Journal of Food Quality
Volume 2021 (2021), Article ID 9916694, 11 pages
https://doi.org/10.1155/2021/9916694

Research Article

Determination of Biochemical Contents of Fresh, Oven-Dried, and Sun-Dried Peels and Pulps of Five Apple Cultivars (Amasya, Braeburn, Golden Delicious, Granny Smith, and Starking)

Zeynep Ergün

Department of Bioengineering, Adana Alparslan Türkeş Science and Technology University, Adana 01250, Turkey

Correspondence should be addressed to Zeynep Ergün; zergun@atu.edu.tr

Received 12 March 2021; Revised 16 April 2021; Accepted 19 May 2021; Published 25 May 2021

Academic Editor: Philippe Jeandet

Copyright © 2021 Zeynep Ergün. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

This study aimed to investigate the biochemical compounds of five different apple cultivars (Amasya, Braeburn, Golden Delicious, Granny Smith, and Starking) grown in the same location and to reveal the differences between peel and pulp in the fresh, oven-dried, and sun-dried samples. Sugar and organic acid contents were analyzed through high-performance liquid chromatography. Besides, antioxidant activity, total flavonoid, and total phenolic contents were analyzed with a spectrophotometer. The results of these analyses showed that the compounds varied between peel and pulp and among fresh, oven-dried, and sun-dried samples. The largest level of total flavonoid was found in Amasya oven-dried peel, and the largest total phenol, antioxidant activity, fructose, and total sugar levels were observed in Amasya sun-dried peel. Moreover, the largest L-ascorbic acid was detected in Amasya oven-dried pulp. Granny Smith oven-dried pulp was rich in citric, succinic acids, and glucose. Braeburn sun-dried pulp, Golden Delicious oven-dried pulp, and Granny Smith sun-dried pulp contained the largest sucrose, malic acid, and total soluble solids levels, respectively. For data interpretation, t-test, ANOVA, Duncan’s test, Kruskal–Wallis H test, and principal component analysis were performed. The peel contained significantly higher total flavonoid, total phenol, and antioxidant activity than pulp. Moreover, among fresh, oven-dried, and sun-dried samples, total phenol, succinic acid, malic acid, and L-ascorbic acid significantly differed. As a whole, the pulp was rich in sugars. The results provide valuable information on the food quality parameters of five different apple cultivars. It was demonstrated that consuming apple with its peel is healthier (in terms of total flavonoid, total phenol, antioxidant activity parameters, and L-ascorbic acid), and drying (especially the sun-drying method) is an effective food storage method for apples.

1. Introduction

Fruits are one of the main nutrition sources in the daily human diet. The quality of the fruits has a substantial effect on the decision-making process of the consumers. Biochemical characteristics are ranked among the inner-quality determiners of fruits, as well as increasing outer quality. This study aims to reveal the biochemical characteristics of five different apple cultivars and to make a comparison between fresh, open-air-dried, and oven-dried peel and pulp samples of these varieties. This study focuses on the parameters of total phenols, total flavonoids, sugars, organic acids, and total soluble solids (TSS) among other biochemical characteristics.

Apple (Malus domestica Borkh.) is one of the members of the Rosaceae family and has been consumed by humans since the early ages. It is one of the most frequently consumed fruits [1]. In addition to its low-calorie levels and high levels of water and fiber, containing essential vitamins, minerals, and trace elements makes the apple a healthy fast food. Phenols, pectin, sugar, acids, and antioxidants containing in apples generally have positive effects on human health [2–4]. Some diseases such as cancer, cardiovascular disease, asthma, and diabetes can be prevented through some biochemical compounds of apple [5–8]. The absorption of gastric secretions, elimination of toxins, and having a diuretic effect are some of the benefits of apple to human health [9]. Besides the health benefits, the apple is an important economic value. According to the FAO data [10], the five largest apple producers can be listed as China, the USA, Turkey, Poland, and India. These countries produced nearly 65% of apples in the world in 2019. In terms of exporting, China, the USA, Italy, Chile, and New Zealand were the largest apple sellers in the same year.

Biochemical compounds of apples determine the quality in terms of not only health but also the economy. Biochemical parameters of apples may be influenced by species, varieties, cultivation conditions, and cultivation techniques [11–15]. Most of the antioxidant activities in apples were fulfilled by polyphenolic antioxidants [16]. Apple peel contains significantly high amounts of polyphenolic compounds in comparison with apple pulp or cores [8, 13, 17, 18]. Total antioxidant activity and phenolic content [16, 19–22] and L-ascorbic acid content [20, 23, 24] may depend on the apple cultivars. Similarly, sugar and acid contents [9], total phenolic levels [25, 26], and TSS [1, 25] also vary between cultivars of apples. Moreover, drying methods may affect some biochemical contents [18]. It is important to determine these parameters due to their impact on apple quality.

