The impact of postharvest dehydration methods on qualitative attributes and chemical composition of Xynisteri grape (Vitis vinifera) must

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Postharvest Biology and Technology
journal homepage: www.elsevier.com/locate/postharvbio
The impact of postharvest dehydration methods on qualitative attributes
and chemical composition of ‘Xynisteri’ grape (Vitis vinifera) must
Savvas Constantinoua, Ana Maria Gómez-Caravacab, Vlasios Goulasa,⁎,
Antonio Segura-Carreterob,c, Stefanos Koundourasd, George A. Manganarisa
a Department of Agricultural Sciences, Biotechnology and Food Science, Cyprus University of Technology, 3603, Cyprus
b Department of Analytical Chemistry, University of Granada, c/Fuentenueva s/n, 18071 Granada, Spain
c Research and Development of Functional Food Centre (CIDAF), Granada, Spain
d Laboratory of Viticulture, School of Agriculture, Aristotle University, 54124 Thessaloniki, Greece
A R T I C L E I N F O
Keywords:
Commandaria
Phenolic compounds
Postharvest dehydration
Bound volatiles
Melanoidins
LC-DAD-qTOF-MS
A B S T R A C T
The objective of this study was to compare the effect of traditional sun-drying method (TM) with four alternative
dehydration methods [(a) multiple horizontal wires (MHW), (b) multiple vertical pallets (MVP), (c) low
greenhouse (LGH) and (d) hot-air dryer treatment (HAD)] on phenolic composition, oenological parameters,
aroma potential and browning compounds of musts obtained from dehydrated grapes (Vitis vinifera cv.
‘Xynisteri’). Dehydrated grapes of the examined cultivar are being used to produce ‘Commandaria’ dessert wine,
a protected designation of origin product in Cyprus. LGH and HAD treatments led to a significant reduction of
the dehydration period. Soluble solid contents were used to monitor the progress of dehydration process; no
changes among the examined dehydration methods in reducing sugar composition were found. Notably, HAD led
to a dramatic rise (3.2-fold) of titratable acidity that was obviously not related only to the concentration effect.
Furthermore, all dehydration methods concentrated total bound volatiles and induced the formation of brown
pigments. Based on the Folin-Ciocalteu index, only HAD and LGH induced a significant increase in total phenolic
content in dehydrated grape musts. Subsequently, forty phenolic compounds were identified and quantified by
LC-DAD-qTOF-MS. Results showed a significant effect of dehydration methods that vary according to the dif-
ferent groups of phenolic compounds considered. Similarly to Folin-Ciocalteu index, HAD and LGH methods
increased significantly the phenolic content in grape musts, whereas MHW and MVP methods increased it
slightly higher than the concentration factor. Flavonols, flavan-3-ols and flavanonols were the most affected
polyphenolic groups. A significant increment of hydroxybenzoic and hydroxycinnamic acids, the predominant
groups of phenolic compounds found in ‘Xynisteri’ grapes, was monitored. Taking into consideration that HAD
cannot be exploited under the existing legal framework, LGH showed the greatest potential for the production of
high quality dehydrated ‘Xynisteri’ grape must.
1. Introduction
Cyprus is one of the oldest vine growing countries with current vi-
neyard areas covering ca. 9.000 ha, almost entirely on own-roots as
Cyprus remains one of the few phylloxera-free wine producing regions
in the world. ‘Xynisteri’ is an indigenous Cypriot white grape (Vitis vi-
nifera L.) cultivar, accounting for ca. 30% of vineyards, with an in-
creasing trend. Apart from white dry wine production, ‘Xynisteri’ con-
tributes, jointly with the local red cultivar ‘Mavro’ (ca. 40% of Cypriot
vineyards), in the elaboration of the dessert wine ‘Commandaria’, a
protected designation of origin (PDO) product with unique organoleptic
properties (Constantinou et al., 2017). In the traditional ‘Commandaria’
production process, grapes are spread on nets placed in large open sites
with suitable orientation and gentle slope in order to be dehydrated;
this procedure lasts approximately two weeks (Constantinou et al.,
2017). High daytime air temperature and low relative humidity are
considered optimum conditions for sun-drying that accelerate berry
dehydration and facilitate control of potential fungal diseases
(Serratosa et al., 2008).
Grape dehydration process for the production of dessert wines
varies worldwide, depending on the style of wine, geographical loca-
tion, grape cultivar and viticultural practices (Esmaiili et al., 2007;
Figueiredo-Gonzalez et al., 2013; Pangavhane and Sawhney, 2002;
Torchio et al., 2016). Dehydration methods can be classified into three
http://dx.doi.org/10.1016/j.postharvbio.2017.09.005
Received 12 June 2017; Received in revised form 5 September 2017; Accepted 12 September 2017
⁎ Corresponding author.
E-mail address: vlasios.goulas@cut.ac.cy (V. Goulas).
Postharvest Biology and Technology 135 (2018) 114–122
Available online 26 September 2017
0925-5214/ © 2017 Elsevier B.V. All rights reserved.
