Analysis of three decades of sugarcane researches focusing on ethanol and bioethanol.

Biofuel production from sugarcane has been researched in terms of the environment. Performing a bibliometric method and visual network analysis, the purpose of this study is to assess the trends in sugarcane and bioethanol researches and recognize the associated hotspots based on 4958 publications from 1991 to 2020. The geographical distribution of articles and per capita sugarcane production in the world were also examined. As a widely used database in the eld of bibliometric, the Scopus database was used in this research to collect relevant literature. The research method includes the bibliometric analysis of documents and citations, subject categories, journals, countries, institutions, authors, geographical distribution status of articles and annual sugarcane production, top articles. Finally, the social network of the author's keywords was reviewed. The VOSviewer software was used for performing graphic maps showing the co-authorship and communication of countries, authors and keywords. The results show a high growth in the number of published documents in this eld and the tendency change from agricultural sciences to environmental sciences in documents and journals. Based on the study's gained results, Brazil is the undisputed leader, publishing 43.8% of the documents in last three decades, followed by the USA and India.


Introduction
In recent years, the world has faced a dramatic rise in fossil fuel consumption. With the increasing use of oil reserves, environmental concerns and energy security issues have intensi ed. Global warming increasing resulted from rising greenhouse gases has prompted researchers to discover alternative fuels against fossil fuels. The crops and plant residues can be used as sustainable biofuels (Neves et al., 2016), one of the proposed alternatives, especially used in transportation. Bioethanol, biodiesel and pure vegetable oils are the three main types of biofuels. The rst-generation biofuels are produced from food sources like corn, sugarcane and sugar beet as main sources. The second-generation biofuels like sugarcane bagasse are not produced from a direct source of food (Correa et al., 2019). Sugarcane is a multi-purpose plant that is used to produce sugar. Its main features include high biomass yield, sucrose content and e ciency in absorbing solar energy (Benjamin et al., 2014).
Sugarcane bagasse is one of the most abundant lignocellulosic substances in tropical countries. Although crop residue biomass is considered as industrial waste, there are advantages in its agricultural development aspect, just like producing cheap and economic fuels (Rabelo et al., 2011). Due to its high cellulose and hemicellulose, bagasse is a promising agricultural product for bioethanol and electricity production (Hamelinck et al., 2005). The Brazilian power plants with boiler systems produce 12 kWh of electricity, 330 kWh of thermal energy and 16 kWh of mechanical energy from each ton of sugarcane bagasse averagely (Turdera, 2013).
Sugarcane is cut when harvesting after 12-16 months of growth. Manual harvesting involves burning the product for collection, which facilitates the cutting process. Manual vesting is still being used in many companies globally, although mechanized sugarcane harvesting is recommended due to worker safety and environmental concerns. Sugarcane is then placed in a chopper to open cells containing sugars and facilitate the process of sucrose extraction (Viana & Perez, 2013a). Grinding is the process of extracting agricultural elements and residues. Usually, the rst extracted group is used to produce sugar and about 55% is used to produce ethanol (Viana & Perez, 2013b). Fuel ethanol or bioethanol is produced from pre-treatment and fermentation of crop products and residues (Demirbas, 2009).
Oil fuels can lead to dangerous environmental issues. These kinds of fuels are non-renewable resources and making concerns and risks, which led to research on alternative renewable fuels such as biofuel (Turdera, 2013). Biofuels contain between 10% and 45% oxygen increasing the fuel e ciency and decreasing the pollution created by hydrocarbons (Demirbas, 2009). The biofuels markets in North America and the European Union are almost entirely dependent on political guidelines and nancial incentives, which are expected to contribute to reducing greenhouse gas emissions, the security of supply and employment targets, including the in uential cases. For example, the EU's latest policy aimed to use at least 10% biofuels by 2020 (Margeot et al., 2009). Energy used in the EU transport sector was 8.4% from renewable sources, end of 2019 (Shares of energy from renewable sources used in transport in Europe -European Environment Agency, n.d.). According to a report released in December 2020, the 10% target for 2020 does not appear to be being met (Merce Almuni (VITO), Tom Dauwe (VITO), Ils Moorkens (VITO), 2020).
The present study aims to analyze previous researches done throughout the world on sugarcane and bioethanol between 1991 to 2020 to help researchers identify the future trends through bibliometric methods. The geographical distribution of articles and per capita sugarcane production in the world were also examined.

