2.1. Physicochemical Properties of Different Kiwifruits
The physicochemical properties of different kiwifruits, including the fresh weight per fruit, soluble solids content (SSC), and dry matter content, are shown in
Table 1. Results showed that significant (
p < 0.05) differences in the fresh weight per fruit, dry matter, and SSC were found among the tested kiwifruits. Briefly, the fresh weight per fruit ranged from 7.10 ± 0.37 g (
A. arguta) to 133.74 ± 5.66 g (
A. chinensis cv. Jinlong). The fresh weights per fruit of
A. macrosperma,
A. polygama, and
A. arguta were much lower than those of
A. chinensis and
A. deliciosa, which is a similar result to previous studies [
18]. Generally, the SSC consists of sugar, acid, vitamins, some minerals, and other soluble solid content, which is an important index of fruit taste quality and consumer acceptability [
14,
19]. The SSC ranged from 13.1% (
A. chinensis cv. Jinyan) to 17.6% (
A. chinensis cv. Hongyang), which is in accordance with a previous study [
14].
A. chinensis cv. Hongyang,
A. chinensis cv. Hongshi,
A. chinensis cv. Jinshi,
A. chinensis cv. Jinlong, and
A. chinensis cv. Jinhong contained much more SSC (>16.1) than those of the other tested kiwifruits. According to some previous studies, the SSC of kiwifruits could be influenced by different environmental conditions and species [
20,
21]. Moreover, the dry matter of 14 different kiwifruits varied from 13.48% (
A. deliciosa cv. Hayward) to 20.35% (
A. arguta).
Furthermore, the content of ascorbic acid is a preferential factor for the evaluation of the quality of fruits [
22]. Kiwifruits may be considered a good source of ascorbic acid. As shown in
Table 1, the ascorbic acid content in different kiwifruits varied from 51.32 ± 0.42 to 390.68 ± 4.24 mg/100 g FW (fresh weight). Briefly, the ascorbic acid content of newly developed
A. chinensis cv. Hongshi,
A. chinensis cv. Jinshi, and
A. chinensis cv. Jinlong were 390.68 ± 4.24 mg/100 g FW, 235.30 ± 8.27 mg/100 g FW, and 335.06 ± 10.39 mg/100 g FW, respectively, which were much higher than that of the most commercialized
A. deliciosa cv. Hayward (59.32 ± 1.89 mg/100 g FW) [
7,
17]. The content of ascorbic acid was not only strongly affected by type of species, but also by different cultivars [
1,
14].
2.2. Phenolic Profiles of Different Kiwifruits
Phenolic compounds are considered the main bioactive components in kiwifruits [
14]. However, the phenolic compounds of different kiwifruit species and newly developed cultivars collected in China have seldom been evaluated. As shown in
Table 2, the total phenolic content (TPC) among the fourteen different kiwifruits differed significantly (
p < 0.05), ranging from 3.75 ± 0.09 mg GAE/g DW to 16.52 ± 0.26 mg GAE/g DW. Results showed that the TPC of different kiwifruits was significantly affected by different species and cultivars. Briefly, the total phenolic content of newly developed
A. chinensis cv. Hongshi,
A. chinensis cv. Jinshi, and
A. chinensis cv. Jinlong were 16.52 ± 0.26 mg GAE/g DW, 13.38 ± 0.20 mg GAE/g DW, and 11.02 ± 0.05 mg GAE/g DW, respectively, which were significantly (
p < 0.05) higher than those of the other tested kiwifruits. However, the lowest level of TPC was found in the most commercialized
A. deliciosa cv. Hayward (3.75 ± 0.09 mg GAE/g DW). Similar studies also showed that the total phenolic content in kiwifruits with red flesh (
A. chinensis cv. Hongyang) and yellow flesh (
A. deliciosa cv. Jinkui) were higher than that of kiwifruits with green flesh (
A. deliciosa cv. Hayward) [
1]. Generally, the genus and cultivar of kiwifruit, climatic conditions, harvest time, and extraction processes could affect the content of phenolic compounds [
14,
23,
24].
Furthermore, in order to properly understand the differences between individual phenolic compounds in tested kiwifruits, the HPLC-DAD analysis was performed. Previous studies have shown that flavan-3-ols (such as (−)-epicatechin, (+)-catechin), procyanidin B1, and procyanidin B2), hydroxybenzoic acids (such as gallic acid and protocatechuic acid), hydroxycinnamic acids (such as chlorogenic acid, neochlorogenic acid, and caffeic acid), and flavonols (such as quercetin-3-rhamnoside, quercetin-3-
O-glucoside, and rutin) have been identified and found as the major phenolic compounds in kiwifruits [
14,
17,
21,
25]. Therefore, a total of fifteen commercially available phenolic compounds, including (+)-catechin, (−)-epicatechin, procyanidin B1, procyanidin B2, quercetin, rutin, kaempferol, quercetin-3-
O-glucoside, quercetin-3-rhamnoside, chlorogenic acid, neochlorogenic acid, cryptochlorogenic acid, caffeic acid, gallic acid, and protocatechuic acid were selected and investigated in each kiwifruit extract according to previous studies.