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distinct groups, namely sun-drying, shade drying and mechanical
drying (Pangavhane and Sawhney, 2002). Other terms that have been
additionally used are: natural withering, on-vine withering and forced
withering (Fregoni, 2005), while Valero et al. (2008) proposed the
following five main classifications for the grape dehydration methods:
sun-drying, warm chamber, fresh chamber, noble rot and ice.
Sun-drying is the most widely used grape dehydration method in
warm grape-growing regions, such as the Mediterranean area
(Figueiredo-Gonzalez et al., 2013). Except for ‘Commandaria’, direct
exposure to the sun is also applied during the production of some
‘Passito’, ‘Malaga’ and ‘Pedro Ximenez’ wines (Bellincontro et al., 2004;
Valero et al., 2008; Mencarelli and Bellincontro, 2013; Scienza, 2013).
The drying process under cover includes natural dehydration of grapes
in shaded conditions as in the case of ‘VinSanto’ and ‘Recioto di Soave’
wine production (Bellincontro et al., 2004; Figueiredo-Gonzalez et al.,
2013). Mechanical drying implies that the drying process is operated in
closed chambers with artificial control for temperature, relative hu-
midity and air-flow as in the case of ‘Amarone’ and ‘Recioto’ wine
production in the Valpolicella area, ‘Moscato Passito’ production in
Piedmont and ‘Vin de paille’ production in the Jura wine region
(Bellincontro et al., 2004; Accordini, 2013; Eberle, 2013; Teissedre
et al., 2013).
In general, traditional sun-drying is very difficult to be controlled
and the lack of consistent conditions often cancels out the differences in
the quality between different grape cultivars or origins, especially in
terms of primary aroma (Mencarelli and Bellincontro, 2013). Previous
study on ‘Malvasia delle Lipari’ sweet wine also highlighted significant
differences between wines produced from sun-dried and shade-dried
grapes, as free and glycosilated volatile fractions were quantitatively
enriched by dehydration under shaded conditions (Piombino et al.,
2010). Figueiredo-Gonzalez et al. (2013) summarized the importance of
dehydration method on colour and phenolic composition of sweet
wines with the traditional sun-drying being associated with strong en-
zymatic oxidation of some phenolic fractions, such as hydroxycinnamic
acids, anthocyanins and flavan-3-ol derivatives, contributing to the
browning of the grapes. Furthermore, considerable production loss and
quality problems may occur during sun-drying due to several factors,
such as rodents, birds, insects, microorganisms and rain incidents over
the drying period (Esmaiili et al., 2007; Frangipane et al., 2012;
Pangavhane and Sawhney, 2002). Other limiting factors of the tradi-
tional sun-dryingprocess is that it can be highly time consuming and
requires large areas.
Taking into consideration the significant effect of dehydration
method on the composition of grape musts and wines and the afore-
mentioned limitations of grape sun exposure, the elaboration of alter-
native grape dehydration processes stands as a challenging perspective
in improving the quality of grape musts, destined for dessert wine
production. Therefore, the potency to exploit new dehydration pro-
cesses for the production of ‘Commandaria’ wine was investigated. In
particular, the impact of five dehydration processes on chemical com-
position parameters of the must obtained from ‘Xynisteri’ dehydrated
grapes was assessed, with special reference to the phenolic composition,
aroma potential and melanoidin content of grapes musts.
2. Materials and methods
2.1. Grapes sampling procedure
Five hundred kilograms of ‘Xynisteri’ grapes were hand harvested
on the 20th of September 2014 with a sugar content of 250 g L−1 from
a 30-year-old, own-rooted vineyard located at ‘Agios Mamas’ village,
Lemesos district, Cyprus (32°94′N, 34°84′E, 600 m). In particular, the
vineyard was planted on a limestone soil, at 2.200 plants/ha
(2.1 × 2.1 m), trained as traditional ‘gobelet’ without irrigation. The
climate of the region is dry with less than 30 mm of summer rainfall
(June to August), while the average midday temperature and air hu-
midity during the summer months of on-vine ripening were ca. 30.0 °C
and 30%, respectively.
From the harvested grapes, three 20 kg batches were collected to
extract the must used as control while the rest was used for dehydration
with different processes as described below. At the end of dehydration,
three 5 kg batches from each process were manually crushed and
pressed in a laboratory vertical press (Torchietto Premitutto, Italy),
similar to industrial models. In all cases, the obtained must was clarified
by centrifugation at 4000 × g for 15 min and preserved at −20 °C until
needed.
2.2. Dehydration methods
The traditional sun-drying method (TM) was compared with dehy-
dration (a) on multiple horizontal wires (MHW), (b) on multiple ver-
tical pallets (MVP), (c) in low greenhouse (LGH) and (d) with the use of
a hot-air dryer (HAD). All methods, except for HAD, were performed at
the same place with the traditional sun-drying, next to the vineyard.
