Global Trends on Food Security Research: A Bibliometric Analysis

The issue of food security has been widely studied by the international community. To reveal the research situation as it pertains to food security objectively, this paper comprehensively utilizes bibliometrics techniques (i.e., Bibliometrix, VOSviewer, and CiteSpace) to explore the research status and development trends in the area of food security. The results show that: (1) food security research has shown an increasing trend during the past 30 years. The 2013–2019 period was shown to be an active period with a high yield of articles, which were published mainly in the developed regions of Europe and America. Particularly, the number of articles published in the United States was far ahead of other countries in the world. (2) Food security research involved agriculture, environmental science and ecology, food science and technology, and business economics. The research topic is an interdisciplinary subject with a good momentum of development and a large space remaining for research. (3) Climate change, poverty, gender, nutrition, and diet structure have been the focuses of food security research in recent years. Food security in China, India, and sub-Saharan Africa has attracted wide attention. (4) Food security research is becoming more and more mature. The research scope extended from food security to food security and water and land resource security. Research topics range from decentralized to systematic. (5) Food security research is likely to gain much attention in the future based on three pillars: food supply, food access, and food use. Sustainability and diversity of food supply, along with dietary restructuring and food conservation initiatives, are expected to be new trends in future research on land management.


Introduction
Food security is directly related to international stability and human well-being, so the issue of food security has been widely studied by the international community. Although global food security has improved in recent years, many countries still suffer from serious food security problems [1]. Especially since the outbreak of COVID-19, food production has been greatly impacted, causing humanity to realize the importance of ensuring food security once again [2]. As productivity and human living standards improve continuously, people's understanding of the concept and connotations of food security has expanded and deepened, so the connotations of food security are characterized by the contemporary era [3]. In 1974, the Food and Agriculture Organization (FAO) defined food security as "ensuring that adequate food for survival and health is available to all at all times". The concept was redefined in 1983 to be "anyone who can afford at any time to buy the basic foods they need". The concept of food security was further refined in 2001 in the belief that food security can be achieved by providing people with economic and material access to food to meet their needs and preferences, for which resources should be socially available. According to the FAO's definition of food security, the first definition focuses on national and global food supply, whereas the second focuses on household and individual access to food. Policy concerns about food security are increasingly considering income, Land 2021, 10, 119 3 of 21 urbanization boosts total grain consumption. However, other studies have found that economic and social progress brought about by urbanization can also lead to more modern agricultural production technologies, in which farmers can use machinery to replace lost labor and chemical fertilizers to promote crop growth [36]. There is also considerable controversy over the development of biofuels and biotechnology [37][38][39].
Finding a problem and recognizing its drivers are often closely linked with the process of finding a solution. In the field of food security, there are also studies on how to ensure food security, which propose corresponding countermeasures and recommendations. The mainstream recommendations can be divided into three categories. The first is that poverty is the root cause of the food security problem [16], and we should start by solving poverty. Second, agricultural technology should be improved, which would result in increased grain output and improved grain quality. Third, grain consumption habits should be changed to adapt to the severe food security environment. Specific proposals include strengthening agricultural stimulus policies, as well as poverty reduction and social security schemes [40]; providing agricultural credit and increasing off-farm employment [41]; providing a variety of fertilizers, crop varieties, and irrigation methods [42][43][44]; developing appropriate agricultural scale operations [45]; optimizing the composition of residents' diets, reducing food waste; adjusting the composition of grain consumption [46,47]; and increasing the convenience of the grain trade [48].
Although many studies have been made on food security in the past, the lack of systematic articles about the research results means that the temporal and content evolution of this research field is not well understood. Although there are some comprehensive articles on food security, their reviews of food security involved only specific aspects [49], and the research methods were relatively simple [50], so most lack an interdisciplinary perspective. For scholars who want to understand the focal points and hotspots of food security research, review articles can improve their understanding of the knowledge framework of the research field more quickly. Combining the advantages of three bibliometric analysis tools-Bibliometrix, VOSviewer, and CiteSpace 5.5-this paper uses keyword frequency statistics, keyword co-occurrence, document clustering, and topical evolutionary methods to draw a research map of food security from the Web of Science (WoS) database. To understand the current research situation in the field of food security intuitively through the atlas, to analyze the influential articles and main research topics in the field of food security, and to explore the knowledge structure in the field of food security, in order to provide guidance and reference for the researchers of food security, hotspots, and frontiers in the field of food security were found, and the research trends were analyzed and proposed.

Data Source
Web of Science is the largest comprehensive information resource in the world. In this paper, we used the Web of Science core set as the data source, selected the literature type as article, set the advanced retrieval type as Ti = (food security OR grain security) [51], and set the deadline as 31 December 2019. After the data reweighting, this method resulted in 2974 references to food security for 1990-2019.

