Total phenolic and flavonoid contents and antioxidant activities of sixteen commercial date cultivars grown in Saudi Arabia

Abstract

The total phenolic and flavonoid contents and flavonoids/phenolics% of sixteen commercial Saudi date cultivars at the Tamer stage were measured. The antioxidant activities of dates using scavenging assays of 1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) were investigated. A strong correlation was existed between antioxidant activity and phenolic concentration of the date cultivars. Date cultivars had different phenolic and flavonoid patterns and exhibited also different antioxidant capacities. All of the tested cultivars were growing in the same district (Al-madinah Al-Munawwarah), accordingly the observed variations are mainly due to the genetically factor. The results concluded that flavonoid compounds were more sensitive toward DPPH assay than ABTS assay. The correlation of the IC₅₀ between DPPH and ABTS radical scavenging were evaluated. The ABTS assay was more sensitive toward phenolic compounds than DPPH assay for the most tested date cultivars, except of the phenolic compounds of cv. Agwah and cv. Safawi had the same sensitivity toward DPPH and ABTS assays. The results confirmed the antioxidant potential of the most commercial Saudi date cultivars.

---

Introduction

Date palm (Phoenix dactylifera, L.) is the most successful fruit tree in many arid regions, being a critical subsistence crop.^1,2 Accordingly, dates are one of the most commonly consumed fruit in the Kingdom of Saudi Arabia (KSA) and other Gulf countries.^3,4 In KSA, the production of dates of several excellent commercial cultivars reached about one million ton in 2010 season.⁴ It is well known that fruit especially dates, is one of the main source of naturally dietary antioxidants such as phenolic compounds that critically required for keeping human health.^3,5,6 Dates are also rich in carbohydrates, fiber and essential minerals such as calcium, iron, magnesium, phosphorus, potassium and zinc.^7–9 The crude protein, carbohydrates and fatty acids were also estimated in seeds of different Saudi date varieties.¹⁰ Plants synthesized phytochemicals such as phenolic compounds mainly as a protective mechanism against environmental stress (heat, drought and excessive UV light), predator and microbiological attack. In dates, as in other fruit, the accumulation of phenolics including flavonoids greatly vary upon genetical, developmental and environmental factors as well as postharvest handling conditions of fruit.³

Phytochemicals of fruit have been possessed significant antioxidant activities.⁵ Accordingly, the antioxidant properties of fruit vary upon their content of phenolic compounds, vitamins C and E, and carotenoids.¹¹ Antioxidants quench reactive free radicals and prevent the oxidation of other biological molecules.¹² There is a growing interest in fruit antioxidant compounds because of its high capacity in scavenging free radicals related to various human diseases.¹³ The antioxidant showed association with lower incidence and lower mortality rates of degenerative diseases in human.¹⁴ Fruit and vegetables rich in phytochemicals and antioxidants reduced risks of developing chronic diseases such as cancer, cardiovascular disorders and neurological diseases.¹⁵ Frequent consumption of fruit and vegetables was associated with a lower risk of cancer, heart disease, hypertension and stroke.¹⁶ The health benefits of fruit and vegetables were attributed to their phenolic compounds and their antioxidant capacities.^17,18 Dates have been used in folk medicine, and evaluated for their role in protection against hypertension, cancer, infections and heart diseases.⁵ Several studies on the antioxidant activity of many date cultivars from different producing countries such as KSA, Algeria, Tunisia and Iran were reported.^{3,7,8,19–24}

