)
Baysal, Ersus, and Starmans (2000)
β-Carotene (50%)
Tomato paste waste
Carotenoids
80 °C, 30 MPa, 5% ethanol, solvent
flow rate (0.792 kg/h)
Lycopene (88%)
β-Carotene (80%)
Tomato seeds
Carotenoids
34.5 MPa, 88 °C
Lycopene
Rozzi, Singh, Vierling, and Watkins
(2002)
Tomato skins and seeds
Carotenoids
30 MPa, 60 °C, solvent
flow-rate
of 0.59 g/min, particle size of 0.36 mm
and feed moisture content of 4.6
trans-Lycopene
Nobre, Palavra, Pessoa, and Mendes
(2009)
Tomato seeds and skins
Carotenoids
46 MPa, 80 °C
Lycopene (90%)
Tomato skins
Carotenoids
62 °C, 45 MPa (450 bar), and 14%
ethanol
trans-Lycopene
Kassama, Shi, and Mittal (2008)
(33%)
Tomato skins
Carotenoids
40 MPa, 100 °C, and 2.5 mL of CO
2
/min
Lycopene
Topal, Sasaki, Goto, and Hayakawa
(2006)
(94%)
Tomato skins
Carotenoids
35 MPa, 75 °C, ethanol (10%) and
olive oil (10%)
Lycopene
(73%)
Apricot by-products
Carotenoids
40 MPa, 50 °C, 2,2-dimethoxypropane,
moisture content 10%
β-Carotene (60%)
Sanal, Güvenç, Salgin, Mehmetoglu
and Çalimli (2004)
Apricot by-products
Carotenoids
31 MPa, 59 °C, 27% ethanol
β-Carotene (60%)
Carrot press cake
Carotenoids
34.5 MPa, 55 °C, 10% ethanol
β-Carotene (99.5%)
Vega, Balaban, Sims, O'Keefe, and
Cornell (1996)
Apple and peach pomaces
Polyphenols
30% ethanol and 40 min
Total phenols (25%)
Citrus peel
Polyphenols
9.5 MPa, 60 °C, 15% ethanol
Naringin
Giannuzzo, Boggetti, Nazareno, and
Mishima (2003)
(14.4 mg/100 g DW)
Grape by-products
(seed, stem, skin and pomace)
Polyphenols
40 MPa, 35 °C, using 5% v/v ethanol
Resveratrol
(19.2 mg/100 g DW)
Grape skin
Polyphenols
40 °C, 11 MPa, 7.5% ethanol,
extraction time 15 min
Resveratrol
(100%)
Grape skins
Polyphenols
45
–46 °C temperature, 1.56–1.6 MPa
pressure and 6
–7% ethanol
12.3%, total phenols (2.156 mg
GAE/100 mL), antioxidants
(1.628 mg/mL) and total anthocyanins
(1.176 mg/mL).