Research Methodology
At present, there are more than ten kinds of literature atlas analysis software, and each has its advantages. Bibliometrix measures scientific literature based on the R language developed by Dr. Massimo Aria of Italy. It can import and process information from Scopus and WoS, analyze the index of scientific literature, and construct the data matrix; the co-citation, coupling, co-, and co-word analyses; and other aspects of research and visualization [52] from recent years in various fields of scientific research [53][54][55]. VOSviewer, developed by Nies Jan van Eck and Ludo Waltman, is widely used in all kinds of co-occurrence analysis software, especially in terms of keyword co-occurrence analysis [56][57][58][59][60][61]. CiteSpace, developed by Professor Chen Chaomei of Drexel University in the United States, is known for its powerful document co-citation analysis. CiteSpace is used by scholars to explore research hotspots, frontiers, and trends and is now widely used in more than 60 fields, including computer science, information science, food, and medicine [62][63][64][65]. To accurately and comprehensively analyze the development status and research hotspots in the field of food security and to develop visualizations thereof, this paper analyzes the research status and hotspot trends in the field of food security using the software packages CiteSpace 5.5, VOSviewer, and Bibliometrix. The detailed steps for this research are shown in Figure 1. matrix; the co-citation, coupling, co-, and co-word analyses; and other aspects of research and visualization [52] from recent years in various fields of scientific research [53][54][55]. VOSviewer, developed by Nies Jan van Eck and Ludo Waltman, is widely used in all kinds of co-occurrence analysis software, especially in terms of keyword co-occurrence analysis [56][57][58][59][60][61]. CiteSpace, developed by Professor Chen Chaomei of Drexel University in the United States, is known for its powerful document co-citation analysis. CiteSpace is used by scholars to explore research hotspots, frontiers, and trends and is now widely used in more than 60 fields, including computer science, information science, food, and medicine [62][63][64][65]. To accurately and comprehensively analyze the development status and research hotspots in the field of food security and to develop visualizations thereof, this paper analyzes the research status and hotspot trends in the field of food security using the software packages CiteSpace 5.5, VOSviewer, and Bibliometrix. The detailed steps for this research are shown in Figure 1.

Main Information
The analysis of the evolution of things can be a yearly tracking of the time series and can be divided into different stages of research. The chronological distribution of the literature quantity can reflect the trend of the research and describe the characteristics of the trend in different stages of development [66]. From 1990 to 2019, the number of research papers published in the field of food security, although slightly fluctuating, maintained

Main Information
The analysis of the evolution of things can be a yearly tracking of the time series and can be divided into different stages of research. The chronological distribution of the literature quantity can reflect the trend of the research and describe the characteristics of the trend in different stages of development [66]. From 1990 to 2019, the number of research papers published in the field of food security, although slightly fluctuating, maintained an overall trend of growth, as seen in Figure 2, depicting the annual publication numbers of the food security articles. This trend can be divided into three research stages: 1990-2006, 2007-2012, and 2013-2019. The period from 1990 to 2006 was one of low output exploration. Before 2006, the increase in the number of articles published was slow, and the annual average number of articles published varied little annually. Research on food security was in the embryonic stage, which shows that the importance of the field of food security had not attracted the attention of most scholars. In 2007-2012, an average number of 84 papers was published, entering the stage of rapid development. The period from 2013 to 2019 was an active period of high yield, with a sharp increase in the amount of literature, with an average annual publication volume of 292.9 articles, which continued to increase. This phenomenon was largely affected by the food crisis in 2008-2012, resulting in an increase in the relevant research [67,68], which also shows that the international academic community is paying more attention to the issue of food security. The average number of citations was the highest in 2004, at 100.1, which shows that the related research in this period has a great influence. The 2004 paper Soil Carbon Sequestration Impacts on Global Climate Change and Food Security, published in Science by Lal R, has been cited as many as 3412 times, making a significant contribution to the average citations of the literature of the period. In addition to increasing food security, the paper suggests that carbon sequestration could offset between 0.4 and 120 million tons of carbon emitted by fossil fuels each year. The results of this study have been widely recognized in various academic fields [69].
an overall trend of growth, as seen in Figure 2, depicting the annual publication numbers of the food security articles. This trend can be divided into three research stages: [1990][1991][1992][1993][1994][1995][1996][1997][1998][1999][2000][2001][2002][2003][2004][2005][2006][2007][2008][2009][2010][2011][2012], and 2013-2019. The period from 1990 to 2006 was one of low output exploration. Before 2006, the increase in the number of articles published was slow, and the annual average number of articles published varied little annually. Research on food security was in the embryonic stage, which shows that the importance of the field of food security had not attracted the attention of most scholars. In 2007-2012, an average number of 84 papers was published, entering the stage of rapid development. The period from 2013 to 2019 was an active period of high yield, with a sharp increase in the amount of literature, with an average annual publication volume of 292.9 articles, which continued to increase. This phenomenon was largely affected by the food crisis in 2008-2012, resulting in an increase in the relevant research [67,68], which also shows that the international academic community is paying more attention to the issue of food security. The average number of citations was the highest in 2004, at 100.1, which shows that the related research in this period has a great influence. The 2004 paper Soil Carbon Sequestration Impacts on Global Climate Change and Food Security, published in Science by Lal R, has been cited as many as 3412 times, making a significant contribution to the average citations of the literature of the period. In addition to increasing food security, the paper suggests that carbon sequestration could offset between 0.4 and 120 million tons of carbon emitted by fossil fuels each year. The results of this study have been widely recognized in various academic fields [69].

