antioxidants
Article
Valorization of Peels of Eight Peach Varieties: GC–MS Profile,
Free and Bound Phenolics and Corresponding
Biological Activities
Dasha Mihaylova 1, * , Aneta Popova 2, *, Ivelina Desseva 3 , Ivayla Dincheva 4 and Yulian Tumbarski 5
1
2
3
4
5
*
Citation: Mihaylova, D.; Popova, A.;
Desseva, I.; Dincheva, I.; Tumbarski,
Y. Valorization of Peels of Eight Peach
Varieties: GC–MS Profile, Free and
Bound Phenolics and Corresponding
Biological Activities. Antioxidants
2023, 12, 205. https://doi.org/
Department of Biotechnology, University of Food Technologies, 4002 Plovdiv, Bulgaria
Department of Catering and Nutrition, University of Food Technologies, 4002 Plovdiv, Bulgaria
Department of Analytical Chemistry and Physical Chemistry, University of Food Technologies,
4002 Plovdiv, Bulgaria
Department of Agrobiotechnologies, AgroBioInstitute, Agricultural Academy, 1164 Sofia, Bulgaria
Department of Microbiology, University of Food Technologies, 4002 Plovdiv, Bulgaria
Correspondence: dashamihaylova@yahoo.com (D.M.); popova_aneta@yahoo.com (A.P.)
Abstract: Sustainability, becoming essential for food processing and technology, sets goals for the
characterization of resources considered as food waste. In this work, information about the GCMS metabolites of peach peels was provided as a tool that can shed more light on the studied
biological activities. In addition, distribution patterns and contribution of the chemical profile and
free and bound phenolic compounds as antioxidant, antimicrobial, and enzymatic clusters in peach
peels of different varieties of Bulgarian origin were studied. The two applied techniques (alkaline
and acid hydrolysis) for releasing the bound phenolics reveal that alkaline hydrolysis is a better
extraction approach. Still, the results indicate the prevalence of the free phenolics in the studied
peach peel varieties. Total phenolics of peach wastes were positively correlated with their antioxidant
activity. The antioxidant activity results certainly defined the need of an individual interpretation
for each variety, but the free phenolics fractions could be outlined with the strongest potential. The
limited ability of the peels’ extracts to inhibit α-amylase and acetylcholinesterase, and the moderate
antimicrobial activity, on the other hand, indicate that the potential of peach peels is still sufficient
to seek ways to valorize this waste. Indeed, this new information about peach peels can be used
to characterize peach fruits from different countries and/or different food processes, as well as to
promote the use of this fruit waste in food preparation.
Keywords: peel; waste recovery; valorization; peach; free and bound phenolics; metabolites
10.3390/antiox12010205
Academic Editor: Julia
González-Álvarez
Received: 12 December 2022
Revised: 8 January 2023
Accepted: 12 January 2023
Published: 16 January 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1. Introduction
Traditionally, plants are known to be abundant in secondary metabolites [1]. Several
studies have designated the use of plants (including fruits and vegetables) rich in bioactive
compounds in the management of non-communicable diseases [2,3]. Some of them are also
produced in their fruits [4]. Knowing the distribution of these metabolites in different parts
of the fruit is a piece of valuable information, especially in the light of a circular economy
and waste recovery. That is why research is no longer limited to the edible part of the fruit,
but also to the generated by-products of fruit processing [5,6]. Moreover, fruit processing
generates a large amount of waste. In fact, according to the FAO, fruit and vegetables
account for up to 45% of food waste, in general [7]. Recently, many articles emphasize
fruit waste as a source of health-promoting biologically active substances that could be
identified, extracted, and further used [8–11]. This is a sustainable approach targeting the
global goal for “zero waste” in the environment.
Phenolic compounds are common in a number of natural raw materials—fruits, vegetables, cereals, herbs, etc. [12,13]. They draw scientific interest due to their biological activities,
Antioxidants 2023, 12, 205. https://doi.org/10.3390/antiox12010205
https://www.mdpi.com/journal/antioxidants
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including anti-inflammatory, antibacterial, neuroprotective, anti-coronavirus, antidiabetic,
etc., as well as their powerful antioxidant properties [13–15]. Polyphenol compounds are
considered as the potential active compounds that inhibit the enzyme acetylcholinesterase,
associated with dementia and Alzheimer’s Disease (AD) prevention [16]. In addition, fruit
isoflavonoids are studied for their pancreatic lipase inhibition [17]. Phenolic acids and
flavonoids are some of the most recognizable phenolic compounds, which can be seen in
soluble free, soluble esterified, and insoluble forms in plants. Polyphenols in the bound
forms are covalently linked to the polysaccharide and protein structural components of
the cell wall. In a number of in vitro antioxidant analyses, bound phenols have shown
significantly higher antioxidant activity than that of soluble phenols [18]. Moreover, bound
phenols can endure in both stomach and small intestine conditions, and reach the colon
intact, where they have been released and exhibit bioactivity [19].
Depending on the species, fruits and vegetables’ edible parts contain between 6.5%
and 76.3% of the total phenols in bound form [18–20]. Of particular interest are fruit peels,
which are believed to hold a substantial amount of non-extractable phenols [21,22]. The
contribution of non-extractable polyphenols to the total polyphenol content of common
fruit peels in different fruits varies from 23 to 82% [21–23]. That is why analyzing only
the free phenolics may underestimate the total phenolic content and the related activities.
Moreover, approximately 24% of the phenolic compounds in fruits are considered to be
present in a bound form [20]. Recently, researchers have shown an increased interest in the
valorization and recovery of food waste [24,25]. Data shows that food waste, as well as
being a global sustainability issue, can also present a valuable research topic.
Fruit and vegetable peels are one of the most widely neglected sources of beneficial
substances. Thus, the objective of the present study was to estimate the phytochemicals
present in peach peels of peaches, flat peaches, and nectarines from different varieties, as
well as to shed light about the amount of bound and free polyphenolic compounds and
their contribution to antioxidant action and enzyme inhibitory potential. This will elucidate
this underestimated fruit waste as a source of biologically active substances and beneficial
actions. Resulting data will help evaluate differences and similarity between peaches, flat
peaches, and nectarines. Results will also promote the possible utilization of peach peels in
various industries, i.e., functional food production and cosmetology, among others.
2. Materials and Methods
2.1. Fruit Samples
Peels from the “Filina” (F), “Ufo 4” (U), “Gergana” (G), “Laskava” (L), “July Lady”
(JL), “Flat Queen” (FQ), “Evmolpiya” (Evm), and “Morsiani 90” (M) varieties grown in
the same plantation were the objects of analysis. “Filina,” “Laskava,” “July Lady,” and
“Evmolpiya” are peaches; “Ufo 4” and “Flat Queen” are flat peaches with white flesh;
“Gergana” and “Morsiani 90” are nectarines [26]. No bactericides were applied to plants
during testing. The samples were collected in 2021 at eating ripeness (fruit growth has
stopped, yellow-orange color appeared, and softening of tissue occurred) in the Fruit
Growing Institute, Plovdiv, BG.
A vacuum freeze dryer (BK-FD12S, Biobase, Jinan, Shandong, China) aided in the
sample preparation. Different peel samples were first freeze-dried, then, powdered and
kept prior to extraction.
2.2. Gas Chromatographic–Mass Spectrometry Analysis (GC-MS)
Lyophilized material (50.0 mg) from each sample was exposed to the following procedure: 500.0 µL methanol, 50.0 µL ribitol, and 50.0 µL n-nonadecanoic acid (internal
standards in concentration 1 mg/mL to quantify polar and non-polar metabolites) were
added, then the resulting mixture was heated using a thermo shaker TS-100 (Analytik Jena
AG, Jena, Germany) for 30 min at 70 ◦ C/300 rpm. A volume of 100.0 µL water and 300.0 µL
chloroform were added after cooling down to room temperature, then the mixtures were
centrifuged (Beckman Coulter, Brea, California, USA) for 5 min at 22 ◦ C/13,000 rpm. The
Antioxidants 2023, 12, 205
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upper phase was intended to analyze the polar (amino and organic acids, carbohydrates)
compounds, whereas the lower phase studied the non-polar (saturated and unsaturated
fatty acids) metabolites. The two attained phases were vacuum-dried at 40 ◦ C in a centrifugal vacuum concentrator (Labconco Centrivap, Hampton, NH, USA).