Orange pomace
Polyphenols
30 MPa, 40 °C, 2% co-solvent
Phenolic compounds
Benelli, Riehl, Smânia, Smânia and
Ferreira (2010)
(36 ± 2 mg GAE/g)
Tea seed cake
Polyphenols
20 MPa, 80 °C, and 60% aqueous
ethanol, extraction time 150 min
Two kaempferol glycosides
(11.4 ± 0.4 mg/g)
Guava (Psidium guajava L.) seeds Polyphenols
10 MPa at 60 °C, ethanol (10%)
Total phenolic content (153 mg
GAE/100 g DW)
Pistachio (Pistachia vera) hull
Polyphenols
45 °C, 355 bar, 15 min, 15% methanol
Total phenolic content (7810 mg
GAE/100 g DW)
Goli, Barzegar and Sahari (2005)
Olive leaves
Polyphenols
33 MPa, 100 °C (CO
2
density 0.70 g/mL),
flow rate 2 mL/min, 140 min, 10%
methanol
Total phenolic content (45%)
Le Floch, Tena, Rios, and Valcárcel
(1998)
Liquid grape seed extract
Anthocyanins
30
–40 °C; 10–13 MPa, pH
Total anthocyanins
2
–4; 25–30% ethyl alcohol;
(85%)
flow rate 25–50 mL/min
Distilled white grape pomace
Monomeric
polyphenols
50 °C, 90 min, 8% ethanol,
solvent-to-solid ratio 1:1
Gallic acid, catechin and epicatechin
White grape seeds
Monomeric
polyphenols
55 °C, 20 min, 20% methanol
Gallic acid, catechin and epicatechin
Grape seeds
Polyphenols
30 MPa, 15% methanol
Low molecular weight polyphenols
(
N90%)
Murga, Ruiz, Beltran, and Cabezas
(2000)
507
H. Wijngaard et al. / Food Research International 46 (2012) 505
–513
There are con
flicting reports on the effect of source material mois-
ture content on the recovery of carotenoids from by-products. For ex-
ample
reported that the extraction yield of trans-
lycopene rose when the moisture content of the sample increased
from 4.6% to 22.8%, possibly due to the modi
fications in the skin struc-
ture mentioned before, implying less available compounds in tomato
waste with lower moisture content. However, for higher moisture
contents the yield decreased, in this case, probably due to the water
preventing contact between CO
2
and tomato particles. In contrast,
reported that extraction yields of
β-carotene
from apricot pomace increased markedly when the moisture content
was decreased from 30 to 10%. The reason for the con
flicting reports is
unclear. However it should be noted that all carotenoids are susceptible
to degradation in the presence of light and heat. Therefore a drying
method such as freeze drying is preferable as this will preserve caroten-
oids in the starting by-product material (
).
It is clear that a variety of extraction conditions can in
fluence extrac-
tion ef
ficiencies of carotenoids from plant by-products. This can make it
dif
ficult to pinpoint optimal conditions using conventional statistical ap-
proaches. However, many authors have utilised response surface meth-
odology (RSM) to minimise the number of experiments required to
identify optimal extraction. For example
used
RSM to increase extraction yields of
β-carotene from apricot pomace
by 10% over a previous experiment which utilised a conventional statis-
tical approach. In summary SC-CO
2
represents a promising technique for
recovery of carotenoids from plant by-products which uses smaller
amounts of solvent than conventional approaches and therefore could
be used to valorise carotenoid rich plant based by-products. A number
of promising sources of carotenoids in plant by-products have been iden-
ti
fied and optimal extraction conditions are broadly in agreement.
3.2. Polyphenols
The world market for polyphenols is signi
ficant. For example,
Leatherhead Food Research (2009)
estimates the current market is
worth approximately $200 million. The majority of polyphenols
extracted for sale as nutraceuticals or for use in functional foods come
from either grapes, apples, olives or green tea and this is re
flected in
the studies that have been performed on optimising SC-CO
2
for their ex-
traction.
