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\DOI{10.5802/crchim.446}
\datereceived{2025-10-16}
\daterevised{2026-01-27}
\dateaccepted{2026-02-13}
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\section*{Declaration of interests}
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\COI{The authors do not work for, advise, own shares in, or receive
funds from any organization that could benefit from this article, and
have declared no affiliation other than their research organizations.}

\dateposted{2026-04-23}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\title{Fatty acid composition, total phenols, and antioxidant capacity
of grape seed oil (\textit{Vitis vinifera} L.) extracted from different
varieties}

\alttitle{Composition en acides gras, teneur totale en compos\'{e}s
ph\'{e}noliques et capacit\'{e} antioxydante de l'huile de p\'{e}pins
de raisin (\textit{Vitis vinifera} L.) extraite de diff\'{e}rentes
vari\'{e}t\'{e}s}

\author{\firstname{Stamber Alvaro} \lastname{Ram\'{i}rez-Revilla}\CDRorcid{0000-0003-3133-3353}\IsCorresp}  
\address{Laboratorio de Calidad de Aguas y Medio Ambiente del Instituto
de Energ\'{i}as Renovables de la Universidad Tecnol\'{o}gica del
Per\'{u}. Av. Tacna y Arica 160, Arequipa, Peru}
\email[S. A. Ram\'{i}rez-Revilla]{sramirezr@utp.edu.pe}

\author{\firstname{Daniela} \lastname{Camacho-Valencia}\CDRorcid{0000-0003-3672-6420}}  
\addressSameAs{1}{Laboratorio de Calidad de Aguas y Medio Ambiente del
Instituto de Energ\'{i}as Renovables de la Universidad Tecnol\'{o}gica
del Per\'{u}. Av. Tacna y Arica 160, Arequipa, Peru}

\author{\firstname{Ariadne\nobreakauthor Maria}\nobreakauthor\lastname{Tacusi-Oblitas Taco}\CDRorcid{0009-0008-9335-7784}}   
\addressSameAs{1}{Laboratorio de Calidad de Aguas y Medio Ambiente del
Instituto de Energ\'{i}as Renovables de la Universidad Tecnol\'{o}gica
del Per\'{u}. Av. Tacna y Arica 160, Arequipa, Peru}

\author{\firstname{Juan Jos\'{e}} \lastname{Mil\'{o}n Guzm\'{a}n}\CDRorcid{0000-0001-7271-0931}}  
\addressSameAs{1}{Laboratorio de Calidad de Aguas y Medio Ambiente del
Instituto de Energ\'{i}as Renovables de la Universidad Tecnol\'{o}gica
del Per\'{u}. Av. Tacna y Arica 160, Arequipa, Peru}

\shortrunauthors

\keywords{\kwd{Grape seed oil}\kwd{Fatty acids}\kwd{Total phenolic
compounds}\kwd{Antioxidant capacity}}

\altkeywords{\kwd{Huile de p\'{e}pins de raisin}\kwd{Acides
gras}\kwd{Compos\'{e}s ph\'{e}noliques totaux}\kwd{Capacit\'{e}
antioxydante}}

\begin{abstract}
This study aimed to valorize grape seed residues from the wine industry
by characterizing cold-pressed grape seed oil obtained from five
\textit{Vitis vinifera} L. varieties (``Malbec'', ``Syrah'',
``Italia'', ``Moscatel'', and ``Criolla'') collected in Arequipa, Peru.
The structural and chemical characteristics of the grape seeds were
analyzed using Fourier-transform infrared spectroscopy (FTIR) and
scanning electron microscopy coupled with energy-dispersive X-ray
spectroscopy (SEM--EDS). The fatty acid composition of the recovered
oil was determined by gas chromatography--mass spectrometry (GC--MS),
total phenolic compounds (TPC) were quantified using the
Folin--Ciocalteu method and antioxidant capacity was evaluated by the
2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging assay.
Microbiological quality and a preliminary cost analysis were also
assessed. The average oil yield obtained by cold pressing was 9.06\%
(90.64 g/kg of seeds). FTIR analysis confirmed the presence of
functional groups mainly associated with lipids (C${=}$O and C--H) and
phenolic compounds (O--H), while GC--MS revealed that the oil was
mainly composed of linoleic acid ethyl ester (64.40\%), ethyl oleate
(11.83\%), and hexadecanoic acid ethyl ester (10.14\%). Among the
evaluated varieties, Syrah oil exhibited the highest total phenolic
content (143.84 mg gallic acid equivalent/kg) and the highest
antioxidant activity. Overall, DPPH radical scavenging values ranged
from 33\% to 46\% among the analyzed oils. No microbiological growth
was detected in the oil samples. The estimated production cost of grape
seed oil was 25.58 USD per liter. These findings indicate that grape
seed residues represent a sustainable source of value-added oil with
antioxidant properties, which supports their potential application
within a circular economy framework.
\end{abstract}

