The Emerging Hemp Industry: A Review of Industrial Hemp Materials and Product Manufacturing

: There is a growing need for resilient and renewable materials to aid society in global sustainability. It is incumbent upon the agricultural and manufacturing industries to work together to achieve this vision. In particular, the hemp plant has been identiﬁed as an emerging industrial crop that will be pivotal in achieving the United Nations Sustainable Development Goals. However, this nascent industry has received an inﬂux of research and development activity, resulting in various methods and practices globally, challenging the repeatability of results, research advancement, standards development, and sustainability assessment. A systematic literature review is conducted to identify and document (1) the various practices for harvesting and converting industrial hemp into materials and products and (2) existing hemp-derived products and those under development. Using the PRISMA methodology, 5295 articles were identiﬁed, and 109 articles were included for review. Unlike


Introduction
The way products are manufactured is steadily revolutionizing as industries globally strive to meet the environmental principles of the United Nations Global Compact, which is concerned with energy, climate change, food, water, and biodiversity [1].There have been concerted efforts globally by countries and industry stakeholders toward achieving common sustainability goals [2].As the world population increases and the demand for products and services rises, industries face the challenge of sustainably meeting the market demand for products and providing the infrastructure for services.Products that can create sustainable value chains, integrating the three pillars of sustainability-environmental, social, and economic-across their up-, mid-, and downstream activities, are in high demand [3].With the advances in technologies in the Information Age, there is greater focus and interest in renewable and resilient materials to enable the development of innovative products [2,4].This is true for even some of the oldest industries, such as construction and textile manufacturing, as they also need more sustainable alternative materials [5,6].
Renewable material sources with practical industrial applications and lower environmental impacts are steadily gaining interest as potential feedstocks for a range of industrial processes and as alternatives to synthetic materials, driving the industry to be more sustainable [4,7].Manufacturers seek to improve their products' green attributes, exploring the use of multi-versatile, natural materials to produce more eco-friendly products instead of relying on conventional or synthetic materials derived from fossil fuels [8].
The hemp plant fits the previous description well as it meets production and human food needs, has environmental benefits, efficiently uses nonrenewable resources, and has good economic viability and social benefits to the farmer and society [9].The hemp plant has been identified as a material for the future industry [7], while the hemp industry has been noted as an emerging agricultural industry of the Fourth Industrial Revolution [5] that will contribute to achieving a majority of the Sustainable Development Goals established by the United Nations [9].Sustainable agriculture is recognized as a critical need in achieving sustainable development [9,10], with hemp ranked highly as an emerging source for producing more sustainable products [11].There has also been surging interest in hemp products and their derivatives, as all parts of the plant can be utilized and processed into valuable products [12], making it versatile in industries such as construction, automobile production, packaging and paper, textiles, food, cosmetics, and personal care products [4,10].
Hemp is a short-season crop that uses less water, unlike other fiber plants like cotton (hemp requires 2.5 times less water than cotton per unit cultivation area) [4].It adapts well to different climatic types and soil conditions [9,10].Its cultivation benefits include the absorption of heavy metals and carbon-sequestering properties [11,12].It has also been known to have nutritional benefits [4] and can be cultivated for its seed, fiber, cannabinoid, or flowers [12].Its myriad consumer and industrial applications, availability, and short life cycle give hemp a good market value; its products can be economically competitive with alternative materials [6,12].It has a greater per-hectare yield and lower agricultural costs (70% reduction) as an alternative to cotton in the textile industry [6].It has advantageous material and processing properties, making hemp paper seven times more recyclable than wood pulp paper, and does not require bleaching, reducing relative environmental impacts [7,12].
The industrial hemp market has seen rapid growth as the material has been evaluated for a variety of industry applications [4].A range of applications for hemp has emerged, with over 25,000 hemp or hemp-based products and uses reported in the literature [13][14][15][16][17][18].Hemp has been applied in filaments for 3D printing, as a harmonic steel cable net replacement, as fiber reinforcement for automotive composite parts, and for the production of carbon nanosheets as a replacement for graphene in supercapacitors [9].However, the utilization of hemp has not been fully developed globally due to the knowledge gap in its cultivation and processing and insufficient data across its life cycle.In addition, there has been a shortage of hemp production [12], a lack of process technology development [10], market competition with alternative crops, and regulatory risks [19].This underutilization presents opportunities for innovation and industrialization [6] to create a sustainable and resilient agriculture system for hemp [10].
Hemp was excluded during much of the Green Revolution, or the Third Agricultural Revolution, in the U.S. due to the restrictions imposed to control the illegal use and cultivation of cannabis [12].This exclusion created a significant research and technology gap for hemp relative to other industrial crops [6].The passage of the 2014 and 2018 U.S. Farm Bills to legalize industrial hemp led to continuously increasing interest in its cultivation and processing, reinvigorating research and development that had been paused since the early 1970s [12].The U.S. Farm Bills describe hemp as "Cannabis sativa L. and any part of that plant, whether growing or not, with a delta-9 tetrahydrocannabinol (THC) concentration of not more than 0.3 percent on a dry weight basis" [20].The cultivation of hemp gradually picked up after 2014 in Kentucky, Colorado, Vermont, and Indiana, with 22 other States joining in hemp cultivation in 2018 [12].Several programs have been initiated to advance the hemp industry and address the variability in data, quality, cost, and agricultural practices to bridge the research and technology gap, enabling informed decisions about hemp production, processing, manufacturing, marketing, and policy [19].These have included investments by the U.S. Department of Agriculture (USDA), for instance, through recent funding of a project to establish a sustainable hemp economy in the rural U.S. West and Tribal lands to the Global Hemp Innovation Center (GHIC) at Oregon State University [21].
These recent policy changes correlate with a growing interest in hemp research over time.A search in Web of Science and ScienceDirect databases for "hemp" over the last two decades is presented in Figure 1, showing only limited research interest between 2000 and 2007.A slow upward trend from 2008 to 2013 can be inferred to result from global interest in hemp, especially in the U.S., as changes to hemp regulations were discussed, and specific U.S. states cultivated it with a license for research purposes.A rapid increase in research publications was seen annually from 2014 to 2017 after the passage of the 2014 U.S. Farm Bill to legalize the cultivation of hemp.There has been an even steeper rise since the passage of the 2018 U.S. Farm Bill, which expanded provisions allowing for the cultivation of hemp for commercial use [19,22].
AgriEngineering 2024, 6, FOR PEER REVIEW 3 through recent funding of a project to establish a sustainable hemp economy in the rural U.S. West and Tribal lands to the Global Hemp Innovation Center (GHIC) at Oregon State University [21].These recent policy changes correlate with a growing interest in hemp research over time.A search in Web of Science and ScienceDirect databases for "hemp" over the last two decades is presented in Figure 1, showing only limited research interest between 2000 and 2007.A slow upward trend from 2008 to 2013 can be inferred to result from global interest in hemp, especially in the U.S., as changes to hemp regulations were discussed, and specific U.S. states cultivated it with a license for research purposes.A rapid increase in research publications was seen annually from 2014 to 2017 after the passage of the 2014 U.S. Farm Bill to legalize the cultivation of hemp.There has been an even steeper rise since the passage of the 2018 U.S. Farm Bill, which expanded provisions allowing for the cultivation of hemp for commercial use [19,22].The hemp industry is currently seeking standards across its value chain.Although there has been a growth in hemp research, there is a relative paucity of review studies.The research literature has focused alternately on hemp cultivation, supply chains, material processing, and market economics.Variation across these efforts in terms of approaches, data, and findings has increased the challenges facing the emerging hemp industry in establishing standards and supporting environmental, economic, and social impact assessments to better inform investors, policymakers, and other stakeholders.No review of the literature has been reported to systematically examine the emerging hemp industry in its totality, which would aid in refocusing production and research activities to meet globally growing demands on the biobased industry.Therefore, a systematic review of the literature on hemp product manufacturing is undertaken with the aim of identifying relevant technologies and processes by analyzing review studies spanning more than twenty years of hemp research from 2000 to 2023.A meta-analysis of the selected articles is provided to understand the current and future trends of hemp products and their production, as highlighted by previous review studies considering cultivation practices, production yield, supply chain management, product sustainability performance, and industry standards and governmental policies.
This paper is organized as follows: Section 2 highlights the systematic literature review methodology and publication statistics of the final selected literature for review.Section 3 presents post-harvest activities, manufacturing processes, and applications of the hemp materials.Section 4 and Section 5 discuss emerging hemp applications and sustainability performance, respectively.Finally, future research opportunities and conclusions are presented in Section 6 and Section 7, respectively.The hemp industry is currently seeking standards across its value chain.Although there has been a growth in hemp research, there is a relative paucity of review studies.The research literature has focused alternately on hemp cultivation, supply chains, material processing, and market economics.Variation across these efforts in terms of approaches, data, and findings has increased the challenges facing the emerging hemp industry in establishing standards and supporting environmental, economic, and social impact assessments to better inform investors, policymakers, and other stakeholders.No review of the literature has been reported to systematically examine the emerging hemp industry in its totality, which would aid in refocusing production and research activities to meet globally growing demands on the biobased industry.Therefore, a systematic review of the literature on hemp product manufacturing is undertaken with the aim of identifying relevant technologies and processes by analyzing review studies spanning more than twenty years of hemp research from 2000 to 2023.A meta-analysis of the selected articles is provided to understand the current and future trends of hemp products and their production, as highlighted by previous review studies considering cultivation practices, production yield, supply chain management, product sustainability performance, and industry standards and governmental policies.
This paper is organized as follows: Section 2 highlights the systematic literature review methodology and publication statistics of the final selected literature for review.Section 3 presents post-harvest activities, manufacturing processes, and applications of the hemp materials.Sections 4 and 5 discuss emerging hemp applications and sustainability performance, respectively.Finally, future research opportunities and conclusions are presented in Sections 6 and 7, respectively.

Methodology
This section presents the systematic approach used to conduct the literature review.The research questions and Boolean search terms used across five major databases are also presented.A meta-analysis was performed on the final selected articles to evaluate their publication statistics, which helped identify the various applications of hemp over the selected timeframe.

Systematic Literature Review
A systematic literature review of review papers in hemp product manufacturing was performed to identify qualitative and quantitative information on the existing technologies and manufacturing processes reported for hemp products.The systematic review was guided by developing a research protocol [23], which includes (1) specifying the review questions, (2) identifying the databases to be queried, (3) applying inclusion and exclusion criteria, (4) evaluating selected papers for quality, and (5) presenting results using the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) methodology, as shown in Figure 2 [24].A checklist according to the updated PRISMA 2020 is provided as Supplementary Material [25].The systematic literature review aims to answer the following five research questions (R1-R5):

Methodology
This section presents the systematic approach used to conduct the literature review.The research questions and Boolean search terms used across five major databases are also presented.A meta-analysis was performed on the final selected articles to evaluate their publication statistics, which helped identify the various applications of hemp over the selected timeframe.

