Optimization of microwave-assisted extraction of phenolics from potato and its downstream waste using orthogonal array design

Abstract

While other extraction methods have been tempted, a microwave-assisted extraction (MAE) method coupled with the orthogonal array design was investigated for efficient extraction of the phenolic compounds in potato downstream wastes. Four parameters were examined for the MAE of the total phenolic content (TPC) and optimized at 60% ethanol, 80°C, 2min, solid-to-solvent ratio 1:40 (g/ml). The MAE was proven more efficient than the conventional solvent extraction by refluxing. The optimized model showed that the downstream wastes, both the supernatant and the residue contained high TPC, particularly the former (11.0±0.26mgGAE/gDW). The antioxidant activities (DPPH and FRAP) closely correlated with the TPC of the samples (r=0.92–0.97). Chlorogenic acid and caffeic acid were found to be the predominant phenolic acids. The extracts of the downstream wastes from potato processing can be a promising candidate for functional foods and nutraceutical ingredients.

Full text

Optimization of microwave-assisted extraction of phenolics from potato
and its downstream waste using orthogonal array design
Tao Wu
a,b,c
, Jun Yan
c,d
, Ronghua Liu
c
, Massimo F. Marcone
a
, Haji Akber Aisa
b
, Rong Tsao
c,
⇑
a
Department of Food Science, University of Guelph, 50 Stone Road East, Guelph, Ontario, Canada N1G 2W1
b
Xinjiang Key Laboratory of Plant Resources and Natural Products Chemistry, Xinjiang Technical Institute of Physics and Chemistry,
Chinese Academy of Sciences, 830011 Urumqi, China
c
Guelph Food Research Centre, Agriculture and Agri-Food Canada, 93 Stone Road West, Guelph, Ontario, Canada N1G 5C9
d
School of Pharmaceutical Science, Jilin University, Changchun 130022, China
article info
Article history:
Available online 5 August 2011
Keywords:
Microwave-assisted extraction
Phenolic
Antioxidants
Potato
Downstream waste
Chlorogenic acid
Caffeic acid
Orthogonal array design
abstract
While other extraction methods have been tempted, a microwave-assisted extraction (MAE) method cou-
pled with the orthogonal array design was investigated for efficient extraction of the phenolic com-
pounds in potato downstream wastes. Four parameters were examined for the MAE of the total
phenolic content (TPC) and optimized at 60% ethanol, 80 °C, 2 min, solid-to-solvent ratio 1:40 (g/ml).
The MAE was proven more efficient than the conventional solvent extraction by refluxing. The optimized
model showed that the downstream wastes, both the supernatant and the residue contained high TPC,
particularly the former (11.0 ± 0.26 mg GAE/g DW). The antioxidant activities (DPPH and FRAP) closely
correlated with the TPC of the samples (r= 0.92–0.97). Chlorogenic acid and caffeic acid were found to
be the predominant phenolic acids. The extracts of the downstream wastes from potato processing can
be a promising candidate for functional foods and nutraceutical ingredients.
Crown Copyright Ó2011 Published by Elsevier Ltd. All rights reserved.
1. Introduction
Potato (Solanum tuberosum) is the fourth largest crop grown
worldwide after rice, wheat and maize (Navarre, Pillai, Shakya, &
Holden, 2011). Most potatoes are peeled when processed as the
raw material for the starch and food industries. During the process-
ing, a large amount of downstream wastes containing potato skins is
generated. Disposal of such large volumes of wastes presents great
sanitary and environmental challenges; in addition to the associated
high cost, it leaves a heavy carbon footprint during transportation to
the landfill sites. On the other hand, downstream wastes can be an
excellent source of bioactive phytochemicals. More than 50% of
the total phenolic content was found in the peels and the adjoining
tissues of the potato tuber (Al-Weshahy & Venket Rao, 2009; Nara,
Miyoshi, Honma, & Koga, 2006). Several studies have demonstrated
that potato peel extract (PPE) can be a potential source of functional
food ingredient for improving human health. For example, PPE
exhibited very strong antioxidant activities nearly equivalent to
those of the synthetic antioxidants (BHA and BHT) (Rehman, Habib,
& Shah, 2004). For this reason, it has been explored as antioxidants to
prevent oxidation of vegetable oils (Mohdaly, Sarhan, Mahmoud,
Ramadan, & Smetanska, 2010; Rehman et al., 2004). PPE pretreat-
ment was found to significantly offset CCl
4
-induced liver injury in
rats (Singh, Kamath, Narasimhamurthy, & Rajini, 2008), to protect
erythrocytes against oxidative damage in vitro (Singh & Rajini,
2008), to show strong inhibitory activity toward lipid peroxidation
of rat liver homogenate (Singh & Rajini, 2004). Despite these activi-
ties of the PPE, waste from the actual potato processing line has not
been evaluated for its phytochemical composition, the efficacies of
recovering these compounds, the potential health benefits and
added values.
