Toward an Environmentally Friendly Future: An Overview of Biofuels from Corn and Potential Alternatives in Hemp and Cucurbits

Abstract

Increased energy consumption and climate change, driven by greenhouse gas emissions, pose significant risks to global sustainability. Concerns about using agricultural land for fuel production and its competition with food production have made feedstocks like corn (Zea mays) highly controversial. This study explores the potential of alternative feedstocks, such as hemp (Cannabis sativa) and cucurbits (family Cucurbitaceae), for biofuel production amidst environmental concerns linked to fossil fuel usage. Hemp is widely acknowledged as a promising feedstock for sustainable biorefinery due to its agricultural adaptability and its ability to produce oil and carbohydrates. Cucurbits seeds are characterized by a high oil content, which can be utilized in the food industry or for energy production as biofuel. As a byproduct of cucurbits processing, a significant number of seeds often remains, which constitutes waste. By examining hemp and cucurbit byproducts and waste, which are suitable for bioenergy production, this research highlights the promise these alternative feedstocks hold for the biofuel industry. Utilizing these resources presents a viable route to diminish dependence on fossil fuels and transition toward a more environmentally sustainable energy future.

1. Introduction

In recent years, the escalating global energy consumption and soaring oil prices have propelled energy-consuming nations toward renewable energy sources, with a particular emphasis on biofuel production [1]. However, concerns regarding the utilization of agricultural land for fuel production, along with its competition with food production, have rendered the use of first-generation feedstocks, such as corn, increasingly contentious. The intensified utilization of land with significant biodiversity, freshwater, fertilizer, and pesticides, along with the associated resource consumption, could lead to deforestation and other environmental challenges as the demand for agricultural output escalates. By utilizing second-generation feedstocks, some of these issues could be addressed [2].

A multitude of helpful bioenergy products, such as biogas, biodiesel, bioethanol, and bio-hydrogen, can be made from biomass as feedstock [3]. Transesterification stands as the primary method for producing biodiesel from plant oils and waste oil [4]. Furthermore, to address climate objectives, like those set forth by the European Commission to decrease net greenhouse gas emissions significantly from 1990 levels by 2030, there is an urgent imperative to diversify and expand renewable energy sources [5].

Exploring alternative biofuel feedstocks, such as hemp (Canabis sativa) and cucurbits (family Cucurbitaceae), holds promise for sustainable energy production. While fossil fuels currently dominate the global energy supply, their finite nature and environmental impact necessitate the exploration of alternative energy sources [6]. The COVID-19 pandemic temporarily disrupted global oil demand, but consumption has since rebounded, highlighting the importance of diversifying energy sources [6]. In the United States, bioethanol production from corn (Zea mays) has emerged as a prominent renewable energy source, but concerns over land use and competition with food production persist [7]. As a result, there is growing interest in second-generation bioethanol production methods, which utilize non-edible plant parts and waste materials to improve land-use efficiency [8,9].

The objective of this paper is to provide an overview of biofuel production, focusing on the evolution of corn-based biofuels and the emerging potential of hemp and cucurbits as alternative feedstocks. By exploring the latest research and developments in this field, we aim to clarify the opportunities and challenges of biofuel production, contributing to a more sustainable energy future.

Overview of Biofuel Production: A Historical Perspective and Current State

Over time, energy production and agricultural commodities production followed separate paths until the production of biofuel began using agricultural feedstock [10]. Humans have been utilizing biofuels, primarily wood, for household purposes since prehistoric times, burning wood for heating, lighting, and cooking meals [11]. Notable archaeological discoveries, such as burned animal remains and stone tools amidst wood ash, indicate the control of fire by prehistoric hominid species over a million years ago [12]. Materials such as wood, straw, hay, cattle dung, and peat were intentionally gathered, dried, and ignited for cooking and heating. In remote areas, large fires and straw torches were essential for warmth and illumination. However, as industrialization advances and stoves become more modern, the use of firewood is likely to decline in any given country or region, mainly because firewood generates smoke, pollutants, and toxic gases [13].

The potential of ethanol as an engine fuel was recognized well before gasoline entered commercial production in 1913 [14]. In 1826, Samuel Morey created an internal combustion engine powered by a blend of ethanol and turpentine, propelling a boat at speeds from 7 to 8 mph [15]. In 1860, Nicolaus August Otto invented an internal combustion engine that operated on ethanol-based fuel, and subsequently, Henry Ford built tractors that ran on the same type of fuel [15]. However, in the United States, the widespread use of ethanol as a fuel was hindered by a tax on alcohol established in the 1860s to finance the Civil War [16]. Despite the dominance of gasoline in the late 1910s, many experts continued to advocate for ethanol’s efficiency and environmental benefits in the fuel sector [11].

Since the beginning of the 21st century, wood chips have been increasingly utilized for heating and electricity generation [17]. This is due to the fact that firewood, although the most efficient method for direct combustion to exploit bioenergy, is not applicable to small-scale systems [11].

