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INFLUENCE OF THE THERMAL PROCESSING ON THE PHYSICO-
CHEMICAL PROPRETIES AND THE ANTIOXIDANT ACTIVITY OF
A SOLANACEAE VEGETABLE: EGGPLANT
LYNDA ARKOUB-DJERMOUNE, LILA BOULEKBACHE-MAKHLOUF
1
, SABRINA ZEGHICHI-HAMRI,
SALIMA BELLILI, FARID BOUKHALFA and KHODIR MADANI
Laboratoire de Biomath
ematiques, Biophysique, Biochimie, et Scientom
etrie (L3BS), Department of Food Sciences, Facult
e des Sciences de la
Nature et de la Vie, Universit
e de Bejaia, Bejaia, 06000, Alg
erie
1
Corresponding author.
TEL: 1213 05 52 93 27 38;
FAX: 1213 34 21 47 62;
EMAIL: lilaboulekbachemakhlouf@yahoo.fr
Received for Publication November 10, 2014
Accepted for Publication September 18, 2015
10.1111/jfq.12192
ABSTRACT
Physico-chemical parameters, antioxidants and antioxidant activity were evaluated
in eggplant fruit (Solanum melongena) before and after domestic cooking methods
(frying, griddling and baking). The results showed large differences among the
three treatments. After the cooking process, total phenolics and flavonols increased
significantly, whereas vitamin C, carotenoid, lycopene and anthocynin contents
decreased significantly. The thermal treated samples showed significant increase in
their chemical composition (pH, total soluble solids, ash and non enzymatic
browning) along with a significant loss in their water and total sugar contents.
Results showed that eggplant improved its antioxidant capacity in all cooking
methods. Taken together, our results suggest that the various thermal treatments
can increase some phytonutrients and antioxidant activity of eggplant.
PRACTICAL APPLICATIONS
Eggplant (Solanum melongena L.) is an important source of phytochemicals in the
Algerian diet, which can be consumed after three cooking processes: frying,
griddling and baking. However, few data are available on the effect of these
domestic cooking methods on its nutritional quality. The purpose of this study is
to offer to the consumers the best cooking way that enhances phytonutrients and
antioxidant activity of eggplant fruit.
INTRODUCTION
Phytochemicals in fruits and vegetables have been receiving
increased interest from consumers and researchers for their
beneficial health effects on human diseases, mainly due to
their antioxidant activity. Consumption of fruits and vegeta-
bles rich on phytochemicals, particularly phenolic com-
pounds, has been linked to reduce the risk of coronary heart
diseases, neurodegenerative diseases and certain forms of
cancers (Halliwell 1994; Hung et al. 2004).
Brinjal eggplant (S. melongena L.) is an agronomically
important nontuberous crop belonging to the Solanaceae
family, which is important for its richness in healthy compo-
nents (Kaur et al. 2014), it is also widely consumed in the
world, particularly in Algeria. Eggplant contains important
phytonutrients such as phenolic compounds which have
high antioxidant capacities (Boulekbache-Makhlouf et al.
2013; Chumyam et al. 2013; Garc
ıa-Salas et al. 2014) and
anthocyanins (Jung et al.2011;Moncadaet al.2013)like
nasunin and delphinidin conjugates (Ichiyanagi et al.2005).
The main polyphenols found in eggplant are phenolic acids
(chlorogenic acid, caffeic acid and p-coumaric acid), but this
vegetable is poor in provitamin A and vitamin E. However,
the presence of vitamins C and B in this fruit has been estab-
lished (Sakakibara et al.2003;Hansonet al. 2006).
Food quality often deals only with the influences of pri-
mary production and industrial processing. Food prepara-
tion at home as final step of the chain, has also great
influence on physico-chemical propreties and antioxidants.
It can change them in both positive and negative way. It is a
common practice that most of vegetables are cooked before
Journal of Food Quality 00 (2015) 00–00 V
C2015 Wiley Periodicals, Inc. 1
Journal of Food Quality ISSN 1745-4557
use. The effect of many cooking ways such as boiling, steam-
ing and microwave cooking, on the antioxidant activity of
some vegetables have been tested (Turkmen et al.2005;
Chuah et al.2008;Mcdougallet al. 2010; Chumyam et al.
2013). However, frying, griddling and baking are also being
used for this purpose.
The choice of this study is based on the fact that the Alge-
rian people use eggplant in the preparation of three tradi-
tional recipes: fried alone or with other vegetables such as
onion, green pepper, potatoes and zucchini “Tchakchouka,”
grilled “salad,” or used in a modern recipe by baking with
some vegetables such as onion, tomato, pepper, potatoes and
zucchini (gratin). In this work, as a continuation of our
studies on eggplant crop (Boulekbache-Makhlouf et al.
2013) we have studied the effects of different domestic cook-
ing methods (frying, griddling and baking) on its physico-
chemical characteristics and its antioxidant activity.
MATERIALS AND METHODS
Chemicals and Sample Treatement
All chemicals were purchased from Sigma (represented by
Algerian Chemical Society, Setif, Algeria). The samples of
eggplant (S. melongena) were purchased from local market,
Bejaia City, Algeria. They were fresh and without infections
or wounds, they were washed by tap water and prepared for
later use. The representative sample of eggplant (5 kg) was
divided into four parts that were treated in different ways.