To date several conventional extraction techniques have been
reported for the extraction of phenolics from the potato peel,
including refluxing at elevated temperature (Al-Weshahy & Venket
Rao, 2009), homogenization (Navarre et al., 2011), and mechanical
shaking (Mohdaly et al., 2010; Navarre et al., 2011; Rehman et al.,
2004). However, these techniques require a long extraction time
and result in low yields of extraction. Pressurised liquid extraction
(PLE) of phenolics has been proposed as a green extraction tech-
nique for antioxidants from potato peel (Singh & Saldaña, in press;
Wijngaard, Ballay, & Brunton, in press). The technology uses high
pressure to maintain water at temperatures between 100 and
374 °C, which demands for complicated and high-cost equipment
in large-scale industrial extractions. Interest in microwave-assisted
extraction (MAE) has increased significantly over the past several
years for food bioactives (Inoue, Tsubaki, Ogawa, Onishi, & Azuma,
2010; Khajeh, 2009; Terigar et al., 2010). In MAE, the principle of
heating is based upon the direct effect of microwaves on molecules
by ionic conduction and dipole rotation (Hemwimon, Pavasant, &
0308-8146/$ - see front matter Crown Copyright Ó2011 Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.foodchem.2011.08.002
⇑
Corresponding author. Tel.: +1 519 780 8062; fax: +1 519 829 2600.
E-mail address: Rong.Cao@agr.gc.ca (R. Tsao).
Food Chemistry 133 (2012) 1292–1298
Contents lists available at SciVerse ScienceDirect
Food Chemistry
journal homepage: www.elsevier.com/locate/foodchem

Shotipruk, 2007). Microwaves are able to penetrate plant matrix
and interact with polar molecules such as water and phenolics,
resulting in a pressure increase inside plant cells. This pressure in-
crease leads to break of cell walls and release of phytochemicals,
whereas the conventional heating depends on its thermal conduc-
tivity. Consequently, extraction time using microwave can be sig-
nificantly reduced and energy saved as compared to conventional
heating methods.
There are many factors affecting the extraction efficiency of
MAE. Typical parameters to be considered in microwave-assistant
extraction include type of solvent, composition of solvent, temper-
ature, extraction time and solid-to-solvent ratio. Orthogonal array
design (OAD), a type of factorial design, has been used to optimize
experimental conditions with fewer numbers of experiments (Guo
et al., 2007; Wang, Liao, Lee, Huang, & Tsai, 2008). OAD is particu-
larly advantageous in rapid characterization of complicated pro-
cesses in fewer experiments.
In the present study, the applicability of MAE to the extraction of
phenolic compounds in downstream wastes from an industrial
potato processing line and how the different operational parame-
ters, i.e. ethanol concentration, temperature, extraction time and
solid-to-solvent ratio affect the total phenolic content (TPC) were
investigated using a multilevel OAD strategy. The phenolic compo-
sition and antioxidant activities of the extracts were also evaluated.
2. Materials and methods
2.1. Sample materials
All fresh (intact) tubers, industrially peeled and cut potato dices
and the downstream waste samples were of the same cultivar Cal-
white, and obtained from the processing lines of a food company in
Toronto, Canada. Samples (at least 0.5 kg each) were collected ran-
domly on 1, April, 2011. The downstream wastes were centrifuged
at 4000 rpm for 5 min (Eppendorf 5810R, Brinkman Instruments
Inc., Westbury, NY), and the residue and supernatant were col-
lected separately. The intact potato tubers were washed with tap
water, cut into pieces in similar sizes as the peeled potato dices
(ca. 1 11cm
3
). All samples (ca. 500 g) were freeze-dried (Bulk
tray dryer, Labconco, USA) and blended into fine powder (particle
size < 0.5 mm) (CBG 100SC, Black & Decker
Ò
Inc., Canada). The
yield of residue and supernatant from the downstream wastes
were 14.58 g and 6.09 g, respectively. These materials were stored
in polyethylene tubes at 20 °C until use. Dried powder of residue
of the downstream waste was used in the optimization model of
the MAE.
