F1000Research F1000Research 2046-1402 F1000 Research Limited London, UK 10.12688/f1000research.52925.1 Research Article Articles Essential oil extraction from onion using ethanol and CO 2 as an extraction fluid mixture

[version 1; peer review: 5 approved with reservations, 1 not approved]

 dos Santos Etiandra Investigation Methodology https://orcid.org/0000-0003-3206-2778 1 Tingeira Teresa Saleme Investigation Methodology 1 da Costa Vicencia de Fátima Cristovão Investigation Methodology https://orcid.org/0000-0002-0407-4598 1 Chiarello Luana Marcele Writing – Original Draft Preparation Writing – Review & Editing https://orcid.org/0000-0002-1213-1150 2 Chivanga Barros António André Investigation Methodology https://orcid.org/0000-0001-5922-5368 a 1 Department of Engineering and Technology (DET), ISPTEC, Avenida Luanda Sul, Rua Lateral S10, Talatona, Luanda, Angola Chemical Engineering Department, Regional University of Blumenau (FURB), Rua: São Paulo, 3250, Itoupava Seca, Blumenau – Santa Catarina, Brazil chivanga.barros@isptec.co.ao

No competing interests were disclosed.

 21 7 2021 2021 10 625 6 7 2021 Copyright: © 2021 dos Santos E et al. 2021 This is an open access article distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The author(s) is/are employees of the US Government and therefore domestic copyright protection in USA does not apply to this work. The work may be protected under the copyright laws of other jurisdictions when used in those jurisdictions.

Introduction

Essential oils are volatile chemical compounds, widely known by their fragrance, as well as by antimicrobial and antioxidant activities. These oils are generally extracted from aromatic plants in procedures using conventional solvents.

Methods

In this study, essential oil was extracted from onion (previously chopped and dried) using a mixture of ethanol and CO 2 as the extraction fluid. The essential oil obtained from the extraction was collected and purified and the mass was determined (by weighing) to evaluate the effect of CO 2 flow on the yield. The essential oil extracted and purified was characterized to determine the acid and refraction indexes, viscosity, and specific mass.

Results

The values obtained for refraction and acid indexes are within limits and similar to the average reported in literature. In all cases, when the CO 2 was used, there was an increase the essential oil recovery. In terms of quality, the products from this process were characterized to determine the density, acid index and refraction index. The results obtained were similar to those published in the literature.

Discussion

The proposed apparatus and CO 2 methodology can be considered a good alternative to boost the extraction of essential oil aiming the obtaining of new products for use as raw materials in different industrial processes. Since this apparatus presents more than double extraction yield than Soxhlet experiment.

 Extraction Oil Solvents Supercritical Fluids Ethanol and CO2 Coordenação de Aperfeiçoamento de Pessoal de Nível Superior Chemical Engineering Course of Instituto Superior Politécnico de Tecnologias e Ciências (ISPTEC) The authors gratefully acknowledge support of this research by Chemical Engineering Course of Instituto Superior Politécnico de Tecnologias e Ciências (ISPTEC) and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior – Brasil (CAPES) – Finance Code 001. Introduction

For Reddy (2019) and Tongnuanchan & Benjakul (2014), the natural essential oils are aromatic compounds present in different parts of medicinal plants, for instance: flowers, leaves, bark, roots and fruits that are separated after hydro-distillation or steam distillation as continuous solvent extraction techniques. Essential oils are a combination of low molecular mass chemical compounds, which contain: alcohols, polyphenols, terpenoids, carbonyl compounds, aliphatic substances, and they have distinct aromas and possess biological properties.

Cavalcanti (2013) and Sharmeen et al. (2021) noted that essential oils are used and applied as additives in the cosmetic, pharmaceutical, food, textile and perfumery industries to incorporate appropriate functional characteristics, ensuring a wider range of organic functions. In fact, most natural essential oils incorporate several functions, with emphasis on their use as natural dyes, nutraceuticals, functional foods, preserving agents, flavorings and fragrances, medicines, vitamin supplements, chemical standards and perfumes, among others.

Essential oils incorporate vegetable oil and are known by having an elevated content of organic and natural substances when obtained from flowers, herbs, fruits and aromatic spices. The extraction procedures involve the use of an extractor and a conventional solvent or supercritical fluid, or both, known as a hybrid mixture, where the proportion can be adjusted to improve the extraction yield ( Uwineza & Waśkiewicz (2020) and Chemat et al. (2019)).

The extraction procedure used directly reflects in the quality of the products obtained, mainly in relation to the degree of purity, and the recovery capacity. The concepts of diffusion and convection mass transference, based on interfacial equilibrium between solid–liquid or solid–gas phases, are used to assess the process efficiency and to define the appropriate extraction equipment.