During the dehydration period that took place during the end of
September – early October, the average midday temperature of the
region was about 27.0 °C, with 40% relative air humidity. During the
initial dehydration period (20 September–5 October), the weather was
sunny with high midday temperature and intermediate air humidity.
Thereafter and especially after October 10th, the weather was occa-
sionally cloudy, temperature significantly decreased and air humidity
increased, resulting in a marked decrease of the dehydration rate.
For the traditional sun-drying method, about 100 kg of grapes were
spread on nets placed in large open sites with southeast orientation and
Fig. 1. Schematic representation of set-up for
three dehydration methods applied: multiple
horizontal wires (A), multiple vertical pallets
(B), low greenhouse (C).
S. Constantinou et al. Postharvest Biology and Technology 135 (2018) 114–122
115
Tássia Nievierowski
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Tássia Nievierowski
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gentle slope. The sun-drying process lasted 10 days. For the Multiple
horizontal wires (MHW), two metal poles with their respective holds on
both sides, placed in the ground at a distance of 2 m (Fig. 1A). Then,
five wires of two meters length each, were placed horizontally with a
spacing of 0.4 m, where bunches were hanged one beside the other,
without any further elaboration. In each wire, ca. 20 kg of fresh grapes
were hanged. The sun-drying process lasted 22 days. Multiple vertical
pallets (MVP) consisted of five pallets of dimensions 1 × 1 m which
placed vertically with 0.4 m spacing one to the other (Fig. 1B). Each
pallet was fitted with plastic net with wooden frames. At the top, a
wooden cover to protect the grapes from rain or excessive sun rays was
placed. The pallets were loaded with ca. 20 kg fruit material each. Due
to the particular structure of this dryer, grapes were exposed directly to
the sun during the morning and afternoon hours, while during the
middle of the day, the grapes were shaded by the roof or by the pallet
above them. The sun-drying process lasted 18 days. Low greenhouse
system (LGH) was composed of a greenhouse of 5 m length, 1 m width
and a height of 0.7 m, constructed from a metal frame and plastic nylon
(Fig. 1C). Inside the greenhouse, a plastic net was placed on the ground
where about 20 kg m−2 of fresh grapes were spread on this. Then, the
LGH was closed without any possibility of ventilation. The dehydration
process lasted 6 days. Finally, for the hot-air dryer (HAD) treatment, ca.
60 kg of fresh grapes were placed on three trays within a laboratory
hot-air dryer (Venticell 111, MMM Group, Germany). Fans were in-
stalled in order to ensure a constant temperature of 40 °C and the de-
hydration process lasted 5 days.
The dehydration period for TM, LGH and HAD was terminated when
soluble solids content attained ca. 36% which is the lower threshold set
by the Cypriot legislation for the production of ‘Commandaria’ wine.
Notably, MHW and MVP dehydration processes were terminated when
the soluble solids content was constant (32.3% and 33.8%, respec-
tively) after 18 and 22 days of dehydration, respectively, in order to
avoid grape deterioration due to adverse weather conditions, as pre-
viously mentioned.
2.3. Calculation of dehydration factor
To study the response of each compound or parameter after the
implementation of the dehydration methods, a dehydration factor (DF)
was calculated by dividing the SSC of must obtained from ‘Xynisteri’
grapes before and after dehydration (Table 1) (Ruiz et al., 2010). De-
hydration factor is a useful index whether compounds were synthe-
sized, degraded or transformed into other compounds during the de-
hydration procedure; a 20% of error around the DF was additionally
taken into consideration as elsewhere described (Constantinou et al.,
2017)
2.4. Qualitative attributes
Soluble solids content (SSC), reducing sugars, titratable acidity (TA)
and pH were determined according to the methods described by the
International Organization of Vine and Wine (OIV, 2012).
For the determination of SSC, a portable digital refractometer
(Master Baume 2594, Atago, Japan) was used. Glucose and fructose
were analyzed by high-performance liquid chromatography (HPLC,
Shimadzu Corporation, Kyoto, Japan), connected to a refractometric
detector (RID, Shimadzu Corporation, Kyoto, Japan). Once samples
were filtered, they were passed over a filter cartridge C18 in order to
remove phenolic compounds. Then, a volume of 20 μL per sample was
injected into the Luna® (30 × 4.6 mm id, 5-μm) column (Phenomenex,
Cheshire, UK). The elution was carried out with a mobile phase of
acetonitrile/water (80/20, v/v), delivered at 1 mL min−1. The de-
termination of titratable acidity (TA) was contacted by potentiometric
titration with 0.1 mol L−1 NaOH up to pH 8.1, using 5 mL juice diluted
in distilled water until final volume of 25 mL. The measurements were
carried out using a DL22 Mettler Toledo titrator (Mettler-Toledo, Inc.,
Columbus, Ohio, USA). The pH values were measured with a pH-meter
(HI 2222, Hanna instruments, Inc., Woonsocket, Rhode Island, USA).
All measurements were carried out in triplicate.