lists optimal conditions for the extraction of polyphe-
nols from various by-products using SC-CO
2
. A variety of potentially
valuable polyphenolic compounds have been targeted in SC-CO
2
extrac-
tion studies including resveratrol (
d) and
kaempferol glycosides, however in many cases the ef
ficiency of
extraction is measured not by the levels of speci
fic compounds but as
total phenolic content. Unlike carotenoids polyphenols are moderately
polar compounds and when extracted by SC-CO
2
a modi
fier usually eth-
anol is added to create a more polar environment. Addition of ethanol
increases the critical temperature of SC-CO
2
and if temperatures and
pressure are not increased to compensate, the mixture remains at or
near its critical point instead of the supercritical point. For extraction
of polyphenols the addition of a moderately polar modi
fier is critical
with most authors reporting zero yields when no ethanol is added
(
Casas et al., 2010; Palma & Taylor, 1999
). Similar to carotenoids ethanol
is usually added at relatively low levels (10
–20%) although extraction
using up to 60% ethanol has been reported (
). In addition
to the bene
fits listed above for addition of a modifier for carotenoid ex-
traction, the addition of a polar modi
fier for polyphenol extraction will
also increase the solubility of the target compounds in the supercritical
solvent. As a result very high yields for modi
fier assisted SC-CO
2
extrac-
tion of polyphenols from by-products have been reported. For example
reported that resveratrol, a well known bioactive
phenolic compound in grape by-products (seed, stem, skin and pom-
ace) was ef
ficiently extracted using SC-CO
2
at high pressure (400 bar)
and low temperature (35 °C) using 5% v/v ethanol as a co-solvent. In
fact using these conditions, the extraction yields were 14.67 and 21.33
times higher than those of a conventional extraction method. Contrary
to most studies,
Goli, Barzegar and Sahari (2005)
reported a signi
ficant
lower extraction yield of phenolic compounds in SC-CO
2
extraction
(45 °C, 355 bar, 15 min) from pistachio (Pistachia vera) hull even after
using 15% methanol as modi
fier in comparison to solid/liquid water ex-
traction. The presence of highly polar compounds in pistachio might
have been the determinant factor in this study.
A wide variety of extraction temperatures have been reported as op-
timal for polyphenol extraction (
, 35
–100 °C) from by-products
of plant origin. However in general most authors report that tempera-
tures between 40 and 60 °C are best. This is slightly lower than the op-
timal conditions for carotenoid extraction and would appear to imply
that polyphenols are less heat stable than carotenoids. Similar to carot-
enoids extraction pressures between 20 and 30 MPa are optimal for ex-
traction of polyphenols. As outlined above the enhanced solubility of
polyphenols at higher pressures is related to an increase in solvent den-
sity at higher pressures.
The type of pomace from which polyphenols are recovered can in-
fluence extraction yields. For example for apple and peach pomaces
Adil, Çetin, Yener and Bayindirli (2007)
reported that yields were
quite low with about 25% of phenols in comparison to 100% using an
ethanolic solid liquid extraction (SLE) (
). In contrast
COOH
HO
OCH
3
Ferulic acid
(c)
Caffeic acid
(b)
COOH
HO
HO
Gallic acid
(a)
COOH
HO
OH
OH
HOOC
OH
HO
OH
O
O
OH
OH
Chlorogenic acid
(e)
OH
HO
OH
Resveratrol
(d)
Fig. 2. Structures of the major phenolic acids and their derivatives extracted from food by-products.
508
H. Wijngaard et al. / Food Research International 46 (2012) 505
–513
Giannuzzo, Boggetti, Nazareno and Mishima (2003)
reported that when
naringin was recovered from fresh citrus peel a higher yield was
obtained when SC-CO
2
was used than with SLE and a similar level to
that obtained by Soxhlet extraction. Polyphenols are structurally di-
verse class of plant metabolites which usually go through various mo-
lecular modi
fications predominantly by hydroxylation, glycosylation
and polymerisation. For example
reported that mo-
nomeric polyphenol levels, such as catechins and phenolic acids were
increased, while polymeric polyphenols were not that well extracted
in SC-CO
2
.
Murga, Ruiz, Beltran and Cabezas (2000)
also reported that
low molecular weight polyphenols were best extracted by SC-CO
2
at
15% of added methanol. Even at these conditions high molecular weight
tannins were not extracted. Using methanol makes the application unu-
sable for food ingredient production. As noted earlier CO
2
is a non-polar
solvent and some authors have utilised SC-CO
2
as a pre-treatment to re-
move non-polar components prior to extraction of polyphenols from
plant by-products. For example
Vatai, Skerget and Knez (2009)
devel-
oped a 2 step process for extraction of phenolic compounds from
grape pomace involving pre-treatment with SC-CO
2
to remove non-
polar compounds followed by solvent extraction of the residue. This
technique gave better recoveries of phenolic compounds than single
step solvent extraction. In summary, it is possible to extract certain
polyphenols from by-products, but the use of modi
fier seems unavoid-
able. Therefore for the recovery of polyphenols other techniques, such
as PLE, may be more interesting.