The novelty of this research comes especially from the investigation of some biochemical compounds through the implementation of different drying methods for the pulp and peel of five different apple varieties. To the best of the author’s knowledge, this is the first study focusing on differences of the total phenol, total flavonoid, sugars, organic acids, TSS, and antioxidants levels of fresh, traditionally open-air-sun-dried, and oven-dried pulp and peel. This is important since apples are consumed peeled and unpeeled. The cultivars used in this study are Amasya, Granny Smith, Braeburn, Starking, and Golden Delicious grown in Isparta, Turkey.

The remainder of this paper is organized as follows: in the second section, the material and methodology are introduced. In the third section, the results are presented and discussed in light of the previous studies. Finally, in the fourth section, the findings are summarized.

2. Materials and Methods

In this research, five apple cultivars (Amasya, Braeburn, Golden Delicious, Granny Smith, and Starking) were used grown in Isparta, Turkey. The fruits were commercially harvested in the 2020 season. The analytical parameters were measured by using mixes of 10 fruits for the cultivars. Fresh, oven-dried, and sun-dried peels and pulps of the cultivars were used for the biochemical analysis. Oven-dried samples were prepared by drying in an oven at 70°C for 10 h. And, sun-dried samples were prepared by drying in open-air conditions in August for 3 days in Adana, Turkey.

2.1. Organic Acid and Vitamin C Content

Organic acid analyses were done using high-performance liquid chromatography equipped with a UV detector (Shimadzu) according to the method of Bozan et al. [27]. Organic acids were extracted by 3% metaphosphoric acid. A Transgenomic ICSep ICE COREGEL-87H3 (250 mm × 4.6 mm, 5 µm) Organic Acid column was used (2032 San Jose, CA 95131, USA). The column temperature was adjusted to 30°C. The mobile phase was 0.05% sulphuric acid with a flow rate of 0.6 mL/min. The organic acid content of samples was determined qualitatively and quantitatively at a wavelength of 210 nm, while C vitamin content was determined at a wavelength of 242 nm by comparison of the external standard calibration curve and the retention time of the standard.

2.2. Individual Sugar Content

The total soluble solid (TSS) content of samples was determined using the digital hand refractometer. Individual sugar contents (glucose, fructose, and sucrose) in one gram of fresh and dried samples were detected using high-performance liquid chromatography (HPLC) (Shimadzu) equipped with a refractive index detector (RID) and Transgenomic COREGEL-87C (250 mm × 4.6 mm, 5 µm). The column temperature was adjusted to 70°C. The mobile phase was ultra-deionized water with a flow rate of 0.6 mL/min. The sugar contents of the samples were determined qualitatively and quantitatively using the refractive index curves according to the external standard and the retention time of the standard using the refractive index detector.

2.3. Determination of Total Phenolic Compounds

The amount of total phenolics of samples was identified by the Folin–Ciocalteu reagent using the modified method of Spanos and Wrolstad [28]. µL of Folin–Ciocalteu’s reagent was mixed with 50 µL of methanolic extracts followed by adding 750 µL of sodium carbonate (20% ). The solution was incubated at room temperature in dark for 2 hours. The absorbance of all samples was measured at 760 nm using Multiskan™ GO Microplate Spectrophotometer. Results were expressed as milligrams of gallic acid equivalent per gram of extract (mg/GAE100 g) [29].

2.4. Determination of Total Flavonoid Compounds

Determination of the total flavonoid content of fresh and dried samples was performed by the method of aluminum chloride colorimetric [30]. Rutin was used as the standard for the calibration curve. 200 µL methanolic extract was diluted with 4600 µL of ethanol and reacted with 2% AlCl₃. The solution was incubated for 45 min at room temperature. The absorbance of the reaction mixtures was read against a blank at 415 nm using Multiskan™ GO Microplate Spectrophotometer.

2.5. Determination of DPPH Radical Scavenging Activity

The ability of hydrogen donating or radical scavenging of samples was measured using DPPH (1, 1-diphenyl 2-picrylhydrazyl) method according to the method of Brand-Williams et al. [31]. About 1950 µL of the 1 mM solution of DPPH was mixed with 50 µL of methanolic extract of samples. The absorbance was measured at 517 nm against the corresponding blank solution as 80% methanol and control. Percentage inhibition of radical DPPH% inhibition was calculated based on control reading by the following equation:

where Ac is the absorbance of the control (standard), As is the absorbance of the sample, and Ab is the absorbance of the blank.