Wood pellets, which are 2–3 cm in length, represent a more processed biofuel compared to wood chips [18]. Besides wood, pellets can be manufactured from grasses, crop residues, and nutshells as well. Their main advantage over firewood and wood chips lies in their high density, low moisture content, and small size, allowing for compact storage and long-distance transportation [19]. However, their high production cost results in a higher price, making them more available in developed countries [11]. The global trade of wood pellets, recognized as a sustainable energy resource, has experienced a significant increase since 2000 [20]. However, the wood pellets production process requires further improvement due to its high cost and complexity, limiting its use for electricity generation.

2. Corn: From Feed to Biofuel

Cultivated across various regions of the world, corn (Figure 1a) is recognized for its significant role not only as a staple food for humans but also for its utility in animal feed and industrial products. Approximately 85% of corn is utilized for animal feeds and bioethanol production [21]. In 2022, the United States allocated around 38% of its 13.7 billion bushels of produced corn to feed and residual, around 38% to ethanol, 14% to exports, and the remaining 10% to other food, seed, and industrial purposes [22].

Although its protein content is not as high as in some other crops, corn can still be nutritionally significant, especially in regions where it constitutes a major part of the diet. The limitation in corn’s protein quality—mainly its deficiency in essential amino acids lysine and tryptophan—is addressed through the development and cultivation of quality protein maize (QPM). QPM varieties have been genetically enhanced to contain higher levels of these amino acids, thus providing a more balanced protein source than conventional corn [23].

Corn kernels (Figure 1b) are notably enriched with vitamins, minerals, antioxidants, and various bioactive substances. Specifically, they are an essential source of carotenoids, phenolic compounds, and phytosterols. However, studies have indicated that the vitamin B content (B1, B2, B3, B5, and B6) in standard varieties of corn cultivated in America falls significantly short of meeting the daily recommended values (B1: 1.2 mg day⁻¹, B2: 1.3 mg day⁻¹, B3: 16 mg day⁻¹, B5: 5 mg day⁻¹, B6: 1.3 mg day⁻¹) [24]. Efforts toward enhancing the nutritional value of corn through biofortification, particularly for provitamin A, vitamin B9 (folic acid), and vitamin E, have been explored by Blancquaert et al. [25].

Corn plays an important role in ethanol production, which has seen a significant increase over the past decade, particularly in the United States, where it accounts for about 40% of corn production [26]. Currently, the United States is the biggest ethanol manufacturer and distributor globally [27]. Due to regulatory caps on the quantity of distillers’ dried kernels with soluble (DDGS), the United States supplies 85% of the DDGS for the global market [27,28]. The output of DDGS has escalated from 10 million tons in 2005 to 38 million in 2016, highlighting the connection between energy production and the use of agricultural residuals in feed efficacy [27,28]. Moreover, corn’s application in the United States extends beyond fodder, with over 130 million tons converted into 52 billion L of anhydrous ethanol for use in E10 fuel [23]. Predominantly, the dry grind method is employed in this transformation, representing 86% of ethanol production in the United States [28].

2.1. Agronomic Practices on Corn Stover Quality and Biofuel Potential

Historically, breeding efforts have primarily focused on improving grain yield for food and feed, with less attention given to stover yield or compositions crucial for biofuel production [29]. Increases in yield per unit area have been primarily attributed to the ability of new hybrids to tolerate increased plant densities and the stresses they induce rather than to increases in harvest index or yield potential per plant [30]. The global average rate of corn grain yield increase was 1.64% or 84 kg ha⁻¹ per year in the period 1989–2008 [31]. In the period 1930–2000 in the United States Corn Belt, the annual grain yield gain per hybrid regressed on the year of hybrid introduction was 77 kg ha⁻¹ [32]. Over the past 45 years, Argentine corn hybrid improvements in grain yield increased at a rate of 107 kg ha⁻¹ per year [33]. This enhancement in yield potential has been primarily attributed to gains in kernel number per unit area throughout the period from 1965 to 2010. Harvest index (HI) contributions to yield were notably crucial during the 1980s and early 1990s but remained stable in the last two decades, in agreement with the work by Munaiz et al. [29].

Corn stover, the residue left behind after corn harvest, is an abundant, inexpensive, and promising biomass source for producing cellulosic ethanol without competing with the food and feed uses of corn [34]. The amount of ethanol produced from corn stover depends on the stover yield, the concentration of cellulose in the stover, and the proportion of cellulose that is released as free glucose, which can then be fermented to ethanol. Estimates of the amount of stover that can be removed from the field can reach up to 70% [35]. Due to the fact that an adequate amount of corn residue should be left in the field for soil preservation, stover quality for cellulosic ethanol may be a more suitable breeding target than stover yield. This means a high concentration of cell wall glucose, a high percentage of glucose released from the stover, and a low concentration of lignin in the cell wall matrix [36]. Corn differs significantly in stover yield [37,38]. There was a 53% difference in stover yield between commercial grain hybrids that were statistically equal in grain yield, stover dry matter, and days to mid-pollen [39]. This suggests that, with careful selection and breeding, it is possible to achieve improvements in both areas simultaneously. However, a notable inverse relationship was observed between stover yield and stover dry matter content, as well as between stover yield and theoretical ethanol potential (TEP, maximum amount of ethanol that can be produced per unit mass of feedstock) of stover [39]. This suggests that, as stover yield increases, there may be a decrease in the quality of stover, specifically regarding its suitability for ethanol production. The correlation between stover glucan, xylan, hemicellulose, cellulose content, and stover TEP was very high and significant, suggesting that there is a significant determinant of stover’s ethanol potential, in agreement with the work by Dien et al. [36].

