The Application of Lean Methods in Corporate Sustainability—A Systematic Literature Review

: This paper reviews the application of lean methods for corporate sustainability and highlights demands for future research. With the help of a systematic literature review, papers at the interface of lean and sustainability were identiﬁed and matched to a standardized list of lean methods to assess their frequency in the context of sustainability. In a further step, papers containing actual case studies were analyzed in more detail regarding speciﬁc application settings, sustainability dimensions, measurability of sustainability impact, and other criteria. The quantitative analysis of 363 publications shows frequent use of lean methods such as just in time and value stream mapping in the context of sustainability, and a surprisingly low use of other approaches such as karakuri, milk run, or chaku chaku. The in-depth analysis of 81 case studies reveals the primacy of intra-company and ecological assessments in the lean context, while social and inter-company aspects remain rather underexposed. This study complements existing research on lean and sustainability by systematically analyzing speciﬁc lean methods in the context of sustainability and by further exploring the sustainability characteristics of such lean applications.


Introduction
Sustainable development is a globally accepted and pursued objective, which is manifested in the United Nations Sustainable Development Goals [1]. These 17 high-level political goals have not only been integrated into regional, national, and local strategies and policies, but have also gained strategic importance in business, see, e.g., [2]. Lately, since the framing by Elkington of the term "triple bottom line" [3], the business challenge of balancing environmental, social, and economic performance is an important aspect of management research and practice, which is often referred to as corporate sustainability. The exact definition and the methodological scope of corporate sustainability are still under discussion to date. They range from "a firm's attempt to respond to environmental and social issues" to "a bundle of activities fully integrated into a firm's overall strategy that contribute effectively to the welfare of current and future generations through protecting and enhancing the resilience of the biosphere, social equity and cohesion, and economic prosperity" [4] (p. 333). Without a doubt, corporate sustainability needs to contribute to ensuring sustainable consumption and production patterns. Using natural resources efficiently and reducing waste generation substantially are key targets to achieve the respective aspects of United Nations Sustainable Development Goal 12 [5]. The wise use of resources and waste elimination are key targets of sustainable development and corporate sustainability. Resource efficiency and waste elimination are equally significant for lean strategies as well.
Lean production originated from scarce resources and the necessity to eliminate waste in all processes [6] (p. 13). Lean production focuses on the customer and strives for continuous improvement of processes and products [7] (p. 7). Therefore, lean methods and tools have "been widely implemented by manufacturing organizations to achieve operational

Methodology
A systematic literature review is a comprehensible and reproduceable research method to determine the current state of science in a subject area and to identify research gaps and further research needs [12,13]. According to Denyer and Tranfield [12], a systematic literature review includes the steps of question formulation, locating studies, study selection and evaluation, analysis and synthesis, and reporting and using results. Figure 1 illustrates the design of the respective literature review. The research questions "Which lean methods are used in the context of corporate sustainability?" and "Which specific characteristics of corporate sustainability are addressed by lean applications?" have been derived in the previous section.  Search string 1: "lean" AND ("green" OR "sustainability" OR "environment" OR "Corporate Social Responsibility") Search string 2: "lean" AND "green" OR "lean" AND "sustainability" OR "lean" AND "environment" OR "lean" AND "Corporate Social Responsibility" the two scientific databases Wiley Online Library and EBSCO EconLit with Full Text were also selected. For the search within the databases, pairs of terms from the fields of lean management and sustainability are chosen and combined into search strings using Boolean operators and brackets. Table 1 shows search engines and terms used for this study. The literature review covers publications between 1990, where the terms lean and lean management were first mentioned by Womack et al. [6], and July 2020. It includes (peer-reviewed) journal articles, conference papers (if published in a journal or a book), book contributions, and book chapters. The review excludes unpublished conference papers, working papers, textbooks, and unpublished articles and textbooks. The search terms were applied to abstract, title, and keywords. A total of 2876 publications was identified in this second step. The titles, abstracts, and keywords of all identified papers were screened using consistent criteria to exclude irrelevant publications and duplicates. The screening led to the exclusion of 159 duplicates, 25 publications that did not meet the formal requirements, and 67 publications that revealed none or only one of the search terms. There were 2262 publications that did not address the topic of lean and sustainability as specified in Section 1; for instance, environment is often used with a different meaning such as "working environment", "competitive environment", etc. Further examples are sustainability in the sense of longer lasting, as well as "lean burn" and "environment", where lean burn refers to the combustion of gases. Therefore, a further 2262 publications were excluded.
The remaining population of 363 publications was examined further to validate and answer the research questions:

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Within the given population, all meta-studies and literature reviews were identified. A qualitative content analysis of these 47 publications was used to better understand if and how previous research has covered the interaction of lean methods and corporate sustainability. Hence, this analysis served as a triangulation of the suitability and focus of the research questions of this paper.