Authors
The authors' information contained in the collected data allows for the identification of lead authors, as well as their institutions and countries in the field of food security. The h-index refers to the number of published papers that have been cited at least h times [70]. The h-index, total citations, number of publications, and year of first publication in the area of food security for the top 10 influential authors who make the largest contribution to the WoS database are shown in Table 1. Figure 3 shows their distribution over time. The volume of the sphere is proportional to the annual number of papers (NP) and the color depth of the sphere is proportional to the annual total article citations (TC) (TC/Y). According to the volume statistics, Sieber S (14 articles) had the largest number of articles, and the researchers studied a wide range of issues in dealing with food security issues, mainly the performance of farmers in Tanzania. Replacing food crops with biofuel-producing crops could have a negative impact on local food security and was shown to increase hunger [71][72][73]. In terms of the h-index, Nord M has the largest influence, with an index value of up to 10, followed by Herrero M and Lal R, who each have an index value of 9. Nord M is mainly devoted to the research of the food security evaluation system,

Authors
The authors' information contained in the collected data allows for the identification of lead authors, as well as their institutions and countries in the field of food security. The h-index refers to the number of published papers that have been cited at least h times [70]. The h-index, total citations, number of publications, and year of first publication in the area of food security for the top 10 influential authors who make the largest contribution to the WoS database are shown in Table 1. Figure 3 shows their distribution over time. The volume of the sphere is proportional to the annual number of papers (NP) and the color depth of the sphere is proportional to the annual total article citations (TC) (TC/Y). According to the volume statistics, Sieber S (14 articles) had the largest number of articles, and the researchers studied a wide range of issues in dealing with food security issues, mainly the performance of farmers in Tanzania. Replacing food crops with biofuel-producing crops could have a negative impact on local food security and was shown to increase hunger [71][72][73]. In terms of the h-index, Nord M has the largest influence, with an index value of up to 10, followed by Herrero M and Lal R, who each have an index value of 9. Nord M is mainly devoted to the research of the food security evaluation system, which primarily takes the form of an interview survey aiming to study the influence of food security on life based on the types and psychological states of the respondents. In terms of total citations, Lal R has the most. Figure 3 shows that high-yield authors in the database published a large number of articles between 2013 and 2019, which coincides with the period of high-yield activity. This shows that food security research has gained high attention. food security on life based on the types and psychological states of the respondents. In terms of total citations, Lal R has the most. Figure 3 shows that high-yield authors in the database published a large number of articles between 2013 and 2019, which coincides with the period of high-yield activity. This shows that food security research has gained high attention.  According to the statistical analysis of the relevant countries, the three countries with the highest number of papers on food security in the WoS database are the United States (1952), Canada (486), and the United Kingdom (464). The United States is far ahead of other countries in terms of the number of documents published, which partly reflects the fact that the United States is more active in the field of food security than other countries. Six of the top 10 countries are developed, whereas the four developing countries are the world's two most populous (China and India) and two are from sub-Saharan Africa (South Africa and Kenya). According to the average citation quantity of the articles, Italy has the highest average citation quantity, and the average citation quantity of articles in According to the statistical analysis of the relevant countries, the three countries with the highest number of papers on food security in the WoS database are the United States (1952), Canada (486), and the United Kingdom (464). The United States is far ahead of other countries in terms of the number of documents published, which partly reflects the fact that the United States is more active in the field of food security than other countries. Six of the top 10 countries are developed, whereas the four developing countries are the world's two most populous (China and India) and two are from sub-Saharan Africa (South Africa and Kenya). According to the average citation quantity of the articles, Italy has the highest average citation quantity, and the average citation quantity of articles in developed countries is generally higher. China and India are the two most populous countries in the world, and their populations are still growing. The international community has been paying close attention to the food security of these two countries [74,75]. Sub-Saharan Africa is the only region in the world where more than one-third of the population suffers from widespread hunger, and smallholder farmers, who produce more than 80% of Africa's food, face caloric and micronutrient deficiencies [76]. South Africa and Kenya are representative of the region, where 61% of households suffer from mild, moderate, or severe food insecurity [77]. Studies from this region study focused primarily on smallholder farmers. The strategies for improving food security were studied Land 2021, 10, 119 7 of 21 from the perspectives of pests, agricultural diversification, food improvement, investment in education, and food storage [14,18,[78][79][80]. Country-by-country articles are included in Table 2 below, and country postings are distributed in Figure 4 below. Darker colors indicate higher number of articles published. Most of the articles are published in European and American countries, whereas China, India, and Japan in Asia have more articles, and some countries in central Africa and central and western Asia have no relevant literature on food security. This result also shows the weakness of food security research in these countries. On the whole, the United Kingdom and the United States cooperate closely, and the level of cooperation between African countries and the developed countries needs to be improved. The different clusters formed also illustrate the differences in food security experienced by individual countries. Countries with similar natural, economic, and social conditions are conducting more exchanges and cooperation.  The statistical analysis of the articles related to food security revealed that food security involves a number of research directions, including agriculture, environmental sciences, and ecology; food science and technology; business economics, etc. It has a good momentum of development and a large space for research. Most of these areas are in agriculture, followed by environmental sciences and technology, and then food science and technology, as shown in Figure 5. The analysis of relevant articles on different research directions showed that the research on food security from the perspective of agriculture is mainly focused on how to improve agricultural production technology. Suggestions include improving new varieties to increase food production, increasing irrigation levels to make rational use of water resources, advocating for the importance of urban and smallholder agriculture, and encouraging the provision of agricultural credits [81][82][83][84][85]. Research The statistical analysis of the articles related to food security revealed that food security involves a number of research directions, including agriculture, environmental sciences, and ecology; food science and technology; business economics, etc. It has a good momentum of development and a large space for research. Most of these areas are in agriculture, followed by environmental sciences and technology, and then food science and technology, as shown in Figure 5. The analysis of relevant articles on different research directions showed that the research on food security from the perspective of agriculture is mainly focused on how to improve agricultural production technology. Suggestions include improving new varieties to increase food production, increasing irrigation levels to make rational use of water resources, advocating for the importance of urban and smallholder agriculture, and encouraging the provision of agricultural credits [81][82][83][84][85]. Research in the environmental sciences and ecology focuses on the impact of climate change on food production; argues that climate change has potentially devastating impacts on biodiversity and food security; and argues for the importance of biological and genetic diversity, opportunities for income generation through the development of biofuels, and the expansion of agricultural production technologies that may lead to welfare gains that increase purchasing power and reduce vulnerability to food price shocks [86][87][88]. Furthermore, food science and technology mainly focus on food nutrition [89,90]. Food security from the perspective of the commercial economy mainly considers food policy and the food trade, as well as the fact that global trade improves national food security and suggests that food security should be improved through measures such as increasing non-farm employment, providing agricultural credits, and developing fisheries [85,[91][92][93][94][95][96].