2.2. Chemicals and standards
Chlorogenic acid, caffeic acid, p-coumaric acid, ferulic acid,
vanillic acid, protocatechuic acid, gallic acid,
L
-ascorbic acid,
1,3,5-tri(2-pyridyl)-2,4,6-triazine (TPTZ), 2,2
0
-diphenyl-1-picrylhy-
drazil (DPPH) free radical, Folin–Ciocalteu (FC) reagent were pur-
chased from Sigma (St. Louis, MO, USA). Sodium acetate, ferric
chloride hexahydrate, sodium phosphate monobasic, sodium phos-
phate dibasic and HPLC-grade solvents, including methanol, formic
acid and ethanol were purchased from Caledon Laboratories
(Georgetown, Ont., Canada). Deionized water (18 M
X
cm) was ob-
tained from a Millipore (Bedfore, MA, USA) Milli-Q plus system. All
other chemical reagents used were of analytical grade.
2.3. MAE procedure
MAE system (Model MES-1000, CEM Corporation Inc., Matthews,
USA) was used. Desired weight of a freeze-dried sample was put into
the 100 mL Teflon
Ò
vessel (CEM Corporation Inc., Matthews, USA)
suited for microwave at a constant power of 300 W. A magnetic stir
bar at low speed (120 rpm) was introduced into the vessel for homo-
geneous heating. A constant solvent volume of 25 mL with varying
sample mass was chosen for the microwave-assisted extraction.
The vessels were placed at the centre of the microwave apparatus
and heated to desired temperature in 1 min, and then held at the de-
sired temperatures for a certain time period according to the exper-
imental design. Temperature was monitored by a fibre optic inserted
directly into the vessel and controlled by an external infrared sensor.
After microwave heating, the mixture in the extraction vessel was
allowed to cool down to room temperature (in ca. 10 min). The
supernatants were filtered through a 0.2-
l
m PTFE membrane filter
(VWR international, Ont., Canada) in preparation for spectrophoto-
metric and HPLC analyses. All procedures were performed in dim
lighting.
2.4. Optimization of MAE
The OAD design applied in this study focused on the effects of
the four most important variables which were set as follows: A:
ethanol concentration (20%, 40%, 60%, 80%); B: irradiation temper-
ature (50 °C, 60 °C, 70 °C, 80 °C); C: extraction time (2 min, 4 min,
6 min, 8 min); D: solid-to-solvent volume ratio (1:40, 1:45, 1:50,
1:55, w/v). The OAD16 (4
4
) matrix used for optimization and the
level settings of individual factor are shown in Table 1. The average
TPC of each trial as well as the average responses for individual fac-
tors at different level was calculated and used to evaluate the effi-
ciency and to optimize experimental conditions of the MAE. All
samples were tested in triplicate.
2.5. Comparison between the conventional extraction (CE) and MAE
procedures
MAE: 0.5 g dried powder of the residue, the supernatant of the
waste, peeled dices, whole potato tuber were extracted according
to the optimized conditions at: ethanol concentration 60%, extrac-
tion time 2 min, temperature 80 °C, and solid-to-solvent ratio of
1:40 (g/mL).
CE: Previous studies used 100% methanol for the efficient extrac-
tion of polyphenols and antioxidants from potato peel (Al-Weshahy
& Venket Rao, 2009). In the present study, 25 mL methanol was
Table 1
Orthogonal array design and result for the four variables studied.
Exp no. A (%,V/V) B (°C) C (min) D (mL/g) TPC (GAE mg/g DW)
1 I I I I 8.8 ± 0.13
2 I II II II 9.0 ± 0.14
3 I III III III 8.5 ± 0.13
4 I IV IV IV 9.8 ± 0.17
5 II I II III 9.1 ± 0.12
6 II II I IV 9.0 ± 0.13
7 II III IV I 9.7 ± 0.17
8 II IV III II 9.7 ± 0.16
9 III I III IV 8.8 ± 0.11
10 III II IV III 9.1 ± 0.13
11 III III I II 9.1 ± 0.15
12 III IV II I 9.7 ± 0.18
13 IV I IV II 2.6 ± 0.02
14 IV II III I 3.0 ± 0.03
15 IV III II IV 3.8 ± 0.04
16 IV IV I III 4.1 ± 0.03
I4050240
II 60 60 4 45
III 80 70 6 50
IV 100 80 8 55
A, ethanol concentration; B, temperature; C, extraction time; D, solid-to-solvent
ratio. I, II, III, IV are the four levels under each parameter. TPC: average total phe-
nolic content (n= 3), GAE: gallic acid equivalent.
T. Wu et al. / Food Chemistry 133 (2012) 1292–1298 1293

added to 0.5 g dried powder of the downstream waste residue, and
extracted under refluxing at 75 °C for 30 min. The mixture was cen-
trifuged and filtered similarly as described above before analysis.
