Comparison of different drying methods on Chinese ginger (Zingiberofficinale Roscoe): Changes in volatiles, chemical profile, antioxidant properties, and microstructure

Abstract

Nowadays,food industry is facing challenges in preserving better quality of fruit and vegetable products after processing. Recently,many attentions have been drawn to ginger rhizome processing due to its numerous health promoting properties. In our study, ginger rhizome slices were subjected to air-drying (AD), freeze drying (FD), infrared drying (IR), microwave drying (MD) and intermittent microwave convective drying (IMCD). Quality attributes of the dried samples were compared in terms of volatile compounds, 6, 8, 10-gingerols, 6-shogaol, antioxidant activities and microstructure. Results showed that AD and IR were good drying methods to preserve volatiles. FD, IR and IMCD led to higher retention of gingerols, TPC, TFC and better antioxidant activities. However, FD and IR had relative high energy consumption and drying time. Therefore, considering about the quality retention and energy consumption, IMCD would be very promising for thermo sensitive material.

Full text

Comparison of different drying methods on Chinese ginger (Zingiber
officinale Roscoe): Changes in volatiles, chemical profile, antioxidant
properties, and microstructure
Kejing An
a
, Dandan Zhao
b,c
, Zhengfu Wang
b,c
, Jijun Wu
a
, Yujuan Xu
a
, Gengsheng Xiao
a,
⇑
a
Sericulture and Agri-Food Research Institute Guangdong Academy of Agricultural Sciences/Key Laboratory of Functional Foods, Ministry of Agriculture/Guangdong Key Laboratory
of Agricultural Products Processing, Guangzhou 510610, PR China
b
College of Food Science and Nutritional Engineering, China Agricultural University, Beijing 100083, PR China
c
National Engineering and Technology Research Center for Fruits and Vegetable Processing, PR China
article info
Article history:
Received 18 May 2015
Received in revised form 4 November 2015
Accepted 6 November 2015
Available online xxxx
Chemical compounds studied in this article:
6-Gingerol (PubChem CID: 442793)
8-Gingerol (PubChem CID: 168114)
10-Gingerol (PubChem CID: 168115)
6-Shogaol (PubChem CID: 5281794)
Zingerone (PubChem CID: 31211)
b-Phellandrene (PubChem CID: 11142)
b-Bisabolene (PubChem CID: 10104370)
a
-Curcumene (PubChem CID: 3083834)
2,2-Diphenyl-1-picrylhydrazyl (PubChem
CID: 74358)
2,2
0
-Azinobis (3-ethylbenzo thiazoline-6-
sulfonic acid) diammonium salt (ABTS)
(PubChem CID: 9570474)
Keywords:
Intermittent microwave & convective drying
Volatiles
Antioxidant activity
Microstructure
Ginger
abstract
Nowadays, food industry is facing challenges in preserving better quality of fruit and vegetable products
after processing. Recently, many attentions have been drawn to ginger rhizome processing due to its
numerous health promoting properties. In our study, ginger rhizome slices were subjected to air-
drying (AD), freeze drying (FD), infrared drying (IR), microwave drying (MD) and intermittent microwave
& convective drying (IM&CD). Quality attributes of the dried samples were compared in terms of volatile
compounds, 6, 8, 10-gingerols, 6-shogaol, antioxidant activities and microstructure. Results showed that
AD and IR were good drying methods to preserve volatiles. FD, IR and IM&CD led to higher retention of
gingerols, TPC, TFC and better antioxidant activities. However, FD and IR had relative high energy con-
sumption and drying time. Therefore, considering about the quality retention and energy consumption,
IM&CD would be very promising for thermo sensitive material.
Ó2015 Elsevier Ltd. All rights reserved.
1. Introduction
Ginger, one of the most ancient spices in the world, has been
widely used as a spice or a common condiment for a variety of
compound food and beverages (Larsen, Ibrahim, Khaw, & Saw,
1999). It is also an important medicine for treating cold, stomach
upset, diarrhea, and nausea. Phytochemical studies show that gin-
ger has antioxidant and anti-inflammatory activities, and some of
them exhibit potential cancer preventive activity (Shukla &
Singh, 2007; Stoilova, Krastanov, Stoyanova, Denev, & Gargova,
2007; Thomson et al., 2002). The characteristic components of gin-
ger include essential oil and oleoresin, which are responsible for its
fragrant and pungent behavior, respectively. Essential oil mainly
consists of monoterpenoid and sesquiterpene hydrocarbons,
whereas oleoresin is composed of non-volatile phenolics known
as gingerols, shogaols and zingerone (Huang, Wang, Chu, & Qin,
2012). The gingerols are identified as the major active components
in fresh ginger. Shogaol series of compounds do not intrinsically
http://dx.doi.org/10.1016/j.foodchem.2015.11.033
0308-8146/Ó2015 Elsevier Ltd. All rights reserved.
⇑
Corresponding author at: Sericulture and Agri-Food Research Institute, Dong
Guanzhuang Yiheng RD., Tianhe District, Guangzhou 510610, PR China.
E-mail address: 418259325@qq.com (G. Xiao).
Food Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Food Chemistry
journal homepage: www.elsevier.com/locate/foodchem
Please cite this article in press as: An, K., et al. Comparison of different drying methods on Chinese ginger (Zingiber officinale Roscoe): Changes in volatiles,
chemical profile, antioxidant properties, and microstructure. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.11.033

exist in fresh sample, as they are derived from the corresponding
gingerols during thermal processing or long-term storage. Gener-
ally, the degradation of gingerol to shogaol takes place either
because of the acidic environment or as a result of the increase
in temperature (Kubra & Rao, 2012). Studies also have proven that
shogaols are more pungent, and exhibit higher antioxidant activity
than gingerols (Guo, Wu, Du, Zhang, & Yang, 2014).