Systematic Literature Review
A systematic literature review of review papers in hemp product manufacturing was performed to identify qualitative and quantitative information on the existing technologies and manufacturing processes reported for hemp products.The systematic review was guided by developing a research protocol [23], which includes (1) specifying the review questions, (2) identifying the databases to be queried, (3) applying inclusion and exclusion criteria, (4) evaluating selected papers for quality, and (5) presenting results using the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) methodology, as shown in Figure 2 [24].A checklist according to the updated PRISMA 2020 is provided as Supplementary Material [25].The systematic literature review aims to answer the following five research questions (R1-R5):  A comprehensive literature search was undertaken for all published articles between January 2000 and April 2023 (sourcing of literature from databases commenced in May 2023, followed by the subsequent comprehensive review).Articles were limited prior to 2000 due to hemp cultivation and use restrictions.Google Scholar, ScienceDirect, Engineering Village, Web of Science, and ASME Digital Collection were targeted as major literature databases for manufacturing and production-focused research.
Table 1 presents the Boolean search terms used to search the databases; the "+" in Google Scholar searches filters out expansions of the root words (e.g., "product" would also return "production," whereas "+product" limits the search to only to those publications where the word "product" appears).The option to search only review papers was selected in all databases except Engineering Village, which did not have this filter.Restricting the search to review papers returned a feasible number of papers for evaluation while considering a wider range of publications on the subject matter within the identified review papers.Google Scholar returned 4294 articles, ScienceDirect returned 935, Engineering Village returned 46, Web of Science returned 16, and the ASME Digital Collection returned 4. Duplicated search results within each database and across all databases were removed, as shown in Figure 2. Python codes were used to extract relevant articles from the downloaded databases for articles with hemp in their titles (Table 2).Also, the downloaded databases were screened using Python codes for the word "hemp" included in the abstracts, as shown in the table.Abstracts of downloaded articles from Google Scholar were not screened, as only a snippet of the abstract was available.For Google Scholar, all filtered articles with "hemp" in the title were included for further screening.For ScienceDirect, Engineering Village, and Web of Science, it was observed that all articles with "hemp" in their titles also had "hemp" in their abstract; therefore, all articles with "hemp" in the abstract were considered for further screening.Articles from the ASME Digital Collection were excluded since "hemp" did not appear in any of their titles or abstracts.For this review study, additional exclusion criteria included unpublished work, informal literature surveys and articles, articles not subjected to peer review, articles not published in English, and the nonavailability of full-text or abstract.In the case of multiple versions of the same publication, only the most recently completed version was considered for inclusion.As a result, there were 109 articles included in the systematic review.An independent evaluator examined the excluded and included articles to ensure the process for article selection was as objective as possible.Next, publication statistics of the final selected articles were evaluated, as presented in the next section.

Review Publication Statistics
The article title, author names, year of publication, journal title, country(ies) of the author(s), and classification from the final selected articles were retrieved for publication statistics evaluation [26].Articles were classified according to the product in focus for the study.The authors' countries were retrieved without double counting (where more than one author is from the same country), giving an overview of the regions where research interest in hemp is ongoing.It can be observed from Figure 3 that the interest in hemp research is high in Asia, Europe, and North America, with lesser interest in Africa, Oceania, and South America.Historically, hemp research and development was heavily focused in Europe [27].For the past two decades, however, it can be seen that over 40% of authors have published research from Asia, compared to about 30% from Europe and about 20% from the Americas.
process for article selection was as objective as possible.Next, publication statistics of the final selected articles were evaluated, as presented in the next section.

Review Publication Statistics
The article title, author names, year of publication, journal title, country(ies) of the author(s), and classification from the final selected articles were retrieved for publication statistics evaluation [26].Articles were classified according to the product in focus for the study.The authors' countries were retrieved without double counting (where more than one author is from the same country), giving an overview of the regions where research interest in hemp is ongoing.It can be observed from Figure 3 that the interest in hemp research is high in Asia, Europe, and North America, with lesser interest in Africa, Oceania, and South America.Historically, hemp research and development was heavily focused in Europe [27].For the past two decades, however, it can be seen that over 40% of authors have published research from Asia, compared to about 30% from Europe and about 20% from the Americas.The final selected articles were also evaluated to identify the conference proceedings and journals that had published the most hemp-related studies (Figure A1).It is observed that authors preferred venues related to fiber, food, energy, and sustainability, which shows the potential of the hemp plant and products in various key research areas.Also, there are no conference proceedings or journals generally preferred for publishing hemprelated research; the highest number of published works (three papers) could be found in eight of the 83 publication venues identified.
Other key insights can be observed from recent trends in the number of publications for various aspects of hemp research.The various materials and products explored are reported in Figure 4, where articles considering the general use of the hemp plant feedstock material are classified as "hemp plant."It is observed that the application of hemp in biocomposites consistently received research interest, with the most publications (11) in 2022; however, no articles were captured for its application in early 2023 when the The final selected articles were also evaluated to identify the conference proceedings and journals that had published the most hemp-related studies (Figure A1).It is observed that authors preferred venues related to fiber, food, energy, and sustainability, which shows the potential of the hemp plant and products in various key research areas.Also, there are no conference proceedings or journals generally preferred for publishing hemp-related research; the highest number of published works (three papers) could be found in eight of the 83 publication venues identified.
Other key insights can be observed from recent trends in the number of publications for various aspects of hemp research.The various materials and products explored are reported in Figure 4, where articles considering the general use of the hemp plant feedstock material are classified as "hemp plant."It is observed that the application of hemp in biocomposites consistently received research interest, with the most publications (11) in 2022; however, no articles were captured for its application in early 2023 when the database search was conducted.Hemp food products have also received interest.In 2023, hemp food research generated the most articles (six), matching the same number appearing in 2022.Hemp applications for buildings and supercapacitors witnessed the next highest interest from researchers.As shown in Figure 5, hemp applications have gained recent interest in biocomposites, food, building materials, textiles, and energy research.
database search was conducted.Hemp food products have also received interest.In 2023, hemp food research generated the most articles (six), matching the same number appearing in 2022.Hemp applications for buildings and supercapacitors witnessed the next highest interest from researchers.As shown in Figure 5, hemp applications have gained recent interest in biocomposites, food, building materials, textiles, and energy research.database search was conducted.Hemp food products have also received interest.In 2023, hemp food research generated the most articles (six), matching the same number appearing in 2022.Hemp applications for buildings and supercapacitors witnessed the next highest interest from researchers.As shown in Figure 5, hemp applications have gained recent interest in biocomposites, food, building materials, textiles, and energy research.

Overview of Hemp Cultivation, Processing, and Applications
Hemp cultivation involves two main components: field operations and plant growth.Field operations include soil preparation, fertilization, testing, sowing, mechanization, and harvesting, while plant growth covers germination, development, and maturity.Effective field preparation requires high organic matter (3.5%) and adequate fertilization to produce high-quality fiber [6].Hemp thrives in deep, well-drained clay or silt-rich soils with a neutral pH and struggles with high salinity, acidity, and compaction [12].Hemp can germinate at low temperatures (1-2 • C) but thrives above 10 • C. Optimal germination and development depend on appropriate seeding dates, soil temperature, moisture, and photoperiod.Plant spacing varies by hemp type (i.e., seed, fiber, and cannabinoids).Hemp seeds are typically sown at 2-2.5 cm depth and 2-3 cm spacing [6].For fiber, 1.9-3.2cm depth and 7.6-17.8cm row spacing to promote tall and slender plants.Maturity and harvesting are discussed in respective sections for fiber, seed, and flower.
The term hemp is most commonly used in connection with the Cannabis sativa plant [28].The term is also used for other fiber-bearing plants not related to Cannabis sativa, such as Manila hemp (Musa textilis) or Sisal hemp (Agave rigida).However, hemp is generally accepted in reference to the Cannabis sativa plant and derivative products.The hemp plant has mainly been cultivated for its fiber extracted from the stem (Figure 6), which contains fibrous tissue (fiber) and a woody core (hurd or shive) [29,30].Other parts of the plant are the seeds, flowers, and leaves.Hemp fiber comprises cellulose, hemicellulose, and lignin, all in varying percentages depending on the cultivar.In agricultural practice, hemp can be rotated with potatoes, flax, sugar beet, and oats [31], in addition to cereals or, preferably, a legume, as an oilseed crop, or after any other crop when grown for fiber [12].Hemp cultivation has become more appealing to farmers than flax due to its lower propensity for crop failure [11].Over 500 constituents have been identified in hemp-some of the secondary metabolites are cannabinoids, flavonoids, stilbenoids, terpenoids, lignans, and alkaloids, and rapidly growing interest has been seen for the valorization of these metabolites [33].The primary product categories with market potential are fiber, oilseed, and pharmaceuticals [36].Applications for hemp can be grouped into traditional, industrial, and emerging (novel) applications [13].Hemp and its derivatives have been used for traditional applications, such as baskets, ropes, animal bedding, biomass, clothing, and paper pulp [37,38], and in industrial applications, such as building materials, foods, biofuel, oil, cosmetics, skincare products, pharmaceuticals, automotive materials, packaging materials, and sporting goods [13,35,39].Some emerging applications are its use in supercapacitors, semiconductors, biomaterials, composites, and cellulose nanofibril bioproducts [30].
To organize the past research, publications were classified according to the key foci discussed in each paper, resulting in three primary categories: material processing and product applications of hemp fiber and hurd (Section 4), material processing and product applications of hemp seeds and flowers (Section 5), and sustainability performance evaluation of hemp materials and products (Section 6).The information presented is exclusively gathered from the 109 articles selected for final review.Section 4 contains a metaanalysis of the articles responding to research questions R1-R3, while Sections 5 and 6 focus on questions R4 and R5, respectively.Hemp feedstock applications can have significant ecological, industrial, and societal value, making it an essential crop in achieving a circular economy [18].Hemp can also be a suitable crop for bioenergy, as it requires low inputs to produce high biomass yields [6].The high volume of biomass production from hemp has been attributed to its ability to grow under various conditions, making it resilient and an emerging crop for utilization in the Fourth Industrial Revolution era to achieve more sustainable production-consumption systems [32].Hemp yield depends on the variety and harvesting period.For instance, it has been reported that oil yield decreases with later harvesting dates, while flower production is increased when pollination is prevented [33].Hemp has been used for soil phytoremediation or rehabilitation of mining excavations, and purifying soil and air while being resistant to various forms of environmental pollution [7,34].It has been shown to reduce phytopathogenic nematodes, fungi, bacteria, protozoans, mites, and insects [34] and suppress competing weeds [12].Hemp plants can absorb heavy metals such as cadmium (Cd), lead (Pb), and zinc (Zn) from contaminated soil, storing them in the plant with higher concentrations in the root than the leaves and shoots [7,35].
Over 500 constituents have been identified in hemp-some of the secondary metabolites are cannabinoids, flavonoids, stilbenoids, terpenoids, lignans, and alkaloids, and rapidly growing interest has been seen for the valorization of these metabolites [33].The primary product categories with market potential are fiber, oilseed, and pharmaceuticals [36].Applications for hemp can be grouped into traditional, industrial, and emerging (novel) applications [13].Hemp and its derivatives have been used for traditional applications, such as baskets, ropes, animal bedding, biomass, clothing, and paper pulp [37,38], and in industrial applications, such as building materials, foods, biofuel, oil, cosmetics, skincare products, pharmaceuticals, automotive materials, packaging materials, and sporting goods [13,35,39].Some emerging applications are its use in supercapacitors, semiconductors, biomaterials, composites, and cellulose nanofibril bioproducts [30].
To organize the past research, publications were classified according to the key foci discussed in each paper, resulting in three primary categories: material processing and product applications of hemp fiber and hurd (Section 4), material processing and product applications of hemp seeds and flowers (Section 5), and sustainability performance evaluation of hemp materials and products (Section 6).The information presented is exclusively gathered from the 109 articles selected for final review.Section 4 contains a meta-analysis of the articles responding to research questions R1-R3, while Sections 5 and 6 focus on questions R4 and R5, respectively.

