\makeatletter
\@ifundefined{HCode}
{\documentclass[screen,CRCHIM,Unicode,Thematic]{cedram} 
\newenvironment{noXML}{}{}
\def\tsup#1{$^{{#1}}$}
\def\tsub#1{$_{{#1}}$}
\def\thead{\noalign{\relax}\hline}
\def\tbody{\noalign{\relax}}
\def\endthead{\noalign{\relax}\hline}
\def\tabnote#1{\vskip4pt\parbox{.90\linewidth}{#1}}
\usepackage[figuresright]{rotating}
\RequirePackage{etoolbox}
\def\jobid{crchim20240016}
\def\inlinefig#1{\includegraphics{#1}}
%\graphicspath{{/tmp/\jobid_figs/web/}}
\graphicspath{{./figures/}}
\usepackage[T1]{fontenc}
\usepackage{comment}
\newcounter{runlevel}
\usepackage{upgreek}
\let\MakeYrStrItalic\relax
\csdef{Seqnsplit}{\\}
\def\refinput#1{}
\def\back#1{}
\def\0{\phantom{0}}
\def\hyphen{\mbox{-}}
\newenvironment{inftab}{}{}
\def\xmorerows#1#2{#2}
\usepackage{multirow} 
\def\nrow#1{\@tempcnta #1\relax%
\advance\@tempcnta by 1\relax%
\xdef\lenrow{\the\@tempcnta}}
\def\morerows#1#2{\nrow{#1}\multirow{\lenrow}{*}{#2}}
\def\botline{\\\hline}
\def\xxbotline{\hline}
\def\Lbreak{\newline}
\def\ubreak{\break}
\def\xvarnothing{\emptyset}
\def\mn{\phantom{$-$}}
\DOI{10.5802/crchim.303}
\datereceived{2024-01-09}
\daterevised{2024-02-21}
\dateaccepted{2024-03-01}
\ItHasTeXPublished
%%%
\gdef\splittabular{\hline 
\multicolumn{6}{r@{}}{(continued on next page)}
\end{tabular}
\end{sidewaystable*}
\setcounter{table}{4}
\begin{sidewaystable*}
\vspone
\caption{(continued)}
%\fontsize{8}{10}\selectfont%\tabcolsep1pt
\begin{tabular}{@{}cccccc@{}}
\hline
Component & 
\parbox[t]{5pc}{\centering Molecular formula} &
\parbox[t]{5pc}{\centering Mol weight} &
\parbox[t]{5pc}{\centering ${}^{4}$Sample 4 (Peak area\%)} &
\parbox[t]{5pc}{\centering ${}^{5}$Sample 5 (Peak area\%)} &
\parbox[t]{5pc}{\centering ${}^{6}$Sample 6 (Peak area\%)}\vspace*{2pt} \\
\hline
}
\def\vspone{\vspace*{20.5pc}}
}{\documentclass[crchim]{article}
\def\CDRsupplementaryTwotypes#1#2{}
\def\CDRdoi{10.5802/crchim.303} 
\newenvironment{sidewaystable*}{\begin{table*}}{\end{table*}}
\def\href#1#2{\url{#1}}
\let\newline\break
\def\ubreak{}
\let\xvarnothing\emptyset
\makeatletter
\let\vspone\relax
\def\xxbotline{\botline}
\def\xmorerows#1#2{\morerows{#1}{#2}}
}
\makeatother

\usepackage[T1]{fontenc}
\usepackage{upgreek}

\begin{document}

\begin{noXML}

%\makeatletter
%\def\TITREspecial{\relax}
%\def\cdr@specialtitle@english{Materials and Energy Valorization of Biomass and Waste : The Path for Sustainability and Circular Economy Promotion}
%\def\cdr@specialtitle@french{Valorisation \'energ\'etique et mati\`ere de la biomasse et des d\'echets : la voie de la durabilit\'e et de la promotion de l'\'economie circulaire}
%\makeatother

\CDRsetmeta{articletype}{research-article}

\title{Sustainability by extraction of bioactive compounds from onion
seeds}

\alttitle{Durabilit\'{e} par extraction de compos\'{e}s bioactifs \`{a}
partir de graines d'oignon}

\author{\firstname{Selma} \lastname{Mestour}}
\address{Laboratoire de l'Ing\'{e}nierie des proc\'{e}d\'{e}s de
l'Environnement, Universit\'{e} Constantine 3, Algeria}

\author{\firstname{Abdeslam-Hassen} \lastname{Meniai}\CDRorcid{0000-0002-0357-3512}\IsCorresp}
\addressSameAs{1}{Laboratoire de l'Ing\'{e}nierie des proc\'{e}d\'{e}s
de l'Environnement, Universit\'{e} Constantine 3, Algeria}
\email[A.-H. Meniai]{meniai@yahoo.fr}

\author{\firstname{Sarra} \lastname{Melloul}}
\addressSameAs{1}{Laboratoire de l'Ing\'{e}nierie des proc\'{e}d\'{e}s
de l'Environnement, Universit\'{e} Constantine 3, Algeria}

\author{\firstname{Mehdi} \lastname{Louaer}}
\addressSameAs{1}{Laboratoire de l'Ing\'{e}nierie des proc\'{e}d\'{e}s
de l'Environnement, Universit\'{e} Constantine 3, Algeria}

\keywords{\kwd{Onion seed}\kwd{Supercritical fluid
extraction}\kwd{Carbon dioxide}\kwd{Extraction yield}}

\altkeywords{\kwd{Graine d'oignon}\kwd{Extraction de fluide
supercritique}\kwd{Gaz carbonique}\kwd{Rendement d'extraction}}

\shortrunauthors

\begin{abstract}
Onion is not just an edible food compound but its seeds can also be an
important source of valuable oil with various sensitive applications
mainly in pharmaceutical, cosmetic and food fields. This has encouraged
the use of a sustainable extraction technique like the supercritical
CO\tsub{2} extraction of oil from local onion seeds, as a clean and
green alternative to organic solvent processes. To achieve this, the
effects of four key operating parameters like pressure, temperature,
particle size and CO\tsub{2} mass flowrate were investigated in ranges
of [150, 250 bars], [35, 65~\textdegree C], [0.25, 0.85~mm] and [50,
100~g/min], respectively, using a Box-Behnken experimental design, for
a constant extraction time of 3~h. This led to optimal operating
conditions of 250 bar, 57~\textdegree C, 0.25~mm and 50~g
CO\tsub{2}/min for the best oil extraction yield value of 24.36\%.
Furthermore, gas chromatography-mass spectrometry (GC-MS) was used to
identify qualitatively and quantitatively the chemical composition of
the onion seed oils. About ten components were identified in the
extracts, but with four major ones, namely 9,12-octadecadienoic 
acid, methyl ester, 9,12-octadecadienoic acid (Z,Z)-, methyl ester,
9-octadecadienoic acid (Z)-, methyl ester and hexadecanoic acid, methyl
ester, present with significant percentages, depending on the operating
parameters values. These fatty acids are well known for their benefits
for human health. Finally this research showed ability of the
supercritical CO\tsub{2} extraction to induce a sustainability
character while being performing in producing bioactive substances,
excluding the use of organic solvent and even replacing classical and
traditional techniques like hydrodistillation.
\end{abstract}

