Pol. J. Food Nutr. Sci., 2019, Vol. 69, No. 4, pp.

359–366
DOI: 10.31883/pjfns/112654
http://journal.pan.olsztyn.pl
Original article
Section: Food Chemistry

Antioxidant Activity of Extracts of Soursop (Annona muricata L.) Leaves,

Fruit Pulps, Peels, and Seeds

Hakime Hülya Orak1*, Ilayda Sevik Bahrisefit2, Temine Sabudak2

1
Department of Food Technology, Vocational School of Technical Sciences, Namik Kemal University, 59030 Tekirdağ, Turkey
2
Department of Chemistry, Faculty of Science and Arts, Namik Kemal University, 59030 Tekirdağ, Turkey

Key words: by-products, flavonoids, phenolics, GC-MS analysis, antiradical activity, reducing power

The total phenolic content (TPC), total flavonoid content (TFC), and the antioxidant activity of soursop (Annona muricata L.) leaf, fruit pulp, seed,
and peel extracts obtained using successive extraction with hexane (Hxn), dichloromethane (DCM), ethyl acetate (EtOAc), and methanol (MeOH)
were determined. The Hxn soursop seed extract was analysed by GC-MS. The highest TPC was determined in MeOH extracts. MeOH and EtOAc
extracts were rich sources of flavonoids. Generally, soursop leaf and fruit pulp extracts had the highest and the lowest both TPC and TFC, respectively.
Fatty acids were dominant in the Hxn seed extract. Among antioxidants, terpenoids ((E)-nerolidolas as dominant) and phytosterols ((3-β)-stigmast-5-
-en-3-ol with high content) were identified. The soursop seed, followed by leaf and peel extracts (MeOH and EtOAc) had the highest DPPH• scaveng-
ing activity, TEAC, FRAP, and CUPRAC. Antioxidant activity of peel extracts (MeOH and EtOAc) was particularly high in β-carotene-linoleic acid
emulsion system. Strong correlations were found between TPC, TFC, TEAC, FRAP, and results of DPPH assay. In conclusion, soursop leaves and fruit
seeds and peels, which are cheap, waste plant material, could be considered as a source of phenolic antioxidants with a high antioxidant activity.

INTRODUCTION Aromatic soursop fruits are readily used culinary. Pulp

is consumed raw and is used to prepare juice, ice-cream or
In recent years, tropical and exotic fruits have been jelly [Benites et al., 2015]. Moreover, different parts of sour-
in the focus of researchers interest. Consumer interest in them sop (leaf, bark, root, fruit, and seed) are used in traditional
increases as well. This is due to the potential health benefits medicine against several ailments including hypertension, in-
of many tropical and exotic plants. One of them is soursop flammation, diabetes, gastrointestinal disorders, respiratory
(Annona muricata L.), commonly called graviola, belonging diseases, and cancers [Coria-Tellez et al., 2018; Chamcheu
to the Annonacea family. Soursop is native to the warmest et al., 2018]. The medicinal activities and the health benefits
areas of South and North America and is now widely dis- of A. muricata L. have been attributed to their phytochemi-
tributed throughout tropical and subtropical regions of Cen- cals including acetogenins, alkaloids, megastigmanes, pheno-
tral and South America, Western Africa, and Southeast Asia lics, cyclopeptides, and essential oils [Moghadamtousi et al.,
[Moghadamtousi et al., 2015; Coria-Tellez et al., 2018]. 2015].
The soursop fruits are quite large (15–20 cm). The pulp The phenolic compounds are the major phytochemicals
contains 55–170 black seeds covered with green peel. Peels responsible for the antioxidant potential of soursop leaves
and seeds are inedible parts of soursop fruit, there is a high and fruits [Coria-Tellez et al., 2018]. Among them, the phe-
amount of by-products from this fruit that have not been nolic acids (mainly hydroxycinnamic acids), flavonoids,
studied as a source of bioactive compounds [Aguilar Hernan- and tannins (including procyanidin dimers) were determined
dez et al., 2019]. However, in recent years, interest in the uti- in A. muricata L. leaves, pulp, and seeds [Marques & Farah,
lization of fruit and vegetable by-products has increased due 2009; Huang et al., 2010; Nawwar et al., 2012; Jiménez et al.,
to the potential high content of nutrients and bioactive com- 2014; Nam et al., 2017]. Solvent extractions are commonly
pounds, such as phenolics, dietary fiber, and vitamins, among used to obtain plant extracts with phenolic compounds.
others [Kosińska et al., 2012; Sagar et al., 2018; Kuchtová These conventional techniques were also applied to soursop
et al., 2018]. The exotic fruit by-products have previously materials [da Silva et al., 2014; Nam et al., 2017]. The polarity
been considered as a source of valuable food additives of nat- of the solvent is one of the important parameters of the ex-
ural origin [Ayala-Zavala et al., 2011]. traction process. There are some reports indicating that
the type of solvent affected the bioactivity of A. muricata ex-
tracts [George et al., 2015; Chamcheu et al., 2018]. Generally,
* Corresponding Author: E-mail: horak@nku.edu.tr (H.H. Orak) hexane and petroleum ether are suitable for the extraction