2.6. Statistical Analysis

The statistics of biochemical analysis were done using independent samples t-test, ANOVA, Duncan’s test, Kruskal–Wallis H test, and principal component analysis (PCA) methods. By employing independent samples t-test, ANOVA, Duncan’s test, and Kruskal–Wallis H test, the differences between (1) peel and pulp samples, (2) fresh, oven-dried, and sun-dried samples, and (3) five apple cultivars were investigated. The methods were chosen due to the normal and nonnormal distribution characteristics of the data. As an output of PCA, the biplot graph is a useful tool to visualize the interrelationships between variables and observations through the generated first and second principal components (PC1 and PC2). In a biplot, which is an output of PCA, the length of the lines gives the variances of the variables. Longer lines mean higher variance. The cosine of the angle and the lines show the correlation between the variables. A closer angle to 90, or 270 degrees, means a lower correlation. A correlation of 1 or −1 is approximated by an angle of 0 or 180 degrees, respectively [32]. The PCA was performed for sugar contents data and organic acids data. For the group comparison tests and PCA, SPSS and XLSTAT software programs were employed, respectively.

3. Results and Discussion

3.1. Organic Acid and Vitamin C Results

Amasya oven-dried pulp, Braeburn sun-dried peel, and Granny Smith sun-dried peel were the top three and found to be rich in L-ascorbic acid contents. Unlikely, Starking fresh pulp, Starking oven-dried peel, and Starking fresh peel were the last three in terms of L-ascorbic acid. While the first three citric acid-containing samples can be listed as Granny Smith oven-dried pulp, Braeburn oven-dried pulp, and Golden Delicious oven-dried pulp; Amasya fresh peel, Braeburn fresh pulp, and Golden Delicious fresh pulp were on the end of the list. Granny Smith oven-dried pulp, Braeburn sun-dried peel, and Granny Smith sun-dried peel samples were found to be the largest succinic acid-containing sample; however, Granny Smith fresh pulp, Golden Delicious fresh pulp, and Starking fresh peel samples were the minimum citric acid-containing ones. In the malic acid content list, Granny Smith sun-dried pulp, Braeburn sun-dried pulp, and Starking sun-dried pulp were the first three, and the Golden Delicious fresh peel, Granny Smith fresh peel, and Braeburn fresh peel were the last three ones (see Table 1). The previous studies argue that the dominant organic acid is malic acid. The results of this study are nearly uniform for Granny Smith [33]. The organic acid content results differed from the former studies for Amasya [33], Granny Smith [33], Braeburn [34], Starking [35], and Golden Delicious [36, 37]. The main result of this difference might be genotypic, weather, and soil differences [2, 38]. In total, by ignoring the cultivar differences, the smallest amounts of L-ascorbic acid, citric acid, and malic acid were observed in the fresh pulp samples, and the smallest amount of succinic acid was observed in the fresh peel samples. This may show that fresh apples contain lower acid contents. And, the largest L-ascorbic acid and citric acid were found in the sun-dried peel, and succinic acid and malic acid were found in sun-dried pulp and oven-dried peel, respectively. The results, out of malic acid, may indicate that sun drying is more effective than the other methods, in terms of having higher levels of acids.

Table 1: Biochemical analysis of five apple cultivars (peel and pulp of fresh, oven-dried, and sun-dried samples).

Total flavonoid content (mg Ru/100 g)
Total phenol content (mg/GAE100 g)
DPPH inhibition (%)
DPPH radical scaving (%)
L-ascorbic acid (g/100 g)
Citric acid (g/100 g)
Succinic acid (g/100 g)
Malic acid (g/100 g)
Fructose (g/100 g)
Glucose (g/100 g)
Sucrose (g/100 g)
Total sugar (g/100 g)
TSS (g/100 g)

Amasya fresh peel
52.30 ± 0.965
88.76 ± 2.475
70.56 ± 3.015
59.05 ± 2.020
0.0029 ± 0.000
0.0025 ± 0.001
0.1220 ± 0.013
0.1575 ± 0.002
7.56 ± 0.030
1.87 ± 0.035
3.80 ± 0.085
13.22 ± 0.080
17.75 ± 0.250

Amasya fresh pulp
33.94 ± 0.610
8.07 ± 0.225
32.87 ± 1.335
21.36 ± 1.335
0.0109 ± 0.000
0.0615 ± 0.001
0.3400 ± 0.005
0.3730 ± 0.001
19.57 ± 0.610
12.55 ± 0.475
16.66 ± 1.075
48.77 ± 2.160
54.50 ± 0.000

Amasya oven-dried peel
207.44 ± 4.13
467.11 ± 4.990
88.24 ± 0.200
76.74 ± 0.205
0.0088 ± 0.002
0.1370 ± 0.015
0.5195 ± 0.038
0.5830 ± 0.011
39.71 ± 0.235
17.26 ± 0.090
19.02 ± 0.175
75.98 ± 0.505
45.25 ± 0.250

Amasya oven-dried pulp
40.35 ± 0.900
238.76 ± 4.465
82.63 ± 3.270
71.13 ± 3.270
0.0548 ± 0.000
0.1520 ± 0.006
0.4850 ± 0.014
0.3510 ± 0.019
25.40 ± 0.315
18.43 ± 0.400
24.55 ± 0.040
68.38 ± 0.680
65.75 ± 0.250