The first batch was kept in its fresh state and others were
prepared due to the common consumption of eggplant in
Algeria: frying, griddling, and baking.The best cooking time
and temperature have been previously established, so the
eggplants have the color and texture of home-made prod-
ucts. All cooking methods were repeated three times.
Frying. Approximately 1.250 Kg of the sample were cut
into small slices, put in a pan containing about 250 mL of
olive oil and the frying was performed at 160–165C during
20 min.
Griddling. About 1.250 Kg of the fresh eggplant cleaned
were placed on a rack at 160C for 30 min.
Baking. The forth batch of fresh eggplant (1.250 Kg) was
well cleaned, cut to slice and placed in an electric domestic
oven previously set at 175C for 50 min.
Determination of Physico-Chemical
Parameters
The eggplant samples were studied to determine the follow-
ing parameters: pH (AFNOR 1982), titratable acidity (Verma
and Joshi 2000), water content (Doymaz et al. 2004), total
soluble solids (TSS), total sugar, total ash (AFNOR 1982)
and the non enzymatic browning index (NEB) (Davoodi
et al.2007).
Preparation of the Extracts
The extract were prepared as prescribed in our previous
study (Boulekbache-Makhlouf et al. 2013). The fresh and
cooked samples (0.5 g) were extracted with 50 mL of 70%
acetone. The extraction was performed at room temperature,
using magnetic blender. After 40 min, the solutions were
centrifuged for 25 min at 4000 3g (10C), the supernatants
were filtered (Whatman paper N
o
4) and stored under refri-
gerated conditions until used.
Quantification of Antioxidants
The amount of total phenolics in the eggplant extracts was
determined using the Folin-Ciocalteu reagent, gallic acid was
used as standard (Velioglu et al.1998).Theresultswere
expressed as mg gallic acid equivalent per 100 g of dry
weight (mg GAE/100 g DW).
The total flavonoids content was evaluated by the meth-
odology of Djeridane et al. (2006). The quercetin was used
as standard and reported as mg quercetin equivalent per
100 g of dry weight (mg QE/100 g DW).
The amount of total carotenoid and lycopene was deter-
mined using the method of Sass-Kiss et al. (2005). Twenty
milliliter of solvent mixture (hexane-acetone-ethanol, 2:1:1,
v: v: v) were added to 2 g of the homogenized samples. After
30 min of agitation, the supernatant was collected and the
residue was added with 10 mL of hexane for a second extrac-
tion. The amount of carotenoids was determined after meas-
uring the absorbance of the supernatant at 450 nm. The
results were expressed as mg b-carotene equivalent per 100 g
of dry weight (mg b-CE/100 g DW). Concerning the
amount of lycopene, it was determined after measuring the
absorbance of the supernatant at 472 nm. The results were
expressed as mg lycopene equivalent per 100 g of dry weight
(mg LE/100 g DW) from the standard curve prepared with
lycopene.
Anthocyanins and flavonols were extracted according to
the procedure reported by Ganjewala et al. (2008). One
gram of each sample was extracted with 1.0 mL of methanol
0.1 N HCl for 30 min and extract decanted, then 20 lL
of the extract was added to 980 lL methanol 0.1 N HCl and
the absorption spectrum recorded in a spectrophotometer.
The concentration of anthocyanin was determined from the
absorbance at 530 nm using a molar extinction coefficient
(e)of38,000L3mol
21
3cm
21
, that of the flavonol glyco-
sides at 360 nm (e520,000 L 3mol
21
3cm
21
, determined
from a pure sample of quercetin 3-glucoside). The results
were expressed as mg quercetin-3-glucoside equivalent per
INFLUENCE OF THE THERMAL PROCESSING ON THE PHYSICO-CHEMICAL PROPRETIES L. ARKOUB-DJERMOUNE ET AL.
2Journal of Food Quality 00 (2015) 00–00 V
C2015 Wiley Periodicals, Inc.
100 g of dry weight (mg EQ3G/100 g DW) and calculated
using the following formula:
C5Abs3MM3DF31;000e3L
Where Abs: Absorbance at 530 nm for anthocyanins and
at 360 nm for flavonols; MM: Molar weight of quercetin-3-
glucoside (464.4 g/mol); DF, Dilution Factor; L, Optical
path; e, Molar extinction coefficient of quercetin-3-glucoside
(e
530
538,000 L 3mol
21
3cm
21
;e
360
520,000 L 3
mol
21
3cm
21
).
The amount of ascorbic acid in the samples was deter-
mined according to the spectrophotometric method of Mau
et al. (2005). Each extract (20 mg) was extracted with 10 mL
of 1% metaphosphoric acid for 45 min at room temperature
and filtered through Whatman No. 4 filter paper. The filtrate
(1 mL) was mixed with 9 mL of 2,6-dichloroindophenol and
the absorbance was measured within 15 s at 515 nm against
a blank. The content of ascorbic acid was calculated on the
basis of the calibration curve using L-ascorbic acid as stand-
ard and the result was expressed as milligram ascorbic acid
equivalent per 100 g of dry weight (mg AAE/100 g DW).
Antioxidant Activity
Several methods have been developed to assay the antioxi-
dant activity of food extracts. In our study, we used two
methods: scavenging of the radical 2,20-azino-bis (3-ethyl-
benzothiazoline)26 sulfonic acid (ABTS) activity and the
reducing power.