In order to extract the saturated and non-saturated fatty acids, 1.0 mL 2% H2 SO4 in
methanol was added to the dried residue of fraction “non-polar metabolites,” then the
mixture was heated for 1 h at 96 ◦ C/300 rpm on a Thermo-Shaker TS-100. After cooling
to the room temperature, the resulting solution was extracted with 3 × 10.0 mL n-hexane.
Organic layers combined were vacuum-dried at 40 ◦ C in a centrifugal vacuum concentrator
(Labconco Centrivap).
Prior to analysis by GC-MS, samples were derivatized by the following two procedures. Firstly, a 300.0 µL solution of methoxyamine hydrochloride (20.0 mg/mL in
pyridine) was added to the fraction “polar metabolites,” and the mixture was heated on
thermo shaker for 1 h at 70 ◦ C/300 rpm. After cooling, 100.0 µL N,O-Bis (trimethylsilyl)
trifluoroacetamide (BSTFA) was added to the mixture, then heated on the thermo shaker
for 40 min at 70 ◦ C/300 rpm. Lastly, 1.0 µL from the solution was injected in the GC-MS.
Secondly, 100.0 µL pyridine and 100.0 µL BSTFA were added to the fraction “non-polar
metabolites” and then heated on the thermo shaker for 45 min at 70 ◦ C/300 rpm. Then,
1.0 µL from the solution was injected in the GC-MS.
GC-MS analysis was carried out using a gas chromatograph 7890A (Agilent) coupled
to a mass selective detector 5975C (Agilent) and HP-5ms silica-fused capillary column
coated with 0.25 µm film of poly (dimethylsiloxane) as the stationary phase (Agilent),
30 m × 0.25 mm (i.d.). The oven temperature program used was as follows: initial temperature 100 ◦ C for 2 min, then 15 ◦ C/min to 180 ◦ C for 2 min, and after that, 5 ◦ C/min to 300 ◦ C
for 10 min, run time 42 min. The flow rate of the carrier gas (Helium) was maintained at
1.2 mL/min. The injector and the transfer line temperature were kept at 250 ◦ C; EI energy:
70 eV, mass range: 50 to 550 m/z at 1.0 s/decade. The temperature of the MS source was
230 ◦ C. The injections were carried out in a split mode 10:1; the injection volume was 1 µL.
Version 2.64 of the AMDIS software, (Automated Mass Spectral Deconvolution and
Identification System, NIST, Gaithersburg, MD, USA) facilitated the comprehension of
the obtained mass spectra and the recognition of the metabolites. Kovats retention index
(RI) with reference compounds in the Golm Metabolome Database (http://csbdb.mpimpgolm.mpg.de/csbdb/gmd/gmd.html, accessed on 25 March 2022) and NIST’08 database
(NIST Mass Spectral Database, PC-Version 5.0, 2008 from National Institute of Standards
and Technology, Gaithersburg, MD, USA) were compared to the GC-MS spectra. The
2.64 AMDIS software verified the RIs of the compounds with a standard n-hydrocarbon
calibration mixture (C8 –C36 , Restek, Teknokroma, Spain).
2.3. Free and Bound Phenolic Compounds Extraction
2.3.1. Extraction of Free Phenolic Compounds
Free phenolic compounds of each variety’s peels were extracted in triplicate as follows:
0.5 g of sample was mixed with 10 mL 80% (80:16, v/v) ethanol, extracted at 50 ◦ C for 30 min
under ultrasound (UST 5.7150 Siel, Gabrovo, Bulgaria) and centrifuged at 10,000× g for
20 min. Phenolic extracts were filtered using filter paper (Whatman No. 1) and evaporated
until near dryness (RV 10, Ika, Staufen, Germany). The final volume of the extracts was
adjusted by adding 10 mL of 85% methanol (85:15, v/v) and stored at −20 ◦ C until further
analysis.
2.3.2. Extraction of Bound Phenolic Compounds
Alkaline Hydrolysis Method
Bound phenolic extracts were obtained according to the method described by Ding et al. [27]
with modifications. The residues of the extraction process for free phenolics were subject to
18 h digestion with shaking at 30 ◦ C, using 2 M sodium hydroxide (25 mL) and under a
stream of nitrogen gas. The samples were acidified to pH 1.5–2.0 with 6 M hydrochloric
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acid and then extracted six times with 25 mL ethyl acetate. The upper layer was collected
each time and combined. The ethyl acetate extracts were dried to complete dryness using a
rotary evaporator at 38 ◦ C (RV 10, Ika, Staufen, Germany). The dried bound extracts were
reconstituted in 10 mL 85% HPLC grade methanol (85:15, v/v) and stored by avoiding light
at −20 ◦ C until analysis.
Acid Hydrolysis Method
Bound phenolic compounds of each variety were extracted using the method reported
previously [28]. The residues of the extraction process for free phenolics were treated with
25 mL of methanol/H2 SO4 (90:10, v/v) at 70 ◦ C for 24 h as the first hour was with sonication,
and the resulting mixtures were neutralized with 10 M sodium hydroxide to pH 12.0 before
being extracted six times with ethyl acetate. The supernatants were combined/merged
and vacuum evaporated to dryness at 38 ◦ C (RV 10, Ika, Staufen, Germany) before being
reconstituted with 10 mL methanol/water (85:15, v/v) and stored by avoiding light at
−20 ◦ C until analysis.
2.4. Determination of Total Phenolic Contents (TPC)
A modified method of Kujala et al. [29] was used to analyze the TPC as described by
Mihaylova et al. [30].
2.5. Determination of Total Flavonoid Content (TFC)
The total flavonoid content was assessed following the description of Kivrak et al. [31].
Results are expressed as µg quercetin equivalents (QE)/g dw, as quercetin (QE) was used
as a standard.
2.6. Determination of Total Monomeric Anthocyanins (TMA)
The TMA content was defined using the pH differential method [32]. Results are
expressed as µg cyanidin-3-glucoside (C3GE)/g dw.
2.7. Evaluation of Antioxidant Activities of Phenolic (Free and Bound) Fractions
2.7.1. DPPH• Radical Scavenging Assay
The slightly modified method of Brand-Williams et al. [33], as described by Mihaylova et al. [30], aided in the identification of the capability of the extract’s to donate
an electron and scavenge 2,2-diphenil-1-picrylhydrazyl (DPPH) radicals. The antioxidant
activity is presented as a function of the concentration of Trolox with equivalent antioxidant
activity expressed as µM TE/g dw.
2.7.2. ABTS•+ Radical Scavenging Assay
The method of Re et al. [34] was used to estimate the extracts’ ABTS•+ radical scavenging activity. The results are expressed as µM TE/g dw with Trolox as standard.
2.7.3. Ferric-Reducing Antioxidant Power (FRAP) Assay
The procedure of Benzie and Strain [35], with slight modification as described by
Mihaylova et al. [30], was carried out in the FRAP assay, recording the absorbance at
593 nm, and expressing the results as µM TE/g dw with Trolox as standard.
2.7.4. Cupric Ion-Reducing Antioxidant Capacity (CUPRAC) Assay
The CUPRAC assay followed the procedure of Apak et al. [36]. Results are expressed
as µM TE/g dw with Trolox as standard.