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The full texts excluding bibliographies of all 363 publications were searched for the number of mentions of lean methods according to VDI guideline 2870 [11] to answer the first research question. If appropriate, common abbreviations and additional terms were added to the search for each method (cp. Table 1).

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Finally, all relevant publications were screened for "real world" case studies. The 81 publications with case study content were identified and included in a full text analysis to identify their characteristics with regard to corporate sustainability to answer the second research question.

Bibliometric Evaluation
The selection and evaluation of all search results revealed that 363 scientific publications are of interest for this study, dating back to a first publication on lean and sustainability published in 1996. Figure 2 shows a steady increase in publications since 2010. The 204 publications from within the last three years (2018-2020) represent 56% of all publications and underline the increasing attention that this topic receives.

Meta-Studies and Literature Reviews
There are 47 out of the 363 publications that are classified as meta-studies and literature reviews. These meta-studies and literature reviews at the interface of lean and corporate sustainability have been analyzed in terms of their topicality, their scope (number of publications considered), and their thematic focus. Table 2 shows that only a few of the existing studies and reviews cover the last three years, which accounts for the majority of all papers identified within this study. Consequently, most existing meta-studies and literature reviews consider far fewer publications compared to this study. Some of the existing literature reviews and meta-studies are industry-specific and, for instance, focus on lean construction (e.g., [14]); others focus on partial aspects, such as lean and green product development or the supply chain (e.g., [15,16]). The existing meta-studies studies remain at a philosophical level (e.g., [17,18]) or address the elimination of waste at an aggregated level (e.g., [19,20]).

Meta-Studies and Literature Reviews
There are 47 out of the 363 publications that are classified as meta-studies and literature reviews. These meta-studies and literature reviews at the interface of lean and corporate sustainability have been analyzed in terms of their topicality, their scope (number of publications considered), and their thematic focus. Table 2 shows that only a few of the existing studies and reviews cover the last three years, which accounts for the majority of all papers identified within this study. Consequently, most existing meta-studies and literature reviews consider far fewer publications compared to this study. Some of the existing literature reviews and meta-studies are industry-specific and, for instance, focus on lean construction (e.g., [14]); others focus on partial aspects, such as lean and green product development or the supply chain (e.g., [15,16]). The existing meta-studies studies remain at a philosophical level (e.g., [17,18]) or address the elimination of waste at an aggregated level (e.g., [19,20]).  [34] Criteria and practices for lean and green performance assessment: Systematic review and conceptual framework 1996-2018  Most reviews assume a positive relationship between lean and sustainability. Lean methods and lean applications are considered to contribute to resource efficiency and cleaner production and, hence, to business development towards sustainability. Sustainability effects mentioned in the reviews are often savings in materials and energy resulting from various optimizations and the avoidance of waste. For instance, methods of the zero defects principle, such as Six Sigma and Poka Yoke, avoid scrap and consequently reduce emissions and increase resource efficiency.
Some of the existing studies mention specific lean methods (e.g., [42]), but fail to provide a systematic overview nor any classification with regard to corporate sustainability. This confirms the need for a thorough investigation of lean methods and corporate sustainability as well as verifies the research questions of this paper. Figure 3 depicts the frequency of lean methods mentioned within the 363 publications on lean and sustainability. The just in time method is mentioned in 58% of all publications, followed by value stream mapping with 47%, and Six Sigma with 40%. A total of 28 methods are mentioned in at least one publication, while nine lean methods, including, e.g., chaku chaku and U layout, are not mentioned at all. Figure 3 depicts the frequency of lean methods mentioned within the 363 publications on lean and sustainability. The just in time method is mentioned in 58% of all publications, followed by value stream mapping with 47%, and Six Sigma with 40%. A total of 28 methods are mentioned in at least one publication, while nine lean methods, including, e.g., chaku chaku and U layout, are not mentioned at all.  Figure 3 also groups the methods according to the lean principles following the VDI 2870 guideline [11]. Each lean method is counted only once per paper, no matter how  Figure 3 also groups the methods according to the lean principles following the VDI 2870 guideline [11]. Each lean method is counted only once per paper, no matter how many times it is mentioned. The total of 1471 references to lean methods by 363 publications equals an average mentioning of 4 different methods per publication. The pull principle (361 mentions) and the zero defects principle (350 mentions) are the most common lean principles with each about one-fourth of all references. This is followed by the flow principle (289 mentions) and continuous improvement (183 mentions). The least frequently mentioned principle is employee orientation and management by objectives, with not a single mention in either of the two included methods.