Nitrogen (N) fertilization has been observed to cause a slight decrease in the concentration of cell wall material in corn stover, primarily due to dilution rather than a reduction in cell wall deposition [40]. The polysaccharides in the stover cell wall decreased as N fertilization increased, with a quadratic decline observed at both locations studied. A decrease of approximately 2% and 4% in polysaccharides was noted at both locations as N fertilizer rates increased from the control to 168 kg N ha⁻¹ and from the control to 269 kg N ha⁻¹. However, N fertilization did not affect the cell wall polysaccharides of cobs at either location. In the same study, there was an increase in stover lignin across all locations, ranging from 144 to 205 g kg⁻¹ adjusted to a cell wall basis [40]. This increase in lignin concentration is significant as it negatively impacts the cell wall’s degradability, thereby affecting hydrolysis efficiency, which is essential for converting biomass to ethanol [40]. Sindelar et al. [41] observed a linear decrease in theoretical ethanol yield from corn stover with increased N fertilizer, by 0.07 to 0.10 L Mg⁻¹ for every 1 kg N ha⁻¹ increase in N fertilization. However, theoretical ethanol yields of cobs were unaffected by N fertilization and were reported to be 9 to 20% higher than stover when averaged across N fertilizer rates. The authors stated that “variation in stover feedstock quality and ethanol output is possible as a result of the interactive effects among management, genetics, and temporal and spatial environmental variation” and “that cobs may be a more viable option for cellulosic ethanol production”, taking N fertilization into account [41].

2.2. Key Determinants of Biomass Degradability for Ethanol Production

The efficiency of enzymatic hydrolysis, a critical step in converting lignocellulosic biomass to fermentable sugars for ethanol production, is significantly influenced by the composition and structure of the plant cell wall, as explained in the article by Vermerris et al. [42].

Plant cell walls are complex molecular structures that include polysaccharides (cellulose and hemicellulose) and phenolic components (lignin and p-hydroxycinnamic acids). Access to polysaccharides within cell walls presents a challenge because they are constrained by phenolic substances like lignin and p-hydroxycinnamic acids, including ferulic and p-coumaric acids [43]. However, lignin content does not always appear to be the negative factor in controlling degradability [44] and only partially explains variations in cell wall degradability among plants from the grass family [45]. In their study, lignin explained only 28% of the in vitro neutral detergent fiber digestibility variation within the regular inbred lines studied. Li et al. [46] stated that more than 50% of the variability in cell-wall lignin content in switchgrass lines could be attributed to differences in cell-wall architecture. The lignification process within the stem can lead to very different degradability profiles, thereby highlighting the importance of the anatomical structure and distribution of lignin within different tissues of the stem [47]. While chemical composition, specifically lignin, is an essential factor, the anatomical structure, particularly as it becomes more complex with plant maturation, plays a crucial role in determining how digestible a cell wall will be [48].

The variability in hemicellulose content across different plant organs significantly impacts the torrefaction process and subsequent biofuel production [49]. It was observed that biomass with larger particle sizes generates more gas; however, the quality of this gas, particularly its hydrogen and carbon monoxide concentrations, may be compromised due to reduced gas diffusion rates. This observation underscores the importance of selecting appropriate biomass components like stalk pith and cob shell, which are rich in energy content. The torrefaction of these components is critical for enhancing biofuel production efficiency. This thermochemical treatment improves biomass properties such as brittleness, hydrophobicity, and resistance to microbial degradation. Additionally, it increases the energy density, homogeneity, and favorable chemical characteristics of the biomass, thereby lowering the costs associated with transportation, storage, and further processing [50].

Li et al. [51] emphasized the role of cell morphology and its functions in their chemical composition. The authors stated that stalk rind and pith, leaf sheath, and blade are suitable biofuel biomass but differ in their tissues, cells, and chemical composition.

However, inorganic elements in the stover and ash are problematic for processing because they cause slagging, fouling, plugging, and equipment deterioration that directly impacts conversion cost, product yield, and quality [52]. Silica, especially since it is water-insoluble and challenging to eliminate, can block filtration structures and cause instrumental defects [53]. Still, they are essential in plant tissue development [54]. Due to the fact that they are not evenly distributed throughout the biomass plant, one possible strategy to remove unfavorable inorganic elements is to introduce selective fractionation processes that have the potential for the effective conversion of corn stover, alongside other lignocellulosic biomass, contrasting the current strategy to homogenize the bulk materials until they become uniform [55].