Fresh gingers usually contain 85–95% of water and are suscep-
tible to microbial spoilage and chemical deterioration (Mishra,
Gauta, & Sharma, 2004). Dehydration of ginger is the most prac-
ticed processing procedure to inhibit microbial growth and delay
deteriorative biochemical reactions. It is also a fundamental pro-
cessing method to obtain new products. Dried ginger can be uti-
lized for manufacturing ginger spices, medicine and cosmetics as
well as food with ginger flavor such as soft drinks and candies.
However, drying process may cause thermal damage and severe
changes in physical, chemical and organoleptic properties of aro-
matic plant. Therefore, the selection of drying method is very
important. According to Mujumdar and Law (2010), drying tech-
nologies have attracted significant research and development
efforts because of the growing demand for better product quality
and lower operating cost, as well as lessened environmental
impact.
The most conventional drying method is hot air drying (AD),
however, its high temperature and long drying cycle usually result
in the degradation of important flavor, color and nutritional com-
pounds. Freeze drying (FD) can yield high quality products, but it
also leads to high energy consumption, high capital cost and long
drying time. Microwave drying (MD) and infrared radiation (IR)
have their own place in drying technology, due to the same trans-
fer direction of temperature and moisture, they can offer many
advantages such as great energy efficiency and high heat transfer
rate. IR has more advantages in uniform heating and high quality
of final products (Sellami et al., 2013). Although MW heating can
readily deliver energy to generate heat within foods, one of its
major drawbacks is the inherent non-uniformity of the electro-
magnetic field (Zhang, Tang, Mujumdar, & Wang, 2006). Since its
local temperature can easily rise to a level that causes scorching,
microwave drying usually has been combined with other tech-
niques including convective hot-air, vacuum, and intermittent
power application to achieve more uniform, high quality and effec-
tive drying (Gunasekaran, 1999; Kaensup, Chutima, & Wongwises,
2002; Soysal, Ayhan, Esßtürk, & Arıkan, 2009).
Microwave-convection drying includes two kinds of form, one
is microwave and air convection drying conducted in stages, the
other being these two dryings carried out simultaneously. In our
study, the second form was adopted. In microwave combined con-
vection drying, microwave energy removes the inner moisture of
material to the surface and convective air helps removing the sur-
face moisture out of drying chamber, which not only increases the
energy efficiency but also reduces the surface temperature of
material. Intermittent application of microwave energy has proven
itself a good method to avoid uneven heating, improve product
quality and increase energy utilization by allowing redistribution
of temperature and moisture profiles within the product during
off times (Gunasekaran, 1999).
Many authors have studied the variations of volatiles, non-
volatiles or antioxidant capacity of ginger induced by drying pro-
cess. Huang et al. (2012) studied the effects of oven drying,
microwaving drying, and silica gel drying methods on the volatile
components of ginger and found that microwave and silica gel can
be used in drying of ginger to maintain the taste and appearance of
fresh ginger. Bartley and Jacobs (2000) reported that the major
effects of drying process on ginger are the reduction in gingerol
content, increase in terpene hydrocarbons and conversion of some
monoterpene alcohols to their corresponding acetates. Gümüsßay,
Borazan, Ercal, and Demirkol (2015) studied thermal dryings and
freeze drying (FD) for ginger in terms of total phenolic content
(TPC), ascorbic acid (AA) and antioxidant capacity. He found freeze
dried gingers have better antioxidant properties than samples trea-
ted by thermal dryings. Yet so far there is no systematic investiga-
tion regarding the effects of drying methods on energy
consumption, volatile and non-volatile components, antioxidant
capacity, and microstructure of ginger at the same time.
The objective of this work was to explore the possibility of using
intermittent microwave combined convection drying (IM&CD) for
processing of ginger product. Therefore, an investigation was build
on the comparison of different drying methods, namely, AD, IR, FD,
MD and IM&CD on the energy consumption and quality conserva-
tion of ginger.
2. Materials and methods
2.1. Reagent and chemicals
Acetonitrile and methanol (HPLC grade) were purchased from
Honeywell (Morris, NJ, USA). Authentic standards of 6-, 8-, 10-
gingerol and 6-shogaol were purchased from Chromadex Inc.
(Irvine, CA, USA). Analytical grade chemicals: Folin–Ciocalteu
reagent; gallic acid; ascorbic acid; 2,2-diphenyl-1-picrylhydrazyl
(DPPH); 2,2
0
-azinobis (3-ethylbenzo thiazoline-6-sulfonic acid)
diammonium salt (ABTS); 2,4,6-tripyridyl-s-triazine (TPTZ) were
procured from Sigma–Aldrich (St. Louis, MO, USA). Meta-
phosphoric acid, sodium carbonate, potassium persulfate and gla-
cial acetic acid were from National Pharmaceutical Corporation
(Beijing, China).
2.2. Samples
The fresh matured gingers (Shandong Laiwu variety) were pur-
chased from China Agricultural University regional market, in
March, 2013. Voucher specimens were preserved at 4 °C before
drying. Raw ginger rhizomes were washed to detach the dirt and
sand adhering to them and blotted up with filter paper to remove
the excess water. Then gingers were cut into cylinder slices with
thickness of 4 ± 0.2 mm and diameter of 34 ± 2.0 mm. The initial
moisture was determined by using a vacuum oven at 70 °C with
13.3 kPa, until the weight of samples was constant. Gingers used
for experiment were from the same batch.
2.3. Drying of the ginger rhizomes
150 g ginger slices were spread out evenly and subject to five
different drying methods, and drying was last until the ginger
moisture content reached to 0.12 ± 0.02 g H
2
O/g d.w.
2.3.1. Hot-air drying (AD)
Ginger slices were dried in an electric thermo static drying oven
(DHA-9070A; Shanghai Jinghong Experiment Instrument Co.,
Shanghai, China) at 60 °C.