2.8. Enzyme-Inhibitory Activities
The Sigma Aldrich method [37] specified by Mihaylova et al. [24] was used to carry
out the α-Amylase (AM)-Inhibitory Assay. An α-Glucosidase (AG)-Inhibitory Assay was
completed as described in the paper by Mihaylova et al. [26]. The in vitro pancreatic-lipase-
Antioxidants 2023, 12, 205
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inhibitory activity was assessed as described by Saifuddin et al. [38] and Dobrev et al. [39],
with modifications explained in previous research [26]. The experimental conditions of the
in vitro AChE-inhibitory assay were built on the method defined by Lobbens et al. [40],
with modifications available by Mihaylova et al. [26]. All results are expressed as the
concentration of extract (IC50 ) in mg/mL that inhibited 50% of the respected enzyme
(α-amylase, α-glucosidase, lipase, and acetylcholinesterase).
2.9. Antimicrobial Activity
2.9.1. Test Microorganisms
Four Gram-positive bacteria (Bacillus subtilis ATCC 6633, Staphylococcus aureus ATCC
25923, Listeria monocytogenes NBIMCC 8632, Enterococcus faecalis ATCC 19433), four Gramnegative bacteria (Salmonella enteritidis ATCC 13076, Escherichia coli ATCC 8739, Proteus
vulgaris ATCC 6380, Pseudomonas aeruginosa ATCC 9027), two yeasts (Candida albicans
NBIMCC 74, Saccharomyces cerevisiae ATCC 9763) and six fungi (Aspergillus niger ATCC
1015, Aspergillus flavus, Penicillium sp., Rhizopus sp., Mucor sp.-plant isolates, Fusarium
moniliforme ATCC 38932) from the collection of the Department of Microbiology at the
University of Food Technologies, Plovdiv, Bulgaria, were selected for the antimicrobial
activity test.
2.9.2. Culture Media
Luria–Bertani agar medium supplemented with glucose (LBG) was prepared as prescribed by the manufacturer (Laboratorios Conda S.A.): 50 g of LBG-solid substance mixture
was dissolved in 1 L of deionized water. The final pH was adjusted to 7.5, then the medium
was autoclaved at 121 ◦ C/20 min.
Malt extract agar (MEA) was prepared as suggested by manufacturer (HiMedia® ,
Thane, India): 50 g of the MEA-solid substance mixture was dissolved in 1 L of deionized
water. pH was corrected to 5.4 ± 0.2, and then the medium was autoclaved at 115 ◦ C/10 min.
2.9.3. Antimicrobial Activity Assay
The agar well diffusion method [41] was implemented in the antimicrobial activity
determination. The test bacteria B. subtilis was cultured on LBG agar at 30 ◦ C; S. aureus,
L. monocytogenes, E. faecalis, S. enteritidis, E. coli, P. vulgaris, and P. aeruginosa were cultured
on LBG agar at 37 ◦ C for 24 h. The yeast C. albicans was cultured on MEA at 37 ◦ C, while
S. cerevisiae was cultured at 30 ◦ C for 24 h. The fungi A. niger, A. flavus, Penicillium sp.,
Rhizopus sp., Mucor sp., and F. moniliforme were grown on MEA at 30 ◦ C for 7 days or until
sporulation.
A small amount of biomass in 5 mL of sterile 0.5% NaCl was homogenized to prepare
the inocula of test bacteria/yeasts, while 5 mL of sterile 0.5% NaCl was placed into the
tubes for the test fungi. After stirring by vortex V-1 plus (Biosan, Riga, Latvia), they were
filtered and transferred in other tubes prior to usage. A bacterial counting chamber Thoma
(Poly-Optik, Germany) established the number of viable cells and fungal spores. Their
final concentrations were adjusted to 108 cfu/mL for bacterial/yeast cells and 105 cfu/mL
for fungal spores, and then inoculated in agar media that was preliminarily melted and
tempered at 45–48 ◦ C. The inoculated media were subsequently transferred in a quantity
of 18 mL, in sterile Petri plates (d = 90 mm) (Gosselin™), and allowed to harden. After
that, six wells (d = 6 mm) per plate were cut, and triplicates of 60 µL of the extracts were
pipetted into the agar wells. The Petri plates were incubated at identical conditions.
The inhibition zones’ diameters around the wells were measured twice on the 24th
and 48th hours of incubation to establish antimicrobial activity. Test microorganisms with
inhibition zones ≥ 18 mm were regarded as sensitive; those with zones ranging from 12
to 18 mm were considered moderately sensitive; and those with zones ≤ 12 mm were
considered resistant.
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2.10. Statistical Analyses
Each sample was triplicated and the results are expressed as the mean ± SD. The
impact of the peach variety and extraction type on the TPC, TFC, TMA, and AOA was
evaluated using a two-factor variance analysis [42]. The Tukey–Kramer post hoc test
(α = 0.05) [42] was used to statistically compare the data. The web-based MetaboAnalyst
platform (www.metaboanalyst.ca, accessed on 27 June 2022) [43] was used to conduct the
PCA and HCA of GC-MS data, as previously described by Mihaylova et al. [26].
3. Results and Discussion
3.1. GC-MS Volatile Profile Characterization of Analyzed Peach Peels
Due to the beneficial properties of metabolites, the interest in metabolite profiling is
constantly growing. Presenting information about the metabolite profile of different parts
of the fruit is a piece of useful knowledge, especially in the light of waste recovery and
resource scarcity. That is why research is no longer limited to the plant’s edible part, but also
includes the generated by-products of fruit processing, such as peels, stones, and pressed
pulp, among others. In the view of the abovementioned, a semi-quantification GC-MS
profile aids in the characterization of the peels of eight peach varieties. The identified
metabolites (Table 1) in the current study are divided into five groups: sugars and sugaralcohols, organic acids, amino acids, phenolic acids, and fatty acids. It is important to
highlight the availability of potentially active substances in the peels, as they are often
regarded as food waste.
Similar to the whole fruit [26], the peach peel is most abundant in sugars. Organic acids
being intermediates in the degradation pathways of amino acids, fats, and carbohydrates,
also affect properties such as the fruit’s color, flavor, and aroma [44]. The organic acids
content in the studied peaches, flat peaches, and nectarines is relatively similar. The
documented contents of organic acids confirm that the skin is flavor-contributing and
might influence the overall consumer’s acceptance by referencing the color of the skin in
terms of visual ripeness. Considering the amino acids content, it is insufficient for human
daily needs, but it is visible that early ripening varieties have peels richer in amino acids
compared to late ripening varieties.
Shikimic acid, one of the major organic acids in all samples, has been recognized
for its neuraminidase inhibition potential [45]. Shikimic acid has also presented its antiinflammatory effect [46]. The predominant fatty acids in the studied peels are the saturated
palmitic, stearic, and behenic acids, as well as the unsaturated linoleic, oleic, and arachidic
acids. Some researchers [47] point out that oleic and linoleic acids possess powerful
inhibitory effects on the α-glucosidase activity, but they are also competitive inhibitors, and
their interactions with α-glucosidase showed a character of static quenching, which directs
them to bind to α-glucosidase to form a complex. Recent research [48] highlights palmitic
acid as a strong α-amylase inhibitor. Fatty acids also have high potency in the therapy
of Alzheimer’s disease due to their inhibition of cholinesterases (AChe and BChe) [49].
Chlorogenic acid is a main phenolic compound frequently present in plants [50]. Expectedly,
chlorogenic acid is a major compound identified in the extracts. It is a result from the
esterification of caffeic with quinic acid. The production of chlorogenic acid reduces the
ability of caffeic and quinic acids to inhibit α-amylase and α-glucosidase [51].
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Table 1. Semi-quantification of polar metabolites (mg/g dw) of eight peach varieties’ peel extracts.