3. Hemp as a Key toward an Environmentally Friendly Future

Hemp (Cannabis sativa) is a multipurpose plant that offers a wide range of different products (Figure 2). Utilizing the entire plant, it finds applications for nutritional, medicinal, and industrial purposes. Hemp’s high biomass (Figure 3a) content and energy yield make it suitable for ethanol production from the whole plant. Moreover, the seeds (Figure 3b) yield high quantities of oil, which can be pressed to produce biodiesel [56]. Hemp holds the potential to be a highly sustainable and ecologically friendly crop. Notably, hemp roots have significant capacity in the absorbing and retention of heavy metals such as lead, nickel, cadmium, and various other harmful substances. In addition to its phytoremediation capabilities, hemp acts as an excellent carbon trap, absorbing more carbon dioxide per hectare compared to agricultural commodities or even woodlands [57]. Each hectare of hemp can absorb 22 tons of CO₂ per hectare/year [58].

Hemp can be characterized as a low-cost feedstock crop with minimal pesticide and fertilizer requirements, making it a weed-competitive crop. It can effectively reduce weed growth and is generally regarded as a pesticide-free crop. From an economic standpoint, hemp is a plant that generates minimal waste, with every component, from the roots to the leaves, holding value [58]. Hemp biodiesel contains deficient levels of sulfur, which reduces sulfur oxide emissions that contribute to acid rain [59]. These factors suggest that biodiesel production from hemp seed oil could mitigate the negative environmental impact of fossil fuels. Additionally, hemp can serve as a significant force for carbon sequestration while offering a wide range of product possibilities.

3.1. Hemp as a Potential Biofuel Crop

Hemp is widely acknowledged as promising feedstock for sustainable biorefinery due to its agricultural adaptability and its ability to produce oil and carbohydrates [60].

Dry matter yields from cultivating hemp can range from 10 to 15 t ha⁻¹ [61]. Estimates suggest that the biomass collected has a higher cellulose content (40–43%) compared to other agricultural feedstocks (37–39%), such as bagasse from sugarcane, corn stover, and bioenergy sorghum [62,63]. Effective and cost-efficient preparation is necessary for breaking down the complex cellulosic constituents of biomass to ensure the economic viability of hemp as a source of bioethanol. Various technologies, including physical, chemical, and physicochemical approaches, have been researched to improve cellulosic sugar extraction and ethanol production. One standard preprocessing method is steam treatment, which can break down the physiochemical bonds that lignin has with carbohydrates and prevents fermentable sugars from degrading [62,64]. Compared to other chemical approaches, this strategy requires fewer capital investments, making steam or hydrothermal pretreatment a more economically viable and eco-friendly technique for cellulosic ethanol production. According to the work by Sipos et al. [65], utilizing steam pretreatment with a 2% SO₂ catalyst at 210 °C for 5 min, hemp can be transformed into ethanol. Subsequently, simultaneous saccharification and fermentation at high solid loading (7.5% water-insoluble solids) yield approximately 171–163 g ethanol kg⁻¹ raw material of hemp [65].

Modeling findings have shown that hemp with 2% lipid content could produce up to 3.95 million gallons of biodiesel per year [60]. Increasing the biomass lipid content to 5% and 10% resulted in biodiesel outputs of 9.88 and 19.91 million gallons, respectively. The break-even unit production cost of hemp biodiesel with 2%, 5%, and 10% lipid-containing hemp was of USD 18.49, USD 7.87, and USD 4.13 per gallon, respectively [65]. The cost of producing biodiesel from 10% lipid-containing hemp was equivalent to soybean biodiesel at USD 4.13/gallon. Additionally, sensitivity analysis found that reducing hemp feedstock costs by 10% could result in a 7.80% reduction in unit manufacturing costs. Furthermore, industrial hemp has been shown to be capable of producing between 307.80 and 325.82 L of total biofuels per hectare of agricultural land, exceeding soybean [60].

Prade et al. [66] conducted a study demonstrating that hemp’s customized biomass energy yield was over double that of energy created by wheat straw in solid fuel. Additionally, for biogas production, hemp’s customized biomass energy yield was comparable to that of corn and sugar beet and 24% and 14% higher than that of lucerne and clover-grass ley. As a solid fuel, hemp was 120% higher compared to wheat straw and similar to that of reed canary grass [66].

Li et al. [56] investigated the utilization of hemp for biodiesel production. Their results showed a conversion rate higher than 99.5%, with a product yield of 97%. They emphasized the attractive features of hemp biodiesel, such as its low cloud point and low kinematic viscosity, which increase its competitiveness.

Das et al. [67] evaluated the bioenergy potential of industrial hemp compared to biomass sorghum, switchgrass, and kenaf. The results indicated that the yield of industrial hemp stem per hectare was similar that of sorghum and switchgrass, with the significant advantage of hemp requiring fewer inputs.