Cooperative Network Analysis
Analysis of National Cooperative Networks Cooperation networks are mainly of three types: country-to-country, institution-toinstitution, and scholar-to-scholar. For this paper, we used the co-authorship function in VOSviewer to obtain the cooperative network, shown in Figure 6, with a minimum of 30 posts and 50 citations. The size of the nodes in the graph represents the amount of text sent from the country, the lines indicate cooperation between countries, the colors represent different groups, and the distance between nodes represents the similarity of research topics. Our observations showed that the inter-country cooperation network on food security is divided into five clusters, with the following main countries: cluster #1, the United Kingdom and Australia; #2, the United States and Canada; #3, South Africa and Kenya; #4, Italy and France; and #5, Germany and Sweden. The compositions of clusters #3-5 are relatively simple, with African and European countries represented. Clusters #1 and #2 are more diverse, come from different continents, and cover a wider range of research topics.

Cooperative Network Analysis Analysis of National Cooperative Networks
Cooperation networks are mainly of three types: country-to-country, institution-toinstitution, and scholar-to-scholar. For this paper, we used the co-authorship function in VOSviewer to obtain the cooperative network, shown in Figure 6, with a minimum of 30 posts and 50 citations. The size of the nodes in the graph represents the amount of text sent from the country, the lines indicate cooperation between countries, the colors represent different groups, and the distance between nodes represents the similarity of research topics. Our observations showed that the inter-country cooperation network on food security is divided into five clusters, with the following main countries: cluster #1, the United Kingdom and Australia; #2, the United States and Canada; #3, South Africa and Kenya; #4, Italy and France; and #5, Germany and Sweden. The compositions of clusters #3-5 are relatively simple, with African and European countries represented. Clusters #1 and #2 are more diverse, come from different continents, and cover a wider range of research topics.