RI
F
Polar Metabolites
G
U
L
JL
FQ
Evm
M
0.209
0.549
0.082
0.435
0.525
0.177
1.240
0.777
ND
0.124
0.024
0.860
0.467
0.710
1.072
0.669
0.827
0.076
8.825
0.171
0.079
0.007
0.117
ND
0.059
0.438
0.600
0.068
0.191
0.009
0.175
0.106
0.142
0.243
ND
0.203
ND
2.607
0.011
0.014
ND
0.009
0.008
ND
0.028
0.011
ND
0.007
ND
0.051
0.033
0.016
0.015
0.012
ND
0.068
0.283
0.228
0.123
0.010
0.034
0.080
0.137
0.359
0.477
ND
0.142
0.005
0.137
0.205
0.416
0.247
0.028
ND
0.046
2.673
0.041
0.027
ND
0.207
ND
0.078
0.711
0.178
ND
0.035
0.011
0.389
0.058
0.211
0.563
0.055
0.287
ND
2.851
0.086
0.052
0.035
0.143
0.115
0.391
0.239
0.417
ND
0.036
0.020
0.217
0.151
0.273
0.352
0.015
0.122
0.044
2.709
0.783
0.589
1.088
1.787
0.913
0.887
0.309
6.357
0.526
0.396
0.731
1.201
0.613
0.596
0.207
4.271
0.619
0.466
0.860
1.413
0.722
0.701
0.244
5.025
0.602
0.453
0.836
1.374
0.702
0.682
0.237
4.885
0.642
0.483
0.892
1.465
0.748
0.727
0.253
5.211
0.662
0.498
0.920
1.511
0.772
0.750
0.261
5.374
0.197
0.127
0.182
0.091
0.269
0.187
0.281
0.157
0.296
0.170
Amino acids
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
1097
1208
1266
1285
1293
1299
1351
1376
1508
1515
1550
1609
1635
1833
1910
1930
2144
2211
Alanine
Valine
Leucine
Isoleucine
Proline
Gycine
Serine
Threonine
Aspartic acid
Methionine
Cysteine
Glutamic acid
Phenyalanine
Arginine
Lysine
Tyrosine
Histidine
Tryptophan
Total
0.705
0.142
ND
0.119
0.161
0.224
0.591
0.201
1.141
0.050
0.007
0.280
0.215
0.171
0.184
0.156
0.384
ND
4.729
0.104
0.307
0.052
0.265
0.297
0.079
0.618
0.932
ND
0.401
0.014
0.522
0.283
0.376
0.857
0.438
0.425
0.148
6.117
Organic acids
1
2
3
4
5
6
7
1305
1344
1477
1818
1841
1855
1946
Succinic acid
Fumaric acid
Malic acid
Shikimic acid
Citric acid
Quinic acid
L-Ascorbic acid
Total
0.728
0.548
1.012
1.662
0.849
0.825
0.287
5.911
0.547
0.412
0.760
1.249
0.638
0.620
0.216
4.441
Sugar alcohols
1
2
1932
2034
Sorbitol
Myo-inositol
0.326
0.187
0.245
0.141
0.232
0.104
1856
1865
1881
1901
Fructose isomer
Fructose isomer
Glucose issomer
Glucose issomer
Sucrose isomer (alpha-D-Glc(1.2)-beta-D-Fru)
Sucrose isomer (alpha-D-Glc(1.2)-beta-D-Fru)
Total
0.972
0.337
1.855
1.397
1.355
0.470
2.586
1.947
1.588
0.550
3.029
2.281
1.984
0.688
3.786
2.851
1.804
0.625
3.442
2.592
1.491
0.517
2.844
2.142
1.389
0.481
2.650
1.996
1.640
0.568
3.129
2.356
2.992
4.171
4.885
6.106
5.551
4.588
4.274
5.047
1.737
2.421
2.836
3.545
3.223
2.663
2.481
2.930
9.290
12.950
15.168
18.960
17.236
14.245
13.272
15.669
Saccharides (mono-, di-)
1
2
3
2620
2833
Saturated and unsaturated fatty acids
1
1719
2
1926
3
2095
4
2099
5
2103
6
2247
7
2311
8
2408
Tetradecanoic acid (Myristic
acid)
n-Hexadecanoic acid
(Palmitic acid)
9,12-(Z,E)-Octadecadienoic
acid (Linoleic acid)
9-(Z)-Octadecenoic acid
(Oleic acid)
9,12,15-(Z,Z,Z)Octadecatrienoic acid
(Linolenic acid)
n-Octadecanoic acid (Stearic
acid)
n-Eicosanoic acid (Arahydic
acid)
n-Docosanoic acid (Behenic
acid)
Total
0.336
0.319
0.166
0.460
0.394
0.259
0.276
0.287
3.953
3.754
1.948
5.411
4.625
3.040
3.246
3.378
2.451
2.327
1.208
3.355
2.867
1.885
2.013
2.095
1.285
1.220
0.633
1.758
1.503
0.988
1.055
1.098
0.445
0.423
0.219
0.609
0.521
0.342
0.366
0.380
1.648
1.565
0.812
2.256
1.928
1.268
1.354
1.409
0.756
0.718
0.372
1.034
0.884
0.581
0.621
0.646
0.939
0.891
0.463
1.285
1.098
0.722
0.771
0.802
11.811
11.217
5.821
16.168
13.819
9.086
9.701
10.095
Antioxidants 2023, 12, 205
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Table 1. Cont.
RI
F
Polar Metabolites
G
U
L
JL
FQ
Evm
M
0.033
0.122
0.055
0.045
0.061
1.638
1.955
0.107
0.096
0.077
0.068
0.050
0.842
1.240
0.042
ND
0.064
0.085
0.022
2.502
2.714
0.123
0.083
0.137
0.044
0.031
2.150
2.569
0.136
0.096
0.026
0.053
0.046
1.214
1.572
0.273
0.185
0.055
0.026
0.055
1.363
1.957
Phenolic acids
1
2
3
4
5
6
1836
1945
2103
2140
2254
3191
Protocatechuic acid
trans-p-Coumaric acid
trans-Ferulic acid
trans-Caffeic acid
trans-Sinapic acid
Chlorogenic acid
Total
0.073
0.157
0.110
0.034
0.072
2.816
3.263
0.187
0.142
0.092
0.128
0.062
3.559
4.170
ND—not detected in the sample, RI—retention index; G—“Gergana”, F—“Filina”, U—“Ufo 4”, JL—“July Lady”,
L—“Laskava”, FQ—“Flat queen”, Evm—“Evmolpiya”, M—“Morsiani 90”. Essential amino acids are marked
with blue color.
3.2. Total Phenolic, Flavonoid and Total Monomeric Anthocyanins Contents of Free and Bound
Insoluble Fractions
Phenolic compound are responsible for both the desirable and undesirable qualities of
the peach fruit [52]. The presented GC-MS profile of the peels revealed the free phenolic
acids (Table 1). Although they are in relatively small amounts, it is important the clear
out their distribution in such parts of the fruit that are often left unconsumed. Phenolic
compounds are one of the main classes contributing to the biological activity of plant
matrices and fruits, in particular [53]. Traditional solvent extraction usually omits high
quantities of bound phenolics that play an essential role in human health benefits. Due to
their beneficial effects, the use of the full spectrum of polyphenolic potential has attracted
considerable attention, and it seems reasonable to apply different recovery techniques.
The peach fruit, itself, is reported as an excellent supply for phenolic components [54].
Peach peels, on the other hand, are yet to be thoroughly characterized. Bearing in mind the
abovementioned, in targeting to reveal the biological potential of the peach peels, several
phenolic compounds profile was assessed.
Based on existing reports [55,56] about the richness of stone fruits in phenolic compounds, the current study focused on the evaluation of their distribution in both soluble
and non-soluble forms. Various techniques, including alkaline, acid, enzymatic, and
ultrasound-assisted hydrolyses, can be applied in order to release the bound insoluble
phenolic fractions from the cell wall [18].
