A Bibliometric Analysis of the Supercritical CO₂ Extraction of Essential Oils from Aromatic and Medicinal Plants: Trends and Perspectives

Abstract

This bibliometric analysis investigates the scope and evolution of research on the supercritical CO₂ extraction of essential oils from aromatic and medicinal plants. Utilizing the Scopus database, we examine publications from 1995 to 2024, revealing advancements in extraction technologies and collaborative efforts across 42 countries. Key findings include the increasing publication rates signifying the method’s growing acceptance and the pivotal role of countries like China and Italy in influencing the research trajectory. The study employs R Studio for data analysis and VOSviewer for network visualizations, uncovering the centrality of terms like “carbon dioxide”, “essential oils”, and “supercritical extraction” in the research discourse. The results offer insights into the method’s efficiency and highlight potential directions for future research in sustainable extraction practices.

1. Introduction

The extraction of essential oils from medicinal and aromatic plants holds significant value in pharmaceuticals, nutraceuticals, and cosmetics due to the therapeutic potential of these oils. Essential oils are complex mixtures of bioactive compounds, including terpenes, phenols, and aldehydes, which contribute to antimicrobial, antioxidant, and anti-inflammatory properties [1]. Conventional extraction techniques, such as steam distillation, hydrodistillation, and solvent extraction are widely used; however, each have limitations. Steam and hydrodistillation, for example, expose compounds to high temperatures, potentially causing thermal degradation and the loss of volatile components. Solvent extraction, while effective for certain compounds, often leaves residual solvents in the final product, which raises safety and environmental concerns [2,3]. Supercritical CO₂ extraction (SFE) has emerged as an efficient and environmentally friendly alternative to these traditional methods. In SFE, carbon dioxide (CO₂) is used in its supercritical state—above its critical temperature (31.1 °C) and pressure (73.8 bar)—where it exhibits both liquid and gas-like properties. This unique state enables CO₂ to diffuse through plant matrices like a gas and dissolve compounds like a liquid, providing a versatile method for extracting thermolabile and volatile compounds without compromising their integrity [4].

During the SFE process, CO₂ is first compressed and heated to reach supercritical conditions. The plant material is then exposed to the supercritical CO₂, which selectively dissolves target compounds based on their solubility and polarity. Following extraction, CO₂ is depressurized to separate it from the extracted compounds, allowing for the recovery of high-purity extracts and the recycling of CO₂ within a closed-loop system. While traditional methods, such as solvent maceration, produce extracts commonly referred to as “essential oils”, supercritical CO₂ extraction yields products known as “oleoresins”. Oleoresins capture a broader spectrum of bioactive compounds, including both volatile and non-volatile constituents that are often lost in conventional essential oil extraction. Adjustments in pressure, temperature, and the addition of co-solvents during the SFE process further enhance extraction efficiency, making it adaptable to diverse plant materials and target compounds [5].

SFE offers several advantages over traditional extraction methods, making it particularly suitable for medicinal plants. First, SFE operates at relatively low temperatures, preserving thermolabile compounds often degraded by high-temperature methods like steam and hydrodistillation. Second, the solvent power of supercritical CO₂ can be precisely tuned by adjusting temperature and pressure, allowing for the selective extraction of specific compounds. This results in purer extracts with fewer unwanted by-products. Additionally, CO₂ is an inert, non-toxic, and easily removable solvent, posing minimal environmental risk and supporting the global shift toward sustainable “green” extraction methods. Finally, SFE systems operate in a closed loop, enabling CO₂ recycling, which makes the process both cost-effective and scalable for industrial applications [6].

The increasing adoption of SFE aligns with a broader emphasis on eco-friendly and efficient extraction processes in fields such as phytotherapy, nutraceuticals, and green chemistry. Recognizing the importance of SFE in advancing essential oil extraction, this study presents a bibliometric analysis covering nearly three decades (1995–2024) of research on SFE applications for essential oils and oleoresins. Through this analysis, we explore research trends, identify influential contributions, and map collaborative networks within this field, providing insights into the scientific trajectory and emerging opportunities in supercritical CO₂ extraction for medicinal and aromatic plants.

2. Materials and Methods

2.1. Data Source

Database: The primary database utilized for this bibliometric study is Scopus, which offers a robust and comprehensive catalog of scientific publications across various disciplines. Scopus is particularly valued in bibliometric research for its extensive coverage, making it an ideal tool for conducting in-depth analyses in the fields of Science, Technology, and Medicine (STM) [7]. Justification for Choosing Scopus: Scopus was selected over other databases due to its broad inclusion of international scientific literature and its capabilities in citation tracking, which are critical for assessing research impact and trends. Unlike databases that specialize in specific fields or regions, Scopus provides a global perspective essential for this study’s aims to map worldwide research activities and collaborations in essential oil extraction using supercritical CO₂ [8,9].

2.2. Data Collection Process

Research Criteria: The research parameters were meticulously defined to focus on scientific literature about the extraction of essential oils using supercritical CO₂ from aromatic and medicinal plants. Keywords such as “supercritical CO₂ extraction”, “essential oils”, “aromatic plants”, and “medicinal plants” were utilized in a structured query to ensure the retrieval of relevant articles. For a comprehensive search, we included both the terms “essential oils” and “oleoresins” in our query, as extracts obtained via supercritical CO₂ are often referred to as “oleoresins” in addition to “essential oils”. This approach allowed us to capture a broader range of publications related to supercritical CO₂-extracted products.

Data Extraction: An advanced search was conducted via the Scopus search interface using the specified keywords. The search was strategically filtered to include only peer-reviewed research articles, reviews, and conference proceedings that are pertinent to the defined study objectives, ensuring the relevance and quality of the data collected [10,11].