\begin{altabstract}
Cette \'{e}tude visait \`{a} valoriser les r\'{e}sidus de p\'{e}pins de
raisin issus de l'industrie vinicole par la caract\'{e}risation d'une
huile de p\'{e}pins de raisin obtenue par pression \`{a} froid \`{a}
partir de cinq vari\'{e}t\'{e}s de \textit{Vitis vinifera} L.
(``Malbec'', ``Syrah'', ``Italia'', ``Moscatel'' et ``Criolla'')
collect\'{e}es \`{a} Arequipa, P\'{e}rou. Les caract\'{e}ristiques
structurales et chimiques des p\'{e}pins de raisin ont \'{e}t\'{e}
analys\'{e}es par spectroscopie infrarouge \`{a} transform\'{e}e de
Fourier et par microscopie \'{e}lectronique \`{a} balayage coupl\'{e}e
\`{a} la spectroscopie de rayons X \`{a} dispersion d'\'{e}nergie. La
composition en acides gras de l'huile obtenue a \'{e}t\'{e}
d\'{e}termin\'{e}e par chromatographie en phase gazeuse coupl\'{e}e
\`{a} la spectrom\'{e}trie de masse (GC-MS), les compos\'{e}s
ph\'{e}noliques totaux ont \'{e}t\'{e} quantifi\'{e}s par la
m\'{e}thode de Folin-Ciocalteu et la capacit\'{e} antioxydante a
\'{e}t\'{e} \'{e}valu\'{e}e par le test de pi\'{e}geage du radical
2,2-diphenyl-1-picrylhydrazyl (DPPH). La qualit\'{e} microbiologique
ainsi qu'une analyse pr\'{e}liminaire des co\^{u}ts ont \'{e}galement
\'{e}t\'{e} \'{e}valu\'{e}es. Le rendement moyen en huile obtenu par
pression \`{a} froid \'{e}tait de 9,06 \% (90,64 g/kg de p\'{e}pins).
L'analyse FTIR a confirm\'{e} la pr\'{e}sence de groupes fonctionnels
caract\'{e}ristiques, tandis que l'analyse GC-MS a r\'{e}v\'{e}l\'{e}
que l'huile \'{e}tait principalement compos\'{e}e d'ester \'{e}thylique
de l'acide linol\'{e}ique (64,40 \%), d'ol\'{e}ate d'\'{e}thyle (11,83
\%) et d'ester \'{e}thylique de l'acide hexad\'{e}cano\"{i}que (10,14
\%). Parmi les vari\'{e}t\'{e}s \'{e}valu\'{e}es, l'huile de Syrah a
pr\'{e}sent\'{e} la teneur la plus \'{e}lev\'{e}e en compos\'{e}s
ph\'{e}noliques totaux (143,84 mg d'\'{e}quivalents d'acide 
gallique/kg) ainsi que l'activit\'{e} antioxydante la plus
\'{e}lev\'{e}e. Globalement, les valeurs de pi\'{e}geage du radical
DPPH des huiles analys\'{e}es variaient de 33 \`{a} 46 \%. Aucune
croissance microbiologique n'a \'{e}t\'{e} d\'{e}tect\'{e}e dans les
\'{e}chantillons d'huile. Le co\^{u}t de production estim\'{e} de
l'huile de p\'{e}pins de raisin \'{e}tait de 25,58 USD par litre. Ces
r\'{e}sultats indiquent que les r\'{e}sidus de p\'{e}pins de raisin
constituent une source durable d'huile \`{a} valeur ajout\'{e}e
pr\'{e}sentant des propri\'{e}t\'{e}s antioxydantes, soutenant ainsi
leur potentiel d'application dans le cadre d'une \'{e}conomie
circulaire.
\end{altabstract}

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\section{Introduction}\label{sec1}

Grapes are widely consumed both in their fresh form and as processed
products, and they represent one of the most extensively cultivated
fruits worldwide. More than 80\% of global grape production is destined
for winemaking~\cite{1,2,3}. Despite its economic \mbox{relevance,} the wine
industry traditionally follows a linear production model in which raw
materials are processed, consumed, and subsequently discarded as
waste~\cite{4}. It is estimated that the production of 750 L of wine
requires approximately one ton of grapes, generating substantial
amounts of by-products such as stems, skins, and seeds. Grape pomace
alone accounts for nearly 60\% of the total waste generated during
winemaking~\cite{5,6}.