A two-way ANOVA aided in the evaluation of the peach variety and type of extraction
on the TPC, TFC, and TMA. The interpretation of the results showed that the single effect
of both factors and their combination was influential (p < 0.05) to the TPC, TFC, and TMA.
The highest TPC was found in soluble phenolics extracts, showing the relationship of the
total phenolic content in peach fruits with the extractable free polyphenols (Figure 1). The
total TPC of the samples varied between 15.56 and 20.49 mgGAE/g dw, accounting mainly
of extractable polyphenols—from 42 to 76%. The TPC of the free soluble polyphenols was
in the range of 6.82 ± 0.13 to 13.12 ± 0.09 mgGAE/g dw. Previous research also points
out that free phenolic compounds are predominant in plant-based extracts [57,58]. The
alkaline hydrolyzed non-extractable polyphenols account for 16 to 31%. Other authors
reported alkaline treatment, as such, of low yield [6]. Depending on the fruit variety, acid
and alkaline hydrolyzed polyphenols contribute in different manners to the fruit’s total
TPC. Furthermore, the total phenolic content of the extractable polyphenols was statistically
superior to that of non-extractable polyphenols within the same variety, valid for all eight.
The samples with the highest TPC were the free fractions of the “Flat Queen” and “Laskava”
varieties, and the lowest was the “Evmolpiya” variety. In terms of total TPC, the highest
values were established in the “Flat Queen” variety, followed by “Laskava,” confirming the
contribution of the free soluble polyphenols to the total TPC. The contribution of bound
phenolics amounted to a range of 7 to 31% of the total phenolics.
The current results suggest that the alkaline hydrolysis method was more effective
compared to the acid one, and efficiently liberated bound phenolic compounds (Figure 1),
which is comparable to other research targeting by-products, i.e., fruit peels [59,60]. Alkaline
Antioxidants 2023, 12, 205
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hydrolysis successfully breaks the ether and ester bonds which link phenolic compounds to
the cell wall, and are commonly spread in fruit peels [18]. This might explain the established
trend in the current study.
Flavonoids are the largest group of polyphenols. In respect to the total flavonoid
content, the free phenolics extracts are with the predominant content in most of the varieties,
ranging from 164.14 ± 4.72 (“Ufo 4”) to 515.83 ± 30.59 µgQE/g dw (“Flat Queen”). The
content of flavonoids was higher when alkaline hydrolysis was applied (Figure 1) for
“Ufo 4” and “July Lady” compared to the soluble phenolics extracts. The contribution
of acidic hydrolyzed flavonoids could be neglected as below the limit of detection for all
the varieties. The TFC of the peach peels varied from 380.58 to 999.38 µgQE/g dw. The
alkaline hydrolyzed fraction displayed a TFC of 0 and 76.93% from the total TFC of the
samples. The established results follow the same trend as for the total phenolic content.
Other researchers also acknowledge the fact that flavonoids in free form are predominant
in plant-based matrices [61].
25
Free
Acid hydrolysis
Alkaline hydrolysis
TPC, mgGAE/g dw
20
15
a
b
bc
bc
c
c
10
ef
fgh
5
jk
ik
de
fg fgh
ef
gi
gij
gi fi
U
JL
L
d
fg fgh
hik gi
k
0
G
F
FQ
Evm
M
(A)
2500
Free
Acid hydrolysis
Alkaline hydrolysis
TFC, µgQE/g dw
2000
1500
a
b
1000
c
d
gh
hi
i
g
l
k
F
U
d
d
e
f
500
i
g
j
d
f
g
k
ij
ij
Evm
M
0
G
JL
(B)
Figure 1. Cont.
L
FQ
Antioxidants 2023, 12, 205
10 of 22
TMA, µgCya-3-glu/g dw
1400
a
Free
Acid hydrolysis
Alkaline hydrolysis
b
1200
1000
c
800
600
d
400
e
200
g
g
fg
g
eg
ef
fg
g
fg
g g
fg
gg
fg g fg
0
G
F
U
JL
L
FQ
Evm
M
(C)
Figure 1. Free and bound (insoluble) phenolics content distribution of eight peach varieties’ peel
ties’ peel
extracts—(A) Total
flavonoids
(TFC,
µgQE/g
— phenolic content (TPC, mgGAE/g dw), (B) Total
) Total
flavonoids
(TFC,
µgQE/g dw)
dw) and (C) Total monomeric anthocyanins (TMA, µg cyanidin-3-glucoside (C3GE)/g dw).
—“Gergana”, F—“Filina”,
U—“Ufo
4”, 4”,
JL—“July
Lady”,
—“Laskava”,
FQ—“Flat
queen”,queen”,
Evm—
G—“Gergana”,
F—“Filina”,
U—“Ufo
JL—“July
Lady”,
L—“Laskava”,
FQ—“Flat
“Evmolpiya”,M—“Morsiani
M—“Morsiani90”.
90”.Different
Differentletters
letters(a–l)
(a– within chart columns indicate signifiEvm—“Evmolpiya”,
cant differences (p < 0.05) between treatments as analyzed by two-way ANOVA and the Tukey test
(n = 3 per treatment group).
The total monomeric anthocyanins content was mainly due to the free extractable
polyphenolics fraction, and the total content was in the range from 327.84 to 1246.77 µgCya3-glu/g dw (“Gergana”). The distribution between soluble and insoluble phenolics
(Figure 1) showed no or limited contribution of the acid hydrolyzed phenolics, and relatively low input of the alkaline hydrolyzed ones, which is not surprising due to the
degradation of the anthocyanin content with the increased temperature and pH value [62].
About 1%, 7%, 31%, 17%, 40%, 49%, 11%, and 0% of TAC were present in bound form
in the peels of the investigated varieties. The uneven distribution of TAC among soluble
and insoluble phenolics fractions confirms the need for personalized/individual evaluation
of the potential of peel waste as a source of biologically active substances.
In brief, most of the
analyses
are synchronous
regarding
the predominance
of thefraction
poseason
“Morsiani
90” variety,
in particular.
The free phenolic
of
tential of
free phenolic
extracts,
revealing
commonly
applied extraction
“Morsiani
90” was
the most
activethe
oneeffectiveness
according toofthe
CUP
techniques. In the current study, alkaline hydrolysis resulted in more bound phenolics,
flavonoids, and monomeric anthocyanins compared to acid hydrolysis. Earlier studies have
established that alkaline hydrolysis is a more comprehensive bound phenolics extraction
technique than acid hydrolysis [60,63,64]. This might be attributable to the fact that alkaline
hydrolysis has the ability to split the ester bonds between phenolic acid and polysaccharide,
and decrease phenolic acids losses [65,66]. However, acid hydrolysis mainly breaks glycosidic bonds. Contrary to the abovementioned, far more bound phenolic compounds were
released by acid hydrolysis from litchi pulp extracts [67] and apple and peach [68], which
for the early season varieties “Gergana,” “Filina,” and “Ufo 4” (free phenolics fractions).
obviously indicates the need for optimal extraction conditions in each particular study.
The variation in the food matrices, as well the difference in the bond types of the bound
phenolics, should have an effect on the extraction process. Additionally, Verma et al. [69]
stated that elevated-temperature acid hydrolysis resulted in the loss of some phenolics. This
may reason the better efficacy of the alkaline hydrolysis in the release of bound phenolics
from the evaluated peach peels compared to acidic hydrolysis.
3.3. Antioxidant Activity (AOA)
The potential to recover the antioxidant activity of the peach peels is reported for
locally grown peach varieties [70,71]. Moreover, authors revealed peach peels with higher
Antioxidants 2023, 12, 205
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antioxidant activity in comparison to the pulp, suggesting that the fruit peel is a nutrient
carrier [70].
