A Comparison Investigation on Antioxidant Activities, Physicochemical Properties and Phytochemical Contents of Kiwifruit Genotypes and Cultivars

ABSTRACT

The present work was conducted to find the physicochemical properties, chemical components and antioxidant capacity of kiwifruit hydroalcoholic extracts (B-31, HO-1-14, J-242, Bruno, Greenlight, Hayward, Monty, Topstar) grown Yalova, Turkey. Phytochemical tests were performed to determine total phenolic, flavonoid and ascorbic acid. Moreover, the kiwifruit extracts were subjected to evaluate their antioxidant potential using different in vitro assays such as total antioxidant, inhibition of lipid peroxidation, reducing power, metal chelating, free radical, H₂O₂ and superoxide anion scavenging activities at 50–500 μg mL⁻¹, and also found EC₅₀ and IC₅₀ values. The soluble solid content (SSC), titratable acidity (TA) and pH were analyzed in all the genotypes and cultivars. The highest total phenolic, flavonoid and ascorbic acid content were found in J-242 (89.53 mg GAE/100 g), J-242 (109.13 mg QE/100 g) and HO-1-14 (64.15 mg/100 g), respectively. The linoleic, palmitic, stearic and oleic acid methyl esters were detected in all kiwifruit extracts by GC/MS. Significant differences were found between genotypes and cultivars in the antioxidant tests. The extracts showed high inhibition of lipid peroxidation and compared to standards, significantly (p < 0.05). The GC/MS analyses confirmed quantitative variability in the antioxidant profiles. The SSC, TA and pH of kiwifruit genotypes and cultivars showed a moderate level. The key parameters for the most active kiwifruit included the following: total antioxidant activity for B-31, H₂O₂ scavenging activity for J-242 and metal chelating activity for Topstar. In conclusion, this study exhibited the important role played by the genotypes and cultivars background on the chemical content and antioxidant profiles.

KEYWORDS:

• Kiwifruits
• genotype and cultivar
• phenolics
• fatty acids
• antioxidant activity

Introduction

Kiwifruit (Actinidia deliciosa A. Chev.) is known the Actinidiaceae family. It is a perennial and deciduous woody vine that is related to the Stellatae section of the Actinidia genus. Most of the Stellatae species developed south of the Yangzi River, probably related to the subtropical flora of Southeast Asia (Ferguson and Seal, 2008). The subtropical forests of China were characterized by low amounts of precipitation in winter or low temperatures with high rainfall and high temperatures in summer (Fang and Yoda, 1991). All Actinidia species are perennial, present vigorous growth and climbing and strangling characteristics and produce edible fruits (Ferguson and Huang, 2007; Ferguson and Seal, 2008). There are over 50 species of kiwifruit, with only a few being of commercial importance. The most familiar widely planted kiwifruit cultivars are A. deliciosa, Hayward, Bruno and Allison selected in New Zealand (Huang, 2016).

There are many cultivars and genotypes within the Actinidia genus. Fruit volume and shape, shell color and hairiness status, fruit, meat color and taste vary widely (Boyes et al., 1997; Ferguson, 1991). This diversity creates a genetic resource for the selection of many new cultivars and genotypes with commercial potential. Although the kiwifruit’s inner color is usually greenish, it is also known to be red-, purple-, yellow- and orange-colored species (Ferguson, 1991). Hayward, Bruno, Greenlight, Monty and Topstar (cultivars) have green fruit flesh color. Fruits are useful for human health with a high content of ascorbic acid (AA) (Towantakavanit et al., 2011; Yildirim et al., 2011), polyphenolics (D’evoli et al., 2015) and flavonoids (Fiorentino et al., 2009).

Kiwifruit as well as avocado, blueberry and macadamia were introduced to the world in the 20th century. In recent years kiwifruit production and consumption have increased noticeably in the world. According to FAO (Izli et al., 2017), the kiwifruit production in 22 different countries in the world is 3,261,474 tons. China, Italy, New Zealand, Chile, Greece, France and Iran are ranked first to seventh in term of the production rate of kiwifruit. Turkey is ranked eighth in the annual production of 31,795 tones.

Generally, the consumption of kiwifruit is preferred because the fresh fruit of the kiwifruit leads to different flavors. Phenolics, pigments and vitamins found in fruit juices affect the acidity, sweetness appearance and nutritional value. The acid determination of the fruit samples is analyzed by measuring the initial pH and titratable acidity (TA) values. While pH level is rapid and true acidity, TA indicates total or potential acidity, including the total number of acid molecules. Soluble solids content (SSC) contains active molecules that dissolve in an aqueous sample. According to commercial standards, the SSC value of a ripe fruit is generally preferred to indicate its sweetness. SSC is expressed by the Brix value, which is defined as the percentage of sucrose and used to determine the eating quality of ripe fruits. The SSC/TA value is an indication of the quality of the fruits and indicates the best time for harvesting (Pal et al., 2015).

The kiwifruit has a rich source of various structures such as vitamins, polyphenols, flavonoids, organic acids and sugars. Phenolic compounds such as caffeic, p-coumaric, ferulic, protocatechuic, syringic and vanillic acids were identified in kiwifruits (Park et al., 2011). Kiwifruit is also a rich dietary source of minerals (K, Ca, Mg and Mn) (Tighe-Neira et al., 2017). Kiwifruits are used for the treatment of many different types of stomach, lung, and liver cancer in traditional medicine (Motohashi et al., 2002). Moreover, kiwifruit extracts had DNA repair capacity in a human intervention study (Freese, 2006) and also displayed antiproliferative action against human cancer cell lines, e.g. HT-29 and HepG2 (Zuo et al., 2012). Kiwifruit is a good source of inhibition of the carcinogenesis process (stomach, lung, and liver), antihyperlipidemia, antihypertensive and exhibits cell effect of oxidative damage of DNA and protection of the body from endogenous oxidative damage (Tighe-Neira et al., 2017; Zhao et al., 2017).

In the first assessment of the genotype and culture kiwifruits grown in the Marmara Region (Yalova-Turkey), we reported the physicochemical, chemical content (total phenolics, flavonoids and ascorbic acid) and antioxidant properties of five cultivars (Bruno, Greenlight, Hayward, Monty, Topstar) and three genotypes (B-31, HO-1-14, J-242). In this work we carried out the chemical investigations of Bruno, Greenlight, Hayward, Monty, Topstar, B-31, HO-1-14, J-242 fruit extracts, and also the results of phytochemical and chemical compositions are evaluated for the antioxidant properties such as total antioxidant activity, reducing power, inhibition of lipid peroxidation, metal chelating, DPPH^∙ and H₂O₂ scavenging activity. Additionally, pH, TA, SSC and SSC/TA were determined in fruit juice of kiwifruit cultivars and genotypes.

Materials and methods

Chemicals

CH₃COOK, Folin–Ciocalteu reagent (FCR), AlCl₃, gallic acid, quercetin, α-tocopherol, trolox, trichloroacetic acid, butylated hydroxytoluene (BHT), butylated hydroxyanisole (BHA), t-butyl-hydroxyquinone (TBHQ), AA, FeCl₃.6H₂O, 1,1-diphenyl-2-picrylhydrazyl (DPPH^·), nitrobluetetrazolium, potassium persulfate, K₃[Fe(CN)₆] and sodium carbonate were purchased from Sigma Chemical Co., St. Louis, MO, USA. All the other chemicals were of analytical grade and supplied from other commercial sources.