Analysis of National Cooperative Networks
The authors' cooperative network diagram is able to reflect the composition of the research team, the strength of the cooperation among the research teams can be judged by the distance between the groups and the connection density, and the size of the research team can be seen from the number of nodes in the groups. The co-authorship function in VOSviewer sets the minimum number of posts to four and the minimum number of citations to 10 in order to obtain the cooperative network ( Figure 7). The images show that the authors of the study on food security have formed four large groups. Study Group #1, led by Palazzo A, studies the relationship between food security and land and water resources, as well as the impact of climate change on food security. This group considers that the main challenges to food security will come from increased demand plus regional differences in the ability to adapt to climate change [97][98][99][100][101]. Study Group #2, headed by Havlik P, focuses on the effects of carbon emissions-induced global warming on food security [102][103][104]. Study Group #3, headed by See L, focuses on the causes of food security problems on the African continent [18,105]. Study Group #4, headed by Herrero M, addresses food security issues and proposes ways to ensure food security through the provision of diversified foods and the empowerment of farmers [106][107][108]. From the point of view of these authors' cooperative clusters, irrespective of theoretical analysis, driving force analysis, or game analysis, they are all interrelated and inseparable. However, we can also find that the theoretical analysis is still weak, especially from the perspective of sustainable development of food security theory less.

Analysis of National Cooperative Networks
The authors' cooperative network diagram is able to reflect the composition of the research team, the strength of the cooperation among the research teams can be judged by the distance between the groups and the connection density, and the size of the research team can be seen from the number of nodes in the groups. The co-authorship function in VOSviewer sets the minimum number of posts to four and the minimum number of citations to 10 in order to obtain the cooperative network ( Figure 7). The images show that the authors of the study on food security have formed four large groups. Study Group #1, led by Palazzo A, studies the relationship between food security and land and water resources, as well as the impact of climate change on food security. This group considers that the main challenges to food security will come from increased demand plus regional differences in the ability to adapt to climate change [97][98][99][100][101]. Study Group #2, headed by Havlik P, focuses on the effects of carbon emissions-induced global warming on food security [102][103][104]. Study Group #3, headed by See L, focuses on the causes of food security problems on the African continent [18,105]. Study Group #4, headed by Herrero M, addresses food security issues and proposes ways to ensure food security through the provision of diversified foods and the empowerment of farmers [106][107][108]. From the point of view of these authors' cooperative clusters, irrespective of theoretical analysis, driving force analysis, or game analysis, they are all interrelated and inseparable. However, we can also find that the theoretical analysis is still weak, especially from the perspective of sustainable development of food security theory less.

Co-Citation Analysis
Co-citation networks can discover influential articles in the field of food security, which are often enlightening and lay a theoretical foundation for the research field. CiteSpace visual analysis software has some advantages in the co-citation function and can analyze the literature that is active in the research field and has great reference value. In this paper, CiteSpace 5.5 was used, the reference node was selected, and a network graph (Figure 8) of food security literature citations was obtained, according to the default values. Each node represents an article, and its size is proportional to the frequency of citation. The colors of the nodes and wires indicate when the article was referenced. The co-citation network for food security revealed four main groups.
The most influential articles in food security research are available through the CiteSpace co-citation statistical function, and the most influential literature in the field of food security research was published around 2010, mostly in top international journals, according to observational studies. Godfrey HCJ (2010) identifies the need for a multifaceted and interconnected global strategy to ensure sustainable and equitable food security [109]. Barrett CB (2010) focuses on how to measure food safety [10]. Pinstrup-Andersen P (2009) discusses the concept of food security as it changes over time and suggests that temporal assessment is a useful way of measuring the well-being of families and individuals, in particular, by combining this measurement with the estimation of household food purchasing and distribution behavior [7]. From the co-citation of these most influential articles, the frequency of co-citation of the theoretical research articles is high, especially after the 2008 food crisis, which lays a foundation for later research.

Co-Citation Analysis
Co-citation networks can discover influential articles in the field of food security, which are often enlightening and lay a theoretical foundation for the research field. CiteSpace visual analysis software has some advantages in the co-citation function and can analyze the literature that is active in the research field and has great reference value. In this paper, CiteSpace 5.5 was used, the reference node was selected, and a network graph (Figure 8) of food security literature citations was obtained, according to the default values. Each node represents an article, and its size is proportional to the frequency of citation. The colors of the nodes and wires indicate when the article was referenced. The co-citation network for food security revealed four main groups.
The most influential articles in food security research are available through the CiteSpace co-citation statistical function, and the most influential literature in the field of food security research was published around 2010, mostly in top international journals, according to observational studies. Godfrey HCJ (2010) identifies the need for a multifaceted and interconnected global strategy to ensure sustainable and equitable food security [109]. Barrett CB (2010) focuses on how to measure food safety [10]. Pinstrup-Andersen P (2009) discusses the concept of food security as it changes over time and suggests that temporal assessment is a useful way of measuring the well-being of families and individuals, in particular, by combining this measurement with the estimation of household food purchasing and distribution behavior [7]. From the co-citation of these most influential articles, the frequency of co-citation of the theoretical research articles is high, especially after the 2008 food crisis, which lays a foundation for later research.