© Copyright by Institute of Animal Reproduction and Food Research of the Polish Academy of Sciences
© 2019 Author(s). This is an open access article licensed under the Creative Commons Attribution-NonCommercial-NoDerivs License
(http://creativecommons.org/licenses/by-nc-nd/3.0/).
360 Antioxidant Activity of Soursop Extracts

of phenolic terpenes. Ethyl acetate is used for the extraction Antioxidant activities of soursop extracts
of low-molecular-weight phenolics (phenolic acids and flavo-
noid aglycons). Methanol, ethanol and their mixtures with DPPH• scavenging activity
water allowed extracting high-molecular-weight phenolics The 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•)
and flavonoid glycosides [Oreopoulou & Tzia, 2007]. scavenging activity of soursop extracts was determined
The aim of our study was to compare extracts obtained by the method of Brand-Williams et al. [1995]. Firstly,
by solvents with increasing polarity from soursop fruit pulps, the methanol (2 mL) and methanolic solution of 1 mM DPPH
fruit by-products (seeds and peels), and leaves in terms radicals (0.25 mL) were mixed. Then, extracts (0.1 mL) in dif-
of their total phenolic and flavonoid contents and their an- ferent concentrations (0.4–2.0 mg/assay) were added. After
tioxidant activity in polar and lipid emulsion systems. Addi- the reaction in dark (20 min), the absorbance was measured
tionally, the antioxidants of hexane seed extract were looked at 517 nm. The EC50 value (the half-maximal effective con-
for using GC-MS analysis. centration) was determined on the basis of the plot of absor-
bance vs. extract concentrations.
MATERIAL AND METHODS
Trolox equivalent antioxidant capacity (TEAC)
Plant material Re et al. [1999] method was used to determine
Leaves and mature fruits of soursop (A. muricata L.) were TEAC. The portions of 2 mL of [2,2’-azinobis-(3-ethylben-
obtained from the Dominica Island in December 2017. Nine zothiazoline-6-sulfonic acid)] radical cation (ABTS•+) re-
fruits were sampled at about 0.6–1.4 kg weight. The soursop agent and 20 μL of soursop extracts (from 1–2 mg/mL ex-
fruits were harvested from natural grown trees in the Domi- tract concentrations) were mixed and incubated at 30°C for
nica Island and transferred by plane. The fruits were pro- 6 min. The absorbance of samples was determined at 734 nm
cessed for analysis four days after harvest. Peels (Pl), seeds and the results were expressed as mmol Trolox equivalents per
(S), and pulp (P) were manually separated from fruits. All g of extract.
parts of the fruits as well as leaves (L) were frozen at -40°C
and dried using a vacuum freeze dryer (FT 33; Armfield, Ferric-reducing antioxidant power (FRAP)
Ringwood, UK). The FRAP assay was carried out according to Benzie &
Strain [1996] procedure. The reaction was performed by mix-
Extracts preparation ing the extract solution (75 μL), distilled water (225 μL),
Dried plant materials were grounded and subjected to suc- and FRAP solution (2.25 mL). The FRAP solution was
cessive extraction with solvents of increasing polarity. Hex- prepared by mixing 2,4,6-Tri(2-pyridyl)-s-triazine (10 mM
ane (Hxn), dichloromethane (DCM), ethyl acetate (EtOAc), in 40 mM HCl; 6 mL), acetate buffer (300 mM; pH 3.6;
and methanol (MeOH) were used one after the other. Extrac- 60 mL), and ferric chloride (20 mM; 6 mL). The mixture
tion was carried out for twelve hours at room temperature with was incubated at 37°C (for 30 min) and the absorbance
pure solvent by using Soxhlet extraction method. The ratio was measured at 593 nm. Ferrous sulfate was used to pre-
of weights of plant material to the solvent was 1:3. Solvents pare calibration curve and the results were evaluated as μmol
were evaporated under vacuum (R-210 Rotavapor, B-491 heat- Fe2+equivalents per g of extract.
ing bath, V-710 vacuum pump; Büchi Labortechnik, Flawil,
Switzerland). Samples were stored at -22°C until analysed. Cupric ion-reducing antioxidant capacity (CUPRAC)
CUPRAC assay was performed according to Apak et al.
Total phenolics content (TPC) [2004] method. For determination of the antioxidant activ-
The content of total phenolics of soursop extracts was ity of soursop extracts, 0.5 mL of CuCl2 solution (10 mM),
evaluated using Folin-Ciocalteu’s reagent. The absor- 0.5 mL of neocuproine ethanolic solution (7.5 mM), 0.5 mL
bance of reaction mixtures was read at 725 nm (Hitachi of ammonium acetate buffer (1 M; pH7.0), and 0.25 mL
U-2000 spectrophotometer 1210002, Tokyo, Japan) [Amaro- of extract solutions (1–2 mg/mL extract concentrations) were
wicz et al., 2004]. The TPC was expressed as mg (+)-catechin added to the test tubes. The volume of the reaction mixtures
equivalents (CE) per g of extract. was adjusted to 2.05 mL with water. Well-mixed tubes were
closed and incubated (30 min at ambient temperature). Ab-
Total flavonoids content (TFC) sorbance readings were done at 450 nm. The results were
The content of total flavonoids of soursop extracts was calculated based on the calibration curve obtained for Trolox
determined according to the procedure described by Zhishen and expressed as mmol Trolox equivalents per g of extract.
et al. [1999]. The extract (250 μL, concentration of 1–10 mg/mL
depending on solvent used) was mixed with distilled water β-Carotene-linoleic acid bleaching
(1.25 mL) and sodium nitrite solution (5%, 75 μL). After The β-carotene-linoleic acid emulsion oxidation was car-
6 min of incubation, aluminium chloride (10%, 150 μL) was ried out according to Miller [1971] procedure with modifica-
added to the mixture followed by sodium hydroxide (1 M, tions [Orak et al., 2019]. Firstly, the β-carotene (1.0 mg) was
500 μL). Samples were immediately diluted with distilled dissolved in chloroform (5 mL). Then, Tween40 (400 mg)
water (2.5 mL). The absorbance was measured at 510 nm. and linoleic acid (40 μL) were added. The chloroform was
The TFC was expressed as mg (+)-catechin equivalents (CE) evaporated and water (25 mL) was added to the residue
per g of extract. with vigorous stirring. For antioxidant activity measurement,