Samples

In this study, kiwifruit cultivars of Hayward, Bruno, Monty, Greenlight and Topstar (A. deliciosa) were obtained from Atatürk Central Horticultural Research Institute (Yalova-Turkey), and also J-242, B-31 and HO-1-14 were developed by the same institute. The fruits were harvested during the harvesting period. Their analyses and extractions were conducted during the eating period.

Preparation of kiwifruit extract

Twenty-five-gram fresh kiwifruit pulp was homogenized using a homogenizer and mixed with 100 mL of 80% ethanol. The fresh kiwifruit was incubated at 25°C for 60 min and then sonicated for 30 min. The homogenized sample was centrifuged at 5000×g (+4°C). The supernatant was filtered through Whatman filter paper (no. 1). The solvent was removed under reduced pressure using lyophilizer (Christ Alpha 1-2 Model; Martin-Christ, Osterode, Germany) at −50°C to get the dried-crude hydroalcoholic extract. The crude extract obtained was stored in screw cap at −20°C until using.

Determination of SSC, TA and pH

SSC was assessed in juice obtained from kiwifruit and determined with Atago Brand Hand refractometer using 3–4 drops (Pal et al., 2015). The TA was determined by titration with 0.1N NaOH according to the previously reported method (Fiorentino et al., 2009; Pal et al., 2015). The result of TA was expressed as a citric acid equivalent. The pH was measured using an pH meter (Eutech pH 700) (Pal et al., 2015).

Determination of total phenolic (TP), total flavonoid (TF) and ascorbic acid (AA)

The TP content was determined by FCR with measurement at 760 nm and calculated as mg of gallic acid equivalents (GAE)/g DW (Singleton and Rossi, 1965). The TF was measured by a colorimetric method at 510 nm and given as mg quercetin equivalents (QE)/g DW (Chang et al., 2002). The AA was evaluated according to using a given method with some modifications and expressed as mg AA/100 g (Hossain and Gottschalk, 2009). Standard calibration curves for TP, TF and AA were plotted, and the quantitative data were calculated according to the standards.

GC/MS analysis

One hundred milligram of kiwifruit dried-crude extracts were taken and dissolved in 1 mL of methanol. The mixture added to hexane (1 mL) and of 2M KOH (3 mL; prepared in methanol). The mixture was incubated at 2500 rpm for 30 s. Also, 500 μL of the supernatant obtained was taken and subjected to GC/MS analysis. GC/MS was performed on an Agilent Technologies GC7890A and equipped with a 5975-triple axis detector MS using HP-5 ms capillary column (30 m ×250 μm ×0.25 μm). Ionization voltage was 70 eV, and helium was the carrier gas at 1 mL min⁻¹. Injection volume was 2 µL at splitless mode. The temperature was kept at 100°C for 10 min, then increased to 200°C at a 10°C/min rate, and held for 10 min, then 25°C/min to 270°C for 50 min (total run time 82.8 min). Compounds in samples were identified and compared with those in the NIST library. Mass spectra were recorded in the m/z 50–550 scanning range (Ozen et al., 2017).

Determination of antioxidant activity

The antioxidant activities of kiwifruit genotype and cultivar extracts were conducted by seven methods (total antioxidant activity, reducing power, inhibition of lipid peroxidation, metal chelating, DPPH^∙, H₂O₂, superoxide anion scavenging activity and compared to ascorbic acid, BHT, BHA, TBHQ, trolox and α-tocopherol at 50–500 μg mL⁻¹. Also, results were expressed as EC₅₀ in μg mL⁻¹ for total antioxidant activity, reducing power and IC₅₀ in μg mL⁻¹ for inhibition of lipid peroxidation, metal chelating, DPPH^∙, H₂O₂, superoxide anion scavenging activity. All antioxidant tests were carried out in triplicate.

Total antioxidant activity by phosphomolybdate assay

The activity was determined by using phosphomolybdenum method related to the reduction of Mo(VI) to Mo(V) by the sample analyte and the subsequent formation of green phosphate/Mo(V) compounds (Prieto et al., 1999). This reaction is turned into a greenish complex (phosphate-Mo(V)) at an acidic pH that measures the reduction in the coloration of Mo(VI) complex with antioxidant sources. The activity was calculated as equivalents of α-tocopherol according to the extinction coefficient of 4 × 10³ M⁻¹ cm⁻¹ and defined as a µmol α-tocopherol/g sample.

Reducing power

The reducing power was estimated using Fe³⁺ reducing power assay (Oyaizu, 1986). It is indicated that the high absorbance of the sample has a good reducing power in the reaction condition, and the reducing power had increased.

Free radical scavenging activity (DPPH˙)

The scavenging ability of the kiwifruit was carried out the method of Blois with minor modification (Blois, 1958). The activity was calculated as a percentage of DPPH^∙ discoloration using the following formula:

$\mathrm{\%}\mathrm{s}\mathrm{c}\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}=\left[\left({{\mathrm{A}}_{532}}_{\left(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}\right)}-{{\mathrm{A}}_{532}}_{\left(\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}\right)}\right)/{{\mathrm{A}}_{532}}_{\left(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}\right)}\right]\times 100.$

H₂O₂ scavenging activity

H₂O₂ scavenging activity was evaluated with regarding Zhao et al. (2006). The activity was calculated as the percentage of H₂O₂ scavenging and used the following equation:

$\mathrm{\%}\mathrm{s}\mathrm{c}\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}=[({\mathrm{V}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}-{\mathrm{V}}_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}})/{\mathrm{V}}_{\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}]\times 100.$

Superoxide anion scavenging activity

The superoxide anion scavenging activity was estimated according to the method of Nishikimi with a minor modification (Nishikimi et al., 1972). Superoxide radicals were generated in a PMS-NADH system by oxidation-reduction reactions. The activity was calculated as a percentage of superoxide scavenging and used the following equation:

$\mathrm{\%}\mathrm{s}\mathrm{c}\mathrm{a}\mathrm{v}\mathrm{e}\mathrm{n}\mathrm{g}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}=[({{\mathrm{A}}_{532}}_{(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l})}-{{\mathrm{A}}_{532}}_{(\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e})})/{{\mathrm{A}}_{532}}_{(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l})}]\times 100$

Metal (Fe²⁺) chelating activity

The Fe²⁺-chelating activity of the sample was measured by according to Dinis’s method (Dinis et al., 1994) with modifications. In this method, ferrozine can form complex with ferrozine-Fe²⁺ complex, effectively. In the presence of chelating polyphenolics, this complex is disrupted with chelator compounds. The activity was calculated as inhibition (%) of Fe²⁺-ferrozine complex and used the following formation:

$\mathrm{\%}\mathrm{c}\mathrm{h}\mathrm{e}\mathrm{l}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{i}\mathrm{t}\mathrm{y}=[({{\mathrm{A}}_{562}}_{(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l})}-{{\mathrm{A}}_{562}}_{(\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e})})/{{\mathrm{A}}_{562}}_{(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l})}]\times 100$

Inhibition of lipid peroxidation

The inhibition of lipid peroxidation was performed using TBA (thiobarbituric acid) method based on inhibition of linoleic acid peroxidation (Choi et al., 2002). The inhibition level of kiwifruit was calculated according to following formula:

$\begin{array}{rl}& \mathrm{\%}\mathrm{I}\mathrm{n}\mathrm{h}\mathrm{i}\mathrm{b}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{f}\mathrm{l}\mathrm{i}\mathrm{p}\mathrm{i}\mathrm{d}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}=[({{\mathrm{A}}_{532}}_{(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l})}-{\mathrm{A}}_{532(\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e})})\\ & /{\mathrm{A}}_{532(\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l})}]\times 100.\end{array}$

Statistical analysis

The differences between extract concentrations (50–500 µg mL⁻¹) and their activities were processed by ANOVA using the SPSS statistical package for Windows (20.0). Multiple comparisons of means were carried out Tukey’s significant difference post-hoc test with α = 0.05 (p < 0.05).

