Essential Oils and the Circular Bioeconomy

Abstract

The average annual trade of over 250 thousand tons of essential oils generates over 250 million tons of distillation residues, posing environmental problems due to incineration or landfill overburden. The circular economy focuses on reducing resource inputs, waste generation, and pollution, for sustainability. Implementing circular economy principles not only mitigates environmental concerns but also creates economic opportunities by utilizing residual biomass. Nonvolatile secondary metabolites, like flavonoids and phenolic compounds, remain in plant material during essential oil distillation. These bioactive substances can be extracted from the biomass distillation residues. Instead of discarding or burning waste from essential oil production, it can be processed to make extracts. The residue can be converted into biochar, a carbon-rich material beneficial for soil improvement. Other end uses include generating combustible bio-oil and using distillation residues for mushroom cultivation. Circular economy practices in the essential oil agroindustry have implications beyond the field itself. By providing raw materials for various sectors and industries, such as agriculture, cosmetics, and pharmaceuticals, this agroindustry can contribute to broader sustainability goals. While the adoption of circular economy principles presents technological challenges, the potential benefits in terms of waste reduction, value addition, and sustainability justify ongoing research and development efforts.

1. Introduction

An annual average of 264 thousand tons of essential oils (HS code 3301) were exported worldwide during the period 2017–2021 [1]. The actual essential oil production figures should be higher because this annual average does not include the amounts not exported and consumed within the essential oil-producing countries. Since the essential oils are isolated from plant material with typical yields in the range of 0.5%, the yearly byproduct of the traded amounts of essential oils should have been approximately 52 million tons of vegetal material. The disposal of these large amounts of material leads to serious environmental problems associated with river pollution, incineration, or landfill overburden. Alternatively, this residual biomass may become a valuable source of products with reduced environmental impact if a highly recommended approach, the application of the principles of circular economy, is used to add value, generate energy, and reduce waste to zero. The technological challenges are massive, but the potential benefits of following the latter principles have been the subject of many scientific articles during the past decade, not only around essential oil production but also in many different trade sectors. Waste and byproducts from food and agro-industries may no longer be associated with pollution and could be transformed and used to fight hunger and malnutrition. This chapter presents examples of the application of circular economy principles at laboratory and production scales in the essential oil agroindustry.

2. Circular economy

The circular economy approach is a closed-loop system that aims to reduce the use of resource inputs, waste generation, pollution, and carbon emissions. It is a vision toward a sustainable society with great responsibility in the production of biomass and end-of-life product recovery. Reduction, repair, remanufacturing, and recycling are normally acknowledged as the representative loops of the circular economy [2]. Some authors refer to these as holistic approaches to add value to biowastes from fruit and vegetable processing [3], of which aromatic plants represent a small percentage. The goal of adding value and reducing waste to zero may invoke participation not only in physical processes related to extraction but also in chemical transformations (hydrolysis), fermentations, and bioprocessing with microorganisms. Some traditional or conventional extraction methods are not recommended due to time consumption, intensive labor demand, their use of large amounts of organic solvents, or elevated temperatures that degrade some compounds of interest. Alternative emerging technologies are faster and have reduced environmental impact. They include processes such as supercritical carbon dioxide extraction, subcritical water extraction, ultrasound mixing, microwave heating, electric pulse discharge, and enzymatic hydrolysis, which frequently lead to improved yields [4]. However, there is still a large room for improvement in scaling up their operation and changing from batch to continuous operation [5].

Plant secondary metabolites do not participate in a direct manner in basic functions such as growth and development but are very important for plant survival. As part of the plant’s secondary metabolites, essential oils attract pollinators, reduce abiotic stress, and protect plants from pests and herbivores, among other direct and indirect roles. Secondary metabolites of lower volatility are not removed from plant material when essential oils are produced, which mostly happens through distillation. These other secondary metabolites include flavonoids, catechols, phenolic compounds, and other nonvolatile bioactive substances that may be recovered from the plant material to become valuable constituents of products for human well-being. A circular economy operation aims to take full advantage of essential oils, nonvolatile secondary metabolites, and secondary metabolite-depleted biomass.