Hemp Fiber and Hurd
Fiber can be extracted from hemp stalks through decortication, often after retting to loosen the fibers from the hurd [40].Researchers have demonstrated that hemp stalks can be separated into hurd, which makes up 65-75% of the stalk, fibers (20-35%), and dust (5%) [14,41].This section highlights the cultivation and harvesting of hemp plants, as well as the extraction and pretreatment processes for the bast fiber and woody core (hurd).It also discusses various applications for hemp fiber and hurd in the textile, pulp and paper, construction, biocomposites, and packaging industries, as well as emerging applications.

Hemp Stalk Harvesting, Pre-Treatment, and Fiber Extraction
In this subsection, various pretreatment techniques after harvesting are discussed as retrieved from the reviewed articles.Hemp fiber and hurd extraction processes depend on end use and available technology to separate the fiber from hurd.

Hemp Stalk Harvesting and Pre-Treatment
During or after the harvest, the stalk can be separated from the other parts of the plant (e.g., the flower and seed), as shown in Figure 7. Depending on the harvesting equipment used, hemp seed and fiber may be harvested separately [42].Typical haymaking machinery is commonly used for harvesting hemp [12].However, as the demand for hemp has increased, dual-purpose machines have been designed to assist in efficiently harvesting hemp [42].Research is also ongoing to design machines for co-harvesting high-quality seeds and stalks [41].To increase hemp fiber content, the Bredemann method for optimizing the crop fiber content has been successfully applied [30].Further, the timing of the harvest of hemp plants significantly impacts fiber quality [12].
Retting, or breaking down of the pectin binding the fibers to the hurd, reduces the effort required for subsequent fiber separation [42].The quality and quantity of fibers yielded strongly depend on the degree of retting [28,43].In practice, hemp stalks can be retted to straws by field (or dew) retting, water retting, enzyme retting, or chemical retting [28].The method will primarily be determined by location and the requirements of the target end use.A comparison of these retting methods is presented in Table 3 and discussed further below.ment used, hemp seed and fiber may be harvested separately [42].Typical hay-making machinery is commonly used for harvesting hemp [12].However, as the demand for hemp has increased, dual-purpose machines have been designed to assist in efficiently harvesting hemp [42].Research is also ongoing to design machines for co-harvesting high-quality seeds and stalks [41].To increase hemp fiber content, the Bredemann method for optimizing the crop fiber content has been successfully applied [30].Further, the timing of the harvest of hemp plants significantly impacts fiber quality [12].Retting, or breaking down of the pectin binding the fibers to the hurd, reduces the effort required for subsequent fiber separation [42].The quality and quantity of fibers yielded strongly depend on the degree of retting [28,43].In practice, hemp stalks can be retted to straws by field (or dew) retting, water retting, enzyme retting, or chemical retting [28].The method will primarily be determined by location and the requirements of the target end use.A comparison of these retting methods is presented in Table 3 and discussed further below.
Field (or Dew) Retting: In field retting, or dew retting, hemp stalks are laid on the field to allow fungi and bacteria to naturally degrade and loosen the adhesion of the pectin binding the fiber to the hurd [28].This retting process depends on the location, the degree of retting required, and subsequent use of land space.Typically, harvested stems are left on the field for 2-8 weeks.They are routinely turned to allow even exposure to light, moisture (rain and dew), and temperature conditions.The stems are assumed to be sufficiently retted when their color is an even dark grey throughout.Manual handling can also be used to examine the ease and cleanliness with which the woody core and fibers separate.The major advantages of field retting are its simplicity and low cost.The field retting of hemp is the most commercially viable method for a fiber hemp crop.The significant disadvantages of the field retting of hemp arise from a comparative lack of control over the retting process, land occupancy, and variable fineness, such as length, strength, and color within a sample, compared to water retting and chemical retting.Greenhouse retting, frost retting, and other forms of field retting are commonly practiced at an experimental scale and in Nordic countries [44].
Water Retting: Water retting gives the best separation and quality in the shortest time [28].Depending on the climate, most water retting is carried out in sealed or open tanks and may be arranged in a series or cascade layout.Using cold water in a tank-based Field (or Dew) Retting: In field retting, or dew retting, hemp stalks are laid on the field to allow fungi and bacteria to naturally degrade and loosen the adhesion of the pectin binding the fiber to the hurd [28].This retting process depends on the location, the degree of retting required, and subsequent use of land space.Typically, harvested stems are left on the field for 2-8 weeks.They are routinely turned to allow even exposure to light, moisture (rain and dew), and temperature conditions.The stems are assumed to be sufficiently retted when their color is an even dark grey throughout.Manual handling can also be used to examine the ease and cleanliness with which the woody core and fibers separate.The major advantages of field retting are its simplicity and low cost.The field retting of hemp is the most commercially viable method for a fiber hemp crop.The significant disadvantages of the field retting of hemp arise from a comparative lack of control over the retting process, land occupancy, and variable fineness, such as length, strength, and color within a sample, compared to water retting and chemical retting.Greenhouse retting, frost retting, and other forms of field retting are commonly practiced at an experimental scale and in Nordic countries [44].
Water Retting: Water retting gives the best separation and quality in the shortest time [28].Depending on the climate, most water retting is carried out in sealed or open tanks and may be arranged in a series or cascade layout.Using cold water in a tank-based system, retting will take only 7-14 days to complete; retting time can be further reduced to 4-5 days using warm water (30-40 °C).It is rare to include chemical additives to enhance the process as they additionally increase the environmental impacts of the process.The primary benefit of this method is that it can be controlled and evenly applied to hemp stems, producing a superior fiber quality compared to field retting.In addition, this method is not dependent on weather conditions; however, downsides include high volumes of water consumption and resulting waste-product effluent.Typically, 20 m 3 of water is used per ton of straw, and another 10 m 3 /ton is used for rinsing.Drying is also required for water-retted straw, incurring significant energy costs.Water consumption can be reduced by recycling water indefinitely by introducing effluent aerobic bacilli into the water and keeping the tank aerated.
Chemical Retting: Chemical retting is commonly performed using an aqueous sodium hydroxide (NaOH) solution, sometimes with a chelating agent, such as ethylenediaminetetraacetic (EDTA) acid, to improve retting efficiency [28].Other chemicals, like sodium carbonate (Na 2 CO 3 ) or sodium sulfite (Na 2 SO 3 ), can also be used in retting solutions.The main factors determining the effectiveness of chemical retting solutions include chemical concentration, solution acidity (pH value), stem-to-liquor ratio, temperature, duration of treatment, and degree of agitation.This method possesses the advantages of water retting, but the process is accelerated to just a few hours.Although the chemical retting of hemp has generally resulted in very good fiber quality and yield, it is usually too costly for many end uses and not eco-friendly due to the chemicals used [28,37].Enzyme Retting: Enzyme retting has only shown success on a laboratory scale and produces fiber quality equivalent to water retting without its drawbacks (water consumption, effluent waste, and malodor) by using bacterial enzymes to facilitate fermentation [28].The use of pectinase from Fusarium oxysporum (fungi) has also been demonstrated for enzymatic retting [45].This method can be practiced in all seasons and avoids the concerns associated with other retting methods, though considered very costly due to the enzymes and equipment needed [28,29].The enzymatic process is more eco-friendly and reduces fiber damage [45].

Hemp Fiber Extraction
After retting, the stalk is dried to a suitable moisture content in preparation for fiber extraction [28].The traditional processing of hemp straw involves breaking it with rollers, scutching it to separate the long fiber (bast) from the short fiber (tow) and hurds, and hackling of the bast fiber through a series of combs to further remove tow and hurd [29].Alternatively, the straw can undergo mechanical processing, known as decortication.The hemp straw is subjected to repeated mechanical forces, separating the bast fiber from the hurd [29,42].There are two main types of decortication methods: hammer milling and roller milling.Recent work has reported that hemp stalks can be decorticated without retting; however, they would need to undergo a degumming process [44].These two main processing methods are described in detail below, with descriptions of common supporting processes provided in Table 4.
Hammer milling uses rotating drums with plates or hammers to beat the fibrous part into smaller particle sizes able to pass through the mesh screens; fibers can also be pneumatically collected in this method.Hammer milling provides a high hourly throughput of about 5-15 tons of straw input with a limitation of fiber length control (output fiber length ranges between small-to medium-sized fibers of 1-10 cm) [29].Throughputs of 3-4 tons/hour have been reported for unretted hemp stalks [28].Fiber damage can result from mechanical impacts on the fiber, leading to inconsistency in fiber properties, which limits its use in textile and non-woven products.Dislocation, node, or slip plane damage types have been defined [46].Hammer milling is always preferred when high yield is required, but low fiber quality must be acceptable [42], e.g., for paper making [28].
Roller milling has improved long-fiber length control (between 30-60 cm) and maintains better fiber quality but exhibits a lower throughput of about 1-3 tons/hour of straw input [28,29].Roller milling uses long corrugated cylindrical rollers that gradually open the straw and collect fiber through slotted screens using continuous agitation.This process has a lower risk of damaging the fiber than hammer rolling; it may not be capable of processing unretted stalks, especially those with moisture content exceeding 15% [28].

Breaking
Fluted and smooth rollers, arranged either horizontally or vertically, crush and break the straw open along its length, forming long, thin strands [28].The output quality of bast fiber from the breaking stage depends on the degree of retting.

Scutching
Bast fiber passes through rotating turbine blades to open the fiber further [28].Most of the hurd is removed by scutching except those closely adhering to the bast fiber.Hurd, dust, and scutched tow are co-products and can be used for textiles, yarns, or ropes.

Fiber opening
Dust and hurd are removed from the tow or bast fiber by feed rollers covered in pins of different pitches, producing finer fiber strands [28].Drawing, slivering, doubling, and winding are other processes used based on the quality and intended use of the fiber.

Carding
An alternative to fiber opening, though more expensive, where the bast fiber passes through single or multiple opening cycles, carding efficiently extracts fiber from the tow [28].Carding is used for thicker yarns like upholstery or woven, nonwoven, and technical products [44].Optimization has improved anisotropy and hemp fiber content [13].