Amasya sun-dried peel
157.8 ± 3.295
477.09 ± 5.195
89.79 ± 0.360
78.28 ± 0.360
0.0141 ± 0.000
0.0075 ± 0.003
0.2230 ± 0.036
0.6455 ± 0.001
42.19 ± 0.900
23.26 ± 1.645
24.93 ± 0.505
90.38 ± 0.235
40.00 ± 0.000

Amasya sun-dried pulp
33.29 ± 0.225
243.73 ± 1.160
89.34 ± 0.385
77.83 ± 0.390
0.0050 ± 0.000
0.0160 ± 0.001
0.1195 ± 0.002
0.2710 ± 0.008
5.80 ± 0.005
3.59 ± 0.030
5.68 ± 0.015
15.06 ± 0.005
19.75 ± 0.250

Braeburn fresh peel
36.76 ± 3.465
213.69 ± 4.700
36.86 ± 2.565
25.35 ± 2.565
0.0015 ± 0.000
0.0430 ± 0.011
0.1765 ± 0.006
0.1015 ± 0.006
3.48 ± 0.040
1.13 ± 0.140
4.75 ± 0.080
9.36 ± 0.255
14.50 ± 0.500

Braeburn fresh pulp
32.01 ± 0.025
8.79 ± 0.375
22.72 ± 3.515
11.22 ± 2.515
0.0015 ± 0.000
0.0025 ± 0.001
0.0800 ± 0.001
0.1895 ± 0.008
5.00 ± 0.620
1.95 ± 0.200
4.20 ± 0.500
11.15 ± 1.325
17.00 ± 0.500

Braeburn oven-dried peel
59.74 ± 0.530
292.41 ± 4.070
53.67 ± 2.350
42.17 ± 3.355
0.0029 ± 0.000
0.0425 ± 0.004
0.5110 ± 0.014
0.2675 ± 0.011
14.72 ± 0.710
10.33 ± 0.995
8.51 ± 0.285
33.55 ± 1.985
48.75 ± 0.250

Braeburn oven-dried pulp
32.91 ± 1.415
191.90 ± 4.105
33.74 ± 1.560
22.24 ± 2.560
0.0037 ± 0.001
0.4330 ± 0.003
0.3505 ± 0.018
0.7805 ± 0.023
28.76 ± 1.315
19.82 ± 0.320
8.94 ± 0.545
57.52 ± 1.095
48.50 ± 0.000

Braeburn sun-dried peel
74.10 ± 3.670
220.65 ± 1.785
74.96 ± 0.590
63.46 ± 0.585
0.0458 ± 0.001
0.1160 ± 0.015
0.7975 ± 0.007
0.3175 ± 0.006
15.49 ± 1.310
3.51 ± 0.645
20.93 ± 1.085
39.92 ± 3.040
38.75 ± 0.250

Braeburn sun-dried pulp
30.52 ± 0.830
113.10 ± 1.250
31.22 ± 2.260
19.72 ± 2.255
0.0304 ± 0.000
0.0230 ± 0.005
0.2450 ± 0.020
0.9175 ± 0.110
29.91 ± 1.940
7.26 ± 1.905
28.57 ± 1.125
65.73 ± 2.970
53.75 ± 0.250

Golden Delicious fresh peel
50.50 ± 2.045
157.12 ± 1.560
39.58 ± 3.965
28.07 ± 3.970
0.0007 ± 0.000
0.0180 ± 0.016
0.1240 ± 0.004
0.1360 ± 0.004
4.99 ± 0.050
2.56 ± 0.000
5.68 ± 0.005
13.23 ± 0.055
7.75 ± 0.250

Golden Delicious fresh pulp
36.42 ± 2.665
40.74 ± 3.305
27.18 ± 3.375
24.38 ± 2.325
0.0009 ± 0.000
0.0020 ± 0.000
0.0745 ± 0.005
0.1530 ± 0.008
6.69 ± 0.545
1.81 ± 0.205
4.36 ± 0.400
12.85 ± 1.150
10.25 ± 0.250

Golden Delicious oven-dried peel
126.47 ± 2.075
211.89 ± 3.765
85.65 ± 1.400
74.07 ± 1.485
0.0016 ± 0.000
0.0720 ± 0.018
0.4165 ± 0.009
0.4625 ± 0.006
17.08 ± 0.765
14.96 ± 0.440
12.40 ± 0.390
44.43 ± 1.600
22.75 ± 0.250