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H.H. Orak et al. 361

the emulsion (250 μL) was vortexed with extract solution or higher TPC than the extracts obtained using other solvents
standard antioxidant (butylated hydroxyanisole, BHA) solu- (except fruit pulp extracts). In the case of fruit pulp, EtOAc
tion (100 μL collected from 1 mg/mL concentration). The oxi- was a more effective (p<0.05) phenolic compound extract-
dation reaction temperature was 42°C, the absorbance of sam- ant. Hxn extracts had the lowest TPC (p<0.05). Converting
ples was monitored in 30 min intervals throughout 180 min TPC of extracts by extraction yields, it can be noted that peels
at 470 nm. The percentage of non-oxidized β-carotene after and leaves were the richest sources of phenolic compounds,
180 min of emulsion oxidation was calculated. followed by pulp and seeds. Higher TPC of soursop fruit pulp
compared to that of seeds was in line with literature data [da
GC-MS analysis Silva et al., 2014]. Moreover, higher TPC in the peels than
GC-MS analysis was done using the HP 6890 instru- in the pulp of fruits of different Annona species (A. cherimo-
ment (Hewlett-Packard, Palo Alto, CA, USA) combined with la L. and A. squamosal L.) was previously reported [Loizzo
a mass selective detector (GCMS-QP2010 Ultra Shimadzu, et al., 2012; Huang et al., 2010].
Kioto, Japan). The HP-5MS capillary column (5% phenyl The TFC of extracts is shown in Table 1. The highest
methyl siloxane, 30 m × 250 μm, film thickness 0.25 μm, TFC was determined in L-MeOH extract (81.32 mg/CEg)
Agilent, Palo Alto, CA, USA) was used. Helium was used as and the lowest one in S-Hxn extract (1.54 mg CE/g). Gener-
a carrier gas. Its flow rate was 1.0 mL/min. The column ini- ally, TFC of fruit pulp and by-products decreased in the fol-
tial temperature was 180°C (1 min after injection). The tem- lowing order L>SPl>P. When the results were compared
perature increased to 250°C with an 8°C/min heating ramp based on the extraction solvent used, MeOH and EtOAc
in a 1 min holding time, and increased to 300°C with 2°C/min extracts had the highest TFC. On the other hand, as could
heating ramp in 10 min. The injections (5 μL) were done be expected, Hxn was the least effective solvent for flavo-
in the split mode with a split ratio of 10:1. For the analysis, noid extraction. Loizzo et al. [2012] reported that TFC/
the 250°C was interface temperature, the 280°C was injector TPC ratios of Annona fruit peel and pulp ranged from 0.3 to
temperature and running time was 49 min. MS scan range 0.6. In our study, similar values were obtained for MeOH
was m/z 20–440 using electron impact (EI) ionization (70 eV) extracts, but TFC/TPC ratios of EtOAc extracts were sig-
and an ion source temperature of 250°C. Components were nificantly higher, i.e. at about 0.9. This indicates good selec-