Results and discussion

Physicochemical properties of kiwifruits genotypes and cultivars

The five cultivars and three genotypes were categorized into their physicochemical SCC, TA, pH and SSC/TA values in Table 1. The physicochemical values of the eight kiwifruits were found significantly different, p < 0.05. J-242, HO-1-14, Hayward and Greenlight had the highest SSC (16.00 ± 0.61), TA (1.99 ± 0.07), pH (3.25 ± 0.10) and SSC/TA (14.09 ± 1.27) values, respectively. These results indicated that genotypes (B-31, HO-1-14 and J-242) were found to have high SSC and TA values when compared to Cultivars (Bruno, Greenlight, Hayward, Monty, Topstar). Hayward, Greenlight and B-31 had the highest pH values. It was known that pH and TA were important contributors to the desired consumer perception (Pal et al., 2015).

Table 1. Physicochemical properties of genotypes and cultivars of kiwifruits.

Genotypes and cultivars of kiwifruits	SSC (%)	TAD (%)	pH	SSC/ TAD
B-31	14.77 ± 0.40^c	1.65 ± 0.11^c	3.15 ± 0.07^de	8.99 ± 0.83^a
HO-1-14	16.00 ± 0.61^d	1.99 ± 0.07^d	2.62 ± 0.06^a	8.04 ± 0.11^a
J-242	15.97 ± 0.59^d	1.65 ± 0.12^c	3.02 ± 0.02^bcd	9.68 ± 0.40^a
Bruno	15.70 ± 0.35^cd	1.49 ± 0.02^bc	2.98 ± 0.05^bc	10.52 ± 0.39^b
Greenlight	14.70 ± 0.44^c	1.05 ± 0.12^a	3.14 ± 0.01^cde	14.09 ± 1.27^ab
Hayward	12.13 ± 0.15^ab	1.25 ± 0.06^ab	3.25 ± 0.10^e	9.75 ± 0.59^ab
Monty	12.50 ± 0.44^b	1.57 ± 0.20^c	2.97 ± 0.06^bc	8.03 ± 1.10^a
Topstar	11.03 ± 0.15^a	1.43 ± 0.05^bc	2.92 ± 0.06^b	7.72 ± 0.25^a

The values of SSC/TA ratio in genotypes were higher than that of cultivars, ranging from 14.09 ± 1.27 to 7.72 ± 0.25. Greenlight had the highest SSC/TA ratio, which indicates a more desirable flavor. The order of SSC/TA ratio was Greenlight > Bruno > Hayward > J-242 > B-31 > HO-1-14 ≈ Monty > Topstar. The SSC/TA ratio is widely reported as an index of fruit quality and maturity determinants (Wojdyło et al., 2017; Yilmaz et al., 2009).

Phytochemicals and antioxidant activities of kiwifruits genotypes and cultivars

Phenolics act effectively as reductive secondary metabolites and antioxidants. These compounds have been found to inhibit the formation of free radicals produced from different sources and contribute antioxidant properties (Petropoulos et al., 2017). The antioxidant potentials of cultivars and genotypes kiwifruits were assessed by total antioxidant activity, reducing power, inhibition of lipid peroxidation, metal chelating, DPPH^∙ and H₂O₂ scavenging activity. The TP, TF and AA contents were quantified as a mg GAE/100 g, mg QE/100 g and mg/100 g, respectively. GC/MS analysis was used to assign the content of fatty acids and phenolics that are responsible for the antioxidant properties of cultivars and genotypes kiwifruits. The results were expressed as mean ± SD of triplicate measurements.

The TP, TF and AA contents of five cultivars and three genotypes kiwifruits are represented in Table 2. Among the kiwifruit genotypes, J-242 showed the highest phenolic and flavonoid content. It was reported that there is a close relationship between the phenolic compounds and the antioxidant activity (Kontogiorgis et al., 2016). Phenolics are active compounds that have a reducing structure as an electron donor or a hydrogen substance. The TP content among the genotype kiwifruit extracts was as follows in descending order: J-242 (89.53 ± 0.39) > B-31 (60.55 ± 0.33) > HO-1-14 (42.91 ± 0.36), whereas the TP content of cultivars kiwifruit extracts is as follows in descending order: Topstar (61.23 ± 1.02) > Hayward (49.52 ± 1.19) > Greenlight (45.07 ± 2.96) > Bruno (32.52 ± 0.81) > Monty (29.58 ± 0.23).

Table 2. Yield, total phenolic, flavonoid and ascorbic acid content of hydroalcoholic extracts in different kiwifruit genotypes and cultivars.

Genotypes and cultivars of kiwifruits	Yield (%)	Total phenolics (mg GAE/100 g)	Total flavonoids (mg QE/100 g)	Ascorbic acid (mg AA/100 g)
B-31	14.76	60.55 ± 0.33	38.79 ± 0.15	63.16 ± 0.22
HO-1-14	13.49	42.91 ± 0.36	50.04 ± 0.22	64.15 ± 0.13
J-242	15.32	89.53 ± 0.39	109.13 ± 0.19	61.76 ± 0.26
Bruno	15.99	32.52 ± 0.81	10.32 ± 0.11	60.38 ± 0.26
Greenlight	14.8	45.07 ± 2.96	90.22 ± 0.07	63.45 ± 0.26
Hayward	11.52	49.52 ± 1.19	46.43 ± 0.47	43.58 ± 0.44
Monty	10.64	29.58 ± 0.23	108.27 ± 0.26	64.05 ± 0.44
Topstar	12.48	61.23 ± 1.02	96.04 ± 0.50	61.22 ± 0.34

The flavonoids are polyphenolic structures that are commonly distributed in the aromatic plants. They may also conduct as a cell cycle inhibitor, scavenge, physiological regulator and chemical messenger (Rodrigues et al., 2016). J-242 had the highest TF (109.13 ± 0.19) of the cultivars tested, followed by Monty (96.04 ± 0.50), J-242 (108.27 ± 0.26) and Topstar (96.04 ± 0.50). Bruno also was found to contain the lowest content of flavonoids (10.32 ± 0.11).

The AA contents of the eight kiwifruit samples were high and ranged from 60.38 ± 0.26 to 64.15 ± 0.13. HO-1-14 was found to contain the highest AA content (64.15 ± 0.13). AA is highly bio-available and is consequently the most important water-soluble antioxidant vitamin in cells, effective scavenging reactive oxygen species. It is important to evaluate the AA content of fruit extracts with antioxidant activities (Lee and Kader, 2000).