In the current study, the peach variety, the type of extraction, and their combination
had impact (p < 0.05) on the AOA (DPPH, ABTS, FRAP, and CUPRAC assays) as studied
by two-way ANOVA. The assessed in vitro antioxidant potential of the investigated peach
peels is the strongest with regard to the free phenolics fractions (Figure 2). According to
the DPPH and FRAP assays, the most potential extract was the free phenolics one, and the
one of the late-season “Morsiani 90” variety, in particular. The free phenolic fraction of
“Morsiani 90” was the most active one according to the CUPRAC assay as well, although
several alkaline extracts had good prospective (Figure 2D). The antioxidant potential
towards the DPPH free radical was in the range of 1.94 ± 0.04 to 43.43 ± 0.28 µMTE/g dw
(Figure 2A). The FRAP assay showed values from 5.14 ± 0.18 to 123.08 ± 4.0 µMTE/g
dw (Figure 2C). According the CUPRAC assay, the results varied from 23.13 ± 0.84 to
122.97 ± 4.71 µMTE/g dw (Figure 2D). When the three abovementioned in vitro assays are
applied, the free phenolics extracts show the greatest potential, significantly different from
the bound phenolic fractions. The ABTS assay, for example, was revealed as most active for
the early season varieties “Gergana,” “Filina,” and “Ufo 4” (free phenolics fractions).
The results for the bound phenolic compounds released by acid hydrolysis were
substantially lower than those obtained by alkaline hydrolysis for most of the varieties. The
discrepancy in the antioxidant activity results confirms the need for more than one assay to
be applied, bearing in mind the different aspects of the antioxidant activity mechanism and
contributing compounds, in particular.
While retrieving the bound phenolic fractions, the conducted alkaline and acid hydrolyses resulted in less antioxidant activity in most of the peach peel extracts. Furthermore,
some authors correlate the total phenolic content and antioxidant potential decrease with
ripening. Nonetheless, more than 50% of the total antioxidant activity is contributed by the
extractable polyphenols [54]. However, the assessed activity is an important contribution
to the general antioxidant potential of the whole peach fruit, and the peels in particular. In
general, the alkaline and acid hydrolyzed fractions showed moderate activity, and no clear
trend could be pointed out. This certainly outlined the need for individual interpretation
of the results for each particular variety.
Tang et al. [59] reported the better potential of the bound phenolics extracts of the pitahaya peel when the alkaline hydrolysis method was applied. More specifically, the authors
established good correlation of the antioxidant activity and the highest phenolic content
achieved. Furthermore, the paper revealed that hydrolysis methods had a significant effect
on the release of phenolics, in preference of the alkaline method.
The present study is validating the fruit wastes potential and fruit peels, in particular.
Therefore, the question set to peel or not to peel [72] seems to have an answer. Peach peels
are worth researching and using, as the latter contribute to a more beneficial absorption of
compounds when the unpeeled fruit is consumed.
3.4. Inhibitory Potential towards α-Glucosidase, α-Amylase, Lipase, and Acetylcholinesterase of
Analyzed Prunus Persica Peels
Based on the results presented in Table 1 (GC-MS profile of the peels), indicating a
possible inhibitory potential, extracts from the peels were analyzed for inhibitory activity
against α-glucosidase, lipase, α-amylase, and acetylcholinesterase (Table 2). The results are
expressed as the concentration in mg/mL that inhibits 50% of the corresponding enzyme
activity. The current finding may be explained by the specific metabolite profile of the peels
(Table 1). For example, the fatty acids content, which is relatively high in the studied peel
extracts, may be the reason for the inhibitory potential towards acetylcholinesterase.
Antioxidants 2023, 12, 205
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Free
Acid hydrolysis
Alkaline hydrolysis
50
45
AOA, µM/g dw
40
b
e
d
d
e
35
a
b
f
30
25
20
a
b
15
10
jl
5
jl
n
n
G
F
m
jl
kl l
JL
L
mm
i
m
ij jk
0
U
FQ
Evm
M
(A)
9000
AOA, µM/g dw
8000
7000
6000
Free
Acid hydrolysis
Alkaline hydrolysis
a
b
c
cd
d
e
e
5000
4000
ij
3000
jk
m
mn
2000
f8
f
gh
hi
m kl
m6
m
U
JL
f
g
nm
m
Evm
M
1000
0
G
F
L
FQ
(B)
Free
Acid hydrolysis
Alkaline hydrolysis
140
AOA, µM/g dw
120
b
b
100
80
c
60
f
g
h
40
20
ce
de
e
e
o
n
lm
o
mn
a
kl
lm lm
JL
L
k kl
j
ij hi
0
G
F
U
(C)
Figure 2. Cont.
FQ
Evm
M
Antioxidants 2023, 12, 205
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Free
Acid hydrolysis
Alkaline hydrolysis
140
b
120
e
AOA, µM/g dw
cd
100
f
f
80
60
bc
h
fg
j
ij
j
bc
bc
de
a
ij
i
k
m
40
n
n
kl
lm
20
0
G
F
U
JL
L
FQ
Evm
M
(D)
Antioxidant
activity
of and
free bound
and bound
phenolics
in eight
peach
varieties’
peel
extracts
Figure 2. Antioxidant
activity
of free
phenolics
in eight
peach
varieties’
peel
extracts
(µMTE/g dw) by (A) DPPH, (B) ABTS, (C) FRAP and (D) CUPRAC assays. Different letters (a–n)–
within chart columns indicate significant differences (p < 0.05) between treatments as analyzed by
—“Gergana”, F—“Filina”, U—“Ufo 4”, JL—“July
two-way ANOVA and the Tukey test (n = 3). G—“Gergana”, F—“Filina”, U—“Ufo 4”, JL—“July
Lady”, L—“Laskava”, FQ—“Flat queen”, Evm—“Evmolpiya”, M—“Morsiani 90”.
Lady”, L—“Laskava”, FQ—“Flat queen”, Evm—“Evmolpiya”, M—“Morsiani 90”.
Table 2. Enzyme-inhibitory activities (α-glucosidase, lipase, α-amylase, and acetylcholinesterase
(AChE)) of free and bound phenolics of eight peach varieties’ peel extracts, IC50 , mg/mL.
Samples
α-Glucosidase
Lipase
α-Amylase
AChE
G
Free
Acid hydrolysis
Alkaline hydrolysis
3.4 ± 0.11 k
-
26.67 *
26.67 *
F
Free
Acid hydrolysis
Alkaline hydrolysis
9.0 ± 0.13 h
-
31.51 *
20.55 ± 0.51 a
31.1 ± 0.22 a
U
Free
Acid hydrolysis
Alkaline hydrolysis
39.7 ± 0.05 b
22.9 ± 0.14 d
5.9 ± 0.04 i
-
15.17 *
15.17 *
JL
Free
Acid hydrolysis
Alkaline hydrolysis
26.1 ± 0.54 c
5.1 ± 0.13 j
-
17.38 ± 0.11 b
21.3 *
17.8 ± 0.52 b
L
Free
Acid hydrolysis
Alkaline hydrolysis
27.2 ± 0.11 c
14.1 ± 0.09 f
9.0 ± 0.10 h
-
31.51 *
-
30.23 *
FQ
Free
Acid hydrolysis
Alkaline hydrolysis
49.3 ± 0.22 a
3.1 ± 0.10 k
-
-
-
Evm
Free
Acid hydrolysis
Alkaline hydrolysis
19.4 ± 0.12 e
2.7 ± 0.08 l
-
-
56.36 *
-
M
Free
Acid hydrolysis
Alkaline hydrolysis
α Glucosidase,
α
55.67 *
11.6 ± 0.14 g
l
2.6
±
0.08
Based on the results presented in Table 1 (GC−MS profile of the peels), indicating a
“-” not detected; Different letters in the same column indicate statistically significant differences (p < 0.05), according to ANOVA (one-way) and the Tukey test (n = 3). *—a concentration that inhibits 30% of the corresponding
glucosidase,
lipase,
α
enzymeagainst
under theαdescribed
conditions.