The antiradicalar activity of the radical ABTS, was deter-
mined by the methodology of Re et al. (1999). The inhibi-
tion percentage was expressed as mg trolox equivalent per
gram of dry weight (mg TE/g DW) and the IC
50
was calcu-
lated as the concentration of extracts causing a 50% inhibi-
tion of ABTS radical.
The reducing power of the extracts was evaluated accord-
ing to the protocol of Kumar et al. (2005), and the results
were expressed as mg trolox equivalent per gram of dry
weight (mg TE/g DW) at different concentrations. The RC
0.5
was calculated as the concentration of extracts reducing 50%
of ferric ions.
Statistical Analysis
All data are reported as mean 6standard deviation of mean
of three replicates. The analysis of variance (ANOVA) at
P<0.05 was calculated using STATISTICA 5.5 to determine
significant differences between the results.
RESULTS AND DISCUSSION
Physico-Chemical Parameters
Acidity, pH, water content, TSS, total sugar, total ash and
the NEB index of raw and cooked eggplant were illustrated
in Table 1.
The pH is a determining factor in the ability of food to be
preserved. Thus, a pH ranged between 3 and 6 is very favor-
able to the growth of yeasts and molds. The result shows that
the pH values of different samples were significantly different
(P<0.05). The pH of cooked eggplant increased slightly
compared with that of the fresh sample. The increase rate cor-
responds to 15.30%, 17% and 22.72% in grilled, baked and
fried eggplant, respectively. This increase could be attributed
to the good extraction of organic acid after softening in the
cooked sample and/or to their degradation, during cooking,
which induce a release of protons. According to Ergezer and
Gocke (2011), the increases of pH could be ascribed to the
reduction of available carboxylic groups of proteins, but also
to the release of calcium and magnesium ions from proteins.
The acidity indicates the maturity of the fruit (it decreases
during maturation) and the ratio of sugars/acidity deter-
mines the gentle character, balanced or sour fruit. The titrat-
able acidity values (Table 1) were significantly different
(P<0.05) and the fresh eggplant showed the lowest one
(1.24 60.09 g citric acid/100 g DW). Titratable acidity of
the fried eggplant was higher (1.86 60.08 g citric acid/100 g
DW) than that of the fresh sample with a value of 33.33%.
This increase can be due to a more extraction of organic
acid after sofetning of the cell wall by thermal treatement
and/or attributed to the reduction of water content thus
leads to an increase in pH value. Nevertheless, the acidity
decreases after baking (0.92 60.07 g citric acid/100 g DW)
TABLE 1. PHYSICO-CHEMICAL PROPRETIES OF FRESH AND COOKED EGGPLANT
Eggplant pH Acidity (%)* Moisture (%)
†
Sugar (%)* TSS (%)* Ash (%)* NEB index
Fresh 4.15 60.04
a
1.24 60.09
b
92.8 60.3
b
20.55 60.23
d
31.48 63.50
a
0.55 60.02
a
0.23 60.01
a
Fried 5.37 60 .05
c
1.86 60.08
c
86.23 60.25
a
11.41 60.12
a
48.41 62.10
b
0.84 60,01
b
0.68 60.05
c
Grilled 4.90 60.09
b
1.40 60.15
b
86.26 60.36
a
12.42 60.59
b
31.68 61.87
a
1.15 60,07
c
0.45 60.01
b
Baked 5.00 60.1
b
0.92 60.07
a
86.46 60.01
a
13.56 60.29
c
63.76 61.71
c
2.18 60,19
d
0.99 60.00
d
Values are averages 6standard deviation of triplicate analysis; different letters in same column indicate significant difference (P<0.05). Results
are ranked in ascending order; d >c>b>a.
*The results are expressed as gram per one hundred gram of dry weight (g/100 g DW).
†
The results are expressed as gram per one hundred gram of fresh weight (g/100 g FW).
TSS, Total soluble solids; NEB, Nonenzymatic browning.
L. ARKOUB-DJERMOUNE ET AL.INFLUENCE OF THE THERMAL PROCESSING ON THE PHYSICO-CHEMICAL PROPRETIES
Journal of Food Quality 00 (2015) 00–00 V
C2015 Wiley Periodicals, Inc. 3
with a rate of 25.80%. Indeed, the acidity of the grilled egg-
plant remaind stable with a value of 1.40 60.15 g citric acid/
100 g DW. These results allow to conclud that cooking can
change the acidity of eggplant in both positive and negative
way depending on the cooking method. In an attempt to
establish a potential relationship between pH value and acid-
ity of eggplant extracts, the correlation coefficient was eval-
uated. Indeed, a high positive correlation was found between
the two parameters (r50.69).
Water is a source of degradation of antioxidants thus the
preservation of the chemical composition in cell is per-
formed by removing water from the fruit with rapid drying
(Tom
as-Barber
an and Espin 2001). After harvesting in the
presence of water, an enzymatic activity may quickly cause
irreversible changes in antioxidants, such as oxidation which
leads to their decomposition or polymerization.
The moisture test permits to know the water content of the
different samples. The results show significant differences
(P<0.05) between water content of the fresh sample and that
of the cooked ones. The fresh eggplant has a moisture content
of 92.80 60.3% and the cooking process caused a significant
(P<0.05) decrease with a percentage ranged from 6.83 to
7.08%. These results were consistent with those reported by
Del Pilar Ram
ırez-Anaya et al. (2015) and Kalogeropoulos
et al. (2010).The main change in the composition of vegeta-
bles during frying is the loss of water due to the evaporation
and absorption of the oil (Kalogeropoulos et al. 2010).