Figure 2 shows the chromatograms of the mixed standards and representative phenolic profiles of
A. chinensis cv. Hongshi. The calibration data (regression equation, linear range, and R
2) and limits of detection for the fifteen phenolic compounds investigated in kiwifruits are summarized in
Table 3. Results showed that the developed HPLC method was suitable for the qualitative and quantitative analysis of phenolic compounds in different kiwifruits. A total of twelve phenolic compounds, including procyanidin B1, procyanidin B2, (−)-epicatechin, (+)-catechin, gallic acid, protocatechuic acid, neochlorogenic acid, chlorogenic acid, caffeic acid, rutin, quercetin-3-
O-glucoside, and quercetin-3-rhamnoside, were identified in different species and cultivars of kiwifruits based on their HPLC retention time and UV spectra, which is similar with previous studies [
14,
17,
21,
25]. The content of individual phenolic compounds in different kiwifruits are summarized in
Table 4. However, as shown in
Table 4, the type and content of phenolic compounds varied significantly by species and cultivars.
As shown in
Table 4, four flavan-3-ols, including two monomeric flavan-3-ols ((−)-epicatechin and (+)-catechin) and two dimeric procyanidins (procyanidin B1 and procyanidin B2), were detected in different species and cultivars of kiwifruits. Results are in accordance with previous studies [
17,
25]. The total flavan-3-ols content in different kiwifruits ranged from 96.07 μg/g DW (
A. deliciosa cv. Cuixiang) to 823.61 μg/g DW (
A. chinensis cv. Hongshi). The highest content of total flavan-3-ols determined in
A. chinensis cv. Hongshi may be attributed to its large part of red flesh, as shown in
Figure 1. Briefly, the content of (−)-epicatechin varied significantly among the tested kiwifruits. The highest content of (−)-epicatechin was measured in
A. chinensis cv. Hongshi (162.61 ± 0.99 μg/g DW). However, (−)-epicatechin was not found in
A. chinensis cv. Honghua,
A. deliciosa cv. Hayward, and
A. arguta. As for (+)-catechin, it was only found in
A. chinensis cv. Hongshi,
A. deliciosa cv. Xuxiang,
A. macrosperma,
A. polygama, and
A. arguta. The highest content of (+)-catechin was measured in
A. arguta (132.08 ± 1.16 μg/g DW). The procyanidin B1 predominated among the flavan-3-ols. Procyanidin B1 content varied significantly (
p < 0.05) among different species and cultivars. The highest content of procyanidin B1 was measured in
A. chinensis cv. Hongshi (446.81 ± 1.51 μg/g DW), which was two times higher than that of the most commercialized
A. deliciosa cv. Hayward (203.68 ± 4.27 μg/g DW). Similarly to procyanidin B1, procyanidin B2 content also varied greatly among the tested kiwifruits, and the highest content was also measured in
A. chinensis cv. Hongshi (182.11 ± 1.16 μg/g DW). However, procyanidin B2 was not found in
A. chinensis cv. Jinshi, Jinhong, and Honghua. Finally, results indicated that (−)-epicatechin and procyanidin B1 were the main flavan-3-ols in kiwifruits, which is in accordance with previous reports [
17,
25]. Results showed that the types and content of flavan-3-ols in kiwifruits were significantly affected by species and cultivars.