2.3.2. Infrared drying (IR)
Ginger slices were put into an infrared radiation chamber (Sent-
tech Infrared Technology Co., Ltd. Taizhou, China) with three red
glass lamps (225 W each).
2.3.3. Freeze drying (FD)
Ginger slices were first frozen at 40 °C for 12 h, and then were
quickly placed into a freeze dryer (LGJ-25C; Beijing Si Huan Scien-
tific Instrument Factory Co., Beijing, China) and dried under 20 Pa
2K. An et al. / Food Chemistry xxx (2015) xxx–xxx
Please cite this article in press as: An, K., et al. Comparison of different drying methods on Chinese ginger (Zingiber officinale Roscoe): Changes in volatiles,
chemical profile, antioxidant properties, and microstructure. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.11.033

absolute pressure. The temperature of the heating plate and the
cold trap were at 25 and 58 °C.
2.3.4. Microwave drying (MD)
Ginger slices were dried in a microwave oven (NJ07-3; Nanjing
Jiequan Microwave Apparatus Co. Ltd., Nanjing, China) with an ini-
tial energy density of 5 w/g until moisture content reached to 50%
w.b. (1.0 g H
2
O/g d.w.), then dried samples with 1 w/g to the termi-
nal point.
2.3.5. Intermittent microwave-convection drying (IM&CD)
Ginger slices were dried in laboratory-setup microwave oven
with an output of 700 W and hot air drying of 60 °C, including a
control unit for microwave pulse ratio (PR) regulation. Fresh sam-
ples were first dried at initial PR = 2 (5 s on–5 s off) to 50% w.b.,
then adjusted PR = 6 (5 s on–25 s off) to the end of drying. The
whole drying process was assisted with hot air.
After each drying, the samples were ground into powder form
sieved with 60 mesh wire screen and kept in dark and dry place
for further analysis.
2.4. Preparation of extract for determination of volatile composition
The volatile components of ginger were extracted by solid-
phase micro-extraction (SPME) method. The SPME manual device
(Supelco Co., Bellefonte, PA) was equipped with a fused silica fiber
coated with polydimethylsiloxane (PDMS). 1.0 g ginger powder
was placed in a 15-mL vial and sealed. The fiber was inserted into
the headspace for 30 min at 40 °C water bath, then it desorbed at
250 °C for 3 min in the injection port of an Agilent GC–MS
(7890A-5975C).
2.5. Determination of volatile flavor composition
Analysis of volatiles was according to the procedure described
by Ding et al. (2012) with modifications. The aroma compounds
were identified using an Agilent J&W DB-5 column
(30 m 0.25 mm 0.25
l
m). The oven temperature programme
was as follows: 50 °C (held for 3 min), then raised to 120 °Cata
rate of 4 °C/min (held for 8 min), then heated to 200 °Cat4°C/
min (held for 3 min) and finally increased to 250 °Cat10°C/min
(held for 3 min). Helium was used as the carrier gas at a flow rate
of 1.0 mL/min. The split ratio was 1:60. The MS fragmentation was
performed by electronic impact (EI) at 70 eV, a source temperature
of 230 °C, scanning rate of 1 scan s
1
and mass acquisition range of
35–550 Da.
The compounds were identified using the Wiley and NIST
libraries (Washington, DC), and also by matching against the pub-
lished data (Bartley & Jacobs, 2000; Ding et al., 2012; Nirmala
Menon et al., 2007). Compounds whose similarity is more than
78 were reported here. The relative amounts present were calcu-
lated on the basis of peak-area ratios.
2.6. Preparation of extract for the determination of HPLC analysis and
antioxidant activity
Extracts were prepared according to the procedure described by
Chan et al. (2008). The ginger powder 5 g was accurately weighed
and placed into a beaker, then 30 mL of 80% aqueous methanol was
added. Ginger was ultrasonically extracted three times for 30 min
each time and filtered. Methanol was removed by drying at 40 °C
in a rotary evaporator. After concentration, the extract was trans-
ferred to a 50 mL volumetric flask and made up to the volume
using 80% methanol. The solution was filtered through 0.45
l
m
organic membrane filter to an auto sampler vial for HPLC analysis.
Similarly, 10 g of each fresh ginger sample was homogenized with
30 mL 80% methanol using a breaking pulper (JYL-CO51 type, Joy-
oung Company, Beijing, China). The extraction procedure was
repeated.
2.7. Determination of 6, 8, 10-gingerol and 6-shagaol by HPLC method
HPLC analysis was performed on an Agilent 1100 liquid chro-
matography system, chromatographic separation was carried out
according to the method of Cheng, Liu, Peng, Qi, and Li (2011).
The chromatographic separation was carried out using the RF-
10AXL HPLC system (Shimadzu Co., Japan). The column used was
a reverse phase column (Sunfire
TM
C
18,
4.6 250 mm i.d., 5
l
m).
The mobile phase was prepared from water (A) and acetonitrile
(B). The gradient program for the HPLC was as follows: 0–5 min,
0–20% B; 5–45 min, 20–90% B; and 45–55 min, 100% B. The flow
rate was 1 mL min
1
, the injection volume was 20
l
L, and the col-
umn temperature was maintained at 30 °C. The detection wave-
length was set at 280 nm.
2.8. Determination of total phenolic content (TPC)
TPC of ginger was determined using the Folin–Ciocalteu assay
according to Singleton, Orthofer, and Lamuela-Raventos (1999).
Samples (400
l
L) were introduced into test tubes followed by
2.0 mL of Folin–Ciocalteu phenol reagents (10 times dilution with
deionized water). After 5 min, 3.0 mL of sodium carbonate (7.5%
w/v) solution was added to the mixture. The absorbance was mea-
sured at 765 nm using a spectrophotometer (UV-726, Shimadzu,
Shanghai, China) after 2 h reaction in darkness. 400
l
L 80% metha-
nol was added instead of sample taken as blank. The amount of
total phenolics was expressed as gallic acid equivalents (GAE,
mg/g of dry sample).