The recent reports have indicated a positive correlation between polyphenolic contents and the antioxidant activities of kiwifruit extracts (Hwang et al., 2017; Lee et al., 2015; Pal et al., 2015) and contained fatty acids (Antunes and Sfakiotakis, 2008; Duman and Ozcan, 2011). The essential fatty acids were thought to contribute to cancer, cardiovascular disease and inflammation (Mosby et al., 2012). Chemical and fatty acid components of kiwifruit genotypes and cultivars are represented in Table 3. The kiwifruit genotypes and cultivars were found as the richest samples in terms of saturated and unsaturated fatty acids. Stearic acid methyl ester, palmitic acid methyl ester, oleic acid methyl ester and linoleic acid methyl ester were found major fatty acids in hydroalcoholic extracts of kiwifruit genotypes and cultivars (Table 3). The fatty acid content was determined to be close to each other when compared between genotypes and cultivars. Until now, the fatty acid compositions of kiwifruit genotypes and cultivars have not been reported and assumed to be comparable to the fatty acid compositions of kiwifruit genotypes and cultivars.

Table 3. Identified chemical components of hydroalcoholic extracts in different genotypes and cultivars of kiwifruits by GC/MS.

RT (min)	Compound name	B-31	HO-1-14	J-242	Greenlight	Bruno	Hayward	Monty	Topstar
		Components (%)
12.508	2-Dodecene	0.94	nd	nd	nd	nd	nd	nd	nd
15.469	Carvacrol	nd	nd	nd	6.47	nd	nd	nd	nd
17.180	1-Tetradecene	1.49	1.15	1.11	0.92	nd	1.00	1.03	0.44
17.465	Methyleugenol	nd	nd	17.54	nd	nd	nd	nd	nd
19.177	α-Himachalene	nd	nd	nd	0.32	nd	nd	nd	nd
20.209	Cetene	0.94	0.72	0.60	0.63	nd	0.64	0.45	6.25
20.292	Spathulenol	nd	nd	nd	3.04	nd	nd	nd	nd
20.359	3-Hexadecene	nd	nd	nd	nd	nd	nd	nd	0.58
20.402	Caryophylleneoxide	nd	nd	nd	2.48	nd	nd	nd	nd
20.695	Cedrol	0.28	nd	nd	tr	nd	nd	nd	nd
21.140	Caryophylleneoxide	nd	nd	nd	0.56	nd	nd	nd	nd
21.610	Longipinocarveol	0.28	nd	nd	0.30	nd	nd	nd	nd
23.245	1-Nonadecene	nd	nd	nd	0.29	nd	0.25	0.20	15.37
23.455	7-Octadecene	nd	nd	nd	nd	nd	nd	nd	1.16
23.715	1-Eicosene	nd	nd	nd	nd	nd	nd	nd	0.40
25.695	Palmitoleic acid, methyl ester	0.36	0.85	0.70	0.68	nd	1.12	nd	0.80
26.181	Palmitic acid, methyl ester	22.19	27.69	24.28	16.47	22.80	21.49	16.48	14.69
31.080	1-Eicosanol	nd	nd	nd	nd	nd	nd	nd	2.05
31.340	Linoleic acid, methyl ester	15.36	17.19	12.30	9.30	12.13	5.09	3.41	3.31
31.474	Linolenic acid, methyl ester	nd	nd	nd	nd	nd	nd	nd	17.22
31.475	Oleic acid, methyl ester	34.55	29.12	37.64	27.43	27.93	33.79	43.61	8.14
31.542	Oleic acid, methyl ester	nd	nd	nd	12.52	nd	19.66	14.54	nd
31.709	10-Octadecenoic acid, methyl ester	nd	nd	nd	1.05	15.69	nd	nd	nd
31.869	Stearic acid, methyl ester	23.06	19.63	4.82	15.00	21.46	16.51	20.28	9.76
32.389	Linoleic acid, ethyl ester	nd	nd	nd	0.17	nd	nd	nd	nd
32.473	Linolenin, 1-mono-	nd	nd	nd	0.38	nd	nd	nd	nd
32.532	Oleic acid, ethyl ester	nd	nd	nd	0.47	nd	0.44	nd	nd
32.749	1-Docosene	nd	nd	nd	nd	nd	nd	nd	3.84
32.858	Behenicalcohol	nd	nd	nd	nd	nd	nd	nd	0.40
33.899	10,13-Eicosadienoic acid, methyl ester	nd	0.53	0.54	nd	nd	nd	nd	nd
33.974	11,14,17-Eicosatrienoic acid, methyl ester	nd	1.34	nd	nd	nd	nd	nd	0.61
34.192	Arachidic acid methyl ester	0.82	0.85	0.47	nd	nd	nd	nd	0.41
34.872	1-tetracosene	nd	nd	nd	nd	nd	nd	nd	0.82
38.706	Heptacosane	nd	nd	nd	0.34	nd	nd	nd	nd
41.222	Squalene	nd	0.93	nd	nd	nd	nd	nd	nd
62.789	13,27-Cycloursan-3-one	nd	nd	nd	nd	nd	nd	nd	4.72

nd: not detected.

Plants include polyphenolics that contain hydroxyl groups conjugated to polyaromatic rings. These compounds control chain-oxidation reactions by chelating trans-metals or donating hydrogen atoms. These metabolites of plants have antioxidant, reducing property, metal chelating and singlet oxygen scavenging. Many investigations have exhibited that the polyphenolics of medical edible plants have antioxidant activities (Kiritsakis et al., 2017). Free radicals, oxidant compounds or anionic and cationic radicals are highly reactive species that are responsible for many cell disorders due to their effects on DNA, lipids and proteins (Srivastava et al., 2017). The antioxidants are substances that neutralize or scavenge the negative effects of free radicals.

Antioxidant assays

The antioxidant activity, evaluated by seven different methods, namely total antioxidant activity, reducing power, inhibition of lipid peroxidation, metal chelating, DPPH^∙, H₂O₂, superoxide anion scavenging activity, is displayed in Tables 4–7.

Table 4. Total antioxidant activity and reducing power of hydroalcoholic extracts of five kiwifruit cultivars and three genotypes at 50–500 µg mL⁻¹.