\begin{altabstract}
L'oignon n'est pas seulement un compos\'{e} alimentaire comestible,
mais ses graines peuvent \'{e}galement constituer une source importante
d'huile pr\'{e}cieuse avec diverses applications sensibles,
principalement dans les domaines pharmaceutique, cosm\'{e}tique et
alimentaire. Cela a encourag\'{e} l'utilisation d'une technique
d'extraction durable comme l'extraction au CO\tsub{2} supercritique de
l'huile de graines d'oignon locales, comme alternative propre et verte
aux processus avec solvants organiques. Pour y parvenir, les effets de
quatre param\`{e}tres de fonctionnement cl\'{e}s tels que la pression,
la temp\'{e}rature, la taille des particules et le d\'{e}bit massique
de CO\tsub{2} ont \'{e}t\'{e} \'{e}tudi\'{e}s dans des plages de [150,
250 bars], [35, 65~\textdegree C], [0,25, 0,85~mm] et [50, 100~g/min],
respectivement, en utilisant un plan exp\'{e}rimental Box-Behnken, pour
un temps d'extraction constant de 3 heures. Cela a conduit \`{a} des
conditions de fonctionnement optimales de 250 bars, 57~\textdegree C, 
0,25~mm et 50~g de CO\tsub{2}/min pour le meilleur rendement
d'extraction d'huile de 24,36~\%. De plus, la chromatographie en phase
gazeuse-spectrom\'{e}trie de masse (GC-MS) a \'{e}t\'{e} utilis\'{e}e
pour identifier qualitativement et quantitativement la composition
chimique des huiles de graines d'oignon. Une dizaine de composants ont
\'{e}t\'{e} identifi\'{e}s dans les extraits, mais avec quatre
principaux, \`{a} savoir l'acide 9,12-octad\'{e}cadi\'{e}no\"{i}que,
l'ester m\'{e}thylique, l'acide 9,12-octad\'{e}cadi\'{e}no\"{i}que
(Z,Z)-, l'ester m\'{e}thylique, l'acide 9-octad\'{e}cadi\'{e}no\"{i}que
(Z)-. ester m\'{e}thylique et acide hexad\'{e}cano\"{i}que, ester
m\'{e}thylique, pr\'{e}sents avec des pourcentages significatifs, en
fonction des valeurs des param\`{e}tres op\'{e}ratoires. Ces acides
gras sont bien connus pour leurs bienfaits pour la sant\'{e} humaine.
Enfin, cette recherche a montr\'{e} la capacit\'{e} de l'extraction au
CO\tsub{2} supercritique \`{a} induire un caract\`{e}re durable tout en
\'{e}tant performante dans la production de substances bioactives,
excluant l'utilisation de solvants organiques et m\^{e}me
rempla\c{c}ant les techniques classiques et traditionnelles comme
l'hydrodistillation.
\end{altabstract}

\maketitle

{\vspace*{2pc}}

\twocolumngrid

\end{noXML}

\section{Introduction}\label{sec1}

The sector of natural plants is characterized by a great potential
which is unexploited to a large extent, due to the still used limited
traditional extraction techniques. This factor has slowed down the
development of the cultivation of natural plants from which great value
bioactive substances can be extracted, for various applications that
bring considerable and sustainable social and economic prospects. The
demand of medicines from natural plants bioactive ingredients is
constantly increasing, hence the need to introduce non conventional
extraction techniques, like supercritical CO\tsub{2} extraction which
has shown many important applications~\cite{1,2,3}. 

The sustainability aspect in various industries like pharmaceutical and
food industries depends to a large extent upon the extraction step
which has to be ``green'', with low energy consumption, short operating
times, compatible with friendly solvents, gives high quality and value
extracts of renewable natural substances with a high purity and enables
the valorization of the generated residues for instance into
fertilizers, promoting circular economy as well.

As an illustration the supercritical CO\tsub{2} extraction of oil from
onion seeds, was considered as a model system in the present study.
This choice was mainly guided by the important local cultivation of
onions scientifically known as \textit{Allium cepa}~L.\ and which have
been grown for over 4000 years and belong to the \textit{Liliaceae}
family~\cite{4,5}. In recent years, the world production of onions has
increased by more than 25\% and now reaching about 44 million tons, the
second most important crop after tomatoes~\cite{5}.  They are an
essential ingredient in cooking traditions of many countries, widely
consumed worldwide due to their distinctive flavor and numerous health
benefits~\cite{6} and are considered as a medicinal plant that can
alleviate or prevent several common illnesses such as atherosclerosis,
asthma, bronchitis, and cough. They are also a rich source of
biologically active phyto molecules~\cite{7} and may exhibit various
activities such as antimicrobial, antifungal, antiviral, antioxidant,
antidiabetic, antitumor, anti-inflammatory, antihypertensive, ant
allergic, and hypolipidemic properties~\cite{6,8}.  All these
properties have been the main motivating factor for the recent research
efforts made to identify new sources of edible oils due to their
nutritional, industrial, and medicinal benefits~\cite{7}, using
appropriate separation techniques like the supercritical CO\tsub{2}
extraction where the great majority of research works was based on the
use of freeze-dried onion powders and rarely onion seeds~\cite{9}.
However many researchers are now getting more interested in the
bioactivity of onions, through the seeds to minimize the effect of the
high water content of onion powder which may lead to low oil extraction
yields, making the extracting process delicate~\cite{6}.  The onion
seeds have considerable oil content, with high percentages of linoleic,
oleic, and palmitic acids, proteins, alcohols, acids, and esters, as
well as sulfur compounds, which aid in controlling cholesterol,
diabetes, cancer, etc. and are characterized by important free amino
acids and antioxidants contents~\cite{4}.

The selection of supercritical CO\tsub{2} extraction for this study was
primarily influenced by its \mbox{environmentally} friendly nature, which
involves minimizing the utilization and generation of harmful
substances while extracting oils from natural plants. This process is
categorized as a green chemistry approach. Supercritical fluids are
often regarded as eco-friendly, and their supercritical phase can be
viewed as an intermediary state that is almost as dense as a liquid and
with transport properties, such as viscosity and diffusion, comparable
to those of a gas~\cite{10}. Among the solvent based extraction
processes, supercritical CO\tsub{2} is commonly favored, especially for
oxidation-sensitive, thermolabile, high-value, or minor components,
owing to its advantageous blend of mild critical conditions regarding
temperature (304.13~K) and pressure (7.38~MPa)~\cite{11}. Another major
advantage of supercritical CO\tsub{2} extraction lies in the ease to
manipulate the operating parameters, mainly the pressure and
temperature to obtain extracts with composition made to measure,
promoting sustainability in sensitive fields like food and
pharmaceutical sectors.