The total sugars content in samples are shown in Table 1.
Few data were available on the influence of cooking treat-
ments on total sugars in eggplant, although sugar is an
important chemical compound with a nutritional value.
Our results showed that the total sugar content of eggplant
decreased significantly after cooking (P<0.05), from
20.55 60.23% to 11.41 60.12%. The highest value was
recorded in the fresh eggplant. All cooking treatments caused
a significant decrease of total sugars content with levels of
34.02%, 39.56% and 44.48% in baked, grilled and fried egg-
plant, respectively. This decrease can be explained by the fact
that sugars are necessary substrate for the NEB and partici-
pate in the Maillard reaction during cooking, resulting in an
increase of NEB index in cooked samples.
The brix or total soluble solid increase after frying and
baking but do not change after griddling (Table 1), the
obtained values showed a significant differences (P<0.05)
with the highest content in the baked sample
(63.76 61.71%) compared with that of the fresh one, which
showed the lowest content (31.48 63.50%). The brix
increased after thermal treatments with different proportion
(34.97%, 50.62% after frying and baking, respectively).
These results were confirmed by Dos Reis et al.(2015)on
the effect of microwave, boiling, steaming and sous vide
processing on broccoli and cauliflower, which is explained
by the higher water loss and sugar concentration. To our
knowledge no results have been reported on the effect of
cooking on the total soluble solid of eggplant.
The ash content is the total quantity of minerals present
in the sample. Table 1 shows that cooking increased signifi-
cantly (P<0.05) the content of mineral matter in eggplant
samples. The fresh sample has a rate of 0.55 60.02%, it
increased in the cooked samples with 34.52%, 38.70% and
74.77% for the fried, grilled and baked eggplant, respectively.
These results were approximate to those reported by Hos-
seini et al. (2014). Mineral components show great changes
during cooking operations, such as boiling, because of their
solubility in water. However, their losses are much lower
during frying in oil, as they are soluble in oil only in small
amounts. The availability of some important minerals, such
as calcium, magnesium, phosphorus and especially iron,
may decrease, partially because of their binding in insoluble
compounds (Vaquero 1998). Water migrating from food
into frying oil is converted into steam, and lost. Due to this
water loss, the wet weight of fried food decreases during fry-
ing. Most mineral components are nonvolatile; therefore, the
content of minerals, expressed as wet weight, would be
expected to rise (Boskou and Elmadfa 2010). The increase
was important in the grilled and the baked samples, as they
are processed in the absence of water (dry cooking), these
cooking methods allowed for a high retention of ash than
frying (wet cooking). In fact, in dry cooking methods, such
as griddling and microwave, the water loss by evaporation
induces a concentration of minerals (Lopes et al. 2015).
Cooking might improve mineral bioavailability by increasing
solubility due to cell wall disruption, protein denaturation
and release of organic acids. For example, iron bioavailability
increased by at least 200% when vegetables such as broccoli,
kale and cabbage were cooked (Reddy and Love 1999).
The results of the NEB index are shown in Table 1. The
degree of NEB of different samples present a significant dif-
ferences (P<0.05), they vary from 0.23 to 0.99. The highest
value was obtained in the baked sample (0.9960.00), fol-
lowed by fried, grilled and fresh eggplant (0.6860.05;
0.45 60.01; 0.23 60.01, respectively). After cooking, the
NEB index increases significantly with a proportion of
48.89%, 66.18% and 76.76% in grilled, fried and baked sam-
ple, respectively. A similar result have been found by Sharma
and Gujral (2011), these reserachers reported that the index
of NEB of barley increases after griddling with a range of
315–774%. Accordingly, Maillard products are produced
during heating and the higher the roasting temperature
results in a greater the browning index. Indeed, the develop-
ment of NEB reactions, such as Maillard reaction, has been
associated to the formation of new active compounds (Man-
zocco et al. 2000). There are two reactions that could result
in the caramelization browning due to sugar–sugar reactions
when heated at high temperatures, and the Maillard reaction
which results from reactions between reducing sugars and
INFLUENCE OF THE THERMAL PROCESSING ON THE PHYSICO-CHEMICAL PROPRETIES L. ARKOUB-DJERMOUNE ET AL.
4Journal of Food Quality 00 (2015) 00–00 V
C2015 Wiley Periodicals, Inc.
proteins and their derivatives (amino acids and amides)
(Quayson and Ayernor 2007). The main variables affecting
the extent of the Maillard reaction are temperature and time
which depend on processing conditions as well as pH, water
activity and type and availability of the reactants which are
based on product properties, but may be changed as a result
of the processing of food and raw materials (Rufi
an-Henares
et al. 2009). The Maillard reaction, has been recently associ-
ated to the formation of compounds with strong antioxidant
capacity. The major groups of reactions leading to browning
are enzymatic phenol oxidation and the so-called NEB. In
the case of Maillard reaction, high antioxidant capacity was
generally associated to the formation of brown melanoidins.
Moreover, it must be kept in mind that polyphenols, ascor-
bic acid and other carbonyl compounds, even if formed dur-
ing oxidative reactions, can take part to the Maillard
reaction itself. The contribution of these compounds to the
formation of heat-induced antioxidants is still unknown
(Manzocco et al. 2000; Lo Scalzo et al. 2010).