In another study by Das et al. [67], the agronomical and economic potential of 11 industrial hemp varieties for biofuel production was evaluated. The results and analyses confirmed that industrial hemp presents a crop with significant potential for biofuel production from both agronomical and economic perspectives.

For the calculation of hemp biodiesel yield, Alcheikh [68] provided the following formula:

Hemp Seed Yield (kg ha⁻¹) × 35% (Oil Content) × 1.163 (L kg⁻¹) × 0.264172 (gallons L⁻¹) × 97%

3.2. Quality of Hemp Oil for Biodiesel

Biodiesel possesses a low cloud point (−5 °C) and kinematic viscosity (3.48 mm² s⁻¹) [56]. This may be attributed to the significant quantity of polyunsaturated fatty acids in hemp seed oil and its unique 3:1 ratio of linoleic to α-linolenic acid [68,69,70]. Sulfur concentration is a significant factor because burning fuels with a greater amount of sulfur produces sulfur oxide molecules, which are major pollutants and the primary cause of acid rain [68,71,72]. Hemp biodiesel is considered a cleaner fuel compared to soybean and rapeseed biodiesels, as evidenced by its significantly lower sulfur content [68,72]. However, hemp biodiesel exhibits slightly higher kinematic viscosity than the European EN 14214 standard [73] of 5 mm² s⁻¹ but lower than the American ASTM D6751 [74] maximum of 6 mm² s⁻¹ (ASTM 2015) [72]. Furthermore, both hemp biodiesel and other biodiesels exhibit poor oxidation stability compared to typical diesel variants, falling below the ASTM D6751 and EN 14214 specifications of 6 and 8 h, respectively. However, these metrics can be easily improved using chemical additives to meet testing requirements [71,72]. Antioxidants, for example, are commonly used to suppress biodiesel oxidative degradation and extend the fuel’s lifespan [68,72]. Additionally, the increased viscosity of biodiesels can be improved by blending them with petro-diesel or less saturated FAME, as well as the use of certain additives [68,71,72]. The chemical and physical characteristics of hemp oil biodiesel are investigated to determine engine emissions as well as the parameters of diesel engines [71,72].

3.3. Status/Potential of Hemp Seed Production for Biofuels

Hemp seeds have been consumed as food by both humans and livestock for at least three thousand years [75]. Historically, hemp oil was used in various products such as soap, paints, varnishes, and lighting oil. In the twentieth century, hemp grain was primarily utilized as birdseed, leading to the discontinuation of hemp oilseed landraces [76].

Throughout history, hemp seed has been used to produce various products, including hemp meal, hemp seed flour, hemp seed protein, and hemp seed powder. Hemp seeds contain approximately 20 to 30% edible oil, 25 to 30% protein, 20 to 25% fiber, and 20 to 30% carbohydrates, along with essential amino acids, vital nutrients, and vitamins [77]. The optimal balance of omega-6 to omega-3 fatty acids in hemp oil makes it a valuable component of human health foods. However, it remains illegal to feed hemp to animals in the United States [78].

Currently, there is a growing interest in hemp grain production due to the increased awareness of its high nutritional value for human health, nutritional potential, and industrial applications. Hemp seeds, previously considered waste products from fiber production, are now being recognized as valuable commodities [79].

Breeding objectives for hemp seeds primarily focus on increasing yield and reducing seed shattering. Advances in breeding for oilseed traits in recent years suggest that significant improvements can be made in oilseed composition [75]. Controlling THC levels is essential in breeding programs, although recent decisions by the European Commission regarding THC content regulations have been subject to change [80].

Currently, all imports of hemp are subject to import license requirements and strict regulations regarding THC content. Hemp seeds for sowing must demonstrate THC content not exceeding 0.2%, while seeds not intended for sowing require authorization from EU countries [80].

In addition to its large-scale production potential and low cost as a waste product, hemp biomass offers advantages over fossil fuels. Biomass is a sustainable, carbon-neutral feedstock, making it an attractive option for biofuels. Industrial hemp is particularly promising as a biomass source, with predictions suggesting a significant increase in demand for biomass and biofuels by 2035 [69].