Boucard & Serth (2005) evaluated the extraction process based on diffusion and convectional mass-transfer phenomena to identify procedures that could enhance the quantity of the products. Shakir (2018) observed that a higher extraction temperature could increment the interaction of solvent and matrix and also the diffusion mass transfer.

Danlami et al. (2014) observed that the fundamental of solvent extraction is based on the contact of solvent and the solid material, allowing the solid materials to be transferred to the liquid phase as soluble compounds. However, in the case of plants, this extraction occurs through concentration differences. In the moment when the mass transfer decreases by the concentration of active sites, the equilibrium is finally reached and therefore the mass transfer will no longer occur.

In recent decades, an increase in international demand for essential oils produced from plants has been observed. This has driven further research to develop new extraction techniques aimed at increasing the oil quality and reducing the production cost.

In general, essential oil extraction involves vapor flow in distillation, hydro-distillation or procedures based on organic solvents, mechanical extraction or supercritical fluids (generally carbon dioxide) to improve the mass transfer during the extraction ( Cavalcanti, 2013).

Filippis (2013) studied supercritical extraction with CO 2 and noted the importance of the production and commercialization of essential oil globally, with a considerable variety of essential oils on the market (more than 90 types). Several important differences in the oils obtained arise from the characteristics of the process applied for the extraction.

In this context, hydro-distillation is characterized by direct contact between two phases, vapor and solid. When the vapor comes into contact with an aromatic plant, the essential oil is dragged into this phase and then recovered in a condensation system ( Figure 1). In this process, to increase efficiency extraction, the processes parameters are controlled, mainly temperature, pressure, vapor flow rate and particle size of the solids. Optimizing the operational conditions in this way generally increases the extraction efficiency and yield. After this step, the essential oil produced can be purified to satisfy international standards ( Golmohammadi et al., 2018).

Description of hydro-distillation process.

Process adapted from Kusuma & Mahfud (2017).

When oil extraction is applied in industrial processes, the particle size is reduced and the essential oil is extracted inside an extractor tank, using appropriate solvents. After this step, the liquid phase is passed to an evaporator to remove the solvent and recover the essential oil. These processes can be applied on an industrial scale only after the associated phenomena have been studied. The difference between the ebullition temperature of essential oil and solvents ensures rapid solvent removal, using a one-stage evaporation processes. The solvent can be recovered, recycled and re-used in same extraction process, leading to a reduced need for new solvents.

For Vieira de Melo et al. (2000), the conventional procedures for essential oils recovery from plant materials, as cited above, can present limitations due to instability of essential oils by heat and by residual organic solvent from extraction. Hence, the use of SCF for essential oils extraction is being evaluated as an alternative to traditional methods.

Supercritical CO 2 offers advantages because it is not toxic, not expensive, non-flammable and odor- and colorless. Also, the critical temperature is near to ambient temperature and it has low viscosity and high diffusivity in relation to liquids. Thus, it has transformed into a common solvent in the process of natural materials ( Vieira de Melo et al., 2000)).

The extraction of compounds from solid matrices is associated with the internal mass transfer (dissolution and diffusion) and external mass transfer (solid particles). Thus, the solute diffused in the supercritical fluid-rich phase can be carried away by the bulk flow. Numerous authors have been published their attempts of model for these supercritical extraction process ( Reverchon & Morone, 1997; Sovová, 1994; Sovová, 2005), and a broad discussion of modelling aspects is presented elsewhere ( Capuzzo et al. (2013); Danlami et al. (2014); Khaw et al. (2017); Reverchon (1996); Vieira de Melo et al. (2000) and Lin et al. (2013)).

Essential oil extraction using high pressure or supercritical fluids (SCF) is an alternative from organic solvent extraction or steam distillation, and has been studied by different industries, especially in the food, pharmaceutical and cosmetic industries ( Ferreira et al. (1999); Pinto et al. (1999) and Cao et al. (2007)).

Khaw et al. (2017) observed the influence of solubilization on fluid density. At a high pressure, SCF presents high density, allowing the dissolution of great quantities of organic compounds. When the density is reduced, those dissolved compounds can be recovered, and it can be performed by decreasing the pressure or increasing the temperature.

For the author cited below, by performing the separation process at lower temperatures it can avoid the degradation of compounds by decreasing the heat exposure, which occur in steam distillation.

Supercritical fluid extraction provides better extraction efficiency, increases the production rate and improves the product quality. These processes, generally hybrid, involve the use of supercritical fluid, compressed air and a co-solvent, which, in combination, can intensify the fluid bubbling in the solid mass and increase the extraction efficiency ( Figure 2).

System used in supercritical extraction.

Process adapted from Borges et al. (2017).