The accumulation of winery by-products reflects inadequate waste
management practices and poses significant environmental concerns.
Consequently, the sustainable valorization of these residues has gained
increasing attention, as it offers an opportunity to transform
production losses into value-added products. Within this context, grape
pomace represents a promising resource for circular-economy strategies.
Grape seeds constitute approximately 38\% of the pomace and can be
effectively reused as a raw material, which enables the recovery of
valuable compounds and contributes to waste reduction while adding
economic value to the wine \mbox{industry~\cite{7,8,9,10}.}

Grape seeds from different varieties exhibit diverse nutritional and
functional properties, particularly due to their content in fatty acids
and phenolic compounds. When discarded without proper treatment, these
components represent a lost opportunity and may contribute to
environmental pollution. The extraction of grape seed oil constitutes
an effective strategy to exploit the full potential of these residues,
allowing their reuse as raw materials for the development of products
across various industrial sectors. Such an approach supports
circular-economy principles by promoting resource efficiency and
contributing to zero-waste objectives~\cite{11,12}.

Grape seeds are recognized as an important source of bioactive
phytochemicals, with an oil content ranging from 8\% to 20\%. Grape
seed oil is characterized by its high content in unsaturated fatty
acids, including linoleic and oleic acids, as well as minor
constituents such as tocopherols and carotenoids, which confer
nutritional and functional benefits~\cite{13}. These properties make
grape seed oil attractive not only for dietary applications but also
for cosmetic formulations, where its antioxidant and emollient
characteristics are particularly valued.

In addition, grape seed oil exhibits notable antioxidant properties
associated with its phenolic compounds, vitamin E content, and
oligomeric proanthocyanidins. These characteristics have driven growing
interest in its application within the pharmaceutical, cosmetic, food,
biofuel, and chemical industries. Beyond enhancing product quality, the
utilization of grape seed oil contributes to the development of greener
production pathways by mitigating environmental impacts and promoting
sustainable resource management~\cite{14,15}.

While grape seed oil is valued for its favorable fatty acid composition
and antioxidant properties, it is important to acknowledge that
vegetable oils may also contain naturally occurring antinutritional
factors, such as phytic acid or tannins, depending on the botanical
source and processing conditions. These compounds may influence
nutrient bioavailability and product stability, highlighting the
importance of comprehensive characterization when considering
food-related applications~\cite{16}.

The objective of this study was to evaluate the valorization potential
of grape seed residues from the wine industry in the city of Arequipa
by assessing the oil yield, chemical composition, antioxidant capacity,
microbiological quality, and economic feasibility of cold-pressed grape
seed oil obtained from different grape varieties.

\section{Materials and methods}\label{sec2}
\subsection{Grape seed samples}\label{ssec21}

Seeds of five different varieties of \textit{Vitis vinifera} were used:
``Malbec'', ``Syrah'', ``Italia'', ``Moscatel'', and ``Criolla'', which
were collected from the company Majes Tradici\'{o}n located in the city
of Arequipa (Peru,  16\textdegree{}13$'$35.30$''$S 
72\textdegree{}27$'$04.49$''$W) after the process of obtaining pisco
and wine was completed. The seeds were dried at room temperature,
cleaned, and stored in airtight bags protected from light until
analysis. The seeds of each grape variety were ground and sifted for
spectroscopic analyses.

\subsection{Oil extraction from grape seeds}\label{ssec22}

Oils were extracted from 500~g of each seed variety using a screw press
driven by a variable speed electric motor with a screw rotating at 60
rpm. The oil was decanted, filtered, and stored at ${-}$10~\textdegree
C in amber glass bottles. 

\subsection{Fourier-transform infrared spectroscopy (FTIR)}\label{ssec23}

FTIR analyses were performed using a Nicolet iS10 mid-infrared
spectrophotometer. Spectra were recorded in the 4000--400 cm$^{-1}$
range in order to identify the main functional groups present in the
grape seed samples.

\subsection{Scanning electron microscopy--energy dispersive
spectroscopy (SEM--EDS)}\label{ssec24}

The surface morphology and pore diameter were evaluated using a Thermo
Fisher FEG-SEM Scios 2 scanning electron microscope. The morphology of
the samples was observed using a backscattered electron detector and a
low vacuum detector at a pressure of 50~Pa and a voltage of 10~kV. 

For chemical analysis, a Thermo Fischer Ultra-Dry energy dispersive
spectroscopy (EDS) detector was used. The sifted seed samples were
prepared as briquettes, mounted on an aluminum sample holder using
conductive carbon tape, and subsequently coated with gold using a
sample metallizer. They were blown with compressed air to prevent the
release of dust particles, and then they were taken to the vacuum
chamber of the electron microscope for observation.

\subsection{Gas chromatography--mass spectrophotometry (GC--MS)}
\label{ssec25}
\subsubsection{Sample Preparation}\label{sssec251}

Once the oils from each grape seed variety were obtained, a pool was
prepared with equal quantities f the five varieties. The oil sample
(pool) was dried with anhydrous sodium sulfate and filtered using 0.45
$\upmu$m syringe filters. Subsequently, 0.20~mL of the sample was
diluted with $n$-hexane in a 10~mL vial. The diluted 10~mL sample was
then transferred to a vial and placed in the GC--MS autosampler for
reading.