identified according to the comparison of their mass spectra tivity of EtOAc for flavonoid extraction from soursop fruits
with those of Wiley 9 and NIST library. The relative percent-
age of separated compounds was determined from Total Ion
Chromatogram by the computerized integrator. TABLE 1. The extract yield, total phenolic content (TPC) and total flavo-
noid content (TFC) of soursop (A. muricata L.) leaves (L), fruit pulp (P),
peels (Pl) and seeds (S) extracts obtained using hexane (Hxn), dichloro-
Statistical analysis methane (DCM), ethyl acetate (EtOAc), and methanol (MeOH).
The MSTAT-C software package was used for statistical
analyses. The results were subjected to ANOVA with a Fisher’s Extract
Extract yield TPC TFC
Least Significant Difference (LSD) post hoc test (p<0.05). (%) (mg CE/g) (mg CE/g)
Moreover, the correlations between variables were determined L-Hxn 3.66 10.92±1.28i 2.62±0.19i
and Pearson correlation coefficients (r) were calculated. L-DCM 1.10 30.60±2.71f 26.46±1.57e

RESULTS AND DISCUSSION L-EtOAc 0.83 73.42±3.48d 65.98±4.79b

L-MeOH 12.07 244.61±7.00a 81.32±3.45a

Extraction yield, total phenolic and total flavonoid
Pl-Hxn 0.59 20.75±0.20h 1.68±0.09j
contents
The yields of hexane (Hxn), dichloromethane (DCM), Pl-DCM 0.23 27.35±0.50g 15.77±0.22f
ethyl acetate (EtOAc), and methanol (MeOH) extracts
Pl-EtOAc 0.25 56.33±4.97e 50.22±2.90c
of leaves (L), fruit pulp (P), seeds (S), and peels (Pl) of sour-
sop were between 0.23% and 64.14% (Table 1). The largest Pl-MeOH 16.50 187.48±6.78c 36.10±1.04ed
yield was obtained for P-MeOH extract (64.14%). MeOH was P-Hxn 0.71 19.84±0.90h 2.16±0.12i
also the most effective solvent for peels (16.50%) and leaves
P-DCM 0.25 26.23±0.96g 13.34±0.28g
(12.07%). Hxn was able to extract the largest amount of mat-
ter from seeds (24.26%). Yields of MeOH extracts determined P-EtOAc 0.26 50.15±4.57e 34.41±2.20d
in our study were in line with those reported for methanol-wa-
P-MeOH 64.14 38.36±2.12f 13.95±0.19g
ter extracts of seeds and pulps of some Annona species fruits;
e.g., A. coriacea L. (14.5% and 20.5%, respectively) and A. syl- S-Hxn 24.26 5.06±1.37j 1.54±0.08i
vatica L. (8.7% and 5.2%, respectively) [Benites et al., 2015] S-DCM 3.01 20.70±5.00h 11.45±0.59h
as well as for methanolic extract of A. muricata L. leaves
(10.30%) [Nam et al., 2017]. S-EtOAc 0.58 53.73±2.81e 48.04±2.11c
The TPC varied in the range of 10.92–244.61 mg CE/g S-MeOH 3.66 202.17±12.99b 56.59±5.29c
in leaf extracts; 20.75–187.48 mg CE/g in peel extracts;
Data are expressed as the mean ± standard deviation for each extract
19.84–50.15 mg CE/g in pulp extracts; and 5.06–202.17 mg (n=3). Values in the same column having different superscript letters dif-
CE/g in seed extracts (Table 1). MeOH extracts had a much fer significantly (p< 0.05). CE: catechin equivalents.