To increase the extraction efficiency with use the supercritical fluids, the extraction process must be optimized, guaranteeing the best operational conditions; mainly the temperature, pressure, flow rate, residence time, solid material size, and ratio between the material and the flow rate of the fluid used as the extractive solvent (organic solvent, supercritical fluid and compressed air).

For Manjare et al. (2019), the low polarity of supercritical CO 2 is an important obstacle to its application, hindering the extraction of polar compounds. However, this could be surpassed by the addition of polar modifiers, as methanol or ethanol together with supercritical CO 2, therefore increasing its solution power.

Under these conditions, it is possible to increase mass transfer rate and improve the performance of the process, guaranteeing the applicability of this type of fluid as an alternative for the improved extraction in industrial processes ( Huang et al. (2016) and Silva et al. (2004)).

De Barros et al. (2014) studied supercritical extraction using laboratory and pilot scale processes. Their parametric studies supported the mathematical models developed for industrial-scale extraction, guaranteeing extraction products with high quality and greater added value. The parameters studied highlight the most appropriate and highest-performing operating conditions, allowing extraction curves to be constructed, and those arising from different procedures can be compared.

Based on these considerations, this paper proposes the extraction of essential oil, using a hybrid solvent system, involving a conventional solvent (ethanol) and a supercritical fluid (CO 2). In this study, the operational parameters (temperature, pressure and solvent flow rate and mass used ratio) were controlled in order to determine the optimal condition, to increase the mass transfer and yield of the essential oil production process. The procedure used in this paper is the same used by Capuzzo et al. (2013).

 Methods

In this study, for the experimental setup, an extractor system was built. The system involves the use of ethanol and supercritical fluid (CO 2), as a hybrid fluid mixture, based on supercritical concepts, and the essential oil was extracted from onion. The best operational conditions were determined, aimed at market efficiency, considering the supercritical flow rate, temperature and mass of onion used in the extraction. The turbulence phenomenon was evaluated in terms of the intensity of the solid-fluid mixture, considered the velocity of the ethanol-CO 2 mixture. The CO 2 was bubbled in ethanol in a distillation balloon, where the mixture received the thermal energy needed to increase the solid to fluid phase transferring. The effect of onion mass on the mass transfer efficiency was also investigated.

The onion sample used in this study was previously chopped and dried in a horizontal dryer with controlled circulating air speed and the air was heated using electrical resistance, in order to improve the efficiency during the extraction process. The results are represented as graphs showing the mass loss over time.

Experimental procedure

Raw material preparation. In the study reported herein, onion ( Allium cepa) was used as the raw material, which was chopped into cubes to obtain an adequate size for essential oil extraction. The chopped onion was placed in a convective dryer, coupled to an analytical balance and temperature control sensors.

The mechanical ventilation system provided airflow and there was a humidity (water concentration) gradient between the air and the onion, to increase the mass transfer from solid particles to the air.

The operation parameters for drying, hot air and mass loss of raw material were controlled. The process was carried out to give an onion mass loss of 50%, with a temperature of 40±1.2ºC, hot air speed of approximately 1.7 m/s, extraction time for 240 min and average particle size of 4 mm.

Procedure for essential oil extraction. The experimental apparatus shown in Figure 3 was used to carry out the extraction of essential oil from the dried onion.

Experimental apparatus constructed for essential oil extraction.

The apparatus built for this study was comprised of three sections: a) section with fluid mixture of solvent and supercritical fluid (CO 2) heated to the temperature required to provide the best conditions for the extraction process; b) the extraction of essential oil section, allowing interaction between solid and fluid phases, where the mass transfer from solid to fluid mixture occurs; c) section for recovery of essential oil, where the extraction fluid mixture carries the essential oil through the heat exchanger to the expansion tank for recover, by condensation, of the fluid mixture, ethanol and essential oil.

The experiments were performed using the apparatus ( Figure 3), applying the following procedure: a) the raw material is placed inside the extraction tank and the water in the condensation system is cooled. Also, during this step, the ethanol inside the balloon is heated; b) cooled water begins to recirculate in the condenser system and when the temperature in the balloon reaches that suitable for extraction, the bottle of the CO 2 valve was opened and the flow of supercritical fluid was bubbled into the ethanol to form the mixture which would pass to the extraction tank; c) the fluid mixture flows in contact with the solid phase and the mass transfer occurs. The flow then passes through the condenser to recover the ethanol and essential oil in the expansion tank.

The methodological sequence described herein was used to carry out the essential oil extraction using onion as the raw material, as well as the effect of operation parameters on the quality of the process was evaluated.

Table 1 show the operational conditions used in this study, based on an experimental plan to evaluate the parameters and their effect on the process efficiency, based on the essential oil extraction capacity.