2.5. Phenol-free glycosyl glucose (PFGG)
Extraction and isolation of PFGG was conducted according to
Whiton and Zoecklein (2002), with slight modifications. A volume of
15 mL of each sample was adjusted to pH 13 using 10 mol L−1 NaOH,
loaded onto 1 g Oasis HLB cartridges, conditioned with 10 mL methanol
(HPLC grade) plus 10 mL Milli-Q water. The cartridge was then washed
three times with 20 mL of water and the glucosides were eluted with
1.5 mL absoluteethanol, followed by 3.5 mL distilled water. Flow rate
was 3 mL min−1.
A volume of 0.5 mL of the glucoside fraction was mixed with 1 mL
of H2SO4 (2.25 mol L−1) in order to prepare solutions for hydrolysis
containing 1.5 M H2SO4 and 10% (v/v) ethanol. A blank was similarly
prepared with 30% (v/v) ethanol in place of the glucoside fraction. The
samples and the blank were heated at 100 °C for 60 min. After cooling,
1 mL of each sample was neutralized with 1 mL of NaOH (3 mol L−1)
and 0.5 mL triethanolamine buffer (0.2 mol L−1). The glucose released
in the hydrolysates was finally determined, using a glucose (HK) assay
kit (GAHK-20, Sigma-Aldrich).
2.6. Browning index and melanoidins content
Grape must was dialyzed using cellulose dialysis tubing that retains
molecules from 12 to 14 kDa. In particular, a quantity of 15 mL of must
was put into the dialysis tubing which was placed in a glass vessel with
1 L of deionized water. The solution was stirred for 12 h at 5 °C. This
procedure was repeated once. The volume of must which remained in
the dialysis tubing was diluted to 50 mL. The brown pigments and
melanoidins of the samples were determined by measuring its absor-
bance at 420 nm and 345 nm, respectively (Rivero-Pérez et al., 2002).
Spectrophotometric measurements of samples were carried out, using
an InfinitePro 200 (Tecan, Mannedorf, CH) microplate reader.
2.7. Total phenolic content
A volume of 100 μL of must was mixed with 5 mL of distilled water,
0.5 mL of Folin-Ciocalteu reagent and 2 mL of 20% (w/v) sodium car-
bonate. The mixture was standing for 30 min in the dark and the ab-
sorbance at 750 nm was registered. Total phenolic content (TPC) was
expressed as mg L−1 gallic acid equivalents of must (OIV, 2012).
2.8. Identification and quantification of individual polyphenols by LC-DAD-
qTOF-MS
An Agilent 1200-LC system (Agilent Technologies, Palo Alto,
California, USA) equipped with a vacuum degasser, auto sampler, a
binary pump and a DAD was used for the chromatographic analyses.
The separation was performed using a Poroshell 120 EC-C18 analytical
Table 1
Calculation of dehydration factor (DF) was determined by dividing the soluble solid
contents (SSC) of must obtained from ‘Xynisteri’ grapes dehydrated with the traditional
sun-drying method (TM), on multiple horizontal wires (MHW), on multiple vertical
pallets (MVP), at low greenhouse (LGH) and with the use of a hot-air dryer (HAD), by SSC
of fresh grape (FG).
Dehydration Process DF DF ± 20%
TM 1.4 1.1–1.7
MHW 1.3 1.0–1.6
MVP 1.3 1.0–1.6
LGH 1.4 1.1–1.7
HAD 1.6 1.3–1.9
S. Constantinou et al. Postharvest Biology and Technology 135 (2018) 114–122
116
Tássia Nievierowski
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column (4.6 mm× 100 mm, particle size 2.7 μm), operating at 25 °C
and at a flow rate of 0.8 mL min−1. The mobile phases used were water
with acetic acid (1%, v/v) (Phase A) and acetonitrile (Phase B) and the
gradient used the following conditions: 0 min, 0.8% B; 2.5 min, 0.8% B;
5.5 min, 10% B; 11 min, 10% B; 17 min, 20% B; 22 min, 30% B; 26 min,
100% B; 28 min, 100% B; 30 min, 0.8% B; and finally a conditioning
cycle of 3 min with the initial conditions. A volume of 6 μL of each
sample was injected. Three replicates of each extract were performed.
MS analyses were done using a 6540 Agilent Ultra-High-Definition
Accurate-Mass qTOF-MS coupled to the HPLC, equipped with Agilent
Dual Jet Stream electrospray ionization (Dual AJS ESI) interface. The
negative ionization mode was used and the conditions were as follows:
drying gas flow (N2), 12.0 L min−1; nebulizer pressure, 50 psi; gas
drying temperature, 360 °C; capillary voltage, 3500 V; fragmentor
voltage and scan range were 3500 V and m/z 50–1300, respectively.
Automatic MS/MS experiments were carried out using the followings
collision energy values: m/z 100, 30 eV; m/z 500, 35 eV; m/z 1000,
40 eV; and m/z 1500, 45 eV. Data elaboration was performed using the
MassHunter Workstation software (Agilent Technologies, Santa Clara,
CA, USA).