2.3. Bibliometric Analysis Methods

Use of R Studio: For the analysis of the bibliometric data extracted from Scopus, R Studio was employed, utilizing specialized packages such as bibliometric. This setup allowed for the extraction and analysis of bibliographic data, the computation of bibliometric indicators such as the H-index and journal impact factor, and the generation of statistical and graphical outputs. Bibliometrix facilitated the assessment of citation networks, keyword co-occurrence, and author co-citation networks, providing a comprehensive toolkit for this study.

Use of VOSviewer: To further augment the analysis, bibliometric data were imported into VOSviewer for network visualizations. This tool was instrumental in mapping the connections between co-authors, co-citations, and keywords, thereby illuminating the most influential contributors, prevalent research clusters, and dominant themes within the field. These visualizations not only highlight the intellectual landscape but also suggest areas for potential future research [12,13].

By employing these sophisticated bibliometric tools and techniques, the methodology ensures a rigorous and thorough exploration of the bibliometric data, revealing the dynamics and evolution of the field of essential oil extraction using supercritical CO₂. This analysis not only highlights the current research status but also guides future studies by identifying emerging trends and potential research gaps [14].

2.4. General Extraction Parameters

The studies analyzed in this research generally apply supercritical CO₂ extraction parameters, often operating at pressures ranging from 100 to 300 bars and temperatures between 30 and 80 °C, depending on the targeted volatile compounds and plant matrices. These conditions are chosen to optimize the extraction process and are known to preserve the integrity of bioactive compounds within the extracts.

2.5. Background Behind Choosing the Keywords

The choice of the keywords “CO₂ supercritical extraction”, “essential oils”, and “aromatic and medicinal plants” is rooted in the increasing demand for environmentally sustainable and efficient extraction methods that preserve the integrity of bioactive compounds derived from natural sources [15,16]. Supercritical CO₂ extraction stands out as a pivotal technique due to its ability to operate at low temperatures, thereby maintaining the natural essence and properties of the extracted oils without the use of harmful solvents. This method is particularly significant for extracting essential oils, which are concentrated plant extracts known for their aromatic properties and therapeutic benefits. These oils are predominantly sourced from aromatic and medicinal plants, which have been utilized historically across various cultures [17,18].

The integration of these keywords encapsulates a modern approach to traditional practices, highlighting a shift towards greener technologies in extracting natural substances from plants that are vital for applications in the pharmaceutical, cosmetic, and food industries [19].

3. Results

3.1. Bibliometric Analysis Overview

This comprehensive table (

Table 1. Bibliometric data summary for document analysis.

Description		Results
Main Information About the Data	Timespan	1995–2024
	Sources	270
	Documents	689
	Annual Growth Rate %	8.79%
	Document Average Age	11
	Average Citations per Doc	27.45
Document Contents	Keywords Plus (id)	4144
	Author’s Keywords (de)	1657
Authors	Authors	2409
	Authors of Single-Authored Docs	11
Authors Collaboration	Single-Authored Docs	14
	Co-Authors per Doc	4.83
Document Types	International Co-Authorships %	21.34%
	Article	606
	Book Chapter	25
	Conference Paper	31
	Retracted	1
	Review	26
Language	English	602
	Chinese	81
	Portuguese	4
	Russian	3

) encapsulates a range of bibliometric data spanning from 1995 to 2024. It includes information gathered from 270 sources, culminating in 689 documents. The dataset shows a healthy annual growth rate in publications of 8.79% and indicates that the average document is around 11 years old. Each document has garnered an average of 27.45 citations, a testament to the impact and reach of the research within the academic community. Keywords are an essential part of bibliometric analysis, and in this dataset, there are 4144 instances of ‘Keywords plus’, which are additional keywords assigned by databases, and 1657 ‘Author’s keywords’ provided by the document authors. The number of unique contributing authors is 2409, with a subset of 11 authors who have the distinction of having published documents on their own. There are 14 documents identified as single-authored, indicating a small proportion of works where a single researcher drove the entire publication process. Collaboration is key in academia, and this is reflected in the average of 4.83 co-authors per document and an international co-authorship rate of 21.34%, highlighting the global nature of research collaboration. Most of the documents are articles, followed by book chapters and conference papers, a single retracted paper, and 26 reviews. Language-wise, the vast majority of the documents are written in English, followed by Chinese and Portuguese, and Russian being less common, demonstrating the linguistic diversity of the contributions to this field.

3.2. Chronological Scope and Interdisciplinary Research Trends

Starting with the advancements, is has been widely recognized that greener technology for extracting natural bioactive compounds from plants is paramount [20,21]. The drive for eco-friendly and generally recognized safe extraction methods has become a significant trend. In this context, supercritical fluid extraction, particularly with carbon dioxide, has been emphasized, due to its efficiency in extracting bioactive compounds like essential oils from various natural plant materials, including herbs, spices, aromatics, and medicinal plants. This method is known for its selectivity and productivity, contingent upon the critical parameters selected for the process and the nature of the bioactive compounds involved [22,23]. The bibliometric analysis conducted on 18 April 2024 meticulously mapped out the evolution of research on CO₂ supercritical essential oil extraction. This study precisely targeted documents that included terms such as “CO₂ supercritical extraction” and “essential oils” in their titles, abstracts, or keywords, ensuring the relevance and specificity of the data collected. The timeline of the analysis spanned from 1995 to the present, a period highlighted for its significant growth in research output. This era began modestly in 1995, with a gradual and consistent increase in publications over the years, illustrating a growing fascination with this field of study. This is depicted in Figure 1.