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362 Antioxidant Activity of Soursop Extracts

TABLE 2. Ferric-reducing antioxidant power (FRAP), Trolox equivalent

and leaves. In previous studies, the presence of flavonoids
antioxidant capacity (TEAC) and DPPH• scavenging activity of soursop
belonging to subclasses of flavan-3-ols and flavonols was (A. muricata L.) leaves (L), peels (Pl), fruit pulp (P), and seeds (S) ex-
determined in soursop leaves, fruit pulp, and peels [Huang tracts obtained using hexane (Hxn), dichloromethane (DCM), ethyl ac-
et al., 2010; Nawwar et al., 2012; Jiménez et al., 2014; Nam etate (EtOAc), and methanol (MeOH).
et al., 2017]. Besides flavonoids, hydroxycinnamic acid
TEAC FRAP EC50 DPPH
derivatives were identified in leaves and pulp [Marques & Extract
(mmolTrolox/g) (μmol Fe2+/g) (mg/mL)
Farah, 2009; Jiménez et al., 2014; Nam et al., 2017]. In turn,
L-Hxn 0.222±0.029i 66.5±0.48l 0.312±0.04c
phenolic terpenoids were found in soursop seeds [Huang
et al., 2010]. L-DCM 0.242±0.008hi 104.0±1.7ij 0.143±0.02h

L-EtOAc 0.474±0.009d 339.7±5.9e 0.136±0.06h

Antioxidant activity of soursop leaf and fruit part
extracts L-MeOH 0.848±0.011b 798.9±2.4b 0.063±0.04k
Five assays in which antioxidants act as free radical scav- Pl-Hxn 0.251±0.018h 102.8±4.3j 0.264±0.02e
engers (TEAC and DPPH assay), as reducing agents (FRAP
and CUPRAC) or as inhibitors of the lipid substrate oxida- Pl-DCM 0.253±0.014h 180.7±4.2h 0.286 ±0.02d
tion (β-carotene-linoleic acid bleaching assay) were used to Pl-EtOAc 0.300±0.005f 284.1±6.8f 0.277±0.10de
determine the antioxidant activities of extracts of soursop
Pl-MeOH 0.438±0.005e 465.2±8.0c 0.090±0.05j
leaves and fruit pulps, peels and seeds.
The DPPH• scavenging activity of the soursop extracts was P-Hxn 0.180±0.012j 75.9±3.0k 0.411±0.03a
expressed as EC50 values. The results are presented in Table 2. P-DCM 0.225±0.029i 117.9±1.8i 0.328±0.05b
The highest antiradical activity against DPPH• with the low-
est EC50 value had the S-MeOH extract (0.044 mg/mL). P-EtOAc 0.280±0.002g 210.4±9.6g 0.307±0.02c
The lowest antiradical activity was determined for the P-Hxn P-MeOH 0.104±0.002k 97.0±2.1j 0.281±0.04d
extract (EC50 0.411 mg/mL). In addition to S-MeOH, other
S-Hxn 0.110±0.009kj 33.2±4.2m 0.231±0.02f
methanolic extracts were also characterized by low EC50 val-
ues, especially in the case of leaf (0.063 mg/mL) and peel S-DCM 0.201±0.006ij 77.5±3.2k 0.191±0.04g
(0.090 mg/mL). The extract obtained with use of ethyl ac-
S-EtOAc 0.572±0.025c 447.4±3.7d 0.115±0.02i
etate and dichloromethane had intermediate EC50 values for
each of the plant materials except peels where antiradical S-MeOH 0.905±0.029a 1100.6± 9.3a 0.044±0.02l
activity of Pl-Hxn and Pl-DCM extracts as well as Pl-DCM Data are expressed as the mean ± standard deviation for each extract
and Pl-EtOAc extracts did not differ significantly (p>0.05). (n=3). Values in the same column having different letters differ signifi-
Given the type of extracted material, the DPPH• scaveng- cantly (p < 0.05).
ing activity decreased generally in the following order:
S>L>Pl>P. The TEAC values shown in Table 2 indicate
the ability of soursop extract to inactivate ABTS•+. Compared TEAC. Plant materials could be ordered as follows: S  L
to DPPH• scavenging activity, the highest TEAC was deter- > Pl >P, if decreasing TEAC values of MeOH and EtOAc
mined for S-MeOH (0.905 mmol Trolox/g) and L-MeOH extracts were considered.
(0.848 mmol Trolox/g) extracts. Moreover S-EtOAc The ability of extracts to reduce Fe3+ (FRAP) and Cu2+
(0.572 mmol Trolox/g), L-EtOAc (0.474 mmol Trolox/mg), (CUPRAC) is shown in Table 2 and Figure 1, respectively.
and Pl-MeOH (0.438 mmol Trolox/g) extracts had high The FRAP ranged from 33.2 to 1100.6 mmol Fe2+/g in seed

