Introduction

In recent times, natural antioxidants have attracted considerable interest among nutritionists, food manufacturers and consumers because of their presumed safety and potential therapeutic values. It has been shown that some phytochemicals, including polyphenols, flavonoids and carotenoids in fruits and vegetables, consumed as part of our daily diet, may reduce the risk of chronic diseases such as cancer and heart disease (Keevil et al., 2000; Veluri et al., 2006; Voutilainen et al., 2006). Grape is one of the most important fruit commodities as an economic plant with good agricultural characteristics. It is mainly processed to juice, wine or raisins (dried grapes). Grapes are becoming increasingly popular as a fruit and are a significant source of nutritional antioxidants, such as flavonoids, phenolic acids, anthocyanins and carotenoids, as well as biologically active dietary components with high antioxidant activities such as vitamin C. Grape has been studied by many researchers (Giovanelli and Brenna, 2007; Orak, 2007; Mulero et al., 2010; Bunea et al., 2012), who found that it is among food products having the highest concentration of TPP compounds and the highest level of total AC . The phenolic profile of grapes was published by some authors (Revilla and Ryan, 2000; Rockenbach et al., 2011). Revilla and Ryan (2000) reported the presence of 17 different phenolic molecules with antioxidant properties (gallic acid, procyanidin B1, trans-caffeoyltartaric acid, (1)-catechin, trans-coumaroyltartaric acid, procyanidin B2, (2)-epicatechin, delphinidin-3-O-glucoside, cyanidin-3-O-glucoside, petunidin-3-O-glucoside, peonidin-3-O-glucoside, malvidin-3-O-glucoside, rutin, quercetin-3-O-glucuronoside, malvidin-3-O-glucoside-acetate, trans-resveratrol, and malvidin-3-O-glucoside-(p-coumarate)). In this context, the aim of this study was to assess the TPP, carotenoid and vitamin C contents, the AC, and their correlations of five fresh table grape varieties from Algeria, with a view to exploiting their potential as a source of natural antioxidants. In addition, some chemical characteristics were analyzed as they are important for the sensory quality of grape at ripeness.

Materials and methods

1. Grape samples

The work was carried out on five different varieties of table grape, namely: Cardinal (red variety), Gros noir, Muscat noir (blue-black varieties), Muscat blanc and Victoria (white varieties) (harvest year 2012) grown at four different sites (Cardinal grapes were harvested in two different sites) in the region of El-Tarf located in northern Algeria (36° 45' 00" N; 81° 10' 00" E). Samples were harvested at maturity (as determined by the owners of the vineyards), which was confirmed later by measuring the sugar content (°Brix). The sampling of grapes was done meticulously and berries were collected randomly from top, bottom, sun-exposed and unexposed clusters on each side of the vine. For each variety, two samples were taken, one for physicochemical analysis and the other for antioxidant analysis. Samples were placed in clean, dry, plastic boxes and quickly transported and stored until analysis.

Table 1 summarizes the main characteristics of the vineyards and sample plots.

2. Physicochemical analysis

Physicochemical analyses were performed on grape juice two days after sampling. The grape juice was obtained by direct pressure of grapes followed by separation of juice from pulp by filtration. The grape juice samples were analyzed for pH, soluble solid content (°Brix), total acidity and density according to Algerian Standards of the Official Algerian Journal NA 301: Grape Juice: Specification (1989).

• pH: determined with a pH-meter (Hanna, pH-210);
• °Brix: determined with a refractometer (Carl Zeiss Jena);
• Acidity: measured by titration with 0.1N NaOH;
• Density: determined with a pycnometer at 20°C.

3. Estimation of ascorbic acid

The oxidation-reduction method described by Njoku et al. (2011) was used for the estimation of ascorbic acid content with some modifications; it is a direct titration of 10 ml of grape juice with iodine (I₂). Results are expressed as mg of vitamin C/100 ml of juice.