Dates pass through several developmental stages designated by the Arabic terms, Hababouk, Kimri, Bisir or Khalal, Rutab and Tamer that represent, respectively, the cell division, cell elongation or the immature green, the mature firm full colored, the soft brown and the hard raisin-like fruit.³ Dates can be consumed at three stages of their development mainly Bisir, Rutab and Tamer depending on cultivar characteristics especially soluble tannin level, climatological conditions and market demand.²⁵ In KSA, most of date cultivars are consumed at the Tamer stage (Fig. 1).^3,6 Several analytical methods have been used in determining and comparison of the antioxidant activity of fruit and vegetables such as DPPH, ABTS, FRAP, TRAP, ORAC and HORAC.^3,6,19,26 In most cases, there are good correlations among the different methods applied for evaluating the antioxidant properties. However, using more than one antioxidant assay was recommended for understanding the principles of antioxidant properties of fruit. Positive correlations between antioxidant activity assayed by DPPH and FRAP and phenolic concentration have been previously reported in dates^3,6,22 and in other fruits.^27–29 High correlation between antioxidant, assayed by DPPH and FRAP techniques, and total phenols was also found in other fruit such as nectarines, peaches and plums²⁷ and guava.²⁹ Whereas, high correlation between antioxidant activity and vitamin C was only found in fruit that contain high vitamin C such as guava.²⁹ However, Gil et al.²⁷ found no correlation between vitamin C and antioxidant activity as determined by DPPH or FRAP assays in nectarines, peaches and plums. Also, Yu et al.³⁰ showed no significant correlation between total phenols concentration and radical scavenging capacity in cereal samples. Such conflictions were attributed to that different phenolic compounds have different responses in the Folin–Ciocalteu method.³¹ Also, not all of the phenolic compounds are active radical scavengers or have the same matrix effect.³²

Fig. 1 Saudi date (Phoenix dactylifera L.) at the Tamer stage.

There some published research focused on a limited number of Saudi date cultivars regarding their total phenolics, flavonoids and the antioxidant activities.^3,6,9,19 The same patterns were also evaluated in seeds of different Saudi date varieties.¹⁰ Quantitative analyses of phenolic and flavonoid compounds found in Saudi date cultivars were determined using HPLC-DAD techniques. Nine free phenolic acids (caffeic acid, ferulic acid, protocatechuic acid, catechin, gallic acid, p-coumaric acid, resorcinol, chlorogenic acid and syringic acid) and five flavonoid compounds (quercetin, luteolin, isoquercetrin, apigenin and rutin) were determined.³³ However, no available information on many other commercially important dates cultivars. Accordingly, the aim of this study was to determine the total phenolic and flavonoid contents and evaluate the antioxidant activity of sixteen highly commercial Saudi date cultivars at the Tamer stage. The correlation of IC₅₀ as phenolic concentration between DPPH and ABTS radical scavenging were also evaluated.

Experimental

Plant material

Fresh date samples from sixteen commercial and widely consumed cultivars at the Tamer stage were collected few days following harvest from a local market in Jeddah, KSA. The experiment was repeated 3 times.

Preparation of methanol extract

Two g dried date peel was extracted by shaking at 150 rpm for 24 h with 20 mL of 80% methanol and filtered through filter paper no. 1. The filtrate designated as methanol extract. The experiment was repeated 3 times.

Estimation of phenolic content

Total phenolic content was measured according to Velioglu et al.³⁴ Fifty μL of the methanol extract was mixed with 100 μL Folin–Ciocalteu reagent, 850 μL of methanol and allowed to stand for 5 min at ambient temperature. A 500 μL of 20% sodium carbonate was added and allowed to react for 30 min. Absorbance was measured at 750 nm. Total phenolic content was quantified from a calibration curve obtained by measuring the absorbance of known concentrations of gallic acid. The results are expressed as mg gallic acid equivalent (GAE)/100 g tissues.

Estimation of flavonoid content

Total flavonoid content was determined using a modified colorimetric method described by Zhishen et al.³⁵ and used catechin as a standard. Methanol extract or standard solution (250 μL) was mixed with distilled water (1.25 mL) and 5% NaNO₂ solution (75 μL). After standing for 6 min, the mixture was combined with 10% AlCl₃ solution (150 μL). One M NaOH (0.5 mL) and distilled water (275 μL) was added to the mixture 5 min later. The absorbance of the solutions at 510 nm was then measured. Total flavonoid content was quantified from a calibration curve obtained by measuring the absorbance of known concentrations of catchin. The results expressed as mg catechin equivalent (CE)/100 g tissues.