4.0
a
3.5
b
3.0
CUPRAC (mmol Trolox/g)

bc
bcd
2.5 bcd
de cde
de de
2.0 efg
ef efg fgh 3.7
gh
1.5 2.9 g
2.6
h 2.4 2.4
1.0 2.0 2.0 2.0 2.1
1.8 1.6 1.7
1.4 1.3 1.4
0.5 0.9

FIGURE 1. Cupric ion reducing antioxidant capacity (CUPRAC) of soursop (A. muricata L.) leaves (L), fruit pulp (P), peels (Pl), and seeds (S) ex-
tracts obtained using hexane (Hxn), dichloromethane (DCM), ethyl acetate (EtOAc), and methanol (MeOH). Data are expressed as mean ± standard
deviation (n=3) for each extract. Bars having different letters differ significantly (p<0.05).

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H.H. Orak et al. 363

100
a
90

80

Non-oxidized β-carotene (%)

70

60

50 b
88.4
40
c
d
30 e
g f
45.1
20 j h h
30.8 i
k 26.3
10 21.9 19.3
16.1 l l
6.5 11.6 nd nd 8.9 nd 11.4
0 4.09

FIGURE 2. Inhibition of β-carotene-linoleic acid emulsion oxidation by soursop (A. muricata L.) leaves (L), fruit pulp (P), peels (Pl), and seeds (S) ex-
tracts obtained using hexane (Hxn), dichloromethane (DCM), ethyl acetate (EtOAc), and methanol (MeOH). Data are expressed as mean ± standard
deviation (n = 3) for each extract. Bars having different letters differ significantly (p<0.05); nd – not detected.

extracts, from 75.9 to 210.4 μmol Fe2+/g in pulp extracts, from et al. [2015] report in which ABTS, DPPH and β-carotene-
102.8 to 465.2 mmol Fe2+/g in peel extracts, and from 66.4 to -linoleic acid bleaching assays of soursop seed and pulp
798.9 mmol Fe2+/g in leaf extracts. The differences between methanol-water extracts were carried out. In turn, Loizzo
CUPRAC of soursop extracts were significant (p<0.05). et al. [2012] found that ethanolic extract from A. cheri-
The values ranged from 0.87 mmol Trolox/g to 3.65 mmol mola L. peel had higher FRAP, DPPH• scavenging activity
Trolox/g. In both assays, again, the S-MeOH and L-MeOH and ability to inhibit oxidation of β-carotene-linoleic acid
extracts exhibited the highest activity and hexane was the least emulsion than extract from pulp which is also accordance
effective in the extraction of compounds with the ability to with our finding. However, in mentioned study the signifi-
reduce metal ions. cant difference between ABTS results for peel and pulp ex-
Antioxidant activity of soursop extracts determined tracts was not noted.
in the β-carotene-linoleic acid emulsion system is shown The results of correlation analysis are shown in Table 3.
in Figure 2. The results are slightly different from those TPC of extracts of soursop leaves and fruit parts was sig-
obtained in the previously discussed assays, because after nificantly correlated (p<0.05) with TFC (r =0.761) as well
180 min of oxidation, the most of non-oxidised β-carotene as with results of antioxidant assays, especially with FRAP
(45.1%) remained in Pl-MeOH extract. Among the EtOAc (r=0.899), TEAC (r=0.872), and emulsion oxidation
extracts, the peel extract also had the highest ability to in- (r=0.865). Weaker correlation was noted only between TPC
hibit emulsion oxidation. However, the antioxidant activity of and CUPRAC (r=0.589). The correlations of TFC with
L-MeOH and S-MeOH extracts was also high; the extracts FRAP, TEAC, and results of emulsion oxidation were also
inhibited β-carotene oxidation at 30.8% and 26.3%, respec- significant (p<0.05), and confirmed by high correlation co-
tively. In turn, all Hxn and some of DCM extracts were not efficients – 0.900, 0.887 and 0.713, respectively. In a previ-
able to inhibit the oxidation of the emulsion. All extracts ous study, strong correlations between TPC and antioxidant
showed a lower antioxidant activity than BHA. activities determined by FRAP and DPPH assays were re-
The higher antioxidant activity of soursop seed extracts ported for soursop leaf extracts obtained with using differ-
(MeOH and EtOAc) compared to pulp extracts determined ent solvents [George et al., 2015]. In turn, Nam et al. [2017]
in our studies in all used assays was in line with Benite found that r values of correlations between TPC and antiox-