\section{Materials and methods}\label{sec2}
\subsection{Chemicals and materials}\label{ssec21}

Carbon dioxide (CO\tsub{2}) with a purity of 99.99\% was supplied by
Air Liquide, Algeria, MeOH, Hexane and KOH were purchased from Sigma
Aldrich and were of reagent grade; The used equipments were a Shimadzu
GCMS-QP2020 instrument, a ROTOFIX 32 centrifuge, a SAYONA SZJ8500-R14
638 CAFE MILL for grinding, a RETSCH sieve shaker and a KERN precision
balance.

\subsection{Description of supercritical CO\tsub{2} extraction}
\label{ssec22}

A description of the used CO\tsub{2} extraction method is presented.
The used supercritical CO\tsub{2} extraction machine (Separex -4219)
was purchased from Separex Company  (Champigneulles, France). The
machine is better described schematically by the simple flowsheet shown
in the following Figure~\ref{fig1}.

\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}Flowsheet of the supercritical CO\tsub{2}
extraction machine: 1: CO\tsub{2} tank; 2: cooler; 3: high pressure
pump; 5: heater; 6: extractor; 7: separators; 8: expansion valve.}
\end{figure}

The supercritical CO\tsub{2} extraction process was carried out
acccording to the following steps:
\begin{itemize}
\item CO\tsub{2} from a cylindrical tank is liquified by going
through a cooler and then compressed by being pumped by a high pressure
pump, making sure that it is at pressure greater than its critical
value of 74 bar.

\item The CO\tsub{2} stream going out the high pressure pump goes
through a preheater to ensure that its temperature is higher than its
crititical value of 31~\textdegree C.

\item The CO\tsub{2} stream exiting the preheater is at the
supercritical state and it goes into the extractor containing the solid
plant material (onion seeds in the present case).

\item The contact supercitical CO\tsub{2}---plant solid material in the
extractor initiates the mass transfer of the solute (oil) into the
supercritical CO\tsub{2} stream.

\item After a certain contact time, the extract phase (oil and
supercritical CO\tsub{2} mixture) goes into two seprators in series
where the pressure is highly decreased to induce a separation of the
oil from CO\tsub{2}.

\item The extracted oil collected from the separators and the flashed
CO\tsub{2} is recycled to be recompressed and brought to the
supercritical conditions for a further extraction run.
\end{itemize}
The operating parameters, mainly temperature, pressure and CO\tsub{2}
flowrate can be controlled and regulated during the process by means of
the necessary instrumentation.

\subsection{Sample preparation method}\label{ssec23}

The plant onion seeds (\textit{Allium cepa}~L.) were obtained from Oued
Souf area in the south eastern Algerian \mbox{Sahara} and were stored at room
temperature after removal of all impurities. Then the seeds were
crushed into small fragments and passed through a vibrating siever to
obtain three different particle diameters of 0.25, 0.55, and 0.85~mm.
The supercritical CO\tsub{2} extraction was performed using the
supercritical fluid extraction machine for a constant time of 3~h.
A mass of 40~g of ground onion (\textit{Allium cepa}~L.) was fed into
the machine to undergo the supercritical CO\tsub{2} extraction which
was fine-tuned using a four-factor Box-Behnken experimental design.

\subsection{The applied experimental design}\label{ssec24}

The design of experiments (DOE) was used to methodically alter
variables like temperature, pressure, particle size and CO\tsub{2}
flowrate, which may impact any considered specific response or outcome
of interest, like the extraction yield for the present case. Controlled
manipulations of the considered variables may reveal cause-and-effect
relationships and identify the factors that exert the most significant
influence on the response variable. Consequently, DOE can statistically
enhance the process reliability while reducing the consumption of raw
materials and time~\cite{12}.

The objective of this study was to optimize and investigate the impact
of the four mentioned parameters (pressure, temperature, particle
diameter and CO\tsub{2} flow rate), on the extracted oil mass and its
quality through the composition. Therefore a BBD (Box-Behnken Design)
was used for this purpose. Its choice has been mainly guided by a
number of advantages like not requiring a lot of runs for the
generation of higher order response surfaces, comparatively to other
factorial techniques, determining the interactions between the
independent variables, using a reduced experiments number, hence an
important time saving. Also for the Box-Behnken design each factor is
characterized by three levels, hence the possibility for a second-order
model only.

Table~\ref{tab1} shows the actual values of the considered independent
parameters in the DOE.

%tab1
\begin{table}
\caption{\label{tab1}Coded and actual independent variables}
\begin{tabular}{cccc}
\thead
\xmorerows{1}{Variables} & \multicolumn{3}{c}{Level} \\\cline{2-4}
& ${-}1$ & 0 & ${+}1$ \\
\endthead
Pressure (bar) & 150 & 200 & 250 \\
Temperature (\textdegree C) & 35 & 50 & 65 \\
CO\tsub{2} flow rate (g/min) & 50 & 75 & 100 \\
Particle size (mm) & 0.25 & 0.55 & 0.85
\botline
\end{tabular}
{\vspace*{.5pc}}
\end{table}

\subsection{Oil characterization}\label{ssec25}
\subsubsection{Preparation of fatty acid methyl ester (FAME)}
\label{sssec251}

The onion seed were subjected to supercritical CO\tsub{2} extraction
for 3~h and subsequently the oil \mbox{converted} to methyl
ester through trans-esterification. The protocol used for this process
involved the use of methanol and hexane, with slight adjustments made
to comply with Francisca Salinas' method which incorporates potassium
hydroxide as a catalyst~\cite{13}.

\subsubsection{GC-MS instrument and analytical conditions}
\label{sssec252}

The operating conditions of the used GC-MS 2010 plus Shimadzu apparatus
are shown in the following Table~\ref{tab2}. 