Figure 1 summarizes common processing options applied to the essential oil value chain within the circular economy approach. The first step is the retrieval of essential oil from plant material, due to its exposure to steam or mechanical compression (in the case of citrus). After a condensation step (in the case of steam distillation), the essential oil is separated by simple decantation from the aqueous phase, hydrosol, which retains small amounts of some essential oil components that are not completely hydrophobic. The residual biomass enters a sequence of value-addition steps that have several potential final products and no waste. A very useful first task is the use of some extraction agent (ethanol, ethanol-water, CO₂) to remove waxes, pigments, flavonoids, and polyphenols from the residual biomass combined into a mixture known as the extract. This complex mixture may be further processed to obtain various fractions that are enriched in certain bioactive compounds. Composting, biochar generation, and biogas, or ethanol production from fermentation, are examples of several value-addition processes that may be applied to the lignocellulosic material that remains after solvent extraction. Thus, the circular economy version of the essential oil value chain has zero waste and several products, such as essential oil, hydrosol, extract, biofuel, biochar, and compost.

Figure 1.

Summary of the processing steps for the complete use of vegetal material in the essential oil agroindustry in observance of the circular economy principles.

3. Hydrosol

Essential oils are decanted from the condensed water used as steam to separate them from the plant material. This condensed water, called hydrosol, hydrolat, distillation wastewater, or floral water, is obtained in amounts at least 50 times larger than the decanted essential oil and contains polar and hydrophilic essential oil components. A comparative study of the compositions of 44 hydrosols and their essential oils showed that in almost half of the hydrosols, the major component was different from that of the oil. Due to solubility differences, the concentrations of these organic substances are much smaller in hydrosol than in essential oil [6]. Some substances found in the hydrosol may result from molecular rearrangements caused by heating during distillation. For example, linalool and α-terpineol were found at higher concentrations in hydrosol than in Lavandula angustifolia essential oil [7].

There is a growing recognition of hydrosols as essential oil coproducts with many applications. Mentha pulegium and Mentha suaveolens hydrosols have shown high insecticidal effects against Toxoptera aurantii, a citrus pest [8]. A study on the control of the Myzus persicae aphid pest showed that the application of Melissa officinalis or M. pulegium hydrosols on eggplant leaves had an inhibitory effect, while the use of Origanum marjorana hydrosol caused 10–15% mortality after 24 h [9]. These and many more recent reports support the increased use of hydrosols in biological agriculture against mushrooms, mildew, and insects. The hydrosol from Cuminum cyminum seeds has caused decreased hatching of root-knot nematodes, a widely spread pest of many plantations [10].

Hydrosols are increasingly recognized as sources of natural ingredients for cosmetic, nutraceutical, and food applications. One important biological activity in this respect is antioxidant capacity. It was found that hydrosols from basil, sage, and rosemary wastes of packaged fresh aromatic plant production contained caffeic acid derivatives, glycosylated luteolin, and other flavonoids, whose presence was manifested in their strong antioxidant capacity, similar to that of pomegranate juice and higher than that of red wine and green tea [11]. Bactericidal activity is also important for certain applications. A study of the hydrosols of basil, cardamom, clove, cinnamon, and thyme showed that they produced inhibitory effects against Salmonella typhi, Staphylococcus aureus, and Escherichia coli [12].

4. Extract

The residual biomass from the distillation of aromatic plants to obtain essential oils contains valuable components such as carotenoids, carbohydrates, lipids, flavonoids, and phenolic acids within a lignocellulosic matrix. The current destiny of most distillation residues is a garbage dump. When this is not the case, common applications are the transformation into biofuel (as raw lignocellulosic bagasse, or with further processing into biochar) or as a major ingredient for composting operations. However, an important and profitable step may be inserted before these treatments to take advantage of the presence of flavonoids and other bioactive molecules in these residues. Extraction techniques may be employed to obtain fractions enriched in these bioactive substances. Maceration, Soxhlet extraction, microwave-assisted extraction, ultrasonic-assisted extraction, supercritical extraction techniques, pulse electric field extraction, enzyme-assisted extraction, molecular distillation, and accelerated solvent extraction are some of the tools employed to obtain extracts from plant materials. Further processing may involve ultrafiltration, nanofiltration, membrane filtration, supercritical antisolvent fractionation, or other physical processes that help to separate extract components according to molecular size. Rosemary (Salvia rosmarinus) is a medicinal and aromatic herb that contains bioactive compounds (carnosic acid, carnosol, rosmarinic acid) of interest in many fields. Many of the previously mentioned techniques have been applied to fresh and residual rosemary biomass and this collective effort permits the comparison of these techniques and their operating conditions according to yield, extract stability, costs, and energy demands [13].