2.9. Determination of total flavonoid contents (TFC)
The TFC was measured according to the method of Dewanto,
Wu, Adom, and Liu (2002) with small modifications. Diluted solu-
tions of extracts of dried ginger 2 mL were put in a 10 mL volumet-
ric flask. Initially, 5% NaNO
2
0.3 mL was added to the volumetric
flask, after 6 min, 10% AlCl
3
6H
2
O 0.3 mL was added, after 6 min,
4% NaOH 2 mL was added. Water (5.4 mL) was added to the reac-
tion flask after 15 min and mixed well. Absorbance of the reaction
mixture was read at 510 nm. 400
l
L 80% methanol was added
instead of sample taken as blank. TFC were determined as rutin
equivalents (mg/g of dry weight).
2.10. Determination of DPPH radical scavenging assay
This assay is based on the measurement of the scavenging abil-
ity of antioxidants towards the stable radical DPPH. It was con-
ducted according to Lim and Murtijaya (2007) with
modifications. Different dilutions of extract 0.4 mL were added to
3.5 mL of 0.14 m mol/LDPPH solution in methanol and shaken vig-
orously. The mixture was allowed to stand for 30 min before mea-
suring the absorbance at 517 nm. Results were also expressed as
IC
50
and ascorbic acid equivalent antioxidant capacity (AEAC).
IC
50
of the extract was determined from the graph of antioxidant
activity (%) against amount of extract (mg). Antioxidant activity
was expressed using the equation:
AA%¼Abs
control
Abs
sample

=Abs
control

100
where control: 3.5 mL 0.14 mM DPPH + 0.4 mL 80% methanol; sam-
ple: 3.5 mL 0.14 Mm DPPH + 0.4 mL extract.
AEAC in mg ascorbic acid/100 g of fresh material with following
equation:
K. An et al. / Food Chemistry xxx (2015) xxx–xxx 3
Please cite this article in press as: An, K., et al. Comparison of different drying methods on Chinese ginger (Zingiber officinale Roscoe): Changes in volatiles,
chemical profile, antioxidant properties, and microstructure. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.11.033

AEAC ðmg ascorbic acid=100gÞ¼IC
50 ðascorbic acidÞ
IC
50 ðsampleÞ
10
5
The IC
50 ascorbic acid
used was determined to be 0.00387 mg/mL.
2.11. Determination of ferric-reducing antioxidant power (FRAP)
The FRAP assay was performed according to the method
reported by Kubra and Rao (2012). The stock solution included
300 Mm acetate buffer (5.1 g C
2
H
3
NaO
2
3H
2
O and 20 mL C
2
H
4
O
2
)
at pH 3.6, 10 mM TPTZ (2,4,6-tripyridyl-s-triazine) solution in
40 mM HCl, and 20 Mm FeCl
3
6H
2
O solution in distilled water.
Then acetate buffer 25 mL and TPTZ 2.5 mL were mixed together
with FeCl
3
6H
2
O 2.5 mL. The temperature of the solution was
raised to 37 °C before it was used. Ginger extracts 100
l
L were
allowed to react with the FRAP solution 4.0 mL for 10 min under
37 °C. The ferric reducing ability was measured by monitoring
the absorbance at 593 nm and the FRAP solution was used as blank.
Results of the FRAP assay were expressed as mg ascorbic acid/g.
2.12. Determination of ABTS antioxidant activity
The ABTS antioxidant activity was carried out using the ABTS
+
radical cation decolorization assay followed the method of Sogi,
Siddiq, Greiby, and Dolan (2013) with some modifications. ABTS
stock solution was dissolved in sodium acetate–acetic acid buffer
(20 Mm pH = 4.5) to make a 7 mM ABTS stock solution. 7 mM ABTS
solution and 2.45 mM potassium persulfate were mixed in 1:1
ratio and allowed to stand in the dark for 12–16 h to produce
ABTS
+
working solution. This solution was further diluted with
80% methanol to reach the absorbance of 0.70 ± 0.02 at 734 nm.
The ABTS
+
working solution 3.6 and 0.4 mL of extracts were mixed
and the absorbance was measured at 734 nm after 30 min in the
dark. The blank was run with 80% methanol. A standard curve
was prepared using Trolox solution (30–90
l
g/mL).
2.13. Microstructure of dried ginger
Microstructure changes of ginger during different drying pro-
cess were analyzed using a scanning electron microscope
(Phenom-World BV, Eindhoven, The Netherlands). To obtain the
SEM images, 5 55mm
3
small pieces were taken from both
the inner parts and surface of dried ginger slices, then glued on
the metal stub, each specimen was coated with a very thin layer
of gold under high vacuum.
2.14. Statistical analysis
Experiment data were analyzed using Origin 8.0 (Microcal Soft-
ware, Inc., Northampton, USA) and Spss18.0 (Chicago, IL, USA). Sig-
nificant differences between samples were analyzed using analysis
of variance (ANOVA) and Duncan’s multiple-range test (P< 0.05).
Principal composition analysis (PCA) and cluster analysis were
conducted using Spss18.0 (Chicago, IL, USA). All experiments were
run in triplicate, and data were reported as the mean ± standard
deviation (SD).
3. Results and discussion
3.1. Comparison of drying time, energy consumption and extraction
yield of different dried gingers
The drying time, energy consumption and extraction yield were
different in the selected drying techniques. As shown in Table 1,FD
had the longest drying time and highest energy consumption, with
drying time of 44.5 ± 2.0 h and energy consumption of
33.7 ± 0.53 kW h/g H
2
O. Air-dried samples went through the sec-
ond longer drying time of 12.0 ± 0.5 h, but its energy consumption
was relatively low, 3.30 ± 0.08 kW h/g H
2
O. The second large
energy consumption was IR process with the drying time of
6.0 ± 0.7 h. MD and IM&CD had both lower drying time and energy
consumption compared with other drying methods, while IM&CD
had lower drying time but higher energy consumption than MD.