Antioxidant Contents
Polyphenols. Table 2 shows that the total polyphenol con-
tents increased significantly (P<0.05) in the grilled and
baked samples, they were about 5956.19 6351.39 and
7793.93 6218.98 mg GAE/100 g DW, respectively. No signif-
icant difference has been detected between the phenolic con-
tent of the fried sample and the fresh one (4838.346670.30
and 4914.83 6388.05 mg GAE/100 g DW, respectively).
According to Sharma et al. (2001) and Abeysinghe et al.
(2007), thermal processing is among the several factors
which could influence the phenolic content of fruit and veg-
etables. The amount of total phenolic contents in grilled and
baked samples rose with a proportion of 18.76% and
37.92%, respectively, while that of the fried ones remained
stable. The order of total phenolic contents was as follow:
Baked >Grilled >Fried 5Fresh (Table 2). Our results con-
firm those obtained by Lo Scalzo et al.(2010)andDaset al.
(2011), who reported a considerable increase in polyphenol
contents (chlorogenic acid, caffeic acid and nasunin) in the
grilled eggplant slices. In the other hand, Del Pilar Ram
ırez-
Anaya et al. (2015) reported an important increase of total
phenolic compounds in deep fried eggplant. This increase
was the result of the simultaneous action of several mecha-
nisms such as the facility with they are extracted in cooked
samples, after strong weakening of cell walls by heat. The
loss after cell burst which facilitates the release of polyphe-
nols and other substances in the cooking medium, the trans-
fer to the foodstuff of the phenols present in the absorbed
olive oil (fried sample) and the effect of concentration in the
food matrix after partial evaporation of moisture (Provesi
et al. 2011). There is an increase in the availability of phenols
physically and chemically linked to the microstructure of the
processed vegetables in comparison to the raw (Mart
ınez-
Hern
andez et al. 2013), whether because of the decomposi-
tion of phenolic compounds linked to the fiber (cellulose
and pectin) (G€
okmen et al. 2009). The breaking of phenol-
sugar glycosidic links giving rise to aglycons also contributes
to the increase in phenol concentration. This last mechanism
is perhaps the main one concerned in the increase of phyto-
nutrient concentrations, which has been suggested to explain
the variations during not only frying, but also oven baking,
microwave cooking, boiling and the culinary preparation of
various green-leaf vegetables, among others (Bunea et al.
2008; Del Pilar Ram
ırez-Anaya et al. 2015). The cooking
may induce degradation and de novo compounds produc-
tion such as Maillard reaction products as well. The occur-
rence of these compounds, which are reactive in a Folin-
Ciocalteu system (Summa et al. 2006), was evidenced by the
brownish color of the extracts from cooked samples (Lo
Scalzo et al.2010).
Flavonoids. The flavonoids concentration in the studied
samples was shown in Table 2. According to the statistical
study, the levels of flavonoids were significantly lower in the
cooked samples. The flavonoid content of the fresh eggplant
decreased with a rate ranging from 37.92% to 40.05% and
the lowest value was found in the cooked eggplant with the
concentrations of 1438.47 610.01 mg QE/100 g DW;
1448.16 68.69 mg QE/100 g DW and 1489.48 617.96 mg
EQ/100 g DW in the baked, fried and grilled samples, respec-
tively, compared with the fresh eggplant which has a concen-
tration of 2399.64 650.80 mg QE/100 g DW. Similar results
TABLE 2. ANTIOXIDANT CONTENT OF FRESH AND COOKED EGGPLANT EXTRACT
Polyphenols Flavonoids Flavonols Anthocyanins
Eggplant (mg GAE/100 g DW) (mg EQ/100 g DW) (mg Q3GE/100 g DW) (mg Q3GE/100 g DW)
Fresh 4838.34 6670.30
a
2399.64 650.80
b
150.72 623.58
a
201.51 616.41
b
Fried 4914.83 6388.05
a
1448.16 68.69
a
266.03 612.52
b
135.93 63.55
a
Grilled 5956.19 6351.39
b
1489.48 617.96
a
143.29 63.25
a
150.98 68.85
a
Baked 7793.93 6218.98
c
1438.47 610.01
a
307.13 640.73
b
189.18 62.42
b
Values are averages 6standard deviation of triplicate analysis; different letters in same column indicate significant difference (P<0.05). Results
are ranked in ascending order; c >b>a.
GAE, Gallic Acid Equivalent; EQ, Quercetin Equivalent; Q3GE, Quercetin-3-Glucoside Equivalent.
L. ARKOUB-DJERMOUNE ET AL.INFLUENCE OF THE THERMAL PROCESSING ON THE PHYSICO-CHEMICAL PROPRETIES
Journal of Food Quality 00 (2015) 00–00 V
C2015 Wiley Periodicals, Inc. 5
have been reported by Boateng et al. (2008) and Gujral et al.
(2013) for different beans on thermal processing. Barakat
and Rohn (2014) found that the highest loss of flavonoids in
broccoli-based bars was observed during frying and micro-
waving/frying by 25 and 33%, followed by baking, micro-
waving and steaming by 16.18 and 21%, respectively. This
loss was due to their leaching (frying oil) and/or their ther-
mal degradation (Yuan et al.2009).
Flavonols. The results showed significant differences
(P<0.05) in the flavonol content of the samples (Table 2).