\subsubsection{GC--MS analysis}\label{sssec252}

To determine the composition of the obtained oils, the following were
used: a GC-2010 Plus gas chromatograph (from Shimadzu), a Shimadzu
AOC-6000 autosampler, a GCMS-QP210 Ultra mass spectrometry detector
(Shimadzu), GG Restek column, RTX-5MS, 30~m ${\times}$ 0.25~mm ID
${\times}$  0.25~$\upmu$m df, serial 1346249, carrier gas Helium UHP (5.0)
${\geq}$99.999\%, LS1-10~$\upmu$L syringe, SPL1 injector, injection flow 
$100~\upmu$L/min, injection volume 1.0~$\upmu$L, split type injection port, temperature
of 220~\textdegree C, column flow of 0.80~mL/min, purge flow 3~mL/min.
The temperature program applied was 50--150~\textdegree C with a
heating rate of  3~\textdegree C/min, then from 150 to 250~\textdegree
C with a heating rate of 3~\textdegree C/min, and finally
250~\textdegree C (total programmed time 126.67 min). The mass detector
was set up as follows: ion source temperature of 250~\textdegree C,
interface temperature of 290~\textdegree C, cut-off time of 2~min, MS
start time 3~min, ionization energy of 70~eV, acquisition mode Scan and
Scan range of 20--500~m/z. Identification of the components was
achieved by comparing the retention indices and the respective mass
spectra with those of the NIST mass spectra library (2014).

\subsection{Total phenolic compounds (TPC)}\label{ssec26}

Total phenolic compounds of the seed oil samples were determined using
the Folin--Ciocalteu colorimetric method described by Singleton and
Rossi (1965) with certain modifications~\cite{17}. For the extraction
of phenolic compounds from each of the oils obtained, the procedures of
Bail \etal\ and Pardo \etal~\cite{18,19} were followed with some
modifications. Each sample (1~g) was mixed with 5~mL of $n$-hexane and
1~mL of methanol:water (MetOH:H\tsub{2}O) (60:40). The mixture was
vortexed, then centrifuged at 6000 rpm for 5 min, then extracted three
times with MetOH:H\tsub{2}O (60:40). All methanolic extracts were
combined and concentrated with heat until dry. Finally, the dry residue
was dissolved in 1~mL MetOH:H\tsub{2}O (60:40) and stored at 
4~\textdegree C in the dark. The same process was performed for the
five oil varieties. For the determination of total phenolic compounds,
0.3 mL of the extract was mixed with 0.5~mL of 2N Folin--Ciocalteu
reagent. After 5 min, 2~mL of a saturated sodium carbonate solution
(20\%) was added. The sample was incubated at 40~\textdegree C for 30
min in the dark. Finally, the absorbance was measured at 765 nm in a
UV--Vis spectrophotometer (Thermo scientific). For the quantification
of phenolic compounds, a calibration curve of gallic acid (4, 6, 8, 10,
12 mg/L) was prepared ($R^{2}=0.9984$). The results were expressed in
mg of gallic acid equivalent (GAE) per kilogram of oil (mg GAE/kg).

\subsection{Antioxidant activity}\label{ssec27}

The DPPH assay was selected as a rapid and widely used method to
provide an initial estimation of the antioxidant capacity of grape seed
oils. The method described by Brand-Williams \etal~\cite{20} used in
different studies to determine the antioxidant capacity of different
plant extracts~\cite{21,22} was also followed as a reference. This
method evaluates the antioxidant activity of the methanolic phase or
polar fraction previously extracted from the grape seed oil samples.

The calibration curve was prepared with Trolox dilutions of 0.1, 0.2,
0.4, 0.6, 0.8, 1.0, and 1.2 mmol/L from a stock solution of 10 mmol/L
Trolox in methanol. Then, 0.150~mL of the different concentrations of
Trolox and 3~mL of the 1 mmol/L DPPH solution in methanol were mixed,
stirred for 30 s, and left in the dark at room temperature. The
absorbance at 520 nm was measured after 60 min. For the samples, 3~mL
of the DPPH radical were mixed with 0.150~mL of the methanolic extract
of the oil to be tested. The mixture was stirred and left in the dark
at room temperature, proceeding to measure the absorbance after 60~min.