Evaluation of antioxidant activity

DPPH radical scavenging assay. Free radical scavenging activity of methanol extract was determined using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) method.³⁶ A methanol extract (0.1 mL) was added to 0.9 mL of freshly prepared DPPH methanol solution (0.1 mM). An equal amount of methanol was used as a control. After incubation for 30 min at room temperature in the dark, the optical density (OD) was measured at 517 nm using a spectrophotometer. Activity of scavenging (%) was calculated using the following formula:

DPPH radical scavenging% = [(OD control – OD sample)/OD control] × 100

IC₅₀ value was the inhibition concentration as μg phenolic concentration of the test sample that decreases 50% of initial radical. The IC₅₀ values were calculated from the dose responses curves.

ABTS radical cation decolorization assay

ABTS (2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)) also forms a relatively stable free radical, which decolorizes in its non-radical form. The spectrophotometric analysis of ABTS˙⁺ scavenging activity was determined according to the method of Re et al.³⁷ ABTS˙⁺ was produced by reacting 7 mM ABTS in H₂O with 2.45 mM potassium persulfate (K₂S₂O₈), store in the dark at room temperature for 16 h. The ABTS˙⁺ solution was diluted to give an absorbance of 0.750 ± 0.025 at 734 nm in 0.1 M sodium phosphate buffer (pH 7.4). Then, 1 mL of ABTS˙⁺ solution was added to crude methanol extract. The absorbance was recorded 1 min after mixing and the percentage of radical scavenging was calculated relative to a blank containing no scavenger. The extent of decolorization was calculated as percentage reduction of absorbance. The scavenging capability of test compounds was calculated using the following equation:

ABTS˙⁺ scavenging (%) = [(OD control – OD sample)/OD control] × 100.

IC₅₀ value is the inhibition concentration as μg phenolic concentration of the test sample that decreases 50% of initial radical. The IC₅₀ values were calculated from the dose responses curves.

Statistical analysis

The statistical analyses were performed by a one-way ANOVA and the Student's t-test. The results were expressed as means ± S.E. Difference are considered significant when P < 0.05.

Results and discussion

Table 1 shows the total phenolic content of sixteen Saudi date cultivars. The total phenolic content ranged from 122 to 247 mg GAE/100 g DW. These concentration were in the order of Safawi > Dahlah > Agwah > Anberi > Saqeei > Loban > Barni > Mabroum > Sefrei > Red Sukarrei > Khlase > Roshodi > Khudari > Sukarrei > Nabtat Ali > Shalabe. There were large significant variations (P < 0.05) in total phenolic content among the studied cultivars. These variations could be attributed to genetical and environmental factors. However, all of the tested cultivars were growing in the same district (Al-madinah Al-Munawwarah), accordingly the observed variations are mainly due to the genetically factor. The similar results were reported by Al-Farsi et al.,³⁸ where the total phenolic contents ranged from 172 and 246 mg GAE/100 g FW for Omani dates. The highest phenolic content was found in Khulas with a mean value of 198 mg GAE/100 g compared with other Saudi cultivars.⁹ The higher phenolic content was reported for date cultivars from USA (572 to 661 mg GAE/100 g FW)³⁹ and Tunisia (209–448 mg GAE/100 g FW).⁴⁰ The lowest phenolic content was reported for Iran cultivars (2.89–6.64 mg GAE/100 g DW), except kharak date (141 mg GAE/100 g DW).⁸ The total phenol contents of seeds of some Saudi date cultivars were found between 1.98 mg GAE/100 g (Barhi cv) and 4.65 mg GAE/100 g (Soughi cv).¹⁰

Table 1 The total phenolic and flavonoid contents and flavonoids/phenolics% in Saudi date cultivars^a

Date cultivar	mg GAE/100 g DW	mg CE/100 g DW	CE/GAE (%)
a GAE, gallic acid equivalent, CE, catchin equivalent values are presented as means ± SD (n = 3).
Agwah	212 ± 12	145 ± 8.5	68
Loban	182 ± 9.5	159 ± 9.2	87
Mabroum	168 ± 11	61 ± 2.3	36
Khudari	142 ± 6	69 ± 3.5	48
Safawi	247 ± 14	191 ± 10	77
Sefrei	161 ± 8	96 ± 5	59
Nabtat Ali	122 ± 0.055	100 ± 6	81
Red Sukarrei	151 ± 10	104 ± 7	68
Shalabe	119 ± 8.5	89 ± 4	74
Saqeei	192 ± 7.2	83 ± 3.5	43
Sukarrei	141 ± 5.4	105 ± 4.3	74
Khlase	149 ± 7.5	27 ± 1.5	18
Roshodi	145 ± 6.6	63 ± 2.3	43
Dahlah	215 ± 10	97 ± 4	45
Barni	180 ± 12	52 ± 2.2	28
Anberi	211 ± 13	72 ± 3.1	34