The rates of flavonols increased with different range after
frying and baking with a proportion of 43.34% and 50.92%,
respectively, while, the decrease was no significant in the case
of the grilled sample. The highest amount was found in the
baked eggplant with a value of 307.13 640.73 mg Q3GE/
100 g DW, followed by the fried, fresh and grilled samples
with the concentrations of 266.03 612.52 mg Q3GE/100 g
DW; 150.72 623.58 mg Q3GE/100 g DW and
143.29 63.25 mg Q3GE/100 g DW, respectively. A similar
result was found by Rodrigues et al. (2009) who reported
that the microwaves treatment causes severe losses of flavo-
nols (16–18%). Nevertheless, baking and frying, do not
affect the flavonols content and moderate microwaves cook-
ing has no effect on these compounds. The increase of flavo-
nol contents may be the result of the breaking of quercetin
3-O-gentiobioside, kaempferol dihexoside (isomers),
kaempferol-3-O-rutinoside, which have been identified in
fresh eggplant, giving rise to their aglycons forms; and the
increase in flavonol levels in fried and baked samples may be
a result of the hydrolysis of delphinidin rutinoside (Garc
ıa-
Salas et al. 2014). Indeed, this compound is an anthocyanin
which is a flavonoid easily degraded at high temperatures.
Indeed, Lo Scalzo et al. (2010) have reported a decrease of
this compound in the grilled samples. But, in this study no
significant difference has been observed in the flavonol con-
tents of the grilled, probably due to the difference on the
temperature and the time of cooking used in both studies.
Anthocyanins. The anthocyanin contents of raw and
cooked eggplant were shown in Table 2. The percentages of
reduction were from 6.12 to 32.54 in cooked samples. The
result showed that the contents of anthocyanin in the differ-
ent batches of eggplant vary from 135.93 63.55 to
201.51 616.41 mg Q3GE/100 g DW, and the highest value
was found in the fresh eggplant. The preservation of natural
pigments after thermal processing is a major quality parame-
ter. A similar result has been found by Harakotr et al.
(2014), these authors reported that boiling and steaming led
to the greatest decreases in the anthocyanin content of pur-
ple waxy corn. In the other hand, it has been reported that
steaming of sweet potato, reduced anthocyanin content by
nearly half of original amount (Kim et al. 2012). Therefore,
the various results indicated the importance of cooking
method on nutrients retention. The loss in anthocyanins
content could be due to the degradation or decomposition
of anthocyanin on thermal treatments (Ioannou et al. 2012).
The stability of anthocyanins and other food pigments
decreased with increasing temperature (Xu and Chang
2009). Jing et al. (2007) have observed a consistent decrease
of protein at 100C in purple corn water extracts, indicating a
possible protein denaturation at high temperatures, which
could result in anthocyanin complexation and precipitation
leading to a decline in anthocyanins content.
Vitamin C. Vitamin C or ascorbic acid is a water soluble
vitamin and posses a good antioxidant propreties. Results
showed that there were significant differences (P<0.05)
between the fresh eggplant and the cooked one. The fresh
eggplant is a good source of vitamin C with a value of
197.97 63.33 mg AAE/100 g DW, but the domestic cooking
causes a significant decrease (P<0.05) with proportions
ranged between 50.06 and 72.30%, agreeing with results
found by Das et al. (2011) who reported a reduction in vita-
min C content in the eggplant grilled for 4–5 min using pro-
fessional grilling apparatus. Loss of vitamin C after cooking
has been observed in several vegetables, such as peas, carrots,
spinach, potatoes, broccoli, fenugreek, peppers and some
selected Thai vegetables (Chuah et al. 2008). Furthermore,
Barros et al. (2011) have noted that the cooking processes
decrease the vitamin C content of the chestnuts with average
decrease of 37% (25–54%) for the boiling process and 33%
(2–77%) for the roasting process. Stir-frying and boiling
caused a significant reduction in vitamin C in red cabbage.
In contrast, steaming and microwave heating did not cause
any significant loss of vitamin C, compared with the fresh-
cut group (Xu et al. 2014). The reduction of the vitamin C
content in cooked eggplant is the result of thermal treatment
which is known to accelerate oxidation of ascorbic acid to
dehydroascorbic acid, followed by the hydrolysis to 2,3-dike-
togulonic acid and eventually polymerization to other nutri-
tionally inactive components (Chuah et al. 2008).
Total Carotenoids. The carotenoid contents (Table 3) of
different samples were ranged between 42.96 mg bCE/100 g
DW and 86.60 mg bCE/100 g DW. The content of fresh egg-
plant was about 86.60 64.26 mg bCE/100 g DW and it
decreased significantly (P<0.05) in all cooked samples with
rate of 50.39%, 48.45% and 39.26% in baked, fried and
grilled eggplant, respectively. Das et al. (2011) have reported
a reduction in b-carotene content in eggplant grilled for 4–5
min using professional grilling apparatus. In the other hand,
Godoy and Rodriguez-Amaya (1998) have determined the
carotenoids composition of raw and cooked (boiled for 3
min) eggplant, they have observed an increase in 13-cis-b-
carotene (from traces to 0.2 60.1 lg/g) and a slight increase
INFLUENCE OF THE THERMAL PROCESSING ON THE PHYSICO-CHEMICAL PROPRETIES L. ARKOUB-DJERMOUNE ET AL.
