O R I G I N A L A R T I C L E
Effect of microwave and oven drying processes on antioxidant
activity, total phenol and phenolic compounds of kiwi and pepino
fruits
Mehmet Musa O¨ zcan1•Fahad Al Juhaimi2•Isam A. Mohamed Ahmed2•
Nurhan Uslu2•Elfadil E. Babiker2• Kashif Ghafoor2
Revised: 28 July 2019 / Accepted: 20 August 2019 / Published online: 27 August 2019 Ó Association of Food Scientists & Technologists (India) 2019
Abstract Kiwi and pepino fruits are most valuable fruits as they contains substantial amounts of nutrients and bioactive compounds. These fruits exhibited several health potentials such as antioxidant, antiinflammatory, antiobe-sity, antihyperlipidemia, and anticancer properties. How-ever, studies on the effect of microwave and conventional drying methods on the antioxidant activity and bioactive compounds of kiwi and pepino fruits are limited. There-fore, this study was conducted to assess the effect of microwave and oven drying methods on antioxidant activity, total phenolic, and phenolic compounds of kiwi and pepino fruits. Drying of the fruit samples was carried out using conventional (70°C for 20 h) and microwave (720 W for 3 min) ovens. 1,1-diphenyl-2-picrylhydrazyl scavenging and colorimetric Folin–Ciocalteu assays were used to assess the antioxidant activity and total phenolic contents, respectively, of fresh and dried fruits. Both drying methods significantly (p \ 0.05) decreased the moisture contents of both fruits compared to untreated controls. Concomitantly, drying methods also enhanced (p \ 0.05) antioxidant activity and total phenolic content of both fruits with the highest improvement being observed for micro-wave-dried fruits compared to untreated controls. In addi-tion, a significant increase was observed in catechin and
1,2-dihydroxybenzene content of kiwi and pepino after drying process. However, microwave drying method reduced the amount of 3,4-dihydroxybenzoic acid in kiwi (ranging from 34.120 to 9.350 mg/100 g) and pepino (varied from 33.414 to 15.445 mg/100 g). Generally, the highest antioxidant activity and phenolic contents were reported in microwave oven dried samples, followed by samples dried in oven and fresh fruits. The results revealed that microwave drying could be more useful in fruit drying than conventional drying. In addition, dried kiwi and pepino fruits contains substantial quantities of phenolic compounds with high antioxidant activity compared to fresh fruits, and thus they are considered as healthy food. Keywords Kiwi Pepino Drying Microwave
Antioxidant activity Total phenol Phenolic compounds HPLC
Introduction
Kiwi fruits (Actinidia deliciosa) are fruiting vine that originated from Asia. It is mostly grown in southern China and its popularity in world today may be attributed to its high level of bioactive compounds. Kiwi fruits are rich in ascorbic acid, folic acid and antioxidants. Pepino (Solanum muricatum Aiton) which is also called mishqui, melon pear or pepinodulce are cultivated in South America and its fruit is edible (Gonzalez et al. 2000; Prohens and Nuez 2001; Huyskens-Keil et al.2006). Pepino fruits contain substan-tial amounts of phenols and flavonoids and possess high free radical scavenging activity with high anti-oxidative stress potentials (Sudha et al.2012). Several vegetable and fruits contain phytochemicals and bioactive components that carry different physiological and biochemical & Mehmet Musa O¨zcan
mozcan@selcuk.edu.tr; iali@ksu.edu.sa
1 Department of Food Engineering, Faculty of Agriculture,
Selcuk University, 42031 Konya, Turkey
2 Department of Food Science and Nutrition, College of Food
and Agricultural Sciences, King Saud University, Riyadh, Saudi Arabia
functions (Ferguson and McRae1991; Tavarini et al.2008; Salinero et al.2009). Kiwi fruits exhibit in vivo antioxidant activity and hypolipidemia (Chang and Liu 2009), ame-liorates chemically-induced toxicity (Kang et al.2012) and nephrotoxicity (Mahmoud 2017), prevent cancer (Moto-hashi et al. 2002), and possess anti-inflammatory effects (Gammon et al.2014). Pepino fruits also exhibit antidia-betic (Hsu et al. 2011), antitumor (Ren and Tang 1999), anti-inflammatory (Ma et al.2016), and antihyperlipidemia (Wang et al. 2012) potentials. Drying is an important operation in fruit processing industry. The purpose of fruit drying is the reduction of moisture content to a level where deterioration reactions are inactivated. Hot air drying is a traditional drying method for vegetables and fruits (Ogura
1993; Orikasa et al. 2008; Lopez et al. 2017). However, application of heat to fruits for an extended period of time often results to various physicochemical changes (Maskan
2001; Ciemniewska-Zytkiewicz et al. 2014). Microwave drying which is done for a relatively short period of time can be a better alternative to the traditional hot air drying method. In addition to the lower energy cost incurred in microwave drying, the process has also been found to be faster compared to the conventional hot air drying treat-ment. To date, various drying methods such as hot-air drying for pepino and kiwi fruits (Di Scala et al. 2011; Orikasa et al. 2008), microwave, infrared-microwave (O¨ ztu¨rk et al. 2017), and connective-infrared (Ozdemir et al.2017) drying of kiwi fruits among others have been used. However, information on the impact of microwave or conventional drying on the antioxidant activity and phe-nolic compounds of kiwi and pepino fruits are scarce and still required further enrichment. Therefore, the aim of present study was to determine the effects of oven and microwave drying systems on bioactive properties and phenolic compounds of kiwi and pepino fruits.