Degumming
Further processing of bast fiber into workable, fine, and soft (cottonized) forms, in which the viscous non-cellulosic (lignin) content is significantly reduced from 8-10% to as little as 0.2%, suitable for weaving, spinning, and blending [45,47].

Fiber cleaning
Bast fiber is cleaned using various combinations and sequences of cleaners (comb shaker table, step cleaner, or tambour turbine), depending on the degree of cleaning required [28].Cleaning is similar to hackling; however, the literature has ascribed hackling to manual or traditional fiber extraction.Tow at this stage is referred to as hackled tow.

Hemp fiber spinning
Fibers are spun into different yarn types depending on the uniformity of fineness, quality, length, and cleanliness [28].Generally, longer fibers are spun into finer yarns and tow into coarse yarns.Hemp spinning can be wet or dry; wet spinning produces finer yarns [48].

Blending
Hemp fiber is combined with cotton or artificial fibers in fabrics to reduce the environmental impacts of textile production [11,28,45], appealing to environmentally conscious consumers.
Figure 8 presents a high-level system model of the extraction steps for traditional and mechanical processing of fiber and its co-products according to the reviewed literature, without the optional steps of fiber opening, carding, drawing, slivering, doubling, and winding.Also, further processes or steps like spinning, weaving, and blending were not included.

Applications of Hemp Fiber and Hurd
There is a growing demand for hemp-based products as alternatives to synthetic fibers and products [39] due to environmental laws resulting from the end-of-life treatment of synthetic materials (e.g., composites) and the development and improvement of manufacturing techniques for bio-fibers [49].Technical applications have gained interest in industrial design, construction, transportation (e.g., automobile, aerospace, and railway), and packaging industries [16,17,41,[50][51][52].Hemp fibers and hurd can also be used for animal bedding, energy, and low-quality papers [14].Hemp-derived composites can be designed into various forms like foams, fabrics, and film membranes, leading to even more applications, such as replacements for synthetic materials, including glass fiber [18,53].Some key drivers for hemp-based composites are the robust acoustic performance of nonwoven fabrics and sound absorption of fiber-reinforced composite products [12,52], as well as their high impact and electrical resistance [54].It has been reported that a 10-30% density reduction can be achieved by replacing metallic parts with natural fibers like hemp [49].
Compared to flax, hemp has thicker, stiffer, longer, and coarser fibers, leading to favorable mechanical properties [13].Depending on the hemp material and manufacturing process, its antimicrobial, pesticide, and insect-repellent attributes make it less vulnerable to biological damage, resulting in a longer lifespan [12].Generally, benefits derived from exploring hemp in the composites industry relate to cost, performance, quality, supply chain resilience, and environmental regulations [49].Mathematical models for the thermal effects have been developed to assist in optimizing process conditions, manufacturing costs, and time for hemp fibers [55].
AgriEngineering 2024, 6, FOR PEER REVIEW 13 Figure 8 presents a high-level system model of the extraction steps for traditional and mechanical processing of fiber and its co-products according to the reviewed literature, without the optional steps of fiber opening, carding, drawing, slivering, doubling, and winding.Also, further processes or steps like spinning, weaving, and blending were not included.

Applications of Hemp Fiber and Hurd
There is a growing demand for hemp-based products as alternatives to synthetic fibers and products [39] due to environmental laws resulting from the end-of-life treatment of synthetic materials (e.g., composites) and the development and improvement of manufacturing techniques for bio-fibers [49].Technical applications have gained interest in industrial design, construction, transportation (e.g., automobile, aerospace, and railway), and packaging industries [16,17,41,[50][51][52].Hemp fibers and hurd can also be used for animal bedding, energy, and low-quality papers [14].Hemp-derived composites can be designed into various forms like foams, fabrics, and film membranes, leading to even more applications, such as replacements for synthetic materials, including glass fiber [18,53].Some key drivers for hemp-based composites are the robust acoustic performance of nonwoven fabrics and sound absorption of fiber-reinforced composite products [12,52], as well as their high impact and electrical resistance [54].It has been reported that a 10-30% density reduction can be achieved by replacing metallic parts with natural fibers like

Application of Hemp Fiber in the Textile Industry
Primary characteristics of hemp fiber include its flexibility, comfort, breathability, durability [56], adaptability, sturdiness, water resistance [12], resistance to molds and UV light [16], and comparatively higher strength than other natural fibers, such as nettle, cotton, and linen [12].These characteristics make it an excellent raw material for the textile industry.However, the industrial use of hemp fiber for textiles requires higher purity with no hurd present [46].It is spun to create various hemp fiber products, followed by weaving or knitting to make fabrics, cordage, yarns, and carpets [12].The fabrics can be processed into clothing and accessories, bags, pillowcases, blankets, shoes, upholstery, wall decor, and ornamental items.Hemp has been demonstrated as an excellent furnishing fabric, especially for drapes [47].In jeans and sportswear, it has been used in 100% hemp fabric or blended with cotton, wool, flax, or synthetics.In response to the COVID-19 pandemic, face masks made from hemp fabric were reported to offer multiple layers of protection, showing the potential of its utilization in healthcare, biotechnology, and safety [57].

Application of Hemp Fiber in Pulp and Paper Industry
When cultivating hemp for pulp and paper, harvesting time has a limited influence on biomass characteristics, and late harvest only maximizes biomass production [46].In addition to harvesting time, one study indicated that pulp quality and yield can be affected by location, breeding, and biomass variation [11].Paper production using 100% hemp fiber or hurd is rare; it is often blended with wood pulp or other substances to enhance paper properties.Recent studies have demonstrated that incorporating the pulp of hemp woody core into hardwood yields enhances tensile index, bursting strength, and softness without sacrificing water absorbency, making it an optimal choice for hand tissues.Furthermore, hemp hurd or fiber can be combined with pine and eucalyptus fiber, resulting in longer and stronger fibers in the pulp and leading to more robust paper.Utilizing hemp waste for pulp and paper is a potential path for achieving a circular economy within the hemp industry [58].
Hemp paper has unique characteristics that differentiate it from other paper types: it has a long shelf life (it can survive hundreds of years [11]), and though it yellows with time, hydrogen peroxide can be used for whitening rather than bleach, mitigating the release of toxins such as dioxin [12,16].The chemical processing used in manufacturing hemp paper is less hazardous when compared with regular wood pulp paper [16].A significant advantage of hemp-based paper is its recyclability; it can be reused 7-8 times, whereas pulpwood paper can only be recycled up to three times [12,16].Hemp paper is known for its high tear strength and wet strength, which makes it an ideal choice for various applications such as currency paper, art paper, bible sheets, tea bags, office paper, specialty nonwovens, and grease-proof paper [11,12].
A novel process identified in the literature for pulp and paper production from hemp fiber is the organosolv pulping process [11].This technique uses organic solvents to dissolve the lignin and hemicellulose in hemp fibers, resulting in high-strength paper.Adding 2% nanocellulose hemp increases a paper's breaking force [48].This process allows for the recovery of high-quality lignin, eliminates the use of harmful sulfur, saves water, and avoids the need for chlorine bleach in the production of hemp paper.The organosolv pulping process of whole hemp stalks produces paper as strong as commercial hardwood and softwood pulps.The BioRegional MiniMill technology has been reported to be a process with relatively low environmental impacts for small-scale pulp and paper production, with the potential for zero-emission output.

Application of Hemp Fiber and Hurd in the Construction Industry
Hemp hurd can be used to create a variety of sustainable building products for the construction industry.In particular, hemp-based building materials have been shown to improve indoor environments due to hemp's breathability [18,59].These materials also have been reported to perform well in regulating heat, moisture, and humidity [60,61] and can be resilient to natural disasters like earthquakes and flooding [12].With careful control of production and use cycles, hemp-based materials are also amenable to circularity, minimizing manufacturing process wastes and maximizing the recoverability of end-of-life product materials [34].Hemp concrete, commonly called hempcrete, combines hemp hurd, binder, and water and has been gaining popularity in building construction.It has been used for non-load bearing walls (blocks, panels, bricks, or formwork for hempcrete) [17,58], flooring screed, roof and window insulation, pavement, mortar, plaster, and fillers, mainly due to its lightweight, thermal insulating, acoustic, and moisture buffering properties [7,18,41].The properties and range of hempcrete products depend on binder type, aggregate-to-binder ratio, size and porosity of the aggregates, and level of compaction [11,12].Spraying, molding, or manually mixing and tamping the mixture are common techniques for pre-manufactured or onsite production [11,13].Most hempcrete is made with lime as the binder [11] due to its low emissions and abundant availability [60].The hempcrete manufacturing setup usually consists of a lime hopper, hemp hoppers, and a water pump.Dry lime and hemp hurd are fed into a chamber using pressurized air, and the mixture is then combined with water and projected into formwork or block molds through a nozzle.Process modifications and operator skills can significantly affect properties like fast dry ability and visual appearance.The use of planetary or helical mixers has been reported for off-site hempcrete manufacturing [38].With lime binders, curing can take between 28 and 45 days, which may be an impractically long time for typical construction projects [60].The reported density for hempcrete products is 250-350 kg/m 3 for wall insulation, 200-250 kg/m 3 for roof insulation, and 375-500 kg/m 3 for floor slab, which is determined by the mix ratios of hemp, binder, and water.
Both synthetic and natural binders can be classified as hydraulic (require water to harden) and non-hydraulic (harden with air) [14].Natural binders include bio-based binders, such as lignin-based, starch-based, plant-protein-based, and paper pulp-based binders [62]; plant-based binders, such as cornstarch (used for panel board) [51,61]; and organic binders, such as sapropel clay which provides good sound absorption properties [13].Synthetic binders include cement and aluminum sulfate binders, which provide higher compressive strength [11].Other synthetic binders have been used for hempcrete, such as magnesium-based binders, cenosphere binders (an alternative to lime binders) [11], and pozzolanic binders, such as pulverized fuel ash, silica fumes, and ground granulated blast furnace slag [61].Additives have also been reported to improve strength and durability [58], and treatment with a sodium hydroxide (NaOH) solution enhances crystallinity by hydrolyzing amorphous compounds, resulting in increased rigidity of the composites [63].
Other hemp plant parts can be used to reinforce the hempcrete.Introducing hemp fibers into the hempcrete mixture can increase the flexural and tensile strength, and the hemp flower could delay hydration, therefore enabling the controlling of the hempcrete setting [18].Façades, curtain walls, and building skins have been reported to be developed from hemp fiber-reinforced composite sandwich panels [61].Hemp-based products, like plywood reinforced with short hemp fibers bonded with lignin-phenol-formaldehyde adhesives prepared by a hot-pressed method, can be processed into various building products, such as wall claddings, sheathing, ceilings, and cabinets [64].Hemp mortars and finer plaster utilizing shorter and thinner particle sizes ranging from 2 to 15 mm and binder-less hemp particle boards have been reported [51].Preliminary 3D-printing research shows that hempcrete is printable with a density as low as 660 kg/m 3 , with adequate buildability and compressive strength for individual walls [4].
Some challenges with hempcrete include variation in density, poor freeze-thaw resistance, incomplete decomposition at end-of-life due to composite mineralization, the limitation for load-bearing applications, and difficulty of onsite production in cold regions [15,60].Data referring to the fire resistance of hemp mortars are limited, potentially due to the high cost of testing, which further leads to a lack of standardization [51].