Phenolic acids, including two hydroxybenzoic acids (gallic acid and protocatechuic acid) and three hydroxycinnamic acids (chlorogenic acid, neochlorogenic acid, and caffeic acid), were found in different species and cultivars of kiwifruits. The content of total phenolic acids ranged from 26.58 μg/g DW (
A. deliciosa cv. Cuixiang) to 315.76 μg/g DW (
A. chinensis cv. Hongshi). Briefly, as shown in
Table 4, chlorogenic acid, as a caffeoylquinic acid, was not only the main individual hydroxycinnamic acid in different kiwifruits, but also the main individual phenolic compound in different kiwifruits, which is in accordance with previous studies [
17,
26]. The content of chlorogenic acid differed significantly (
p < 0.05) among the tested kiwifruits, ranging from 7.70 ± 0.04 μg/g DW (
A. arguta) to 235.75 ± 5.44 μg/g DW (
A. chinensis cv. Hongshi). Meanwhile, the content of neochlorogenic acid also varied significantly (
p < 0.05) among the tested kiwifruits. The highest content of neochlorogenic acid was detected in
A. chinensis cv. Jinlong (133.72 ± 3.98 μg/g DW). However, neochlorogenic acid was not found in
A. deliciosa cv. Cuixiang,
A. deliciosa cv. Xuxiang,
A. deliciosa cv. Hayward,
A. macrosperma, and
A. polygama. Furthermore, caffeic acid was only found in
A. macrosperma,
A. polygama, and
A. arguta, which indicated that the species significantly affected the types of phenolic compounds in kiwifruits. Moreover, gallic acid was the main individual hydroxybenzoic acid found in different kiwifruits, which is in accordance with a previous study [
17]. The content of gallic acid varied significantly (
p < 0.05) among the tested kiwifruits. The top three samples were
A. chinensis cv. Jinshi (53.76 ± 0.43 μg/g DW) >
A. chinensis cv. Jinyan (49.13 ± 0.27 μg/g DW) >
A. chinensis cv. Jinlong (26.69 ± 0.45 μg/g DW). However, gallic acid was not detected in
A. macrosperma and
A. arguta. Additionally, protocatechuic acid was only detected in
A. chinensis cv. Hongshi,
A. chinensis cv. Jinshi, and
A. polygama, and the highest content was measured in
A. chinensis cv. Hongshi (15.76 ± 0.18 μg/g DW).
Furthermore, three flavonols, including quercetin-3-rhamnoside, quercetin-3-
O-glucoside, and rutin were identified in the tested kiwifruits. The content of total flavonols varied greatly among the tested kiwifruits. The main flavonol of the tested kiwifruits was quercetin-3-rhamnoside, which is consistent with previous studies [
25,
27]. The highest content of quercetin-3-rhamnoside was detected in
A. chinensis cv. Hongshi (41.94 ± 1.44 μg/g DW). However, quercetin-3-rhamnoside was not detected in
A. chinensis cv. Jinlong and
A. deliciosa cv. Hayward. Moreover, rutin was only found in
A. macrosperma,
A. polygama, and
A. arguta. Similarly to rutin, quercetin-3-
O-glucoside was also only detected in
A. chinensis cv. Hongshi,
A. deliciosa cv. Xuxiang,
A. polygama, and
A. arguta.
2.3. Antioxidant Capacities of Different Kiwifruits
The contribution of kiwifruits to health improvement has been partly attributed to their antioxidant capacity [
13]. Generally, the antioxidant capacity of fruits is affected by different mechanisms of action of their antioxidant constituents [
8]. Therefore, the antioxidant capacity was evaluated by different methods.
Table 2 also summarized the antioxidant capacities of different kiwifruits. Significant differences (
p < 0.05) were found in the tested kiwifruits. Briefly, the antioxidant capacities ranged from 32.95 ± 0.29 μmol Trolox/g DW (
A. deliciosa cv. Hayward) to 160.36 ± 6.15 μmol Trolox/g DW (
A. chinensis cv. Hongshi) in ABTS assay, from 13.12 ± 0.07 μmol Trolox/g DW (
A. chinensis cv. Jinhong) to 87.38 ± 4.32 μmol Trolox/g DW (
A. chinensis cv. Hongshi) in DPPH assay, and from 27.24 ± 0.41 μmol Trolox/g DW (
A. chinensis cv. Jinhong) to 149.97 ± 6.98 μmol Trolox/g DW (
A. chinensis cv. Hongshi) in FRAP assay, respectively. These results are similar with previous studies [
24,
28].
A. chinensis cv. Hongshi showed the highest antioxidant capacities, followed by
A. chinensis cv. Jinshi and
A. chinensis cv. Jinlong, while
A. deliciosa cv. Hayward showed the lowest antioxidant capacities, regardless of assay methods. The differences in antioxidant capacity of the tested kiwifruits could be preliminarily attributed to the different content of phenolic compounds and ascorbic acid. As shown in
Table 2, the antioxidant capacities of the tested kiwifruits evaluated by ABTS assay, DPPH assay, and FRAP assay were positively correlated with the TPC, respectively. Generally, phenolic compounds and ascorbic acid play important roles in fruits’ antioxidant capacities [
14]. Previous studies have shown that both total polyphenols and ascorbic acid are major contributors to the total antioxidant capacity in kiwifruits [
28]. In particular, it was reported that chlorogenic acid showed a better correlation with antioxidant activity (DPPH, ABTS, and FRAP) [
29]. Thus, it seems likely that the high content of chlorogenic acid could be one of the major contributors to the antioxidant capacities of kiwifruits. Results suggested that