Materials and methods
Control sampleNo heat treatment was applied to the control sample.
Materials
Kiwi (Actinidia deliciosa) and pepino (Solanum muricatum Aiton) fruits of the same varieties used in this present study were purchased from a local market in Konya, Turkey. All reagents used were purchased from Sigma Laboratory and were of analytical grade (Sigma-Aldrich Co., St. Louis, MO, USA).
Methods
Drying process Oven drying processKiwi and pepino fruits were peeled, washed, and sliced into slices with 2–3 mm dimeter using a knife. Slices (10 mm thicknesses) (about 100 g for each fruits) were dried in an oven (Nu¨ve FN055 Ankara, Turkey, 55 L volume) at 70°C for 20 h. Moisture content was measured at 100 ± 5°C. Microwave drying process
The kiwi and pepino fruits were manually cleaned, pitted and sliced into 3–4 mm slices with a knife. In this process, a microwave oven (Arc¸elik ARMD 585, Turkey) with the dimension cavity of 34.5 9 34.0 9 22.5 cm and capable of generating a powerof 720 W 2450 MHz was used for drying the fruit slices. Briefly, 100 g sliced fruits for each type were placed in beakers (50 ml each), and then the beakers were set appropriately on a rotary plate of the microwave oven at equal distances on a 28 cm diameter circumference. Thereafter, the samples were heated at 2450 MHz for 5 min. After that, the samples were dried at 720 W for 3 min in the microwave oven until the moisture content was below 20%. Overheating was avoided and heating was controlled by monitoring the color change. After cooling the fruit samples at room temperature, they were then kept frozen (- 25°C) under nitrogen in sealed bottles until being used for further chemical analyses. Fresh fruits of kiwi and pepino were used as untreated controls. Additionally, fresh and dried samples (1 kg for each sam-ple) were analysed for antioxidant, total phenolics and phenolic compounds.
Bioactive compounds extraction
The method described by Herraiz et al. (2016) was employed to extract bioactive compounds from fruit sam-ples. 1 g of ground fruit samples was mixed with 5 ml of a mixture of 70% methanol and 30% water (v/v) followed by vortex mixing for 2 min and then sonication for 30 min. The mixture was then centrifuged at 6000g for 15 min and the process was repeated twice. After centrifugation, the supernatants were collected and filtered prior to analysis. Total phenolic content determination
Total phenolic content of kiwi and pepino fruit extracts was assessed using Folin–Ciocalteu (FC) reagent as described by Yoo et al. (2004). In this assay, Folin–Ciocalteu reagent
(1 ml) was added to samples and vortexed for 5 min. Afterwards 10 ml of 7% Na2CO3(w/v) were mixed with
the sample by vortex. Finally, sample volume was 25 ml using distilled water. The mixture was left to stand for 1 h at room temperature in the dark. Standard of known con-centrations of gallic acid was prepared and treated in the same way of the samples. Then the absorbance of the standard and samples were measured at 750 nm using a spectrophotometer and the results were calculated from the linear equation of the gallic acid standard curve and expressed as mg gallic acid equivalent (GAE)/100 g. Determination of scavenging activity
The scavenging activity of kiwi and pepino extracts were quantified according to the method described by Lee et al. (1998) using 1,1-diphenyl-2-picrylhydrazyl (DPPH). Briefly, 2 ml of methanolic DPPH was mixed with the extract. After that, the mixture was vortexed and allowed to stand for 30 min at room temperature. Blank was treated in the same way with the omission of the extract. Then the absorbance of the samples and the blank was measured at 517 using a spectrophotometer and the results were expressed as percentage of the inhibition.
Phenolic compounds determination
The bioactive phenolic compounds of kiwi and pepino extracts were separated on an Inertsil ODS-3 (5 lm; 4.6 9 250 mm) column connected to HPLC system (Shi-madzu, Kyoto, Japan) and the absorbance of the analyses was detected using PDA detector. The mobile phase con-sisted of a mixture of 0.05% acetic acid in water and acetonitrile. The system was run at a flow rate of 1 ml/min and 30°C. The sample injection volume was 20 ll and peaks were recorded at 280 and 330 nm after giving a total running time of 60 min for each sample.
Statistical analysis
Results obtained were subjected to Analysis of Variance (ANOVA) using JMP version 9.0 (SAS Inst. Inc., Cary, NC, USA) and expressed as mean ± standard deviation of triplicate samples. Means were compared using Duncan’s multiple range tests and probability was accepted at p\ 0.05. Pearson’s correlation coefficients were calcu-lated for detecting the correlations between different vari-ables using Stat View software and multivariate analysis was performed using HJ-biplot methods included in the MULTBIPLOT software (Vicente-Villardo´n 2010) as described in the instruction manual.