Keyword Co-Occurrence Analysis
Keywords can provide core information about the content of the article, and two or more keywords appearing in the same paper at the same time are called a keyword cooccurrence [110]. A keyword co-occurrence analysis can identify research hotspots and monitor the transfer of research frontiers in the field of scientific knowledge [111,112]. In this study, we used the co-occurrence feature in VOSviewer, set to at least 15 author keywords, to obtain a keyword co-occurrence graph. In the keyword co-occurrence graph, a larger node indicates a higher frequency of keyword occurrence. The 20 most frequently used keywords are listed in Table 3. The analysis of keywords generally includes two analysis routes-the frequency of individual keywords, from which we can understand the research hotspot in this field, or the number of keyword co-occurrences, which relies on a combination of analyses to understand the main research topics. Table 3 shows that climate change is the most frequently used keyword in addition to food security, followed closely by food insecurity, nutrition, poverty, and others. The issue of food security in China, India, Ethiopia, and sub-Saharan Africa has drawn wide attention from the national community. There are differences in the causes of and approaches to food security issues due to local backgrounds in natural and socio-economic conditions. Given the cooccurrence of keywords in Figure 9, poverty is an important cause of food shortages in Ethiopia, and the main way to solve the problem of food security is to get rid of war chaos and develop the economy [51,81]. Differences in the status of men and women in sub-Saharan African countries result in the division of roles and the impact of adverse natural conditions on food security; the problem of food shortages can be further improved by changes in farmers' concepts, the development of small-scale farmers, and the improvement of new crop varieties [94]. As the two most populous countries in the world, China and India are both developing countries, and the limitations of population and economic conditions are obvious [74]. Therefore, it is necessary to formulate reasonable food policies and perfect food sovereignty. At the same time, attention should also be paid to the threats

Keyword Co-Occurrence Analysis
Keywords can provide core information about the content of the article, and two or more keywords appearing in the same paper at the same time are called a keyword co-occurrence [110]. A keyword co-occurrence analysis can identify research hotspots and monitor the transfer of research frontiers in the field of scientific knowledge [111,112]. In this study, we used the co-occurrence feature in VOSviewer, set to at least 15 author keywords, to obtain a keyword co-occurrence graph. In the keyword co-occurrence graph, a larger node indicates a higher frequency of keyword occurrence. The 20 most frequently used keywords are listed in Table 3. The analysis of keywords generally includes two analysis routes-the frequency of individual keywords, from which we can understand the research hotspot in this field, or the number of keyword co-occurrences, which relies on a combination of analyses to understand the main research topics. Table 3 shows that climate change is the most frequently used keyword in addition to food security, followed closely by food insecurity, nutrition, poverty, and others. The issue of food security in China, India, Ethiopia, and sub-Saharan Africa has drawn wide attention from the national community. There are differences in the causes of and approaches to food security issues due to local backgrounds in natural and socio-economic conditions. Given the co-occurrence of keywords in Figure 9, poverty is an important cause of food shortages in Ethiopia, and the main way to solve the problem of food security is to get rid of war chaos and develop the economy [51,81]. Differences in the status of men and women in sub-Saharan African countries result in the division of roles and the impact of adverse natural conditions on food security; the problem of food shortages can be further improved by changes in farmers' concepts, the development of small-scale farmers, and the improvement of new crop varieties [94]. As the two most populous countries in the world, China and India are both developing countries, and the limitations of population and economic conditions are obvious [74]. Therefore, it is necessary to formulate reasonable food policies and perfect food sovereignty. At the same time, attention should also be paid to the threats posed by climate change and the excessive use of pesticides and fertilizers to agricultural production and sustainability. posed by climate change and the excessive use of pesticides and fertilizers to agricultural production and sustainability.  As shown in Figure 10, burst keywords (frequently quoted keywords over a period) can indicate leading-edge topics. As seen from Figure 10, twenty-one burst keywords emerged in the field of food security research from 1990 to 2019. Nutrition and hunger are two of the keywords that emerged in the early days, and they appear and last approximately the same amount of time, suggesting that nutrition and hunger have drawn attention to food security in the academic world and that they represent two of its aspects. Over time, keywords such as conflict, aid, urban agriculture, overweight, and smallholder farmer emerged, reflecting the research focus at different times. Keywords such as food safety, food access, sustainable development, food waste and availability are among the most recent and are found in ongoing research, indicating that food safety, food access, and use sustainability and availability have been the focus of recent academic attention and maybe for a while, as scholars address the issue of food security at the point of entry. The emergence of a word from the original scattered specific research topics into food As shown in Figure 10, burst keywords (frequently quoted keywords over a period) can indicate leading-edge topics. As seen from Figure 10, twenty-one burst keywords emerged in the field of food security research from 1990 to 2019. Nutrition and hunger are two of the keywords that emerged in the early days, and they appear and last approximately the same amount of time, suggesting that nutrition and hunger have drawn attention to food security in the academic world and that they represent two of its aspects. Over time, keywords such as conflict, aid, urban agriculture, overweight, and smallholder farmer emerged, reflecting the research focus at different times. Keywords such as food safety, food access, sustainable development, food waste and availability are among the most recent and are found in ongoing research, indicating that food safety, food access, and use sustainability and availability have been the focus of recent academic attention and maybe for a while, as scholars address the issue of food security at the point of entry. The emergence of a word from the original scattered specific research topics into food security, food availability, diversification, sustainability, effectiveness, and other systematic research indicates that studies have made great progress in terms of their breadth and depth. It shows that the research on food security is becoming mature in recent years. security, food availability, diversification, sustainability, effectiveness, and other systematic research indicates that studies have made great progress in terms of their breadth and depth. It shows that the research on food security is becoming mature in recent years.