In terms of extraction of yield, FD samples had the highest yield,
followed by IM&CD, MD, AD and IR samples. According to Asami,
Hong, Barret, and Mitchell (2003), freeze drying has higher extrac-
tion efficiency due to the rupture of cell structure caused by ice
crystals formed within plant matrix. In general, IM&CD had lower
drying time, lower energy consumption and higher extraction of
yield.
3.2. Effect of drying process on volatile components composition
In the analysis of fresh ginger, 48 compounds were extracted
and identified (Supplementary Table 1). The main compounds of
the fresh ginger were zingiberene (22.76%), b-phellandrene
(12.40%), b-sesquiphellandrene (7.01%), geranial (14.50%),
a
-curcumene (2.78%) and b-bisabolene (3.25%) (Supplementary
Table 2). The relatively high content of zingiberene and
b-phellandrene are account for the odor of the fresh ginger, which
was consistent with the result of Huang et al. (2012). However,
Bartley and Jacobs (2000) have reported that main flavor fractions
of Australian-grow ginger were geranial, zingiberene, zingerone
and (E,E)-
a
-farnesene. Nishimura (1995) adopted n-hexane to
extract the characteristic odorants in fresh ginger and found that
linalool, geraniol, geranial, neral and isoborneol presented high
value in flavor factors. The differences in the main volatile compo-
sition may be attributed to the different origins of ginger, methods
of extraction and kinds of solvents used (Ding et al., 2012).
Different drying methods resulted in different changes of the
volatile compounds, but there was a same trend after drying that
the relative percentage of sesquiterpenes compounds (zingiberene,
b-sesquiphellandrene,
a
-farnesene, and
a
-curcumene) showed
considerable increase while monoterpenes (b-phellandrene, cam-
phene) decreased significantly. This could be attributed to the syn-
thesization of short-chain alkenes and isomerization of similar
compounds. In the hot-air drying of 60 °C, 49 compounds were
found. It was worth noting that the concentrations of zingiberene,
a
-curcumene, b-bisabolene, b-sesquiphellandrene and a-farnesene
were lower compared with other dried samples (Supplementary
Table 2). This was possibly owing to long exposure to high temper-
ature air resulting in the degradation of sesquiterpenes to
monoterpenes. We also found many esters such as Propanoic acid,
2-methyl-, 3,7-dimethyl-2,6-octadienyl ester and bornyl acetate
were only formed in hot air drying. This was attributed to long
time exposure to oxygen promoting the esterification of alcohols
to corresponding esters, which was also proved by Ding et al.
(2012). In the IR drying, 47 compounds were extracted and identi-
fied. We found appearance of many new volatile compounds and
disappearance of original compounds (Supplementary Table 1).
According to Sellami et al. (2011), IR drying of Laurus nobilis leaves
at 65 °C may cause oxidation process and chemical rearrange-
ments, which may also explain the phenomenon we found in gin-
ger. We also found that IR drying preserved the levels of
sesquiterpenes (zingiberene, b-sesquiphellandrene, b-bisabolene
and
a
-curcumene), which was proved by Yoshikawa et al. (1993).
In microwave, the concentrations of major sesquiterpenes were
well retained, even marginally increased as well as the concentra-
tions of monoterpenes (b-phellandrene, camphene, 1s-
a
-pinene).
This indicated that higher temperature inside the products would
accelerate the release of volatile compounds owing to cell damage,
which was consistent with the report of Kubra and Rao (2012).
4K. An et al. / Food Chemistry xxx (2015) xxx–xxx
Please cite this article in press as: An, K., et al. Comparison of different drying methods on Chinese ginger (Zingiber officinale Roscoe): Changes in volatiles,
chemical profile, antioxidant properties, and microstructure. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.11.033

There were 41 and 43 compounds detected in FD and IM& CD dried
gingers, indicating the two processes decreased the varieties of
total volatiles significantly. Though former research reported
freeze drying was an optimal technique for volatile retention
(Liapis & Bruttini, 1995), we found the long vacuum treatment of
FD changed the categories and quantities of ginger volatiles signif-
icantly. In the IM&CD process, volatile compounds were also
greatly affected. According to Sellami et al. (2011), many volatiles
have more affinity to the water fraction contained in plant samples
and were lost accompanied with evaporating water during drying
process. Thereby, in the IM&CD process, microwave accelerates the
release of volatile compounds and hot air speeds up the evapora-
tion of surface moisture, which increase the loss of volatile
compounds.
Cluster analysis was carried out to characterize the correlation
between different drying methods (Fig. 1). The longer euclidean
distance, the lower similarities of the samples are. As shown in
Fig. 1, AD had shortest euclidean distance with fresh samples, fol-
lowed by IR, MD, FD and IM&CD. The dendrogram indicated that
drying methods could be regrouped into three clusters. The first
cluster included AD and IR, with shorter euclidean distance of
26.13 (Supplementary Table 3). The second cluster was composed
of FD, IM&CD and MD, with the most similar profiles of FD and IM
&CD. The third cluster consisted of MD, AD and IM &CD. Therefore,
AD is the most preferable method, followed by IR and MD, while FD
and IM &CD were not advisable drying methods for volatile preser-
vation of ginger.
In general, the mild heating process led to less impact on the
volatile components, despite its long treatment time, whereas
the intense drying method exerted significant effect on volatiles.
In addition, the vacuum treatment also had substantial influence
on volatile compounds. Based on above findings, more work is
required to improve the effect of IM&CD on ginger volatiles.