4. Determination of total carotenoids

Carotenoids were extracted by the method described by Sass-Kiss et al. (2005). Briefly, 20 ml of mixed extraction solvents (hexane/acetone/ethanol, 2:1:1) was added to 10 g of fresh crushed grapes. After stirring for 30 min, the upper phase was recovered. 10 ml of hexane was added for a second extraction. The mixture of the two hexane phases was used for the determination of total carotenoids by spectrophotometry at 420 nm. Concentrations of carotenoids are estimated by reference to the calibration curve using β-carotene as standard and the results are expressed as µg/g of fresh weight (µg/g FW).

5. Sample preparation and extraction for phenolic and antioxidant analysis

Frozen grapes were washed with distilled water, dried with a cloth and crushed in a domestic mixer. A mixture of 10 g of crushed grapes, 20 ml of extraction solvent (methanol 80%) and 0.1 ml/10 ml of solvent (v/v) of concentrated HCl (to avoid oxidation of the phenolic compounds) was placed in a water bath with stirring. After agitation, the liquid extract was separated from the solid residues by centrifugation at 3000 rpm/15 min; the extraction procedure was repeated three times. The final extract, which consisted of a mixture of three supernatants, was transferred into opaque vials and stored in a fridge (4°C) until further analysis. The extraction procedure was performed twice.

5.1 Determination of total polyphenols (TPP)

Total phenolic content in the extract was determined using Folin-Ciocalteu (FC) reagent according to the method described by Nickavar et al. (2008) with some modifications. Briefly, 50 µl of freshly prepared extracts was mixed with 2.5 ml of Folin-Ciocalteu reagent (diluted 1/10) and incubated at room temperature. After 10 min, 2 ml of sodium carbonate solution (7.5%) was added. The final solution was mixed thoroughly and allowed to remain in the dark for 30 min. The absorbance was measured at 765 nm with a spectrophotometer. Gallic acid was used as standard phenolic compound (0.1 - 0.9 mg/ml) and the results were expressed as mg of gallic acid equivalent (GAE)/g FW. The assay was performed in two replicates for each extract.

5.2 Determination of total flavonoids (TF)

TF were measured by a colorimetric assay developed by Kim et al. (2003), with modifications. Briefly, 250 µl of extract or standard solution of catechin at different concentration (20 - 260 µg/ml) and 1 ml of distilled water were mixed in a 10 ml test tube. The following were successively added: at zero time, 75 µl of 5% NaNO₂; at 5 min, 75 µl of 10% AlCl₃; and at 6 min, 500 µl of 1N NaOH. The solution was then immediately diluted by adding 2.5 ml of distilled water and mixed thoroughly. The absorbance of the mixture, pink in color, was directly measured in a spectrophotometer at 510 nm against a blank sample and the results were expressed as catechin equivalents (mg CE/g FW). Samples were analyzed in two replicates for each extract.

5.3 Antioxidant assay

The antioxidant activities of the whole grape extracts were determined using reducing power according to the method described by Jayaprakasha et al. (2001). Briefly, 1 ml of grape extract, 2.5 ml of phosphate buffer (0.2 M, pH 6.6) and 2.5 ml of potassium ferricyanide solution (1% w/v) were mixed in a test tube and reacted for 20 min at 50°C. At the end of incubation, the tubes were immediately cooled and 2.5 ml of trichloracetic acid (TCA) (10%) was added. After, 2.5 ml of mixture was mixed with 2.5 ml of distilled water and 500 µl of ferric chloride (0.1% w/v) and reacted for 10 min. The absorbance was then measured at 700 nm against a blank sample. Ascorbic acid was used as standard (0.1 - 0.9 mg/ml) and the results were expressed as ascorbic acid equivalent antioxidant (mg AAEA/g FW). The assay was performed in two replicates for each extract.

6. Statistical analysis

All data are reported as mean ± standard error of two replicates. The statistical comparison of data was performed by a one way analysis of variance (ANOVA) at p<0.05 with STATISTICA 5.5 (StatSoft Inc., Oklahoma, USA) in order to determine significant differences between the results. The LSD (Least Significant Difference) test was applied to reveal significant differences for each parameter among the studied grape varieties. To determine whether the bioactive compounds contributed to the (AC), Pearson’s correlation coefficients were calculated at 5%.

Results and discussion

1. Physicochemical analysis

The results for the physicochemical analysis of the grape juices are shown in Table 2 as mean ± standard deviation.

