Bibliometric Analysis of Research History, Hotspots, and Emerging Trends on Flax with CiteSpace (2000-2022)

ABSTRACT With the growing health and environmental consciousness, flax (Linum usitatissimum L.) has attracted more attention owing to its great potential in the food, health care, and material industry. For such an important crop, it is crucial to understand its development history, current status, and hotspots and finally find the future directions of flax research. This paper mainly analyzed the published articles (collected from the Web of Science) related to flax from 2000–2022 and the cited references by these articles using the software of CiteSpace. Results showed that the number of studies on flax kept increasing and increased rapidly from 2010 to 2022. Canada and France are the leading countries in flax research with more than 970 articles published during the period. By analyzing the high-frequency keywords, five important research areas were found: (1) flax fiber quality and its application in composites, (2) chemical composition and products of flaxseed, (3) tolerance of flax to stress and genetics, (4) cellulose and lignin, (5) fiber-reinforced composites and flax fabric. With the strongest citation bursts, bio-composite with flax straw has become the hottest research area for flax. In the future, efforts should still be made to the simplified and efficient production of flax owing to the higher labor cost, and more attention should be paid to healthier flaxseed food and flax-based environmentally friendly biomaterials. Finally, decreasing the cost of cultivation and pre-processing and developing end products with higher values would greatly promote the development of the whole flax industry.


Introduction
Flax (Linum usitatissimum L.) is one of the most important traditional crops with multi-uses (Mahendran et al. 2013;Wang et al. 2012Wang et al. , 2017Zhao et al. 2022). According to the main applications, flax is generally classified into three intraspecies: fiber-type flax (for fiber extraction), linseed types (for oil extraction), and dual-purpose flax (for both fiber and oil). It is well known that flax fiber features high hygroscopicity, good air permeability, antianaphylaxis, antistatic, antibacterial, and low harm to the human body, which makes flax a good source of natural fiber in the textile industry. Flax fiber and flax shives can also be used in the worldwide manufacture of pulp and paper (Judt 1993;Sain, Fortier, and Lampron 2002). Recently, the research on flax-fiber-based bio-composites has become popular (Makhlouf et al. 2020;Sanjay et al. 2018), although flax fiber had been applied to composites since the end of the last century (Mieck, Nechwatal, and Knobelsdorf 1995). Apart from the above applications, flax also plays vital roles in the medical industry (Chytilova et al. 2013;Colli et al. 2012) and the food industry (Fofana et al. 2011;Karimzadeh Soureshjani et al. 2019;Meng et al. 2016;Ramcharitar et al. 2005). When flax is not used for food, it would be a good candidate for phytoremediation due to its high resistance to several adversities (Guo et al. 2020;Zhao et al. 2022).
The abrupt COVID-19 has seriously disordered the production of flax. According to the FAOSTAT, the cultivation area for flax decreased to some extent for several of the main production countries, such as the Netherlands, Russian Federation, Italy, and France, but increases in production area also were found for China and Belgium. The decrease in cultivation area for flax was not only related to the stagnant economy caused by COVID-19 but also the complex production-processing flows and high labor cost (Pudełko, Mańkowski, and Kołodziej 2014), which has seriously impacted the development of the flax industry. Hence, more attention should be paid to the high-yield cultivation techniques of flax, the initial processing technology for flax fiber, and the high-value products from flax fibers or seeds. Only in this way, can the farmer be willing to grow flax and achieve more income and the flax industries be able to obtain adequate raw materials for further process of producing the high-value commodity. Then the products manufactured from flax can be circulated in the world and bring convenience and health to humans. To achieve the above goals, it is very necessary to understand the development history and current status of flax research and discuss the hot spots and future trends of flax research.
Bibliometric analysis is a wide-used and efficient method of identifying research trends and hotspots based on published literature (Li et al. 2017). CiteSpace is a free information-visualizing bibliometric software based on Java language and is developed by Professor Chaomei Chen from Drexel University. It can be used to analyze the potential knowledge behind a vast of scientific literature. Through this software, people can generate visual maps through different analyses, find the relationship between various information intuitively, and finally find the research hotspots, development trends, core authors, and research institutions in this field via a diversified, time-sharing, and dynamic manner (Chen 2004(Chen , 2017. Although there are several bibliometric studies related to flax, they are focused on the application of natural fibers in concrete (Majumdar, Thakur, and Majumdar 2022;Nwankwo et al. 2023), and systematic reviews on flax are still needed. Therefore, in this paper, an in-depth analysis of flax-related research from 2000 to 2022 was conducted with the help of CiteSpace software using the bibliometric method. It is expected that, by this analysis, the development history of flax can be well understood, the hot and focus as well as the trends of flax research would be found, and finally constructive ideas and suggestions for the promotion of the flax industry could be provided.