The quantification was performed according to our previous study
for grape musts (Constantinou et al., 2017). Notably, the response of
the standards could be different from the response of the derivatives
present in grape must samples, and consequently, the quantification of
these compounds is only an estimation of their actual concentrations.
2.9. Statistical analysis
The R-statistic software package was used. In this way, LSD multiple
range test was performed to study the changes in analytical parameters,
phenolics and aroma potential in must before and after the dehydration
processes. The results given are the average of three independent tests
and are expressed as mean ± standard error.
3. Results and discussion
3.1. Qualitative attributes
SSC was used as index to monitor dehydration processes as it is
defined by the relevant legislation for the production of ‘Commandaria’
wine. Grape must obtained from grapes dehydrated with HAD method
had higher SSC than those of the grapes dehydrated with the other
methods, as a result of the highest dehydration rate (Table 2). On the
contrary, in MWH and MVP methods, dehydration proceeded slowly
producing grape musts with lower SSC than the statutory threshold.
Furthermore, results showed no effect of dehydration method on
composition of reducing sugars in grape musts (Table 2).
In grape must glucose and fructose represent ca. 95% of the total
dissolved solids, which gives the °Brix reading; a range of other car-
bohydrates including pectins, dextrans and pentoses are also found in
grapes. In overripe grapes, the amount of glucose and fructose often
appears higher than the corresponding °Brix results. This is due to the
fact that °Brix is measured as a percentage by weight, which is greatly
influenced by the density of the must, while glucose and fructose
measured as weight by volume and are independent of must density.
The dehydration process also had a significant effect on TA and pH
values; as TA ranged between 2.6 and 8.2 g L−1 tartaric acid equiva-
lents. In particular, HAD method led to a significant increment of TA,
higher than the concentration effect (Tables 1 and 2). In general, rapid
dehydration process is usually accompanied by a steep increase in acid
concentration and cell death (Chkaiban et al., 2007). In TM method, the
increase of TA was also higher than the concentration effect, while in
the rest of methods, dehydrated grape musts increase in TA was only
associated with the concentration effect.
The pH values in dehydrated grape musts ranged between 3.81 and
4.02. No strong correlation between pH values and TA was found,
possibly due to the percentage of acid salification. According to Miele
and Rizzon (2013), as grape skins degrade, potassium is released into
the must where it reacts with tartaric acid and thereby increases wine
pH. Hence, the dehydration process, by affecting the extent of cell de-
gradation of the internal layers of the skin, can ultimately lead to
changes in wine tartaric stability and the choice of stabilization method
to be applied.
3.2. Aroma potential
Results indicated differences in aroma potential of ‘Xynisteri’ de-
hydrated grape musts, as a result of the dehydration method applied.
More specifically, the higher aroma potential, as estimated by the
PFGG, was observed in must obtained from grapes dehydrated with
HAD (195.7 ± 11.7 mmol L−1), followed by LGH and TM
(138.8 ± 7 mmol L−1 and 124.4 ± 4.8 mmol L−1, respectively)
(Table 3). Previous study reported that the PFGG in musts obtained
from other white grape cultivars ranged from 50 to 250 mmol L−1
(Arevalo Villena et al., 2006). On the other hand, MHW and MVP
methods produced musts with PFGG values non statistically significant
different from those obtained with TM method. However, taking into
consideration the DF (Table 2), it can be deduced that the aroma po-
tential of grapes underwent no substantial changes related to the im-
plementation of the different dehydration procedures. Previous study
demonstrated an increaseof 20% for bound terpenoids in wines, which
were produced under shade-drying conditions with significant reduc-
tion of the average ambient temperature, in comparison to wines pro-
duced from grapes dehydrated under direct sunlight (Piombino et al.,
2010). Thus, provided that a 20% of error around the DF was ad-
ditionally taken into consideration, our data cannot pinpoint marked
differences between dehydration methods. However, it should be
mentioned that volatiles of grapes dried at high temperature derive not
only from fresh grapes, but mainly from the oxidative degradation of
unsaturated fatty acids and the Maillard reaction (Guiné et al., 2010). In
Table 2
Soluble solids content (SSC), reducing sugars, titratable acidity (TA) and pH of must obtained from ‘Xynisteri’ grapes (FG), before and after dehydration with the traditional sun-drying
method (TM), on multiple horizontal wires (MHW), on multiple vertical pallets (MVP), at low greenhouse (LGH) and with the use of a hot-air dryer (HAD). Each value is the
mean ± standard error of three biological repeats.