Operational conditions used to carry out the experiments.
Nº Parameters Exp. 1 Exp. 2 Exp. 3
1 Initial onion mass (g) 255.8 614.97 184.73
2 Pressure (mm Hg) 760.0 760.0 760.0
3 Temperature (ºC) 63.4 62.6 55.4
4 CO 2 flow rate (L/min) 0.5 1.44 3.60
6 Ethanol mass in balloon (g) 480 481 478
7 Size of raw material particles (mm) 4.0 4.0 4.0

The performance of each experiment was evaluated based on Equation 1.

η ( % ) = M a s s o f o i l e x t r a c t e d ( g ) I n i t i a l m a s s o f b i o m a s s i n t h e e x t r a c t e d ( g ) ∗ 100 ( 1 )

The performance of each extraction essay was related to the operation parameters mainly the bubble intensity of CO 2 in ethanol, temperature and fluid mixture flow rate (measured at the inlet and outlet of the expansion tank).

Essential oil purification. The essential oil produced in the extraction process describe herein was purified using simple distillation and filtration, supported by technical norms. The procedure applied provides products with a high degree of purity, suitable for application as a raw material in other industrial processes.

Characterization of essential oil. The essential oil extracted and purified was characterized to determine the acid and refraction indexes, viscosity, and specific mass. The analysis procedures are described in the standard test method ASTM D 974 and the European standard EN14103.

 Results and discussion

The results of the experimental assays described above are reported in this section along with a discussion, supported by data available in the literature, to better contribute to future scientific and technological developments.

Drying of raw material

Table 2 shows the results obtained from the drying of the raw material (onion) used in this study, based on experiments conducted using the procedure described above. The results show the mass loss over time, characterized by a loss of the water present in the structure of the raw material. The runs were conducted with control of the operation parameters.

Results obtained for drying of raw material over time.
Nº Time (min) Mass (g) Mass loss (%) Evolution of mass (%)
0 0 1784.8 0 100
1 30 1543.5 11.46 88.53
2 60 1340.2 20.52 79.47
3 90 1161.3 27.72 72.27
4 120 1094.7 34.38 65.61
5 150 965.1 40.82 59.17
6 180 940 45.28 54.72
7 210 917.2 49.32 57.68
8 240 892.4 52.41 47.59

The data in Table 2 were obtained during the drying process (240 min) and the mass was measured at 30-min intervals. Globally, for this assay, a mass loss of 892.4g corresponds to 54.42% of the initial raw material mass. These data were used to construct the graphs in Figure 4, which show the mass loss during the drying time. The drying curve indicates that a greater mass was transferred from the solid to gas phase, occurred during first hour of drying, equivalent to 39.15% of the global mass loss. In the second hour, the mass loss represents 26.45% of the global mass, in the third hour 20.80% and in the last hour 13.5%.

Drying curve obtained from experimental data.

The decrease in water mass during drying, according to Figure 4, was associated with the mass transfer phenomenon, where the flow rate is directly related to the concentration gradients of water, between the solid and gas phases, associated with this process. In this process, mass transfer by convection at the solid surface predominates, and this is proportional to the solid area through the transfer coefficients. Equation 2 can thus describe the mass transfer, where the flow transference is related with global coefficient and the concentration gradient.

N A = k c Δ C A ( 2 )

Where N A is the flow rate of water (kmol/m 2s), k c is the global coefficient and Δ C A is the concentration difference between the solid particle and the airflow.

The mass loss rate was greatest during the first 180 min when approximately 65.60% of the total loss occurred. The mass transfer rates then decreased substantially leading to relative stability. The reduction in mass transfer rates is related to the drying time, which can be adjusted to guarantee the optimal condition for the essential oil extraction, as described herein.

 Essential oil extraction

Experimental apparatus. The experimental apparatus described above was used to carry out the procedure applied to obtain the essential oil extract from dried onion ( Figure 4).

Once the apparatus had been built, we ascertained the limits operational based on experimental planning, mainly the characteristics of the mixture, supercritical fluid flow rate and the ratio of mass of conventional solvent and supercritical fluid that gives the best extraction performance.

Experimental data acquisition. Initially, the conventional procedure tested the interaction of ethanol (vapor) rate and onion (solid) mass transference from solid to vapor phase. This process, established according the solubility of the solid in the solvent, occurs through mass transfer by diffusion and convection at the solid and vapor interface. In this initial study, a Soxhlet apparatus was used to extract the essential oil from onion particles, with a global yield of the 27.93%.

Using the same conditions ( Table 3), the yield increased in non-linear fashion of CO 2 performance operation and reach over 60% (yield).