Methodological Approach
Bibliometric is a method by which the research process and its historical course are investigated (Ye et al., 2014). Impact factor (IF) and H-index are used for evaluation. IF was used to investigate the features of publication statistics for journals and authors, which evaluates qualitatively based on a journal's citations and publications in the last two years (Buela-Casal & Zych, 2012). H-index is de ned as the h number of articles that have at least h number of citations each. H-index was proposed by Hirsch (2005), used in scientometrics as a quantitative measurement for evaluating a researcher, giving an estimate of the importance, signi cance and broad impact of a scientist's cumulative research contributions (Hirsch, 2005). Another index used in this study is CiteScore. Cite Score estimates the average citations received per document published in this title. Cite Score values are based on the number of citations in a mentioned year (2020) to documents published in three prior years (2014)(2015)(2016), divided by documents count in these three previous years (2017)(2018)(2019). (How are CiteScore metrics used in Scopus? -Scopus: Access and use Support Center, n.d.). This bibliometric study was performed based on keywords using CiteScore 2019 data extracted from Scopus as an appropriate database due to its extensive resources and discipline. The selected data were evaluated through VOSviewer software. Analysis and interpretation were performed based on countries, institutions, authors, journals and keywords. Keywords and countries were visually extracted through the relationship between different social networks and clusters in the authors' groups. Currently, Derwent, WoS and Scopus are used as data sources amongst the databases (Wang et al., 2021). The Scopus database was chosen for this research.
The data were analyzed using Visualization of Similarities (VOS) viewer software version 1.6.15. This software is freely available for making bibliometric maps to evaluate the communication and cooperation networks between countries and authors, as well as the keywords used; furthermore, the research elds can be interpreted.

Results And Discussion
In january 2021, seeking the words "cane" or "sugarcane" and "ethanol" or "bioethanol" through the Scopus database for 30 years from 1991 to 2020, included in articles title, abstract and keywords, a total of 4958 documents were found. The most signi cant share was assigned to English documents (95.03%), followed by Portuguese documents (9.98%).

The evolution of publications over the years
These 4958 documents included 3555 articles (71.70%), 537 conference papers (10.83%), 338 book chapters (6.81%) and 295 review articles (5.94%). Other research items such as business articles, short surveys, books, and conference reviews make up less than 5% in total. percentage occurring during those years. The number of publications in this decade is higher than the total of all publications in previous years. That while, the number of publications increased by about 1000%. The production and demand for sugarcane also increased in 2001s. Consequently, the crop production system needed to improve by replacing the mechanized sugarcane harvesting instead of burning and manual harvesting, which coincided with the mentioned scienti c milestone (Martíni et al., 2020). The global ethanol production from sugarcane, beet and sugar was about 20 billion liters in 2001, which grew to 40 billion liters by 2005 (Demirbas, 2009). In 2010, the last year of the 2001s, global ethanol production reached more than 88 billion liters (Alternative Fuels Data Center: Maps and Data -Global Ethanol Production, n.d.). This 340% increase in production in this decade was also in uential in research in this eld. For this reason, the 2001s became a turning point. These changes and developments can be the reason for the acceleration of research in this eld. In 2007, the European Union set a target of reducing GHG[1] emissions by 20% from 1990 levels, including a 10% share of biofuels for each member state (Demirbas, 2009). On January 23, 2008, a proposal was issued by the European Parliament and the Council to promote the use of renewable energy sources. The rst measure of sustainability de ned in the European proposal was a 35% reduction in GHG emissions for biofuels compared to their fossil fuel samples (Demirbas, 2009). In 2009, the European Union (EU) approved a directive (Directive 2009/30 / EC) through which, from 2011/01/01, the level of gasoline at the pump will be changed to 10% ethanol (E10) in 27 EU countries (EUR-Lex -32009L0030 -EN -EUR-Lex, n.d.) In 2011s, global ethanol production increased by 34%, which is not comparable to the previous decade. Also, the number of publications in those years grew slightly more than 25%. In 2016, the Paris Agreement was rati ed, which commits for the rst time to all countries to reduce greenhouse gas emissions (The Paris Agreement | UNFCCC, n.d.). Given this trend, political incentives were expected to require the rapid adoption of renewable energy systems to replace fossil fuels. For this reason, throughout these years, the general trend of publications has been increasing, as far as the highest number of publications is in 2017 with 428 documents. But since 2017, the increasing trend was broken and the number of publications reduced. This year, the USA ceased all participation in the Paris Agreement. Figure Table 1, a total of 3885 documents were cited at least once, which is equal to 78.35% of the total. In 1994In , 1997In , 1998In and 1999 all documents have at least one citation; 39.26% for documents with at least 10 citations and 3.81% for at least 100 citations. For further study, the three decades of 1991, 2001 and 2011s were considered. The investigation showed 93.33% of all 1991s documents had at least one citation. In 2011s, more than 80% of documents had at least one citation and most documents with more than 100 citations are in 2001s. (Table. 1 respectively. ≥ 100, ≥ 10, ≥1 = Number of papers with equal or more than 100, 10 and 1 citations.