Results and discussion
Effect of microwave and oven drying methods on antioxidant and total phenolics of kiwi and pepino fruits
The antioxidant activity, moisture and total phenolic con-tents of fresh and dried kiwi and pepino fruits are presented in Table1. Fresh kiwi and pepino fruits were used and analyzed as untreated controls. The results showed that antioxidant activity of fresh kiwi was higher than fresh pepino (p \ 0.05), while the total phenolic and moisture contents were higher in fresh pepino fruits compared to kiwi (p \ 0.05). Regardless of the drying method used, drying had significant (p \ 0.05) effect on antioxidant activity, moisture and total phenolic contents of both kiwi and pepino fruits in comparison with control samples (fresh fruits). The moisture content was significantly reduced (p \ 0.05) during drying processes of both fruits with the highest reduction being observed in microwave dried fruits, which could be due to the high heat emitted in short time during microwave drying compared to low heat production for long time in conventional oven drying. The antioxidant activity and total phenolic content were significantly higher in dried fruits than fresh fruits, which might be due to the concentration of antioxidant compounds during drying processes (Nunes et al. 2016). In addition, heating pro-cesses could also destroy the integrity of the cell structure of kiwi and pepino fruits thereby promoting the release of phenolic compounds in extraction solution and conse-quently increased phenolic and antioxidant activity of the extracts (Arslan and O¨ zcan2010; Szychowski et al.2018). In current study, microwave-dried fruits had significantly higher antioxidant activity and total phenolic content compared to oven-dried fruits (p \ 0.05). Similarly, Ben-lloch-Tinoco et al. (2013) reported that microwave heating led to a better antioxidant capacity in kiwi fruit than con-ventional heating. In addition, Arslan and O¨ zcan (2010) reported that both conventional and microwave ovens drying increased the total phenolic contents of onion slices with the greatest values being observed in microwave-dried samples. Moreover, Ghanem et al. (2012) observed that microwave dehydration enhances the total phenolic content of dried peels of Thompson navel orange compared to fresh peels. In contrast, a reduction in the total phenolic and antioxidant activity after thermal drying methods has also been reported in dried fruits such as kiwi (Izli et al.2017), pepino (Di Scala et al.2011), chokeberry (Samoticha et al.
2016), sour cherries (Wojdylo et al.2014). The differences between these studies could be attributed to the variation in the fruit types, phenolic composition, dehydration methods,
dehydration conditions, extraction and analysis methods of total phenolic and antioxidant activity.
Effect of microwave and oven drying methods on phenolic compounds of kiwi fruits
The phenolic compounds in kiwi fruits as affected by drying methods are presented in Table2. The phenolic profile of fresh kiwi fruits indicated that (?)-catechin (46.910 mg/100 g), 1,2-dihydroxybenzene (44.801 mg/ 100 g), 3,4-dihydroxybenzoic acid (34.120 mg/100 g) and gallic acid (29.406 mg/100 g) were the dominant (p \ 0.05) phenolic compounds in this fruit, which is comparable to that reported previously in kiwi fruits (Park et al. 2011). The drying methods significantly (p \ 0.05) affected the phenolic profile of kiwi fruits. With view exceptions, both conventional and microwave ovens drying
caused a significant increase in phenolic compounds of kiwi fruits compared to controls. Conventional oven drying increased gallic, 3,4-dihydroxybenzoic, and trans-cinnamic acids, while microwave drying reduced them compared to controls (p \ 0.05). The reduction of these compounds during microwave heating could be due to their degrada-tion by high heat generated by microwave oven. Micro-wave oven drying increased syringic acid, while conventional oven drying reduced it in dried kiwi fruits compared to fresh ones (p \ 0.05). The highest (p \ 0.05) values of (?)-catechin, 1,2-dihydroxybenzene, syringic acid, caffeic acid, rutin trihydrate, trans-ferulic acid, and kaempferol were observed in microwave-dried kiwi fruits. Overall, drying of kiwi fruits with conventional and microwave ovens significantly enhanced the phenolic compounds which could be attributed to the concentration of these compounds due to the evaporation of moisture
Table 1 Antioxidant activity and total phenolic content of kiwi and pepino fruits
Sample Process Moisture (%) Antioxidant activity (%) Total phenolic content (mg/100 g DW)
Kiwi Control 81.343 ± 0.248*c 16.239 ± 0.004c 623.100 ± 0.018c
Oven 21.016 ± 0.569a** 79.344 ± 0.016b 1034.225 ± 0.003b
Microwave 20.457 ± 0.765b 92.343 ± 0.002a 1117.319 ± 0.009a