This figure (Figure 1) presents the juxtaposed trends in annual publications and their citations in the field of essential oil CO₂ supercritical extraction and reveals a complex narrative of research focus, impact, and scientific development over nearly three decades (from 1995 to 2024).

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Emerging Interest (1995–2000): The field begins modestly with two publications in 1995, and an average citation count of 2.50, indicating growing interest. By 1998, with eight publications, citation impact peaks at 4.09, signaling seminal research that garnered widespread attention within the academic community.

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Expansion and Diversification (2001–2010): The number of publications rises steadily, reaching 26 by 2010, while the citation average sees variation, underscoring the field’s diversification. This could reflect the foundational nature of early work, with some studies continuing to accrue citations and others, perhaps due to their more incremental nature, accruing fewer.

○

High Activity with Varied Impact (2011–2016): A significant uptick in publications occurs, peaking at 43 documents in 2016. This is contrasted by a variable citation average, peaking at 3.71 in 2010 but dropping to lower figures like 1.36 in 2012, suggesting not all publications are influential. The high citation counts in 2016, with a 3.48 average, may indicate a culmination of impactful work or the recognition of previous studies.

○

Recent Trends and Decline in Citations (2017–2024): The publication count continues its ascent, reaching 45 in 2023, yet citation averages trend downward, particularly dropping to 0.26 at the start of this year, 2024. This could hint at a field approaching saturation, where the large volume of publications may not equate to high-impact contributions. It might also signal shifting research priorities or the emergence of new methodologies. The marked decrease in both publications and citations in 2024, while not necessarily shocking due to it being early in the year and there being a likelihood that more studies are underway, invites scrutiny and may presage significant shifts in research direction.

The graph shown in Figure 2 displays the citation distribution across a set of documents according to Scopus data, highlighting their impact on essential oil CO₂ supercritical extraction. On the horizontal axis, the documents are organized in descending order based on the number of citations received, with the document receiving the highest citations placed at the leftmost. The vertical axis quantifies the total citations each document has garnered. The blue curve, representing these citation counts, shows a steep decline initially, suggesting that a few documents are significantly more cited than the others, possibly indicating seminal works with substantial influence. The purple 45-degree line signifies where the number of citations equals the rank of the document; for instance, the 50th document would ideally have 50 citations.

The intersection of this line with the blue curve, marked by a star and labeled “H-index = 72”, indicates the H-index of the dataset. This means that 72 documents have each received at least 72 citations, pointing to a significant scholarly impact within the field. The sharp peak at the beginning followed by a plateau suggests that while a few top-performing articles dramatically shape the citation landscape, a broad base of publications consistently contributes to the field’s development. This H-index and the overall citation trend provide essential insights into the research impact and thematic significance of the work conducted, illustrating the balance between productivity and citation impact that the H-index metric captures.

Overall, the increasing number of research publications in the field, along with the variable citation rates year over year, indicates a domain that is constantly developing. This dynamic indicates that the field is not static but is characterized by ongoing changes in focus and relevance. The inconsistency in citation rates—sometimes high, sometimes low—suggests that while research is being steadily produced, its influence and significance vary. The implication here is clear: for the field to progress meaningfully, there must be a concerted effort towards pioneering new research avenues and rigorously evaluating the contributions of this research for quality and impact. These evolving trends reflect how the field’s prominence and direction are influenced by new scientific discoveries, advancements in technology, and changes in economic and societal needs.

The scholarly works surveyed encompassed a range of scientific disciplines as presented in Figure 3, with Chemistry leading the way, now adjusted to represent approximately 330 documents, which is 24.4% of the research corpus, followed closely by Chemical Engineering with about 221 documents (16.3%), and Agricultural and Biological Sciences at roughly 218 documents (16.1%). The diversity of the field is further underscored by significant contributions from Biochemistry, Genetics, and Molecular Biology with around 104 documents (7.7%), Physics and Astronomy with about 91 documents (6.7%), and Pharmacology, Toxicology, and Pharmaceutics with approximately 80 documents (5.9%). Moreover, intersecting domains such as Medicine represented about 69 documents (5.1%), Engineering with about 72 documents (5%), Environmental Science with around 49 documents (3.6%), and Energy with approximately 29 documents (2.1%) were included. Additionally, other research areas such as Materials Science, Immunology, Microbiology, and various fields collectively contribute 126 documents (7.1%), demonstrating a broad engagement across the scientific landscape. The graphical elucidation in Figure 3 casts a spotlight on the extensive yet focused scholarly engagement across these diverse scientific territories. This compilation of research underscores the dynamic and steadfast interest from the scientific community, reflecting a spirited pursuit of innovation and a deeper understanding within the specialized realm of CO₂ supercritical essential oil extraction.

3.3. Pioneering Research: Key Articles and Leading Journals in Supercritical CO₂ Extraction of Essential Oils

Table 2. Top 10 journals on Scopus for research on CO₂ supercritical extraction essential oil.