Subject categories
As revealing in Figure 2, in 1991s, subject categories were more related to biochemistry, immunology and microbiology. In 2001s, two groups of chemical engineering and agricultural and biological science were among the major groups. But in 2011s, the two main groups of environmental science and energy performed 16% of all publications, indicating that researchers have focused on energy and the environmental elds in recent years.

Journals
Focusing on publishing articles' references, journals in this eld were distributed in various elds such as environmental science, engineering, fuel, agriculture, energy, socioeconomic, etc. It shows that sugarcane and ethanol have attracted the attention of many authors in various elds. 15 important publications are presented in Table 2 alingside their characteristics. In terms of quality, these journals are also ranked with IF and H-index. Bio-resource Technology has the highest-ranking with 187 documents. This journal also has the best ranking based on H-index among these 15 publications.  According to Figure 3, the top seven journals' annual publications with the highest TP during 1991-2020, more than 40% of Bioresource Technology journal documents were from the last ve years. The International Sugar Journal had four publications in the previous ve years. More than 72% of its publications were from 2001s. It demonstrates that this journal was more popular in 2001s than it was in 2011s and its third rank in Table. 2 is the outcome of the documents of this journal in 2001s.
The journal's categories including Agricultural and Biological Sciences show that these issues were of interest in 2001s but have not been hot topics recently.
The Journal of Cleaner Production's rst issue was published in 2010 and TP of this journal are from 2010 to 2020. More than 63% of this journal's publications from the last three years and its categories include Environmental Science, Renewable Energy and Sustainability, which show the attractiveness of these elds to researchers in recent years.