4. Pressurised liquid extraction (PLE)
During PLE, pressure is applied, allowing the use of temperatures
above the boiling point of solvents (
Mendiola, Herrero, Cifuentes, &
). Extracting at elevated temperatures can be advanta-
geous due to changes in mass transfer and surface equilibria. Higher
extraction temperatures will increase the mass transfer rate and ex-
traction rates, because higher temperatures generally imply: i) an in-
crease in capacity of solvents to solubilise solutes, ii) an increase in
diffusion rates, iii) better disruption of solute
–matrix bonds, iv) a de-
crease in viscosity of the solvent and v) decrease in surface tension
(
Ramos, Kristenson, & Brinkman, 2002; Richter et al., 1996
). The ap-
plied high pressure, usually ranging from 4 to 20 MPa, ensures the
solvent maintains in the liquid state at the applied temperature
(
). This is the main reason for the use of pressure,
although pressure has also been reported to help driving the solvent
into the pores of the matrix and enhance analyte solubility (
& Turner, 2011; Ramos et al., 2002
). The extra pressure effects seem
negligible though (
) and therefore researchers
usually apply one constant pressure in PLE experiments.
PLE is also known as pressurised solvent extraction, subcritical sol-
vent extraction or accelerated solvent extraction. When 100% water is
used as a solvent, PLE is generally called superheated water extraction,
subcritical water extraction, pressurised low polarity water extraction
or pressurised hot water extraction (
).
Anthocyanin (a)
Procyanidin (b)
O
O
HO
OH
OH
OH
OH
OH
OH
OH
OH
HO
HO
OH
O
OH
O-glu
R
1
R
2
OCH
3
O
O
OH
O
O
HO
HO
OH
OH
O
HO
H
3
C
O
HO
OH
Hesperidin (c)
OH
O
O
OH
O
O
HO
HO
O
O
H
3
C
HO
OH
OH
OH
Naringin (d)
Fig. 3. Structures of
flavonoids and their derivatives commonly found in food by-products.
509
H. Wijngaard et al. / Food Research International 46 (2012) 505
–513
PLE has mainly been used to optimise analytical extractions
(
Alonso-Salces et al., 2001; Luthria, 2008
), although a few commercial
applications exist in the
field of extraction of flavourings from natural
products as an alternative to steam distillation (
). PLE generally requires less time and a lower consumption of
organic solvents than conventional techniques (
). This is the reason why PLE may have commercial interest as al-
ternative extraction method to obtain bioactive compounds from by-
products.
for example patented the potential
of the use of PLE in order to extract polyphenols from fruit and vege-
table by-products.
In order to recover bioactive compounds from food by-products,
food grade solvents such as aqueous ethanol or water can be used. At-
tempts to recover bioactive compounds from by-products have been
published, focusing mainly on polyphenolic compounds as is discussed
below.
4.1. Polyphenols
As mentioned before, the world market for polyphenols is signi
fi-
cant, which generates interest in non-conventional extraction tech-
niques. PLE is one of the techniques that can be used in order to
extract polyphenols from by-products and has thus been studied by
various researchers. An overview is shown in
. The main pa-
rameters that are important in PLE, are temperature, pressure,
flow
rate and extraction time (
). It should be
noted that optimal conditions depend on applied reference methods
and analytical procedures. Therefore yields higher than 100% have
been reported, since apparently, the conventional standard extraction
procedures were not extracting all of the polyphenols present. Keep-
ing this in mind, general trends of the in
fluence of various processing
parameters on extraction ef
ficiency can be noted.