Sample		50 µg mL^−1	100 µg mL^−1	250 µg mL^−1	500 µg mL^−1	r^2
Total antioxidant activity (µmol α-tocopherol/g sample)
Genotypes	B-31	69.92 ± 2.25^f	76.65 ± 3.76^b	83.07 ± 1.55^b	105.42 ± 1.54^a	0.98
	HO-1-14	58.46 ± 1.86^e	85.60 ± 0.10^c	113.03 ± 1.13^e	129.06 ± 1.19^b	0.85
	J-242	70.32 ± 1.14^f	86.82 ± 1.00^c	113.40 ± 2.61^e	121.42 ± 0.90^b	0.83
Cultivars	Bruno	48.32 ± 1.66^ab	66.86 ± 1.81^a	74.18 ± 1.02^a	103.31 ± 1.83^a	0.94
	Greenlight	43.49 ± 1.16^a	83.16 ± 0.67^c	111.83 ± 1.00^e	143.50 ± 1.74^c	0.88
	Hayward	54.52 ± 1.59^cd	66.22 ± 2.37^a	85.37 ± 1.45^b	123.76 ± 1.14^b	0.99
	Monty	54.20 ± 2.15^bcd	85.40 ± 1.84^c	100.34 ± 1.43^d	179.13 ± 4.10^e	0.96
	Topstar	50.53 ± 2.07^bc	69.07 ± 1.81^a	92.81 ± 0.87^c	170.07 ± 0.72^de	0.98
Standards	Ascorbic acid	78.50 ± 3.46^g	133.68 ± 2.58^f	325.68 ± 1.81^j	452.53 ± 1.11^h	0.94
	BHA	111.78 ± 2.33^i	182.23 ± 1.43^h	352.70 ± 3.12^i	394.75 ± 0.56^g	0.83
	BHT	83.86 ± 1.74^g	99.89 ± 2.50^d	154.05 ± 1.77^g	165.95 ± 2.47^d	0.84
	TBHQ	121.46 ± 2.54^j	148.21 ± 2.07^g	247.65 ± 1.80^h	436.25 ± 1.03^i	0.99
	Trolox	92.75 ± 1.97^h	119.44 ± 1.00^e	122.22 ± 1.68^f	225.83 ± 1.82^f	0.91
Reducing power, 700 nm
Genotypes	B-31	0.0186 ± 0.0009^a	0.0448 ± 0.0006^b	0.0747 ± 0.0003^a	0.0871 ± 0.0007^a	0.82
	HO-1-14	0.0258 ± 0.0007^a	0.0628 ± 0.0057^c	0.0465 ± 0.0006^a	0.1881 ± 0.0237^a	0.83
	J-242	0.0158 ± 0.0009^a	0.0583 ± 0.0048^c	0.1344 ± 0.0080^a	0.1565 ± 0.0299^a	0.83
Cultivars	Bruno	0.0460 ± 0.0091^b	0.0468 ± 0.0003^b	0.0651 ± 0.0006^a	0.1037 ± 0.0219^a	0.98
	Greenlight	0.0263 ± 0.0195^a	0.0465 ± 0.0003^b	0.0473 ± 0.0003^a	0.0615 ± 0.0004^a	0.76
	Hayward	0.0164 ± 0.0072^a	0.0358 ± 0.0005^a	0.0692 ± 0.0049^a	0.0853 ± 0.0053^a	0.85
	Monty	0.0114 ± 0.0048^a	0.0308 ± 0.0005^a	0.0360 ± 0.0047^a	0.0488 ± 0.0003^a	0.81
	Topstar	0.0141 ± 0.0005^a	0.0486 ± 0.0008^b	0.0685 ± 0.0081^a	0.0972 ± 0.0095^a	0.88
Standards	Ascorbic acid	0.1174 ± 0.0015^c	0.2745 ± 0.0015^e	1.1797 ± 0.0025^c	1.8804 ± 0.0067^bc	0.96
	BHA	0.3877 ± 0.0022^g	0.7237 ± 0.0026^j	1.5163 ± 0.0025^c	2.8796 ± 0.0239^c	0.99
	BHT	0.2955 ± 0.0004^e	0.6797 ± 0.0024^i	1.2818 ± 0.0009^c	1.8219 ± 0.0037^bc	0.94
	TBHQ	0.3309 ± 0.0012^f	0.6028 ± 0.0030^g	1.4251 ± 0.0042^c	2.4024 ± 0.0017^c	0.99
	Trolox	0.2314 ± 0.0003^d	0.4457 ± 0.0012^f	1.0125 ± 0.0028^bc	1.6281 ± 0.0035^bc	0.98
	α-tocopherol	0.1336 ± 0.0013^c	0.2274 ± 0.0016^d	0.5226 ± 0.0013^ab	0.9937 ± 0.0046^ab	0.99

Each data was evaluated as a superscript letter (a–h) and was significantly different at p < 0.05.

BHA: butylated hydroxytoluene; BHT: butylated hydroxytoluene; TBHQ: t- butyl-hydroxyquinone.

Table 5. Free radical and hydrogen peroxide scavenging activity of hydroalcoholic extracts of five kiwifruit cultivars and three genotypes at 50–500 µg mL⁻¹.

Sample		Free radical scavenging activity (%)
		50 µg mL^−1	100 µg mL^−1	250 µg mL^−1	500 µg mL^−1	r^2
Genotypes	B-31	6.80 ± 1.94^b	10.09 ± 0.77^a	20.27 ± 2.82^d	35.47 ± 1.83^c	0.99
	HO-1-14	8.32 ± 2.12^bc	7.96 ± 1.81^a	18.96 ± 0.16^cd	35.64 ± 1.01^c	0.98
	J-242	14.80 ± 0.10^d	22.00 ± 1.05^a	34.31 ± 3.67^e	48.91 ± 0.41^d	0.97
Cultivars	Bruno	10.80 ± 1.12^cd	11.17 ± 2.16^a	17.18 ± 0.60^cd	33.70 ± 1.83^c	0.97
	Greenlight	2.61 ± 0.54^a	6.00 ± 1.38^a	15.04 ± 1.22^bc	19.30 ± 3.50^ab	0.89
	Hayward	8.16 ± 1.32^bc	8.64 ± 0.82^a	9.83 ± 2.28^a	15.84 ± 2.06^a	0.73
	Monty	5.88 ± 0.40^ab	7.59 ± 0.81^a	10.57 ± 0.84^ab	21.96 ± 2.51^b	0.97
	Topstar	5.27 ± 0.92^ab	8.90 ± 2.35^a	9.81 ± 2.17^a	14.88 ± 0.53^a	0.91
Standards	Ascorbic acid	52.23 ± 1.25^e	31.79 ± 1.64^a	47.03 ± 0.47^f	50.74 ± 1.13^d	0.78
	BHA	79.67 ± 0.83^i	82.68 ± 0.65^b	83.71 ± 1.44^f	84.89 ± 0.75^f	0.73
	BHT	15.98 ± 0.54^d	33.10 ± 1.71^a	55.83 ± 0.38^g	69.30 ± 0.52^e	0.88
	TBHQ	77.58 ± 0.48^f	83.02 ± 1.40^b	84.43 ± 7.08^h	88.18 ± 1.05^g	0.94
	Trolox	82.30 ± 1.17^hi	83.40 ± 0.15^b	84.53 ± 0.82^h	85.41 ± 1.22^f	0.88
	α-tocopherol	79.62 ± 2.59^hi	81.57 ± 0.88^b	82.00 ± 1.59^h	83.42 ± 1.05^f	0.82
		Hydrogen peroxide scavenging activity (%)
Genotypes	B-31	34.39 ± 3.43^ab	42.50 ± 4.18^a	65.91 ± 3.21^b-f	68.55 ± 3.86^bc	0.79
	HO-1-14	45.45 ± 4.50^a-d	50.91 ± 5.14^ab	56.36 ± 3.86^abc	59.14 ± 7.68^ab	0.83
	J-242	48.23 ± 9.96^a-e	58.18 ± 4.50^ab	60.45 ± 2.57^a-e	62.41 ± 2.89^abc	0.60
Cultivars	Bruno	34.41 ± 5.46^ab	47.41 ± 1.25^ab	53.50 ± 3.54^a	58.32 ± 9.32^ab	0.74
	Greenlight	45.68 ± 1.61^a-d	49.00 ± 1.29^ab	59.09 ± 0.64^a-d	63.73 ± 3.47^bc	0.90
	Hayward	38.18 ± 7.71^abc	42.73 ± 2.57^a	49.55 ± 1.29^b	60.07 ± 6.33^bc	0.98
	Monty	35.23 ± 4.82^ab	40.80 ± 5.50^a	59.55 ± 4.50^a-d	64.00 ± 4.50^bc	0.84
	Topstar	35.09 ± 5.79^ab	48.91 ± 8.36^ab	53.86 ± 4.82^ab	65.23 ± 0.32^bc	0.86
Standards	Ascorbic acid	62.95 ± 8.04^cde	69.32 ± 0.96^b	76.59 ± 0.96^fg	82.09 ± 0.64^c	0.89
	BHA	73.86 ± 0.32^e	74.77 ± 0.32^b	75.23 ± 0.32^ef	80.50 ± 0.71^c	0.92
	BHT	70.68 ± 0.32^de	71.36 ± 0.64^b	72.05 ± 0.96^def	75.00 ± 1.28^c	0.97
	TBHQ	34.09 ± 6.43^ab	53.82 ± 7.07^ab	68.64 ± 0.64^b-f	76.05 ± 1.35^c	0.78
	Trolox	30.91 ± 3.21^a	54.23 ± 0.32^ab	71.14 ± 0.32^c-f	78.32 ± 1.99^c	0.75
	α-tocopherol	48.86 ± 2.89^a-e	55.45 ± 0.64^ab	76.41 ± 0.06^f	83.37 ± 0.63^c	0.87