6Journal of Food Quality 00 (2015) 00–00 V
C2015 Wiley Periodicals, Inc.
in 9-cis-b-carotene (from 0.1 60.1 lg/g to 0.2 60.1 lg/g).
b-carotene is subjected to isomerization and oxidation, fol-
lowed by cleavage because of its unsaturated structure, par-
ticularly under the influence of heat and light during
processing or storage. The main degradation products iden-
tified are cis-isomers, mainly 13-cis-and9-cis-b-carotene
(Achir et al. 2015). Several studies have reported the effect of
cooking on the carotenoid contents of vegetables. The heat
treatment causes cis/trans isomerization of carotenoids, alter-
ing their biological activities and discolors the food (Rodri-
guez-Amaya and Kimura 2004). According to the present
study, the fried and grilled samples have presented significant
losses in their carotenoid contents. The cooking time may
enhance the oxidation of carotenoids by prolonging the time
of contact of the vegetable with the heat. Indeed, griddling
usually takes longer to achieve ideal palatability of foods,
and this might be reflected in the decrease of carotenoids in
the vegetables subjected to this cooking method. In the case
of the fried sample, most likely because carotenoids are
extremely hydrophobic molecules (€
Otles and C¸agindi 2007);
frying may decrease their concentrations by leaching carote-
noids into the oil, in addition to their instability at the high
temperatures usually reached in this process (Murador et al.
2014).
Lycopene. Lycopene is highly susceptible to oxidative deg-
radation because of its highly conjugated polyenic structure.
Except for the fried sample, results showed that lycopene
content was significantly affected by the cooking procedure
(P<0.05) (Table 2). No significant difference has been
detected between the fried and the fresh samples
(12.84 61.66 mg LE/100 g DW and 13.51 60.62 mg LE/
100 g DW, respectively). This result was confirmed by the
study conducted by Nguyen and Schwartz (1998), indeed,
these researchers have reported that the presence of fat is a
factor that slows the isomerization reaction and protects the
trans-andcis lycopene isomers against oxidation. All-trans
lycopene was found to be more stable in sanflower oil or
olive oil compared with oil-in-water emulsion (Ax et al.
2003; Colle et al. 2010). Nevertheless, Mayeaux et al.(2006)
have reported that the lycopene in tomato slurry was severely
degraded during frying. Approximately 70% and 75% of
lycopene was lost during frying for 2 min at 145 and 165C,
respectively. This can be explained by the high frying tem-
peratures which leads to the production of hydroperoxide
free radicals by haut oils, and accelerate the degradation of
lycopene as well.
The lycopene contents decreases with a range of 38.47%
and 41.90% in baked and grilled eggplant, respectively
(Table 2), similar results have been reported in the literature
(Sahlin et al.2004;Mayeauxet al. 2006; Murador et al.2014).
During food processing, lycopene may isomerize to cis-iso-
forms with the presence of heat and/or oil, or during dehy-
dration. With long heating times or temperatures above 50C,
degradation proceeds faster than isomerization. The degra-
dation of total lycopene in oleoresin from tomato samples
increased significantly (P<0.05) from 25C to 100C. Lyco-
pene at 25C and 50C may degrade mainly through oxidation
without isomerization (Hackett et al. 2004). In any case the
autoxidation of either, the all-trans or cis isomer intermedi-
ates, was likely the major pathway for lycopene degradation.
Thus, the different rate of lycopene degradation may depend
from the other components present in the oil phase of oleor-
esins tomato, protective antioxidant (such as tocopherols) or
high temperature generated free radicals, which can affect
the lycopene degradation. Anguelova and Warthesen (2000)
reported that during storage of tomato powder at 75–100C,
several cis-isomers of lycopene were formed from all-trans
lycopene. Temperature and time dependent isomerization of
all-trans lycopene to cis-isoforms was reported in different
tomato oleoresins (Hackett et al. 2004). Moreover, Xianquan
et al. (2005) observed a large decrease in the concentration of
all-trans-lycopene during heating at 150C, and no lycopene
was detected after 10 min. Furthermore, Mayeaux et al.
(2006) have reported that lycopene stability decreasing as the
temperature increased from 100 to 150C and as time
increased from 0 to 60 min, lycopene is not stable during
long heating times and rapidly decomposed at a heating tem-
perature of 150C and above. The proposed pathway of lyco-
pene degradation consists of two stages: isomerization and
auto-oxidation due to the unsaturated double bonds
(Bo
skovic
´1979). However, at 88C cooking temperature,
increases of trans and cis lycopene was found during 30 min
heating (Dewanto et al. 2002). This suggested that mild ther-
mal processing could simultaneously increase lycopene con-
centration in tomato by increasing the free and bioaccesible
form while degrading lycopene through oxidation. Thus, the
heating temperature and time may play an important role in
lycopene concentration in eggplant. The stability of lycopene
in food products depends on their lycopene isomers profile
in a complex manner, the composition of the source
TABLE 3. VITAMIN C AND CAROTENOIDS CONTENT OF FRESH AND
COOKED EGGPLANT
Vitamin C Carotenoids Lycopene
Eggplant
(mg AAE/100 g
DW)
(mg b-CE/100 g
DW)
(mg LE/100 g
DW)
Fresh 197.97 63.33
d
86.60 64.26
c
12.84 61.66
b
Fried 54.82 64.21
a
44.64 60.11
a
13.51 60.62
b
Grilled 92.92 61.01
c
52.60 62.19
b
7.46 60.72
a
Baked 81.47 61.12
b
42.96 60.45
a
7.09 60.15
a
Values are averages 6standard deviation of triplicate analysis; differ-
ent letters in same column indicate significant difference (P<0.05).