Essential oil recovery in each experiment.
Nº Initial mass (g) Mass of essential oil recovered (g) CO 2 speed (mL/min) Extract yield (%) Density (g/mL) Refraction index
Exp 1 255.80 59.22 0.5 23.15 1.132 1.3455
Exp 2 614.97 213.52 1.44 34.70 1.130 1.3535
Exp 3 184.73 118.15 3.60 63.96 1.127 1.3465

In all of these experiments, the operating time was the same. Aris et al. (2019) and Hassanien (2019) evaluated the effect of fluid flow rate using supercritical CO 2 and concluded that when CO 2 flow rate increased, the extract yield increased by 3.698% for every 0.7 mL/min. According to the cited authors, under conditions of low pressure and high temperature, a rise in the flow rate of 8 mL/min would increase extract yield. Maran & Priya (2015) and Gadkari & Balaraman (2017) observed that with an increase in the flow rate, the film thickness surrounding the solid particle reduced, thus decreasing the external mass transfer resistance around the solid. The solute could thus easily move to the bulk solvent, enhancing the solubilization of the solute in the solvent and increasing the extract yield.

Özkal & Yener (2016) observed that when the flow rate of the supercritical fluid was increased considerably, there was an increase in the convection between solid and CO 2. In this case, this causes damage to the weak parts of the solid particles, leading to freer solute being removed from the solid particle during extraction.

Table 3 shows the yield effect by the CO 2 flow rate in terms of essential oil recovery, related to the role of the solubility of the oil on the concentration in the supercritical fluid flow.

In a study by Inamuddin & Asiri (2020) and Da Silva et al. (2016), the extraction yield with ethanol, at high CO2 solvent pressures, increased with a decrease in pressure in the binary CO2-ethanol system, and the extraction yield was high.

In the study reported herein, a hybrid system was used, with supercritical fluid and ethanol as the extraction fluid. In this case, the bubbling of the supercritical fluid in a conventional solvent was used to form the mixture, which was applied in the extraction of essential oil. Interaction between the solid and vapor phases was verified.

In all cases evaluated, the essential oil was characterized in terms of the acid index and refraction index ( Table 3). The values obtained for the three runs were 1.132; 1.130 and 1.127 g/mL, which are within limits reported in literature. The average value obtained for the acid index in these the experiments was 3.19 mg KOH/g, while data obtained from the literature show an average value 3.56.

 Conclusions

We can conclude:

The essential oil from supercritical fluid and conventional solvent mixture extraction provided good yields;

The use of CO 2 in the extraction processes is a good alternative to improve the extraction performance, due to the turbulence imposed on the mixture in the extractor, which increases the mass transfer between solid and fluid phases;

The mass transfer by convection in essential oil extraction, based on the results of this study, is more effective than that by diffusion, mainly with the use of a supercritical fluid in the system; and

The apparatus proposed for this study represents a good alternative to improve the extraction of essential oil, with a view to obtaining new products for use as raw materials in other industrial processes.

 Data availability

All data underlying the results are available as part of the article and no additional source data are required.

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Braz J Chem Eng. 2000;17(3). 10.1590/S0104-66322000000300014 10.5256/f1000research.56257.r257046 Reviewer response for version 1 Tatiane Raspe Djéssica 1 Referee State University of Maringa,, Maringa, Brazil

Competing interests: No competing interests were disclosed.

 6 5 2024 Copyright: © 2024 Tatiane Raspe D 2024 This is an open access peer review report distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. recommendation approve-with-reservations

1) Knowing that the term "essential oil" is only and exclusively permitted when obtained by hydrodistillation (aqueous distillation), do the authors think it is appropriate to use this term in the title of the work? The results were compared with Soxhlet using a solvent (other than water), however this is not appropriate for obtaining essential oil. The introduction cites "vegetable oil" as a result of these techniques. If this is correct, vegetable oil has volatile compounds that provide aroma to the sample, which is not essential oil. I suggest that the text be revised, as it is not adequate.

2) The introduction is too long. The references cited are not current (5 years) and compare essential oil extraction with extraction of oils and plant extracts, hence the confusion in the terms used. I suggest revising the text.

3) It is not necessary to highlight in the Summary that the onion was "previously chopped and dried".

4) In the abstract the authors say "The essential oil obtained from the extraction was collected and purified and the mass was determined (by weighing) to evaluate the effect of CO2 flow on the yield". Was the ethanol evaporated?

5) Where was the onion purchased? Add geographic location.

6) Why were the brand and model of the equipment, and the origin of the solvents, not mentioned? It is not possible to reproduce the experiments without this information.

7) How many replications were made of each experiment? I suggest adding the deviation of the replicas in the answers, as well as data from some statistical test.

8) The work presents a very long introduction and poor discussion of the results. Why was the supercritical fluid flow rate evaluated? Is it relevant? Why? What physical influence does this variable have? Discuss this data further.