Each data was evaluated as superscript letters (a–h) and was significantly different at p < 0.05.

BHA: butylated hydroxytoluene; BHT: butylated hydroxytoluene; TBHQ: t- butyl-hydroxyquinone.

Table 6. Metal chelating activity of hydroalcoholic extracts of five kiwifruit cultivars and three genotypes at 50–500 µg mL⁻¹.

Sample		Metal chelating activity (%)
		50 µg mL^−1	100 µg mL^−1	250 µg mL^−1	500 µg mL^−1	r^2
Genotypes	B-31	24.37 ± 0.57^d	26.45 ± 0.23^f	32.46 ± 0.35^cde	44.19 ± 0.88^b-e	0.99
	HO-1-14	13.67 ± 2.72^bc	18.20 ± 0.31^bc	25.36 ± 0.29^a-e	36.00 ± 9.17^ab	0.98
	J-242	15.82 ± 1.56^c	16.70 ± 2.18^b	36.07 ± 0.54^cde	43.94 ± 0.53^b-e	0.90
Cultivars	Bruno	16.22 ± 1.83^c	20.10 ± 1.18^b-e	28.57 ± 0.54^b-e	39.28 ± 0.58^a-d	0.99
	Greenlight	18.91 ± 0.68^cd	23.42 ± 0.25^ef	30.77 ± 0.36^b-c	56.55 ± 0.22^de	0.97
	Hayward	3.32 ± 0.66^a	9.08 ± 0.82^a	23.53 ± 4.09^abc	24.02 ± 2.27^a	0.75
	Monty	6.61 ± 1.93^a	18.41 ± 0.68^abc	36.98 ± 0.46^e	56.64 ± 0.39^de	0.96
	Topstar	14.93 ± 0.64^bc	16.03 ± 1.31^b	27.93 ± 0.52^b-e	30.24 ± 1.26^ab	0.83
Standards	Ascorbic acid	13.84 ± 0.22^bc	22.40 ± 0.39^def	36.49 ± 4.95^de	38.74 ± 0.92^abc	0.78
	BHT	14.85 ± 1.93^ab	23.17 ± 2.14^bcd	25.97 ± 4.18^ab	56.43 ± 6.18^b-e	0.93
	BHA	17.69 ± 0.55^c	21.82 ± 2.87^ef	25.63 ± 4.47^a-e	38.15 ± 6.16^b-e	0.98
	TBHQ	16.74 ± 1.25^bc	22.54 ± 0.74^ef	25.21 ± 4.83^a-e	46.93 ± 1.08^cde	0.94
	Trolox	8.01 ± 3.11^cd	20.88 ± 1.45^de	19.66 ± 9.96^a-d	44.76 ± 10.37^ab	0.88
	α-tocopherol	16.09 ± 5.92^c	22.73 ± 0.46^ef	34.92 ± 5.18^cde	58.98 ± 3.44^e	0.99
		Inhibition of lipid peroxidation (%)
Genotypes	B-31	11.99 ± 1.99^ab	26.73 ± 5.53^ab	31.09 ± 8.60^ab	81.69 ± 0.64^cd	0.93
	HO-1–14	19.19 ± 5.04^bc	36.51 ± 4.06^bc	45.87 ± 8.85^cd	73.06 ± 2.59^cd	0.95
	J-242	14.35 ± 3.80^abc	27.83 ± 6.46^ab	40.49 ± 5.14^bc	50.26 ± 5.25^ab	0.87
Cultivars	Bruno	18.33 ± 1.63^bc	54.95 ± 1.69^de	67.50 ± 2.82^fg	75.51 ± 0.71^cd	0.65
	Greenlight	5.33 ± 2.81^a	39.29 ± 3.39^bc	63.19 ± 6.23^efg	86.98 ± 0.09^cd	0.86
	Hayward	22.85 ± 3.54^cde	25.70 ± 7.69^ab	73.91 ± 2.74^g	78.50 ± 0.70^cd	0.79
	Monty	28.75 ± 7.16^de	33.47 ± 7.70^ab	60.05 ± 2.28^d-g	75.52 ± 2.40^cd	0.94
	Topstar	46.45 ± 4.10^g	54.31 ± 6.12^de	62.40 ± 6.15^efg	84.96 ± 1.16^cd	0.98
Standards	Ascorbic acid	4.91 ± 1.30^a	21.44 ± 1.19^a	24.49 ± 1.81^a	38.79 ± 7.42^a	0.84
	BHA	57.26 ± 0.68^h	62.19 ± 1.70^f	67.44 ± 1.13^fg	91.31 ± 0.52^d	0.96
	BHT	46.27 ± 0.77^h	48.92 ± 0.64^cd	50.38 ± 0.72^cde	67.36 ± 1.81^bc	0.92
	TBHQ	20.29 ± 0.82^bcd	26.74 ± 1.84^ab	40.03 ± 1.69^bc	50.73 ± 1.34^ab	0.95
	Trolox	30.75 ± 1.74^ef	39.05 ± 2.03^bc	47.11 ± 1.34^cd	48.37 ± 2.74^ab	0.72
	α-tocopherol	39.05 ± 1.59^fg	47.65 ± 2.32^cd	52.47 ± 2.35^def	56.09 ± 7.16^ab	0.78

Each data was evaluated as a superscript letter (a–f) and was significantly different at p < 0.05.

BHA: butylated hydroxytoluene; BHT: butylated hydroxytoluene; TBHQ: t-butyl-hydroxyquinone.