3.4. Methods of Hemp Oil Extraction for Biodiesel Production

Hemp seed oil can be obtained from either whole or dehulled hemp seeds. Standard extraction methods are generally designed to efficiently extract oil and produce high-quality oil [56,66]. Pressing is one of the oldest processes for extracting hemp oil, with seed preparation and processing components influencing oil extraction efficiency and quality [56,66]. The opportunities for improving hemp seed processing and producing fractions from hemp seed oil will be comparable to those for better-known oilseeds such as soybean, rapeseed/canola seed, sunflower seed, safflower seeds, flax seeds, and palm kernels [66,81]. Hemp seeds have been subjected to a variety of oil extraction techniques, including solvent extraction, mechanical pressing, supercritical CO₂ utilization, and microwave or ultrasound-assisted processing [68,82]. Triacylglycerols, the lipids that store plant seed oil, must be released from the seeds by decreasing or breaking the cell wall [56,68,81]. A range of lipid extraction methods can be used to recover lipids from different organic compounds. Various methods are being employed to enhance the procedure by obtaining the maximum oil extraction from hemp seeds at the most affordable price [66,82,83]. A few novel methods for oil extraction have emerged as a result of recent technical advancements: supercritical fluid extraction, ultrasound-assisted extraction, and microwave-assisted extraction [70,82,83]. Transesterification processes can be catalyzed by enzymes, acids, or alkali-base compounds [56,70]. Some characteristics of fuel, like turbidity, cloud point, and color, can be affected by impurities from polluted source oils [70,82]. Wet washing to eliminate water-soluble substances, dry washing with certain adsorption materials, filtration, and ion exchange are some of the techniques used to eliminate contaminants either before or after transesterification [56,70,82].

4. Cucurbits as a Key toward an Environmentally Friendly Future

Cucurbitaceae is a plant family comprising approximately 115 genera and over 960 species, exhibiting variations in shape, size, color, and taste [84]. Primarily, cucurbits fruits of various ripeness stages are utilized (Figure 4a), and their flesh is employed to create various dishes such as soups, salads, roasted and boiled dishes, pickled, fermented, and dried products [85,86]. As a byproduct of cucurbits processing (e.g., in juice production), a significant number of seeds often remains, which constitutes waste. However, cucurbits seeds (Figure 4b) are characterized by a high oil content, which can be utilized in the food industry or for energy production as biofuel [87].

Fedko et al. [88], in their study on plant species from the genus Cucurbita spp., indicate that seed content in the fruit varies depending on the species, as follows: Cucurbita maxima 2.04–10.71%, Cucurbita moschata 2.73–3.19%, Cucurbita pepo 3.20–8.44%, and Cucurbita pepo var. turbinata 2.63–6.80%. Ikanović et al. [86] mention that seeds constitute approximately 10% of the total fruit mass, with cucurbits seed yields ranging from 600 to 1000 kg ha⁻¹, while in the study by Sure et al. [89], seed yields ranged from 400 to 1686 kg ha⁻¹. According to Berényi [90], cucurbits seed yields range from 800 to 1000 kg ha⁻¹.

4.1. Oil Content in Cucurbits Seeds: Potential for Biofuel Production

The potential of a species to be used as a raw material for biofuels, such as biodiesel production, depends on the oil content in its seeds. According to FAO, seeds containing more than 17% oil can be utilized as feedstock for biodiesel synthesis [91].

Azam et al. [92] investigated the possibility of biodiesel production from 23 plants, two of which belonged to the Cucurbitaceae family, Citrulus colocynthis and Cucurbita maxima, with oil contents in their seeds of 21.0% and 36.6%, respectively. Mohammed et al. [87] reported that the oil content in cucurbits seeds is approximately 41.08%, which is lower than the 48% reported by Bikash et al. [93]. Jafari et al. [94] found oil contents ranging from 33.0% to 46.0% in seeds of ten varieties of Cucurbita pepo. In addition to plants from the genus Cucurbita spp., other plants from the Cucurbitaceae family can also be used for biodiesel production, such as wild melon (Cucumis melo var. agrestis) with a seed oil content of 29.1% [95]. The oil content in Citrulus lanatus seeds is reported to be of 52.2% [96]. In a study by Fedko et al. [88], the oil content in seeds of Cucurbita maxima was 32.6%, Cucurbita pepo var. turbinata 32.7%, and Cucurbita pepo 31.5%. The same authors noted significant variability in oil content among different varieties within a species. Hagos et al. [97] obtained an oil content of 43.6% in seeds of Cucurbita maxima, suggesting that variations in oil content may result from various factors such as extraction procedures, extraction solvent, and growing conditions.

Ibeto et al. [98] emphasized that the clarity of oil is advantageous as biodiesel feedstock compared to oils that are very dark in color. Ikanović et al. [86] reported that cold-pressed oil from cucurbit seeds is dark green with a red fluorescence. Karaye et al. [99] reported light yellow oil from seeds of Cucurbita pepo and dark brown oil from Lagenaria breviflora. Ibeto et al. [98] noted yellow oil from seeds of Cucurbita pepo, dark brown oil from Luffa cyilindrica, and light brown oil from Cucumis melo. Hagos et al. [97] reported that cucurbit seed oil contains two colors: green and red.

The economic feasibility of biodiesel production from cucurbit seeds depends on seed yield and oil yield achieved during processing. Winayanuwattikun et al. [100] reported an average oil yield of 235.3 kg ha⁻¹ for Cucurbita moschata. Berényi [90] stated that oil yield in cucurbit seeds could exceed 300 kg ha⁻¹, while Ikanović et al. [86] reported that 300–400 L of oil could be obtained per hectare. Sure et al. [89] achieved oil yields of 100–600 kg ha⁻¹ in their study with Cucurbita pepo var. styriaca.