Sources Journals	Articles	Total Publication/Document	Total Citation	Quartile	H-Index	Sjr 2023
Journal of Supercritical Fluids	91	939	7818	Q2	133	0.66
Journal of Essential Oil Research	29	217	1117	Q2	59	0.52
Journals of Industrial Crops and Products	27	4494	43,674	Q1	173	0.91
Journal of Chinese Medicinal Materials	22	N/A	N/A	Q4	25	00
Journal of Flavour and Fragrance Journal	21	205	1059	Q2	84	0.45
Journal of Molecules	19	27,362	184,529	Q1	227	0.74
Journal of Food Chemistry	12	74	139	Q1	324	1.75
Journal of Food Engineering	10	1247	14,682	Q1	217	1.16
Journal of Natural Product Research	10	3067	13,899	Q1	71	0.41
Journal of Essential Oil-Bearing Plants	9	493	1938	Q3	39	0.43

provides a comparative snapshot of various journals within the field of supercritical CO₂ extraction and essential oils research. It showcases the journals’ research output, influence, and quality through metrics such as the number of articles published in 2023, total citations, and the Scimago Journal Rank (SJR). The Journal of Industrial Crops and Products stands out with the highest number of documents and citations, indicating significant influence and research activity in Q1 quality, which suggests a robust reputation in the academic community. The Journal of Supercritical Fluids shows a strong focus on the subject, with a high H-index reflecting the consistent citation of its articles, denoting impact and maturity in the field. The presence of “N/A” for “Zhong Yao Cai” suggests incomplete data, which might be due to various factors such as recent indexing or database discrepancies. The SJR values give an insight into the relative prestige of these journals, with the Journal of Food Chemistry showing the highest rank, implying high visibility and citation rates for its publications.

Table 3. Ten most-cited articles on essential oil CO₂ supercritical extraction.

Article Title	Total Citations	Year of Publication	Journal	Authors
Supercritical fluid extraction and fractionation of essential oils and related products.	692	1997	Journal of Supercritical Fluids	Reverchon, E., et al.
Antioxidant activity of extracts obtained by different isolation procedures from some aromatic herbs grown in Lithuania.	507	1998	Journal of the Science of Food and Agriculture	Dapkevicius, A., et al.
Comparison of essential oils of clove buds extracted with supercritical carbon dioxide and other three traditional extraction methods.	484	2007	Food Chemistry	Guan, W., et al.
Chemical Composition and Antimicrobial Activity of Rosmarinus officinalis L. Essential Oil Obtained via Supercritical Fluid Extraction.	332	2005	Journal of Food Protection	Santoyo, S., et al.
Generation, Capture, and Utilization of Industrial Carbon Dioxide.	308	2010	ChemSusChem	Hunt, A.J., et al.
Reverse osmosis applications: Prospect and challenges.	273	2016	Desalination	Wenten, I.G., et al.
Flavour compounds of Lavandula angustifolia L. to use in food manufacturing: Comparison of three different extraction methods.	227	2009	Food Chemistry	Da Porto, C., et al.
Comparison of essential oil composition of Carum copticum obtained by supercritical carbon dioxide extraction and hydrodistillation methods.	225	2004	Food Chemistry	Khajeh, M., et al.
Towards more rational techniques for the isolation of valuable essential oils from plants.	214	1999	TRAC Trends in Analytical Chemistry	Luque de Castro, M.D., et al.
Supercritical Fluid Extraction of Plant Flavors and Fragrances	213	2013	Molecules	Capuzzo, A., et al.

aggregates the most cited articles on the supercritical CO₂ extraction of essential oils, showcasing pivotal contributions to the field. Leading the table is Reverchon, E.’s 1997 article, which, with 692 citations, reflects its enduring influence on extraction technologies. Dapkevicius, A.’s work on antioxidant activities from 1998 follows with 507 citations, emphasizing the method’s effectiveness in capturing essential oils’ health-promoting properties. Guan, W.’s comparative study on clove bud oils extraction from 2007, with 484 citations, and Santoyo, S.’s exploration of rosemary oil’s antimicrobial activity from 2005, cited 332 times, highlight the diverse applications and benefits of the extraction method. These studies, along with Hunt, A.J.’s 2010 paper on CO₂ utilization (308 citations) and Wenten, I.G.’s 2016 article on reverse osmosis applications (273 citations), underline the cross-disciplinary impact and evolving nature of supercritical CO₂ technology. The remaining articles focus on the specificity of extraction outcomes and methodological advancements, such as Da Porto, C.’s 2009 analysis of Lavandula angustifolia’s flavor compounds (227 citations), Khajeh, M.’s 2004 comparison of Carum copticum oil composition (225 citations), and Capuzzo, A.’s 2013 study on plant flavors and fragrances (213 citations). These papers demonstrate the technique’s capacity to maintain the integrity of complex compounds and cement the role of supercritical CO₂ extraction as a cornerstone in both academic research and industry application, as reflected in the high citation counts.

3.4. Global Dynamics and Impactful Contributions in CO₂ Supercritical Extraction Research

Table 4. Ten most-relevant authors.

Author	Documents on CO2 Supercritical Extraction Oil Extraction	Total Documents	H-Index	Citations	Country	% International Collaboration
Alessandra Piras	29	164	34	4417	Italy	67.90%
Silvia Porcedda	29	150	32	3085	Italy	75.00%
Bruno Marongiu	26	154	36	3950	Italy	93.30%
Ernesto Reverchon	19	386	66	17,438	Italy	20.50%
José M. del Valle	11	87	30	2830	Chile	42.50%
Rimantas Venskutonis	11	245	50	10,714	Lithuania	36.40%
Eduardo Cassel	10	85	23	1684	Brazil	30.90%
D. Falconieri	10	81	24	1743	Italy	83.00%
M. Angela A. Meireles	10	334	63	13,138	Brazil	30.50%
Ying Wang	10	66	15	917	China	35.40%