This experiment was to compare the extraction efficiency with that
of the MAE. Both procedures were run in triplicate.
2.6. TPC assay
TPC of the extracts was assessed using the FC assay similar to
the procedures reported (Wang, Meckling, Marcone, Kakuda, &
Tsao, 2011). Briefly, gallic acid was used as a standard curve was
generated with concentrations ranging from 50 to 500
l
g/L. For
the analysis, 25
l
L of sample extract or gallic acid standard,
125
l
L methanol and 125
l
L 0.2 N FC reagent were added into a
well of a 96-well microplate, which was mixed well and incubated
for 8 min at room temperature, followed by adding 125
l
L 7.5%
Na
2
CO
3
. The plate was then incubated in the dark for 30 min at
room temperature. The absorbance was then measured at
765 nm using a UV–Visible microplate kinetic reader (EL 340,
Bio-Tek Instruments Inc., Winooski, VT, USA). The results were ex-
pressed in mg gallic acid equivalent/g dry weight (mg GAE/g DW).
All samples were tested in triplicate.
2.7. DPPH radical scavenging assay
The DPPH radical scavenging capacity assay was based on a pre-
viously described method (Wang et al., 2011) with some modifica-
tion. Briefly, 100
l
L of a methanolic solution of DPPH (0.065 mM)
was mixed with 20
l
L of a solution of plant extract and let stand
for 30 min at room temperature before the absorbance was re-
corded at 517 nm using the aforementioned microplate kinetic
reader. All samples were diluted 10 times before analysis. The rad-
ical scavenging activity of the extracts was calculated as follows:
percent scavenging (%) = [A
0
(A
1
A
S
)]/A
0
]100, where A
0
is
the absorbance of DPPH alone, A
1
is the absorbance of DPPH + ex-
tract and A
S
is the absorbance of the extract only. All samples were
tested in triplicate.
2.8. Ferric reducing antioxidant power (FRAP) assay
The FRAP assay was determined according to a method using
96-well microplates (Wang et al., 2011). Briefly, 10
l
L standard
or sample extract were mixed with 300
l
L of ferric-TPTZ reagent
(prepared by mixing 300 mM acetate buffer, pH 3.6, 10 mM TPTZ
in 40 mM HCl and 20 mM FeCl
3
6H
2
O at the ratio of 10:1:1 (v/v/
v)) in a well. The plate was incubated at 37 °C for the duration of
the reaction. The absorbance readings were taken at 593 nm at 0
and 4 min using the same microplate kinetic reader as described
above.
L
-Ascorbic acid was used to prepare the standard curve with
concentrations ranging from 50 to 500
l
mol/L. The antioxidant
activities were expressed as
l
mol ascorbic acid equivalent per
gram dry weight (
l
mol AAE/g DW). All samples were tested in
triplicate.
2.9. HPLC analysis
Quantitative analysis of individual compounds was carried out
in an Agilent 1100 HPLC system equipped with a DAD detector
(Agilent Technologies, Waldbroon, Germany). The separation was
achieved on a Luna 3
l
m ODS (4.6 mm 150 mm) column (Phe-
nomenex, Torrance, CA, USA) at ambient temperature. The mobile
phase consisted of 3.6% formic acid in water (solvent A) and meth-
anol (solvent B). The gradient used is as fellows: 5% B 0–2 min, 5–
17% B 2–15 min, 17–55% B 15–19 min, 55–75% 19–24 min, 75–
100% B 24–25 min, and 100–5% B 25–30 min. The flow rate was
0.9 mL/min and the injection volume was 10
l
L. The monitoring
wavelength was 280 nm for the phenolic acids. Identification of
each compound was based on congruent retention time, spectro-
scopic and mass spectrometric data.
2.10. LC-ESI-MS
LC-ESI-MS experiments were carried out using a Finnigan LCQ
DECA ion trap mass spectrometer (ThermoFinnigan, San Jose, CA)
equipped with electrospray ionization (ESI) source. To reach its
optimum performance, the sheath gas and auxiliary flow rates
were set at 82 and 45 (arbitrary units), respectively. The capillary
voltage was set at 4 kV, and its temperature was controlled at
350 °C. The entrance lens voltage was fixed at 12 V, and the multi-
pole RF amplitude was set at 730 V. The ESI needle voltage was
controlled at 4.5 kV. The tube lens offset was 29 V, the multipole
lens 1 offset was 8 10 V, and the multipole lens 2 offset was
11.5 V. The electron multiplier voltage was set at 1010 V for ion
detection. LC conditions were the same as described above for
the HPLC.