%tab2
\begin{table*}
\caption{\label{tab2}The protocol for the GCMS analysis}
\begin{tabular}{cc}
\thead
Apparatus & GC-MS \\
\endthead
Injector temperature (\textdegree C) & 225\vspace*{2pt} \\

Column temperature (\textdegree C) & \parbox[t]{27pc}{\centering The
oven was preheated to 100~\textdegree C and maintained at that
temperature for a minute. Subsequently, the temperature was increased
to 250~\textdegree C and sustained for 15 min. Finally, the oven was
kept at this temperature for another 14 min. Overall, the oven was
operated for 30 min}\vspace*{2pt} \\

Injection mode & Split, column flow ratio 10:1\vspace*{2pt} \\

Injection volume & 2~${\upmu}$l with a flow rate of 0.8~ml/min\vspace*{2pt} \\

Pressure (kPa) & \parbox[t]{27pc}{\centering At the starting
temperature of the furnace, the pressure exerted by the column of
carrier gas (Helium) at its head was 200~kPa}\vspace*{2pt}
\botline
\end{tabular}
{\vspace*{12pt}}
\end{table*}

Data acquisition and processing was performed by means of specific
computer software Minitab and the analyses and interpretation of the
results were carried out automatically by the gas chromatography-mass
spectrometer (GC-MS). The onion seed oils were separated based on the
major components increasing molecular weight, according to the used
operating conditions~\cite{13}.

\section{Results and discussion}\label{sec3}
\subsection{BBD analysis results}\label{ssec31}

The process was optimized using the Box-Behnken design that involved a
set of 27 experiments. The obtained results showed that the onion seeds
oil extraction yield was influenced by the four considered key
parameters: temperature, pressure, particle diameter and CO\tsub{2}
flow rate. The obtained yield values ranged from 11.6 to 24.36\%,
indicating the significance of examining these parameters. A detailed
experimental design matrix in terms of the four factors, each at three
levels was generated using the BBD approach and the results are shown
in Table~\ref{tab3}. The response (extraction yield) is expressed as
second-order polynomial function of the four independent variables as
follows:{\vadjust{\pagebreak}}
{\begin{eqnarray}\label{eq1}
y(\%) &=& 34.1 + 0.159X_{1} + 0.035X_{2} - 24.3X_{3} -  0.364X_{4} \nonumber \\
&& - \, 0.00517X_{1}X_{1} - 0.000348X_{2}X_{2} +  13.46X_{3}X_{3} \nonumber \\
&& + \, 0.00284X_{4}X_{4} + 0.00216X_{1}X_{2} +  0.006X_{1}X_{3} \nonumber \\
&& - \, 0.00222X_{1}X_{4} + 0.0286X_{2}X_{3} +  0.000204X_{2}X_{4} \nonumber \\
&& - \, 0.034X_{3}X_{4}
\end{eqnarray}}\unskip
where $y$ is the predicted response for oil extraction yield (\%),
$X_{1}$, $X_{2}$, $X_{3}$ and $X_{4}$ are the coded independent
variables, $T$ (\textdegree C), $P$ (bar), $d$ (mm) and $Q$ (g/min),
respectively.

%tab3
\begin{table*}
\caption{\label{tab3}The experimental design matrix}
\begin{tabular}{cccccccc}
\thead
Run & $X_{1}$ & $X_{2}$ & $X_{3}$ & $X_{4}$ & 
 $y_{\mathrm{exp}}$ (\%) &
 $y_{\mathrm{calc}}$ (\%) & 
 Residual (\%) \\
\endthead
\01 & 35 & 150 & 0.55 & 75 & 17.876 & 19.013 & ${-}$1.137 \\ 
\02 & 65 & 150 & 0.55 & 75 & 11.604 & 13.097 & ${-}$1.493 \\ 
\03 & 35 & 250 & 0.55 & 75 & 18.047 & 19.256 & ${-}$1.209 \\ 
\04 & 65 & 250 & 0.55 & 75 & 18.253 & 19.820 & ${-}$1.567 \\ 
\05 & 50 & 200 & 0.25 & 50 & 24.320 & 25.041 & ${-}$0.721 \\ 
\06 & 50 & 200 & 0.85 & 50 & 21.199 & 21.937 & ${-}$0.738 \\ 
\07 & 50 & 200 & 0.25 & 100\0 & 22.241 & 24.206 & ${-}$1.965 \\ 
\08 & 50 & 200 & 0.85 & 100\0 & 18.09\0 & 20.082 & ${-}$1.992 \\ 
\09 & 35 & 200 & 0.55 & 50 & 22.017 & 21.620 & \mn0.397 \\ 
10  & 65 & 200 & 0.55 & 50 & 20.664 & 20.609 & \mn0.055 \\ 
11  & 35 & 200 & 0.55 & 100\0 & 23.161 & 21.940 & \mn1.221 \\ 
12  & 65 & 200 & 0.55 & 100\0 & 18.473 & 17.599 & \mn0.874 \\ 
13  & 50 & 150 & 0.25 & 75 & 19.854 & 20.666 & ${-}$0.812 \\ 
14  & 50 & 250 & 0.25 & 75 & 24.350 & 23.291 & \mn1.059 \\ 
15  & 50 & 150 & 0.85 & 75 & 16.411 & 16.194 & \mn0.217 \\ 
16  & 50 & 250 & 0.85 & 75 & 22.625 & 20.535 & \mn2.090 \\ 
17  & 35 & 200 & 0.25 & 75 & 24.039 & 23.051 & \mn0.989 \\ 
18  & 65 & 200 & 0.25 & 75 & 21.974 & 20.321 & \mn1.654 \\ 
19  & 35 & 200 & 0.85 & 75 & 19.371 & 19.382 & ${-}$0.011 \\ 
20  & 65 & 200 & 0.85 & 75 & 17.408 & 16.760 & \mn0.648 \\ 
21  & 50 & 150 & 0.55 & 50 & 21.407 & 19.921 & \mn1.486 \\ 
22  & 50 & 250 & 0.55 & 50 & 22.695 & 22.894 & ${-}$0.199 \\ 
23  & 50 & 150 & 0.55 & 100\0 & 19.902 & 18.066 & \mn1.836 \\ 
24  & 50 & 250 & 0.55 & 100\0 & 22.210 & 22.059 & \mn0.151 \\ 
25  & 50 & 200 & 0.55 & 75 & 20.968 & 19.830 & \mn1.138 \\ 
26  & 50 & 200 & 0.55 & 75 & 18.174 & 19.830 & ${-}$1.656 \\ 
27  & 50 & 200 & 0.55 & 75 & 20.455 & 19.830 & \mn0.625 \\ 
Optimum & 57 & 250 & 0.25 & 50 & 24.361 & 25.483 & ${-}$1.122
\botline
\end{tabular}
\end{table*}

Table~\ref{tab3} shows that the predicted and experimental yield values
were reasonably close, particularly under optimum conditions where the
values were 25.483 and 24.361\%, respectively.

ANOVA analyses showed that the determined model was weak with a
coefficient of determination ($R^{2}$) equal to 81.37\%. The first
analysis included the Fisher test, hence the comparison the $F$ values
calculated by Minitab software with an $F$ (critical) obtained from the
Fisher-Snedecor table using ($p-1$) and ($n-p$) degrees of freedom, for
a pre-defined level of significance (0.05). The present model had 15
coefficients, with $p-1=14$ and $n=27$ experiments, resulting in 
$n-p=12$ degrees of freedom. The tabulated $F$ value was 2.62. However,
the obtained $F$ value was higher than the tabulated one of 3.74. This
confirms the reliability of the proposed model and its predictive
capability.