Antioxidant activity is expressed as the percentage of inhibition or
percentage of free radical uptake, which corresponds to the amount of
DPPH radical neutralized by the extract at a given concentration, as
determined by the following equation:
{\begin{equation*}
(\%~\mathrm{inhibition}) = 
\dfrac{A_{\mathrm{negative~control}} - A_{\mathrm{sample}}}
      {A_{\mathrm{negative~control}}} \times 100
\end{equation*}}\unskip

\subsection{Microbiological analysis}\label{ssec28}

Samples of grape seed oil from each variety were collected and stored
in sterile amber glass bottles until analysis. For the microbiological
evaluation, 1~mL of each oil sample was aseptically transferred using a
micropipette and inoculated onto MC-Media Pad culture media. The
inoculated pads were incubated at 37~\textdegree C for 24 h. After
incubation, microbial growth was assessed by visual inspection and
colony counting using a digital colony counter. The same procedure was
applied to the pooled oil sample.

\subsection{Cost analysis}\label{ssec29}

A preliminary cost analysis was conducted to estimate the production
cost of grape seed oil obtained by cold pressing under laboratory-scale
conditions. A base quantity of 500 g of grape seeds was considered for
the calculation. The total cost included raw material handling, labor,
energy consumption, and equipment depreciation. Labor costs were
estimated assuming one operator working for 2 h during the extraction
process. Energy costs were calculated based on electricity consumption
(kWh) of the equipment used. Equipment depreciation was estimated
according to laboratory usage. The total cost was extrapolated to one
liter of oil produced.

\subsection{Statistical analysis}\label{ssec210}

All experiments were performed in triplicate and results were expressed
as averages with standard deviations. For this, ANOVA was used by
employing a significance level of $\alpha=0.05$.

\section{Results and discussion}\label{sec3}
\subsection{Extraction of oil from different varieties of grape seeds
(\emph{Vitis vinifera} L.)}\label{ssec31}

Grape pomace collected from the Majes Tradici\'{o}n company consisted
mainly of seeds, dried skins, and stems. The seeds were manually
separated from the remaining pomace, thoroughly cleaned, packaged,
labeled, weighed, and stored by variety in airtight bags prior to oil
extraction. Grape seed oil was extracted in triplicate for each variety
by cold pressing. Briefly, seeds from each variety were introduced into
the hopper of the cold-pressing machine, and the extracted oil was
collected. The oil was subsequently filtered using filter paper and
sterile gauze to remove solid impurities and stored at 4~\textdegree C
in amber glass containers to protect it from light and heat.

The extraction yield of grape seed oil was determined for each variety
(Figure~\ref{fig1}). Oil yields differed among grape varieties, with
Malbec showing the highest yield (13.69\%), followed by Criolla
(9.51\%), Moscatel (8.23\%), Italia (7.75\%), and Syrah, which
exhibited the lowest yield (6.15\%). The average oil yield was 9.07
${\pm}$ 2.85\%. These values fall within the oil content range of
8--20\% reported in the literature for grape
seeds~\cite{18,23,24,25,26,27,28}.

\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}Oil yield (\%) from grape seeds of different
\textit{Vitis vinifera} varieties (mean values, $n=3$).}
\end{figure}

\subsection{Fourier-transform infrared spectrophotometry (FTIR)}
\label{ssec32}

Fourier-transform infrared spectroscopy (FTIR) was used to obtain
information about the functional groups present in the biomass
(Figure~\ref{fig2}).

\begin{figure*}
\vspace*{-4pt}
\includegraphics{fig02}
\vspace*{-4pt}
\caption{\label{fig2}FTIR spectra of the different grape seed
varieties: (a) Malbec, (b) Syrah, (c) Italia, (d) Moscatel, (e)
Criolla.}
\vspace*{-4pt}
\end{figure*}

Figure~\ref{fig2} shows the FTIR spectra of grape seeds from the
different \textit{Vitis vinifera} varieties, which present very similar
spectral profiles, indicating comparable chemical structures among the
samples. In all analyzed varieties, a broad absorption band with a peak
around 3260~cm$^{-1}$ is observed, corresponding to O--H stretching
vibrations present in polysaccharides and/or lignins, as previously
reported~\cite{29,30,31}. A band around 3010~cm$^{-1}$ is related to
C--H stretching vibrations of \textit{cis}-double bond groups (${=}$CH),
while the bands between 2953 and 2852~cm$^{-1}$ are assigned to
CH\tsub{2} stretching vibrations associated with lipid chains or
lignins~\cite{32}.

The absorption band at approximately 1744~cm$^{-1}$ corresponds to
C${=}$O stretching vibrations of ester functional groups, confirming the
presence of fatty acids and their glycerides, as well as contributions
from pectins and lignins~\cite{33,34}. Bands observed around 
1600~cm$^{-1}$ are associated with aromatic C${=}$C stretching vibrations
and O--H bending modes, related to phenolic compounds and pectic
substances~\cite{33,35}. The fingerprint region between 1500 and
800~cm$^{-1}$ exhibits multiple overlapping bands from complex organic
constituents; however, due to its complexity, this region was not used
for detailed compound differentiation.