Firstly the extracting ef
ficiency depends on the type of compound
to be extracted. As shown in
, optimal conditions for anthocy-
anins (
b) and
flavonols differ. This is to be
expected as with any compound, the solubility of polyphenols de-
pends on various parameters, such as polarity, size and the character-
istics of the extraction medium. The characteristics of the solvent can
be changed by adjusting the concentration of solvents used, for exam-
ple the temperature. The solvent used is also of great importance to
extraction yield. As shown in
aqueous ethanol and water are
the major liquids used in PLE of polyphenols from by-products. By
using water only, rules and regulations for using organic solvents as
means of extraction can be avoided. In addition, using 100% water is
eliminating the cost of ethanol itself and the process cost of evaporat-
ing off organic solvents.
The temperature range that can be used for SWE ranges from the
boiling point (100 °C) of water to its critical point (374 °C). The critical
pressure of water is 218 bar. The pressure required to maintain water in
the condensed form depends on the temperature applied: for 200 °C a
minimum pressure of 1.5 MPa is required, while extraction at 300 °C re-
quires a minimum pressure of 8.5 MPa (
). The thermo-
dynamical properties of water change dramatically when the tempera-
ture is increased, especially the dielectric constant. Dielectric constants
that are similar to organic solvents such as methanol can be achieved at
200 °C (
).
Hydroethanolic mixtures are the other liquid often used in PLE of
bioactives (
). By using solvent mixtures, advantages of both
can be utilised. For example one solvent can improve the solubility of
the solute (ethanol), while the other solvent can assist in desorption
of the solute from the matrix (water) (
). The
use of ethanol reduces the boiling point and affects the polarity of the
solvent. The ethanol concentration has therefore a large effect on the
extraction yield of polyphenols (
).
The temperature also affects extraction yield. As explained in the in-
troduction paragraph on PLE, mass transfer rates and extraction rates can
be enhanced by using an elevated temperature.
shows that opti-
mal PLE temperatures ranged between 100 and 180 °C, with one excep-
tion at 40 °C. A required high temperature for ef
ficient extraction was
con
firmed by
Monrad, Howard, King, Srinivas and Mauromoustakos
, who reported that a temperature higher than 80 °C at a pres-
sure of 6.8 MPa was required to enable an ef
ficient procyanidin extrac-
tion from red grape pomace when using PLE.
Rivas-Gonzalo, Ibáñez and García-Moreno (2006)
found that the sum
of individual polyphenols of winery by-products also increased at elevat-
ed temperatures. In addition, the solubility of phenolic acids such as gal-
lic, chlorogenic, caffeic, ferulic and coumaric acids (
) increased with
enhanced temperatures, but at temperatures higher than 180 °C, the
phenolic acids were degraded (
). This showed
that increased temperatures can also have disadvantages and this should
be taken into consideration when optimising the process. It is especially
of importance for thermo-labile bioactive compounds, such as anthocya-
nins, which can be degraded at relatively low temperatures.
reported degradation of anthocyanins in hydroethanolic
solutions at temperatures higher than 120 °C, while
mentioned degradation of anthocyanins at temperatures higher
than 100 °C. They did not use the same solvents (
), which may ex-
plain the difference in results. In any case, at atmospheric pressure con-
ditions, anthocyanins were already degraded at temperatures higher
than 50 °C (
), which shows one of the advantages of
using PLE.
Another consequence of using high temperatures (around 140
–
150 °C) is that certain compounds, such as Maillard reaction products
and 5-(hydroxymethyl)furfural (HMF) can be produced de novo when
using PLE (
Monrad et al., 2010b; Wijngaard & Brunton, 2009
). These
compounds may not be desired. This should be taken into account
when designing an optimal process.
Flow rate is another parameter that can be adjusted. Although
most researchers do not optimise
flow rate, the important role it
may play when extracting polyphenols was emphasised by
Table 2
Ef
ficiency of and conditions used to extract polyphenols from by-products of plant origin using pressurised liquid extraction.