Countries, institutions and authors
Details of the top 20 countries of the publication and citations are presented in Table. 3. The type of independent publications (IP) refers to those all written by authors of institutions from one country. In contrast, the type of collaborative publications (CP) represents documents written by authors from different countries.
The highest number of CP (472) is assigned to Brazil, accounting for 22.32% of Brazilian publications, followed by the USA (309) and the UK (92). Among the top 20 countries, Canada has no CP and Sweden has the highest one with 73.56%.
The seven major industrialized nations known as the G7 are among the top 20 countries, including USA, Britain, France, Italy, Germany, Canada and Japan. Interm of TC / TP, Canada is higher than other countries with a value of 39.3, denoting that it have received the highest recognition per publication, followed by Netherlands (38.73), Sweden (38.02), Cuba (32.74) and the USA (31.75). TP of the six countries with the highest annual production are presented in Figure 4. In 1991s Thailand had no document and China had only one. For three decades, China was the fourth-largest producer of documents, but it did not play a signi cant role in 1991-2000. It shows that China's development has been very rapid, reaching 206 publications during 2011-2020. As revealed in Figure 4, Brazil has widened its distance from other countries over the years. It can be said that Brazil started to distance itself from other countries since  A total of 44 countries were identi ed with more than 10 documents with 310 international links ( Figure 5). Among them, Brazil and the USA had the strongest links (146 links). Brazil has links with 34 other countries with a total link strength of 1329. Brazil is in the same group as Chile, Colombia, Peru and Italy. USA has 38 links and its total link strength is 368. This country has the most links with other countries.
The geographical distribution of papers and sugarcane production is presented in Figure 6. As shown, USA and countries from South America, Southeast Asia and Western Europe are among the countries where the relevant papers have been distributed.
In this term, Brazil is the top country with 2174 documents, followed by the USA, India, China and the UK, far behind Brazil. So that the total publications of these four countries do not reach the publications of Brazil. Brazil's leading position in scienti c research in this eld is determined by the number of institutions and authors with the most publications. Twelve of the top fteen institutes, institutes and eight of the top ten authors were from Brazil.
These three institutions are located in the Brazilian state of Sao Paulo. The next two institutions belong to Sao Paulo state. All ve institutions with the highest number of publications belong to this state with 35.21% of the world's total documents. The reason for this ranking and special attention of the Brazilian state of Sao Paulo could be that Sao Paulo has 53% of Brazilian sugarcane under cultivation (Nadaleti et al., 2020).
The production of more than 50% of Brazilian sugarcane in Sao Paulo's state has become a factor in increasing research in this eld in this state. A total of 13015 authors contributed to sugarcane and bioethanol research from 1991 to 2020. The top ten authors of the dataset are presented in Table. 5. All data for TP, TC, country of researchers, publications release year range of one author (PR) and H-index evaluation of these researchers are enclosed in this Table. These indicators are used for two important reasons.
First, these indicators can measure the overall status of publications and are commonly used in bibliometric research. Second, the characteristics of documents can be assessed with a strong theoretical H-index.
Bonomi, A. has the highest number of documents and citations and Cerri, C.E.P. has the rst place in H-index, both from Brazil.
Among them, eight authors are from Brazil, which shows Brazilian researchers' attention and their leadership in this eld. It should be noted that although Switzerland is not one of the top ten countries in the set, Ensinas, A.V. from Switzerland ranks second in terms of the number of documents.
Vosviewer analyzed the contributions of 117 authors with at least 10 publications. 94 of them were related and their coauthorship map can be seen in Figure 7.
23 researchers had no collaboration with anyone. According to the analysis, there were 362 links in 10 clusters with total link strength of 1657. Bonomi. A had the highest total link strength of 275 with 75 publications and 31 links.  Table. 6 lists the top 10 most cited articles from 1991 to 2020. These articles are from 2001 to 2012. Most of these articles deal with ethanol production from waste, the amount of greenhouse gases produced in this way and ethanol as a sustainable energy source.