To ensure the validity of the factors in the model, their $p$-values
should be examined and indicate significance when less than 0.05. Upon
evaluation, it was discovered that all the terms in the full quadratic
model were significant except for the flowrate term, which was found to
be insignificant with a $p$-value of 0.213. The model $p$-value was
calculated as 0.014, while the $p$-values for temperature, pressure,
and particle diameter were 0.024, 0.005, and 0.005,
respectively~\cite{14}.

For a comparison purpose of supercritical CO\tsub{2}, the cold-pressed
onion seed oil showed antioxidant properties with a total phenolic
content (TPC) of 3.35~mg GAE/g, a higher value compared to those of
seed oils from cold-pressed black raspberry, parsley and milk thistle.
Furthermore, the cold-pressed onion seed oil yield was found to be
21.1\%, higher than the yield of 13.5\% shown by Soxhlet
extraction~\cite{10,15}. However in the present study and as shown in
Table~\ref{tab4}, an even higher extraction yield of up to 24.5\% was
achieved by supercritical CO\tsub{2} extraction from local onion seeds,
at operating conditions of pressure, temperature, diameter, and
flowrate of the supercritical CO\tsub{2} corresponding to the optimum
of oil extraction yield and quality, highlighting the significance of
ground onion seed oil in various industries, including pharmaceuticals,
parapharmaceuticals, cosmetics and food. Also Table~\ref{tab4} shows
the comparison of the total phenolic contents (TPC), obtained by the
three methods 
(supercritical CO\tsub{2}, Soxhlet and cold pressing
extractions~\cite{5,15}). Clearly the results show that the non
conventional supercritical CO\tsub{2} extraction was much more
performing basing on the considered two criteria (extraction yield and
TPC), confirming the effectiveness of its sustainability and
encouraging its further development. This is in agreement with results
of many research works reported in the literature and dealing with
different systems such as in~\cite{16}. However studies based on oil
extraction from onion seeds by supercritical CO\tsub{2} extraction are
very few, according to the\break literature.

%tab4
\begin{table*}
\caption{\label{tab4}Comparison of onion seeds oil extraction by
supercritical CO\tsub{2}, Soxhlet and cold pressing extractions
\vspace*{-2pt}}
\begin{tabular}{cccc}
\thead
Extraction technique & 
Oil yield (\%) & 
\parbox[t]{7pc}{\centering Total phenolic content (mg GAE/g)} & 
Reference\vspace*{2pt} \\
\endthead
Supercritical CO\tsub{2} extraction & 24.36 & 36.15 & Present work \\
Soxhlet extraction & 13.50 & 1.84--6.87 & \cite{5} \\
Cold pressed extraction & 21.10 & 3.35 & \cite{15}
\botline
\end{tabular}
\end{table*}

Also these results confirmed the ability of the BBD model in selecting
the appropriate range for each parameter.

Generally the composition of extracts obtained through supercritical
CO\tsub{2} extraction is influenced by various factors, such as the
plant species and the adopted operating conditions like temperature,
pressure, seed size and CO\tsub{2} flowrate, for the present study.
This influence can be seen through the obtained extract properties like
their nature, color, viscosity, density, etc. This is confirmed by the
three samples of onion seeds oil extracts of Figure~\ref{fig2}, where
the differences can easily be seen.

\begin{figure}
\vspace*{2pt}
\includegraphics{fig02}
\vspace*{2pt}
\caption{\label{fig2}Different extracts of onion seeds by supercritical
CO\tsub{2} extraction according to the operating parameters.}
\vspace*{2pt}
\end{figure}

The effects of the operating parameters are discussed in details in the
next section as follows.

\subsection{The influence of the four selected parameters on the oil
yield of onion seeds}\label{ssec32} 

The solid matrix consisting of onion seeds underwent a mass transfer in
contact with the supercritical CO\tsub{2} in the extractor. This
transfer occurred through molecular diffusion due to concentration
gradients. Figure~\ref{fig3} shows the oil extraction yield variations
with time at given and optimal operating conditions. The curves are
showing two phases: a first one \mbox{corresponding} to an important essential
oil mass transfer into CO\tsub{2} due to its availability at the
external solid surface, hence a rapid increase of the extraction yield;
a second one corresponding to the essential oil exhaust at the external
surface and its extraction from the bulk of the solid where the process
is dominated by diffusional and internal mass transfer resistances.\looseness=1

\begin{figure}
\vspace*{2pt}
\includegraphics{fig03}
\vspace*{2pt}
\caption{\label{fig3}Extraction kinetics of onion seeds (a)~at
35~\textdegree C, 150~bar, 0.55~mm, 75~g/min; (b)~at optimum conditions
57~\textdegree C, 250~bar, 0.25~mm, 50~g/min.}
\end{figure}

The performance of any supercritical fluid extraction process depends
upon the operating parameters mainly temperature, pressure, particle
size and solvent flowrate. The present study is mainly concerned by two
performance criteria: the oil extraction yield and the composition.
Therefore it is important to investigate the effect of each parameter
on these two criteria and determine operating ranges corresponding to
high yield and quality of extracts via the composition.

\subsubsection{The effect of pressure on the extraction yield}
\label{sssec321}

The effect of pressure on oil extraction yield was considered leading
to the observation of various findings. First, at a low temperature of
35~\textdegree C, increasing the pressure from 150 to 250~bar,
respectively, resulted in nearly constant yields of 17.876--18.047\%,
corresponding to experimental Runs 1 and 3, respectively. However, at a
higher temperature of 65~\textdegree C, increasing pressure from 150 to
250 bar in experimental runs 2 and 4, respectively, resulted in a yield
increase from 11.604\% to 18.253\%. 

Second, in experimental Runs 13 and 14 at a temperature of
50~\textdegree C, a pressure increase from 150 to 250 bar, respectively
led to a remarkable increase in oil extraction yield from 19.854\% to
24.350\% while maintaining a flow rate of 75~g/min and a pressure of
0.25~mm. 

Clearly this result shows the great dependency of the solvent power of
supercritical CO\tsub{2} on its density which increases with pressure
at constant temperature. 

\subsubsection{The effect of temperature on the extraction yield}
\label{sssec322}

At fixed parameters at [150 bar, 0.55~mm, 75~g/min] for experimental
Runs 1 and 2, it was observed that an increase in temperature from 35
to 65~\textdegree C led to a decrease in the oil extraction yield from
17.876 to 11.604\%, respectively. Similar results were obtained for
experimental Runs 17 and 18 at fixed operating conditions of [200 bar,
0.25~mm, 75~g/min] and for experimental Runs 19 and 20 under the
conditions of [200 bar, 0.85~mm, 75~g/min], indicating that an increase
in temperature had negatively affected the extracted oil mass at
pressures of 150 and 200~bar. 