3.3. Effect of drying methods on the quantities of 6-, 8-, 10-gingerol
and 6-shogaol of ginger extract
The major pharmacologically active and pungent components
of ginger are 6-gingerol, 8-gingerol, 10-gingerol and 6-shagaol
(Yu, Huang, Yang, Liu, & Duan, 2007). Molecular structure of gin-
gerol consisted of b-hydroxyl keto functional group which is ther-
mally labile (Puengphian & Sirichote, 2008). According to Huang,
Chung, Wang, Law, and Chen (2011), the higher drying tempera-
ture would promote the decomposition of 6-gingerol or the trans-
formation of 6-gingerol to 6-shogaol. As shown in Fig. 2, the
quantity of 6-gingerol was 5.91 mg/g in fresh ginger, after drying,
the 6-gingerol was decreased significantly, especially in micro-
wave drying, only 2.12 mg/g. FD samples had a higher content of
3.54 mg/g, followed by IR, IM&CD and AD samples of 3.44, 3.21
and 2.50 mg/g respectively. It can be inferred that the high temper-
ature and long drying time would promote the degradation and
conversion of 6-gingerol. The quantity of 8-gingerol and 10-
gingerol also showed decreasing trend after drying process except
for FD process. As shown in Fig. 2, 8-gingerol and 10-gingerol were
2.52 and 2.62 mg/g in fresh ginger, 2.52 and 2.74 mg/g in FD sam-
ples, and there was no significant differences between fresh and FD
samples. IR, IM&CD, AD and MD samples were with average
content of 8-gingerol and 10-gingerol of 2.48 and 2.52 mg/g, 2.43
and 2.5 mg/g, 2.15 and 2.33 mg/g, and 1.35 and 1.05 mg/g. It
indicated that 8-, 10-gingerol were much more stable than
6-gingerol during drying, except in the intense heating process of
MD, whose higher temperature accelerated the decomposition
Table 1
Drying time, energy consumption and extraction yield of different dried gingers.
Drying methods AD IR FD MD IM&CD
Drying time (h) 12.0 ± 0.5
b
6.0 ± 0.7
c
44.5 ± 2.0
a
1.8 ± 0.3
d
1.5 ± 0.2
e
Energy consumption (kW h/g H
2
O) 3.30 ± 0.08
c
12.23 ± 0.24
b
33.7 ± 0.53
a
2.7 ± 0.12
e
3.21 ± 0.1
d
Extraction yield (v/w%) 2.69 ± 0.32
cd
2.58 ± 0.19
d
3.55 ± 0.22
a
2.97 ± 0.65
c
3.35 ± 0.55
b
AD: hot air drying; IR: infrared drying; FD: freeze drying; MD: microwave drying; IM&CD: intermittent microwave-convection drying. Values are means ± SD (n= 3). For each
column, values followed by the same small or capital superscript letter did not share significant differences at P< 0.05 (Duncan’s test).
Fig. 1. Ward connection spectrum of the similarity of total volatile components
between different drying methods.
d
a
b
b
0
1
2
3
4
5
6
Content (mg/g d.w.)
Drying methods
6-gingerol
8-gingerol
10-gingerol
6-shogoal
Fresh AD IR FD MD IM&CD
a
c
a
bc
b
b
a
e
d
c
b
Fig. 2. The changes of 6-, 8-, 10-gingerol, and 6-shogaol content of ginger extract
during AD, IR, FD, CM and IM&CD drying process. AD: hot air drying; IR: infrared
drying; FD: freeze drying; MD: microwave drying; IM&CD: intermittent micro-
wave-convection drying. For each column, values followed by the same letter (a–c)
are not statistically different at P< 0.05 as measured by Duncan’s test.
K. An et al. / Food Chemistry xxx (2015) xxx–xxx 5
Please cite this article in press as: An, K., et al. Comparison of different drying methods on Chinese ginger (Zingiber officinale Roscoe): Changes in volatiles,
chemical profile, antioxidant properties, and microstructure. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.11.033

and transformation of 8-gingerol and 10-gingerol significantly. As
shown in Fig. 2, the content of 6-shogaol was 0.09 mg/g in fresh
ginger, then increased to 0.214, 0.209, 0.221, 0.384 and
0.243 mg/g after AD, IR, FD, MD and IM&CD process. This phe-
nomenon proved that 6-shogaol was normally not present in fresh
ginger but dehydrated from gingerols during thermal drying or
storage (Bhattarai, Tran, & Duke, 2001). We can see microwave
drying resulted in the largest increase of 6-shogaol, followed by
IM&CD, indicating that higher temperature would be advantageous
for conversion of gingerol to shogaol, which was agreed with the
reported of Huang et al. (2011) and Cheng et al. (2011). We have
measured the temperature changes of ginger in the entire MD
and IM&CD process, and found that temperature in IM&CD process
was much lower and more stable than that in MD process, espe-
cially at the end of drying (data were shown in Supplementary
Fig. 2), which can well explain that why IM&CD had better reten-
tion of chemical profiles than MD process.
3.4. Effect of drying methods on the total phenolic, flavonoids content
of ginger extract
Different drying treatments were shown variable effects on
total phenolic (TPC) and total flavonoids content (TFC) of ginger
samples. As shown in Table 2, the content of TPC in fresh ginger
was 11.97 ± 0.33 mg GAE/g d.w., which is similar to the report of
Gümüsßay et al. (2015), who found the TPC of ginger from Turkey
was 13.51 ± 0.62 mg GAE/g d.w. There were many researchers
who got different results (Hinneburg, Damien, & Hiltunen, 2006;
Puengphian & Sirichote, 2008). These differences were possibly
due to different genetics, varieties and regions of ginger. In our
study, the TPC of freeze dried gingers was increased significantly
compared with fresh ones, while other thermal drying caused a
significant decrease in TPC (P< 0.05). According to Asami et al.