Evolutionary Path of Research Topics and Identification of Research Frontiers
Thematic evolutionary analysis studies a theme's content, intensity, and structure, showing that the theme changes with time, evolutionary relations, and tendency. Topical evolutionary analysis plays an important role in sorting out the development of the field, grasping the direction of the field and proposing its trends. The Sankey diagram, also known as the Sankey energy split diagram, is a type of flowchart (see Figure 11). In this paper, the evolution of food security research topics over time is visualized using the Sankey diagram, and the evolution of food security research topics over the whole research process is analyzed with the help of the thematic evolutionary diagram. Qualitatively speaking, the topical evolutionary path of the domain was identified based on the evolutionary trend (including the problem-solving, problem-finding, and development paths).

Evolutionary Path of Research Topics and Identification of Research Frontiers
Thematic evolutionary analysis studies a theme's content, intensity, and structure, showing that the theme changes with time, evolutionary relations, and tendency. Topical evolutionary analysis plays an important role in sorting out the development of the field, grasping the direction of the field and proposing its trends. The Sankey diagram, also known as the Sankey energy split diagram, is a type of flowchart (see Figure 11). In this paper, the evolution of food security research topics over time is visualized using the Sankey diagram, and the evolution of food security research topics over the whole research process is analyzed with the help of the thematic evolutionary diagram. Qualitatively speaking, the topical evolutionary path of the domain was identified based on the evolutionary trend (including the problem-solving, problem-finding, and development paths).
The ways to solve the problem mainly include the following: In light of the problem-solving evolutionary path, the issue of food security in Africa has received early attention. To solve the problem, first through the provision of food aid and then through the development of urban agriculture, researchers turned to small farmers. The problems of hunger and nutrition have made people aware of the importance of food security. Studies have found that climate change and poverty are important causes, and sustainable development programs should be developed. Solving the problem of poverty and adjusting the dietary structure are conducive to solving the problem of food security. The development of aquaculture is also conducive to alleviating environmental food security problems.
The main problem-finding paths include the following: Determining the path of the problem can help us understand its cause. As seen from the main evolutionary paths, the problems of drought, poverty, health, and nutrition have drawn attention to the issue of food security. The issue of food security in China, Asia, and southern Africa has drawn wide attention. Poverty limits children's growth and raises concerns about food security.
The development spin-off paths mainly include the following: The main problem-finding paths include the following: The path can be divided into two types: development and derivation. Development is a process of progress and change from the appearance of things; it is constant renewal and a continuous process of change. Derivation refers to the evolution of a new substance from the parent substance. Similarly, the evolution of food security research themes has resulted in developments in spinoffs, such as path 1, where the issue of hunger has given rise to concerns about food security, which has led to concerns about food safety, and the corresponding fields of study have gradually expanded. Because the connotations of food security are smaller than those of food safety, the essence of food security is food safety, and the focus of food safety is food [113]; thus, this evolutionary path belongs to the development path. In the same way, attention to food and nutrition issues is derived from food security, and the research area is expanding, which belongs to the development path. Paths 3 and 4 derive from the initial food security and agricultural policies, and the issues of water security and the food trade, respectively. They are derivative processes of the theme and thus belong to the evolutionary path of the derivative theme [16,114].
The width of the line in the evolutionary graph can reflect changes in the quantitative studies. It can be found that the scope of research topics is gradually narrowing, and there is no inevitable relationship between the maximum amount of research topics. The early food security research was mainly caused by nutrition and health problems in Africa, Brazil, and China, and China and Brazil received the most attention. By the 2000s, food security in African countries remained a serious problem, and research into poverty and food aid related to food security increased. In the past decade, food shortage, food trade and consumption, urban agriculture and small-scale farming have become the focus of most studies.
Keyword co-occurrence networks can intuitively describe the frequency and cooccurrence of keywords, burst words can express the time and duration of the emergence of keywords, and the evolutionary path can clearly depict the specific research context. The whole research process in the field of food security can be understood by means of a comprehensive observation of co-occurrence maps, emergent keyword maps, and topical evolutionary paths. The observations clearly show that the main theme of the study revolves around the exploration of the drivers of food security and the search for solutions to food security problems. Global food and nutrition issues have received wide attention, particularly in Africa, Latin America, and parts of Asia, where food security is a major concern, with studies showing that by 2018 there were more than 820 million undernourished people in the world-people who face chronic food shortages. Malnutrition and severe food insecurity are increasing in almost all regions of Africa and, although to a lesser extent, in Latin America and western Asia [115]. There are many reasons for food security, such as climate change, poverty, drought, gender, the food trade, and agricultural policy. Food aid, including short-term emergency relief and planned food aid to help address medium-term food shortages, is usually a major component of food security strategies in developing countries [94]. Studies have found that the WTO food trade regime has an impact on the trade policies of its members and on food-related rights; therefore, it may have an impact on food security. The changing trade environment undermines the traditional role of the world food aid system in promoting food security. Many developing countries, particularly the least-developed countries and net food importers, are likely to face greater food difficulties than before, at least in the short term [116]. Part of the study area is devoted to the issue of how to ensure food security and suggesting corresponding countermeasures and suggestions. Starting from the definition of food security, the mainstream view holds that the countermeasures and suggestions for food security should be based on three pillars-supply, access, and use. From the perspective of food supply, improvements should be made in agricultural production techniques to increase food production and quality and to facilitate food storage and trade flows, including providing diversified fertilizers and crop varieties, improving irrigation methods [42][43][44], developing appropriate agricultural scale operations [45], providing agricultural credits to promote agricultural production and increase grain production [41], supporting small-scale farmers to integrate organically with modern agriculture [117], paying attention to aquaculture [43], expanding grain storage and logistics facilities [118], and improving the international food security trade order [48,92]. In terms of access to food, poverty has been identified as the root cause of food insecurity, and access to food should be made more affordable by increasing people's income [16]-including through enhanced agricultural incentives, poverty reduction, and social security programs [40,119]-and by increasing nonfarm employment [41,120]. From the perspective of food use, food consumption habits should be changed to adapt to the severe food security environment, including optimizing the composition of residents' diets and adjusting the structure of grain consumption [46,47]. Government policy plays an important role in solving the problem of food security. The government, as the maker of public policy, can better solve the problem of food security by providing public products, and make relevant laws and regulations to restrict and regulate the important links of food supply, food access, and food use; therefore, it is important to find a solution to the problem of food shortage from the perspective of government policy. For instance, in China, the most populous country in the world, the government has formulated some relevant policies to address food security issues, such as setting a red line for protecting 1.8 billion mu of arable land, demarcating permanent basic farmland and developing high-standard farmland, and has thus solved problems of food shortages and made great contributions to poverty reduction [121,122].
The research focus on food security has been changing over time, and some emerging research hotspots and topics are not shown in the Figures. Our results suggest that we should deal with the problem of food security from the viewpoint of sustainable development and encourage people to save food. Topics closely related to food security include water resource security and nutrition security, which have attracted much academic attention, and research on the water-energy-land nexus is becoming increasingly popular.