presents a bibliometric overview of the key authors contributing to the field of CO₂ supercritical extraction of essential oils. The H-index is a metric used to measure both the productivity and citation impact of a researcher’s publications. It is defined as the maximum value of H such that the given author has published H papers that have each been cited at least H times. The international collaboration % reflects the proportion of a researcher’s publications that were co-authored with researchers from other countries or regions, indicating the extent of their global research network and interdisciplinary cooperation. From Table 4, we discern that Alessandra Piras is a notable contributor to the field of CO₂ supercritical extraction. She has authored 29 documents that are specialized in this subject, which are part of her total count of 164 publications. Her work has attracted an impressive 4417 citations, underscoring its substantial impact on the field. Additionally, Piras’s international collaboration rate of 67.90% underscores her widespread global engagement and the extensive reach of her research network. Silvia Porcedda, also from Italy, has a similar number of documents in the specialized field (29), with a slightly lower total document count (150) and citation count (3085), yet a higher rate of international collaboration at 75%. This reflects focused and globally connected research efforts. Bruno Marongiu, another Italian scholar, has 26 documents in the field, with a notable total citation count of 3950. Marongiu stands out for his exceptionally high international collaboration percentage at 93.30%, the highest in the table, underscoring his work’s extensive reach and interdisciplinary nature. Ernesto Reverchon has fewer specific documents in the field (19) but a considerable total of 386 documents, indicating a broad research interest. His work has garnered a substantial 17,438 citations, reflecting significant influence and a high H-index of 66, indicative of consistent quality in research output. José M. del Valle from Chile and Rimantas Venskutonis from Lithuania both have contributed 11 specific documents to the field, with Venskutonis having a higher citation count, reflecting his research’s strong impact. Their moderate international collaboration percentages suggest a balanced local and global research focus. Eduardo Cassel, D. Falconieri, and M. Angela A. Meireles, each with 10 specific field documents, vary in their total document count and citation impact. Notably, Falconieri from Italy has a high international collaboration rate of 83%, implying strong international partnerships. Lastly, Ying Wang from China, with 10 documents and a citation count of 917, shows a growing presence in the field with a substantial 35.40% international collaboration. Collectively, these figures underscore the dynamism of CO₂ supercritical extraction research, with a clear Italian dominance in the top-contributing authors, reflecting the country’s strong research emphasis on this technology. High citation counts across the board indicate a robust and active field with extensive international collaborations that signify its global significance and cross-border scientific exchange.

The network visualization suggests a pattern of scholarly collaboration (Figure 4), with Alessandra Piras and Silvia Porcedda occupying central positions, which likely indicates that they are key collaborators or lead authors within the field of CO₂ supercritical extraction. Their central placement suggests they are pivotal in orchestrating research projects or have a strong influence within this scientific community. Ernesto Reverchon is connected but less centrally, implying more selective or specialized collaborations. The thinner lines to other nodes may suggest interdisciplinary research endeavors or a specialized niche within the field. The names positioned more on the periphery, like Danilo Falconieri, may represent researchers with less frequent interactions with the core group, or they might be new entrants to this collaborative network. Overall, the map indicates an active collaboration network with varying levels of integration and interaction, highlighting the collaborative and interconnected nature of scientific research in this area.

3.5. Bibliometric Mapping Keywords

The study’s methodology encompassed two distinct types of bibliometric mapping: co-occurrence and co-authorship. Co-occurrence mapping, sometimes referred to as a semantic network, examines the interrelationships between keywords used in the literature. This approach illuminates the thematic ties and the conceptual framework of the research area [24]. On the other hand, co-authorship mapping analyzes the collaborative networks among authors, their respective countries or institutions, revealing the collective contributions to specific research domains [25,26]. In the co-occurrence mapping, all keywords were considered as the unit of analysis, aided by the full counting method. The study also set some limitations in the analysis. For instance, a minimum of five (5) occurrences of a keyword was set as a limiting factor. Thus, out of 5072 keywords from 689 articles, only 442 keywords satisfied the threshold. In the study of CO₂ supercritical essential oil extraction, the co-occurrence map shown in Figure 5 shows more than just a deep analysis tool that highlights the connections between different research topics. It clearly shows the key terms important to this field and how they cluster together, pointing out the main focus areas like specific extraction technologies, the properties of essential oils, and their various uses. The terms ‘carbon dioxide’ and ‘essential oils’ stand out on this map, showing that they are central topics in this area of study. Also, words like ‘antioxidant activity’ and ‘antimicrobial’ are noticeable, indicating a significant interest in what essential oils can do functionally. The connecting lines between these terms show not just overlaps but deep links that represent wider trends and interests in the research community. The map created in Figure 5 using the VOS viewer tool guides researchers through the complex details of essential oil studies, indicating current hot topics and new areas that might be worth investigating. It also helps policymakers and those who fund research to see the big picture, making it easier to decide where to direct resources to support emerging or critical areas of study. Additionally, shifts in focus are highlighted, such as an increase in terms related to ‘supercritical extraction’ or ‘volatility’, suggesting a shift towards new extraction methods or deeper exploration into the chemical properties of oils. Building upon this foundation (Figure 5), a ‘cluster’ in the co-occurrence map refers to a collection of thematically related terms that frequently appear together in the literature, demonstrating a common focus within the research community. Each cluster, color-coded for distinction, encapsulates a specific thematic area within the domain of essential oil extraction research [27,28]. Cluster 1, highlighted in red, appears to be at the heart of the extraction process, encompassing critical terms such as “carbon dioxide”, “essential oils”, and “supercritical”. This red cluster signals a collection of studies primarily focused on utilizing carbon dioxide in the oil extraction process, most likely emphasizing the advanced supercritical fluid extraction techniques which are renowned for their efficiency and minimal environmental impact. Cluster 2 in green: With “unclassified drug”, “limonene”, and “antifungal activity”, this cluster delves into the pharmaceutical applications of essential oils. Research here is likely centered on the therapeutic properties of oils like limonene, known for its potential in treating fungal infections. Cluster 3 in blue: Featuring “article”, “solvent”, “plant oils”, and “vegetable oil”, this cluster appears to focus on the various mediums and methods for essential oil extraction. It highlights the scholarly discourse on solvent choices and their implications on the yield and quality of plant and vegetable oil extracts. Cluster 4 in bright yellow: Encompassing “chemistry”, “oils”, “volatile”, and “plant extracts”, this cluster probably explores the chemical analysis of oils and their volatile components, essential for understanding the composition and sensory attributes of plant extracts. Cluster 5 in purple: With “gas chromatography”, “mass spectrometry”, and “distillation”, this cluster suggests a methodological focus on the analytical techniques used to identify and quantify the components of essential oils. The presence of these terms indicates detailed investigations into the precise compositions of oils post-extraction. These interpretations reflect the diverse and interconnected themes of the research within CO₂ supercritical essential oil extraction, showing how each cluster represents a distinct yet interrelated facet of the field.