---

The total flavonoid content of the sixteen date cultivars was shown in Table 1. The results showed that the total flavonoid content varied from 27 to 199 mg CE/100 g DW. The order of the total flavonoid content was Safawi > Loban > Agwah > Sukarri > Red Sukarrei > Nabtat Ali > Dahlah > Sefrei > Shalabe > Saqeei > Anbari > Khudari > Roshodi > Mabroum > Barni > Khlase. The high significant variations (P < 0.05) in total flavonoids content among the tested cultivars were detected. A seven-fold difference in total flavonoid content between Safawi and Khlase cultivars was reported. Different of total flavonoid contents were detected for date cultivars from Algeria (15.22–299.74 mg QE/100 g DW),²³ Iran (1.62–81.79 mg CE/100 g DW)⁸ and Tunisia (6.41–54.46 mg QE/100 g FW).⁴¹

Several methods have been used to evaluate the antioxidant activity of different plants. Usually, these methods measured the ability of antioxidants to scavenge the free radicals. In the present work, DPPH and ABTS were used to evaluate the antioxidant capacity of sixteen Saudi dates. Fig. 2 shows the structure of DPPH and ABTS. Table 2 revealed clearly that the cultivars tested have different free radical scavenging capacities using DPPH assay. Significant differences (P < 0.05) in the antioxidant activity were detected among the different cultivars. The DPPH assay IC₅₀ value (the inhibition concentration as μg GAE of the test sample that decreases 50% of initial radical) was detected for different cultivars. IC₅₀ value of date cultivars varied from 1.65 to 8.25 μg GAE. The order of DPPH IC₅₀ value was Agwah < Shalabe < Nabtat Ali < Loban < Safawi < Red Sukarrei < Dahlah < Sukarrei < Anberi < Roshodi < Sefrei < Khudri < Barni < Saqeei < Mabroum < Khlase. The correlation coefficient (R²) between phenolic concentrations of date cultivars and DPPH scavenging activity ranged from 0.957 to 0.999 indicating the strong correlation. It has been reported that different cultivars of dates exhibited different potent DPPH scavenging capacities.^8–10,22,23 The ABTS assay IC₅₀ values of date cultivars varied from 0.5 to 4.45 μg GAE (Table 3). The order of ABTS IC₅₀ value was Dahlah, Red Sukarrei < Sukarrei < Roshodi < Shalabe < Mabroum < Sefrei < Khudari < Khalase < Agwah < Loban < Nabtat Ali < Anberi < Saqeei < Barni < Safawi. The correlation coefficient (R²) between phenolic concentration of date cultivars and ABTS scavenging activity ranged from 0.950 to 0.997 indicating a strong correlation. The correlation analyses indicated that there was a linear relationship between antioxidant activity measured by ABTS and the phenolic concentrations of various date cultivars in Iran.⁸

Fig. 2 The structure of DPPH and ABTS.

Table 2 Antioxidant effect of GAE of Saudi date cultivars on reduction of DPPH radical scavenging^a