4.2. Quality of Cucurbits Oil for Biodiesel

For cucurbit seeds to be used as a raw material for biodiesel, they must meet certain quality criteria, such as specific physicochemical properties dependent on the fatty acid composition, iodine values, cetane number of the esterified oil, and saponification number.

Veličković et al. [101] state that, in the oil from Cucurbita maxima seeds, the most abundant fatty acid is linoleic acid (46.1%), followed by oleic acid (29.4%), palmitic acid (14.8%), and stearic acid (6.0%). Berényi and Tulok [102], in their study, report the highest content of linoleic acid (44–46%), followed by oleic acid (36–37%), palmitic acid (11%), and stearic acid (6–7%).

The level of unsaturation of fatty acid methyl esters (FAMEs) is measured as the iodine (I₂) value (g I₂ 100 g⁻¹ oil). The presence of unsaturated fatty acid components in FAMEs is necessary to prevent solidification. If the degree of unsaturation is higher, FAMEs are not suitable for biodiesel because, as unsaturated biomolecules, they react with oxygen and convert to peroxides, resulting in material polymerization into plastic, which can clog the engine at high temperatures during combustion [92]. Hagos et al. [97] report an iodine value of 114 g I₂ 100 g⁻¹ for cucurbit seed oil. Karaye et al. [99] obtained 112.94 g I₂ 100 g⁻¹ for Cucurbita pepo and 104.06 g I₂ 100 g⁻¹ for Lagenaria breviflora in their study. Yusuf et al. [103] and Azam et al. [92] report iodine values of 182.74 g I₂ 100 g⁻¹ and 121 g I₂ 100 g⁻¹ for Cucurbita maxima oil, respectively, while Ibeto et al. [98] and Winayanuwattikun et al. [100] report 150.37 g I₂ 100 g⁻¹ and 95.19 g I₂ 100 g⁻¹ for Cucurbita pepo.

The cetane number (CN) represents the combustion quality of fuel, which is defined as the ignition delay time of the fuel in the engine. A higher CN indicates better fuel quality and shorter ignition delay time. A lower CN leads to incomplete combustion and slower engine heating. An excessively high CN indicates that combustion can occur before air and fuels are correctly mixed, resulting in incomplete combustion and smoke. Biodiesel standards of the USA (ASTM D 6751) [74], Germany (DIN 51606) [104], and the European Organization (EN 14214) have set the minimum limit as 47, 49, and 51, respectively [92]. Karaye et al. [99] obtained CNs of 47.00 for Cucurbita pepo and Lagenaria breviflora, while Azam et al. [92] reported a CN of 47.58 for Cucurbita maxima, and Winayanuwattikun et al. [100] report 51.87 for Cucurbita pepo.

The saponification number (SN) is defined as the amount of potassium hydroxide (KOH) required for saponifying one gram of fat [97]. The same author reports a saponification value of 191 mg KOH/g oil for pumpkin seed oil, while Karaye et al. [99] obtained 36.52 mg KOH g⁻¹ for Cucurbita pepo and 41.78 mg KOH g⁻¹ for Lagenaria breviflora in their oil study. Yusuf et al. [103] and Azam et al. [92] report SN values of 210.38 mg KOH g⁻¹ and 191.5 mg KOH g⁻¹ for Cucurbita maxima oil, respectively, while Ibeto et al. [98] and Winayanuwattikun et al. [100] report 162.69 mg KOH g⁻¹ and 202 mg KOH g⁻¹ for Cucurbita pepo. Biodiesel obtained from oils with high saponification values causes harmful exhaust emissions during combustion in the engine [105].

Density refers to the mass per unit volume of the oil and is a crucial indicator of oil quality for biodiesel [93]. Møller [106] suggests that fatty acids exhibit varying molecular weights, and the inclusion of methyl esters of lauric and myristic acids decreases density [107]. This high density potentially results in a greater volume of fuel within the combustion chamber at the same injection pressure and volume [108]. The biodiesel standard specifies that the fuel should have a density between 860 and 900 kg m⁻³ [109]. Barik et al. [110] state that biodiesel density depends on the fatty acid composition as well as their purity. In a study of three Cucurbita pepo varieties, the most prevalent fatty acids were palmitic, stearic, oleic, and linoleic acids, constituting 96–99% of the total fatty acids [111].

Hagos et al. [97] from Ethiopia reported linoleic (50.7%), oleic (18.8%), palmitic (17.9%), and stearic acid (12.4%) in Cucurbita maxima seed oil. Veličković et al. [101] state that linoleic acid (46.1%) is the most abundant fatty acid in Cucurbita maxima seed oil, followed by oleic acid (29.4%), palmitic acid (14.8%), and stearic acid (6.0%). Berényi and Tulok [102], in their study, report the highest content of linoleic acid (44–46%), followed by oleic acid (36–37%), palmitic acid (11%), and stearic acid (6–7%).