2.11. Statistical analysis
Statistical analyses were performed with Statistix for Windows
version 9.0 (Analytical Software, Tallahassee, FL, USA). The results
from the OAD analysis were analyzed by one-way analysis of var-
iance (ANOVA). The Fisher test value (F-value) was obtained from
the ANOVA test generated by the software. Results were expressed
as mean ± SD of three independent extractions. Differences in phe-
nolic compounds or activities were considered significant at
p< 0.05.
3. Results and discussion
3.1. Optimization of MAE conditions
Phenolic substances of plants are usually extracted by using
aqueous organic solvents such as methanol, ethanol, acetone and
ethyl acetate (Proestos & Komaitis, 2008). However, organic sol-
vents such as methanol are toxic and are not acceptable for foods.
The use of ethanol has several advantages over the use of other sol-
vents, including higher extraction efficiency, environmental com-
patibility and lower toxicity and cost. However, the percentage of
ethanol in water as an extraction solvent can affect the extraction
efficiency (Proestos & Komaitis, 2008). For this reason, different
ethanol concentrations were tested in the present MAE optimiza-
tion process.
The OAD optimization of the MAE conditions was based on the
maximum TPC yield of the sample. All parameters (A–D) were
tested in a wider range (data not shown) prior to OAD optimization.
This helped narrowing down the ranges of the parameters tested.
After implementing the 16 experimental trials based on the OAD
16 (4
4
) matrix (Table 1), the average responses (TPC) to all four indi-
vidual factors were obtained at the set levels (Fig. 1A–D).
In Fig. 1A, the efficiency in TPC increased with increasing etha-
nol concentration and plateaued near 60% ethanol, after which, the
extraction yield started to decrease slowly, but drastically beyond
80% ethanol. This suggests phenolic compounds are more soluble
in 60% ethanol. Pure ethanol (100% ethanol) only had a fraction
of what was achieved by the 60% ethanol. A similar effect was re-
ported for the extraction of phenolic compounds from wheat bran
(Wang, Sun, Cao, Tian, & Li, 2008).
As seen in Fig. 1B, a noticeable increase in extraction efficiency
was observed as the temperature was increased from 50 to 80 °C,
and the TPC value was the highest at 80 °C. At a higher temperature,
the solubility of phenolic compounds could be enhanced and the
1294 T. Wu et al. / Food Chemistry 133 (2012) 1292–1298

viscosity of extracts was decreased, thus accelerating the release and
dissolution of these compounds. However, high temperatures can
lead to degradation of certain phenolic compounds (Cunningham,
McMinn, Magee, & Richardson, 2008). In order to prevent the loss
of the bioactives, minimize the adverse effects of processing and re-
duce the running costs, 80 °C was set as the highest in this study.
Duration of extraction did not seem to have significant effect on
the TPC yield (Fig. 1C). The extraction efficiency showed only slight
fluctuation as extraction time was increased from 2 to 8 min, i.e.
the extraction was completed in 2 min. Shorter extraction time is
also favourable as it reduces energy consumption.
Fig. 1D shows the effect of different solid-to-solvent ratios be-
tween 1:40 and 1:55 (g/mL). The solvent volume in the studied
range played a less important role in the MAE extraction of TPC.
In other words, the phenolic compounds of potato are highly solu-
ble within our tested ratio range.
To verify whether the effect of individual factors on MAE effi-
ciency is statistically significant, an ANOVA was used to interpret
the experimental data obtained from the OAD optimization. The
significance of each factor was evaluated by calculating the Fvalue
and the results were summarized in Table 2. As seen in Table 2, the
influence by the parameters on the mean extraction yields of TPC
decreased in the order of: A (ethanol concentration) > B (tempera-
ture) > C (extraction time) > D (solid-to-solvent ratio) according to
the Fvalues. The most significant parameter (at the 95% confidence
level) was found to be the ethanol concentration, followed by
temperature. There was no significant difference in extract time
and solid-to-solvent ratio (at the 95% confidence level) compared
to other experimental variables. The ANOVA result was in good
accordance with what was observed in Fig. 1. Based on this analysis,
and considering the TPC extraction efficiency, the cost of energy and
the feasibility of experiment, the optimum conditions of extraction
were therefore determined as follows: ethanol concentration 60%,
extraction time 2 min, temperature 80 °C, and solid-to-solvent ratio
of 1:40 (g/mL).