Golden Delicious oven-dried pulp
40.63 ± 0.765
78.78 ± 0.300
55.94 ± 1.160
44.44 ± 1.165
0.0025 ± 0.001
0.2545 ± 0.047
0.3370 ± 0.045
0.6520 ± 0.035
32.55 ± 1.335
18.60 ± 1.090
11.63 ± 0.615
62.77 ± 3.040
80.25 ± 0.250

Golden Delicious sun-dried peel
91.80 ± 4.075
195.74 ± 2.025
84.85 ± 2.200
73.35 ± 2.205
0.0139 ± 0.000
0.0660 ± 0.025
0.4420 ± 0.011
0.2890 ± 0.028
21.21 ± 1.580
9.04 ± 1.295
21.99 ± 1.235
52.22 ± 4.110
55.75 ± 0.250

Golden Delicious sun-dried pulp
32.66 ± 0.225
70.24 ± 2.105
48.32 ± 1.870
36.81 ± 1.870
0.0083 ± 0.000
0.0130 ± 0.003
0.4555 ± 0.207
0.5200 ± 0.254
39.05 ± 2.910
11.87 ± 4.640
27.96 ± 1.700
78.88 ± 5.855
56.75 ± 0.250

Granny Smith fresh peel
59.26 ± 0.805
45.86 ± 0.090
52.08 ± 8.380
40.58 ± 8.385
0.0024 ± 0.000
0.0070 ± 0.001
0.2330 ± 0.018
0.1215 ± 0.004
3.69 ± 0.185
1.66 ± 0.010
4.72 ± 0.265
10.07 ± 0.445
16.75 ± 0.250

Granny Smith fresh pulp
30.81 ± 0.180
26.97 ± 1.640
41.15 ± 2.230
29.77 ± 2.105
0.0022 ± 0.000
0.0055 ± 0.001
0.0765 ± 0.009
0.2380 ± 0.028
5.34 ± 0.015
2.10 ± 0.065
3.82 ± 0.020
11.26 ± 0.065
15.50 ± 0.000

Granny Smith oven-dried peel
139.12 ± 8.66
283.71 ± 7.060
76.15 ± 1.775
64.65 ± 1.775
0.0043 ± 0.000
0.0580 ± 0.000
0.6100 ± 0.066
0.3155 ± 0.018
13.73 ± 0.155
12.49 ± 0.190
7.67 ± 0.860
33.88 ± 1.200
48.25 ± 0.250

Granny Smith oven-dried pulp
43.86 ± 1.350
116.43 ± 5.895
53.10 ± 0.080
41.59 ± 0.080
0.0080 ± 0.001
0.4370 ± 0.053
1.1010 ± 0.008
0.2485 ± 0.001
37.60 ± 0.750
24.02 ± 0.180
4.76 ± 0.130
66.38 ± 1.050
60.25 ± 0.250

Granny Smith sun-dried peel
127.89 ± 2.69
474.50 ± 5.845
75.70 ± 5.545
64.19 ± 4.545
0.0375 ± 0.000
0.0885 ± 0.004
0.6675 ± 0.008
0.1915 ± 0.001
13.73 ± 0.875
8.60 ± 0.300
19.58 ± 0.780
41.90 ± 1.950
51.75 ± 0.250

Granny Smith sun-dried pulp
35.25 ± 0.605
110.30 ± 5.125
56.44 ± 3.445
44.93 ± 3.445
0.0209 ± 0.000
0.0465 ± 0.001
0.2095 ± 0.020
1.5655 ± 0.049
29.65 ± 2.100
17.05 ± 2.915
23.37 ± 1.885
70.06 ± 6.905
33.75 ± 0.250

Starking fresh peel
77.96 ± 1.440
169.16 ± 4.170
82.89 ± 0.135
71.39 ± 0.135
0.0003 ± 0.000
0.0960 ± 0.014
0.0700 ± 0.003
0.1960 ± 0.001
4.69 ± 0.095
2.64 ± 0.020
2.83 ± 0.155
10.15 ± 0.035
16.50 ± 0.000

Starking fresh pulp
32.70 ± 0.180
2.33 ± 0.125
31.81 ± 3.325
20.31 ± 3.325
0.0005 ± 0.000
0.0040 ± 0.000
0.1175 ± 0.000
0.1690 ± 0.001
5.10 ± 0.175
2.65 ± 0.125
2.55 ± 0.020
10.30 ± 0.325
15.00 ± 0.000

Starking oven-dried peel
157.75 ± 7.665
365.83 ± 9.565
88.68 ± 0.480
77.18 ± 0.480
0.0005 ± 0.000
0.0385 ± 0.002
0.2095 ± 0.006
0.7480 ± 0.021
16.75 ± 0.525
11.10 ± 1.255
9.36 ± 0.005
37.20 ± 1.770
18.75 ± 0.250