Literature collecting method
The literature analyzed in this paper was collected on May 21, 2022, from the Web of Science (WoS) (the database is purchased by the Chinese Academy of Agricultural Sciences) at the Institute of Bast Fiber Crops, Chinese Academy of Agricultural Sciences with the following process: firstly, choose the ["Web of Science Core Collection"] as the database; secondly, set the ["Topic"] as ["flax" or "Linum usitatissimum"]; thirdly, set the ["publication date"] as ["1975-01-01" to "2022-05-21"]; fourthly, click the ["search"] button to search for publications, then set the document sort as ["Article"]. The reason why we set "Topics" as the search field was that "Topic" searches in WoS can not only search for text in titles as well as in abstracts, addresses, and keywords but also search for the article keywords provided by the author(s) and additional keywords generated based on the article's reference list and thus, retrieving literature with "Topics" can obtain more complete information for a certain theme (Jonkers and Derrick 2012). After the processes mentioned above, 9589 records were found, then these records were exported in the form of a "Plain Text File" with the content of "Full Record and Cited References." To remove the duplicated records, the download files were then processed with the data processor bound to the CiteSpace software, after removing the duplicated ones, 9585 unique records remained. The number of records between 2000 and 2022 is 8120.

Data analysis
The numbers of published literature per year from 1975 to 2022 or from 2000 to 2022 were obtained from the WoS directly using the online analyzing function. The preliminary analysis of the records concerning the contributors in terms of countries, institutes, and authors was also directly got from the WoS. The figures for the above analysis were plotted with the software of Origin 2021.
Further analyses for the records from 2000 to 2022 were carried out with the software of CiteSpace 6.1.2. These analyses contained the collaboration network between countries, keyword co-occurring network, and clustering, citation bursts of keywords and cited references, and top 20 high-frequency keywords.

Numbers of articles published each year from 2000 to 2022
To know the development history of flax research better, we retrieved the flax-related papers from all the years in our current subscription (from 1910 to 2022). Results showed that the earliest paper on flax we searched was published in 1975 and the number of published articles related to flax each year keeps increasing from 1975 (17 articles) to 2021 (708 articles) ( Figure 1a). Especially, the research records rapidly increased with an average of 515 articles per year from 2011 to 2021. This indicated that flax is paid more and more attention today.
Since the search date is on 21 May 2022, the number of published articles is 245, which is far smaller than that of 2021 ( Figure 1b). However, a good polynomial relationship between the publishing year and the number of published articles was found. Hence, the number of articles published in 2022 can be predicted by the equation: Y = 3603181 -3612.22805 × X + 0.90535 × X 2 (in this equation, Y refers to the number of published papers per year, X refers to the publishing year, R 2 = 0.987). The predicted number of published papers in 2022 is 765 with a 95% confidence interval of [733,797].

Main contributors to the research of flax in terms of countries, affiliations, authors, research areas, categories, and journals from 2000 to 2022
Visualization analyses for the search results from 2000 to 2022 were carried out via the WoS website ( Figure 2). The numbers of published papers differed greatly between countries, and Canada and France ranked first with 972 records among the top ten countries. Scientific research can contribute to the production of flax, and vice versa. As a result, the above countries are also the main producers of flax in the world. Among the affiliations, the Centre National de La Recherche Scientifique from France (401) ranks first in terms of paper numbers which also proves the status of France in flax research. Among the high-productivity authors, Christophe Baley (Baley C) from France published 117 papers and ranked first.
Results showed that about 31% of the searched records (2570 records) fell into the research areas of "Materials Science," followed by "Agriculture" (1401 records) and other research areas. This indicated that flax is playing a more important role in the material industry. Among the top ten Web of Science Categories, "materials science composites" ranks first with 1110 records 1110, which also proves the increasing importance of flax in material research. Based on the analyses of the research area and Web of Science Categories, the highest number of studies are related to flax fiber, which indicates that fiber is still the most important usage of flax. The most popular journals for flax studies are Industrial Crops and Products (248 records), Composites Part A: Applied Science and Manufacturing (208 records), Journal of Natural Fibers (165 records), and so on. The three journals mentioned here all rank Q1 according to the Journal Citation Reports 2021, which suggests authors are apt to publish their findings in famous journals. According to the publications, most of the articles are related to composites or polymers, which are the new and promising applications of flax.