(2003), the increased extraction efficiency would promote the
extraction of active ingredients in dried samples, which would
increase the content of total phenols detected in FD samples. In
the thermal drying, IR, IM&CD and AD process resulted in losses
of TPC of 5.17%, 5.76% and 19.05%, respectively, and MD caused a
highest loss of 29.74%. According to Lim and Murtijaya (2007), heat
generated from microwave drying was intense and rapid, which
could cause severe thermal degradation of phenolic compounds.
Besides, activation of oxidative enzymes (polyphenoloxidase and
peroxidase) during drying process may lead to the loss of phenolic
complexes. According to Toor and Savage (2006), changes in chem-
ical structure of phenols, such as bingings of phenols to proteins
could also result in a loss of phenolic content. However, Kubra
and Rao (2012) observed an increase in TPC of MW-dried gingers
with MW power levels (385–800 W) increased. He explained that
this was ascribed to MW energy causing breakdown of cellular
constituents, resulting in higher release of polyphenols from the
matrices. Many researchers have found that TPC in various plant
spices have irregular change under different drying process
(Chan et al., 2008; Dewanto et al., 2002). Therefore, we can get
the conclusion that drying process results in high or low levels of
TPC depending on the type of plant material and the location phe-
nolic compounds present in the cell.
In terms of TFC, it had a different trend with TPC. TFC of fresh
ginger was 13.49 ± 0.36 mg Rutin/g, after drying, IM&CD led to
highest TFC of 15.42 ± 0.87 mg Rutin/g, followed by IR samples
with 14.52 ± 0.23 mg Rutin/g (Table 2). It worth noting that freeze
dried samples had lower TFC than IM&CD and IR samples, and
there was no significant differences between FD and fresh samples.
MD caused a loss of TFC of 6.97%, while AD caused the highest loss
of 10.45%, which confirmed that the loss of such macromolecules
might be caused by the combination of the duration and tempera-
ture (Schieber, Keller, & Carle, 2001). According to Toor and Savage
(2006), IM&CD has the penetrability of microwave radiation, which
would cause the breakdown of cellular constituents, making flavo-
noids more accessible during the extraction. Besides, the shorter
duration and less intense heating of IM&CD make it more advanta-
geous in preserving flavonoids. According to Niwa, Kanoh, Kasama,
and Neigishi (1988), far infrared may have the capability to break
down covalent bonds and liberate antioxidants such as flavonoids,
polyphenols carotene, tannin, ascorbate or flavoprotein from
repeating polymers, which would increase TFC in IR dried samples.
3.5. Effect of drying methods on the antioxidant activity of ginger
extract
In the present study, antioxidant activity of ginger extracts was
evaluated using DPPH, FRAP and ABTS assay. The DPPH free radical
is a stable free radical, which has been widely accepted as a tool for
estimating the free radical-scavenging activity of antioxidants. In
the DPPH test, the highest AEAC value (lowest IC
50
) was observed
in freeze dried samples, followed by IM&CD (3.58 ± 0.11 mg AA/
g d.w., IC
50
1.08 ± 0.03 mg/mL extract), IR (3.55 ± 0.14 AA mg/g d.
w., IC
50
1.09 ± 0.02 mg/mL extract) and MD (3.42 ± 0.19 AA mg/
g d.w., IC
50
1.13 ± 0.06 mg/mL extract) samples, whereas AD
method gave the lowest free radical scavenging ability (Table 2).
The DPPH scavenging ability had higher correlation with TPC
(R
2
= 0.866), and less correlation with TFC (R
2
= 0.594). Many
researchers have found the high correlations between TPC, TFC
and antioxidant activity (Velioglu, Mazza, Gao, & Oomah, 1998),
whereas others found there was no relationship (Kähkönen et al.,
1999).
As shown in Table 2, the highest FRAP value (22.14 ± 0.27 g Vc/
g d.w.) was observed in IR dried samples, followed by IM&CD
(21.91 ± 0.54 Vc g/g d.w.), FD (20.88 ± 1.19 Vc g/g d.w.), and AD
(17.41 ± 1.78 Vc g/g d.w.) samples, whereas the microwave drying
was found to exert the most negative effect. FRAP value had high
correlation with TPC (R
2
= 0.741) and TFC (R
2
= 0.850).
The values of ABTS obtained in terms of Trolox equivalent were
lower than those obtained by the FRAP assay but the overall trend
was similar (Table 2). The highest antioxidant capacity was found
in IM&CD samples, followed by FD, IR, and AD samples, whereas
the microwave drying also exerted lowest antioxidant capacity.
Table 2
Changes of total phenolic, flavonoids content, and antioxidant activity of ginger during AD, IR, FD, CM and IM&CD.
Fresh AD IR FD MD IM&CD
TPC (mg GAE/g d.w.) 11.97 ± 0.33
b
9.69 ± 0.54
d
11.35 ± 0.66
c
13.83 ± 0.31
a
8.41 ± 0.35
e
11.28 ± 0.40
c
TFC (mg Rutin/g d.w.) 13.49 ± 0.36
c
12.08 ± 1.17
d
14.52 ± 0.23
b
13.32 ± 0.52
c
12.55 ± 0.74
d
15.42 ± 0.87
a
IC
50
(mg/mL extract) 1.11 ± 0.05
c
1.15 ± 0.09
a
1.09 ± 0.02
d
1.05 ± 0.11
e
1.13 ± 0.06
b
1.08 ± 0.03
d
AEAC (mg AA/g d.w.) 3.49 ± 0.27
c
3.37 ± 0.23
d
3.55 ± 0.14
b
3.69 ± 0.21
a
3.42 ± 0.19
d
3.58 ± 0.11
b
FRAP (g Vc/g d.w.) 19.37 ± 0.81
d
17.41 ± 1.78
e
22.14 ± 0.27
a
20.88 ± 1.19
c
15.66 ± 1.21
f
21.91 ± 0.54
b
ABTS (mg Trolox/g d.w.) 64.45 ± 5.15
d
62.22 ± 3.21
e
66.79 ± 4.40
c
68.65 ± 11.55
b
60.06 ± 14.43
f
71.68 ± 6.11
a
AD: hot air drying; IR: infrared drying; FD: freeze drying; MD: microwave drying; IM&CD: intermittent microwave-convection drying. Values are means ± SD (n= 3). For each
column, values followed by the same small or capital superscript letter did not share significant differences at P< 0.05 (Duncan’s test).