9) " the bubbling of the supercritical fluid in a conventional solvent was used to form the mixture, which was applied in the extraction of essential oil. Interaction between the solid and vapour phases was verified", what is the physical explanation of this interaction? It is necessary to explore this topic further and better.

10) Discuss the results of acid number and refractive index. Just citing is poor. What do these values mean? What does the legislation establish? Other authors, using what techniques, found "similar" values? For which oils? Were they really essential oils, or were they vegetable oils?

11) In the abstract it is mentioned "Since this apparatus presents more than double extraction yield than Soxhlet experiment". Where is this data? Why were they not presented and discussed? Where was this information taken from?

12) The conclusion only repeats phrases cited in the text. Prepare a coherent and appropriate conclusion for the work.

Is the work clearly and accurately presented and does it cite the current literature?

Partly

If applicable, is the statistical analysis and its interpretation appropriate?

No

Are all the source data underlying the results available to ensure full reproducibility?

Partly

Is the study design appropriate and is the work technically sound?

Partly

Are the conclusions drawn adequately supported by the results?

Partly

Are sufficient details of methods and analysis provided to allow replication by others?

Partly

Reviewer Expertise:

Emerging/unconventional extraction techniques.

I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.

 10.5256/f1000research.56257.r257060 Reviewer response for version 1 Mas'ud Fajriyati 1 Referee https://orcid.org/0000-0002-6116-3351 Politeknik Negeri Ujung Pandang, Makassar, South Sulawesi, Indonesia

Competing interests: No competing interests were disclosed.

 3 4 2024 Copyright: © 2024 Mas'ud F 2024 This is an open access peer review report distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. recommendation approve-with-reservations

The Introduction section is too long and doesn’t focus on the main point of the study, namely Essential oil extraction using ethanol and CO 2 as an extraction fluid mixture. The introduction section should clearly highlight the importance of Essential oil extraction using ethanol and CO 2 as an extraction fluid mixture.

It’s best to cite several studies related to this extraction technique along with its advantages compared to conventional extraction in terms of essential oil yield and the physicochemical properties of the product.

Please state the significance of this study, and also re-state the aim of the study.

Why discuss drying onions? Just mention the % water content of the onion as a sample. Table 2 and figure 4 do not need to be displayed and discussed in this article, they must focus on the core problem, namely essential oil extraction using ethanol and CO 2 as an extraction fluid mixture.

In table 1 it’s stated that there were 3 unit experiments, but the operating conditions were different (temperature, ratio of sample and ethanol). Why?

 Unlike its current form, this article requires major revision to clarify the methods, results, discussion, and conclusions of this work.

Cite several studies that support the statement “the yield increased in the non-linear fashion of CO 2 performance operation and reached over 60% (yield).”

Cite references that support the results of this work.

Results and discussion are very short and unfocused, making it difficult to summarize the results of the work.

Conclusions are unclear.

Is the work clearly and accurately presented and does it cite the current literature?

Partly

If applicable, is the statistical analysis and its interpretation appropriate?

Not applicable

Are all the source data underlying the results available to ensure full reproducibility?

Partly

Is the study design appropriate and is the work technically sound?

Partly

Are the conclusions drawn adequately supported by the results?

No

Are sufficient details of methods and analysis provided to allow replication by others?

Partly

Reviewer Expertise:

Extraction, food technology, analytical chemistry

I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.

 10.5256/f1000research.56257.r257040 Reviewer response for version 1 Idris Sitinoor Adeib 1 Referee Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia

Competing interests: No competing interests were disclosed.

 1 4 2024 Copyright: © 2024 Idris SA 2024 This is an open access peer review report distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. recommendation approve-with-reservations

This article focusing on extracting essential oil from onion using supercritical CO2 + ethanol as the fluid in supercritical extraction and comparing the yield with the one from the conventional method (Soxhlet).

The introduction was enough as it highlighted the conventional and current trends and research on the techniques and methods used in extracting essential oil. However, no writing on onion in the introduction part which is also crucial as to highlight why onion is chosen for this research. 

Methodology was good written, however the layout configuration of Figure 1, Figure 2 and Figure 3 can be better. Process flow diagram (PFD) form or PNID would be better. The caption for Figure 1 and Figure 2 should be revised. The word pump was incorrectly spelled in Figure 1. Equation 1 should be type using correct symbol for multiplication.

The term 'non-linear fashion' used in the second paragraph in Experimental data acquisition section should be validated with a statistical value.

Conclusion (c) should be discussed more in Discussion part as the term diffusion was more stated in the Introduction part but not being discussed together with the results in the discussion part. Maybe a graph or a sketch can be done to highlight the diffusion and convection process during the extraction process.