3.2. Comparison of the proposed MAE and conventional extraction
methods
Conventional solvent extraction is a time-consuming process
(typically > 30 min) that relies on heat to increase the mass trans-
fer rate from outside to inside of sample matrix in the extraction
0
2
4
6
8
10
2468
Extraction time (min)
Total phenolic content (mg/g)
0
2
4
6
8
10
Temperature (°C)
Total phenolic content (mg/g)
0
2
4
6
8
10
50 60 70 80
40 50 60 70 80 90 100
Ethanol concentration (%)
Total phenolic content (mg/g)
B
A
C
0
2
4
6
8
10
1:40 1:45 1:50 1:55
Solid-to-solvent ratio
Total phenolic content
D
Fig. 1. Effect of ethanol concentration (A), temperature (B), extraction time (C) and solid-to-solvent ratio (D) on the total phenolic content in the MAE of the residue portion of
the downstream waste.
Table 2
Fvalues from ANOVA results.
Sum of squares Fvalue
A 101.748 687.486
*
B 2.248 15.189
*
C 0.348 2.351
D 0.148 1
A, ethanol concentration; B, temperature; C, extraction time; D, solid-to-solvent
ratio. Fcritical value (95%) = 9.28
*
P< 0.05.
T. Wu et al. / Food Chemistry 133 (2012) 1292–1298 1295

system, whereas microwave-assisted extraction is a fast extraction
process (2 min) that relies on heat from inside to outside of sample
matrix due to the fact that microwave energy can penetrate mate-
rials through molecular interaction. Our results are in accordance
with findings by other researchers for the extraction of phenolic
compounds from different plant materials by MAE (Pérez-Serradil-
la & Luque de Castro, 2011). A significant increase (10.4%) in TPC
(10.3 ± 0.3 mg GAE/g DW) was obtained for the MAE as compared
to that of the same sample extracted using the CE method
(9.6 ± 0.2 mg GAE/g DW) (P< 0.05) (Table 3). The volume of solvent
used by MAE extraction was lower (1:40, g/mL) than the volume
used by the conventional solvent extraction (1:50, g/mL). The HPLC
profiles of MAE and CE extracts were similar (Fig. 2), which sug-
gests that no phytochemicals were degraded under the optimized
MAE conditions as compared to the conventional extraction meth-
od. All these indicate that MAE is an excellent extraction method
with reduced extraction time, and solvent and energy consump-
tion. A yield of 3.68 mg GAE/g DW has been reported for potato
peels extracted by pressurized liquid extraction (Wijngaard et al.,
in press), which is only 37.5% of what was found in the residue
or 35.7% of the content in the supernatant portion of the industrial
downstream wastes in the present study. Others have reported
even lower TPC in potato peels. TPC in the peel of six potatoes vari-
eties as reported by Al-Weshahy and Venket Rao (2009) were from
1.51 to 3.33 mg GAE/g DW. While cultivar difference exists, the
significantly higher TPC values reported in the present study may
be largely due to the fact that our waste samples were from
mechanical abrasion of the potato tubers rather than manually
peeled ones which may inevitably contain some adjoining flesh.
Thinner skin contained more TPC than the adjoining tissues of
the potato tuber (Friedman, 1997).
3.3. Phenolic compositions and antioxidant activities
3.3.1. Total phenolic content
Calwhite is a very high yielding variety which is excellent for bak-
ing and French frying, thus widely used in the processed foods indus-
try. As shown in Table 3, the TPC value of the whole potato was
higher than that of the commercially peeled potato dices of the same
variety. The TPC of the whole potato tuber was 2.1 ± 0.16 mg GAE/
g DW, a concentration coincides with those reported for other white
potato varieties (1.8 to 3.8 mg GAE/g DW) (Navarre et al., 2011). The
TPC of the downstream wastes of the present study (containing
mostly the skins) was nearly 5-fold higher than that of the whole po-
tato tuber and the peeled potato dices on a dry matter basis. How-
ever, what is most interesting and reported here for the first time,
is that the liquid portion (the supernatant) of the downstream waste
had the highest TPC value (11.0 ± 0.26 mg GAE/g DW), which was
even higher than the residue portion of the same sample (10.3 ±
0.35 mg GAE/g DW) (Table 3). This indicates that a large portion
of the phenolic compounds in potato skins or peels are highly water
soluble, an important fact to consider when developing strategies in
recovering these compounds from the industrial wastes for uses in
value-added products.
3.3.2. Concentration and identification of the phenolic acids
The HPLC profiles of the MAE extracts were similar regardless of
the sample types (Fig. 2). Most peaks also appeared between 18
and 25 min. A major class of phenolic compounds in potato is the
hydroxycinnamic acid derivatives (Navarre et al., 2011). Chloro-
genic acid (5-O-caffeoylquinic acid) was present in the highest
concentration followed by caffeic acid in the Calwhite variety.