Acidity varied significantly (p<0.05) and ranged from 0.217 ± 0.01% (Victoria) to 0.518 ± 0.02% (Muscat noir), and pH varied between 3.81 ± 0.01 (Cardinal) and 3.39 ± 0.01 (Muscat blanc). Muscat noir showed the second lowest pH and the highest acidity, which explains the characteristic taste of this variety. pH data are in agreement with the grape juices (pH 3.30 - 3.51) studied by Burin et al. (2010).

Total soluble solids (°Brix) of the different grape juices showed significant differences (p<0.05) and ranged from 17.05 ± 0.07% (Gros noir) to 21.01 ± 0.01% (Muscat noir), and density varied between 1.062 and 1.087 for the same variety, respectively. According to the CODEX STAN 247 (2005), the minimal °Brix value of grape juice is 16%. All grape juices analyzed in this study present value higher than 16%, indicating that the samples are in accordance with the legislation and are a good source of sugars. The Muscat noir variety contains the highest total soluble solid content, which gives it a relatively sweet taste. The differences in the chemical composition of juices can be attributed to genetic differences among varieties and physiological factors (Del Caro et al., 2004). The chemical composition (total soluble solids, sugar, acids,…) of Flame grapefruit significantly varies with the fertilization treatment (Dou et al., 2005).

2. Ascorbic acid and total carotenoid contents

The ascorbic acid and total carotenoid contents of the six grape samples are shown in Table 3.

2.1 Ascorbic acid content

Grapes are known to be a nutrient source of vitamin C in dietary intake. Here, the ascorbic acid content ranged from 12.33 ± 0.01 (Victoria) to 30.80 ± 4.98 mg/100 ml of grape juice (Gros noir). The ascorbic acid content varies between 27.7 and 40.0 mg/100 ml among grapefruit juices (Dou et al., 2005), whereas it ranges from 4.4 to 57.2 mg (%) in organic and conventional grape juices (Dani et al., 2007). Several factors influence the ascorbic acid content, including preharvest factors, such as climatic conditions (sunlight exposure and weather) and farming practices (fertilizers), maturity at harvest, harvesting method, postharvest handling conditions (storage), species, cultivars and tissues (Lee and Kader, 2000), as well as genotype (Sharique and Beigh, 2009). All these factors are responsible for the wide variation in vitamin C content of fruits and vegetables.

2.2 Carotenoid content

Levels of carotenoids were very different among the five grape varieties studied (p<0.05). With regard to their concentrations (Table 3), we detected significant amounts of carotenoids in all grape varieties at harvest, in both red and white ones. The results ranged from 5.99 ± 0.12 (Victoria) to 16.11 ± 0.34 μg/g (Muscat noir). As climatic conditions were uniform for all the cultivars, this trend could be due to the differences in varieties and viticultural conditions (soil characteristics, irrigation and fertilization treatment), which vary from one site to another. The Victoria variety showed the lowest carotenoid content in comparison with the other varieties studied. This may be due to the fact that this variety was treated for powdery mildew caused by Erysiphe necator and received intensive treatment every 4 days.

There are relatively few studies regarding grape carotenoid composition. The quantitative carotenoid data obtained were not in agreement with the literature data. Crupi et al. (2010) studied the carotenoid concentration in some wine grapes and obtained values for total carotenoids in the range of 720 - 2052 μg/kg depending on grape variety. In their study on three Italian grape varieties (Erbaluce, Barbera and Nebbiolo) at harvest, Giovanelli and Brenna (2007) found amounts of 1.1, 9.1 and 4 µg/g of dry weight, respectively, which approach those found here for Cardinal (8.72 µg/g ± 0.20) and Victoria (5.99 µg/g ± 0.12). In their work on the skin of nine wine and table grape varieties cultivated in organic and conventional agriculture, Bunea et al. (2012) found that white grape varieties have a higher carotenoid content compared with the blue-black cultivars. However, in their work on the skin of three grape varieties (white, red and black), Lancaster et al. (1997) reported a concentration of 0.005 mg/g (on fresh mass basis) for these three varieties. The profile and the amount of grape carotenoids could be influenced by several factors, such as: plant variety, climatic conditions, stage of maturation, soil features (Kamffer et al., 2010), and extraction methods, especially the extraction solvent, time and pH (Kamffer et al., 2010). There are studies showing that soil irrigation has less influence on carotenoid profile compared with the type of soil and its water holding capacity. Soil with a low water holding capacity can lead to an increase in carotenoid concentration (Lee et al., 2007).