Table 7. EC₅₀ values of total antioxidant activity, reducing power and IC₅₀ values of free radical scavenging activity, H₂O₂ scavenging activity, metal chelating activity and inhibition of lipid peroxidation of five kiwifruit cultivars and three genotypes.

Sample		Total antioxidant activity	Reducing power		Free radical scavenging activity	H2O2 scavenging activity	Metal chelating activity	Inhibition of lipid peroxidation
		EC50 (¼g mL^−1)			IC50 (μg mL^−1)
Genotypes	B-31	17.96 ± 2.28	104.24 ± 2.61		35.12 ± 1.84	49.42 ± 3.67	48.95 ± 0.51	138.93 ± 3.13
	HO-1-14	55.48 ± 1.07	138.88 ± 7.65		33.06 ± 1.28	9.73 ± 2.29	90.13 ± 3.12	115.98 ± 5.16
	J-242	31.54 ± 1.41	128.83 ± 9.90		28.39 ± 1.31	3.31 ± 0.98	81.73 ± 1.20	93.39 ± 3.80
Cultivars	Bruno	58.69 ± 1.58	90.14 ± 7.95		37.10 ± 1.50	22.34 ± 2.39	82.99 ± 1.03	75.74 ± 4.88
	Greenlight	89.57 ± 1.14	53.11 ± 5.14		41.16 ± 1.89	9.83 ± 1.93	102.79 ± 0.37	135.89 ± 1.01
	Hayward	77.46 ± 1.64	118.36 ± 4.50		84.49 ± 1.62	31.30 ± 6.73	117.08 ± 1.96	78.49 ± 1.71
	Monty	115.98 ± 4.88	95.76 ± 2.56		42.25 ± 1.14	42.41 ± 4.83	148.6 ± 0.87	91.79 ± 5.14
	Topstar	127.02 ± 1.37	123.31 ± 4.71		18.72 ± 1.49	35.22 ± 4.82	40.81 ± 0.93	46.57 ± 4.19
Standards	Ascorbic acid	139.34 ± 2.24	173.69 ± 3.04		39.27 ± 1.12	3.41 ± 0.65	107.58 ± 2.08	57.81 ± 1.96
	BHT	97.96 ± 1.86	167.87 ± 7.80		12.46 ± 0.91	0.97 ± 0.10	69.65 ± 3.51	29.91 ± 0.98
	BHA	52.37 ± 2.12	134.16 ± 1.83		10.10 ± 0.79	4.20 ± 0.64	113.92 ± 3.61	56.12 ± 1.42
	TBHQ	135.73 ± 1.86	160.37 ± 2.53		1.20 ± 0.02	58.66 ± 4.18	108.98 ± 4.48	88.02 ± 3.35
	Trolox	92.18 ± 1.62	154.22 ± 1.93		1.19 ± 0.34	0.77 ± 0.09	115.73 ± 5.47	18.63 ± 1.96
	α-tocopherol	–	170.50 ± 2.19		3.50 ± 1.53	6.62 ± 1.21	117.23 ± 3.75	13.58 ± 2.93

BHA: butylated hydroxytoluene; BHT: butylated hydroxytoluene; TBHQ: t-butyl-hydroxyquinone.

Total antioxidant activity by phosphomolybdate assay

A total antioxidant activity test was performed with the phosphomolybdate test, which is one of the frequently used tests in recent years (Bunzel and Schendel, 2017). The experiment is based on the reduction of phosphomolybdate by antioxidant acting by the formation of spectrophotometrically measured specific green phosphate/Mo(V)-complex (Prieto et al., 1999). The total antioxidant activities of different ecotypes of genotype and cultivar kiwifruit extracts were expressed as µmol α-tocopherol/g in Table 4. The activities were found to increase dose-dependent and exhibited a significant (p < 0.05). The total antioxidant activity varied from 105.42 ± 1.54 to 129.06 ± 1.19 µmol α-tocopherol/g sample for genotype kiwifruit extracts and ranged from 103.31 ± 1.83 to 179.13 ± 4.10 µmol α-tocopherol/g sample for cultivar kiwifruit extracts at 500 µg mL⁻¹. As seen in Table 4, Monty and Topstar exhibited potent antioxidant activities. The activities values were found as 105.42 (r²; 0.98) for B-31, 129.06 (r²; 0.85) for HO-1-14, 121.42 (r²; 0.83) for J-242, 103.31 (r²; 0.94) for Bruno, 143.50 (r²; 0.88) for Greenlight, 123.76 (r²; 0.99) for Hayward, 179.13 (r²; 0.96) for Monty, 170.07 (r²; 0.98) for Topstar, 452.53 (r²; 0.94) for AA, 394.75 (r²; 0.83) for BHA, 165.95 (r²; 0.84) for BHT 436.25 (r²; 0.99) for TBHQ, 225.83 (r²; 0.91) as a µmol α-tocopherol/g sample for trolox at 500 µg mL⁻¹. Regarding kiwifruit extracts, the results of activity were practically close to the other previous reports (Fiorentino et al., 2009).

The smaller the EC₅₀ value, the greater the total antioxidant activity. As seen for EC₅₀ values in Table 7, genotypes exhibited lower EC₅₀ values, forming good green phosphate/Mo(V)-complex activity than cultivars. Results from the present study indicated that genotypes showed strong total antioxidant activity forming an effective green phosphate/Mo(V)-complex with EC₅₀ values due to rich chemical components, flavonoids and phenolics.

Reducing power

Reductive power is a typical electron transfer-based method that measures the reduction of the blue-colored Fe²⁺ complex to a Fe³⁺-ligand complex by an electron in an acidic environment at 700 nm. It was determined that the kiwifruit extracts increased the reduction power depending on the concentration (p < 0.05). However, cultivars and genotypes displayed lower reducing power than standards at 50–500 μg mL^-1 (Table 4). Certain kiwifruits such as B-31, Bruno, Greenlight and Monty exhibited promising reducing power according to EC₅₀ values (Table 7). Decreasing Fe³⁺-Fe²⁺ is in order of HO-1-14 (r²; 0.83) > J-242 (r²; 0.83) > Bruno (r²; 0.98) > Topstar (r²; 0.88) > B-31 (r²; 0.82) > Hayward (r²; 0.85) > Greenlight (r²; 0.76) > Monty (r²; 0.81) at 500 µg mL⁻¹. The reducing power of HO-1-14 had close to AA and also showed a higher reducing power than α-tocopherol. The results demonstrate that HO-1-14 had the ability to reduce Fe³⁺ on polyphenolic contents and also electron-releasing properties to neutralize free radicals. The genotype extracts exhibited considerably higher reducing power than cultivar extracts. The effective reducing capacity (EC₅₀ values) of their extracts may contribute to the protective effect or reduce transition metal ions. The reducing power of kiwifruit cultivars and genotypes was found to be closer to that reported in previous studies of different kiwifruits (Pal et al., 2015).