G—“Gergana”,
F—“Filina”, U—“Ufo 4”, JL—“July Lady”, L—“Laskava”,
FQ—“Flat queen”, Evm—“Evmolpiya”, M—“Morsiani 90”.
Antioxidants 2023, 12, 205
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The samples that are marked with “-” are not active, or it is impossible to calculate IC50 .
None of the extracts were able to inhibit the action of lipase. The action of alpha-glucosidase
was suppressed by most of the samples. All extracts of “Ufo 4” and “Laskava” peels were
active, resulting in IC50 concentration in the range of 5.9 to 39.7 mg/mL. The alkaline
hydrolysis of bound phenolics of the “Morsiani 90” sample seems to possess the best
inhibitory activity toward alpha-glucosidase—2.6 mg/mL. The alkaline hydrolysates of
“Filina” and “July Lady” samples were both able to inhibit α-amylase, at IC50 —20.55 ± 0.51
and 17.38 ± 0.11, respectively, and AChE at IC50 —31.1 ± 0.22 and 17.8 ± 0.52, respectively.
3.5. Antimicrobial Activity of Peach Peel Extracts
It has been proposed that phenolic compounds (i.e., flavonoids and phenolic acids)
can exhibit antimicrobial properties [73]. Thus, the antimicrobial activity of the peach peel
extracts was evaluated (Table 3) as an indicator of the biological potential of the peach
peels. No particularly high inhibition was detected to calculate the minimal inhibitory
concentration (MIC). However, the antibacterial potential is valuable for fruits in order to
recover injuries and/or to have prolonged shelf life [74]. In respect of both Gram-positive
and Gram-negative bacteria and yeasts, the free phenolics fraction showed no activity.
The inhibitory effect of the bound phenolics was more pronounced against Bacillus subtilis
ATCC 6633, Listeria monocytogenes NBIMCC 8632, Pseudomonas aeruginosa ATCC 9027, and
Saccharomyces cerevisiae ATCC 9763. Other authors have reported the antibacterial activity
of Prunus persica varieties to be more sensitive against Staphylococcus aureus and Listeria
monocytogenes [75]. This is consistent with the current findings, as well as their relation
to the polyphenolic content and their antioxidant activity. Koyu et al. [76] also reached
a minimum inhibitory activity for Prunus leaves against Escherichia coli, Staphylococcus
aureus, Staphylococcus epidermidis, Enterococcus faecalis, Enterococcus faecium, and Candida
albicans. In line with the reports of Mocanu et al. [77], the better reaction towards B. subtilis
may be a result of elevated flavonoid concentrations. Molecular weight, polarity, and side
groups command the specific inhibitory effect of each phenolic compound [78]. Phenolic
compounds, such as acids (ferulic acid, p-coumaric acid, among others), alcohols (guaiacol,
catechol, vanillyl alcohol), and aldehydes (vanillin, syringaldehyde), are regarded as the
most potent inhibitors of microbial growth [79]. Ferulic and p-coumaric acids are the
second and third most abundant in the studied samples after the chlorogenic acid (Table 3).
Organic acids may be responsible for their activity against Gram-positive and Gramnegative bacteria, due not only to their quantity and diverse biochemical nature, but also
their ability to lower pH [80].
Regarding the antifungal activity of the peach extracts, results show limited activity
(Table 3). For instance, the bound phenolics fractions of all varieties possess better activity
compared to the free phenolics fractions. The peach peels show better activity toward
Rhizopus sp. and Fusarium moniliforme.
None of the samples inhibited the growth of the fungi Mucor sp., and as regarding
A. flavus, only bound phenolic extracts under alkali conditions exhibit activity (inhibition
zones of 8 mm). Like in other reports [81], the current findings may suggest a link between
the antimicrobial activity and the fatty acid and flavonoid content of the studied extracts.
Antioxidants 2023, 12, 205
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Table 3. Effect of free and bound phenolics of eight peach varieties’ peel extracts on antimicrobial
potential toward bacteria, yeast and fungi.
Sample (Inhibition Zone, mm) *
Test Microorganism/
Samples
G
F
U
JK L
FQ EvmM G
F
Free
U
JK L
FQ EvmM G
Acid Hydrolysis
F
U
JK L
FQ EvmM
Alkaline Hydrolysis
Gram (+) bacteria
Bacillus subtilis
ATCC 6633
Staphylococcus aureus
ATCC 25923
Listeria monocytogenes
NBIMCC 8632
Enterococcus faecalis
ATCC 19433
-
-
-
8
-
-
8
8
9
10 10 10 10 10 10 9
10 11 9
9
10 10 9
10
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
8
9
8
8
-
-
-
-
-
-
-
-
-
-
-
-
8
8
8
8
8
9
9
8
8
9
8
9
9
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
11 -
8
-
-
-
-
Gram (−) bacteria
Salmonella
enteritidis ATCC 13076
Escherichia coli
ATCC 8739
Proteus vulgaris
ATCC 6380
Pseudomonas
aeruginosa
ATCC 9027
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
9
-
8
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
8
-
8
-
-
-
-
11 11 10 12 12 9
9
-
12 12 12 15 15 9
9
-
13 14 13 13 13 8
-
9
Yeasts
Candida albicans
NBIMCC 74
Saccharomyces
cerevisiae ATCC 9763
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
8
8
-
-
-
9
9
8
-
-
-
-
-
-
-
-
-
-
-
-
-
-
8
8
8
8
-
-
-
8
8
8
8
8
9
9
8
8
8
8
8
8
8
8
8
9
9
8
9
-
8
9
-
8
9
-
8
-
8
8
-
Fungi
Aspergillus niger
ATCC 1015
Aspergillus flavus
Penicillium sp.
Rhizopus sp.
Fusarium moniliforme
ATCC 38932
Mucor sp.
-
-
-
-
-
-
-
-
8
8
8
8
- - - - 10 10 -
- - - - 10 10 -
-
-
9
8
9
8
8
-
- - 9 9 8
10 10 -
- - 8
8 8 9
10 10 9
-
-
-
-
-
-
-
-
8
8
8
9
9
8
8
8
10 10 10 10 10 -
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
* dwell = 6 mm; G—“Gergana”, F—“Filina”, U—“Ufo 4”, JL—“July Lady”, L—“Laskava”, FQ—“Flat queen”,
Evm—“Evmolpiya”, M—“Morsiani 90”.
3.6. Correlation between Phenolic Compounds Content and Antioxidant Activity
Several components are contributing to the antioxidant activity, usually such of phenolic nature. The carried correlation analysis is based on the Pearson correlation coefficient,
also referred to as Pearson’s r, in order to express the strength and direction of the linear
relationship of correlation. Total phenolics compounds content was positively and significantly correlated to all of the antioxidant assays (r = 0.5059–0.8856, p ≤ 0.01). The positive
relationship between total phenolics content and antioxidant activity was also previously
stated [59,82]. No significant correlation between TPC and TFC was observed (r = 0.3249,
p > 0.05). No significant correlation was established between TFC and TMA, or between
TFC and ABTS either, pointing out that the contribution of the TFC is relatively low to the
established activities. Previous reports also [59] stated no significant correlation between
TFC and the antioxidant activities in fruit peel extracts. The relatively low content of TFC in
the free and bound fractions is possibly contributing to the weak or non-existent correlation
between TFC and the other assays.
Among the antioxidant activity assays, the ABTS assay has a moderate correlation,
significant at p ≤ 0.05, to the FRAP and CUPRAC. The ABTS was also significantly correlated to another antiradical assay (DPPH assay) at p ≤ 0.01. This pointed to the ABTS
bound phenolics of eight peach varieties’ peel extracts.
Antioxidants 2023, 12, 205
16 of 22
assay as a not very appropriate method by which to evaluate the potential of the particular
free and bound phenolics in peach peels. The strongest positive correlation was observed
between the DPPH and FRAP assays, and between the TPC and DPPH (Table 4).