Application of Hemp Fiber and Hurd in Biocomposites
Biodegradable, sustainable, and recyclable materials are in high demand in the composites industry [65].In fact, about 25% of hemp fibers produced globally are used for composites in the automotive, aerospace, construction, textile, or sports industries [39].Composites reinforced with hemp fiber have been processed into products used for automobile interior linings, door frames, seatbacks, dashboards, trunk covers, engine covers, sun visors, air filters, and spare tire covers [29,50]; products used for luggage boxes, carpets, floor mats, package trays, headlining, storage tanks, bath tubes, toilet seating, wing box geotextiles, insulation mats, and furniture [16,66]; and other consumer products [51,60,65].Hemp has also been used in high-value products, including protective armor for ballistic applications, e.g., bulletproof vests and helmets [50,67], and functional automobile parts, such as brake pads [68,69].Hemp fiber spun into continuous yarn and utilized in woven or nonwoven fabrics can be used as preforms [29] for processing into bio-composite products using resin transfer molding [70], press molding [16], compression molding, sheet molding [37], injection molding [8], pultrusion [71], and hand lay-up [72], depending on the matrix used.Hemp has also been used to replace fiberglass as a biofiber additive in plastic recreational sports products for skis, snowboards, canoes, bike frames, tennis racquets [11,27], and orthotic devices [11,73].Additionally, hemp fillers have been added to polymer composites for prostheses to mitigate bacterial attachment [74].Costs of manufacturing natural fiber composites are reduced by more than 30% compared to synthetic fiber composites due to reductions in processing time, process energy use, and equipment maintenance [37].
The use of hemp fibers in reinforced composites has been shown to reduce weight and improve availability, eco-friendliness, and biodegradability over synthetic fibers [13,37].Hemp bio-composites have been reported to have favorable mechanical properties, including strength and durability [16], ability to withstand high mechanical and thermal stresses [51], ease of fabrication, and excellent structural rigidity [46,70].Natural materials have been utilized in an array of applications to replace non-biodegradable fiberglass and carbon fiber.Hemp fiber also has good waterproofness and heating and cold endurance in the summer and winter [12,34].It is also considered to be thermally stable [11], and its resistance to fire depends on the composite density and binder used [51,61].A well-designed hemp fiber-reinforced polymer composite may have better energy absorption when compared with metals [70].Blending the properties of hemp fiber and hurd at various ratios changes the polymerization degree to produce hemp material for the desired end use [31].However, increased moisture content with hemp-based composites leads to decreased tensile and flexural properties [37].Retted and treated hemp feedstocks are preferred to overcome some of the challenges of using hemp materials for composite [11,15].
The matrix used for hemp fiber composites can be classified as thermoplastic, thermoset, and biodegradable polymeric resins [65,75].Hemp fiber-reinforced thermoplastic composites are flexible and rigid, have a higher modulus, and have good mechanical strength [13].Hemp thermoplastic matrix composites have been reported to have better performance when compared with hemp thermoset matrix composites according to cost, specific strength, recyclability, corrosion resistance, and design capability, with only temperature as a drawback-thermoplastic composites begin to degrade above 150 • C [11].Thermoplastic matrix material can be reused in granulated form for injection molding or extrusion processes, while thermosetting matrix composites can be reused as fillers [76].Compression molding can be used for hemp thermoset and thermoplastic molded components [49,77].Fully (100%) bio-based composite products are often made using plant-based resins, ensuring a more eco-friendly hemp-based composite [51,61].Bio-based resins are preferred due to their low cost, lightweight, high specific strength and modulus, and renewable source [43].Biodegradable polymers used for hemp-reinforced composites include cashew nut shell liquid, polylactic acid, starch, cellulose ester, and euphorbia oil, and, when combined with other fibers, lignin-based epoxy, soy-based resins, and epoxidized linseed and soyabean oil can be used [13,39,65].To further improve the fire-resistant properties of hemp, hemp fire retardants like phosphoric acid, sol-gel coating, and ammonium polyphosphate are added to the hemp fiber composite [78].
The major challenges of fibers for high-value applications are their inherited hydrophilic nature, lower thermal instability, and higher variation in properties compared with synthetic fibers [72].The properties of hemp fibers can be improved for better surface quality, dimensional stability, biological, ultraviolet exposure, and chemical resistance [49].To address these inherent limitations, fiber for bio-composite applications can be treated using hybridization, physical, and chemical treatment methods [65,72].Hybridization combines one or more resins with more than one filler material, using them in engineering applications due to the resulting high strength-to-weight ratio [77].The physical approach improves the mechanical bonding of fiber to polymer without changing the chemical composition of fibers; these methods are stretching, steam explosion, clantering, cold plasma treatment, and corona treatment [65,69,77].
Steam explosion as a hemp stalk pre-treatment process has been reported to have a similar effect as water retting [44], though exhibiting higher efficiency and lower energy [48].In addition to its application for pre-treatment of hemp stalks, steam explosion has also been reported as an osmotic degumming treatment for hemp straw and fiber [79].Chemical treatment involves using a reagent (with or without a catalyst) to create surface compatibility between the hydroxyl groups of the fiber and the polymer resin's functional groups, commonly using the alkalization method [65].In addition, propionylation, acetylation, and coupling agents, such as maleated polypropylene (MAPP) treatments, have been reported to reduce hydrophilicity [11,80].Other efficient chemical treatments are also available for hemp fiber modification [37,72].Extracted hemp fiber can be further improved using physico-chemical treatments [39,48].Researchers have explored reverse genetics to understand improvement options for hemp fiber [29].While genetic development of new cultivars and optimization of hemp genotypes have been studied to improve fiber quality like fiber elongation and other properties [30,46,56], the development of new varieties using conventional breeding methods takes a significant amount of time, about 7-12 years [81].
Combining hemp with other fibers or synthetic materials has been explored to generate various desired properties [82].Hemp hurd with polyvinyl alcohol solution has been reported to give UV-shielding to a hemp bio-composite [48], while hybridized hemp fiber mats and aluminum sheets with epoxy resin have been observed to have good electromagnetic interference shielding properties [11].An increase in fiber content (>11% fiber volume) resulted in increased tensile strength [65].Combining hemp and bagasse fiber with a polymeric matrix made with the hand lay-up method has been used for automobile composites [66].Hybrid composites of hemp fiber have been observed to have better properties than unhybridized fibers [83,84], and improvements in strength and stiffness properties have been reported for hybridized hemp composites [29].
Hemp has been used to reduce the environmental impacts of fossil-based products; for instance, recycled polyester has been blended with hemp fiber to produce reinforced composites [13].Polyethylene terephthalate (PET)-hemp fiber composites also have been reported to be processed with injection molding [85].Crushed hemp yarn-epoxy composite tubes prepared using pin filament winding were observed to have high strength and modulus [70].The wrapping spinning process has been used for hemp fiber, lyocell, and polylactic acid (PLA) composites, while compression molding techniques have been used for hemp fiber and PLA [48].The hand lay-up method is commonly adopted for hemp fiber when using epoxy resin, and other processing techniques reported are vacuum-infused and hot-pressed methods [55].

Application of Hemp Fiber and Hurd in Packaging
In the packaging industry, special interest has been seen in applying hemp fiber and seed oil to design and develop packaging materials with advanced properties and performance [48] and antimicrobial properties [74].With active antibacterial constituents like alkaloids, flavones, and saponins found in hemp fiber, it is effective against pathogenic bacteria such as Escherichia coli, Staphylococcus aureus, and Pseudomonas aeruginosa [11].Other reported active packaging materials use artificial chemicals or substances to provide antibacterial properties for packaging, while hemp active packaging can be processed without these additives [74].Hemp packaging eliminates the risk of chemical substances migrating into the contained product if used for human consumption.The injection molding process can manufacture hemp packaging to contain foods like salads, meats, and ready-made food products.A recent application of hemp is for mycelium-based composites, which have applications in packaging and construction [18].Bio-based plastics can be laminated with cast hemp paper for eco-friendly and sustainable packaging solutions.Hydrothermal and mechanically generated hemp hurd nanofibers are utilized for sustainable barrier coatings and films [4].
Composite packaging films made by combining alkali-treated hemp fiber with polyethylene or polypropylene have been reported to have high mechanical stability and low water vapor transmission rate [48].Finer hemp hurd obtained during the fiber extraction process can be used as cushioning packaging material.The hurd particle size, foaming agent, and type of adhesive used affect packaging properties.Hemp polymeric composites can be processed for packaging by combining hemp tow with a heated thermoplastic matrix in a radial flow (turbulent) mixer, which is subsequently rolled into a sheet or other forms.Current packaging processes and compatible materials with hemp feedstock reported in the literature are shown in Table 5. Hemp produces horticultural mulch, primarily for garden surface treatment of vegetables, flowers, and ornamental plants grown in containers [12].Hemp mulch is lightweight, retains moisture effectively, and helps suppress weed growth.It is biodegradable and contributes to soil humus as it decomposes, enriching the soil and making it pH-neutral.Hemp-based fleeces or mulch cloth are biodegradable and can be used as weed-control mulching mats.These hemp products are environmentally friendly and effective substitutes for plastic mulching.In addition, hemp has gained popularity as an animal bedding option owing to its water absorption capacity (it can collect up to five times its mass in moisture) and non-allergenic properties.Hemp hurds give a sturdy base to animals while being dust-free, reducing microbial growth, causing less irritation than other bedding options, and decomposing quickly.Though hemp bedding is more expensive, it offers better function, which helps to justify the cost [18].A summary of hemp products processed from fiber and hurd is presented in Table 6.Table 6.Hemp fiber and hurd products.