Is the work clearly and accurately presented and does it cite the current literature?

Yes

If applicable, is the statistical analysis and its interpretation appropriate?

I cannot comment. A qualified statistician is required.

Are all the source data underlying the results available to ensure full reproducibility?

Yes

Is the study design appropriate and is the work technically sound?

Yes

Are the conclusions drawn adequately supported by the results?

Partly

Are sufficient details of methods and analysis provided to allow replication by others?

Yes

Reviewer Expertise:

Extraction of vegetable oil, extraction of essential oil, supercritical fluid extraction

I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.

 10.5256/f1000research.56257.r257054 Reviewer response for version 1 Sawangkeaw Ruengwit 1 Referee Chulalongkorn University, Bangkok, Thailand

Competing interests: No competing interests were disclosed.

 29 3 2024 Copyright: © 2024 Sawangkeaw R 2024 This is an open access peer review report distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. recommendation reject

This article proposed the extraction method using ethanol and carbon dioxide at atmospheric pressure to extract the essential oil from the dried onion. The extractions were conducted in three experiments with different flow rates of carbon dioxide. However, the characterization of extracted oil was doubtful. Because the extraction yield obtained at the flow rate of 3.6 mL/min were 2-fold higher than observed from the Soxhlet extraction, this statement is questionable. The reviewer suggests the comments for improvement of this article below.

Introduction:

Condense the introduction and ensure that it clearly highlights the importance of onion essential oil.

Explicitly mention the significance of onion essential oil in various applications to provide context for the study.

 Figures:

Ensure that Figure 1 accurately depicts the supercritical carbon dioxide extraction process.

Verify that Figure 2 accurately represents the hydro-distillation process.

 Aim of the Study:

Revise the aim of the study to accurately reflect the extraction conditions. The extraction process at 760 mm Hg is not under supercritical condition of ethanol-carbon dioxide mixture.

 Methods and Experimental Procedure:

Avoid repetition and redundancy in the Methods section.

Provide clear and accurate methods for characterizing essential oil properties ASTM D974 and EN14103 are employed to determine acid number and the fatty acid methyl ester (FAME) content of oil feedstock used in biodiesel production, respectively.

 Results and Discussion:

Ensure that data presented in Table 2 and Figure 4 are not redundant. If the drying curve is not directly relevant to the focus of this study, consider moving it to supplementary data.

Clarify what is meant by "non-linear fashion" in the context of the results or discussion.

Provide more detail about the Soxhlet extraction method used for comparison, including relevant parameters and conditions.

Is the work clearly and accurately presented and does it cite the current literature?

Partly

If applicable, is the statistical analysis and its interpretation appropriate?

Not applicable

Are all the source data underlying the results available to ensure full reproducibility?

Partly

Is the study design appropriate and is the work technically sound?

Partly

Are the conclusions drawn adequately supported by the results?

No

Are sufficient details of methods and analysis provided to allow replication by others?

No

Reviewer Expertise:

Supercritical fluid extraction, Biodiesel production, Subcritical water extraction, Lipid, Essential oil

I confirm that I have read this submission and believe that I have an appropriate level of expertise to state that I do not consider it to be of an acceptable scientific standard, for reasons outlined above.

References : Supercritical CO2 Extraction Optimization of Onion Oil Using Response Surface Methodology. Chemical Engineering & Technology .2012;35(4) : 10.1002/ceat.201100217 646-652 10.1002/ceat.201100217 10.5256/f1000research.56257.r257049 Reviewer response for version 1 Gasparetto Henrique 1 Referee Universidade Federal de Santa Maria, Santa Maria, Brazil

Competing interests: No competing interests were disclosed.

 21 3 2024 Copyright: © 2024 Gasparetto H 2024 This is an open access peer review report distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. recommendation approve-with-reservations

In my opinion, the article is very short and inconclusive. Although I believe it can be indexed after a major revision. Here are some considerations to improve the work:

- Please provide recent references for oil extraction using ethanol and supercritical CO2. In my opinion, solvent extraction is better for industry than supercritical CO2. Although the results are worth studying.

- It is unclear how the authors chose the operational conditions mentioned in Table 1; is it derived from some experimental planning design or solely from the prior literature?

- Please mention the technical norms in the "essential oil and purification" section. It would be better to describe them also.

- In Table 2, it's unnecessary to mention the mass loss in g over time; only the % is enough. Indeed, I recommend that the authors present only Fig. 4. as the drying kinetic is not the main result of the work.

- Why do the authors not estimate parameters for the drying kinetics? Several models can be easily linearized to obtain mass transfer coefficients.

- The extraction conditions are very unclear. Is it part of a central composite or rotational design?

- How about the amount of ethanol and CO2? Is ethanol used as a co-solvent at which amounts?