The FTIR analysis confirms the presence of functional groups associated
with lipids and phenolic compounds across all grape seed varieties,
indicating their suitability as a raw material for oil extraction.

\subsection{Scanning electron microscopy--energy dispersive
spectroscopy (SEM--EDS) analysis}\label{ssec33}

SEM micrographs of the grape seeds from the different \textit{Vitis
vinifera} varieties revealed a compact and irregular surface morphology
with a heterogeneous and rough texture (Figure~\ref{fig3}). Such
features reflect the complex structural organization of the seed
matrix. No pronounced morphological differences were observed among the
analyzed varieties.

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}SEM micrographs and corresponding EDS spectra of
grape seeds from different \textit{Vitis vinifera} varieties: (a)
Malbec, (b)~Syrah, (c) Italia, (d) Moscatel, and (e) Criolla.}
\end{figure*}

The corresponding EDS spectra indicated that carbon and oxygen were the
predominant elements in all samples, confirming the organic nature of
the grape seeds. Minor amounts of inorganic elements such as potassium,
calcium, nitrogen, phosphorus, and aluminum were also detected, which
can be attributed to naturally occurring mineral constituents of the
plant material. The elemental composition determined by EDS is
summarized in Table~\ref{tab1}.

%tab1
\begin{table}
\caption{\label{tab1}Elemental composition (wt\%) of grape seeds from
different \textit{Vitis vinifera} varieties determined by SEM--EDS}
\tabcolsep=3pt
\begin{tabular}{cccccccc}
\thead
& \multicolumn{7}{c}{Weight by element (\%)} \\ \cline{2-8}
Sample & C & O & K & Ca & N & Al & P \\ 
\endthead
Malbec &   45.21 & 50.73 & 1.63 & 2.43 & -   & -  & - \\ 
Syrah &    38.10 & 46.95 & -  & 2.18 & 12.77 & -  & - \\ 
Italia &   37.51 & 47.42 & 1.62 & -  & 13.45 & -  & - \\ 
Moscatel & 47.43 & 49.04 & 1.40 & -  & -   & 0.74 & 1.39 \\ 
Criolla &  37.04 & 48.69 & 2.33 & -  & 11.94 & -  & -
\botline
\end{tabular}
\tabnote{Note: C ${=}$\ carbon; O ${=}$\ oxygen; K ${=}$\ potassium; Ca ${=}$\
 calcium; N ${=}$\ nitrogen; Al ${=}$\ aluminum; P ${=}$\ phosphorus.}
\end{table}

\subsection{Gas chromatography--mass spectrophotometry
(GC--MS) analysis}\label{ssec34}

The fatty acid composition of the pooled grape seed oil sample was
analyzed by GC--MS, and the results are presented in Figure~\ref{fig4}
and Table~\ref{tab2}. The chromatographic analysis revealed the
presence of saturated, monounsaturated, and polyunsaturated fatty acid
derivatives, with a clear predominance of unsaturated compounds. The
main constituents of the oil were linoleic acid ethyl ester (49.64\%),
ethyl oleate (10.26\%), and hexadecanoic acid ethyl ester (10.00\%),
indicating that the oil is rich in essential and nutritionally relevant
fatty acids. Compound identification was performed by comparing
retention indices and mass spectra with data from the NIST mass
spectral library, using similarity index values between 85 and 100,
which indicates reliable identification of the detected components.

\begin{figure*}
\includegraphics{fig04}
\vspace*{-2pt}
\caption{\label{fig4}Chromatogram obtained for the pooled sample of
grape oils from mixed seeds.}
\end{figure*}