Emerging Applications of Hemp Fiber and Hurd
Details presented in this section are based on a meta-analysis of the 109 selected articles relevant to emerging areas and applications for hemp in response to question R3.The depletion of fossil fuels and the need for more environmentally friendly, clean, and sustainable energy storage systems have made manufacturing supercapacitors with hemp an emerging area of research [12,88].Due to its hydrophilic nature and porosity, hemp cellulose can be embedded with noble metals such as silver and gold nanoparticles for catalysis, optoelectronics, photovoltaics, packaging, electronics, and biomedical (antimicrobial) applications [89].Hemp biomass can be transformed into activated carbon through either chemical or physical carbonization and activation processes [40,88].Additionally, hemp-based activated carbon can incorporate transitional metal oxides as electrode materials to improve energy storage performance in hybrid supercapacitors [88].Hemp-derived activated carbon has potential applications in supercapacitor cells [55] for electric vehicles, machine power sources, turbocharging, and cordless devices [12].These supercapacitors have the advantage of being lightweight, portable, foldable, and disposable.They can operate efficiently across temperature ranges of −40 to 150 • C and withstand severe environmental conditions, making them potential candidates for applications in space exploration, military, communications, and satellite systems [88].The automotive and electronics sectors are expected to be pivotal drivers in the further development of this technology.
Additionally, hemp is gaining interest as a potential material for creating textile-based electrochemical energy storage devices [40].These include textile-based supercapacitors and hybrid supercapacitor batteries that can be used for portable and wearable applications.Enzymes, macromolecules, and some polymers can also modify natural cellulose for specific end uses [89].Hemp fibers have been chemically activated using ZnCl 2 or H 3 PO 4 at varying concentrations and temperatures [40,88].Hemp hurd can also be activated chemically using KOH at 700-800 • C or hydrothermal processes.The electrodes developed from the latter method are recommended because they perform better than those from hemp fibers [88].An alternative method for transforming hemp hurd is the thermochemical method, which involves processing it under an inert argon atmosphere at 450-550 • C with lengths ranging from 0.25 to 1 mm.Hemp-based activated carbon can be used as negative electrodes.In contrast, hemp-based materials infused with transition metal oxides can serve as positive electrodes in hybrid supercapacitors to improve the overall specific capacitance, cyclic performance, energy density, and power density delivery of the supercapacitor.
Graphene has superior performance to biomass-activated carbon; however, hemp biochar-activated carbon's cost and environmental benefits make it an attractive alternative [88].Another study reported improved energy storage performance using hemp-based carbon nanosheets in ionic liquid electrolytes, surpassing that of activated graphene or carbon-based supercapacitors [40].Nevertheless, there are still challenges to be addressed in the preparation of hemp active electrode materials, capacitive retention, and the implementation of hemp-based electrochemical energy storage devices on a larger scale, which could support the increase in the demand for the production of wearable and portable flexible/non-flexible devices, its materials development, power management interface, recycling, biodegradability, and circular economy [40].

Hemp Seeds and Flowers
Hemp seeds and extracts can be physically and chemically processed into functional food and non-food products, while the whole hemp plant is a source of fats, proteins, fibers, and many bioactive compounds [32,48,90].Some hemp plants cultivated for special medicinal purposes have been reported to be grown indoors to ensure the production cycle all year round; however, there are ongoing studies to understand how the quality differs from field hemp plants, especially the effect of pesticide or fungicide [91].Hemp seed has been reported to contain approximately 30% protein, 25% carbohydrate, and 27-38% oil [11,15].The seed can also be a significant supplement in various products, such as snacks, drinks, bakery food, and culinary products [90].Hemp seed was listed as one of the top five most popular edible seeds globally [92], and its protein has been identified as one of the emerging sustainable protein sources [93].Hemp oil extracted from the hemp flower may be used for human consumption, in animal feed, or as biofuel for energy sources [94].Hemp oil has many other uses in cosmetics, medicine, and crop protection [12].Hemp leaves have also been reported as potential raw materials for food products, though their application is strictly regulated [95].Hemp seed oil and hemp oil have been reported to have antimicrobial activity against bacteria, potentially increasing the shelf-life of their product by controlling food-borne spoilage for food storage and packaging [74].Similar applications are also valuable for medical devices, water purification systems, healthcare products, hospitals, dental office equipment, and household sanitation [48,74].
Several food manufacturing industries are beginning to use hemp seeds and oil in their processes [17], making hemp one of the prospective key industrial crops for developing sustainable and resilient food systems [4].In the food industry, hemp is a superfood with high nutritional value and several health benefits [38,96].Hemp-based food products and supplements have recently gained popularity in Europe [97].Hemp seed proteins are suitable for human and animal consumption since they are primarily high-quality proteins [16] and are gluten-free [4].Hemp-based food products are becoming appealing alternatives for environmentally and nutritious-conscious consumers [98].Hemp is considered a lowallergenic food source and can be incorporated for various uses in the food industry [92,98].Hemp seeds can be consumed raw (hulled seed), sprouted or powdered as flour [17], cooked, roasted [99], or dehulled (hemp heart or kernel) [7].Food researchers are currently finding ways to improve the sensory attributes of hemp-based foods-overcoming the unusual color, bitter taste, and viscosity [95].
Examples of hemp-based products and their benefits include hemp milk (anti-obesity), hemp leaves (anti-fatigue), hemp oil enriched in cannabidiol (anti-cancer, anti-inflammatory, antioxidant, and anti-acne [18]), hemp oligopeptides (anti-diabetic), and hemp leaf and seed extracts (anti-bacterial and anti-inflammatory) [95].Hemp seed oil can protect against Ultraviolet (UV) A and B radiation [34].There has been interest in its use as an emulsifier, a meat analog (e.g., extruded hemp/soy meat), and membrane formation [4].Membrane formation is applied in active packaging by combining hemp seed oil and gelatin film by solution casting, and it has been reported to act as an antibacterial barrier against various microorganisms [48].Hemp efficacy against other medically related diseases like SARSCoV-2 has also been reported [100].
Cannabidiol (CBD) oil is one of the nonintoxicating cannabinoid compounds produced from hemp; it is usually not addictive, unlike THC [101], and has been reported to have soothing and calming properties [7].THC is generally not present in hemp seed oil; however, its presence may be a result of the seed hulls contacting the cannabinoid-containing resin parts (leaves and flowers) during maturation, harvesting, and processing [102].Ongoing research is investigating emerging uses for CBD and hemp oils [103].Characterizing the major cannabinoids in hemp CBD oil or similar products is essential for companies marketing cannabis-derived products and determining legality status [104].Hemp-based products are suitable for all skin types and can be found in various personal care items, such as hair-care products, soaps, moisturizers, cosmetics, and antiperspirants [16].The market growth for hemp is expected to receive a positive impact from the increased production of a variety of hemp-based products, such as soaps, shampoos, body lotions, and UV skin protectors [17].Hemp powder with antibacterial properties has been infused in mouthwash, toothpaste, antiseptic ointment, toilet bars, and foot powders [74].

Hemp Seed Harvesting, Pre-Processing, and Extraction Processes
Hemp seed matures three to four months after it is planted, which can be affected by factors like sowing time, environmental conditions, irrigation practice, and the cultivar [11,12].More than 40 hemp cultivars have been reported, with Finola being the most commonly cultivated commercially [105].To maximize hemp oil yield and to avoid issues with the combine during harvesting, hemp plants are usually harvested when about 50-90% of the seeds have matured [12].After harvesting, seeds are dried to about 8% moisture content [30].Smaller seed lots are air-dried in a dehumidified space below 40 • C, while, for larger seed lots, air is forced through the seeds from a perforated bed.After drying, the seeds are cleaned to remove the hulls, small seeds, dust, and foreign objects [105].Seeds are then processed by hulling, pressing, and crushing [14].Whole seeds are crushed or hulled to separate the heart from the hulls, which can be used as fertilizer or cattle feed [11].Pretreatment, grinding, pressing, extraction, and other processing techniques can be carried out to produce oil, protein, or even biofuel from the seed [105].Various extraction methods and pretreatment techniques have been proposed to increase the utility of hemp seed in the food industry to overcome oxidative rancidity due to an abundance of unsaturated fatty acids.For example, microwaving or enzyme hydrolysis can improve oxidative stability without affecting fatty acid composition.The various extraction processes for hemp seed are shown in Figure 9 for oil, food, protein, and bioenergy products.

Pretreatment
In ultrasound-assisted extraction (UAE), ultrasonic waves are propagated through the cells, creating cavitation and improving solvent penetration into the sample matrix.This leads to increased yield and faster extraction times [106].Alternatively, microwave-assisted extraction (MAE) involves using microwaves to infiltrate the seed's pores, trapping the solvent within these spaces for even and rapid heating.This process accelerates extraction kinetics, reducing extraction durations, increasing extraction efficiencies, decreasing expenses, and diminishing solvent usage.UAE and MAE can more effectively extract tocopherols and other oil-soluble antioxidant compounds, such as polyphenols, in the hemp seeds.Thermal degumming processes are commonly used to remove phospholipids from hemp seed oil, but they can accelerate oxidation and reduce oil shelf life [105].
carried out to produce oil, protein, or even biofuel from the seed [105].Various extraction methods and pretreatment techniques have been proposed to increase the utility of hemp seed in the food industry to overcome oxidative rancidity due to an abundance of unsaturated fatty acids.For example, microwaving or enzyme hydrolysis can improve oxidative stability without affecting fatty acid composition.The various extraction processes for hemp seed are shown in Figure 9 for oil, food, protein, and bioenergy products.

Pretreatment
In ultrasound-assisted extraction (UAE), ultrasonic waves are propagated through the cells, creating cavitation and improving solvent penetration into the sample matrix.This leads to increased yield and faster extraction times [106].Alternatively, microwaveassisted extraction (MAE) involves using microwaves to infiltrate the seed's pores, trapping the solvent within these spaces for even and rapid heating.This process accelerates extraction kinetics, reducing extraction durations, increasing extraction efficiencies, decreasing expenses, and diminishing solvent usage.UAE and MAE can more effectively extract tocopherols and other oil-soluble antioxidant compounds, such as polyphenols, in the hemp seeds.Thermal degumming processes are commonly used to remove phospholipids from hemp seed oil, but they can accelerate oxidation and reduce oil shelf life [105].

Oil Extraction
Hemp seed oil extraction methods can be grouped as solventless extraction (cold press) and solvent-based extraction (e.g., supercritical fluid extraction (SFE) using CO 2 , solvent extraction using isopropanol, hexane or dimethyl ether, and Soxhlet extraction) [106][107][108].Each method has its advantages and disadvantages, and the choice depends on the end product and extraction yield [108].
Solventless Extraction: Cold pressing is the simplest, oldest, and most commonly used extraction method [107], with a recovery rate of 60-80% [105] and a yield of 27-31.5% [109].The cold pressing method is also the most common commercially, as it is considered natural and safe for food processing.With this method, seeds move through a conventional screw press without the use of chemical solvents or heat treatments, which helps retain more of the seeds' beneficial components [108].The drawback of cold pressing is the low yield potential (60-80%) of the extractable oil.Enzyme-assisted cold-pressing (EACP) has been shown to enhance the yield and nutritional quality of hemp seed oil [106,107].Using this technique, tocopherols in the oil increased from 4.8% to 14.1% compared to cold pressing.Ohmic heating and pressurized liquid extraction have also been reported [109].
Solvent-based Extraction: SFE employs a gas-like medium such as carbon dioxide (CO 2 ) at or close to its critical temperature and pressure to extract substances from solid matrices [106].This method can extract tocopherols more efficiently than traditional cold pressing.SFE has the highest economic profitability [105] due to producing foodquality hemp seed oil; however, it is costly and time-consuming [36].A combined UAE and supercritical CO 2 extraction provided significantly higher antiradical capacity than Soxhlet solvent extraction, although it was lower when the supercritical extraction was applied alone [105].Considering an initial economic cost-benefit analysis, supercritical CO 2 extraction is most efficient, followed by Soxhlet extraction [108].
A study found that extracting oil from hemp seeds through supercritical CO 2 , cold pressing, or solvent extraction yields oils with similar fatty acid compositions; however, oil obtained using supercritical CO 2 extraction had elevated tocopherol content yet reduced pigmentation compared to cold-pressed oil [107].To get a qualitatively better oil, UAE or MAE are used, followed by SFE [106].Many solvents have effectively achieved high yields when extracting hemp seed oil [108], e.g., N-hexane, petroleum ether, dimethyl ether (DME), ethanol, and isopropanol.Various optimizations with these solvents have been reported in the literature regarding extraction time, temperature, and other extraction conditions.The UAE solvent extraction method gives the highest extraction yield [105].DME is gaining attention for oil extract due to its extraction abilities on wet feedstock for grounded and ungrounded hulled hemp seeds [109].
A comparison of the solvent-extracted oil from untreated hemp seed and the hemp seed previously subjected to an MAE showed improved oil yield and carotenoid and tocopherol contents [107].The fatty acid composition was not altered, and the resistance to oxidation increased.N-hexane solvent extraction may produce a contaminated final product unsuitable for human food or animal feed [36].Soxhlet extraction is a conventional method that involves the selection of a solvent that is heated to reflux and then used to inundate the solid material to extract the desired compounds, including volatile compounds [108].In a study, Soxhlet extraction provided an optimum fatty acid profile and improved energy efficiency [105].For scale-up of extraction, UAE and Soxhlet extraction have been reported as the best methods to achieve a desirable ratio of omega-6 to omega-3 polyunsaturated fatty acid [108].