TABLE 3. Pearson’s correlation coefficients (r) between total phenolic content (TPC), total flavonoid content (TFC), and results of antioxidant assays
of extracts of soursop (A. muricata L.) leaves and fruit pulp, peel and seed.

TPC FRAP TEAC CUPRAC DPPH• (EC50) Emulsion oxidation

TFC 0.761 0.900 0.887 0.646 -0.680 0.713

TPC 1 0.899 0.872 0.589 -0.719 0.865

FRAP 1 0.968 0.724 -0.753 0.739

TEAC 1 0.731 -0.807 0.640

CUPRAC 1 -0.655 0.388

DPPH (EC50)
•
1 -0.477

FRAP: ferric-reducing antioxidant power; TEAC:Trolox equivalent antioxidant capacity; CUPRAC: cupric ion reducing antioxidant capacity.

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364 Antioxidant Activity of Soursop Extracts

TABLE 4. Chemical compounds of soursop (A. muricata L.) seed hexane TABLE 5. The chemical class distribution of the compounds of soursop
extract identified by GC-MS. (A. muricata L.) seed hexane extract.

Peak RT Compound % Chemical class of compounds Distribution (%)

1 8.83 Decane 0.57
Fatty acids 67.17
2 12.41 Undecane 0.34
Unsaturated fatty acids 43.40
3 16.12 Tridecane 0.45
Saturated fatty acids 23.77
4 18.29 (E)-2-Decenal 1.28
5 19.43 (E,E)-2,4-Decadienal 3.23 Terpenoids 13.23

6 23.18 Tetradecane 0.55 Alkanes 2.87

7 26.46 Pentadecane 0.14 Alkenes 2.35
8 26.78 β-Bisabolene 0.83 Aldehydes and ketones 5.15
9 27.26 β-Sesquiphellandrene 0.53
Alcohols 0.74
10 28.46 (E)-Nerolidol 3.62
Esters 4.26
11 29.33 1-Heptadecene 0.24
Amides 3.88
12 29.57 n-Octadecane 0.34
13 32.52 Heptadecane 0.15 Epoxides 0.27