Source
Target compounds
Extraction conditions
Yield
Reference
Red grape pomace
Procyanidins
6.8 MPa, 140 °C, 50% ethanol
Total procyanidins (115%)
Red grape pomace
Anthocyanins
6.8 MPa, 100 °C, 50% ethanol
Total anthocyanins (112%)
Red grape pomace
Anthocyanins
10 MPa, 100 °C, 5 min, 0.1% HCl in water
Total anthocyanins (100%)
Apple pomace
Polyphenols
10.3 MPa, 102 °C, 5 min, 60% ethanol
Total
flavonols (130%)
Dried grape skin
Polyphenols
110 °C, 40 s, 100% water
Total anthocyanins (100%)
110 °C, 40 s, sulfured water
Winery by-products
Polyphenols
6
–7 MPa, 150 °C, 100% water
Procyanidins + catechins (38%)
Onion waste
Flavonols
5 MPa, 120 °C, 15 min, 100% water + enzyme hydrolysis
Quercetin (106%)
Potato peel
Phenolic acids
6 MPa, 180 °C, 60 min, 100% water
Phenolic acids (177%)
Pomegranate peel
Polyphenols
10.2 MPa, 40 °C, 5 min, 100% water
Total phenolic content (100%)
Onion skin
Flavonols
9
–13.1 MPa, 160 °C, 15 min, 100% water
Quercetin (92%)
a
Sulfured water contained 1400
μg/mL Na
2
S
2
O
3
.
510
H. Wijngaard et al. / Food Research International 46 (2012) 505
–513
King, Howard and Monrad (2011)
. They discovered that optimisation
of the
flow rate is of major importance in order to effectively dissolve
a solute such as quercetin in SWE.
In addition sulphured water (water with an added 1400
μg/mL
Na
2
S
2
O
3
), can enhance extraction of certain polyphenols, such as an-
thocyanins (
). The mechanism is poorly understood, but it is
thought that the solubility is increased due to improved diffusion
through cell walls (
), and/or the dielectric constant
of the solvent reduced. The reaction products of anthocyanins with
sulphites are more soluble in aqueous solvents than their parent an-
thocyanins (
) resulting in higher extraction yields.
Sequential procedures can also be used to enhance extraction yields.
For example SWE at 100 °C, followed by an extraction at 150 °C was
found to be optimal to extract
flavonols and gallic acid (
a) (
). In summary, mainly temperature and type of solute
and extraction medium were reported to be of high importance when
extracting polyphenols from by-products.
5. Novel non-thermal and microwave assisted extraction
In addition to pressurised liquid extraction techniques discussed
above, other techniques exist that may assist in the extraction of bio-
active compounds, such as pulsed electric
fields (PEF), ultrasound
waves and microwaves. These techniques could for example be ap-
plied as a preliminary step in the extraction of bioactive compounds.
In PEF, material located between two electrodes is exposed to a
strong electrical
field. If the stress caused by the electrical field on the
membrane is large enough pore formation occurs. The pore formation
can be reversible or irreversible, and depends on the conditions of PEF
treatment, such as electric
field strength, pulse duration and the num-
ber of pulses. Pore formation enhances cell permeability (
). PEF has been extensively investigated as a non-
thermal preservation technique. On the other hand, the use of PEF in
the recovery of bioactive compounds from by-products is not well stud-
ied up to now. The technique has been mainly used to extract polyphe-
nols from grape by-products. In red grape by-product the level of
anthocyanins was 60% increased when PEF was applied as a pre-
treatment of 1 min at 25 °C in combination with a conventional thermal
extraction at 70 °C for 1 h (
). When white grape
skins were treated with PEF at a temperature of 20 °C, 10% more poly-
phenols than non-treated samples were extracted (
). PEF seems a potential technique to use as pre-treatment in the
extraction of polyphenols from by-products, although industrial scale
equipment is still under development and the technique does not
apply to solid products (
Ultrasound assisted extraction (UAE) can also be used in the food in-
dustry to perform extractions. The technique uses high frequency sound
waves (higher than 20 kHz). When the ultrasound waves are strong
enough, bubbles are formed in the liquid. Eventually the formed bub-
bles cannot absorb the energy any longer and will collapse:
“cavitation”
takes place. This collapse causes a change in temperature and pressure
within the bubble and hence energy for chemical reactions is generated.