Dehydration Process Soluble Solid Contents (%) Reducing sugars (g L−1) Titratable acidity (g L−1 tartaric acid equivalents) pH Dehydration days
Glucose Fructose
FG 25.1 ± 0.7d* 120 ± 3.2d 122 ± 3.6d 2.6 ± 0.1d 3.71 ± 0.1d –
TM 35.5 ± 0.9b 188.1 ± 4.7b 188.7 ± 5.3b 4.8 ± 0.2b 3.92 ± 0.1b 10
MHW 33.3 ± 0.8c 174.1 ± 4.1c 175.1 ± 4.6c 3.6 ± 0.2c 3.87 ± 0.1bc 22
MVP 33.8 ± 0.7c 177.2 ± 4.3c 178.3 ± 4.2c 3.7 ± 0.1c 3.81 ± 0.1c 18
LGH 36.0 ± 0.9b 190.9 ± 4.5b 192.2 ± 4.9b 3.7 ± 0.2c 4.02 ± 0.1a 6
HAD 40.7 ± 1.1a 221.1 ± 5.7a 222.5 ± 5.5a 8.2 ± 0.3a 3.55 ± 0.1e 5
* Values within each column followed by the same letter are not statistically significant according to Duncan’s multiple range test at a significance level of p 50 °C) on the surface of grape skins during
midday, can result in the formation of melanoidins (Serratosa et al.,
2011), typically during the last stages of the Maillard reaction (Rivero-
Pérez et al., 2002; Wang et al., 2011). Serratosa et al. (2008), described
the difficulty to unravel the contribution of each pathway to the
browning of the dehydrated grapes, since the grapes contain high
concentrations of sugars during the dehydration process that may in-
hibit the browning action of PPO (Radler, 1964); moreover, the high
temperature and the gradual decrease of water activity can facilitate the
progress of the Maillard reaction (Rivero-Pérez et al., 2002). Regarding
LGH-treated grapes, their low amount of brown pigments and mela-
noidins content can be linked with a decline of enzymatic reactions,
since this method is closed-air in comparison with the other methods
which are open-air (Figueiredo-Gonzalez et al., 2013).
3.4. Total phenolic contents
All dehydration methods resulted in increased total phenolics con-
tent in the must, compared to fresh grapes (FG) (Table 3). After dehy-
dration with HAD and LGH, a 3.4 and 2.4 fold increase of total phe-
nolics was monitored, respectively, compared to FG. In particular, total
phenolic content of musts obtained from grapes dehydrated with HAD
and LGH methods were 1468.0 ± 61.3 and 1042.2 ± 2.0 mg L−1
GAE, respectively. The hydrolysis of some polymerized phenols and the
induction of some metabolic pathways that occur during the dehydra-
tion period may contribute to a further increment of phenolics
(Constantinou et al., 2017; Panceri et al., 2013). The dehydration with
TM, MHW and MVP led to significant, yet less prominent increases, in
‘Xynisteri’ phenolic content compared to FG. TM-dehydrated grapes
possessed the lowest phenolic content among the dehydration methods
applied. The prolonged dehydration period maybe responsible for the
degradation/oxidation of phenolic compounds as PPO enzyme activity
remains high during the sun-drying process (Wang et al., 2016).
3.5. Influence of dehydration method on grape polyphenols
Forty phenolic compounds were identified by HPLC-DAD-qTOF-MS
in ‘Xynisteri’ musts obtained from all dehydration processes applied.
The identification of each peak was based on their relative retention
time values, their UV–vis spectra, their mass spectra, and information
from the literature. The retention time, molecular formula, experi-
mental and calculated m/z, score, error, MS/MS fragments and pro-
posed compound are summarized in our previous work (Constantinou
et al., 2017). The phenolic groups where the identified compounds
belong are: hydroxybenzoic acids, hydroxycinnamic acids, flavonols,
flavan-3-ols, flavanonols and lignans. According to Constantinou et al.
(2017), all phenolic compounds, except for gallic acid hexoside (isomer
3) had previously been identified in ‘Xynisteri’ grape must.
In the current study, the hydroxybenzoic acids were theTable 3
Phenol free glucosyl-glucose, browning index, melanoidins and total phenols content in must obtained from ‘Xynisteri’ grapes (FG), before and after dehydration with the traditional sun-
drying method (TM), on multiple horizontal wires (MHW), on multiple vertical pallets (MVP), at low greenhouse (LGH) and with the use of a hot-air dryer (HAD). Each value is the
mean ± standard error of three biological repeats.
Dehydration process Phenol free glucosyl-glucose (mmol L−1) Browning Index (a.ua) Melanoidins (a.ua) Total phenols (mg L−1 GAEb))
Fresh Grape 107.5 ± 7.0d*** 0.250 ± 0.01e 0.759 ± 0.024d 427.0 ± 14.5e
TM 124.4 ± 4.8c 0.528 ± 0.007b 1.126 ± 0.009b 513.3 ± 14.2d
MHW 116.9 ± 4.3cd 0.438 ± 0.016c 1.140 ± 0.03b 586.9 ± 13.1c
MVP 112.1 ± 7.0cd 0.521 ± 0.04b 1.215 ± 0.044a 607.7 ± 6.8c
LGH 138.8 ± 7.0b 0.288 ± 0.04d 0.885 ± 0.047c 1042.0 ± 2.0b
HAD 195.7 ± 11.7a 0.570 ± 0.008a 1.202 ± 0.015a 1468.0 ± 61.3a
a a.u: absorbance units.
b GAE: gallic acid equivalents.