Pepino Control 93.250 ± 0.240a 11.443 ± 0.007c 1511.183 ± 0.007b
Oven 22.357 ± 0.320b 88.136 ± 0.001b 1591.049 ± 0.004b
Microwave 21.681 ± 0.960b 90.618 ± 0.001a 1812.793 ± 0.004a
*Mean ± standard deviation
**Values within each column followed by different letters are significantly different (p \ 0.05)
Table 2 Phenolic compounds
of kiwi fruits (mg/100 g) Phenolic compounds Control Drying oven Microwave oven
Gallic acid 29.406 ± 1.267*b 30.016 ± 0.366a 27.630 ± 0.222c
3,4-Dihydroxybenzoic acid 34.120 ± 0.332b** 40.125 ± 0.863a 9.350 ± 0.517c
(?)-Catechin 46.910 ± 0.877c 86.188 ± 2.509b 89.307 ± 0.613a
1,2-Dihydroxybenzene 44.801 ± 2.573c 76.646 ± 1.408b 104.628 ± 0.750a
Syringic acid 2.585 ± 0.195b 1.434 ± 0.038c 3.335 ± 0.123a
Caffeic acid 1.592 ± 0.094c 3.814 ± 0.126b 7.318 ± 0.516a
Rutin trihydrate 0.387 ± 0.027c 1.060 ± 0.039b 2.102 ± 0.118a
p-Coumaric acid 0.061 ± 0.002b 0.186 ± 0.008a 0.114 ± 0.005a
trans-Ferulic acid 0.363 ± 0.014c 1.600 ± 0.015b 2.183 ± 0.042a
Apigenin 7 glucoside 0.347 ± 0.017c 1.498 ± 0.082a 1.068 ± 0.037b
Resveratrol 0.386 ± 0.017b 0.632 ± 0.017a 0.494 ± 0.022b
Quercetin 1.865 ± 0.213b 3.302 ± 0.109a 1.256 ± 0.024b
trans-Cinnamic acid 0.717 ± 0.057b 1.170 ± 0.046a 0.509 ± 0.004c
Naringenin 2.479 ± 0.125b 6.053 ± 0.246a 6.671 ± 0.125a
Kaempferol 8.096 ± 0.063c 15.320 ± 0.041b 19.825 ± 0.111a
Isorhamnetin 8.503 ± 0.228c 18.498 ± 0.238a 16.204 ± 0.393b
*Mean ± standard deviation
from the fruits. In agreement with our findings, Szychowski et al. (2018) found that freeze-, convective-, and vacuum-microwave drying methods increased the phenolic com-pounds, particularly phenolic acids, in quince fruits. In addition, Gao et al. (2012) reported that microwave dying process increased the contents of protocatechuic acid, catechin, and epicatechinin dried jujube fruits compared to fresh ones. Moreover, Slatnar et al. (2011) observed that oven and sun drying increased chlorogenic acid, catechin, epicatechin, kaempferol-3-O-glucoside, rutin, quercetin-3-O-glucoside in dried fig fruits in comparison with fresh fruits. The enhancement of phenolic compounds during drying processes could be attributed to the breakdown and disruption of cell walls by heat or microwave effects and consequently eases the release and extractability of these phenolic compounds during rehydration processes (Szychowski et al.2018). The increased value of phenolic compounds in dried kiwi fruits over that in fresh fruits, suggest the potential benefits of these fruits on human health as they could protect human body from dangerous free radicals.
Effect of microwave and oven drying methods on phenolic compounds of pepino fruits
The phenolic profile of pepino fruits as affected by con-ventional and microwave ovens drying are shown in Table3. The results revealed that 1,2-dihydroxybenzene (53.277 mg/100 g) is the most dominant (p \ 0.05) phenolic compound in pepino fruits followed by
3,4-dihydroxybenzoic acid (33.414 mg/100 g), (?)-cate-chin (32.183 mg/100 g) and gallic acid (17.501 mg/100 g) which is partially comparable to those reported previously in pepino fruits (Herraiz et al. 2016). Drying greatly (p \ 0.05) affected the phenolic compounds of pepino fruits. Regardless of the drying method used, drying sig-nificantly (p \ 0.05) increased the values of all phenolic compounds compared to undried controls. Conventional oven drying enhanced the contents of gallic and 3,4-dihy-droxybenzoic acids, while microwave oven drying reduced them compared to control samples. The highest (p \ 0.05) values of (?)-catechin, kaempferol, gallic, 3,4-dihydroxy-benzoic, caffeic, and p-coumaric acids were observed in pepino fruits dried with conventional oven. Whereas, fruits dried with microwave oven showed the highest (p \ 0.05) values of 1,2-dihydroxybenzene, rutin trihydrate, apigenin 7 glucoside, resveratrol, quercetin, naringenin, isorham-netin, syringic, trans-ferulic and trans-cinnamic acids. The increment of these phenolic compounds in dried pepino fruits over fresh fruits could be attributed to the concen-tration of these compounds as heating removes moisture and concentrated the components of the fruits. Similarly, Nunes et al. (2016) observed higher phenolic compounds in dried guava powders compared to fresh fruits and attrib-uted that to concentration of the phenolic compounds due to water losses by heating processes. Moreover, Slatnar et al. (2011) stated that oven drying process increased the concentration of most phenolic compounds, with exception of cyanidin-3-O-rutinoside, in fig fruits. Furthermore, Szychowski et al. (2018) found increased values of
Table 3 Phenolic compounds
of pepino fruits (mg/100 g) Phenolic compounds Control Oven Microwave
Gallic acid 17.501 ± 0.581b 36.573 ± 0.674a 13.489 ± 0.513c