- The authors do not report the Soxhlet methodology in the procedure for oil extraction.

- Some considerations about solvent flow are well compared with the literature, but in my opinion, they do not provide any exciting information. It is kind of trivial that increased solvent speed will increase oil extraction. However, what about the density and refraction indexes? What do they mean? How could the authors explain those differences concerning the operational variables? Besides, are temperature and pressure fixed?

- Were experiments performed in multiple attempts?

- I recommend the authors report the acid index separately for each experiment and not the average. The acid value and refractive index could respond to how the operational variables influence onion essential oil extraction, as there isn't any other characterization technique. FTIR could be an exciting tool for briefly screening the oils extracted, while GC-MS could offer a profound understanding of the essential oil characteristics.

-  The experimental results must be scrutinized cautiously and not just presented.

- The article is inconclusive; no key conclusion is reported in the "Conclusions" section.

Is the work clearly and accurately presented and does it cite the current literature?

Partly

If applicable, is the statistical analysis and its interpretation appropriate?

Not applicable

Are all the source data underlying the results available to ensure full reproducibility?

Partly

Is the study design appropriate and is the work technically sound?

Partly

Are the conclusions drawn adequately supported by the results?

No

Are sufficient details of methods and analysis provided to allow replication by others?

Partly

Reviewer Expertise:

Process separation; green chemistry; chemical engineering.

I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.

 10.5256/f1000research.56257.r91263 Reviewer response for version 1 Pessela Benevides Costa 1 Referee Departamento de Biotecnología y Microbiología de los Alimentos, Instituto de Ciencias de la Alimentación, CIAL-CSIC, Autonomous University of Madrid, Madrid, Spain

Competing interests: No competing interests were disclosed.

 20 8 2021 Copyright: © 2021 Pessela BC 2021 This is an open access peer review report distributed under the terms of the Creative Commons Attribution Licence, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. recommendation approve-with-reservations

I think the subject of this experiment is important. Even so, I think that the organization of the data should be improved.

I believe very sincerely, even if I am not an expert in the English language, it must be somewhat improved for a better understanding of the work.

The authors should explain why they use 3 masses of the starting material for the extraction of essential oils in the experiment.

Another aspect that must be presented has to do with purification experiments. Some analytics that compare a commercial essential oil with those obtained in this experiment is necessary, for example an FTIR experiment or a thin layer chromatography would be essential to better understand this experiment.

Is the work clearly and accurately presented and does it cite the current literature?

Yes

If applicable, is the statistical analysis and its interpretation appropriate?

Not applicable

Are all the source data underlying the results available to ensure full reproducibility?

Yes

Is the study design appropriate and is the work technically sound?

Yes

Are the conclusions drawn adequately supported by the results?

Partly

Are sufficient details of methods and analysis provided to allow replication by others?

Partly

Reviewer Expertise:

Biotechnology, Screnning of enzymes, Enzyme purifications; Reconbinant process on biotecnhology.

I confirm that I have read this submission and believe that I have an appropriate level of expertise to confirm that it is of an acceptable scientific standard, however I have significant reservations, as outlined above.

Barros António ISPTEC, Angola

Competing interests: No competing interests were disclosed.

 21 8 2021

Thank you for your consideration and contribution on this paper. We evaluate your comment and I answer here.

We make three experiments with better evaluation of the influence of CO 2 flow rate on extraction essential oil yield (%). For this case, was used the range of CO 2 flow rate, respectively 0,5; 1,44 and 3,6 ml/min, with one increase relation of 0,94 ml/min between the first and second CO 2 flow rate and 2,66 ml/min between the second and last CO 2 flow rate. In this increment, it is possible to evaluate the operational conditions used to relate it with mass transference associated with the essential oil yield. With this performance we can know the relevance of CO 2 flow rate with the mass transference and its relation with the turbulence systems. The results showed that when the CO 2 flow rate increase, the turbulence mixture in the system increases too, and that results in increased essential oil yield (%). For this case, the turbulence affect more effectively of mass transference than the contact time between the two phases involved in this study.

We need to evaluate the quality of essential oil produced here using all tool or equipment commonly used for this type of study, but our laboratory has many limitations in term of special equipment for this ending. The laboratory, same with its big dimensions, don’t have FTIR or chromatography or infrared or other equipment. We use only, to evaluate essential oil produced in this study, the procedure associated with equipment’s disposed in laboratory. For this, we evaluated the density, viscosity and refraction index parameters associated with the laboratory conditions. Even so, the results from this study were compared with those referenced in the literature and all that gave the same performance. In these conditions we can confirm the quality of essential oil produced in this study. In future we will establish relation with other laboratory in Angola or in another country to develop joints projects to minimize these situations.