Collaborations between the countries on flax studies
With the development of economic globalization, scientific collaborations among countries are becoming more and more common. Collaboration network analysis between countries was carried out with CiteSpace software (Figure 3). In Figure 3, each circle represents a country, the larger circle means more papers are published by the country and more lines between two countries mean more collaborations between them. The purple lines outside the circles mean the countries are of higher centrality and contributing more to the flax studies and the red part in each circle means citation burst (Wu et al. 2017).
Results showed that France (centrality, 0.28), the USA (0.20), Canada (0.13), Italy (0.13), and England (0.12) have higher centrality than the other countries and this means these countries play more important roles in flax studies in the world. Citation bursts may indicate the degree of attention from the scientific communities and thus can present the research hotspot to some extent (Chen, Dubin, and Kim 2014

Research hotpots and emerging trends on flax
Co-occurring networks of keywords for the citing papers and clustering were performed with CiteSpace software (Figure 4), which can be used to explore past, present, and future trends in recommendation systems (Kim and Chen 2015). Results showed that the largest five clusters can be labeled as "mechanical properties," "flaxseed," "flax rust," "laccase," and "drilling" (clustered by the "All in one" function and choosing the keywords to label the clusters). Each cluster referred to an important research area and the labels are only some representatives. The cluster of the "mechanical properties" mainly contained the studies focused on fiber application (Andersons, Spārniņš, and Poriķe 2009), physicochemical properties of fiber-based composites (Andersons, Spārniņš, and Joffe 2006), and so on. This suggests increasing attention from the material industry is being paid to flax fiber for its advantages, such as natural properties, degradable, environmentalfriendly, and reproducible. The cluster of the "flaxseed" mainly contained studies relative to chemical composition (Zuk et al. 2019), seed product quality (Rajan, Sobankumar, and Nair 2014), and so forth. Hence, the multiuse of flaxseed has also been exploited, nowadays, flaxseed is playing a significant role in the food, fodder, and medicine industries. The cluster of the "flax rust" covered the studies about tolerance to stress , molecular biology , and so on. The fourth cluster of "laccase" contained studies concerning lignin (Mahendran et al. 2013), cellulose pulp (Istomin et al. 2016), and so on. The last cluster of "drilling" mainly included studies about fiber-reinforced composite (Liu et al. 2012), flax fabric (Firouzsalari et al. 2021), and so forth. Owing to the more comprehensive exploitation of flax, the demand for flax products would increase, which requires the elevation of yield for both flax fiber and seed. Therefore, more efficient cultivation technologies and high-yield varieties for flax are needed and more efforts should be made to the related studies.
The high-frequency keywords always represent the research hotspots. By analyzing the high-frequency keywords relative to flax from 2000 to 2022 (Table 1), five hotspots can be found: "genetics of flax" (e.g. Expression, Identification, Growth, Plant), "fiber quality" (e.g. Flax fiber, Natural fiber, Tensile, Strength, Quality, Performance, Behavior, Tensile property), "flaxseed products" (e.g. Gla, Fatty acid, Oil), "biocomposite with flax fiber" (e.g. Biocomposite, Mechanical property, Performance, Composite, Natural fiber), "cellulose and its application" (e.g. Cellulose). These five hotspots covered genetic development, crop yield and quality, and material science, which indicated the theory and application for flax are together developed. All of these would be helpful to the further development of the flax industry.
Citation bursts can not only indicate the degree of attention from the scientific communities to a previous paper but can also identify bursting keywords as indicators of emerging trends (Chen, Dubin, and Kim 2014). When the citation of some keywords or references comes suddenly increases, the study contents related to them would usually become research hotspots or important research areas. The top 23 keywords with the strongest citation bursts related to flax were detected ( Figure 5). The citation bursts can be divided into three stages: during the first stage, the studies were focused on the cultivation and yield of flax (the first ten keywords in Figure 5, e.g. "cell wall" and "yield"); during the second stage, nutritive values of flaxseed,  cellulose from flax fiber became the research priority and the flax fiber-based composite has also emerged (the eleventh to the nineteenth keywords in Figure 5, e. g. "polymer-matrix composites" and "linolenic acid"), and for the third stage, bio-composites has become the most popular and important research front (the last six keywords in Figure 5, e. g. "hybrid composite" and "mechanical behavior"). These three stages not only showed a course of flax exploitation but also indicated the diversification of applications of flax. Citation bursts suggest the possibilities that the scientific communities have paid or are paying special attention to the underlying contribution (Chen, Dubin, and Kim 2014). The top 25 cited references with the strongest citation bursts were also found with the CiteSpace software ( Figure 6). Similar to the citation bursts of keywords, citation bursts of references indicate these references are of vital importance and present the research hotspots during some specific periods. Some of the early citation bursts for the references are related to the response and resistance of flax to flax rust. For example, Catanzariti et al. (2006) identified genes encoding haustorial expressed secreted proteins (HESPs) by screening a flax rust haustorium-specific cDNA library (with a burst strength of 21.8) and Dodds et al. (2006) found direct protein interaction underlies gene-for-gene specificity and coevolution of the flax resistance genes and flax rust avirulence genes (with a burst strength of 19.1). Some of the early and medium-term citation bursts are related to the genetics of flax. For instance, Cloutier et al. (2009) Bodros et al. (2007) found that flax fiber-reinforced thermoplastics had better tensile properties compared with those without flax fiber addition (with a burst strength of 23.3). For the latest citation burst, Sanjay et al. (2018) discussed the benefits of natural fiber-reinforced polymer composites and insisted that future research should concentrate on eco-friendly materials with sufficient properties to replace synthetic polymer composites for expanding new areas of applications (with a burst strength of 17.7). Like the keywords, references with citation bursts also reflected the evolution of the research theme on flax. The changes in the references with citation burst indicated the shift of research focus.
Recently, polyhydroxybutyrate (PHB), a biodegradable bacterial polymer, has been extensively studied because of its potential biomedical applications (Szopa et al. 2009). However, the fragility of PHB severely limits its usefulness. Fortunately, when the bio-fibers are incorporated into PHB mechanical properties can be significantly improved Szopa et al. 2009). Owing to the excellent performance, flax fiber has also been incorporated into PHB and studies have shown that this can well enhance the composites Ventura et al. 2018).
With the increasing attention to global environmental problems, the demand for "green" materials and products is growing (Ngo et al. 2016). Especially, the new emerging pollution of micro-and nano-plastic (Ding et al. 2022;Gong et al. 2019;Li et al. 2020;Ye et al. 2021) has made people realize that we have been surrounded by what we have invented and depended on. Hence, renewable resources and recycled materials must be paid more attention to than ever before. Owing to the advantages of natural fibers, such as reasonable mechanical properties, low density, economical feasibility, environmental benefits, and renewability, they have been receiving more attention (Ngo et al. 2016).