Main research elds
As shown in Table. 6, authors from New Zealand, the USA and Brazil each have their names mentioned twice and once from other countries.
Among the articles "biodiesel from microalgae beats bioethanol" (2008) by Y. Chisti (Chisti, 2008) (Sims et al., 2010) was the only participatory article written with Canada and New Zealand's participation, with 905 citations ranked 6th. Due to the identi ed limitations in the production of rst-generation biofuels from food products, this study has a review on the second-generation biofuel technologies (Sims et al., 2010). The most recent article in Table. 6 "bioethanol production from agricultural waste: an overview", is related to 2012, which ranks second in citation per year with 106.75. In this review, available technologies for bioethanol production from agricultural waste are discussed (Sarkar et al., 2012). Evaluating and analyzing keywords help to understand the hot topics of research. It has been proven to be essential for monitoring the development of research topics. The authors determine the keywords according to the purpose of the article (Li et al., 2009).
In order to understand popular words and topics, the relationships between them and keywords were investigated by social network analysis software.
Page 13/23 86 keywords were used more than 20 times. Close and similar words were removed from them, 75 keywords remained and the relationship between them is given in ve clusters, each marked with a speci c color in Figure 8.
It was found that the words ethanol, sugarcane, bioethanol, sugarcane bagasse and biofuels were the most repetitive words. After them, the words fermentation, enzymatic hydrolysis and biomass were prominent.
In order to focus more on the author's keywords, they were evaluated over three decades (Figure 9). The similar words and the ones from the initial search (such as bioethanol, ethanol and sugarcane) were removed for better evaluation. Figure 9 visually shows the extracted results obtained from VOSviewer.
In 1991s, author-keywords were focused on fermentation, biomass, yeast, sugar cane bagasse, pretreatment and molasses. Fermentation is the most frequent author-keyword with occurrences (9) and total link strength (15).
In 2001s, as the number of publications increased, so did the number of keywords. In addition to the author-keywords used in 1991s, there are biofuels, renewable energy and sustainability related and close to each other. In another group, saccharomyces cerevisiae, molasses and alcoholic fermentation could be seen.
In 2011s, greenhouse gas emissions, life cycle assessment, enzymatic hydrolysis, land-use change, LCA and anaerobic digestion were used a lot. The used words related to the environment and sustainability increased in 2001s and 2011s, indicating that parts of studies focused on these elds and they became hot topics.

Conclusion
According to data from the Scopus search database, the literature review was based on sugarcane and bioethanol studies from 1991 to 2020. Various aspects were investigated, including language, institutions, eld of subject, authors, keywords and countries.
Brazil was known as the leader in sugarcane and ethanol studies with the largest number of authors and institutes. It also had many international collaborations with the USA.
Among the leading manufacturing institutions (the Universidade de Sao Paulo -USP, Universidade Estadual de Campinas and Laboratório Nacional de Biorrenováveis) all from Brazil with the highest publications, were ranked from rst to third, respectively.
Although the highest number of documents were published in 2011s, the 2001s saw the largest increase in the number of documents. From 24 documents in 2001 to 322 documents in 2011, the highest growth rate was reached during 1991 to 2020.
The political incentives and related laws were among the factors that cause research to uctuate. The more international laws and treaties in eld of bioenergy, alternative fuels for fossil fuels, agreements to reduce greenhouse gas emissions, related tax exemptions and guiding laws in this eld, the more growth can be expected in studies in this eld. It seems that the USA withdrawal from the Paris Agreement (2017) had a negative impact on research, reaching a record of the highest number of documents in 2017, but since then number of publications have declined. In the following years, the USA's rank as the secondlargest document producer of sugarcane and ethanol was threatened by India and China.
In 2011s, Environmental sciences and energy were the most important parts of the study and in keywords, focus of studies was on sustainable development, LCA and environment. According to the maps, subjects such as "life cycle evaluation" seem signi cant in the future.
In the subject category of documents, studies in 1991s focused more on biochemistry, genetics, immunology and microbiology, which changed to Environmental Science and Energy by the 2011s. Agricultural and Biological Sciences journals became more popular in 2001s and environmental science and energy journals became more popular in 2011s.
The overview of the research could help the researchers to have a better understanding of the priorities of their future researches.

Declaration of interests
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.

Figure 1
Evolution of publications and citations from 1991 to 2020.

Figure 2
Result of Subject categories analysis in the last three decades.

Figure 3
The yearly output of the top seven productive journals in sugarcane and bioethanol research from 1991 to 2020.

Figure 4
Comparison of publications of the top 6 countries in sugarcane and bioethanol research from 1991 to 2020.

Figure 6
Geographical distribution of papers and sugarcane production. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 7
Bibliographic coupling of authors whit more than ten publications.

Figure 8
Co-authorship map between 75 author keywords with more than 20 repetitions.

Figure 9
Author keywords network visualization map.