Two minor peaks (Peaks 1 and 2) had the same parent ion of m/z
353 in the LC-ESI-MS study, and the same UV spectrum as that
of peak 3 (chlorogenic acid), thus identified as neochlorogenic acid
(3-O-caffeoylquinic acid) and cryptochlorogenic acid (4-O-caf-
feoylquinic acid), respectively according to the elution pattern in
a recent report (Navarre et al., 2011)(Fig. 2). Peak 3 was identified
as chlorogenic acid by congruent retention time, UV spectral and
mass spectrometric data of the standard. Similarly, Peaks 4, 5
and 6 were identified as caffeic acid, ferulic acid and p-coumaric
acid, respectively (Fig. 2). Previous report had also found gallic
acid, vanillic acid and protocatechuic acid in potato (Mattila & Hell-
ström, 2007), however, these compounds were not detected in the
Calwhite variety of the present study. Concentrations of the indi-
vidual phenolic acids in the various potato samples are reported
in Table 3. The total phenolic index (TPI, sum of the individual con-
centrations) of the samples were slightly lower than the TPC which
can be caused by either the interference of reducing non-phenolic
compounds in the extracts or the arbitrary use of gallic acid as the
standard, or both, in the latter assay. However, the TPI values, and
particularly the chlorogenic acid concentrations, followed exactly
the same trend as those of the TPC (Table 3). About 80.5–84.1%
of the TPI in the downstream wastes were chlorogenic acid in
either the supernatant or the residue, whereas the percentages of
chlorogenic acid in the whole potato tuber or its peeled dices were
80.3% and 92.8%, respectively (Table 3). Even though the percent-
ages of chlorogenic acid in TPI were similar, the absolute amount
of chlorogenic acid in the waste samples, e.g. supernatant
(6.63 mg/g DW) was more than 50-fold of that in the peeled dice
(0.13 mg/g DW, Table 3). Chlorogenic acid and caffeic acid are
therefore the predominant phenolic acids of the Calwhite potato.
3.3.3. Antioxidant capacity
Numerous health beneficial effects of potato phenolics have
been described, including the antioxidant activity (André et al.,
2009; Lachman, Hamouz, Orsák, Pivec, & Dvorák, 2008; Lachman
et al., 2009; Rumbaoa, Cornago, & Geronimo, 2009a; Rumbaoa,
Cornago, & Geronimo, 2009b). Two in vitro methods were used to
evaluate the antioxidant activities of the MAE extracts of the Cal-
white potato and its downstream wastes. As shown in Table 4,
MAE extracts of the supernatant and the residue of the downstream
wastes showed significantly higher antioxidant activities in both
assay systems as compared to the whole potato tuber and peeled
Table 3
Content of phenolic compounds in this study (mg/g DW).
TPC Chlorogenic acid Caffeic acid Ferulic acid p-Coumaric acid TPI % Chlorogenic acid of TPI
RW
a
9.6 ± 0.34 4.66 ± 0.13 0.64 ± 0.03 0.41 ± 0.02 0.03 ± 0.00 5.73 81.3
RW
b
10.3 ± 0.39 4.71 ± 0.11 0.64 ± 0.02 0.44 ± 0.01 0.06 ± 0.00 5.85 80.5
SW
b
11.00 ± 0.26 6.63 ± 0.15 0.93 ± 0.03 0.30 ± 0.01 0.02 ± 0.00 7.88 84.1
PD
b
2.1 ± 0.13 0.13 ± 0.00 0.01 ± 0.00 ND ND 0.14 92.8
PT
b
1.5 ± 0.11 0.61 ± 0.02 0.04 ± 0.00 0.08 ± 0.01 0.03 ± 0.00 0.76 80.3
RW, residue of the waste; SW, supernatant of the waste; PD, peeled dices; PT, whole potato tuber. ND, not detected. TPC, the total phenolic contents (expressed as gallic acid
equivalents); TPI, total phenolic index (sum of the individual concentrations).
a
Conventional extraction.
b
Microwave extraction.
1296 T. Wu et al. / Food Chemistry 133 (2012) 1292–1298

dices. However, the difference between the two portions of the
waste (supernatant and solid residue) was insignificant on a dry
matter basis. The antioxidant activities of the waste extracts were
approximately three times higher than those of the whole potato
tuber and peeled dices in radical scavenging capability as measured
by the DPPH assay (Table 4). Similar trend was found in the FRAP
assay, except the FRAP values of the waste extracts were even more
significantly high than those of the whole potato tuber and peeled
dices, by 10- to 20-fold (p< 0.001) (Table 4). The DPPH radical scav-
enging activity of the MAE extracts correlated well with the TPC
values with correlation coefficients of r= 0.97, 0.95, 0.93 and 0.95,
respectively for the supernatant, residue of the waste, the whole
potato and the peeled flesh dices. FRAP values of The MAE extracts
were similarly highly correlated with the TPC with r= 0.92, 0.96,
0.94 and 0.95, respectively. The antioxidant activities also corre-
lated with the TPI in the same fashion, indicating that phenolic
acids are the components responsible for the antioxidant effect of
potato, a result in agreement with findings by others (Navarre
et al., 2011).