Extremely high temperatures of 5000 °C and pressures of 1000 bar have
been measured. The process is affected by mechanical forces surround-
ing the bubble when a solid matrix is present (
). Due to the cavitation process plant cell walls can be pen-
etrated, which provides an easier cell access. In addition, ultrasound can
result in swelling of the plant material, which in turn can enhance ex-
traction (
Various studies have tested ultrasound and its effect on the extrac-
tion of polyphenols. Ultrasound has been shown to enhance the ex-
traction of bioactive compounds such as polyphenols from different
plant by-products in several studies (
Khan, Abert-Vian, Fabiano-Tixier,
). Orange peels
were treated with ultrasound to extract the
flavanones hesperidin
c) and naringin (
d). Temperature, ethanol:water ratio and
sonication power were optimised using RSM. A temperature of 40 °C,
a 4:1 (v/v) ethanol:water ratio and a sonication power of 150 W were
determined as optimal. These conditions resulted in an enhanced ex-
traction of 38% for naringin and 41% for hesperidin when compared to
non-sonicated samples (
When ultrasound was applied to apple pomace, a by-product of the
apple cider industry, catechins were 20% better extracted than when a
conventional extraction was used. In addition, a scale up test was per-
formed with an ultrasonic bath of a volume of 30 L. (
Bourvellec, Renard, & Chemat, 2010
). Ultrasound has also been success-
fully applied to enhance the extraction of carotenoids from different by-
products of plant origin.
Sun, Liu, Chen, Ye and Yu (2011)
investigated
the effect of variety of factors (particle size, solvent type, solid:solvent
ratio, temperature, extraction time, acoustic intensity, height of liquid
and duty cycle of ultrasound exposure) on the ultrasound assisted ex-
traction yield of all-trans-
β-carotene from citrus peel. All tested UAE
conditions yielded higher all-trans-
β-carotene than that of conven-
tional solid/liquid extraction, with a maximum of ca. 11
μg all-
trans-
β-carotene/g DW at an extraction time of 120 min. The height
of the liquid present in the sample beaker showed an inverse rela-
tionship with extraction yield in UAE. The authors reported that
this may be explained by the fact that the cavitation intensity de-
creases with increasing height due to the attenuation of the waves
caused by absorption and scattering.
Microwaves are electromagnetic waves, which are usually operat-
ed at a frequency of 2.45 GHz. Microwaves can access biological ma-
trices and interact with polar molecules, such as water, and
generate heat. The temperature will rise, which generally leads to en-
hanced extraction ef
ficiency (
). An overview of
results of MAE of polyphenols from by-products is shown in
By using MAE the extraction time can be reduced in comparison to
conventional extraction methods (
Ballard, Mallikarjunan, Zhou, &
). MAE extraction ef
ficiency
principally depends on microwave energy, treatment time and tem-
perature used. It should be noted that in general temperature in-
crease enhances extraction rates of solutes due to the same reasons
Table 3
Ef
ficiency of and conditions used to extract polyphenols from by-products of plant origin using microwave assisted extraction.