3. Determination of total polyphenols (TPP)

The TPP content of grape cultivars are shown in Fig. 1. TPP per g of fresh grape ranged from 3.35 ± 0.21 mg GAE in Gros noir to 1.21 ± 0.04 mg GAE in Cardinal 1. The phenolic concentrations of the other grape varieties were as follows: Muscat noir (3.04 ± 0.06 mg GAE), Cardinal 2 (1.85 ± 0.04 mg GAE), Victoria (1.82 ± 0.13 mg GAE) and Muscat blanc (1.58 ± 0.11 mg GAE).

Based on the TPP, the five grape cultivars studied may be classified into two groups: one exhibiting clearly higher phenolic contents (berries with a strong purple color, i.e., Gros noir and Muscat noir), which is due to the maximal accumulation of anthocyanins and flavonols during the last ripening stage, thus contributing to high polyphenol contents (Marinova et al., 2005), and one exhibiting low levels (i.e., Victoria, Muscat blanc, Cardinal 1 and Cardinal 2). These results are in agreement with those found by Dani et al. (2007), who reported higher polyphenol contents in red grape juices from Vitis labrusca compared with white ones.

The TPP content of Gros noir and Muscat noir was about 2.8- and 2.5-fold higher than that of Cardinal 1, respectively. Our observation is lower than the 3.7-fold reported by Orak (2007) in red grapes. The averages of TPP of grape were significantly different at a level of p<0.05. The highest content of TPP in the grape extracts might account for the better results found for their antioxidant activity. In fact, it had been reported that the antioxidant activity of plant materials is well correlated with the content of phenolic compounds (Barros et al., 2007).

Mulero et al. (2010) found in their study on the red grape variety Monastrell 982.0 ± 58.97 mg TPP/kg fresh weight of total grape berry, and Marinova et al. (2005) found 184.1 and 213.3 mg/100 g of fresh mass from white and black grapes (Vitis vinifera), respectively, which is close to or lower than our results. However, Yang et al. (2009) found TPP ranges between 201.1 and 424.6 mg GAE/100 g of fresh grape for 14 different varieties of wine grapes, which is higher than the results found in this study. Several factors such as genetic differences among different cultivars and species (Du et al., 2012; Orak, 2007), geographical origin and clones (Stefanovits-Bányai et al., 2003), tissue analyzed, extraction time, temperature and solvent (Bucić-Kojić et al., 2011), developmental stage (Prvulović et al., 2011), soil component (Imre et al., 2012) and vegetative rootstocks (Jakobek et al., 2009) could influence the phenolic content.

4. Determination of total flavonoids (TF)

The range of TF per g of fresh weight in all tested grapes varied between 0.40 ± 0.01 and 1.09 ± 0.04 mg CE (Fig. 1). Gros noir exhibited the highest flavonoid content (1.09 ± 0.04 mg CE), followed by Muscat noir (0.84 ± 0.03 mg CE), Victoria (0.46 ± 0.03 mg CE), Cardinal 2 (0.46 ± 0.06 mg CE), Muscat blanc (0.45 ± 0.04 mg CE) and Cardinal 1 (0.40 ± 0.01 mg CE). The TF contents of the grape cultivars were significantly different at a significance level of p<0.05.

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Figure 1. Contents of total polyphenols (TPP) and total flavonoids (TF) in various grape cultivars. The data are displayed with mean ± standard deviation (bars). Values marked by different letters are significantly different (p<0.05). TPP and TF are expressed as mg of gallic acid equivalents (GAE)/g and mg of catechin equivalents (CE)/g of fresh weight, respectively.