14 34.16 Tetradecanoic acid 0.29 Total 99.92

15 35.33 Nonadecane 0.13
16 38.01 2-Nonadecanon 0.15
17 38.65 Hexadecenoic acid, methyl ester 0.21 idant activity (FRAP, ABTS, and DPPH assays) of extracts
18 39.69 Pentadecanoic acid 19.92
of different parts of A. muricata L. were higher compared
to those determined for the TFC – antioxidant activity cor-
19 40.39 1-Nonadecene 1.17
relation. In the present study, the FRAP, TEAC, and CU-
20 41.04 Hexadecanal 0.26 PRAC values were significantly (p<0.05) correlated with
21 41.99 Heptadecanoic acid 0.16 each other, wherein the highest r value (0.968) was noted
9,12-Octadecadienoic for TEAC and FRAP correlation (Table 3). Strong, nega-
22 42.83 0.17
acid(Z,Z), methyl ester tive correlations were found between EC50 values of DPPH
23 42.99 6-Octadecenoic acid, methyl ester 0.31 assay and FRAP and TEAC. This finding was in line with
24 43.89 9,12-Octadecadienoic acid (Z,Z) 15.58 literature data [Nam et al., 2017]. Additionally, the lower
r value was determined for correlations between emul-
25 44.06 9-Octadecenoic acid 27.82
sion oxidation results and results of CUPRAC (r=0.388)
26 44.47 Octadecanoic acid 3.40 and DPPH assay (r=-0.477) (Table 3).
27 44.59 (Z)-9-Octadecenoic acid, ethyl ester 1.70
28 44.84 Hexadecanamide 0.64 GC-MS analysis of hexane extract of soursop seeds
The hexane extract of soursop seeds was obtained with
29 45.01 Hexadecanoic acid, butyl ester 0.20
a high yield (Table 1). The TPC and TFC of this extract were
30 45.20 1-Nonadecene 0.94
low. Despite this, it showed some antioxidant activity. There-
31 45.88 Octadecanal 0.23 fore, GC-MS analysis of hexane seed extract was carried out
32 47.57 Heneicosane 0.20 in search of potential antioxidants.
33 48.72 6,9-Octadecadienoic acid, methyl ester 0.39 The GC-MS analysis allowed identifying 44 compounds
in the hexane extract. These compounds were character-
34 48.84 (Z)-9-Octadecenamide 3.04
ized by their retention time (RT), their molecular formula,
35 49.38 Octadecanamide 0.20 and contents which were calculated based on peak area (%)
36 49.61 1-Eicosanol 0.61 (Table 4). According to chemical class distribution, fatty
37 51.31 Di-(9-octadecenoyl)-glycerol 0.13 acids were most abundant (67.17%), followed by terpe-
noids (13.23%), aliphatic hydrocarbons (alkanes/alkenes)
38 53.04 Phthalic acid mono-2-ethylhexyl ester 0.27
(5.22%), aldehydes and ketones (5.15%), esters (4.26%), al-
39 53.40 Oxirane, hexadecyl 0.27 cohols (0.74%), and amides (3.88%) (Table 5). Unsaturated
40 56.12 Humulane-1,6-dien-3-ol 0.52 fatty acids constituted 43.40% of all determined compounds.
41 57.18 Glyceryl trioleate 0.67 The content of saturated fatty acids was 23.77%. Oleic acid
42 58.39 (3-β)-Stigmast-5-en-3-ol 7.45
(9-octadecenoic acid; 27.82 %) and linoleic acid (9,12-octa-
decadienoic acid; 15.58%) were the major unsaturated fatty
43 59.29 Bis (2-ethylhexyl) phthalate 0.34
acids in the soursop seed hexane extract. Especially, linoleic
44 59.90 Urs-12-ene 0.28 acid is known as an essential fatty acid with an important
RT: Retention times metabolic role [Eromosele & Eromosele, 2002]. The high

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H.H. Orak et al. 365

content of oleic and linoleic acids in the hexane seed extract contents. Strong correlations were found between total pheno-
confirmed previous findings. Both acids were found as domi- lic and flavonoid contents and antioxidant activity determined
nant in A. muricata L. seed oil [da Silva & Jorge, 2017; Pinto as TEAC, FRAP, and DPPH• scavenging activity.
et al., 2018]. In turn, the percentage of stearic acid (pentadec- Our study showed that soursop leaves and soursop by-prod-
anoic acid) in total fatty acids was low compared to the re- ucts from fruit processing (seeds and peels) have the potential
sult presented in our study (19.92%). Among phytosterols, to be used to obtain extracts with a high antioxidant activity.
the content of 3-β-stigmast-5-en-3-ol (7.45%) was the highest
in the hexane extract (Table 4). da Silva & Jorge [2017] noted ACKNOWLEDGEMENTS
that this compound was the major phytosterol of soursop
seed oil. The antioxidant activity of 3-β-stigmast-5-en-3-ol The authors would like to thank to Professor Dr. Adnan
examined both in vitro (DPPH and ABTS assays) and in vivo Orak for the statistical analyses. The authors would also like
had already been reported [Ayaz et al., 2017]. Its anti-prolif- to thank Burhan Karaman from the Dominica Island for pro-
erative properties were noted as well [Moon et al., 2008]. Ter- viding plant material.
penoids are another class of compounds with recognized an-
tioxidant activity; they were detected in the analysed soursop RESEARCH FUNDING
hexane seed extract. The main terpenoid in the extract was
(E)-nerolidol (Table 4). Chan et al. [2016] reviewed various This research did not get outsourcing.
biological activities of this sesquiterpene alcohol, including its
antioxidant activity. The major aldehydes in the extract were CONFLICT OF INTEREST
identified as (E)-2-decenal (1.28%) and (E,E)-2,4-decadienal
(3.23%). Caboni et al. [2012] reported a high nematicidal ac- Authors declare that they have no conflict of interest.
tivity of both compounds. In turn, Cheng et al. [2008] sug-
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