Date cultivar	IC50 (μg GAE)	Correlation coefficient (R^2)
a GAE, gallic acid equivalent; R^2, correlation coefficient IC50: is the inhibition concentration as μg GAE of the test sample that decreases 50% of DPPH radical. values are presented as means ± SD (n = 3).
Agwah	1.65 ± 0.04	0.957
Loban	4.37 ± 0.12	0.999
Mabroum	7.9 ± 0.24	0.984
Khudari	6.9 ± 0.18	0.995
Safawi	5.2 ± 0.095	0.990
Sefrei	6.8 ± 0.16	0.992
Nabtat Ali	4.17 ± 0.08	0.999
Red Sukarrei	5.22 ± 0.10	0.997
Shalabe	4.1 ± 0.095	0.899
Saqeei	7.34 ± 0.21	0.985
Sukarrei	5.66 ± 0.15	0.989
Khlase	8.25 ± 0.21	0.994
Roshodi	6.67 ± 0.13	0.989
Dahlah	5.53 ± 0.14	0.980
Barni	7.18 ± 0.24	0.989
Anberi	6.0 ± 0.12	0.998

---

Table 3 Antioxidant effect of GAE of Saudi date cultivars on reduction of ABTS radical scavenging^a

Date cultivar	IC50 (μg GAE)	Correlation coefficient (R^2)
a GAE, gallic acid equivalent; R^2, correlation coefficient IC50: is the inhibition concentration as μg GAE of the test sample that decreases 50% of ABTS radical. values are presented as means ± SD (n = 3).
Agwah	1.62 ± 0.025	0.986
Loban	1.68 ± 0.016	0.992
Mabroum	1.19 ± 0.014	0.987
Khudari	1.35 ± 0.018	0.989
Safawi	4.45 ± 0.12	0.976
Sefrei	1.32 ± 0.03	0.970
Nabtat Ali	1.83 ± 0.025	0.989
Red Sukarrei	0.5 ± 0.008	0.997
Shalabe	1.04 ± 0.032	0.974
Saqeei	2.04 ± 0.06	0.993
Sukarrei	0.57 ± 0.013	0.980
Khlase	1.53 ± 0.016	0.989
Roshodi	0.85 ± 0.018	0.993
Dahlah	0.5 ± 0.02	0.975
Barni	2.26 ± 0.095	0.950
Anberi	1.95 ± 0.023	0.991

---

From the above results, there was significant correlation between flavonoid/phenolic% (CE/GAE%) of date cultivars and the potent of antioxidant activity as IC₅₀ GAE (Table 1–3). This correlation indicated that the decrease of CE/GAE% was accompanied with increasing of IC₅₀ especially for DPPH, and vice versa. This correlation was reported for most date cultivars such as Khlase (CE/GAE 18%, IC₅₀ 8.25 μg), Mabroum (CE/GAE 36%, IC₅₀ 7.9 μg), Loban (CE/GAE 87%, IC₅₀ 4.36 μg) and Shalabe (CE/GAE 74%, IC₅₀ 4.1 μg). These results indicated that flavonoid concentration is the main components responsible for scavenging DPPH radicals, in contrast to ABTS. The results concluded that flavonoid compounds were more sensitive toward DPPH assay than ABTS assay. These results are within line with those of Benmeddoura et al.²³ who also reported that flavonoid concentration was significantly correlated with DPPH assay, reflecting the potent contribution of flavonoids to the antioxidant capacity of date cultivars. The role of flavonoids of date cultivars on the antioxidative potentials using ABTS assay was investigated by Biglari et al.⁸ They reported that, in pistachio cultivars, the significant strong and positive correlations between total phenolics and total flavonoids, and DPPH indicated that a substantial part of total phenolics consisted of flavonoids was mainly attributed to phenolics and particularly to flavonoids,⁴² and this finding agreed with others.^43,44 Furthermore, in the present work, the ABTS assay was more sensitive toward phenolic compounds than DPPH assay for most date cultivars tested. The differences in the IC₅₀ (μg GAE) between DPPH and ABTS assays of date cultivars were observed, except of Agwah and Safawi (Fig. 3). The DPPH IC₅₀ and ABTS IC₅₀ for phenolics of Agwah (IC₅₀ 1.65 and 1.62 μg GAE, respectively) and Safawi (IC₅₀ 5.2 and 4.45 μg GAE, respectively) were very close. These results indicated that the phenolic compounds of Agwah and Safawi had the same sensitivity towards DPPH and ABTS assays and the phenolic compounds of other date cultivars might have different potentials toward the two assays. This is might be due to that each individual phenolic compound has different contribution to the antioxidant activity.⁴⁵ In conclusion, the total phenolic and flavonoid contents of sixteen commercially important Saudi dates of several cultivars were detected. The antioxidant activities using DPPH and ABTS assays of Saudi dates were determined. A strong correlation existed between antioxidant activity and phenolic concentration of dates. The ABTS assay was more sensitive toward phenolic compounds than DPPH assay for the most tested date cultivars. The results showed that flavonoid content was the main components responsible for scavenging DPPH radicals. The phenolic compounds of cv. Agwah and cv. Safawi had the same sensitivity toward DPPH and ABTS assays.