Contrary to the previous case, when the pressure increased to 250 bar
(experimental Runs 3 and 4 at operating conditions of [250 bar, 
0.55~mm, 75~g/min]), the increase of temperature from 35 to 65~\textdegree
C had no adverse impact on the yield with just a slight yield increase
of 0.206\%. This indicates that temperature did not affect the oil
extracting yield at a the fixed pressure of 250 bar along with other
fixed parameters, similarly to the reported supercritical CO\tsub{2}
extraction of Syzygium aromaticum~\cite{17}. 

The impact of temperature on extraction effectiveness relies on the
extraction pressure, with a negative effect seen at low pressure. This
suggests that the density of CO\tsub{2} takes precedence over the vapor
pressure of the solute. Conversely, at higher pressure, this phenomenon
is reversed. Thus, predicting the influence of temperature on the
solubility of a substance in a supercritical fluid is more challenging
than that of pressure. However, the relationship between temperature
and extraction yield is not always simple, and an optimal temperature
range may exist before yield drops at higher temperatures. This is due
to possible degradation or denaturation of the targeted compound or
induced changes to the solvent or solid matrix properties, which can
reduce the extraction yield~\cite{18}. 

Finally from the obtained results it can be confirmed that: the solvent
power of supercritical CO\tsub{2} depends mainly on its density which
increases with pressure at constant temperature conditions and
decreases with temperature at constant pressure conditions. Similar
behavior was also observed by Frohlich et~al.\ for the supercritical
CO\tsub{2} extraction of Syzygium aromaticum~\cite{17}. 

\subsubsection{Effect of particle size on the extraction yield}
\label{sssec323}

For experimental Runs 5 and 6 the considered conditions were
[50~\textdegree C, 200 bar, 50~g/min] with two particle diameters of
0.25 and 0.85~mm. A decrease in the oil extraction yields from 24.32 to
21.20\% for 0.25 and 0.85~mm diameter values, respectively, was
observed. This is simply explained by the greater mass transfer area
for the smaller diameter. This was also observed for other experimental
Runs like, 7, 8, 13, 14, 15 and 16. Therefore the size of particles can
be regarded as a key factor that can greatly influence the
supercritical fluid extraction yield and the smaller are the particle
sizes the higher is the mass transfer area, hence an increase of the
diffusion rate and a decrease in the diffusion path, resulting in a
better extraction~\cite{10}.

\subsubsection{The effect of flowrate on extraction yield}
\label{sssec324}

Fundamentally the flowrate can influence the mass transfer resistance
of the solute into the supercritical CO\tsub{2}, hence the saturation
of this latter. Therfore the oil extraction yield can be affected by
the flowrate of CO\tsub{2} passing through the solid matrix, where an
increase in fluid flowrate can reduce resistance to mass transfer and
the fluid may be saturated when leaving the solid matrix. This leads to
solute solubility equilibrium, hence a maximum yield.

In terms of the seed morphology, raising the flowrate did not lead to
an increase in yield, except for experimental Runs 9 and 10, carried
out under identical conditions of [35~\textdegree C, 200 bar, 0.55~mm]
but with different solvent flowrates. A yield increase from 22.017 to
23.161\% was observed with an increase in flowrate, confirming the
results reported in previous works~\cite{19,20}. The change in
CO\tsub{2} flowrate induced noticeable changes in the viscosity and the
color of the extracts. Therefore in terms of the overall interpretation
of the experimental design and identification of the effects of each
parameter, qualitative analyses were needed to confirm its impact on
oil composition, as shown in Figure~\ref{fig4}. In terms of the effect
on the extraction process, Figure~\ref{fig5} demonstrates that the
yield increased with a decrease in temperature from 65 to
35~\textdegree C, an increase in pressure from 150 to 250 bar, a
decrease in diameter from 0.85 to 0.25~mm and an increase/decrease in
flowrate from 75 to 100~g/min and 75 to 50~g/min, respectively. The
lowest yield was observed at a flow rate of 75~g/min, while the highest
ones were achieved at 50 and 100~g/min.

\begin{figure}
\includegraphics{fig04}
{\vspace*{-4pt}}
\caption{\label{fig4}Yield main effects plot $y$ (\%).}
{\vspace*{-4pt}}
\end{figure}

\begin{figure*}
{\vspace*{-4pt}}
\includegraphics{fig05}
{\vspace*{-4pt}}
\caption{\label{fig5}Optimum conditions for supercritical CO\tsub{2}
extraction of onion seeds oil.}
\end{figure*}

Furthermore, the accuracy of this interpretation can be checked through
the requirements derived for conducting the most efficient experiment,
easing the retrieval of the highest possible yield.

\subsection{Qualitative and quantitative analyses}
\label{ssec33}

Gas chromatography-mass spectrometry (GC-MS) is a powerful analytical
technique that combines two separate methods, gas chromatography and
mass spectrometry, to efficiently separate and identify qualitatively
and quantitatively complex chemical compound mixtures~\cite{21}. 

This method has found widespread applications in the pharmaceutical
industry for various purposes, including analytical research and
development, quality control, quality insurance, production, bulk
drugs, and formulations~\cite{22}. 

The separation of compounds according to their physical and chemical
characteristics is demonstrated in the chromatogram, whereas the
\mbox{molecular} weight and structure of the compounds are revealed through
information provided by the mass spectrum~\cite{23,24}.

One of the main features of supercritical fluid extraction is that the
composition of the extracts is greatly dependent upon the operating
conditions like the pressure, temperature, particle size and flowrate
for the present study, hence the number of the 
different resulting chromatograms shown in Figure~\ref{fig6} shows just
a chromatogram example whereas all the
other obtained ones are provided as Supplementary Material,
to avoid a discontinuity in the main text.
These chromatograms were used to find the respective compositions of
each extract and obtain significant insights from the data, by
carefully interpreting the set of information resulting from GCMS
analysis. 

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig06}
{\vspace*{-2pt}}
\caption{\label{fig6}GC/MS chromatogram showing the sequence of peaks
of fatty Acid methyl ester compounds from onion seed oils for Sample
one (Run 13: 50~\textdegree C, 150~bar, 0.25~mm, 75~g/min).}
{\vspace*{-2pt}}
\end{figure*}

Then the pattern of mass fragmentation of the detected elutants
molecule ions can be observed in the spectrum of Figure~\ref{fig6}.
This could be achieved by comparison of the retention times which
depict sequential peaks of separated analyte components and the mass
spectra with those of known compounds appearing in a reference library
or database~\cite{21}. Table~\ref{tab5} shows the analyses results
concerning three distinct samples extracted at different operating
conditions. In total ten major components were identified. 