Starking oven-dried pulp
48.79 ± 3.175
237.66 ± 1.105
87.18 ± 0.305
75.67 ± 0.305
0.0006 ± 0.000
0.1705 ± 0.134
0.6520 ± 0.239
0.6270 ± 0.093
34.84 ± 0.805
23.82 ± 0.905
10.17 ± 0.405
68.83 ± 0.305
29.25 ± 0.250

Starking sun-dried peel
77.60 ± 3.210
217.78 ± 1.815
89.60 ± 0.040
78.10 ± 0.040
0.0038 ± 0.000
0.0480 ± 0.002
0.2415 ± 0.002
0.5135 ± 0.047
20.32 ± 2.030
10.49 ± 1.325
12.72 ± 1.190
43.53 ± 4.545
29.00 ± 0.000

Starking sun-dried pulp
31.60 ± 0.250
169.72 ± 1.710
51.74 ± 0.565
40.23 ± 0.560
0.0050 ± 0.000
0.0275 ± 0.001
0.6225 ± 0.011
0.8450 ± 0.039
37.03 ± 3.430
21.78 ± 2.995
17.47 ± 0.575
76.27 ± 5.850
75.75 ± 0.250

Results as mean ± standard deviations of triplicate measurements.

3.2. Individual Sugar Content Results

In the fresh pulp group, the largest fructose, glucose, sucrose, and total sugar levels were observed in the Amasya cultivar. And the minimum levels were observed in Braeburn, Golden Delicious, Starking, and Golden Delicious, respectively. According to the fresh peel sample, the largest and minimum fructose was found in Amasya and Braeburn, the largest and minimum glucose was found in Starking and Braeburn, the largest and minimum sucrose was found in Golden Delicious and Starking, and the largest and minimum total sugar was found in Amasya and Golden Delicious, respectively. In the oven-dried pulp group, the largest fructose, glucose, sucrose, and total sugar levels were observed in Starking, Granny Smith, Amasya, and Starking, respectively. The minimum levels were in Amasya (for fructose and glucose), Granny Smith, Braeburn, and Starking, respectively. Besides, for the oven-dried peel group, the largest fructose, glucose, sucrose, and total sugar levels were observed in the Amasya apple cultivar. And the minimum levels were observed in Granny Smith, Braeburn, Granny Smith, Braeburn, and Golden Delicious, respectively. Among the sun-dried pulp samples, the largest levels of fructose, glucose, sucrose, and total sugar were found in Golden Delicious, Starking, Braeburn, and Golden Delicious samples, respectively. On the other hand, the minimum levels were observed in the Amasya sample. For the sun-dried peel group, the largest observations were found in Amasya, in terms of fructose, glucose, sucrose, and total sugar levels. The minimum fructose and glucose levels were detected in Granny Smith, sucrose level in Starking, and total sugar in Braeburn samples. Golden Delicious oven-dried pulp, Starking sun-dried pulp, and Amasya oven-dried pulp are the top three samples; Braeburn fresh peel, Golden Delicious fresh pulp, and Golden Delicious fresh peel are the last three samples, in terms of TSS (see Table 1). Most of the former studies are on fresh pulp or whole (peel + pulp) fresh fruit. The result of this study is parallel with those for Amasya [39], Granny Smith [40], Braeburn [34, 38, 41, 42], Starking [39], and Golden Delicious [36, 37, 39, 40]. The TSS results of this study are similar for Granny Smith and Golden Delicious [39]. The reason for the differences might be differences in cultivars, position, and exposition of the fruits in the crown, culture technology, and weather conditions [9]. In total, by ignoring the cultivar differences, while the lowest sugar content (fructose, glucose, sucrose, and total sugar) and TSS level were found in fresh peel samples, the highest levels were observed in the sun-dried pulp.

3.3. Total Phenolic Results

Amasya sun-dried peel, Granny Smith sun-dried peel, and Amasya oven-dried peel samples were found to be the first three with their phenolic contents. But, Braeburn fresh pulp, Amasya fresh pulp, and Starking fresh pulp had the lowest level of total phenolic (see Table 1). The results were parallel with previous studies, with small differences for Amasya [33], Granny Smith [20, 33, 43, 44], Braeburn [34, 41], Starking [35, 44], and Golden Delicious [8, 20, 36, 45, 46]. If the differences caused by the apple cultivars were neglected, the sun-dried and oven-dried peels contained the largest total phenols. But, the pulp of fresh apples contained the lowest amount.

3.4. Total Flavonoid Results

The largest levels of total flavonoid levels were observed in the samples of Amasya oven-dried peel, Amasya sun-dried peel, and Starking oven-dried peel samples. On the other hand, the lowest levels were observed in the samples of Starking sun-dried pulp, Granny Smith fresh pulp, and Braeburn sun-dried pulp (see Table 1). In general, the results are similar to the literature on flavonoids in Amasya [33] and Granny Smith [33]. As the same with the literature [8, 20, 36, 41], in this study, the peels contained more total flavonoids than pulp. When the cultivar discrepancies were ignored, the flavonoid performance of oven-dried peel was the best, while sun-dried pulp was the worst.