3,4-Dihydroxybenzoic acid 33.414 ± 1.345b 38.083 ± 0.709a 15.445 ± 0.503c
(?)-Catechin 32.183 ± 3.566c 87.981 ± 0.408a 36.949 ± 2.942b
1,2-Dihydroxybenzene 53.277 ± 3.297c 109.643 ± 2.490b 119.267 ± 0.075a
Syringic acid 8.377 ± 1.233b 8.971 ± 0.063b 11.029 ± 0.263a
Caffeic acid 1.916 ± 0.180c 8.474 ± 0.318a 7.102 ± 0.114b
Rutin trihydrate 1.777 ± 0.167c 2.410 ± 0.045b 10.517 ± 0.372a
p-Coumaric acid 0.533 ± 0.076c 8.379 ± 0.384a 2.575 ± 0.103b
trans-Ferulic acid 3.487 ± 0.569b 2.112 ± 0.030b 10.184 ± 0.099a
Apigenin 7 glucoside 4.890 ± 0.778c 9.422 ± 0.496b 21.144 ± 0.883a
Resveratrol 1.851 ± 0.296c 2.516 ± 0.160b 10.900 ± 0.144a
Quercetin 3.200 ± 0.387c 10.853 ± 0.582b 12.905 ± 0.247a
trans-Cinnamic acid 0.775 ± 0.044b 2.037 ± 0.088a 2.530 ± 0.020a
Naringenin 3.372 ± 0.138b 2.675 ± 0.082c 4.081 ± 0.172a
Kaempferol 10.707 ± 0.200c 20.759 ± 0.291a 19.208 ± 0.276b
Isorhamnetin 6.221 ± 0.439c 21.386 ± 0.469b 22.589 ± 0.348a
*Mean ± standard deviation
phenolic acids in dried quince fruits compared to fresh ones and they attributed that to the release of phenolic com-pounds from cellular structure during drying processes. Again, enhancing the phenolic compounds of pepino fruits during drying processes could be of potential health ben-efits to consumers of dried pepino fruits due to the pro-tective roles of phenolic compounds against free radicals. Correlations
The correlation coefficient analysis indicated various cor-relations among the antioxidant, moisture, total phenolic contents, and individual phenolic compounds of kiwi and pepino fruits (Table4). Regardless of the drying methods and fruit types, correlation analysis among traits showed diverse correlations (positive, weak, and negative). Mois-ture content negatively correlated with antioxidants (r2= - 0.991, p \ 0.001), 1,2-dihydroxybenzene (r2= - 0.868, p \ 0.05), caffeic acid (r2= - 0.840, p \ 0.05), kaempferol (r2= - 0.897, p \ 0.01), and isorhamnetin (r2= - 0.939, p \ 0.01) indicating that reduced moisture content in dry fruits resulted in increased values of these phytochemicals. It is clearly evident that during drying processes moisture content was reduced for both fruits, while antioxidant and bioactive compounds were con-comitantly increased. Antioxidant activity showed positive correlations with 1,2-dihydroxybenzene (r2= 0.923, p\ 0.01), caffeic acid (r2= 0.896, p \ 0.05), kaempferol (r2= 0.945, p \ 0.01), and isorhamnetin (r2= - 0.938, p\ 0.01) suggesting the great contribution of these phe-nolic compounds to the antioxidant activity of kiwi and pepino fruits. 1,2-Dihydroxybenzene also positively cor-related with caffeic acid (r2= 0.965, p \ 0.001), kaemp-ferol (r2= 0.969, p \ 0.001), and isorhamnetin (r2= - 0.902, p \ 0.01) demonstrating concurrent increase in these phytochemicals during drying processes of kiwi and pepino fruits. Caffeic acid also showed positive correla-tions with kaempferol (r2= 0.975, p\ 0.001), and isorhamnetin (r2= - 0.851, p \ 0.05). Total phenolic contents revealed positive correlations with syringic acid (r2= 0.915, p \ 0.01), apigenin 7 glucoside (r2= 0.833, p\ 0.05), and quercetin (r2= 0.798, p \ 0.05) indicating concurrent increase of total phenolic and these bioactive compounds during drying processes of kiwi and pepino fruits. Syringic acid also showed positive correlations with apigenin 7 glucoside (r2= 0.861, p \ 0.05), and quercetin (r2= 0.818, p \ 0.05). Rutin trihydrate showed strong positive (p \ 0.001) correlations with trans-ferulic acid (r2= 0.979), apigenin 7 glucoside (r2= 0.947), and resveratrol (r2= 0.987) suggesting significant interaction between these bioactive compounds in kiwi and pepino fruits. Trans-ferulic acid also positively correlated with apigenin 7 glucoside (r2= 0.927, p\ 0.01), and
resveratrol (r2= 0.972, p \ 0.001). Apigenin 7 glucoside also positively correlated with resveratrol (r2= 0.973, p\ 0.001), quercetin (r2= 0.923, p \ 0.01), and trans-cinnamic acid (r2= 0.910, p \ 0.01). Resveratrol showed positive (p \ 0.05) correlation with quercetin (r2= 0.828), and trans-cinnamic acid (r2= 0.833). Quercetin and trans-cinnamic acid were positively correlated (r2= 0.985, p\ 0.001) and also kaempferol and isorhamnetin were also positively (r2= 0.877, p \ 0.01) correlated with each other. With exception to negative correlation of moisture, correlation analysis observed in the current study generally showed positive correlations between antioxidants, total phenolic, and individual phenolic compounds suggesting the great interaction between these parameters in dried kiwi and pepino fruits. Despite of some minor differences, the correlation findings in this study are, in general, compa-rable to those reported previously in many other dried fruits (Izli et al. 2017; Nunes et al. 2016; Szychowski et al.
2018).