6K. An et al. / Food Chemistry xxx (2015) xxx–xxx
Please cite this article in press as: An, K., et al. Comparison of different drying methods on Chinese ginger (Zingiber officinale Roscoe): Changes in volatiles,
chemical profile, antioxidant properties, and microstructure. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.11.033

ABTS value also presented high correlation with TPC (R
2
= 0.710)
and TFC (R
2
= 0.848).
3.6. Morphology of fresh and dried Zingiber officinale Roscoe
Structures of fresh ginger were observed by light microscopy
(LM). As shown in Fig. 3A, an oil cell appeared in ginger tissue
structure. According to Azian, Mustafa Kamal, and Azlina (2004),
for a fresh ginger tissue of 220 220
l
m there appeared an oil cell
with the size of 25 25
l
m. As shown in Fig. 3B, abundant starch
grains were present within the fresh ginger tissue. Both micro-
graphs of (A) and (B) showed distinct wall of the parenchyma indi-
cating no cell fracture of the fresh tissue.
Structural changes of dried gingers were observed by scanning
electron microscope (SEM) shown in Fig. 3a–e. In general, dehydra-
tion caused fractures of parenchyma cell wall, the oil cell was
absent and the starch grains were scattered all over the tissue after
drying. As shown in Fig. 3a, air-dried ginger showed more dense
structures, the cell parenchyma structure was severely damaged
and the starch grains were not well preserved. In the freeze drying,
as shown in Fig. 3c, the skeleton structure of ginger was well
retained because the removal of water occurs by sublimation from
frozen substances with the simultaneous effect of the vacuum.
Therefore, the freeze dried samples had less-dense texture and rel-
ative complete cell structure, which was agreed with the observa-
tion of Huang et al. (2011). In the microwave drying, due to the
rapid conversion of microwave radiation, the inner moisture was
difficult to evaporate outside. Therefore, the accumulated inner
moisture caused cellular structures crosslinked together and starch
grains showed a higher degree of gelatinization (Fig. 3d). In the IR
and IM&CD samples (Fig. 3b and e), the cellular structures were
well retained and much similar to that of freeze dried samples.
Starch grains were preserved intact and showed little degree of
gelatinization. This was because of the less intense heating and
shorter duration of IR and IM&CD.
3.7. Principal component analysis
Principal component analysis (PCA) was applied to observe any
possible clusters within these five drying methods. The clustering
variety was considered caused by the entire physico-chemical
and antioxidant properties. As shown in Fig. 4, the cumulative
contribution of the first and the second principal components
attained 97.99%. PC1 was highly contributed by 6-gingerol
(0.818), 8-gingerol (0.718), 10-gingerol (0.610), 6-shogaol
(0.466), TPC (0.797), TFC (0.975), AEAC (0.712), FRAP (0.942)
and ABTS (0.907). PC2 was mainly correlated to drying time
(0.996), energy consumption (0.964), extraction yield (0.551).
The PC1 and PC2 scores of FD were found to be much higher than
other drying methods as it had higher content of 6, 8, 10-gingerol,
6-shogaol, TPC, TFC, higher antioxidant activities and higher
extraction yield at the cost of higher drying time and energy con-
sumption. MD was highly negatively correlated with both PC1
and PC2, as it had negative effects on active component content
and antioxidant activity with less drying time and energy
consumption. It was worth noting that IM&CD and IR could be
clustered into one group as they had highly positive correlations
with PC1, indicating they had very good impact on physico-
chemical and antioxidant properties.
Fig. 3. Light microscopy image of fresh ginger: (A) (250); B (400); scanning electron micrographs of dried gingers: (a) AD (1000); (b) IR (1000); (c) FD (500); (d) MD
(200); (e) IM&CD (1000). AD: hot air drying; IR: infrared drying; FD: freeze drying; MD: microwave drying; IM&CD: intermittent microwave-convection drying.
K. An et al. / Food Chemistry xxx (2015) xxx–xxx 7
Please cite this article in press as: An, K., et al. Comparison of different drying methods on Chinese ginger (Zingiber officinale Roscoe): Changes in volatiles,
chemical profile, antioxidant properties, and microstructure. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.11.033

4. Conclusions
Based on the results of present investigation, we conclude that
drying methods and conditions have profound effect on the quality
and energy consumption of the dehydrated product. Compared
with AD and MD process, FD, IR and IM&CD had higher retention
of chemical profiles, antioxidant activity and cellular structures,
which was attributed to their less intense heating. However, FD
and IR had relatively higher energy consumption and drying time,
especially freeze drying. Therefore, IM&CD is a very promising
technology for high sensitive products like fruits and vegetables
due to its higher efficiency, good quality retention and lower cost,
which had a broad market prospect for commercial-scale
application.
Acknowledgments
Special thanks were given to Prof. Wang Zhengfu for his techni-
cal support of the intermittent microwave & convective drying
equipment and helpful recommendations for the experiment. We
also thank the financial support of Province Natural Science Fund
of Guangdong (2014A030310208) and Province Science and Tech-
nology Plan Projects of Guangdong (2013B020203001) for this
research.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.foodchem.2015.
11.033.
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Please cite this article in press as: An, K., et al. Comparison of different drying methods on Chinese ginger (Zingiber officinale Roscoe): Changes in volatiles,
chemical profile, antioxidant properties, and microstructure. Food Chemistry (2015), http://dx.doi.org/10.1016/j.foodchem.2015.11.033