Fig. 3 The differences in the IC₅₀ (μg GAE) between DPPH and ABTS assays of date cultivars: 1 Agwah, 2 Loban, 3 Mabroum, 4 Khudari, 5 Safawi, 6 Sefrei, 7 Nabtat Ali, 8 Red Sukarrei, 9 Shalabe, 10 Saqeei, 11 Sukarrei, 12 Khlase, 13 Roshodi, 14 Dahlah, 15 Barni, 16 Anberi. Points are presented as means ± SD (n = 3).

Conclusion

This study confirm the antioxidant potentials of some highly commercial Saudi dates, which are good sources of natural antioxidant compounds suitable for food and pharmaceutical applications.

References

1. C. C. T. Chao and R. R. Krueger, Hortic. Sci., 2007, 42, 1077–1082.
2. T. Youssef and M. A. Awad, J. Plant Growth Regul., 2008, 27, 1–9.
3. M. A. Awad, A. D. Al-Qurashi and S. A. Mohamed, Sci. Hortic., 2011, 129, 688–693.
4. A. M. R. Elsabea, Ann. Agric. Sci., 2012, 57, 153–159.
5. P. K. Vayalil, Crit. Rev. Food Sci. Nutr., 2012, 52, 249–271.
6. S. A. Mohamed, M. A. Awad and A. D. Al-Qurashi, Sci. Hortic., 2014, 167, 91–99.
7. W. Kchaoua, F. Abbèsa, C. Blecker, H. Attia and S. Besbes, Ind. Crops Prod., 2013, 45, 262–269.
8. F. Biglari, A. AlKarkhi and A. M. Easa, Food Chem., 2008, 107, 1636–1641.
9. F. AL Juhaimi, K. Ghafoor and M. M. Özcan, Environ. Monit. Assess., 2014, 186, 2165–2170.
10. F. AL Juhaimi, K. Ghafoor and M. M. Özcan, Int. J. Food Sci. Nutr., 2012, 63, 84–89.
11. F. Saura-Calixto and I. Goni, Food Chem., 2006, 94, 442–447.
12. F. Shahidi, Chemistry, Health Effects, and Applications. AOCS Press, Urbana, IL, 1997.
13. E. M. Silva, J. N. S. Souza, H. Rogez, J. F. Rees and Y. Larondelle, Food Chem., 2007, 101, 1012–1018.
14. J. Javanmardi, C. Stushnoff, E. Locke and J. M. Vivanco, Food Chem., 2003, 83, 547–550.
15. M. Valko, D. Leibfritz, J. Moncol, M. T. Cronin, M. Mazur and J. Telser, Int. J. Biochem. Cell Biol., 2007, 39, 44–84.
16. J. Salta, A. Martins, R. G. Santos, N. R. Neng, J. M. F. Nogueira, J. Justino and A. P. Rauter, J. Funct. Foods, 2010, 2, 153–157.
17. J. A. John and F. Shahidi, J. Funct. Foods, 2010, 2, 196–209.
18. O. Kaisoon, S. Siriamornpun, N. Weerapreeyakul and N. Meeso, J. Funct. Foods, 2011, 3, 88–99.
19. A. R. Al-Najadaa and S. A. Mohamed, Sci. Hortic., 2014, 170, 275–280.
20. E. A. Amira, S. E. Behija, M. Beligh, L. Lamia, I. Manel, H. Mohamed and A. Lotfi, J. Agric. Food Chem., 2012, 60, 10896–10902.
21. A. Amors, M. T. Pretel, M. S. Almansa, M. A. Botella, P. J. Zapata and M. Serrano, Food Sci. Technol. Int., 2009, 15, 65–72.
22. A. Mansouri, G. Embarek, E. Kokkalou and P. Kefalas, Food Chem., 2005, 89, 411–420.