The efficient recovery of bioactive compounds from the distillation residual biomass is an important subject of current research in the field of aromatic plants. A search of scientific publications from 2000 to July 2023 on “aromatic plant” in Scopus produced 28,973 results, more than 25% of which (6556) included the word “waste.” Many works have determined the compositions of extracts obtained from the biowaste of individual aromatic species at the laboratory scale with various techniques. Polyphenols are frequent constituents, and their presence is related to the antioxidant or antimicrobial activities determined for the extracts. It is now clear that the solid wastes from essential oil distillation are rich sources of bioactive molecules with applications in the cosmetic, food, pharmaceutical, hygiene, and other industries. The challenges are scaling up the extraction processes and developing extract fractionation processes toward enrichment in specific sets of compounds or isolating individual substances. The following are examples of the general situation of the addition of value to aromatic plant distillation waste through the production of extracts.

The term “polyphenols” designates phenolic substances that contain several hydroxyl groups but does not imply that they have a polymeric nature. A more general denomination is “phenolic compounds.” They include hydroxybenzoic acids, hydroxycinnamic acids, phenylpropanoids, coumarins, flavonoids, and phenolic terpenes [14].

Rosa alba waste from steam distillation and supercritical CO₂-extracted fresh flowers and steam distillation wastes from L. angustifolia, M. officinalis, and Ocimum basilicum essential oil production were dried at 50°C, ground, sieved (0.5 mm), and macerated with 70% ethanol (1:6 w/v) at 60°C (1 h) and room temperature (24 h). The CO₂-extracted Rosa alba afforded the extract with the highest polyphenol content (11 g/L), followed by melissa waste (6.6 g/L). Flavonoids such as rutin (1.2 g/L), catechin (1.1 g/L), and quercetin-3-glucoside (0.7 g/L) were quantified in these extracts using HPLC-DAD. Basil waste produced the extract with the highest rosmarinic acid content (1.2 g/L). Other phenolic acids, such as gallic, ferulic, and 3,4-dihydroxybenzoic acids, were found in the extracts at concentrations between 0.1 and 0.6 g/L. Thus, the distillation wastes from these aromatic plants are a rich source of polyphenols that could be used as supplements to increase antioxidant activities in food [15].

Ultrasonic agitation (37 kHz, 30°C) and 70% methanol were used to obtain extracts from residual distillation biomass of six aromatic species (M. officinalis, Mentha spicata, Origanum vulgare, Salvia fruticosa, S. rosmarinus, and Satureja thymbra). LC-MS analysis of these extracts identified a total of 48 compounds, including 20 phenolic acids, 26 flavonoids, and 2 phenolic diterpenes. Phenolic acids varied from 3817 mg/100 g (S. rosmarinus) to 14,462 mg/100 g (M. spicata). Flavonoids varied from 747 mg/100 g (S. fruticosa) to 3112 mg/100 g (Satureja thymbra) [16]. Rosmarinic acid is a frequent component of residual biomass extracts, determined at concentrations between 0.7 and 154 mg/g of extract [17]. Its higher concentrations have been reported in residual biomass extracts of M. officinalis (93.3 mg/g), M. spicata (96.6 mg/g), and Thymus vulgaris (105 mg/g) [18].

Residual rosemary hydrodistillation biomass was dried, ground, and extracted with ethanol under ultrasonic agitation. LC-MS analysis showed that 60% of the chromatographic area was represented by carnosol (35.6%), carnosic acid (12.1%), cirsimaritin (9.1%), and genkwanin (4.7%). The absence of rosmarinic acid was attributed to its thermal degradation evidenced by the presence of caffeic acid (0.9%) in the extract, and at its dissolution in the hydrosol [19]. The antioxidant activity of this extract was high, similar to that of the extract obtained from red grape pomace. It had insect antifeeding effects on Leptinotarsa decemlineata Say (Coleoptera:Chrysomelidae) (polyphagous/olyphagous chewing insects) and the aphid Myzuspersicae sulzer (Hemiptera:Aphididae).

The unfractionated rosemary extract can be used as an antioxidant or as a natural crop protectant, among many other applications. However, there are continuous efforts to isolate its main components or to obtain enriched fractions. Increased yields of ursolic acid (15.8 mg/g), rosmarinic acid (15.4 mg/g), and oleanolic acid (12.2 mg/g) were obtained in ultrasound-assisted extraction by varying the pH, ethanol%, temperature, and solvent:solid ratio [20].