%tab5
\begin{sidewaystable*}
\vspone
\caption{\label{tab5}Fatty acid profile of onion seeds analyzed by
GC-MS}
\begin{tabular}{cccccc}
\thead
Component &
\parbox[t]{5pc}{\centering Molecular formula} & 
\parbox[t]{5pc}{\centering Mol weight} & 
\parbox[t]{5pc}{\centering ${}^{1}$Sample 1 (peak area\%)} & 
\parbox[t]{5pc}{\centering ${}^{2}$Sample 2 (peak area\%)} & 
\parbox[t]{5pc}{\centering ${}^{3}$Sample 3 (peak area\%)} \\
\endthead
9,12-Octadecadienoic acid, methyl ester & C\tsub{19}H\tsub{34}O\tsub{2} & 294 & 24.78 & 37.98 & 21.68 \\ 
9,12-Octadecadienoic acid (Z,Z)-, methyl ester & C\tsub{19}H\tsub{34}O\tsub{2} & 294 & 33.68 & 21.86 & 27.76 \\ 
9-Octadecadienoic acid (Z)-, methyl ester & C\tsub{19}H\tsub{36}O\tsub{2} & 296 & 22.89 & 21.37 & 3.62 \\ 
Hexadecanoic acid, methyl ester & C\tsub{17}H\tsub{34}O\tsub{2} & 270 & 11.79 & 11.99 & 31.61 \\ 
Methyl stearate & C\tsub{19}H\tsub{38}O\tsub{2} & 298 & 3.83 & 3.64 & 1.12 \\ 
(Z)-Methyl hexadec-11-enoate & C\tsub{17}H\tsub{32}O\tsub{2} & 268 & 0.15 & 0.15 & / \\ 
\parbox[t]{20pc}{\centering Cyclopropaneoctanoic acid, 2-2-[(2-ethylcyclopropyl)methyl] cyclopropyl]methyl]-, methyl ester} & C\tsub{22}H\tsub{38}O\tsub{2} & 334 & 1.26 & / & / \\ 
11,14-Eicosadienoic acid, methyl ester & C\tsub{21}H\tsub{38}O\tsub{2} & 322 & 0.39 & / & / \\ 
6-Octadecenoic acid, methyl ester,(Z)- & C\tsub{19}H\tsub{36}O\tsub{2} & 296 & 0.77 & / & / \\ 
Eicosanoic acid, methyl ester & C\tsub{21}H\tsub{42}O\tsub{2} & 326 & 0.46 & 0.40 & / \\ 
13-Oxabicyclo[10.1.0]tridecane & C\tsub{12}H\tsub{22}O & 182 & / & 1.46 & / \\ 
7-Tetradecenal,(Z)- & C\tsub{14}H\tsub{26}O & 210 & / & 0.80 & / \\ 
8,11,14-Docosatrienoic acid, methyl ester & C\tsub{23}H\tsub{40}O\tsub{2} & 348 & / & 0.35 & 1.21 \\ 
Methyl tetradecanoate & C\tsub{15}H\tsub{30}O\tsub{2} & 242 & / & / & 0.13 \\ 
\parbox[t]{20pc}{\centering 8,9,9,10,10,11-Hexafluoro-4,4-dimethyl-3,5-dimethyl-3,5-dioxatetracyclo[5.4.1.0(2,6).0(8,11)]dodecane} & C\tsub{12}H\tsub{12 }F\tsub{6} O\tsub{2} & 302 & / & / & 0.20 \\ 
4-(3-Oxocyclohexyl)butyric acid, methyl ester & C\tsub{11}H\tsub{18}O\tsub{3} & 198 & / & / & 12.22 \\ 
9-Tetradecenal,(Z)- & C\tsub{14}H\tsub{26}O & 210 & / & / & 0.45 \\ 
Total & / & / & 100\% & 100\% & 100\% \\\xxbotline 
Component & 
\parbox[t]{5pc}{\centering Molecular formula} &
\parbox[t]{5pc}{\centering Mol weight} &
\parbox[t]{5pc}{\centering ${}^{4}$Sample 4 (Peak area\%)} &
\parbox[t]{5pc}{\centering ${}^{5}$Sample 5 (Peak area\%)} &
\parbox[t]{5pc}{\centering ${}^{6}$Sample 6 (Peak area\%)}\vspace*{2pt} \\\xxbotline
9-Octadecadienoic acid (Z)-, methyl ester & C\tsub{19}H\tsub{36}O\tsub{2} & 296 & 30.85 & 32.19 & 29.17 \\ 
9,12-octadecadienoic acid, methyl ester & C\tsub{19}H\tsub{34}O\tsub{2} & 294 & 0.72 & 53.91 & 52.6 \\ 
hexadecanoic acid, methyl ester & C\tsub{17}H\tsub{34}O\tsub{2} & 270 & 12.45 & 9.23 & 8.88 \\ 
\splittabular
Methyl stearate & C\tsub{19}H\tsub{38}O\tsub{2} & 298 & 3.51 & 3.23 & 3.34 \\ 
9,12-octadecadienoic acid (Z,Z)-, methyl ester & C\tsub{19}H\tsub{34}O\tsub{2} & 294 & 51.01 & / & / \\ 
11,14-Eicosadienoic acid, methyl ester & C\tsub{21}H\tsub{38}O\tsub{2} & 322 & 0.61 & / & / \\ 
6-octadecenoic acid, methyl ester, (Z)- & C\tsub{19}H\tsub{36}O\tsub{2} & 296 & 0.44 & / & / \\ 
Myristate{$<$}methyl-{$>$}; Tetradecanoic acid {$<$}methyl-{$>$}ester & C\tsub{15}H\tsub{30}O\tsub{2} & 242 & 0.27 & / & / \\ 
\parbox[t]{20pc}{\centering 8,9,9,10,10,11-Hexafluoro-4,4-dimethyl-3,5-dimethyl-3,5-dioxatetracyclo[5.4.1.0(2,6).0(8,11)]dodecane} & C\tsub{12}H\tsub{12}F\tsub{6}O\tsub{2} & 302 & 0.15 & / & / \\ 
9-Octadecenal & C\tsub{18}H\tsub{34}O & 266 & / & 0.65 & / \\ 
Tridecanoic acid, methyl ester & C\tsub{14}H\tsub{28}O\tsub{2} & 228 & / & 0.25 & / \\ 
9-Tetradecenal, (Z)- & C\tsub{14}H\tsub{26}O & 210 & / & 0.54 & 0.6 \\ 
Eicosane & C\tsub{20}H\tsub{42} & 282 & / & / & 1.31 \\ 
Docosane,11-decyl- & C\tsub{32}H\tsub{66} & 450 & / & / & 1.92 \\ 
Tetrapentacontane,1,54-dibromo- & C\tsub{54}H\tsub{108}Br\tsub{2} & 914 & / & / & 0.57 \\ 
(Z)-Methyl hexadec-11-enoate & C\tsub{17}H\tsub{32}O\tsub{2} & 268 & / & / & / \\ 
\parbox[t]{20pc}{\centering Cyclopropaneoctanoicacid, 2-[[2-[(2-ethylcyclopropyl)methyl] cyclopropyl]methyl]-, methyl ester} & C\tsub{22}H\tsub{38}O\tsub{2} & 334 & / & / & / \\ 
Eicosanoic acid, methyl ester & C\tsub{21}H\tsub{42}O\tsub{2} & 326 & / & / & / \\ 
13-Oxabicyclo[10.1.0]tridecane & C\tsub{12}H\tsub{22}O & 182 & / & / & / \\ 
7-Tetradecenal,(Z)- & C\tsub{14}H\tsub{26}O & 210 & / & / & / \\ 
8,11,14-Docosatrienoic acid, methyl ester & C\tsub{23}H\tsub{40}O\tsub{2} & 348 & / & / & / \\ 
Methyl tetradecanoate & C\tsub{15}H\tsub{30}O\tsub{2} & 242 & / & / & / \\ 
4-(3-Oxocyclohexyl)butyric acid, methyl ester & C\tsub{11}H\tsub{18}O\tsub{3} & 198 & / & / & / \\ 
Total & / & / & 100\% & 100\% & 100\% 
\botline
\end{tabular}
\tabnote{
${}^{1}$Sample one   (Run 13: 50~\textdegree C, 150~bar, 0.25~mm, and 75~g/min); 
${}^{2}$Sample two   (Run 14: 50~\textdegree C, 250~bar, 0.25~mm, and 75~g/min); 
${}^{3}$Sample three (Run optimum: 57~\textdegree C, 250~bar, 0.25~mm, and 50~g/min);
${}^{4}$Sample four  (Run 1: 35~\textdegree C, 150~bar, 0.55~mm, and 75~g/min);
${}^{5}$Sample five  (Run 2: 65~\textdegree C,150~bar, 0.55~mm, and 75~g/min);
${}^{6}$Sample six   (Run 17: 35~\textdegree C, 200~bar, 0.25~mm, and 75~g/min).}
\end{sidewaystable*}