Conclusions and perspectives
The following conclusions can be drawn from the analysis of flax-related research papers published between 2000 and 2022: (i) the theoretical research about flax continues to increase and flax is still an indispensable industrial crop; (ii) the research focus on flax has changed from primary products of flax (fiber and seed) to senior ones (food, paper, bio-composite) during this period; (iii) flax-fiber reinforced polymer (such as PHB) and composites, as well as the link between process and flax fiber properties have become the most popular research focus.
In the future, efforts are needed to: promote the mechanization and automation in the cultivatingharvesting-processing of flax fiber to decrease the cost; develop methods of deep processing of the waste materials after oil pressing and fiber extraction to reduce the related environmental problems and generate innovative, new products dedicated to the food and feed industry and medicine; breed new flax varieties for higher yield, quality, and special use adapting the changing climate; design and popularize flax-based products with higher values and bigger audience. Owing to environmental problems, such as macro-, micro-, and nano-plastic pollution, green materials will be paid more and more attention. As one of the outstanding green materials, flax fiber would cut a conspicuous figure in biodegradable products. However, the paradox between the adhesion of hydrophilic plant fibers and the hydrophobic chemistry of polymer matrices and the moisture sensitivity and tendency of the fibers to swell due to their hydrophilic nature are challenging the use of natural fibers in the composite industry. These problems need to be solved in the future. On the other hand, health is an everlasting topic to humans, flax will shine in the health-product industry due to its high nutrient values.

Highlights
• More and more attention has been paid to flax for its multi-applications • Research focus on flax has changed from primary products of flax to senior ones • Flax-fiber reinforced polymer become the popular research front of flax research • Flax will play more important roles in bio-composites, medicine, food industry.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
The work was supported by the Central Public-interest Scientific Institution Research Fund .