4.3. Methods of Cucurbit Oil Extraction for Biodiesel Production

There are various methods for extracting oil from cucurbit seeds: cold-pressed extraction [86,93,112], solvent extraction [86], supercritical fluid extraction, enzyme-assisted aqueous extraction, and ultrasound-assisted extraction [113].

Before oil extraction, the seeds need to be separated from the pulpy part of the fruit, cleaned, and washed. Then, the seeds are dried, either naturally or in a vacuum oven, at a temperature of 60–75 °C until the moisture content reaches 2% [93,113].

Cold-pressed extraction is a traditional extraction method that uses mechanical pressure to squeeze oil droplets from the seeds [86]. The advantages of this method include low cost, shorter extraction time, no need for chemical solvents, environmental friendliness, and obtaining high-quality oil at a lower temperature [112]. However, the degree of automation is low, and a large number of seeds is required. In addition to cold pressing with screw presses, Ikanović et al. [86] also mention pressing with cylindrical presses after preheating to 120 °C using the so-called “virgin or Styrian oil” process.

Solvent extraction has a high degree of automation, low cost, simple operation, and high oil extraction rate. Solvents such as hexane, ethanol, methanol, chloroform, acetone, isopropyl ether, and petroleum ether are used, with n-hexane being cited as the best solvent in terms of oil yield [96,109,112]. The drawbacks of this method include its high consumption of chemical solvents, a large number of solvent residues (n-hexane), unpleasant odor and taste of the oil, and the fact that it is less environmentally friendly than the previous method.

Supercritical fluid extraction with carbon dioxide (SC-CO₂) is an economical and environmentally friendly technology that produces high-quality oil with reduced extraction time [114]. The disadvantages include high equipment and operating costs and limited-scale application.

Enzyme-assisted aqueous extraction is an environmentally friendly, safe, and harmless method, with a much better quality of the obtained oil than that obtained with solvent extraction [115]. This method uses enzymes such as pectinases, cellulases, or proteases that hydrolyze and degrade the cell walls of pumpkin seeds, increasing permeability. However, the enzymes used in this method are expensive, a large amount of wastewater is generated, emulsion is formed, extraction efficiency is low, and oil yield is low [115].

Ultrasound-assisted extraction is a modern and highly efficient method that is environmentally friendly and uses less chemical solvents [116]. It is based on the cavitation effect [117], disrupting the primary cell walls of the seeds, facilitating solvent penetration into the sample, and accelerating the release of intracellular oil. However, this method has high energy consumption, requires expensive equipment, and achieving a higher industrial production volume with it is challenging [113].

5. Conclusions and Future Perspectives

The harmful environmental impacts associated with fossil fuel production emphasize the urgent need for the development of alternative energy sources. While corn has traditionally served as the primary raw material for biofuel production, its dual role as both a food source and biofuel feedstock raises concerns regarding food security and land use competition. Therefore, the diversification of our raw material sources to include alternative plant materials such as hemp and cucurbits emerges as a promising strategy. Hemp and cucurbits offer several advantages as biofuel feedstocks. They are rich in biomass and contain oils suitable for bioenergy production, providing a renewable and potentially sustainable source of energy. Additionally, these plant species often thrive in diverse climates and require minimal inputs, making them attractive options for biofuel production. Furthermore, the utilization of byproducts or waste from hemp and cucurbits for bioenergy production aligns with the principles of circular economy and waste valorization, contributing to overall sustainability goals.

Beyond land-dependent feedstocks, algae emerge as a promising avenue for third-generation biofuels [118]. Algae, particularly microalgae, boast high lipid content [119], rapid growth rates, and exceptional photosynthetic capabilities [120]. After oil extraction, residual algal biomass serves as a valuable byproduct that is suitable for animal feed, thus enhancing the economic feasibility of large-scale cultivation [121]. Microalgae excel in photosynthesis, harnessing more solar energy than conventional crops and aiding in CO₂ sequestration and wastewater management. However, scaling up algae-based biofuel production necessitates supportive regulations and policies, alongside leveraging algal biomass in biorefineries to produce food, fertilizers, and other valuable co-products, thereby reducing production costs while optimizing biomass utilization [118].

In the realm of biofuels, fourth-generation biofuel production involves the genetic modification of algae to enhance biofuel yield [122]. This step became imperative due to the low yield and high cost of algal biofuel production [123]. Algae engineering in many countries is regulated under the Toxic Substances Control Act [123], while the environmental impact of these organisms is assessed by the US Department of Agriculture (USDA) [124]. Henley et al. [125] asserted that using these organisms in contained bioreactors is safe for large-scale agriculture and the environment. However, genetically modifying algae for biofuel production also occurs in open ponds [126,127], with assessments of potential risks conducted by the Environmental Protection Agency and USDA [128,129]. Studies in open-pond cultivation are a necessary step toward the industrial-scale production of fourth-generation biofuels [123].