The significantly higher phenolic concentrations and antioxi-
dant activities of the waste extracts suggest that there is merit to
recover these phenolic compounds and find applications in foods
or food supplements. Chlorogenic acid is of particular interest as
the single predominant phenolic compound in the waste. Chloro-
genic acid is a very common phenolic acid in many edible plants
such as apples, sunflowers, coffee beans and cacao, and has been
one of the major health-promoting ingredients of functional foods
and beverages (Liu, Liu, Shi, & Zhou, 2010). Chlorogenic acid has
been shown to possess anti-diabetic properties in vitro and
in vivo (Karthikesan, Pari, & Menon, 2010), to suppress pulmonary
eosinophilia (Kim et al., 2010), and to be a potential agent for the
treatment of osteoarthritis (Chen et al., 2011). Chlorogenic acid
and caffeic acid have also been shown to inhibit hepatitis B virus
replication (Wang et al., 2009), to improve body weight, lipid
metabolism and obesity-related hormone levels in high-fat fed
mice, while activities by chlorogenic acid seemed to be more po-
tent than caffeic acid (Cho et al., 2010). However, others have
shown that caffeic acid is a stronger antioxidant than chlorogenic
acid (Sato et al., 2011). These health beneficial activities, together
with the results found in the present study, indicate that chloro-
genic acid and caffeic acid-rich extracts of the downstream wastes
from potato processing can be a promising candidate for functional
foods and nutraceutical ingredients.
In conclusion, MAE was proven a more effective and efficient
extraction technique than the conventional solvent extraction by
refluxing. Using an orthogonal array design, MAE parameters were
optimized at 60% ethanol, 80 °C, 2 min, and solid-to-solvent ratio
1:40 (g/mL). This model was successfully applied to potato and
its downstream waste samples. The fact that the whole potato
had higher total phenolic content and individual phenolic acids
than the peeled flesh indicates that the majority of the phenolics
18 19 20 21 22 23
0
100
200
300
400
500
Residue of the wastea
Residue of the wasteb
Supernatant of the wasteb
Potato peeled diceb
Potato tuberb
1,2
3
4 5 6
Fig. 2. Chromatograms of the OAD optimized MAE extracts of the whole potato tuber, peeled dices, supernatant of the waste and residue of the waste. Peaks:
1 = neochlorogenic acid, 2 = cryptochlorogenic acid, 3 = chlorogenic acid, 4 = caffeic acid, 5 = ferulic acid, 6 = p-coumaric acid. (a) Conventional extraction and (b) microwave-
assisted extraction.
Table 4
Antioxidant activities of potato and waste extracts (n= 3).
Sample % Radical scavenging
c
FRAP (
l
mol AAE/g DW)
RW
a
93.08 ± 0.8 61.2 ± 1.5
RW
b
96.04 ± 0.5 77.2 ± 1.8
SW
b
95.17 ± 0.6 78.5 ± 2.2
PD
b
34.96 ± 0.1 3.6 ± 0.2
PT
b
34.43 ± 0.1 7.2 ± 0.3
RW, residue of the waste; SW, supernatant of the waste; PD, peeled dices; PT, whole
potato tuber. FRAP, Ferric reducing antioxidant power.
a
Conventional extract.
b
Microwave extract.
c
DPPH assay.
T. Wu et al. / Food Chemistry 133 (2012) 1292–1298 1297

are in the skin of the potato tuber. This was further confirmed in
the present study that the industrial downstream wastes of potato
had up to 5-fold higher total phenolic contents than the whole or
peeled potato on a dry weight basis. The antioxidant activities of
these samples highly correlated with the total phenolic content
and the total phenolic index. The strong antioxidant and other
health beneficial activities of chlorogenic acid and caffeic acid,
two main phenolic acids in the downstream waste, may signify
the importance and great potential of recovering and utilizing ex-
tracts containing these compounds as ingredients for functional
foods and nutraceuticals. Future studies will focus on scale up
recovery of these phenolics and application in various formulations
of foods or beverages.
Acknowledgments
The authors thank Advanced Food & Materials Network, Canada
for their financial support.
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