In Figure 6, the word cloud presents a graphical representation of the most frequent and salient terms extracted from a corpus of literature concerning the supercritical CO₂ extraction of essential oils. The varying sizes of the words directly correspond to their prevalence within the documents studied. The dominance of terms such as “supercritical fluid extraction”, “carbon dioxide”, and “essential oils” indicates that these are the core subjects of focus in the field. Techniques such as “gas chromatography” and “mass spectrometry” are also prominently featured, suggesting their widespread use in analyzing the chemical profiles of extracts. Smaller yet notable terms like “antioxidant” and “volatile” denote specific attributes of essential oils that are of significant research interest, likely due to their implications in product quality and preservation. This visualization effectively encapsulates the thematic essence and methodological approaches in the domain, providing a clear visual summary of the key focus areas in the literature on supercritical CO₂ extraction from aromatic and medicinal plants.

Table 5. Bibliometric insights into the collaborative and thematic landscape of supercritical CO₂ extraction in aromatic and medicinal plant research.

Keywords	Cluster Number	Links	Total Link Strength	Occurrences
carbon dioxide	1	441	6768	437
essential oil	4	433	5536	342
supercritical fluid extraction	1	438	5465	375
essential oils	1	400	4093	307
article	3	398	4050	184
extraction	1	405	3008	206
gas chromatography	5	396	2686	131
nonhuman	2	360	2420	94
distillation	5	388	2417	120
unclassified drug	2	344	2235	84

presents the results of a keyword co-occurrence analysis within the research field of supercritical fluid extraction, specifically examining the context of essential oils. Each parameter in the table plays a critical role in interpreting bibliometric data:

Links: The number of links for a keyword indicates how many times it has appeared in conjunction with other keywords. A higher number of links implies that the keyword is commonly studied in many other terms [29,30].

Total link strength: This is a cumulative measure that represents the strength of the keyword’s connections within its cluster [31]. It accounts for the number of times a keyword co-occurs with others, highlighting the weight of its associations in the research network [32].

Occurrences: This reflects the number of documents in the analyzed set where the keyword appears. It provides a measure of the keyword’s prevalence in the literature [7].

From interpreting the table, we see that “carbon dioxide” has 437 occurrences and the highest total link strength of 6768, underscoring its primary role in supercritical CO₂ extraction research. The term “essential oil” registers 342 occurrences, while “supercritical fluid extraction” shows 375 occurrences, illustrating their significant presence in the field. Keywords like “extraction”, “gas chromatography”, and “distillation” reflect their methodological importance with 206, 131, and 120 occurrences, respectively. In contrast, “nonhuman” and “unclassified drug” appear less frequently, with 94 and 84 occurrences, pointing to more specialized research avenues.

3.6. Most-Relevant Affiliations

The data visualized in Figure 7 provide an analytical snapshot of institutional research output in the domain of CO₂ supercritical essential oil extraction. This chart is instrumental in comparing the contributions of various leading research affiliations, unveiling a pronounced dominance by European institutions, especially from Italy and Serbia. The University of Novi Sad in Serbia emerges as a significant hub of scholarly activity, delivering a substantial 61 articles, indicative of the region’s focus and specialization in this technology. Italy asserts a remarkable influence with the Università degli Studi di Cagliari and the University of Cagliari contributing 82 articles, which may be reflective of Italy’s rich heritage in botanical studies and its forward-looking approach to phytochemical research. Additionally, the University of Belgrade adds to Serbia’s impressive academic output with 39 publications, highlighting Serbia’s active and growing engagement in the field. The chart also brings to light the Kaunas University of Technology in Lithuania, contributing 35 articles, revealing the country’s pivotal role in the broader narrative of supercritical fluid technology’s evolution. China’s Beijing Normal University, with a notable 30 articles, illustrates the nation’s substantial investments and strategic focus on advancing the natural extracts sector. The academic contributions from Brazil, with Pontificia Universidade Católica do Rio Grande do Sul and the Federal University of Santa Catarina producing 27 and 26 articles, respectively, signal Brazil’s expanding capabilities and burgeoning interest in the realm of natural product extraction. Italy’s University of Salerno further enriches the European contribution with 23 publications, while Beijing Forestry University amplifies China’s strong presence in the field with 22 articles. This diverse consortium of academic institutions underscores a dynamic and synergistic international research network that is propelling forward the comprehension and utilization of CO₂ supercritical essential oil extraction. The high volume of publications from specific nations such as Italy and Serbia points to areas of concentrated academic vigor and excellence. The broad spectrum of contributions from China mirror the country’s comprehensive engagement with the field, whereas Brazil’s ascending research trajectory reflects a strategic embrace of natural product extraction technology. Collectively, the bar chart and accompanying data weave a narrative of a rich and diversified global research fabric. This fabric spans across continents, fostering a robust collective advancement in the niche yet critical field of supercritical CO₂ extraction, bolstered by significant contributions from an array of nations known for their research prowess and innovative spirit.