Results are ranked in ascending order; d >c>b>a.
AAE, Ascorbic Acid Equivalent; b-CE, b-carotene equivalent; LE, Lyco-
pene Equivalent.
L. ARKOUB-DJERMOUNE ET AL.INFLUENCE OF THE THERMAL PROCESSING ON THE PHYSICO-CHEMICAL PROPRETIES
Journal of Food Quality 00 (2015) 00–00 V
C2015 Wiley Periodicals, Inc. 7
matrices, the temperature range and the treatment time can
affect the stability of lycopene. Conversely, thermal isomeri-
zation of lycopene is known to improve its bioavailability
from food matrices (Mayeaux et al. 2006).
Antioxidant Activity. The inhibition percentage of the
radical cation ABTS
•1
by the extract is presented in Fig. 1.
The baked eggplant has exhibited the highest activity
(155.11 61.41 mg TE/g DW), followed by grilled, fried and
fresh samples (149.87 610.02 mg TE/g DW;
141.61 66.09 mg TE/g DW; 121.5864.09 mg TE/g DW,
respectively). This result is confirmed by the evaluation of the
IC
50
value (Table 4) which were about 86.58 61.10 mg/mL;
110.80 67.81 mg/mL and 110.93 61.56 mg/mL, respectivly
compared with the raw eggplant (244.20614.15 mg/mL).
However, Jim
enez-Monreal et al. (2009) have reported that
the baked (38 min at 200C), grilled (10 min) and fried egg-
plant (10 min) kept their antioxidant capacity against
ABTS
•1
radical; the difference between their results and those
obtained in this work can be due to the temperature and/or
thetimeofcookingusedinbothstudies.
The results of reducing power expressed as mg trolox
equivalent per gram of dry weight of extract at different con-
centrations, are shown in Fig. 2. The baked sample presented
the highest (P<0.05) activity with a concentration of
267.23 65.99 mg TE/g DW, followed by the grilled, fried
and fresh eggplant with a proportion of 162.73 64.29 mg
TE/g DW, 128.06 62.89 mg TE/g DW and
114.98 613.53 mg TE/g DW, respectively. The divergence
registred between the fresh and cooked samples is probably
due to the high concentration of phenolic compounds in the
cooked extract. The values of RC
0,5
(expressed in mg/mL) of
different extracts were summarized in Table 4. Based on
these results, the baked eggplant present the best reducing
capacity (56.62 60.41 mg/mL) followed by the grilled, fried
and fresh samples with the concentrations of
89.82 61.02 mg/mL; 98.46 65.56 mg/mL and
260.73 68.99 mg/mL, respectively. No results have been
reported on the effect of the three cooking methods on the
reducing power of eggplant.
Several studies have reported the increase in antioxidant
activity of some cookeed vegetables (Huang et al. 2007;
Rocha-Guzm
an et al. 2007). This increase may be a conse-
quence of the liberation of high amounts of antioxidant
components due to the thermal destruction of cell walls and
sub cellular compartments; or the production of stronger
radical-scavenging antioxidants by thermal chemical reac-
tion; suppression of the oxidation capacity of antioxidants
by thermal inactivation of oxidative enzymes; or the forma-
tion of novel compounds such as Maillard reaction products
with antioxidant activity (Jim
enez-Monreal et al.2009).
CONCLUSIONS
In conclusion, this study clearly shows that nutrients and
health-promoting compounds in eggplant were significantly
affected by cooking. The phenolic and flavonol contents were
increased while flavonoid, anthocynin, carotenoid, lycopene
andascorbicacidcontentsdecreaseataratedependingon
the cooking method. The cooked eggplant can be classified in
this order baked >grilled >fried>fresh and the oven cook-
ing is the better way for enhancing the antioxidant properties
of eggplant. Cooking also increased most physico-chemical
FIG. 1. ABTS ANTIRADICAL POWER OF RAW AND COOKED
EGGPLANT
TABLE 4. THE IC
50
AND THE CR
0,5
OF FRESH AND COOKED
EGGPLANT
Eggplant
ABTS
IC
50
(mg/mL)
Reducing power
RC
0.5
(mg/mL)
Fresh 244.20 614.15
c
260.73 68.99
c
Fried 110.80 67.81
b
98.46 65.56
b
Grilled 110.93 61.56
b
89.82 61.02
b
Baked 86.58 61.10
a
56.62 60.41
a
Values are averages 6standard deviation of triplicate analysis; differ-
ent letters in same column indicate significant difference (P<0.05).
Results are ranked in ascending order; c >b>a.
FIG. 2. REDUCING POWER OF RAW AND COOKED EGGPLANT
INFLUENCE OF THE THERMAL PROCESSING ON THE PHYSICO-CHEMICAL PROPRETIES L. ARKOUB-DJERMOUNE ET AL.
8Journal of Food Quality 00 (2015) 00–00 V
C2015 Wiley Periodicals, Inc.
properties (pH, total soluble solid, ash and non enzymatic
browning) while it decreased the water and total sugar
contents.
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