INFLUENCE OF THE THERMAL PROCESSING ON THE
PHYSICO-CHEMICAL PROPERTIES AND THE ANTIOXIDANT ACTIVITY OF
A SOLANACEAE VEGETABLE: EGGPLANT
LYNDA ARKOUB-DJERMOUNE, LILA BOULEKBACHE-MAKHLOUF1, SABRINA ZEGHICHI-HAMRI, SALIMA BELLILI, FARID BOUKHALFA and KHODIR MADANI
Laboratoire de Biomathematiques, Biophysique, Biochimie, et Scientometrie (L3BS), Department of Food Sciences, Faculte des Sciences de la Nature et de la Vie, Universite de Bejaia, Bejaia, 06000, Algerie
1Corresponding 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; Garcıa-Salas et al. 2014) and anthocyanins (Jung et al. 2011; Moncada et 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; Hanson et 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 properties and antioxidants. It can change them in both positive and negative way. It is a common practice that most of vegetables are cooked before
use. The effect of many cooking ways such as boiling, steam-ing and microwave cooksteam-ing, on the antioxidant activity of some vegetables have been tested (Turkmen et al. 2005; Chuah et al. 2008; Mcdougall et 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 (frycook-ing, griddlcook-ing and bakcook-ing) on its physico-chemical characteristics and its antioxidant activity.
MATERIALS AND METHODS
Chemicals and Sample TreatementAll 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 3 g (10C), the supernatants were filtered (Whatman paper No4) 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). The results were 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) of 38,000 L 3 mol213cm21, that of the flavonol glyco-sides at 360 nm (e 5 20,000 L 3 mol213cm21, determined from a pure sample of quercetin 3-glucoside). The results were expressed as mg quercetin-3-glucoside equivalent per
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 (e530538,000 L 3 mol21 3 cm21; e360520,000 L 3
mol213cm21).
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 IC50was
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 RC0.5
was calculated as the concentration of extracts reducing 50% of ferric ions.
Statistical Analysis
All data are reported as mean 6 standard 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 ParametersAcidity, 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 (Ttitrat-able 1) were significantly different (P < 0.05) and the fresh eggplant showed the lowest one (1.24 6 0.09 g citric acid/100 g DW). Titratable acidity of the fried eggplant was higher (1.86 6 0.08 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 6 0.07 g citric acid/100 g DW)
TABLE 1. PHYSICO-CHEMICAL PROPERTIES OF FRESH AND COOKED EGGPLANT
Eggplant pH Acidity (%)* Moisture (%)† Sugar (%)* TSS (%)* Ash (%)* NEB index
Fresh 4.15 6 0.04a 1.24 6 0.09b 92.8 6 0.3b 20.55 6 0.23d 31.48 6 3.50a 0.55 6 0.02a 0.23 6 0.01a
Fried 5.37 6 0 .05c 1.86 6 0.08c 86.23 6 0.25a 11.41 6 0.12a 48.41 6 2.10b 0.84 6 0,01b 0.68 6 0.05c
Grilled 4.90 6 0.09b 1.40 6 0.15b 86.26 6 0.36a 12.42 6 0.59b 31.68 6 1.87a 1.15 6 0,07c 0.45 6 0.01b
Baked 5.00 6 0.1b 0.92 6 0.07a 86.46 6 0.01a 13.56 6 0.29c 63.76 6 1.71c 2.18 6 0,19d 0.99 6 0.00d
Values are averages 6 standard 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).
with a rate of 25.80%. Indeed, the acidity of the grilled egg-plant remaind stable with a value of 1.40 6 0.15 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 (r 5 0.69).