To powerfully determine the interrelationships between the antioxidant activity, total phenolic and moisture con-tents, and bioactive compounds in fresh and dried kiwi and pepino fruits, biplot analysis was performed by comparing the eigenvalues of PC1 andPC2 of principal component analysis (PCA) for drying treatments, fruit types, and quality parameters (Fig. 1). PCA results revealed an excellent contribution of the axes (PC1, 53.76%, and PC2, 26.19%) to the plotted components as the total variability of the data was more than 79%. In the biplot, the angles between the vectors of the parameters indicated the cor-relations between them, in which acute angle (\ 90°) indicated positive correlation, obtuse ([ 90°) or strait (180°) angles indicated negative correlation, and right angle (90°) indicated no correlation (Mutwali et al.2016). Hierarchical clustering analysis shows the interaction between the treatments and fruit types and their influence on the antioxidant activity, moisture and total phenolic contents, and phenolic compounds of the fruits. These interactions and impacts are clearly indicated by four dis-crete clusters in the figure. The first cluster (upper right quadrant of the graph, squire symbol) contains the micro-wave drying treatment of pepino fruits. This treatment strongly contributed to greater enhancement of total phe-nolic, resveratrol, apigenin 7 glucoside, rutin trihydrate, quercetin, and syringic, trans-ferulic, and trans-cinnamic acids in pepino fruits compared to kiwi fruits. Acute angles between the vectors of these bioactive compounds sug-gested strong positive correlations between them. The second cluster (lower right quadrant of the graph, circle symbol) contains the conventional oven drying treatment of pepino fruits which greatly increased the antioxidant activity, kaermpferol, isorhamnetin, 1,2-dihydroxyben-zene, and caffeic and p-coumaric acids in pepino fruits.
Table 4 Correlations coefficient among the antioxidants, total phenolic, and phenolic compounds of fresh and dried kiwi and pepino fruits Moist Antiox TPC GA Dihy A (? )-Cat Dihyb Syr A Caffe A Rutin trihyd p -Coum A Trans- Feru A Apig 7 Glu Resv Querc Trans -Cinn A Naring Kaemp Antiox -0.991*** TPC -0.298 0.388 GA -0.253 0.189 -0.47 Dihy A 0.328 -0.41 -0.224 0.467 (? )-Cat -0.689 0.648 -0.212 0.793 0.059 Dihyb -0.868* 0.923** 0.645 -0.028 -0.534 0.392 Syr A -0.029 0.128 0.915** -0.502 -0.183 -0.461 0.472 Caffe A -0.840* 0.896* 0.56 0.188 -0.454 0.528 0.965*** 0.412 Rutin trihyd -0.386 0.446 0.721 -0.672 -0.543 -0.391 0.662 0.717 0.479 p -Coum A -0.373 0.409 0.556 0.395 0.241 0.288 0.547 0.586 0.661 0.206 Trans -Feru A -0.271 0.338 0.766 -0.787 -0.513 -0.492 0.569 0.748 0.367 0.979*** 0.122 Apig 7 Glu -0.334 0.397 0.833* -0.55 -0.313 -0.384 0.65 0.861* 0.507 0.947*** 0.464 0.927** Resv -0.298 0.353 0.734 -0.669 -0.414 -0.47 0.586 0.766 0.404 0.987*** 0.258 0.972*** 0.973*** Querc -0.443 0.487 0.798* -0.207 -0.067 -0.135 0.693 0.818* 0.623 0.781 0.742 0.722 0.925** 0.828* Trans -Cinn A -0.498 0.522 0.736 -0.198 -0.023 -0.107 0.677 0.729 0.583 0.786 0.677 0.722 0.910** 0.833* 0.985*** Naring -0.564 0.545 -0.119 -0.026 -0.466 0.523 0.316 -0.463 0.244 -0.009 -0.459 -0.001 -0.241 -0.148 -0.351 -0.283 Kaemp -0.897** 0.945** 0.585 0.138 -0.44 0.572 0.969*** 0.359 0.975*** 0.473 0.572 0.386 0.480 0.388 0.576 0.556 0.402 Isorham -0.939** 0.938** 0.488 0.144 -0.23 0.485 0.902** 0.292 0.851* 0.577 0.559 0.46 0.593 0.533 0.716 0.764 0.288 0.877** Significant level: * p \ 0.05; ** p \ 0.01; *** p \ 0.001
Again acute angles between these phytochemicals indi-cated strong positive correlations between them. The latter group of parameters in this cluster also positively corre-lated with those in the first cluster as they formed acute angles. These findings demonstrated that drying methods exhibited different effects on the antioxidant activity and phenolic compounds of pepino fruits as microwave drying improved some and conventional drying improved the others. The third cluster (upper left quadrant of the graph, diamond symbols) contains fresh pepino and kiwi fruits those characterized by higher moisture contents than dried fruits of both types. With exception to 3,4-dihydroxy-benzoic acid, moisture has negative or no correlation with all other parameters as evident from right, obtuse, or straight angles. The fourth cluster (lower left quadrant of the graph, tringle symbols) contains conventional and microwave drying treatments of kiwi fruits. In this cluster, conventional oven drying contributed to the increment of 3,4-dihydroxybenzoic and gallic acids, while microwave oven drying enhanced the naringenin and (?)-catechin in dried kiwi fruits. Overall, the findings showed that both heating treatment and fruit type affected the antioxidant, total phenolic and phenolic compounds of the dried fruits. Both drying methods significantly enhanced most of the antioxidant compounds in fruits and the enhancement is more noticeable in pepino fruits than kiwi fruits. This could be due to the variation in the moisture contents,
phytochemical composition, and cellular structure of these fruits, in addition, to the differences in the heating rate and type between both treatments. Variations in the environmental, agronomical, genotypes and drying pro-cesses are suggested as the factors affecting the antioxi-dant and phenolic composition of dried fruits (Bennett et al. 2011).