*** Values within each column followed by the same letter are not statistically significant according to LSD multiple range test at a significance level of pcontent, while the dehydration
with the other methods showed a significant increase (1.7 to 6.0-folds)
which was higher than the concentration effect (Fig. 2). The most sig-
nificant changes were the increase in coutaric acid glucoside under TM,
the increase in caftaric acid isomer and fertaric acid under MHW and
the increase in caftaric acid isomer and caftaric acid under MVP, LGH
and HAD. Significant decreases were also recorded for deferuloyl
hexoside pentoside under MVP and for caffeic acid dihexoside under
LGH.
Flavonols are a class of flavonoid compounds widely found in Vitis
vinifera L. grape berry skins, being usually present solely as 3-glucosides
(Castillo-Muñoz et al., 2007). Flavonol contents were significantly
lower than phenolic acid contents; however, they play an important
role in wine co-pigmentation and are useful markers in grape taxonomy
(Flamini et al., 2013). In the present study, total flavonol content was
1.29 mg L−1 for must obtained from FG, while after dehydration with
TM, MHW, MVP, LGH and HAD their concentrations were 4.54 mg L−1,
5.69 mg L−1, 6.61 mg L−1, 25.44 mg L−1 and 16.80 mg L−1, respec-
tively (Table 4). In all samples, the main flavonol was quercetin glu-
curonide. DF’s revealed a significant increase of flavonol content in all
musts obtained from dehydrated grapes, which was not only related to
the concentration effect (Fig. 2).
Flavonol contents in dehydrated grape musts increased from 3.5-
fold (TM) to 19.7-fold (LGH). This dramatic increase in flavonols con-
tent can be related to the diffusion from solid portions of the grapes to
their pulp at high temperatures. Furthermore, the postharvest berry
dehydration selectively affects specific phenylpropanoid pathways in
skins and also the physiological responses which may be related to
stress adaptation are partially modulated by the rate and the intensity
of the water loss (Bonghi et al., 2012). Additionally, the significant
enrichment of flavonols on LGH-dehydrated grapes can be attributed to
the increment of temperature and relative humidity within this close-air
type system (Bellincontro et al., 2009; Mencarelli et al., 2010; Peinado
et al., 2010; Serratosa et al., 2011; Marquez et al., 2012). A previous
study, also, has shown that different temperatures had modulated their
metabolism in ‘Aleatico’ grape berries subjected to postharvest dehy-
dration (Antelmi et al., 2010).
Flavan-3-ols, also called condensed tannins or proanthocyanidins,
are of great importance for wine quality due to their astringent, bitter
properties and their role in color stability (Lorrain et al., 2013). In the
present study, the contribution of flavan-3-ols in total phenolic content
determined in musts obtained from grapes dehydrated with TM, MHW,
MVP, LGH and HAD was 0.49%, 0.43%, 0.34%, 0.77%, 3.33% and
2.94%, respectively, since these compounds are mostly detected in the
skins and seeds of grape berries (Harbertson et al., 2002). Catechin was
the sole flavan-3-ol found in must obtained from fleshly harvested and
TM- and MHW- dehydrated grapes (Table 4). Furthermore, in must
obtained from grapes dehydrated with MVP and HAD, catechin ac-
counted for 69.4% and 50.0% of total flavan-3-ols, respectively, while
in the must obtained from grapes dehydrated with LGH, the procya-
nidin dimer (isomer 1) and catechin were the major flavan-3-ols, ac-
counting for 89.4% of total. Based on DF, flavan-3-ols underwent no
substantial changes after the implementation of TM and MHW. On the
other hand, under MVP, LGH and HAD, flavan-3-ol contents rise was
substantially higher than the concentration effect (Fig. 2); a 21.6-fold
and 23.8-fold increase of flavan-3-ol contents were found for LGH and
HAD methods, respectively.
Astilbin was determined in must obtained from FG, as well as in
must obtained from grapes dehydrated with TM, MHW, MVP, LGH and
HAD at 0.01 mg L−1, 0.02 mg L−1, 0.02 mg L−1, 0.02 mg L−1,
0.04 mg L−1 and 0.45 mg L−1, respectively. Taking into consideration
the DF, it can be concluded that astilbin had a significant increase after
dehydration with MHW, LGH and HAD methods, while it showed no
significant variation under TM and MVP (Table 4).
Piceid was the only stilbene found (Table 4), in accordance with
previous study of our group (Constantinou et al., 2017). In particular,
piceid was detected only in must obtained from grapes dehydrated with
Fig. 2. Comparison of phenolic composition
in ‘Xynisteri’ grape musts after the comple-
tion of dehydration with traditional method
(TM), multiple horizontal wires (MWH),
multiple vertical pallets (MVP), low green-
house (LGH) and hot-air dryer (HAD).