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 (Tomas-Barberan 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 6 0.3% 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 Ramırez-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 6 0.23% to 11.41 6 0.12%. 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 6 1.71%) compared with that of the fresh one, which showed the lowest content (31.48 6 3.50%). 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 6 0.02%, 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 (0.99 6 0.00), fol-lowed by fried, grilled and fresh eggplant (0.68 6 0.05; 0.45 6 0.01; 0.23 6 0.01, 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
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 (Rufian-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 6 351.39 and 7793.93 6 218.98 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 (4838.34 6 670.30 and 4914.83 6 388.05 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 5 Fresh (Table 2). Our results con-firm those obtained by Lo Scalzo et al. (2010) and Das et 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 Ramırez-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 (Martınez-Hernandez 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 Ramırez-Anaya et al. 2015). The cooking may induce degradation and de novo compounds produc-tion such as Maillard reacproduc-tion 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 6 10.01 mg QE/100 g DW; 1448.16 6 8.69 mg QE/100 g DW and 1489.48 6 17.96 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 6 50.80 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 6 670.30a 2399.64 6 50.80b 150.72 6 23.58a 201.51 6 16.41b
Fried 4914.83 6 388.05a 1448.16 6 8.69a 266.03 6 12.52b 135.93 6 3.55a
Grilled 5956.19 6 351.39b 1489.48 6 17.96a 143.29 6 3.25a 150.98 6 8.85a
Baked 7793.93 6 218.98c 1438.47 6 10.01a 307.13 6 40.73b 189.18 6 2.42b
Values are averages 6 standard deviation of triplicate analysis; different letters in same column indicate significant difference (P < 0.05). Results are ranked in ascending order; c > b > a.
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 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 6 40.73 mg Q3GE/ 100 g DW, followed by the fried, fresh and grilled samples with the concentrations of 266.03 6 12.52 mg Q3GE/100 g DW; 150.72 6 23.58 mg Q3GE/100 g DW and 143.29 6 3.25 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 (Garcıa-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 6 3.55 to 201.51 6 16.41 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 properties. 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 6 3.33 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 vita-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 6 4.26 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 6 0.1 lg/g) and a slight increase
in 9-cis-b-carotene (from 0.1 6 0.1 lg/g to 0.2 6 0.1 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- and 9-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 6 1.66 mg LE/100 g DW and 13.51 6 0.62 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- and cis 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; Mayeaux et 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 (Boskovic´ 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 6 3.33d 86.60 6 4.26c 12.84 61.66b Fried 54.82 6 4.21a 44.64 6 0.11a 13.51 6 0.62b Grilled 92.92 6 1.01c 52.60 6 2.19b 7.46 6 0.72a Baked 81.47 6 1.12b 42.96 6 0.45a 7.09 6 0.15a
Values are averages 6 standard 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.
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•1by the extract is presented in Fig. 1. The baked eggplant has exhibited the highest activity (155.11 6 1.41 mg TE/g DW), followed by grilled, fried and fresh samples (149.87 6 10.02 mg TE/g DW; 141.61 6 6.09 mg TE/g DW; 121.58 6 4.09 mg TE/g DW, respectively). This result is confirmed by the evaluation of the IC50value (Table 4) which were about 86.58 6 1.10 mg/mL;
110.80 6 7.81 mg/mL and 110.93 6 1.56 mg/mL, respectivly compared with the raw eggplant (244.20 6 14.15 mg/mL). However, Jimenez-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•1radical; the difference between their results and those obtained in this work can be due to the temperature and/or the time of cooking used in both studies.
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 6 5.99 mg TE/g DW, followed by the grilled, fried and fresh eggplant with a proportion of 162.73 6 4.29 mg TE/g DW, 128.06 6 2.89 mg TE/g DW and 114.98 6 13.53 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 RC0,5(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 6 0.41 mg/mL) followed by the grilled, fried and fresh samples with the concentrations of 89.82 6 1.02 mg/mL; 98.46 6 5.56 mg/mL and 260.73 6 8.99 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-Guzman 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 reacforma-tion products with antioxidant activity (Jimenez-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 and ascorbic acid contents decrease at a rate depending on 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 enhanccook-ing the antioxidant properties of eggplant. Cooking also increased most physico-chemical
FIG. 1. ABTS ANTIRADICAL POWER OF RAW AND COOKED
EGGPLANT
TABLE 4. THE IC50AND THE CR0,5OF FRESH AND COOKED
EGGPLANT Eggplant ABTS IC50(mg/mL) Reducing power RC0.5(mg/mL) Fresh 244.20 6 14.15c 260.73 6 8.99c Fried 110.80 6 7.81b 98.46 6 5.56b Grilled 110.93 6 1.56b 89.82 6 1.02b Baked 86.58 6 1.10a 56.62 6 0.41a
Values are averages 6 standard 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.
properties (pH, total soluble solid, ash and non enzymatic browning) while it decreased the water and total sugar contents.
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