Marinova et al. (2005) found 36.5 and 77.1 mg/100 g of fresh mass from white and black grapes (Vitis vinifera), respectively, which is higher than our results. Here, the TF levels are much lower than those found by Du et al. (2012) (228.7 and 607.7 mg of rutin equivalents/100 g of fresh grapes of eight different grape varieties) and lower than those found by Yang et al. (2009). Grape flavonoid composition appears to vary greatly among tested fruits depending on their genetic origin, the time of fruit collection, and the fruit parts used (peel and edible parts) (Lu et al., 2006). The presence and/or concentration of flavonoids can be affected by differences among cultivars, agricultural practices, assay protocols (Kim et al., 2003) as well as sunlight and temperature exposure (Makris et al., 2006).

5. Antioxidant capacity (AC) / Reducing power

Fig. 2 shows the reducing power of grape methanolic extracts. In this assay, the yellow color of the test solution changes to various shades of green and blue depending on the reducing power of each compound. The presence of reducers (i.e., antioxidants) causes the conversion of the Fe³⁺/ferricyanide complex used in this method to the ferrous form. Therefore, by measuring the formation of Perl’s Prussian blue at 700 nm, we can monitor the Fe²⁺ concentration; a higher absorbance at 700 nm indicates a higher reducing power. The total AC per g of fresh grape varied between 3.69 ± 0.13 mg AAEA in Cardinal 1 and 16.58 ± 0.07 mg AAEA in Gros noir, with intermediate values for the other four varieties: 11.85 ± 0.36, 9.19 ± 0.12, 6.26 ± 0.09 and 5.04 ± 0.23 mg AAEA in Muscat noir, Muscat blanc, Cardinal 2 and Victoria, respectively.

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Figure 2. Reducing power of grape methanolic extract. The data are displayed with mean ± standard deviation (bars). Values marked by different letters are significantly different (p<0.05). Reducing power is expressed as mg of ascorbic acid equivalent antioxidant/g of fresh weight.

Methanolic extracts from Gros noir, Muscat noir and Muscat blanc showed higher reducing power values than those from Victoria and Cardinal 1 and 2 (Fig. 2). There was no clear trend in terms of AC and phenolic content between white and red grape cultivar as a group. It is the individual cultivar that matters. For example, Muscat blanc (white grape) had a lower TPP content than Cardinal 2 (red grape) but a higher reducing power. This can be explained by the fact that Muscat blanc had high carotenoid levels. In this case, it is not possible to rank white and red grape cultivars as a group in terms of AC, as this activity is more related to individual cultivar. This is in accordance with the results found by Gil et al. (2002) on five different cultivars of white-flesh nectarines and yellow-flesh nectarines from California. It was reported that the reducing properties are generally associated with the presence of reductones, which have been shown to exert antioxidant action by breaking the free radical chain by donating a hydrogen atom. Grape varieties with high reducing power might contain higher amounts of reductones, which could react with free radicals to stabilize and block radical chain reactions (Barros et al., 2007). Thus, the AC of an extract cannot be predicted on the basis of its total phenolic content.

6. Correlation

The TPP content of the different varieties studied followed the same order as that of TF (Gros noir > Muscat noir > Cardinal 2 > Victoria > Muscat blanc > Cardinal 1), resulting in a high positive correlation coefficient (r² = 0.937; Fig. 3a). A similar relation was found by Kim et al. (2003) in six cultivars of plums (r² = 0.934). Total AC was strongly correlated with total phenolics (r² = 0.820; Fig. 3b). A similar relation was found in 16 red grape cultivars (r² = 0.756; Orak, 2007), in nectarine, peach and plum cultivars from California (r² > 0.90; Gil et al., 2002), and in some plum cultivars (r² = 0.938; Kim et al., 2003). Dani et al. (2007) also observed a positive correlation between TPP (Folin-Ciocalteu) and AC (DPPH) in different red grape juices. The same authors have shown that for grape juices made from Vitis labrusca grapes, considered an antioxidant source, the biological activity is influenced not only by the phenolic levels but also by the agricultural method used.

AC was also correlated with TF content (r² = 0.881; Fig. 3c); Kim et al. (2003) also found good correlation (r² = 0.942) between TF and AC of plums and apples with a high significance level (p<0.001).

These results imply that polyphenols and flavonoids may play an important role in free radical scavenging activity, thus contributing significantly to the AC of the grape extracts.