Conclusions
In this study, the literature related to food security in the WoS database from 1990 to 2019 was analyzed using the advantages of different software. From the macroscopic understanding of the development of food security research, we noted the main research group and the country, and the correlation between the distribution of research topics. We aimed to understand the research focus and evolution path of food security from the microcosmic point of view, and analyze the main articles and the research results of the main authors. It is expected that this paper will provide relevant researchers with a comprehensive reference and help scholars to understand the latest content on food security and to conduct more in-depth research based on the existing studies and future research trends. We have drawn the following conclusions: (1) The number of papers published in the field of food security fluctuated slightly in the past 30 years, but the general trend continued to increase. A high-yield active period for papers on food security was from 2013 to 2019, with a sharp increase in the number of publications. The United States published the largest number of articles, far ahead of the rest of the world. China, India, and Japan were the largest contributors in Asia, whereas some countries in central Africa and in west and central Asia have not published articles on food security, indicating a weak research capacity in these countries. (2) From the perspective of cooperative networks, five main groups formed between countries and four main research groups formed between authors and authors. At present, the degree of cooperation and data sharing among countries is not high, the existing research base cannot be fully developed, leading to more repetitive work. Therefore, the national partnership urgently needs to be strengthened and improved. The most influential articles in the field of food security research were published around 2010, and most of the journals in which the articles were published were top international journals. The issue of food security in China, India, Ethiopia, and sub-Saharan Africa has drawn wide attention from the national community, and climate change, agriculture, nutrition, poverty, sustainability, gender, hunger, and dietary diversity are among the most discussed topics in food security. Nutrition and hunger are two of the earlier buzzwords, whereas food safety, food access, sustainable development, food waste, and availability have been the focus of attention in food security research in recent years. It is worth noting the amount of past research on food security based on the theory of sustainability is low, as well as the lack of food security evaluation research. Regional food security evaluation is not a one-time evaluation, but a dynamic evaluation process dependent upon changes in natural and social economic conditions. Monitoring and periodic assessments are therefore necessary to analyze the factors that impede food security at different times. (3) The study of food security has become increasingly mature and has made great progress in recent years, changing from specific research subjects to research on food