Protein Extraction
Extracting protein from hemp seeds involves the following steps: defatting or degreasing of seeds, optional pretreatment (ultrasound or microwave-assisted extraction), extraction, and precipitation of the proteins [99].Degreasing removes fatty acids and helps increase extraction efficiency due to the formation of lipid-protein complexes, which may reduce the rate of extraction.Degreased hemp seed produces protein meal of 30-50% protein content [105].After degreasing, proteins and fibers dominate the seed cake or meal.Extraction of hemp seed proteins utilizes chemical and enzyme-assisted methods [99].Extraction using alkaline followed by acid or isoelectric precipitation and micellization extraction are commonly used to prepare hemp seed protein isolates with high protein content.Recently, Reverse Micellization (RM) technology has been applied for protein extraction as it is convenient and cost-effective [4].RM extracts protein through forward extraction, which encapsulates soluble proteins in the inner aqueous core, and backward extraction recovers protein by disrupting the RM process.

Hemp Milk Processing
Like the processing of other plant-based milk, hemp seeds are wet-milled, filtered, sterilized, homogenized, packaged, and stored cold [110].Gums are added to stabilize the milk, and salt and sweeteners are used to enhance sensory properties.New technologies have been implemented to reduce particle size and viscosity and to inactivate microorganisms and enzymes.Researchers have used a combination of novel thermal technologies to achieve high-quality hemp-based milk products.A variety of primary products can be produced from hemp seeds, flowers, and leaves, from which a range of consumer products can be derived (Table 7).Hemp oil cake, also called seed cake or hemp meal (cold pressing [101] and enzyme-assisted low-temperature pressing [95]) Pasta, tortilla chips, salad dressings, snack products (crackers, cookies, and gluten-free biscuits), frozen desserts, hemp milk, fertilizer, animal feed, and supplements [14,35] [ 90,95] Hemp seed oil (enzyme hydrolysis and freeze-drying [95]) Hemp seeds are high in fiber, making them valuable feed for livestock [32].The seeds also contain omega-3 and omega-6, making them a good source of nutrition for birds and fish (and have been used for fish bait) [11,14,46,48].Hemp oil has been incorporated in pig feed and hemp seed used for alpine goats; however, high fiber content limits its use, particularly in pig and poultry feed [48].Cannabinoid contamination is the primary concern that prevents the approval of hemp feed for most livestock; restrictions are expected to ease as more regulators understand that seeds are not a source of cannabinoids [18].

Emerging Applications of Hemp Seeds for Bioenergy
Hemp can be processed into sustainable bioenergy products like bioethanol, biogas, biochar, solid biofuel, biohydrogen, and biodiesel [18,32,46].The quality of these fuels depends on the biomass characteristics, chemical composition, morphology [46], pretreatment methods, and extraction technique [87].The cellulose, hemicellulose, and lignin present in hemp biomass are sources of fermentable sugars that can be processed into bioethanol [115].The high cellulose content in industrial hemp biomass makes it an excellent alternative for bioethanol production compared to other agricultural feedstock options [87].Hemp seed oil cake can be processed into biofuels due to its high fiber content, proteins, and secondary metabolites [113].Hemp seed oil biofuel can achieve conversion rates of 97% biodiesel and was reported to have the potential to produce fewer emissions when burned compared with conventional diesel [18].The use of hemp bioethanol and biodiesel can significantly reduce greenhouse gas (GHG) emissions [41].Hemp's heat of combustion has been reported to be equivalent to maize (18 MJ kg −1 ), less than Miscanthus sp.(19.8 MJ kg −1 ), and greater than Jerusalem artichoke (16.5 MJ kg −1 ) [16].Hemp biodiesel production was estimated to yield over 800 L/ha/year, greater than sunflower, soybean, peanut, or rapeseed [32].A study reported ethanol from hemp hurd as relatively efficient for passenger vehicles, like other lignocellulosic materials such as alfalfa stems, poplar, Ethiopian mustard, and flax shives [86].
Hemp bioenergy processing involves the preparation and pretreatment of biomass using a biological, chemical, or physical method or a combination of two or more methods, with the biological-chemical method reported to be the most efficient [115].After drying and primary pretreatment of the raw hemp biomass, it undergoes pulverization (crushing, grinding, and sieving) to reduce particle sizes with increased surface area for efficient secondary pretreatment, which facilitates enzyme digestibility.The biomass particles then undergo Simultaneous Saccharification and Fermentation (SSF).This physicochemical pretreatment enables a faster transesterification process when compared with the biological pretreatment [102].Biodiesel produced through transesterification [12] from the oil extract of hemp seeds has been reported to meet the ATSM D6751 and EN 14214 standards and is a superior fuel to conventional diesel, except for its oxidation stability, which can be resolved by adding antioxidants [32].Hemp biodiesel can be used as a vehicle fuel and to produce heat and electricity [7].Table 8 shows the different pretreatments considered for hemp materials.Bioethanol and biogas can be produced through microbiological processes (fermentation and anaerobic digestion) or thermal processes (combustion), which can be further used to produce biochar, solid fuels, and biohydrogen [7,12,32,36].Among biorefinery processes, anaerobic digestion is the most extensively utilized for biomethane production, owing to its high efficiency, operational flexibility, and environmental advantages compared to other methods [102].The hemp waste slurry from bioethanol production via anaerobic digestion can fertilize hemp and other crops [41].
Hemp biochar carbonized at 400-600 • C can be used for high-heating value products like solid biofuel [4].If carbonized at a range of 800-1000 • C, a graphite-like microstructure forms, giving it electrical conductivity suitable for electrical uses.Pyrolysis has been reported for producing hemp bio-oil from biochar; however, it has not been implemented commercially [41].Biochar has multiple uses, such as fertilizer, an adsorbent, for CO 2 sequestration, and as a material filler for products such as concrete.The saccharification process for extracting bioenergy resources is shown in Figure 10.
There are also novel emerging applications for hemp seed oil, such as its use in polyurethane (PU) and chitosan for wound dressing, which offers excellent barrier properties, oxygen permeability, and antibacterial protection against infection [74].Cannabinoids and alkaloids found in hemp seed oil can also be included in antibiotics to treat bacterial infections.The wide range of emerging applications for hemp seed has been reported to improve the quality of life in various areas, such as lifestyle, fitness, healthcare, and daily life [40].
Hemp biochar carbonized at 400-600 °C can be used for high-heating value products like solid biofuel [4].If carbonized at a range of 800-1000 °C, a graphite-like microstructure forms, giving it electrical conductivity suitable for electrical uses.Pyrolysis has been reported for producing hemp bio-oil from biochar; however, it has not been implemented commercially [41].Biochar has multiple uses, such as fertilizer, an adsorbent, for CO2 sequestration, and as a material filler for products such as concrete.The saccharification process for extracting bioenergy resources is shown in Figure 10.There are also novel emerging applications for hemp seed oil, such as its use in polyurethane (PU) and chitosan for wound dressing, which offers excellent barrier properties, oxygen permeability, and antibacterial protection against infection [74].Cannabinoids and alkaloids found in hemp seed oil can also be included in antibiotics to treat bacterial infections.The wide range of emerging applications for hemp seed has been reported to improve the quality of life in various areas, such as lifestyle, fitness, healthcare, and daily life [40].

Hemp Flower Oil Extraction Processes
The production and purification of cannabinoids from hemp flowers were reported to involve primary and secondary extraction [114].Primary extraction involves a preprocessing stage, where the flower is ground into powder and then decarboxylated to activate the cannabinoids.For hemp flowers, the harvest time depends on the variety's flowering behavior [12].Hemp crude oil (hemp oil) is extracted using the supercritical CO 2 method discussed earlier.The hemp oil contains numerous volatile chemicals, mainly monoterpenes, sesquiterpenes, and other terpenoid-like molecules that can be separated by distillation.Some other essential chemical constituents are myrcene, pinene, limonene, caryophyllene, humulene, and terpinolene.The secondary extraction process involves the post-processing stage, where the winterization process is used to remove waxes and color compounds, and a rotary evaporation process is then applied.Lastly, the distillation process generates approximately 90% purified CBDs that can be used for edibles, vapeables, and tropical products [114].A chromatographic-based method is used to isolate CBDs to about 99%, which can be used for pharmaceutical applications.Figure 11 shows the primary and secondary extraction processes for hemp flowers.

Hemp Flower Oil Extraction Processes
The production and purification of cannabinoids from hemp flowers were reported to involve primary and secondary extraction [114].Primary extraction involves a pre-processing stage, where the flower is ground into powder and then decarboxylated to activate the cannabinoids.For hemp flowers, the harvest time depends on the variety's flowering behavior [12].Hemp crude oil (hemp oil) is extracted using the supercritical CO2 method discussed earlier.The hemp oil contains numerous volatile chemicals, mainly monoterpenes, sesquiterpenes, and other terpenoid-like molecules that can be separated by distillation.Some other essential chemical constituents are myrcene, pinene, limonene, caryophyllene, humulene, and terpinolene.The secondary extraction process involves the postprocessing stage, where the winterization process is used to remove waxes and color compounds, and a rotary evaporation process is then applied.Lastly, the distillation process generates approximately 90% purified CBDs that can be used for edibles, vapeables, and tropical products [114].A chromatographic-based method is used to isolate CBDs to about 99%, which can be used for pharmaceutical applications.Figure 11 shows the primary and secondary extraction processes for hemp flowers.