23. Z. Benmeddoura, E. Mehinagi, D. Le Meurlay and H. Louaileche, J. Funct. Foods, 2013, 5, 346–354.
24. A. M. Martn-Snchez, S. Cherif, J. Ben-Abda, X. Barber-Vallés, J. A. Pérez-Alvarez and E. Sayas-Barber, Food Chem., 2014, 158, 513–520.
25. M. A. Awad, Postharvest Biol. Technol., 2007, 43, 121–127.
26. M. Ciz, H. Cizova, P. Denev, M. Kratchanova, A. Slavov and A. Lojek, Food Control, 2010, 21, 518–523.
27. M. I. Gil, F. A. Tomas-Barberan, B. Hess-Pierce and A. A. Kader, J. Agric. Food Chem., 2002, 50, 4976–4982.
28. M. Leontowicz, S. Gorinstein, H. Leontowicz, R. Krzeminski, A. Lojek, E. Katrich, M. Ciz, O. Martin-Belloso, R. Soliva-Fortuny, R. Haruenkit and S. Trakhtenberg, J. Agric. Food Chem., 2003, 51, 5780–5785.
29. K. Thaipong, U. Boonprakob, K. Crosby, L. Cisneros-Zevallos and D. H. Byrne, J. Food Compos. Anal., 2006, 19, 669–675.
30. L. Yu, J. Perret, B. Davy, J. Wilson and C. L. Melby, J. Food Sci., 2002, 67, 2600–2603.
31. M. P. Kähkönen, A. I. Hopia, H. J. Vuorela, J. P. Rauha, K. Pihlaja, T. S. Kujala and M. Heinonen, J. Agric. Food Chem., 1999, 47, 3954–3962.
32. C. Stushnoff, A. E. McSay, P. L. Forsline and J. J. Luby, Acta Hortic., 2003, 623, 305–312.
33. I. Hamad, Life Sci. J., 2014, 11, 1268–1271.
34. Y. S. Velioglu, G. Mazza, L. Gao and B. D. Oomah, J. Agric. Food Chem., 1998, 46, 4113–4117.
35. J. Zhishen, T. Mengcheng and W. Jianming, Food Chem., 1999, 64, 555–559.
36. C. Ao, A. Li, A. A. Elzaawely, T. D. Xuan and S. Tawata, Food Control, 2008, 19, 940–948.
37. R. Re, N. Pellegrini, A. Proteggente, A. Pannala, M. Yang and C. Rice-Evans, Free Radical Biol. Med., 1999, 26, 1231–1237.
38. M. Al-Farsi, C. Alasalvar, M. Al-Abid, K. Al-Shoaily, M. Al-Amry and F. Al-Rawahy, Food Chem., 2007, 104, 943–947.
39. X. Wu, G. Beecher, J. Holden, D. Haytowitz, S. Gebhardt and R. Prior, J. Agric. Food Chem., 2004, 52, 4026–4037.
40. E. B. Saafi, A. El Arem, M. Issaoui, M. Hammami and L. Achour, Int. J. Food Sci. Technol., 2009, 44, 2314–2319.
41. N. Chaira, M. I. Smaali, M. Martinez-Tome, A. Mrabet, M. A. Murcia and A. Ferchichi, Int. J. Food Sci. Nutr., 2009, 60, 316–329.
42. E. Tsantili, K. Konstantinidis, M. V. Christopoulos and P. A. Roussos, Sci. Hortic., 2011, 129, 694–701.
43. G. Ballisteri, E. Arena and B. Fallico, Molecules, 2009, 14, 4358–4369.
44. A. Tomaino, M. Martorana, T. Arcoraci, D. Monteleone, C. Giovinazzo and A. Saija, Biochimie, 2010, 92, 1115–1122.
45. R. Blomhoff, M. H. Carlsen, L. F. Andersen and D. R. Jacobs, Br. J. Nutr., 2006, 96(S2), S52–S60.

---