Prevalence of Lean Methods and Concepts in Sustainability Contexts
A matrix included in the Supplementary Material shows the individual lean methods in each paper.

Sustainability Characteristics of Lean Case Studies
All 363 publications were screened for "real world" case studies, i.e., descriptions of lean implementation within company settings. The 81 publications that included case study content were subjected to a full text analysis for identifying common features and corporate sustainability characteristics within the cases (cp.  The vast majority of case studies describes applications in manufacturing companies with a huge variety of products and industries covered. Automotive, metal processing, electronics manufacturing, paper, and food are mentioned several times. Furthermore, four case studies from the construction industry and two from the logistics sector can be identified.
The full text analysis illustrates that all case studies deal with the environmental dimension of sustainability; hence, no study focuses on the social and/or economic dimension alone. The ecological dimension is combined with the economic dimension in 20 cases and with the social dimension in 9 cases. All 3 dimensions of sustainability are combined in 14 case studies.
All case studies describe one or more application(s) within a single company, while less than 15% of the studies consider either downstream or upstream supply chain actors and processes. Most case studies report results of applying lean methods with regard to sustainability qualitatively, quantitatively, or both. Energy savings are the most frequently mentioned sustainability-related effect of applying lean, followed by material savings, emission reductions, and water use decrease. The fact that cost savings and waste reduction are also on the list of most frequently reported effects mirrors the stronghold of the ecological and economic as well as the absence of the social dimension of sustainability, as mentioned before. The sustainability effects are often measured in specific indicators including energy savings in MWh/a (e.g., [61]), water savings in m3 (e.g., [62]), or Carbon Footprint in kg CO 2 -eq. (e.g., [63]).
While just in time is the most frequently mentioned method in all papers on lean and sustainability, value stream mapping dominates the case studies, followed by Six Sigma and single minute exchange of die (SMED). The case studies also feature adaptations and refinements of lean methods for sustainability purposes, e.g., "sustainable value stream mapping" [64], "energy value stream mapping" [65,66], "environmental value stream mapping" [67] and "resource value stream mapping" [68], and combinations of lean and sustainability methods, e.g., combinations of quick changeover (SMED) and carbon footprint [69], or Six Sigma and environmental life cycle assessment [70].