Hemp Sustainability Performance
Details presented in this section were generated from a meta-analysis of the 109 articles relevant to the sustainability performance of hemp selected in response to question

Hemp Sustainability Performance
Details presented in this section were generated from a meta-analysis of the 109 articles relevant to the sustainability performance of hemp selected in response to question R4.It has been reported that cultivating hemp fiber can lead to a 77% cost savings compared to cotton cultivation due to a higher yield and reduced water, fertilizer, pesticide, and land use [4,116].Hemp fiber yield is 25-500% higher than cotton [116], making hemp economically competitive [41].Due to its rapid growth, the biomass accumulated by hemp crops significantly absorbs atmospheric CO 2 [34,62].Carbon emissions per kg of hemp hurd production range from 0.085-0.19kg of CO 2 , offset by 1.5-2.1 kg of CO 2 sequestration during growth [61].Hemp production uses 11,400 MJ/ha of energy, around half of that required by similar crops, such as cotton [51,61].Hemp plants are also energyeffective because of their capacity to control weeds, reduce pesticide usage, and enhance soil health [12].Hemp has also been used for phytoremediation, removing heavy metals from the soil [16], making it a promising sustainable industrial crop [117].Further development of cultivation and processing technologies will enable hemp to outperform other crops.
Renewable, low-carbon materials with extended life cycles that promote healthier buildings are needed in the construction industry [64].Globally, the construction and use of buildings and roads consume about half of the raw materials and energy; natural and ecofriendly materials such as hemp-based products could significantly reduce environmental impacts in the construction industry [11].A study reported that a hemp-lime wall requires up to 394 MJ of energy and sinks up to 35 kg CO 2 over a 100-year life span (resulting from the continuous hardening process of the lime binder) [38].In contrast, an equivalent Portland cement-based concrete wall requires 560 MJ of energy with an additional release of 52.3 kg CO 2 [11].Hemp concrete has very low embodied carbon and energy, making it ideal for green building applications [60], and it can also reduce daily indoor relative humidity variations and lower energy consumption by 45% compared to cellular concrete [72].The material is also recyclable at the end of the building's life [61], and incineration after use can save more energy and reduce GHG emissions compared with its fossil-based counterpart [48].To make hemp concrete more sustainable, sourcing waste lime from other industries through industrial symbiosis has been reported to further reduce its carbon footprint [18].A comprehensive review of the energy and environmental performance of hemp-based building materials has been reported [14].
Hemp fiber feedstock encourages holistic plant utilization, reducing deforestation, water consumption, and GHG emissions, while promoting carbon sequestration and enabling cultivation through regenerative agricultural methods [116].These advantages lead to significantly lower environmental impacts compared to those typically seen in the textile industry.Utilizing hemp process and product waste in manufacturing other products promotes a circular economy [55].Hemp biofuels have been reported to reduce GHG emissions significantly and as excellent fossil fuel alternatives [41].A study evaluating the environmental impact of flax and hemp fiber reported hemp to have higher environmental impacts due to the agricultural operations, emissions associated with fertilizer production, and chemicals used in pulp production [118].These relatively high impacts could be reduced by mitigating fertilizer use and standardizing cultivation, fiber extraction, and product production practices.

Future Research Opportunities
The unique properties of hemp material have generated new research interest in its applications in developing novel sustainable products, such as sporting goods and musical instruments, due to its higher vibrational damping capacity [11,72].Interest in the processing, properties, and application of composites made from hemp materials appears to be growing more quickly in Asia and North America than in Europe [27].Future demand is expected for new hemp varieties with attributes resistant to environmental stresses and pests and the ability to efficiently optimize agricultural treatments to improve nutrient uptake [34].The comprehensive literature review herein was undertaken for all published articles between January 2000 and April 2023; thus, new materials and applications reported in later 2023 do not appear in the foregoing.It was observed that the number of published articles increased by 13% in 2023 over 2022 in ScienceDirect, though the number of articles decreased slightly (−6%) over the same time period in the Web of Science, as shown in Figure 1.Overall, there has been an increasing trend in research publications on the topic.
More research will be needed to investigate how hemp seed extraction processes, such as pH shifting, high-pressure homogenization, and ultrasound treatment, affect hemp seed-based food products' physical and chemical properties, sensory qualities, toxicity, and health-derived benefits [105,108].Other areas of exploration for manufacturing and sustainability engineers will be the use of hemp materials for various packaging needs, e.g., food packaging and closures, and in conducting comparative life cycle assessments with their synthetic alternatives [114].Although a gate-to-grave life cycle assessment study has been carried out on hemp-based mycelium packaging box inserts [119], more studies evaluating the impacts of other emerging hemp-based packaging options, such as active packaging, will be in high demand as the move for sustainable packaging moves to the forefront, especially in the food industry.
Previous review studies have consistently emphasized the limited available studies investigating the improvement of the mechanical performance of hemp concrete and the development of theoretical prediction methods for compressive strength [60,61].The hemp concrete end-of-life phase should be studied to understand degradation mechanisms and fire resistance standards for other countries outside of Europe.Hemp building materials have been reported to have good energy absorption behavior; however, there are limited studies on structural integrity against large failure scenarios for more resilient buildings [60].Most data on hemp cultivation and building materials are collected in Europe [61], and other studies have highlighted a lack of data on hemp [42,120].Data sharing across various hemp value chains from different parts of the world, especially in the US, is encouraged for a more accurate techno-economic assessment of the hemp construction industry and other hemp industries.Research efforts are also needed to investigate challenges with manufacturing techniques, which limit the use of hemp for some industrial products.For example, drying time in producing hempcrete blocks in cold regions must be reduced while maintaining structural integrity, and the design of stalk milling equipment must be improved to increase both fiber quality and process yield.
Research interest in the energy and environmental performance assessment of hemp products is increasing [14].Comparative life cycle assessment studies have been recommended for hemp-based products and other conventional products [8].A lack of data has limited the comprehensive assessment of the energy and environmental performance of hemp plants, materials, and products.Similarly, there have been limited social assessment studies on hemp material and products [36].However, health issues associated with the handling or processing of hemp have been reported; there is a need for a comprehensive social assessment of the hemp industry to enable manufacturers to design more sustainable processes for hemp manufacturing and policymakers and investors to make more informed decisions in this nascent industry.
To ensure the continued acceptance and growth of hemp-based products globally, manufacturers processing hemp materials should collaborate on making process data, processing methods, and materials available for research [114].Research in the efficient development of training curricula to serve the future workforce needs of the industry will be a key focus area.The aforementioned research efforts will help promote and drive the technological innovation needed for the emerging hemp industry.Research to support policy development for the safer use of hemp medical and health products for humans and animals will also be key areas of effort [121].Consequently, the regulations and guidelines for the cultivation, processing, and use of hemp will improve as research continues to demystify hemp across its various value chains.Key stakeholders such as hemp product converters or processers continue to improve labeling, especially for CBD products [122], for testing by regulatory bodies.As the number of environmentally conscious consumers grows globally and concerns increase over product composition, raw materials sourcing, and production processes, transparency and active engagement of consumers will be key components for the hemp industry to outperform other alternative or conventional products.

Conclusions
The legalization of hemp in the United States and other regions of the world has led to increased research interest globally; however, there exists variability and limited data within the published literature for advancing the state-of-the-art hemp industry.To address this challenge, a systematic review of the literature on hemp product manufacturing was performed to identify qualitative and quantitative studies reporting the existing production technologies, manufacturing processes, and products.More specifically, all published articles identified between January 2000 and April 2023 were reviewed to address four research questions (R1-R4) related to hemp materials and products, technologies or manufacturing processes, emerging or newly developed materials or topics in sustainability, and future research scope for hemp products.After considering appropriate inclusion and exclusion criteria, 109 articles were selected for review.Publication statistics from this review reveal a steadily growing research interest in hemp in North America and Asia.Hemp uses for bio-composites and functional food have received the most interest, while current research efforts have been geared towards emerging applications of hemp, such as supercapacitors.
This review study identified the hemp plant as a resilient and emerging industrial crop.Existing and emerging hemp products were identified for all hemp parts (stalk, flower, leaf, and seed).The conversion of the hemp materials and manufacturing processes were also discussed in detail, and variations in research articles were highlighted for hemp stalk retting, decortication, and hemp fiber pretreatment and extraction, as well as fiber and hurd's application in the textile, composites, construction, packaging, pulp and paper, and other manufacturing industries.Hemp leaves, seed, and flower extraction processes were highlighted for producing body care products, food and food supplements, pharmaceutical products, and bioenergy resources.
As the nascent industry continues to grow, there has been substantial research interest in the sustainability of the hemp industry.Thus, the sustainability performance of hemp from the studies reviewed was also evaluated to understand how favorably hemp materials and products may compete with other materials based on cost and environmental impacts.Future research needs were identified for the hemp industry that would help to bridge the gap created by the earlier restrictions on hemp, which excluded it from the green revolution.The hemp plant will continue to find new applications due to its unique properties for sporting goods, musical instruments, portable and wearable energy storage devices, semiconductors, nanomaterials, active packaging, bio-composite orthotic devices, and medical applications, among others.Developing new hemp varieties and optimizing extraction processes for fiber, seed, and oil will open even more research opportunities and product applications.Comprehensive environmental, economic, and social impact assessments will be in high demand, enabling stakeholders and policymakers to make better-informed decisions about the hemp industry.Collaborative efforts between producers, manufacturers, and researchers, including the design of education programs for students and farmers, will drive technological innovation for the hemp industry.The limitations present in this systematic literature review include the possibility of not capturing recently developed processes or products due to the focus on review articles during the time period of January 2000-April 2023.

Supplementary Materials:
The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/agriengineering6030167/s1.Funding: This research was funded by the Agriculture and Food Research Initiative (AFRI), Sustainable Agricultural Systems (SAS), grant number 13333755/Project accession no.1027531 from the USDA National Institute of Food and Agriculture (NIFA).

Figure 3 .
Figure 3. Review publication statistics by country.

AgriEngineering 2024, 6 ,Figure 6 .
Figure 6.Field of hemp plants (left) with a closeup view of its flower (center) and stalk showing fibrous and woody hurd parts (right).

Figure 6 .
Figure 6.Field of hemp plants (left) with a closeup view of its flower (center) and stalk showing fibrous and woody hurd parts (right).

Figure 8 .
Figure 8. Hemp fiber extraction processes: (a) mechanical processing of unretted stalk, (b) mechanical processing of retted straw, and (c) traditional processing of hemp fiber.

Figure 8 .
Figure 8. Hemp fiber extraction processes: (a) mechanical processing of unretted stalk, (b) mechanical processing of retted straw, and (c) traditional processing of hemp fiber.

Figure 9 .
Figure 9. Extraction processes and products from hemp seed.

Figure 10 .
Figure 10.Saccharification process for the extraction of biomass bioenergy.

Figure 10 .
Figure 10.Saccharification process for the extraction of biomass bioenergy.

Table 1 .
Databases and the Boolean search terms.

Table 2 .
Screening results for the occurrence of "hemp" in the title and abstract.

Table 4 .
Description of common techniques for processing industrial hemp.

Table 7 .
Products made from hemp seeds, flowers, and leaves.

Table 8 .
Pretreatment methods for various hemp materials.