A. chinensis cv. Hongshi,
A. chinensis cv. Jinshi, and
A. chinensis cv. Jinlong could be potential resources of antioxidants for the production of health-benefiting products.
2.4. Inhibitory Effects on Digestive Enzymes of Different Kiwifruits
Pancreatic lipase is the most important enzymes responsible for triglyceride digestion. Consequently, the suppression and delay of triglyceride digestion and absorption through inhibition of lipase is a key approach to the control of hyperlipidaemia and obesity [
10]. Previous studies have shown that the consumption of kiwifruit plays a significant role in the levels of plasmatic lipids [
13], and that kiwifruit extracts exhibit a strong inhibitory effect on pancreatic lipase [
8]. However, the inhibitory effects on lipase of different species and cultivars of kiwifruits collected in China have seldom been investigated and compared. As shown in
Figure 3A, significant differences (
p < 0.05) were found among the tested kiwifruits in the inhibitory activities toward pancreatic lipase. The IC
50 values of inhibitory effects on pancreatic lipase varied from 3.12 ± 0.09 mg/mL to 7.44 ± 0.11 mg/mL among the tested kiwifruits. The difference in IC
50 values may have resulted from the different levels of TPC in kiwifruits, which suggests that the inhibitory effects on pancreatic lipase were associated with the high content of phenolic compounds [
9]. Among the tested kiwifruits,
A. chinensis cv. Hongshi showed the highest pancreatic lipase inhibitory activity, whereas the lowest activity was measured in
A. macrosperma. In addition,
A. chinensis and
A. deliciosa showed more potent inhibitory effects on pancreatic lipase than other species (
A. macrosperma, A. polygama, and
A. arguta). Furthermore, the inhibitory effects on pancreatic lipase of the tested kiwifruits (except for
A. macrosperma) still presented much lower IC
50 values when compared with the commercial orlistat drug (IC
50 = 6.34 mg/mL). Previous studies have reported that proanthocyanidins are effective lipase inhibitors in grape seed extract and strawberry extract [
30,
31]. In addition, previous studies have shown that quercetin and its derivatives exert an inhibition effect on pancreatic lipase [
32,
33]. Therefore, it seems likely that proanthocyanidins (procyanidin B1 and procyanidin B2) and quercetin derivatives (quercetin-3-rhamnoside and quercetin-3-
O-glucoside) present in kiwifruits could be the major contributors toward the pancreatic lipase inhibition effects of kiwifruits due to their high content.
α-Glucosidase is a key enzyme responsible for the breakdown of oligosaccharides and disaccharides into monosaccharides suitable for absorption. Therefore, the inhibition of α-glucosidase is one of the main strategies to counteract metabolic alterations related to hyperglycaemia and type II diabetes [
34]. Previous studies have shown that the kiwifruit extract exhibits a strong inhibitory effect on α-glucosidase [
8,
14]. However, different kiwifruits’ inhibitory effects on α-glucosidase have seldom been investigated. As shown in
Figure 3B, significant differences (
p < 0.05) were found among the tested kiwifruits in the inhibitory activities toward α-glucosidase. There was a wide range of α-glucosidase inhibition, with the IC
50 values ranging from 9.11 ± 0.24 mg/mL to 66.73 ± 0.09 mg/mL. Particularly,
A. polygama showed the highest inhibitory activity on α-glucosidase, which might be attributed to the high content of quercetin-3-
O-glucoside. A previous study showed that the flavonols, especially quercetin-3-
O-glucoside, were found to possess high inhibition activity on α-glucosidase [
9]. Moreover, previous studies have also indicated that chlorogenic acid and its structural isomer are major contributors to the inhibitory activities on α-glucosidase [
32,
35]. The high content of chlorogenic acid and neochlorogenic acid in kiwifruits may explain the α-glucosidase inhibitory effect of kiwifruits. Caffeic acid and catechin might also play important roles in α-glucosidase inhibition effects [
29,
32,
35]. The order of the most potent α-glucosidase inhibitors was as follows:
A. polygama >
A. chinensis cv. Hongshi >
A. chinensis cv. Jinshi >
A. chinensis cv. Jinlong. Furthermore, compared with the positive inhibitor (acarbose standard, IC
50 = 4.63 mg/mL), the tested kiwifruits exerted a moderate inhibitory effect towards α-glucosidase. However, kiwifruits exhibited stronger antidiabetic activity than orange, mandarine, apple, banana, pineapple, plum, pear, pomelo, and red grapefruit [
8].