Scavenging activity on DPPH˙

DPPH˙ scavenging activity has been widely conducted the estimation of free radical scavenging activity of plant and food extracts (Ozen, 2010; Pal et al., 2015). The activity was assessed by DPPH₂ formation by bleaching reaction by kiwifruit extracts. Therefore, natural antioxidants can act at different stages and mechanisms, such as giving hydrogen, singlet oxygen quenching, chelating and free radical scavenging (Di Mascio et al., 1991). As it was in reducing power and DPPH˙ scavenging activities of genotype kiwifruits were higher than those of the cultivated kiwifruit samples (Tables 5 and 7). The free radical scavenging activities of kiwifruit samples were significantly found to increase in a dose-dependent manner (p < 0.05). The activities varied from 14.88 ± 0.53 to 48.91 ± 0.41% for kiwifruit extracts and ranged from 50.74 ± 1.13 to 85.41 ± 1.22% for standard antioxidants. A higher activity value (48.91%, r²; 0.97) was found for J-242 extract at 500 µg mL⁻¹. The genotypes showed lower DPPH˙ scavenging activity than standards. The recorded IC₅₀ values of genotypes were more effective than that of cultivars but Topstar exhibited the most effective free radical scavenging activity (8.72 µg mL⁻¹) compared to the AA (39.27 µg mL⁻¹) due to the phenolic compounds. It has been reported that natural antioxidant components reduce DPPH˙ due to their hydrogen donating ability (Habashy et al., 2018).

H₂O₂ scavenging activity

In biological systems, H₂O₂ forms in vivo by antioxidant enzymes and indicates a precursor to produce the hydroxyl radicals (˙OH). The ˙OH is able to cause tissue damage and react with the most biomolecule, cell death and cross cell membrane (Oktay et al., 2003). The scavenge of ˙OH is very important for the elimination of cell. Therefore, we have aimed to investigate the H₂O₂ scavenging activity of the kiwifruit extracts. The H₂O₂ scavenging activity of the eight kiwifruit extracts (cultivars and genotypes) exhibited an activity close to that of standard antioxidants (Table 5). Their activities were done in a dose-dependent manner in the test, significantly (p < 0.05). The activities varied from 59.14 ± 7.68 to 68.55 ± 3.86% for genotypes and ranged from 60.07 ± 6.33 to 65.23 ± 0.32% for cultivars. A higher activity value (68.55%, r²; 0.79) was found for B-31 extract at 500 µg mL⁻¹. The H₂O₂ scavenging efficacy of kiwifruit extracts and standards at the high dose increased in the following order: α-tocopherol > AA > BHA > trolox > TBHQ > BHT > B-31 > Topstar > Monty Topstar > Greenlight > J-242 > Hayward > HO-1-14 > Bruno. The IC₅₀ values of kiwifruit extracts and standards varied from 0.77 to 58.66 µg mL⁻¹. The H₂O₂ scavenging activities were registered as IC₅₀ of 3.31 µg mL⁻¹ (J-242), 9.73 µg mL⁻¹ (HO-1-14) and 9.83 µg mL⁻¹ (Greenlight) and found to be more effective than the other kiwifruit extracts. The IC₅₀ value of J-242 was found to be more effective H₂O₂ scavenging activity than that of α-tocopherol (6.62 μg mL⁻¹), AA (3.41 μg mL⁻¹) and BHA (4.20 μg mL⁻¹).

Metal (Fe²⁺) chelating activity

Due to lipid peroxidation, iron generally increases the extent of Fenton reaction, which is affected by many diseases and also acts as a potent catalyst (Young and Woodside, 2001). Therefore, the chelating activity of kiwifruit extracts was performed by evaluating their ability to compete with ferrozine for Fe²⁺ and refraining the formation of a complex at 562 nm. If there is a high degree of reduction in the absorbance, it is an indication of an effective chelating activity. The distinction between different concentrations of kiwifruit extracts (50–500 µg mL⁻¹) was fixed to be important, statistically (p < 0.05). The kiwifruit cultivars and genotypes exhibited higher chelating activity than BHA, TBHQ and trolox (Table 6). As Monty and Greenlight extracts had important TP and TF contents that involve the presence of many hydroxyl groups and capable of chelating metal ions, they exhibited high iron-chelating activity. Ferrozine-Fe²⁺ is a stable complex. In the test, the presence of kiwifruit cultivars and genotypes extracts capable of chelating with Fe²⁺ deteriorated the formation of the ferrozine-Fe²⁺ complex and caused the loss of the red color of the ferrozine-Fe²⁺ complex. In the metal chelating test, the more effective was found for IC₅₀, 40.81 µg mL⁻¹ for Topstar and followed by IC₅₀, 48.95 µg mL⁻¹ for B-31 than that of AA, BHT, BHA, TBHQ, trolox, and α-tocopherol (Table 7). The enhancement of the metal chelating activity can influence the TP, TF and AA contents of B-31 and Topstar. These chelating results are comparable to those of different kiwifruit species (Bursal and Gulcin, 2011).

Lipid peroxidation inhibition

Many physical and chemical phenomena can promote oxidation that proceeds continuously in the presence of potentially toxic substrate(s) due to inhibiting defense mechanisms. Target substances are oxygen, phospholipids, polyunsaturated fatty acids, DNA and cholesterol (Antolovich et al., 2002). Lipid peroxidation, being a complex free radical chain phase, involves an array of radicals and is measured by the amount of peroxide and the primary product of lipid oxidation produced during the initial stages of oxidation (Frankel, 2014). The eight kiwifruit genotypes and cultivars affected the non-enzymatic linoleic peroxidation at 50–500 µg mL⁻¹ and inhibited the linoleic peroxidation induced by Fe²⁺ in a dose-dependent manner, significantly (p < 0.05). The activity extent of kiwifruit extracts and standards in the comparable level at all concentrations is shown in Table 6. These outcomes expressly exhibited that Greenlight had the highest inhibition of linoleic peroxidation level compared with AA, BHT, TBHQ and α-tocopherol, but lower than that of BHA at 500 µg mL⁻¹. Among the kiwifruit extracts for inhibition of linoleic peroxidation level, the Topstar, which is rich in TP and TF, was found to be more active (IC₅₀: 46.57 µg mL⁻¹) than that of AA and TBHQ. Also, chemical components supported inhibition of linoleic peroxidation in the assay (Table 3). The B-31 had the highest IC₅₀ value (138.93 µg mL⁻¹) among all kiwifruit extracts.

Conclusion

The objective of the present study was to investigate the antioxidant properties of commonly consumed eight kiwifruit genotype and cultivar extracts using six different antioxidant assays. The results exhibited that kiwifruit extracts contained the important quantity of phenolic, flavonoid, AA and chemical composition determined to produce the best antioxidant activities. These research findings may afford a useful basis to affirm genotype and cultivar properties of natural food as dietary supplements and functional foods. Therefore, such information will assist kiwifruit genotypes and cultivars in regard to the health benefiting qualities of fresh fruit consumption as a daily dietary source of natural antioxidants. For this reason, kiwifruits are healthy edible foods when consumed daily and should be regarded as a valuable natural antioxidant source. In this very elegant study, the protective effects of the extracts of genotype and cultivar kiwifruits against oxidative damage were determined by different antioxidant tests. This can be attributed to the physicochemical properties, phytochemical contents and fatty acids of the first time for genotype and cultivar kiwifruits. In conclusion, some genotype and cultivar can be suggested as a potential natural source and chemoprevention diet.

Additional information

Funding

The authors are grateful to the Scientific and Technological Research Council (TUBITAK;114Z683) and Ondokuz Mayis University (PYO.FEN.1904.13.003 and PYO.FEN.1904.13.006) for their financial support.