Thyme (T. vulgaris) is a common aromatic plant used in the traditional medicine, pharmaceutical, and food industries [21]. Residual biomass from thyme distillation was macerated with 75% ethanol (1:10 w/v) at room temperature for 1 day. The extract was obtained with a 3.85% yield with a phenolic acid content of 62 mg of gallic acid equivalents per gram of dry extract. LC-MS analysis revealed that its main components were rosmarinic acid (105 mg/g) and rutin (87 mg/g) [18].

Culinary herbs of many cultures include some types of oregano, of which there are several species from various origins. Carvacrol and thymol are the most common oregano essential oil components and are also the main contributors to the bioactive properties of this herb. The variability in origin and habitat is reflected in the reported 20-fold variation in carvacrol content found in comparisons of essential oils obtained from different oregano species [22]. Mexican oregano is mainly represented by Lippia graveolens, whose essential oil is rich in thymol in carvacrol. The analogous species in northern South America is Lippia origanoides, which in Brazil is more commonly recognized as Lippia sidoides, a synonym according to genetic studies [23]. Similar to other species, there are many L. origanoides and L. graveolens chemotypes, which are distinct populations within the same species with different secondary metabolite profiles. There are reports of at least five L. origanoides chemotypes, some of which have no thymol or carvacrol in their essential oils [24, 25].

One approach to the complete utilization of L. origanoides categorizes essential oil and hydrosol as products of the distillation process and directs the residual plant material to various purposes, such as extraction, composting, or combustion material for steam generation. A patent has been granted for this process, specifically applied to L. origanoides [26].

Extraction with ethanol-modified supercritical CO₂ of the residues from thymol-rich L. origanoides distillation afforded a resin that contained 20 g of flavonoids/kg. When applied to the phellandrene-rich L. origanoides chemotype, oleoresin contained 31 g of valuable pinocembrin/kg [27]. This relatively large content of pinocembrin motivated further studies on solubility [28] in CO₂, and mass transfer models [29]. A recent report of this work showed that the use of two coexisting fluid phases with various proportions of ethanol, water, and CO₂, afforded an extract containing 145 g pinocembrin/kg, which is approximately a fivefold increase relative to the concentration obtained with ethanol-modified CO₂ [30].

5. Soil amendments

The pyrolysis of biomass under low oxygen conditions produces biochar, which is a carbonaceous material with agricultural applications due to its porous surface and nutrient content. The estimated annual residual biomass from the distillation of Mentha arvensis, Mentha citrata, and M. piperita is 10.5 thousand million tons [31]. These wastes are burned or composted, but both approaches face implementation problems. The use of aromatic plant biomass has been associated with antigerminating attributes upon composting [32]. Proper burning to secure complete combustion requires a considerable initial investment. An alternative to burning and composting is a two-step sequential approach in which solvent treatment of the residual biomass is used to obtain an extract with antioxidant capacity, and the plant material is subsequently heated under an inert atmosphere to obtain a biochar. The biochar is used for soil amendment [33].

An interesting alternative approach that reduces investment and operating costs is to involve the distillation plants themselves in biochar production. The ash pit of a distillation plant with 500 g plant material capacity was used to hold stainless steel boxes (25 L) that contained biomass waste (10 kg) and was provided with eight holes for the release of volatile compounds. A residence time of 2 h was suitable to produce biochar in up to 60% yield and 0.26–1.6 g/m³ density [34].

Crop cultivation leftovers from pruning and plant selection at a commercial production of fresh basil, rosemary, and sage were used for essential oil and hydrosol production and the distillation residue was employed in on-farm composting. Temperature monitoring showed that 30 days was sufficient to obtain usable compost [35].

6. Other end uses

Combustible bio-oil was obtained by pyrolysis of Cymbopogon flexuosus distillation wastes in a fixed-bed reactor. Due to the low lignin content of the biomass, the bio-oil had a lower content of phenolic compounds than those obtained from other biomasses. It also had low concentrations of polyaromatic hydrocarbons and nitrogenous compounds. Combustion heat in the order of 30 MJ/kg was measured for this bio-oil [36].

The use of lavender (L. angustifolia Miller) distillation residues in mushroom production was examined by cultivating Pleorotus ostreatus on substrates composed of shredded lavender waste and barley straw, maintained at 24°C and 65–70% humidity for 50–60 days. Once the mushrooms were removed, the exhausted substrate was used as a soil conditioner or to produce biocompost. The entire mushroom production process was established to be cost-effective [37].