Source
Target compounds
Extraction conditions
Yield
Reference
Longan peel
Polyphenols
95% ethanol, 80 °C, 500 W, 30 min, 2.45 GHz
Total phenolic content (115%)
Grape seed
Polyphenols
66 °C, 200 s, 30 W
Total phenolic content (13.5%)
Satsuma peels
Flavonoids
70% ethanol, 140 °C, 7 min, 1000 W, 2.45 GHz
Narirutin (93%)
Hesperidin (91%)
Peanut skins
Polyphenols
30% ethanol, 30 s, 855 W
Total phenolic content (122%)
Grape skins
Anthocyanins
40% methanol, 100 °C, 5 min, 500 W
Total anthocyanins (118%)
Wine lees
Polyphenols
75% ethanol, 200 W, 17 min
Total phenolic content
Pérez-Serradilla and
Luque de Castro (2011)
Mandarin pomace
Phenolic acids
Dry, 125 W, 5 min, 2.45 GHz
Total phenolic acids (99%)
Hayat, Zhang, Chen, et al. (2010),
Hayat, Zhang, Farooq, et al. (2010)
511
H. Wijngaard et al. / Food Research International 46 (2012) 505
–513
as mentioned with PLE. But as with PLE very high extraction temper-
atures can generate unwanted compounds. For example,
reported that the proportion of poly-
phenol in the extract of tea residue increased from 25.3% to 74.4%
when the temperature was increased from 110 °C to 230 °C using
MAE. However, 5-(hydroxymethyl)furfural, a potentially harmful
compound was also formed at 230 °C. Therefore, caution should be
taken while increasing the temperature in MAE.
The effect of microwave energy and treatment time on the extrac-
tion of phenolic compounds from citrus mandarin peel and pomace
has been described in the studies of
Hayat, Zhang, Chen, et al. (2010)
and
Hayat, Zhang, Farooq, et al.(2010)
. The results showed that an in-
crease of microwave energy and time signi
ficantly increased the con-
tent of free phenolics in the extracts while decreasing the bound
phenolic content. This indicated that microwave treatments had
cleaved and liberated some of the bound phenolics from the tissue ma-
trix, thus allowing them to be available in free form in the extracts.
However, higher microwave energy and longer treatment time resulted
in degradation of some
flavonol compounds. The greatest advantage of
MAE over conventional maceration and/or solid/liquid extraction is ex-
treme reduction of extraction time while obtaining similar or higher ex-
traction ef
ficiency. More research needs to be carried out on this topic
though, especially on the effect of microwave technology and the ex-
traction of individual polyphenols and other bioactive compounds.
6. Conclusions
Selection of the most sustainable extraction technique for plant
by-products depends on the source and the bioactive compound to
be extracted. SC-CO
2
is the best technique used for apolar compounds
such as carotenoids, while PLE is more suited for more polar compounds,
such as polyphenols. PLE is less studied and still requires further research
before application, especially on possibilities of up-scaling. To our knowl-
edge PLE is not currently used at all on an industrial scale. Analytical sys-
tems are readily available and some laboratories have made their own
pilot scale version (
). SC-CO
2
is an older technique
and is already used on industrial scale. In addition to batch systems, the
present industrial scale SC-CO
2
extractors can consist of continuous or
semi-continuous systems. In addition, it has to be noted that bioactive
compounds are usually a minor part of the by-product. In particular
plant by-products mainly consist of carbohydrates which may also
have a commercial application. Therefore at an industrial scale sustain-
able solutions should make use of all usable streams present. For exam-
ple,
extracted polyphenols from apple pomace with
resins after which the remaining pomace was used for pectin extraction.
Some researchers have proposed the use of multiple pressurised
fluid steps to separate various compounds in a sustainable way. These
steps would involve standard SC-CO
2
, subcritical water extraction, su-
percritical
fluid chromatography and supercritical membrane separa-
tion (
). In addition, the use of technologies such
as PEF, ultrasound, microwave assisted extraction, in combination
with pressurised
fluid extraction techniques mentioned above could
further enhance extraction ef
ficiencies. These could be applied either
as a pre-treatment step or simultaneous with the extraction methodol-
ogy applied. For example from a practical point of view, it is possible to
use a combination of PEF and PLE. There are many ways of combining
steps and it would be very interesting for science and industry to opti-
mise the extraction of bioactive compounds on a larger scale. The
scale up and combination of potential techniques are therefore priori-
ties for further research as they could provide sustainable solutions for
an increasing waste problem.
Acknowledgements
The authors would like to acknowledge the Food Institutional Re-
search Measure (FIRM) 2006programme from the Irish Department
of Agriculture, Fisheries and Food for the funding of this literature
research.
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