3.5. Antioxidant Capacity Results

According to DPPH free radical scavenging capacity, the highest antioxidant activities were measured for Amasya sun-dried peel, Starking sun-dried peel, and Amasya sun-dried pulp samples, and the lowest levels were observed in Braeburn sun-dried pulp, Golden Delicious fresh pulp, and Braeburn fresh pulp samples. Furthermore, according to the DPPH inhibition method, the highest antioxidant activities were measured for Amasya sun-dried peel, Starking sun-dried peel, and Amasya sun-dried pulp samples. Starking fresh pulp, Braeburn sun-dried pulp, and Braeburn fresh pulp samples had the lowest level of antioxidant capacity (see Table 1). 2–5 times higher total antioxidant capacity levels were observed in the literature, in peels than pulps [11]. Although some differences with the previous studies were detected, with a general evaluation, the result of this study is similar [8, 33, 41, 45, 46]. By ignoring the cultivar variations, the sun-dried peel samples displayed the best performance and the lowest antioxidant capacity was observed in fresh pulps in terms of antioxidant capacity.

3.6. Statistical Results

The results of the investigation of the differentiations between pulps and peels can be seen in Table 2. According to the results, total flavonoid, total phenol, and antioxidant activity parameters differ. These are significantly high in the peel. But the rest of the parameters do not significantly differ. Additionally, some descriptive statistics, such as mean and standard deviation, were also shown in the table.

Table 2: t-test results.

Compounds
Sample
Mean
Std. deviation
t
Prob.

Total flavonoid
Peel
103.15
50.03
5.195
0.000

Pulp
35.71
5.35

Total phenol
Peel
270.89
131.21
3.896
0.001

Pulp
110.50
87.65

% DPPH inhibition
Peel
72.76
18.99
3.027
0.005

Pulp
49.69
21.83

% DPPH radical scavenging
Peel
61.25
18.99
2.992
0.006

Pulp
38.77
21.29

Citric acid
Peel
0.01
0.01
−0.086
0.932

Pulp
0.01
0.01

Succinic acid
Peel
0.06
0.04
−1.197
0.242

Pulp
0.11
0.15

Malic acid
Peel
0.37
0.22
0.245
0.808

Pulp
0.35
0.28

L-ascorbic acid
Peel
0.35
0.21
−1.526
0.139

Pulp
0.53
0.39

Fructose
Peel
16.55
11.96
−1.317
0.199

Pulp
22.82
13.52

Glucose
Peel
9.21
6.53
−1.148
0.261

Pulp
12.48
8.58

Sucrose
Peel
12.50
7.43
−0.150
0.882

Pulp
12.98
9.39

Total sugar
Peel
38.27
24.03
−1.043
0.306

Pulp
48.28
27.39

TSS
Peel
32.46
16.22
−1.309
0.201

Pulp
42.40
23.67

The results of the investigation of the difference among fresh, oven-dried, and sun-dried samples are shown in Tables 2 and 3. ANOVA and Kruskal–Wallis H test were performed according to the different characteristics of data.

Table 3: ANOVA results.

df
Mean square
F
Sig.

Total flavonoid
Between groups
2
5091.682
2.377
0.113

Within groups
26
2142.214

Total
28

Total phenol
Between groups
2
84508.292
6.302
0.006

Within groups
26
13408.715

Total
28

Succinic acid
Between groups
2
0.068
7.876
0.002

Within groups
26
0.009

Total
28

Malic acid
Between groups
2
0.346
8.336
0.002

Within groups
26
0.042

Total
28

L-ascorbic acid
Between groups
2
0.45
5.927
0.008

Within groups
26
0.076

Total
28

Due to the normal distribution of the total flavonoid, total phenol, succinic acid, malic acid, and L-ascorbic acid, ANOVA was performed. According to results, total phenol, succinic acid, malic acid, and L-ascorbic acid differed among fresh, oven-dried, and sun-dried samples. After the results of ANOVA, to clearly see which groups significantly differ, Duncan’s test was performed. The results are shown in Table 4.

Table 4: Duncan’s test results.

Groups
Total phenol
Succinic acid
Malic acid
L-ascorbic acid

1
2
1
2
1
2
1
2

Fresh
74.7456

0.0266

0.1436

0.1864

Sun-dried

229.2815
0.0452

0.4024

0.6076

Oven-dried

248.4450

0.1795

0.5192

0.5036

Sig.
1.000
0.719
0.664
1.000
1.000
0.219
1.000
0.415

Means for groups in homogeneous subsets are displayed. a: uses harmonic mean sample size = 9,643. b: the group sizes are unequal. The harmonic mean of the group sizes is used. The subset for alpha = 0.05.