%tab2
\begin{table*}
\caption{\label{tab2}Chemical composition of the pooled sample of grape
oils from mixed seeds\vspace*{-2pt}}
\fontsize{9pt}{10.8pt}\selectfont
\begin{tabular}{ccccccccc}
\thead
Peak \# & Name & 
\parbox[t]{3.4pc}{\centering Retention time (min)} & 
Identification$^{\mathrm{a}}$ & 
\parbox[t]{3pc}{\centering Similarity index (SI)} & Area & 
\parbox[t]{2pc}{\centering Area (\%)} & Height & 
\parbox[t]{2.1pc}{\centering Height (\%)}\vspace*{2pt} \\ 
\endthead
\01 & Hexanal & \0\03.266 & RI, MS & 92 & \0\056\,209 & 1.76 & \026\,138 & 3.95 \\ 
\02 & \parbox[t]{7pc}{\centering Octanoic acid, ethyl ester}\vspace*{2pt} & \042.214 & RI, MS & 93 & \0\077\,542 & 2.43 & \016\,887 & 2.55 \\ 
\03 & 2,4-Decadienal & \048.148 & RI, MS & 92 & \0\079\,097 & 2.48 & \016\,871 & 2.55 \\ 
\04 & 2,4-Decadienal, (E,E)- & \049.351 & RI, MS & 95 & \0152\,739 & 4.79 & \030\,162 & 4.56 \\ 
\05 & \parbox[t]{7pc}{\centering Decanoic acid, ethyl ester}\vspace*{2pt} & \053.540 & RI, MS & 90 & \0\060\,984 & 1.91 & \015\,584 & 2.36 \\ 
\06 & \parbox[t]{7pc}{\centering Hexadecanoic acid, ethyl ester}\vspace*{2pt} & \090.057 & RI, MS & 95 & \0318\,703 & 10.00 & \047\,114 & 7.13 \\ 
\07 & \parbox[t]{7pc}{\centering Linoleic acid, ethyl ester}\vspace*{2pt} & \098.351 & RI, MS & 95 & 1582\,034 & 49.64 & 311\,245 & 47.08 \\ 
\08 & Ethyl oleate & \098.626 & RI, MS & 94 & \0327\,122 & 10.26 & \066\,012 & 9.98 \\ 
\09 & \parbox[t]{7pc}{\centering Octadecanoic acid, ethyl ester}\vspace*{2pt} & \099.793 & RI, MS & 90 & \0\086\,303 & 2.71 & \018\,174 & 2.75 \\ 
10 & Pentyl linoleate & 106.860 & RI, MS & 89 & \0118\,572 & 3.72 & \027\,935 & 4.23 \\ 
11 & Linolelaidic acid & 108.479 & RI, MS & 87 & \0190\,658 & 5.98 & \048\,237 & 7.30 \\ 
12 & \parbox[t]{7pc}{\centering 9-Octadecenoic acid, 1,2,3-propanetriyl ester, (E,E,E)-}\vspace*{2pt} & 108.636 & RI, MS & 86 & \0\058\,715 & 1.84 & \016\,610 & 2.51 \\ 
13 & Isopropyl linoleate & 114.091 & RI, MS & 87 & \0\078\,268 & 2.46 & \020\,177 & 3.05 \\ 
& & & & & 3186\,946 & 100.00 & 661\,146 & 100.00
\botline
\end{tabular}
\tabnote{$^{\mathrm{a}}$Method used for identification of compounds; 
RI: retention index data from literature; MS: mass spectrum.}
\vspace*{-2pt}
\end{table*}

The dominance of linoleic acid derivatives is consistent with previous
reports on grape seed oil, which is known for its high content in
polyunsaturated fatty acids~\cite{36}. The presence of oleic acid
derivatives contributes to the overall fatty acid profile of the oil,
complementing the predominance of polyunsaturated fatty acids. The
identification of fatty acid ethyl esters can be attributed to the
extraction and analytical conditions in GC--MS analyses of vegetable
oils~\cite{37}.

\subsection{Microbiological analysis}\label{ssec35}

No microbial growth was detected in any of the grape seed oil samples
obtained from the five analyzed varieties after 24 h of incubation.
Similarly, the pooled oil sample showed no detectable growth of
colonies. These results indicate that the grape seed oils obtained by
cold pressing met acceptable microbiological quality under the
evaluated conditions. Grape seed oil has been shown to exhibit
antimicrobial activity, which supports the relevance of including
microbiological evaluation in this study~\cite{38}.

\subsection{Analysis of total phenolic compounds (TPC) and antioxidant
activity}\label{ssec36}

The results of TPC and antioxidant activity of the five varieties of
grape seeds are shown in Table~\ref{tab3}. The results are in a range
of 112--144 mg GAE/kg of oil, with the highest amount in the Syrah
variety and the lowest in the Criolla variety. These results are higher
than those found by Rombaut \etal~\cite{39} for type 2 and 3 grape
seeds with maximum values of approximately 90 mg GAE/kg. However, for
type 1 grape (121 mg GAE/kg of oil), it is within the range found in
this study. In Brazil, four varieties of oils obtained in the local
market presented a maximum value of 28 mg GAE/100 g, a value lower than
the samples of this study obtained by cold compression~\cite{40}. On
the other hand, Kiralan \etal\ obtained a TPC value of 554~mg~GAE/kg,
which is higher than the values found in this study. These differences
can be explained by factors such as the different varieties of seeds
used, place and season of collection, the extraction conditions
(temperature, time, speed of rotation of the equipment), oil
conservation (temperature and conservation container used), and finally
the method of extraction of the polyphenols from the sample because,
being hydrophilic in nature, they are not very soluble in
oil~\cite{41}.