In our investigation, another correlation was found between AC and carotenoid content (r² = 0.541, r = 0.74; Fig. 3d) and between AC and vitamin C content (r² = 0.514, r = 0.72; Fig. 3e), which indicates that these compounds also contribute to the AC of grape. In contrast, Gil et al. (2002) did not find a correlation between vitamin C or carotenoids and AC in their study, while Rufino et al. (2010), in their work on 18 non-traditional tropical fruits from Brazil, did find a positive correlation between vitamin C content and AC (ABTS, r = 0.70 and FRAP, r = 0.70). A similar result was found by Jabłońska-Ryś et al. (2009) in their study on seven wild fruits from the Lublin region (Poland) (ABTS, r² = 0.925 and FRAP, r² = 0.984).

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Figure 3. Simple linear regression analysis. Correlations between TPP and TF (a); TPP and AC (b); TF and AC (c); carotenoids and AC (d); vitamin C and AC (e). TPP: total polyphenols; TF: total flavonoids; AC: antioxidant capacity (reducing power); FW, fresh weight.

Conclusion

Five grape varieties cultivated in Algeria were compared regarding TPP, TF, carotenoids, vitamin C and antioxidant activity. The white grape varieties have a high carotenoid content (except Victoria), as high as the blue-black varieties, while the blue-black varieties have a higher TPP content and a higher AC than the white grape varieties. Vitis vinifera grapes originating from table grape varieties can be used as a potential source of natural antioxidants, which can be attributed to their content of phenolic compounds. In turn, they may provide health-promoting effects to consumers, since it has been suggested that a higher consumption of fruits and vegetables with high phytochemical content can inhibit, prevent or retard chronic diseases. The grapes in our study showed substantial antioxidant activity. An increased consumption of these fruits is therefore recommended in our diet. The extension of this work to future vintages should refine these conclusions and further studies on individual phenolic compounds are needed to elucidate the different antioxidant mechanisms and possible synergism.

Acknowledgements:

The authors would like to thank Dr Yves Cadot for his help in interpreting results.