7. Citrus

Citrus fruits constitute a separate case because their essential oils are normally obtained by mechanical compression of the peel, not by steam distillation. Citrus fruits are the largest fruit crop in the world (over 100 million tons per year) [38] and their main destiny is the food industry. After processing, the peels, seeds, and membrane residue represent approximately 60% of the original fresh fruit mass. Essential oil is present in the colored exterior part of the peel, called the flavedo. The main component of citrus essential oils is limonene (above 80%, depending on the species). Citrus essential oils have antioxidant, antidiabetic, insecticidal, antifungal, and antibacterial properties, which make them valuable ingredients for applications in the pharmaceutical, sanitary, cosmetic, agricultural, and food industries [39]. The inner walls of the citrus fruit peel (albedo) are rich (~30%) in pectin. Pectin is a mixture of acid and neutral-branched polysaccharides that contain glucuronic acid. Pectin is used as a thickener, emulsifier, gelling agent, or fat substitute in the food and beverage industry. Citrus peel also contains flavonoids for which antioxidant, anticancer, anti-inflammation, and cardiovascular protection activities have been reported [40]. Thus, processing citrus fruit residual material with a zero-waste approach may lead to value addition through the isolation of essential oil, pectin, flavonoids, and other bioactive molecules.

The most common primary approaches to treat citrus fruit waste involve essential oil separation, followed by the use of the residue for composting and for animal food. Some of the limiting factors to using citrus peel waste in composting are its low nitrogen content, and its antimicrobial properties (associated with limonene), which have a negative impact on soil microorganisms. Animal food is another potential use of citrus peel waste, but there have been mixed positive and negative experiences. A study in which broiler finisher birds were fed with increasing amounts of orange peel concluded that the inclusion of sweet orange peels had adverse effects on the growth rate and nutrient utilization by the birds [41]. This was attributed to the decrease in feed intake because the compounded diets became unpalatable upon the inclusion of sweet orange peels, some of them with certain fermentation levels. The processing of citrus peel waste through anaerobic digestion and consecutive fermentations may lead to the isolation of several bioactive products, but it requires the previous removal of limonene, which can inhibit microbial activity [42].

The essential oil in the peel of citrus fruits may be isolated by mechanical pressing (cold press), which is the most common method, although higher yields are obtained with hydrodistillation and steam distillation. The latter have higher costs due to the energy needed, but when complete biomass utilization is the goal, these techniques have the advantage that the residual biomass has low amounts of limonene and can be subjected to liquid fermentation without further processing. An interesting alternative is microwave-assisted distillation, which due to its shorter duration has lower total energy costs. In one of its implementations, the moisture content of the vegetal material is sufficient to absorb the supplied microwave energy, and no additional water is needed. This lowers operation costs and the volume of the treatment chamber required [43].

8. Conclusions

The examples presented outline the importance of embracing circular economy principles within the essential oil agroindustry to mitigate environmental problems and enhance sustainability. The substantial amounts of residual biomass generated during essential oil production can be transformed into valuable resources through innovative approaches and advanced technologies. The circular economy approach offers a solution to the environmental challenges posed by residual biomass from essential oil production. Advanced extraction techniques play an important role in recovering bioactive compounds from residual biomass. These compounds, such as flavonoids and phenolic acids, have diverse applications in industries such as cosmetics, pharmaceuticals, and food. Hydrosols, byproducts of essential oil distillation, have emerged as valuable coproducts because their insecticidal, antioxidant, and antibacterial properties make them versatile assets in various sectors. Circular economy principles extend to the utilization of residual biomass for producing biochar and compost. These products have applications in soil conditioning, agriculture, and energy generation, contributing to waste reduction and resource efficiency.

In conclusion, the integration of circular economy principles in the essential oil agroindustry holds immense potential to alleviate environmental concerns and contribute to sustainable development. By extracting maximum value from residual biomass, the industry can create a more environmentally friendly and economically viable production process. Through innovative techniques, collaboration across sectors, and ongoing research, the vision of circular and sustainable essential oil agroindustry can become a reality.

Acknowledgments

The authors are thankful for the funding provided by Ministerio de Ciencia, Tecnología e Innovación, Ministerio de Educación Nacional, Ministerio de Industria, Comercio y Turismo, and ICETEX, Programm Ecosistema Científico-Colombia Científica from Fondo Francisco José de Caldas, Grant RC-FP44842-212-2018, and the General Royalties System, BPIN-2018000100044.

Conflict of interest

The authors declare no conflict of interest.