S. Constantinou et al. Postharvest Biology and Technology 135 (2018) 114–122
120
HAD at concentration of 0.94 mg L−1, accounting for 0.8% of total
phenols. According to Mencarelli et al. (2010), at high dehydration
temperature (i.e., 30 °C), biosynthesis for defense compounds, such as
stilbenes, occurs rapidly, but immediately declines thereafter due to the
physical alteration of cells and rapid enzymatic and non-enzymatic
oxidation, which lead to cell death.
Regarding lignans, the isolariciresinol-β-4′-Ο-glucopyranoside was
only found in grape musts. In general, lignans possess a good anti-
oxidant activity and their presence in wines depends partly on growing
facilities and variety of the grapes used (Nurmi et al., 2003). In the
present study, the contribution of isolariciresinol-β-4′-Ο-glucopyrano-
side in must obtained from FG and from grapes dehydrated with TM,
MHW, MVP, LGH and HAD was 1.50%, 1.35%, 1.27%, 1.14%, 0.71%
and 0.74%, respectively. According to DF, this increase in must ob-
tained from grapes dehydrated with HAD was higher than the con-
centration effect, while no substantial change of isolariciresinol-β-4′-Ο-
glucopyranoside was observed in must obtained from grapes dehy-
drated with TM, MHW, MVP and LGH (Table 4).
Overall, results of this study denote a great impact of dehydration
methods on phenolic composition of ‘Xynisteri’ grape musts. In general,
dehydration caused a significant accumulation of phenolic compounds
due to concentration effect, but it also induces qualitative and quanti-
tative changes in dehydrated grape musts. Thus, postharvest dehydra-
tion methods may lead to a marked enrichment in total phenolic con-
tent of dehydrated grape musts; HAD and LGH had the most evident
changes in an array of attributes, compared to freshly harvested grapes.
The concentration of hydroxybenzoic acids, the most abundant group of
polyphenols, was influenced only by HAD and LGH, while all dehy-
dration methods induced significant changes in flavonol content.
Overall, each group of phenolic compounds was affected in a different
manner by the dehydration method applied, indicating different me-
chanisms underlying the changes in polyphenolic composition during
dehydration. This is in agreement with previous work that highlights
that the effects of withering may vary according to the different groups
of phenolic compounds considered (Corradini and Nicoletti, 2013).
Apart from condensation effects due to water loss, cell degradation
of the internal layers of the skin, which occurs during the dehydration,
facilitates the extraction of these compounds from the skins to the grape
must and could be responsible for the enrichement of grape must in
some phenolic compounds under specific dehydration methods ex-
amined (Panceri et al., 2013). Furthermore, hydrolysis reactions of
some polyphenols of high-molecular weight may partially explain the
increase of their contents (Serratosa et al., 2008). Conversely, some
phenolic compounds are involved in other reactions, such as oxidation
or transformation, contributing to their degradation during dehydration
(Serratosa et al., 2008). Finally, the postharvest dehydration process
affects the expression of a number of genes involved in general meta-
bolism, regulatory processes, response to biotic and abiotic stimuli and
stress, hormonal signaling and secondary metabolism (Mencarelliet al.,
2010; Bonghi et al., 2012; Zenoni et al., 2016).
4. Conclusions
This comparative study highlights the importance of the postharvest
dehydration method on the composition of primary and secondary
metabolites in grape musts. Results demonstrated that the HAD process
caused the most evident changes, producing dehydrated grape must of
high phenolic content and aroma potential, yet resulting in a higher
browning effect of the must. Among the non-mechanical dehydration
methods, which according the current legislation can be used for
‘Commandaria’ production, LGH method showed the most considerable
potential for the production of high quality sun-drying grapes. This
method seems to produce dehydrated grape musts with appreciably
higher phenolic and aroma potential, as well as lower concentration of
brown pigments and melanoidins. Furthermore, it shortens the duration
of dehydration time and protects against several factors (i.e rodents,
birds, insects and rain incidents over the drying period), while the cost
of application is not high. These promising results for LGH can be the
starting point for the design and development of optimized solar
greenhouse for the dehydration of ‘Xynisteri’ grapes. The use of a
ventilation system, multiple racks and inclined north wall reflection
could further improve the performance of the present low greenhouse.
Acknowledgments
The authors would like to thank Mr. Nikos Polydorou, Revecca
Winery and Kyperounda Winery Ltd for providing fruit material and
their infrastructure for the experimentation.
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	The impact of postharvest dehydration methods on qualitative attributes and chemical composition of ‘Xynisteri’ grape (Vitis vinifera) must
	Introduction
	Materials and methods
	Grapes sampling procedure
	Dehydration methods
	Calculation of dehydration factor
	Qualitative attributes
	Phenol-free glycosyl glucose (PFGG)
	Browning index and melanoidins content
	Total phenolic content
	Identification and quantification of individual polyphenols by LC-DAD-qTOF-MS
	Statistical analysis
	Results and discussion
	Qualitative attributes
	Aroma potential
	Browning index and melanoidin contents
	Total phenolic contents
	Influence of dehydration method on grape polyphenols
	Conclusions
	Acknowledgments
	References