3.7. Geographical Distribution

Table 6. Top ten countries with the highest number of publications.

Country	Cluster	Links	Total Link Strength	Documents	Citation	Average Article Citations
CHINA	3	5	7	155	1683	14.9
ITALY	6	11	25	81	3785	49.10
BRAZIL	2	9	14	72	1981	27.70
PORTUGAL	2	10	27	40	1569	45.40
IRAN	2	14	18	38	1291	33.60
SERBIA	4	7	15	37	886	24.60
SPAIN	2	8	17	30	1445	62.20
UNITED STATES	3	13	17	29	791	19.50
INDIA	5	8	9	27	421	18.8
FRANCE	6	6	15	19	544	41.40

details the scholarly contributions of various countries to the field of CO₂ supercritical essential oil extraction. It ranks countries based on the number of documents published, the total citations received, and average citations per article, highlighting the key players in research output and impact. The varying shades of color across the countries depict the volume of research activity in the field, with darker shades representing higher activity and lighter shades indicating lower activity.

The retrieved 689 articles about CO₂ supercritical essential oil extraction were obtained from more than 42 contributing countries. Table 6 presents the top 10 countries that contribute to 76.63% of the total publications. These countries published 368 articles and received 14,396 citations. The analysis of the data reveals a dynamic interplay of research contributions among various nations. From the table, we discern that China leads in the number of published documents, contributing 155 papers, which is visually supported by the world map in Figure 5 where China is represented with a dark hue, indicating a high level of output. Italy, despite having a lower volume with 81 papers, is notable for its high average citations per article at 49.10, suggesting the impactful nature of its research. This is represented on the map with a vibrant color denoting high activity and influence. Brazil also emerges as a significant contributor with 72 papers, showing considerable engagement in the field, which is reflected on the map with a correspondingly strong color. Portugal, with 40 documents, demonstrates substantial average citations per article at 45.40, indicating not just quantity but the quality of research contributions. On the other hand, Spain’s highest average citations at 62.20 for its 30 documents suggest deep, impactful research. Iran has been credited with 38 documents and a total of 1291 citations. With an average of 33.60 citations per article, Iranian researchers have demonstrated a consistent impact on the field, suggesting that their work is both prolific and resonates well with the scientific community, being referenced frequently in other studies. Serbia’s contribution includes 37 documents and a total of 886 citations, averaging 24.60 citations per article. This indicates that Serbian research, while slightly lower in volume compared to Iran, still holds a significant place in the global research landscape, contributing valuable insights and findings that are acknowledged by their peers. Furthermore, the United States’ role as a research nexus, with 29 documents and significant citation counts, along with its myriad of international research partnerships, is also highlighted on the map in Figure 8. India and France also demonstrate significant engagement in the field, with India producing 27 documents and France 19, suggesting a focus on supercritical CO₂ extraction research in these regions. The United States, with a relatively modest document count of 29 and a lower average citation rate of 19.50, might reflect a focused but niche approach to research in this domain (Figure 8).

Figure 9 is a network map that displays bibliometric relationships between countries. The size of the nodes represents the volume of research output, while the thickness of the lines between nodes indicates the strength of collaborative ties. The placement and proximity of the nodes reveal clusters of countries that frequently collaborate or share similar research interests. The scale at the bottom acts as a legend. Each color corresponds to a specific range of years or the collaborations that were established. The visualization in Figure 9 indicates a robust network of collaboration and research output in the field of CO₂ supercritical essential oil extraction. Italy, Brazil, and Portugal emerge as significant contributors with 11, 9, and 10 collaborative links, respectively, reflecting their active engagement in the research community. The total link strength values of 25 for Italy, 14 for Brazil, and 27 for Portugal underscore the depth and consistency of their international research relationships, with Portugal displaying particularly strong ties relative to its number of connections. Iran and the United States also demonstrate substantial collaborative engagement, with Iran having 14 links and a total link strength of 18, and the United States with 13 links and a total link strength of 17. These figures suggest not only the breadth of research partnerships but also meaningful interactions, as evidenced by the strength of these connections. The considerable link strength indicates that collaborations are likely to be recurrent and influential, contributing significantly to the advancement of the field. The prominence of the nodes also suggests their significant output in terms of research publications. The color gradients on the lines suggest the evolution of these collaborations over the studied timeframe, reflecting how relationships have been initiated or strengthened across different years. The landscape of co-authorship analysis shows that while some countries may lead in volume, others lead in influence, and yet others demonstrate the power of partnerships. It is a global dialog in which each participant, from the most to the least prolific, plays a vital role in advancing the collective understanding of supercritical CO₂ extraction techniques. This analysis not only highlights the current state of international collaboration but also underscores the potential for future partnerships in driving innovation and discovery in this dynamic field.