Conclusion
The results obtained in this present studies showed that drying methods had significant effects on the moisture, antioxidants and phenolic compounds present in kiwi and pepino fruits. Microwave and conventional heating increased significantly the antioxidant activity and total phenolic contents of both fruits. Principle component analysis results showed that drying methods exhibited varied effects on the antioxidant activity and phenolic compounds of pepino fruits as microwave drying improved some and conventional drying improved the others. Both Kiwi and pepino fruits are rich source bioactive com-pounds and the present studies has revealed that the physicochemical and nutritional properties of kiwi and pepino fruits could be preserved if proper drying method is used. Microwave heating showed higher antioxidant and total phenolic compounds which could be a better
Fig. 1 HJ-biplot (PCA)
analysis for antioxidant activity, total phenolic and phenolic compounds in dried pepino and kiwi fruits dried with
conventional (oven) and microwave ovens
alternative to the conventional heating methods which could affect the stability of bioactive components of the fruits.
Acknowledgements The authors would like to extend their sincere
appreciation to the Deanship of Scientific Research at King Saud University for its funding this Research Group No. (RG-1439-80).
References
Arslan D, O¨ zcan MM (2010) Study the effect of sun, oven and
microwave drying on quality of onion slices. LWT Food Sci Technol 43:1121–1127
Benlloch-Tinoco M, Igual M, Rodrigo D, Martı´nez-Navarrete N (2013) Comparison of microwaves and conventional thermal treatment on enzymes activity and antioxidant capacity of kiwifruit puree. Innov Food Sci Emerg Technol 19:166–172 Bennett LE, Jegasothy H, Konczak I, Frank D, Sudharmarajan S,
Clingeleffer PR (2011) Total polyphenolics and anti-oxidant properties of selecteddried fruits and relationships to drying conditions. J Funct Foods 3:115–124
Chang WH, Liu JF (2009) Effects of kiwifruit consumption on serum lipid profiles and antioxidative status in hyperlipidemic subjects. Int J Food Sci Nutr 60(8):709–716
Ciemniewska-Zytkiewicz H, Brys J, Brys A, Sujka K, Koczon P (2014) Effect of roasting process on moisture content and color of Polish in shell hazelnuts. Akad Gıda 12(1):6–10
Di Scala K, Vega-Galvez A, Uribe E, Oyanadel R, Miranda M, Vergara J et al (2011) Quality characteristics changes of pepino fruit (Solanum muricatum Ait) during convective drying. Int J Foods Sci Technol 46:746–753
Ferguson AR, Mcrae EA (1991) Vitamin C in actinidia. Acta Hortic 297:481–487
Gammon CS, Kruger R, Conlon CA, von Hurst PR, Jones B, Stonehouse W (2014) Inflammatory status modulates plasma lipid and inflammatory marker responses to kiwifruit consump-tion in hypercholesterolaemic men. Nutr Metab Cardiovasc Dis 24(1):91–99
Gao Q-H, Wu C-S, Wang M, Xu B-N, Du L-J (2012) Effect of drying of Jujubes (Ziziphus jujuba Mill.) on the contents of sugars, organic acids, a-Tocopherol, b-Carotene, and phenolic com-pounds. J Agric Food Chem 60:9642–9648
Ghanem N, Mihoubi D, Kechaou N, Mihoubi NB (2012) Microwave dehydration of three citrus peel cultivars: effect on water and oilretention capacities, color, shrinkage and total phenols content. Ind Crops Prod 40:167–177
Gonzalez M, Ca´mara M, Prohens J, Ruiz J, Torija E, Nuez F (2000) Colour and composition of improved pepino cultivars at three ripening stages. Gartenbauwissenschaft 65:83–87
Herraiz FJ, Villano D, Plazas M, Vilanova S, Ferreres F, Prohens J, Moreno DA (2016) Phenolic profile and biological activities of the pepino (Solanum muricatum) fruit and its wild relative S. caripense. Int J Mol Sci 17:394–408
Hsu C-C, Guo Y-R, Wang Z-H, Yin M-C (2011) Protective effects of an aqueous extract from pepino (Solanum muricatum Ait.) in diabetic mice. J Sci Food Agric 91:1517–1522
Huyskens-Keil S, Prono-Widayat HP, Ludders A, Schreiner M (2006) Posthatvest quality of pepino (Solanum muricatum Ait.) fruit in controlled atmosphere storage. J Food Eng 77:628–634 Izli N, Izli G, Taskin O (2017) Drying kinetics, colour, total phenolic
content and antioxidant capacity properties of kiwi dried by different methods. Food Measure 11:64–74
Kang WY, Hong HJ, Lee YJ (2012) Anti-oxidant activities of kiwi fruit extract on carbon tetrachloride-induced liver injury in mice. Korean J Vet Res 52(4):275–280
Lee SK, Mbwambo ZH, Chung HS, Luyengi L, Games EJC, Mehta RG (1998) Evaluation of the antioxidant potential of natural products. Comb Chem High Through Screen 1:35–46