%tab3
\begin{table}
\caption{\label{tab3}TPC and antioxidant activity of grape seed oils
from different varieties}
\begin{tabular}{ccc}
\thead
Sample & \parbox[t]{6pc}{\centering TPC (mg GAE/kg of oil)} & \parbox[t]{6pc}{\centering DPPH radical scavenging (\%)}\vspace*{2pt} \\ 
\endthead
Syrah    & 143.84 & 45.71 \\ 
Italia   & 118.02 & 34.10 \\ 
Moscatel & 115.75 & 32.82 \\ 
Criolla  & 112.35 & 36.19 \\ 
Malbec   & 127.18 & 39.92 
\botline
\end{tabular}
\end{table}

Likewise, the antioxidant capacity of the samples varies from 33\%
(Moscatel) to 46\% (Syrah). Konuskan \etal\ reported lower antioxidant
capacity values of 7--18\% for six grape seed oil samples from
different varieties obtained by cold pressing~\cite{8}. Meanwhile,
other studies presents antioxidant capacity results of 19\%--30\% for
the samples obtained by solvent extraction~\cite{42}. The values of the
samples in this study are in this range, even though the extraction
method is different. Therefore, it is established that the extraction
method, whether by solvent or by cold compression, influences the
antioxidant activity of grape seed oil. Akca and Akpinar found
\mbox{approximately} 29\% \mbox{antioxidant} capacity in a commercially available
grape oil sample, showing that the one obtained by cold compression in
our study has higher\break values~\cite{43}. 

\subsection{Preliminary cost analysis}\label{ssec37}

Based on the considered parameters, the estimated total production cost
of grape seed oil obtained by cold pressing was 25.58 USD per liter.
This cost includes labor associated with sample handling and oil
extraction, energy consumption required for the operation of the
cold-pressing equipment, and laboratory-related costs such as equipment
depreciation. Raw material costs were not included, as grape seeds were
considered an agro-industrial residue from the wine industry. The
reported value reflects laboratory-scale conditions and provides an
initial indication of the economic feasibility of valorizing grape seed
residues. Although production costs may vary under industrial-scale
processing due to differences in scale, efficiency, and operational
conditions, the result obtained suggests that cold pressing represents
a potentially viable and environmentally friendly approach for grape
seed oil production.

Beyond oil extraction, the solid residue remaining after cold pressing
(grape seed cake) represents an additional opportunity for valorization
within a circular economy framework. Oilseed cakes have been widely
recognized as promising sources of value-added food components,
including dietary fiber, proteins, and bioactive compounds, depending
on the raw material and processing conditions~\cite{44}. The integrated
utilization of both grape seed oil and the residual cake could further
enhance the sustainability and economic potential of winery
by-products.

Although this study provides relevant insights into the valorization of
grape seed residues from the wine industry, some limitations should be
acknowledged. The fatty acid composition was determined using a pooled
oil sample, which does not allow differentiation among individual grape
seed varieties. Antioxidant activity was evaluated using the DPPH
assay, which provides an initial estimation of radical scavenging
capacity; complementary antioxidant assays could offer a more
comprehensive evaluation. In addition, the cost analysis represents a
preliminary estimation based on laboratory-scale conditions and may
vary when applied to larger-scale or industrial processing.

\section{Conclusions}\label{sec4}

This study demonstrates that grape seed residues from the wine industry
constitute a valuable raw material for the sustainable production of
cold-pressed grape seed oil. The grape seeds used in this work were
obtained as by-products of local wine production in southern Peru,
highlighting an opportunity to transform agro-industrial residues that
are typically discarded into value-added products. The recovered oil
exhibited a favorable fatty acid profile, significant levels of
phenolic compounds, and notable antioxidant activity, particularly in
oils obtained from the Syrah variety. The absence of microbiological
growth further supports the quality and stability of the extracted
oils.

In addition to the chemical and functional characterization,
preliminary application tests were carried out using the obtained grape
seed oil in the formulation of an exfoliating soap and a cosmetic
cream. These exploratory formulations showed satisfactory performance,
highlighting the suitability of grape seed oil as a functional
ingredient with emollient and antioxidant properties. Such applications
reinforce the potential use of this oil in cosmetic products, while its
lipid composition and antioxidant capacity also support its
applicability in food-related formulations.

Furthermore, the preliminary cost analysis indicates that cold pressing
represents an economically viable and environmentally friendly approach
for the valorization of winery residues. Overall, the integration of
grape seed oil recovery into local wine production chains may
contribute to waste reduction, promote circular economy strategies, and
add value to agro-industrial by-products through practical and scalable
applications.

\section*{Acknowledgments}

The authors would like to thank Majes Tradici\'{o}n for providing the
samples for this research study.

%\section*{Declaration of interests}
%The authors do not work for, advise, own shares in, or receive funds
%from any organization that could benefit from this article, and have
%declared no affiliations other than their research organizations.

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\refinput{crchim20250824-reference.tex}

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