4. Discussion

The bibliometric analysis that was conducted provides a panoramic view of the evolving trends in research on the supercritical CO₂ extraction of essential oils from aromatic and medicinal plants. The chronological scope of the study spans from 1995 to 2024, revealing significant growth and diversification within the field. Initially, the field captured a nascent interest in 1995, reflecting the pioneering stages of adopting supercritical CO₂ techniques for essential oil extraction. A notable peak in citation impact in 1998 suggests the publication of seminal research within the academic community, perhaps due to breakthroughs in technology or compelling findings regarding extraction efficiencies or product quality. As we advance into the 2000s, the steady rise in the number of publications indicates an expansion and maturation of the research field, with an increasing diversification of study topics. This era might be characterized by the establishment of fundamental methodologies and the exploration of various plant materials, which, as shown in Table 2, have attracted considerable scholarly attention. The variable citation average during this period reflects the natural evolution of a field as it grows; some studies may continue to garner attention over time, while others attract less, possibly due to their incremental contributions. The uptick in research publications between 2011 and 2016 highlights a period of heightened activity within the field, where a surge in interest leads to a proliferation of studies. However, the citation impact during these years varies, indicating that not all studies have an equal influence on the field. The high activity paired with the variable impact points to a critical evaluation phase where the field collectively gauges the quality and relevance of the research being produced. The recent trend from 2017 to 2024 shows an interesting paradigm: While the number of publications remains high, the average citation per document exhibits a declining trend, particularly in the year 2024. This could be attributed to a myriad of factors such as the saturation of certain research topics, the emergence of alternative extraction technologies, or shifts in scientific priorities that influence the citation dynamics. Geographically, bibliometric analysis uncovers a robust network of international collaborations and contributions. China’s predominance in the volume of publications, as illustrated in the geographical distribution map, could suggest a strong national focus on advancing supercritical CO₂ extraction technologies. In contrast, Italy’s significant average citation rate per article indicates the impactful nature of its research contributions, despite producing fewer publications than China. This distinction points to the qualitative difference in research outputs across countries and underscores the importance of examining both quantity and impact metrics in bibliometric studies. Moreover, the analysis of co-authorship networks reveals the intricate connections between researchers across the globe. The visualization maps, such as the network visualization map, provide a graphical representation of these collaborations, emphasizing the importance of knowledge exchange and collective scientific efforts. The evolution of research in the field of supercritical CO₂ extraction of essential oils is reflective of broader scientific, technological, and societal trends [33,34]. It showcases the importance of sustainable and innovative extraction methods that align with current environmental and health consciousness [35]. The dynamic and multidisciplinary nature of the field promises continued advancements and discoveries, with implications for both academic research and industrial applications. The extracts obtained via supercritical CO₂, often referred to as oleoresins, show promising applications across various sectors. In the cosmetic industry, these extracts are used to formulate natural products without chemical residues, meeting the growing consumer demands for safe and health-friendly products. In the pharmaceutical industry, oleoresins are valued for their bioactive properties, which can be leveraged in the development of health and wellness products. Lastly, in the food sector, oleoresins are incorporated as natural flavorings and additives, contributing to solvent-free formulations that comply with strict food safety standards. The study’s findings are a testament to the power of bibliometric analyses in informing researchers, industry professionals, and policymakers about the landscape of scientific research. It encourages the scientific community to continue pioneering studies that push the boundaries of existing knowledge and contribute to the technological advancement of essential oil extraction methods.

Strengths and Limitations

This bibliometric analysis of supercritical CO₂ extraction research in essential oils offers several notable strengths. First, it provides a comprehensive overview of key contributors to the field, including prominent researchers, institutions, and countries, helping to map the global landscape of research in this area. Additionally, by identifying collaborative networks and co-authorship patterns, this study highlights the partnerships and interdisciplinary collaborations that drive advancements in supercritical extraction methods. Another strength is the identification of frequently used keywords, which gives insight into major research themes and emerging trends within the field. This can guide future research directions by pinpointing popular areas of study as well as potential gaps in the literature. However, this analysis also has limitations that should be acknowledged. The study relies solely on data from the Scopus database, which, while comprehensive and widely regarded for its quality, may not cover all relevant publications available in other databases, such as Web of Science or Google Scholar. Consequently, certain studies or researchers may be underrepresented, especially if their work is published in journals not indexed by Scopus. Additionally, this analysis does not differentiate between studies where supercritical CO₂ extraction is the primary focus and those where it is only a tangential aspect, potentially including papers with varying levels of relevance to the central topic. Future studies could address this limitation by incorporating multiple databases and implementing more refined criteria to ensure that only highly relevant studies are included.

In summary, while this study offers valuable insights into the development and impact of supercritical CO₂ extraction research in essential oils, acknowledging these limitations is crucial for contextualizing the findings and guiding further research.

5. Conclusions

The bibliometric analysis presented in this article has provided a detailed exploration of the scientific literature surrounding the use of supercritical CO₂ extraction for essential oils from aromatic and medicinal plants. Through the rigorous examination of publication trends, citation impacts, keyword occurrences, and international collaborations, this study has illuminated the trajectory of research in this burgeoning field from its nascent stages to its current status. Spanning three decades of scholarly efforts, the study underscores a progressive increase in research volume, pointing to the growing significance and recognition of supercritical CO₂ extraction techniques. The emergence of Italy as a central node of impactful research, despite fewer overall publications, reflects the country’s contribution to high-quality studies in the field, while China’s extensive body of work underscores its role as a prolific research hub. This geographic diversity is emblematic of the global scientific community’s commitment to advancing the field through cooperative efforts and knowledge sharing. The word cloud and network visualization maps provide a compelling snapshot of the thematic field and collaborative networks that characterize this area of study. They reveal not only the prevalence of specific research themes but also the intricate web of partnerships that span across countries, highlighting the interconnected nature of modern scientific inquiry. As the field continues to evolve, it remains essential for future research to push the boundaries of current methodologies, explore emerging trends, and address the gaps identified by this bibliometric study. The drive towards more sustainable, efficient, and environmentally friendly extraction processes is evident and aligns with the global shift towards greener practices. In summary, this bibliometric analysis reflects on past achievements in supercritical CO₂ extraction research and acts as a clarion call for future innovation. The continued dedication to exploring the potential of essential oils and the refinement of extraction technologies will benefit the scientific community and have far-reaching implications for industries reliant on these natural compounds. Through the lens of bibliometric analysis, the scientific narrative of this field is woven, highlighting the cumulative efforts that shape the trajectory of research and technology.