References

• Barros L., Ferreira M.J., Queiros B., Ferreira I.C.F.R. & Baptista P. (2007). Total phenols, ascorbic acid, b-carotene and lycopene in Portuguese wild edible mushrooms and their antioxidant activities. Food Chemistry, 103, 413-419. doi:10.1016/j.foodchem.2006.07.038
• Bucić-Kojić A., Planinić M., Tomas S., Jokić S., Mujić I., Bilić M. & Velić D. (2011). Effect of extraction conditions on the extractability of phenolic compounds from lyophilised fig fruits (Ficus carica L.). Polish Journal of Food and Nutrition Sciences, 61, 195-199. doi:10.2478/v10222-011-0021-9
• Bunea C I., Pop N., Babeş A.C., Matea C., Dulf F.V., Bunea A. (2012). Carotenoids, total polyphenols and antioxidant activity of grapes (Vitis vinifera) cultivated in organic and conventional systems. Chemistry Central Journal, 6, 1-9. doi:10.1186/1752-153X-6-66
• Burin V.M., Falcão L.D., Gonzaga L.V., Fett R., Rosier J.P. & Bordignon-Luiz M.T. (2010). Colour, phenolic content and antioxidant activity of grape juice. Ciência e Tecnologia de Alimentos, 30, 1027-1032. doi:10.1590/S0101-20612010000400030
• CODEX STAN 247 (2005). Norme Générale Codex pour les Jus et les Nectars de Fruits. Codex Alimentarius.
• Crupi P., Coletta A. & Antonacci D. (2010). Analysis of carotenoids in grapes to predict norisoprenoid varietal aroma of wines from Apulia. Journal of Agricultural and Food Chemistry, 58, 9647-9656. doi:10.1021/jf100564v
• Dani C., Oliboni L.S., Vanderlinde R., Bonatto D., Salvador M. & Henriques J.A.P. (2007). Phenolic content and antioxidant activities of white and purple juices manufactured with organically- or conventionally-produced grapes. Food and Chemical Toxicology, 45, 2574-2580. doi:10.1016/j.fct.2007.06.022
• Del Caro A., Piga A., Vacca V. & Agabbio M. (2004). Changes of flavonoids, vitamin C and antioxidant capacity in minimally processed citrus segments and juices during storage. Food Chemistry, 84, 99-105. doi:10.1016/S0308-8146(03)00180-8
• Dou H., Jones S., Obrea T. & Rouse B. (2005). Influence of various phosphorus and potassium rates on juice vitamin c, β-carotene, lycopene and sugar concentrations of flame grapefruit. Proceedings of the Florida State Horticultural Society, 118, 372-375.
• Du B., He B.J., Shi P.B., Li F.Y., Li J. & Zhu F.M. (2012). Phenolic content and antioxidant activity of wine grapes and table grapes. Journal of Medicinal Plants Research, 6, 3381-3387.
• Gil M.I., Tomas-Barberan F.A., Hess-Pierce B. & Kader A.A. (2002). Antioxidant capacities, phenolic compounds, carotenoids, and vitamin C contents of nectarine, peach, and plum cultivars from California. Journal of Agricultural and Food Chemistry, 50, 4976-4982. doi:10.1021/jf020136b
• Giovanelli G. & Brenna O.V. (2007). Evolution of some phenolic components, carotenoids and chlorophylls during ripening of three Italian grape varieties. European Food Research and Technology, 225, 145-150. doi:10.1007/s00217-006-0436-4
• Imre S.P., Kilmartin P.A., Rutan T., Mauk J.L. & Nicolau L. (2012). Influence of soil geochemistry on the chemical and aroma profiles of Pinot noir wines. Journal of Food, Agriculture & Environment, 10, 280-288.
• Jabłońska-Ryś E., Zalewska-Korona M. & Kalbarczyk J. (2009). Antioxidant capacity, ascorbic acid and phenolics content in wild edible fruits. Journal of Fruit and Ornamental Plant Research, 17, 115-120.
• Jakobek L., Seruga M., Voca S., Sindrak Z. & Dobricevic N. (2009). Flavonol and phenolic acid composition of sweet cherries (cv. Lapins) produced on six different vegetative rootstocks. Scientia Horticulturae, 123, 23-28. doi:10.1016/j.scienta.2009.07.012
• Jayaprakasha G.K., Singh R.P. & Sakariah K.K. (2001). Antioxidant activity of grape seed (Vitis vinifera) extracts on peroxidation models in vitro. Food Chemistry, 73, 285-290. doi:10.1016/S0308-8146(00)00298-3
• Kamffer Z., Bindon K.A. & Oberholster A. (2010). Optimization of a method for the extraction and quantification of carotenoids and chlorophylls during ripening in grape berries (Vitis vinifera cv. Merlot). Journal of Agricultural and Food Chemistry, 58, 6578-6586. doi:10.1021/jf1004308
• Kim D.O., Jeong S.W. & Lee C.Y. (2003). Antioxidant capacity of phenolic phytochemicals from various cultivars of plums. Food Chemistry, 81, 321-326. doi:10.1016/S0308-8146(02)00423-5
• Lancaster J.E., Lister C.E., Reay P.F. & Triggs C.M. (1997). Influence of pigment composition on skin color in a wide range of fruit and vegetables. Journal of the American Society for Horticultural Science, 122, 594-598.