Discussion and Interpretation
This literature review was conducted as a standardized analysis with clear framework conditions using pertinent literature databases and full text access for all query results. The disregard of non-English language publications and publications not listed in the literature databases could be considered a limitation of this research.
The results of this literature review complement existing research on lean and sustainability by systematically analyzing specific lean methods in the context of sustainability and by further exploring the sustainability characteristics of such lean applications. The triangulation of the identified 47 meta studies additionally showed the relevance and result of the interrelationships between lean and sustainability.
Within corporate sustainability, various lean methods could be of relevance, e.g., to increase energy and resource efficiency. A detailed overview of the qualitative and quantitative sustainability effects of the lean applications is provided in the Supplementary Material. It is noticeable that certain lean methods are mentioned more frequently than others (cp. Figure 3). Overall, this is consistent with the experience of the authors regarding the frequency of use of specific lean methods. However, it is surprising that certain methods are mentioned in detail only very rarely, even though they appear to be clearly related to sustainability.
JIT, or just in time, occurs most frequently, which points to the ambivalence of the method from a sustainability point of view, where the reduction of storage space and over-ordering is offset by the environmental impact of additional transport processes. The high frequency of the JIT-Principle (210 references) can furthermore be explained by its high relevance for lean and production systems in general [71] (p. 145), [7] (p. 69).
A powerful and often used lean analysis method is value stream mapping (172 references), which combines information and material flows, determines key figures, and identifies wastage. The full text analysis of lean case studies in the context of sustainability shows that value stream mapping is further developed in terms of sustainable, energy, or environmental value stream maps as in [64,66]. Other lean methods remain underrepresented in terms of their further development. Furthermore, value stream mapping is predestined for a combination with sustainability methods such as material flow cost accounting, but only two combinations are described for this purpose in case studies. Material flow cost accounting maps material and energy flows of a production system and assigns all related costs to the products of the system and materials losses, in order to identify the cost saving potentials of resource efficiency measures [72,73].
Surprisingly, the lean principle autonomation (jidoka), which is historically older than JIT, is only ranked in the middle range (47 references). Jidoka stops machines or processes before defects are produced to avoid the passing on of errors [74] (pp. [69][70], which inevitably leads to environmental performance improvements by avoiding wastage material and energy. An unexpectedly low named method with only three mentions is the milk run [75] (pp. 236-237). Empty runs and the number of trips can be reduced in both the supply chain and internal factory logistics. The little attention paid to this methodology may also be attributed to the relatively low application of lean methods in supply chain contexts. About 85% of all case studies focus solely on the operations of one company, without considering any downstream or upstream aspects (Table 3). Neither from a lean nor sustainability perspective is the focus on the operation processes of a single company sufficient. Lean implementation often follows the so-called "line back" approach [76] (p. 113), which means that lean methods are applied for an assembly line and then gradually expanded to logistics, further areas of the factory, and towards suppliers. Likewise, methodologies of sustainability require a whole life cycle perspective, which includes the direct supply chain and even further up-and downstream processes of environmental and social relevance (e.g., [77]). The very low number of lean applications that include up-or downstream aspects show that there is still a lot of potential for further development in this area.
Karakuri is a widespread lean method and combines the characteristics of kaizen and sustainability [78]. By using natural forces, such as gravity, spring force, or magnetism, karakuri helps to save resources and energy. Thus, karakuri has a strong connection to sustainable implementations and solutions [79]. Synonyms for karakuri are low cost automation (LCA) and low cost intelligent automation (LCIA). The karakuri methods, as well as LCA and LCIA, are mentioned in only 2 out of 363 publications and are not discussed in-depth.
The lean method U-layout (U cell) [80] (pp. 71-79) is not specified in any of the identified publications. From a sustainability point of view, using small flexible assembly cells offer advantages, in particular in combination with the assembly method chaku chaku [74] (p. 70), which is not mentioned within any of the 363 publications. U cells and chaku chaku reduce distances for people and logistics, and at lower volumes, so individual cells can be completely shut down in terms of energy savings.
In general, this literature review shows that the main interface of lean and sustainability lies in the operational and production-related area and is, thus, closely related to resource-and eco-efficiency. Therefore, the ecological and economic dimensions of sustainability, plus the combination of both, clearly outweigh the social dimension (which is merely a side note). This finding is in line with previous literature reviews (e.g., [25,39,48]). However, this disregards the fact that lean methods, e.g., through ergonomics, can have a direct influence on social performance in terms of worker protection and satisfaction and can, thus, contribute very well to sustainability. Moreover, the operational focus of the lean and sustainability interface neglects the potential advantages and disadvantages of lean in unforeseeable and disruptive changes of the business environment. In consequence, this literature review does not provide answers to whether lean strengthens or weakens the resilience of supply chain systems in situations such as the current pandemic.

Conclusions and Outlook
Within a systematic literature review, papers at the interface of lean and sustainability have been identified to analyze the application of lean methods in the context of corporate sustainability. The quantitative analysis shows the prevalence of only a few lean methods, such as just in time and value stream mapping. Other methods, such as karakuri, milk run, or chaku chaku, are mentioned only very rarely, even though they appear to be clearly related to sustainability. The in-depth analysis of case studies has revealed a focus on intra-company applications and effects regarding the ecological dimension of sustainability. Upstream and downstream processes in the supply chain, as well as social implications for lean methods and concepts, remain underrepresented.
The combination of lean and sustainability is a current and relevant topic for practice. Sustainability in manufacturing companies is increasingly coming into focus. In this context, ecological and economic issues go hand in hand. As shown in the case studies, lean also has an impact on corporate sustainability and the saving of resources and greenhouse gases.
Further research topics can be derived from the results of this literature review. A promising next research step is to analyze the impact mechanisms of lean methods on sustainability performance. How does lean impact different aspects of sustainability and what is the measurable impact? Which sustainability metrics are affected? This opens up opportunities for conceptual work as well as for empirical research in the form of systematic surveys of companies that already apply lean methods and measure sustainability performance. A more detailed investigation into the potential of individual lean methods in the context of sustainability is another promising field of research. Here, case studies on the use and benefits of karakuri, for example, could improve the state of research.
The findings could be incorporated into the development of an integrated framework of lean and sustainability concepts and methods, which considers the entire value chain and starts at the early innovation stages and product development processes. Such a framework embraces the social dimension of sustainability, as lean methods and principles also focus on social aspects and people.
In the near future, the success of lean methods will also depend on their ability to contribute to corporate sustainability. This requires a systematic integration of lean and sustainability, which this paper is aimed at contributing to.