Table 4. Pearson correlation matrix of phenolic compounds and antioxidant activity of free and
bound phenolics of eight peach varieties’ peel extracts.
Variables
TPC
TFC
TPC
TFC
TMA
DPPH
ABTS
FRAP
CUPRAC
1
0.3249 ***
1
TMA
DPPH
ABTS
FRAP
CUPRAC
0.5957 *
0.8856 *
0.5059 *
0.8231 *
0.6251 *
** correlation
is ***
significant
****
0.0608 ***≤ 0.01;
0.5701
*
0.1461
0.7241at* ≤ 0.05;
0.5234
1
0.6286 *
0.5618 *
0.4695 *
0.3833 *
1
0.4303 *
0.9562 *
0.7453 *
1
0.2979 ** of 0.2376
**
GC−MS
1
0.7155 *
1
TPC, total phenolics content; TFC, total flavonoids content; TMA, total monomeric anthocyanins; DPPH, antioxidant activity determined by the DPPH assay; ABTS, antioxidant activity determined by the ABTS assay; FRAP,
ferric reducing antioxidant power; CUPRAC, cupric ion reducing antioxidant capacity. * Correlation is significant
at p ≤ 0.01; ** correlation is significant at p ≤ 0.05; *** correlation is not significant (p > 0.05).
3.7. Principal Component Analysis (PCA) and Hierarchical Cluster Analysis (HCA) of GC-MS
and Phenolic Compound and AOA Assays Data
In order to verify the sample differences or resemblances, principal component analysis
(PCA) and hierarchical cluster analysis (HCA) of the volatile compounds identified were
High positive
loadtoscores
in PC1
which
“Laskava”
fromPC1
the
utilized.
According
the PCA
plot (Figure
(Figure 3),
3), the
firstdistinguish
two principal
components
(33.1%) and PC2 (18.6%) summed up 51.7% of the total variance of all identified volatile
negative
scoresininthe
PC1
clearly differentiate
“Filina”
thethe
others.
“Gercompounds
analyzed
peach peels. the
When
takingpeels
into from
account
totalThe
flavonoid
content,
phenolic
content,
total
monomeric
anthocyanins,
gana”
peelstotal
stood
out from
the rest
by the
high negative
scores and the antioxidant assays
applied, the PCA plot reveals 50% of the total variance in the analyzed samples, namely
27.5% for PC1 and 22.5% for PC2.
(A)
(B)
Figure 3. Principal component score plot (A) and eigenvector load values (B) of GC-MS data of
volatile compounds of peach (Prunus persica L.) peels for the eight peach peel varieties.
Antioxidants 2023, 12, 205
17 of 22
High positive load scores in PC1 (Figure 3), which distinguish “Laskava” from the
other studied peels, are shown by quinic acid, oleic acid, and fructose isomer 2. The high
negative scores in PC1 clearly differentiate the “Filina” peels from the others. The “Gergana”
peels stood out from the rest by the high negative scores of a number of amino acids in
PC2.
Figure 4A,B reveal the PCA score plots of TPC, TMA, TFC, and AOA assays of
peach (Prunus persica L.) peels from the studied samples. The content of total monomeric
anthocyanins, as well as the FRAP antioxidant assay, can be characterized
as important
) of GC−MS
data of
for the varieties “Laskava” and “July Lady,” due to their high values. The total flavonoid
content, on the other hand, is less dependent for the “Morsiani 90” peels. The ABTS values
are not defining for the peels of the “Flat Queen” variety.
The peels of the two nectarine varieties (“Gergana” and “Morsiani 90”) were grouped
in the same cluster, due to their phytochemical similarity, while “July Lady” and “Laskava”
were clustered in another. The results from the HCA show significant differences from
the ones established for the whole fruit of the same varieties [83]. A clear distinction
varieties “Laskava”
and “July Lady,” due to their high values. The total flavonoid content,
of nectarines compared to peach and flat peach peels is shown in the current results
on the other
hand,
less dependent
for thethe
“Morsiani
90” content,
peels. The
values
are not
(Figure
5A).isWhen
taking into account
total flavonoid
totalABTS
phenolic
content,
the “Flat Queen”
variety. assays applied, the heatmap (Figure 5B)
total monomericfanthocyanins,
and the antioxidant
shows that peaches from the same ripening period are clustered together.
(A)
(B)
Figure 4. Principal component score plot (A) and eigenvector load values (B) of TPC, TMA, TFC, and
AOA assays of peach (Prunus persica L.) peels for the eight peach peel varieties.
The statistically independent variables (assays or compounds) can be drawn from the
heatmaps (Figures 4 and 5). Peels from each variety are characterized with different linear
The peels
of the two
nectarine
(“Gergana”
and
“Morsiani
90”)
were grouped
relationships.
Considering
thevarieties
clade arrangement
in the
Figures,
it can be
assumed
that
in the same
cluster,
due
to
their
phytochemical
similarity,
while
“July
Lady”
and
phenolic compounds and antioxidant assays lead to more distinct differences between
the“Lasestablished
clades.
kava” were
clustered
in another. The re
Antioxidants 2023, 12, 205
18 of 22
(A)
(B)
Figure 5. Heatmap of the clustering result of peach peels from eight varieties. (A) GC-MS data of volatile
) GC−MS
of
compounds and (B) TPC, TMA, TFC, and AOA assays. Values were normalized
by log10data
transformation.
4. Conclusions
This study represents new information concerning the phytochemical peculiarities of
peels from four native Bulgarian peach varieties and four introduced in the geographical
region of the Thracian valley. Information about the GC-MS metabolites can shed more
light on the studied biological activities. The current findings are one of the few reports
on the topic of free and bound phenolics in several peach peel varieties, including flat
peaches, peaches, and nectarines. The results are consistent with other existing literature
stating that alkaline hydrolysis is a better extraction approach for distributing bound
phenolics. The results also indicate the prevalence of the free phenolics in the studied peach
peel varieties. The present findings confirm, yet again, the well-known facts about the
health-promoting properties of polyphenols, and the fact that fruit by-products can provide
potential accessible sources of antioxidants for direct consumption. Furthermore, peach
peels could be considered useful natural sources of bioactive compounds with prospective
activities. In any case, they are worth contemplating for waste recovery. This study can be
seen as a stepping stone in the context of functional foods enriched with natural extracts
obtained through effective extraction.
Antioxidants 2023, 12, 205
19 of 22
Author Contributions: Conceptualization, D.M. and A.P.; methodology, D.M., I.D. (Ivelina Desseva),
Y.T. and I.D. (Ivayla Dincheva); software, D.M. and A.P.; validation, D.M., A.P. and I.D. (Ivelina
Desseva); formal analysis, D.M., I.D. (Ivelina Desseva), Y.T. and I.D. (Ivayla Dincheva); investigation,
D.M., I.D. (Ivelina Desseva), Y.T., A.P. and I.D. (Ivayla Dincheva); resources, D.M. and A.P.; data
curation, D.M. and A.P.; writing—original draft preparation, D.M. and A.P.; writing—review and
editing, D.M., A.P., I.D. (Ivelina Desseva), Y.T. and I.D. (Ivayla Dincheva); visualization, D.M. and
A.P.; supervision, D.M.; project administration, D.M.; funding acquisition, D.M. All authors have
read and agreed to the published version of the manuscript.
Funding: This research and the APC were funded by the Bulgarian National Science Fund, grant
number KΠ-06-H37/23.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data presented in this study are available on request from the
corresponding author.
Acknowledgments: The results presented in this work were supported by the Bulgarian National
Science Fund, project no. KΠ-06-H37/23 (granted to Dasha Mihaylova). The authors are grateful
to Argir Zhivondov and his team from the Fruit Growing Institute, Plovdiv (Bulgaria), for kindly
providing the peach samples.
Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or
in the decision to publish the results.
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