Based on the GCMS analysis results, it has been discovered that among
the 10 components obtained, the following four primary components were
present in significant percentages in all extracts obtained under
varying conditions:{\vspace*{-2pt}} 
\begin{enumerate}[(1)]
\item 9,12-octadecadienoic acid, methyl ester.
\item 9,12-octadecadienoic acid (Z, Z)-, methyl ester.
\item 9-octadecadienoic acid (Z)-, methyl ester.
\item Hexadecanoic acid, methyl ester (Palmitic acid Methyl ester).{\vspace*{-2pt}} 
\end{enumerate}
By changing the pressure applied during the extraction process, both
chemical and physical characteristics of the oil could be modified. For
instance, when the pressure was raised from 150 to 250 bar, the primary
compounds remained present at both pressures, but at different
percentages. For example, 9, 12-Octadecadienoic acid, methyl ester was
present at 24.78\% at 150 bar, but increased to 37.98\% at 250 bar.
This was also valid for the compositions of the secondary compounds. 

By increasing the flow rate from 50 to 75~g/min, the quality of the
extract was impacted. This was evident from the increase in the mass
percentage of compound 9-octadecadienoic acid (Z)-, methyl ester from
3.62 to 21.37\%. Therefore CO\tsub{2} flow rate had a more significant
impact on the quality of the extract than the on the quantity.

Fatty acids are critical molecules of essential components for the
human body. They not only do serve as a primary source of energy, but
they also have crucial roles in the building and operation of cell
membranes, and in the production of hormones and other signal
molecules. Saturated fats are typically solid at room temperature and
commonly found in animal-based products whereas unsaturated fatty acids
are usually as liquid at room temperature and are present in
plant-based foods like nuts, seeds, and vegetable oils. They are widely
acknowledged to be more beneficial for health than saturated fats, as
studies have linked them with a reduced risk of heart disease and other
chronic conditions~\cite{25}. According to the result obtained in the
present study, it is important to note that the onion seed extracts
contain a remarkably high proportion of these unsaturated fatty acids.
A literature search did not show any study dealing particularly with
the quality of FFA obtained by different extraction methods, for
comparison purposes. However the results reported in the literature
concerning the extraction of walnut oil by cold pressing, Soxhlet
extraction, and ultrasonic extraction, confirmed the effect of the
selected extraction technique on the quality of the extracted
oil~\cite{26}. Knowing that 9, 12-Octadecadienoic acid (Z, Z), also
known as Omega 6, is considered to be the most significant among the
various fatty acids due to its numerous biological functions which
induce its impact on the inflammatory cascade, its ability to reduce
oxidative stress, as well as its role in providing neuro and
cardio-vascular protections~\cite{27}.

\section{Conclusion}\label{sec4}

The present study had clearly shown that many natural plants,
vegetables, fruits, etc., may be great sources of valuable substances
with important applications in sensitive fields like pharmaceutical,
cosmetic and food industries. However in order to obtain the valuable
substances from these natural sources, performing fluid based
extraction techniques are required. One of these non conventional
techniques is the supercritical CO\tsub{2} extraction which was used in
the present work to extract oil from onion seed as a model system.

The results did put into evidence one of the important features of the
supercritical CO\tsub{2} extraction consisting of manipulating the
operating parameters in order to achieve extracts with a high purity
and a targeted composition, confirming the merit of fluids at
supercritical state. The ``green'' aspect of the supercritical
CO\tsub{2} extraction had also been confirmed, excluding the use of
organic solvents, preserving the environment by a valorization of
process residues, avoiding the release of CO\tsub{2} into the
atmosphere,~etc. All these factors can be exploited to promote
sustainability.\looseness=-1

Finally through this study it can be seen that sustainability may be
induced by means of non conventional techniques like the supercritical
fluid extraction which is characterized by the possibility to
manipulate the composition of the extracts according to preset goals
and also to valorize the extraction residues as biosorbents,
biofertilizers, etc. promoting a circular economy. 

{\vspace*{-2pt}}

\section*{Declaration of interests}

{\vspace*{-2pt}}

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.

\back{}

{\vspace*{-2pt}}

\section*{Supplementary data}

{\vspace*{-2pt}}

Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.  

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crchim-303-suppl.pdf}}

{\vspace*{-2pt}}

\bibliographystyle{crchim}
\bibliography{crchim20240016}
\refinput{crchim20240016-reference.tex}

\end{document}