Because the data of antioxidant activity parameters, citric acid, fructose, glucose, sucrose, total sugar, and TSS was not normally distributed, the Kruskal–Wallis H test was used to investigate the difference between being fresh, oven-dried, and sun-dried. The results are shown in Table 5. Kruskal–Wallis H test showed that the parameters given in Table 5 significantly differ in fresh, oven-dried, and sun-dried samples.

Table 5: Kruskal–Wallis H test results.

% DPPH inhibition
% DPPH radical scavenging
Citric acid
Fructose
Glucose
Sucrose
Total sugar
TSS

Kruskal–Wallis H
9.691
9.113
13.408
14.947
16.542
17.323
15.119
12.838

df
2
2
2
2
2
2
2
2

Asymp. sig.
0.008
0.010
0.001
0.001
0.000
0.000
0.001
0.002

In addition to the group comparison tests of biochemical compounds, to visualize the interrelations between variables and observations, PCA was performed with organic acids (see Figure 1) and sugar data (see Figure 2). According to the PCA results of organic acids, PC1 and PC2 can explain 44.32% and 24.71% of the total variance, respectively. 69.03% of the total variance was able to be explained by the first two principal components. The correlations between variables were positive but not strong. Some certain acid levels were maximum when the samples dried in the oven or sun.

Figure 1: Biplot graph of organic acids (scores and loading plots) obtained from principal component analysis (abbreviations; A-Fpe: Amasya fresh peel; A-Fpu: Amasya fresh pulp; A-Ope: Amasya oven-dried peel; A-Opu: Amasya oven-dried pulp; A-Spe: Amasya sun-dried peel; A-Spu: Amasya sun-dried pulp; B: Braeburn; Go: Golden Delicious; Gr: Granny Smith; S: Starking).

Figure 2: Biplot graph of sugars (scores and loading plots) obtained from principal component analysis (abbreviations; A-Fpe: Amasya fresh peel; A-Fpu: Amasya fresh pulp; A-Ope: Amasya oven-dried peel; A-Opu: Amasya oven-dried pulp; A-Spe: Amasya sun-dried peel; A-Spu: Amasya sun-dried pulp; B: Braeburn; Go: Golden Delicious; Gr: Granny Smith; S: Starking).

According to the PCA results of sugars, PC1 explained 83.56% of the variance, while PC2 explained 14.72%. Thus, 98.28% of the total variance was able to be explained by the first two principal components. The correlations between variables were positive. Higher levels of certain sugars were observed especially in pulps. Peels had the lowest sugar, as expected. The pulp observations dispersed near to the sugars; however, the peel observations dispersed apart from these sugars.

4. Conclusions

Biochemical characteristics are among the inner-quality determiners of fruits, as well as increasing outer quality. Due to their vitality to determine the fruit quality, investigating the biochemical compounds has substantial health and economic importance. In line with that, biochemical compounds of five different apple cultivars (Amasya, Braeburn, Golden Delicious, Granny Smith, and Starking) grown in the same plantation in Isparta were investigated. The novelty of this research comes especially from the investigation of some biochemical compounds through the implementation of different drying methods for the pulp and peel of five different apple varieties. In detail, the total phenol, total flavonoid, sugars, organic acids, TSS, and antioxidants levels were identified and quantified. Amasya oven-dried peel contained the largest level of total flavonoid. The largest total phenol, antioxidant activity, fructose, and total sugar levels were observed in Amasya sun-dried peel. The highest L-ascorbic acid was detected in Amasya oven-dried pulp. Granny Smith oven-dried pulp was rich in citric, succinic acids, and glucose. Braeburn sun-dried pulp, Golden Delicious oven-dried pulp, and Granny Smith sun-dried pulp contained the largest sucrose, malic acid, and total soluble solids levels, respectively. The results provide valuable information on the food quality parameters of five different apple cultivars. Among the organic acids, malic acid was dominant, and among the sugars, the concentration of fructose is much higher than other sugars. Total flavonoid, total phenol, and antioxidant activity parameters are significantly high in the peel. Total phenol, succinic acid, malic acid, and L-ascorbic acid significantly differed among fresh, oven-dried, and sun-dried samples. As is expected, higher sugar levels were observed in the pulp. Moreover, if the cultivar and peel-pulp differences were ignored and the apples were evaluated as a whole, the oven-drying method performed better for total flavonoids, total phenols, and malic acid contents, while the sun-drying method outperformed in terms of antioxidant capacity, fructose, glucose, sucrose, total sugars, TSS, L-ascorbic acid, citric acid, and succinic acid. In light of the results of the research, consuming apple with its peel is healthier. Besides, the effectiveness of fruit drying among the food storage methods was also shown.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The author declares that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

The author thanks Professor Dr. N. Ebru Kafkas for her invaluable support and guidance for this research.

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