• Lee S.K. & Kader A.A. (2000). Preharvest and postharvest factors influencing vitamin C content of horticultural crops. Postharvest Biology and Technology, 20, 207-220. doi:10.1016/S0925-5214(00)00133-2
• Lee S.H., Seo M.J., Riu M., Cotta J.P., Block D.E., Dokoozlian N.K. & Ebeler S.E. (2007). Vine microclimate and norisoprenoid concentration in Cabernet Sauvignon grapes and wines. American Journal of Enology and Viticulture, 58, 291-301.
• Lu Y., Zhang C., Bucheli P. & Wei D. (2006). Citrus flavonoids in fruit and traditional chinese medicinal food ingredients in China. Plant Foods for Human Nutrition, 61, 57-65. doi:10.1007/s11130-006-0014-8
• Makris D.P., Kallithraka S. & Kefalas P. (2006). Flavonols in grapes, grape products and wines: burden, profile and influential parameters. Journal of Food Composition and Analysis, 19, 396-404. doi:10.1016/j.jfca.2005.10.003
• Marinova D., Ribarova F. & Atanassova M. (2005). Total phenolics and total flavonoids in Bulgarian fruits and vegetables. Journal of the University of Chemical Technology and Metallurgy, 40, 255-260.
• Mulero J., Pardo F. & Zafrilla P. (2010). Antioxidant activity and phenolic composition of organic and conventional grapes and wines. Journal of Food Composition and Analysis, 23, 569-574. doi:10.1016/j.jfca.2010.05.001
• Nickavar B., Alinaghi A. & Kamalinejad M. (2008). Evaluation of the antioxidant properties of five Mentha species. Iranian Journal of Pharmaceutical Research, 7, 203-209.
• Njoku P.C., Ayuk A.A. & Okoye C.V. (2011). Temperature effects on vitamin C content in citrus fruits. Pakistan Journal of Nutrition, 10, 1168-1169. doi:10.3923/pjn.2011.1168.1169
• Official Journal of Algerian Republic (1989). NA 301: Grape Juice: Specification.
• Orak H.H. (2007). Total antioxidant activities, phenolics, anthocyanins, polyphenoloxidase activities of selected red grape cultivars and their correlations. Scientia Horticulturae, 111, 235-241. doi:10.1016/j.scienta.2006.10.019
• Prvulović D., Malenčić D., Popović M., Ljubojević M. & Ognjanov V. (2011). Antioxidant properties of sweet cherries (Prunus avium L.) - Role of phenolic compounds. World Academy of Science, Engineering and Technology, 59, 1149-1152.
• Revilla E. & Ryan J.M. (2000). Analysis of several phenolic compounds with potential antioxidant properties in grape extracts and wines by high-performance liquid chromatography–photodiode array detection without sample preparation. Journal of Chromatography A, 881, 461-469. doi:10.1016/S0021-9673(00)00269-7
• Rockenbach I.I., Gonzaga L.V., Rizelio V.M., de Souza Schmidt Gonçalves A.E., Genovese M.I. & Fett R. (2011). Phenolic compounds and antioxidant activity of seed and skin extracts of red grape (Vitis vinifera and Vitis labrusca) pomace from Brazilian winemaking. Food Research International, 44, 897-901. doi:10.1016/j.foodres.2011.01.049
• Rufino M.S.M., Alves R.E., de Brito E.S., Pérez-Jiménez J., Saura-Calixto F. & Mancini-Filho J. (2010). Bioactive compounds and antioxidant capacities of 18 non-traditional tropical fruits from Brazil. Food Chemistry, 121, 996-1002. doi:10.1016/j.foodchem.2010.01.037
• Sass-Kiss A., Kiss J., Milotay P., Kerek M.M. & Toth-Markus M. (2005). Differences in anthocyanin and carotenoid content of fruits and vegetables. Food Research International, 38, 1023-1029. doi:10.1016/j.foodres.2005.03.014
• Sharique A. & Beigh S.H. (2009). Ascorbic acid, carotenoids, total phenolic content and antioxidant activity of various genotypes of Brassica Oleracea encephala. Journal of Medical and Biological Sciences, 3, 8 p.
• Stefanovits-Bányai É., Tulok M.H., Hegedûs A., Renner C. & Varga I.S. (2003). Antioxidant effect of various rosemary (Rosmarinus officinalis L.) clones. Acta Biologica Szegediensis, 47, 111-113.
• Veluri R., Singh R.P., Liu Z., Thompson J.A., Agarwal R. & Agarwa C. (2006). Fractionation of grape seed extract and identification of gallic acid as one of the major active constituents causing growth inhibition and apoptotic death of DU145 human prostate carcinoma cells. Carcinogenesis, 27, 1445-1453. doi:10.1093/carcin/bgi347
• Voutilainen S., Nurmi T., Mursu J. & Rissanen T.H. (2006). Carotenoids and cardiovascular health. American Journal of Clinical Nutrition, 83, 1265-1271.
• Yang J., Martinson T.E. & Liu R.H. (2009). Phytochemical profiles and antioxidant activities of wine grapes. Food Chemistry, 116, 332-339. doi:10.1016/j.foodchem.2009.02.021