Lopez J, Vega-Galvez A, Bilbao-Sainz C, Chiou B-S, Uribe E, Quispe-Fuentes I (2017) Influence of vacuum drying temperature on: physico-chemical composition and antioxidant properties of murta berries. J Food Process Eng 40(6):1–9
Ma CT, Chyau C-C, Hus C-C, Kuo S-M, Chuang C-W, Lin H-H, Chen J-H (2016) Pepino polyphenolic extract improved oxida-tive, inflammatory and glycative stress in the sciatic nerves of diabetic mice. Food Funct 7:1111–1121
Mahmoud YI (2017) Kiwi fruit (Actinidia deliciosa) ameliorates gentamicin-induced nephrotoxicity in albino mice via the activation of Nrf2 and the inhibition of NF-kB (Kiwi and
gentamicin-induced nephrotoxicity). Biomed Pharmacother
94:206–218
Maskan M (2001) Drying, shrinkage and rehydration characteristics of kiwifruits during hot air and microwave drying. J Food Eng 48:177–182
Motohashi N, Shirataki Y, Kawase M, Tani S, Sakagami H, Satoh K, Kurihara T, Nakashima H, Mucsi I, Varga A, Molna´r J (2002) Cancer prevention and therapy with kiwifruit in Chinese folklore medicine: a study of kiwifruit extracts. J Ethnopharmacol 81(3):357–364
Mutwali NIA, Mustafa AA, Gorafi YS, Mohamed Ahmed IA (2016) Effect of environment and genotypes on the physic chemical quality of the grains of newly developed wheat inbred lines. Food Sci Nutr 4(4):508–520
Nunes JC, Lago MG, Castelo-Branco VN, Oliveira FR, Torres AG, Perrone D, Monteiro M (2016) Effect of drying method on volatile compounds, phenolic profileand antioxidant capacity of guava powders. Food Chem 197:881–890
Ogura N (1993) Theory and method of preseving for food. In: Ogura N (ed) Food processing study. Kenpakusha Co., Ltd, Tokyo, pp 3–35 (in Japanese)
Orikasa T, Wu L, Shiina T, Tagawa A (2008) Drying characteristics of kiwifruit during hot air drying. J Food Eng 85:303–308 Ozdemir MB, Aktas M, Sevik S, Khanlari A (2017) Modeling of a
convective-infrared kiwifruit drying process. Int J Hydrogen Energy 42:18005–18013
O¨ ztu¨rk S, S¸akıyan O¨, Alifakı YO¨ (2017) Dielectric properties and
microwave and infrared-microwave combination drying charac-teristics of banana and kiwifruit. J Food Process Eng 40(3):e12502
Park YS, Leontowicz H, Leontowicz M, Namıesnik J, Suhaj M, Cvikrova M, Martincova O, Weisz M, Gorinstein S (2011) Comparison of the contents of bioactive compounds and the level of antioxidant activity in different kiwifruit cultivars. J Food Compos An 24:963–970
Prohens J, Nuez F (2001) Improvement of mishqui (Solanum muricatum) earliness by selection and ethephon applications. Sci Hortic 87:247–259
Ren W, Tang DG (1999) Extract of Solanum muricatum (Pepino/ CSG) inhibits tumor growth by inducing apoptosis. Anticancer Res 19:403–408
Salinero MC, Vela P, Sainz MJ (2009) Phenological growth stages of kiwifruit (Actinidia deliciosa ‘Hayward’). Sci Hortic 121:27–31 Samoticha J, Wojdyło A, Lech K (2016) The influence of different the drying methods on chemical composition and antioxidant activity in chokeberries. LWT Food Sci Technol 66:484–489 Slatnar A, Klancar U, Stampar F, Veberic R (2011) Effect of drying
of figs (Ficus carica L.) on the contents of sugars, organic acids, and phenolic compounds. J Agric Food Chem 59:11696–11702
Sudha G, Priya MS, Shree RBI, Vadivukkarasi S (2012) Antioxidant activity of ripe and unripe pepino fruit (Solanum muricatum Aiton). J Food Sci 77(11):C1131–C1135
Szychowski PJ, Lech K, Sendra-Nadal E, Herna´ndez F, Figiel A,
Wojdyło A, Carbonell-Barrachina A´ A (2018) Kinetics,
biocom-pounds, antioxidant activity, and sensory attributes of quinces as affected by drying method. Food Chem 255:157–164
Tavarini S, Degl’Innocenti E, Remorini D, Massai R, Guidi L (2008) Antioxidant capacity, ascorbic acid, total phenols and carote-noids changes during harvest and after storage of Hayward kiwifruit. Food Chem 107:282–288
Wang ZH, Hsu CC, Yin MC (2012) Aqueous extract from pepino (Solanum muricatum Ait.) attenuated hyperlipidemia and cardiac oxidative stress in diabetic mice, ISRN Obes 490870
Wojdyło A, Figiel A, Lech K, Nowicka P, Oszmian´ski J (2014) Effect of convective and vacuum–microwave dryingon the bioactive compounds, color, and antioxidant capacity of sour cherries. Food Bioprocess Technol 7:829–841
Yoo KM, Lee KW, Park JB, Lee HJ, Hwang IK (2004) Variation in major antioxidants and total antioxidant activity of Yuzu (Citrusjunos SiebexTanaka) during maturation and between cultivars. J Agric Food Chem 52:5907–5913
Publisher’s Note Springer Nature remains neutral with regard to
