Dual process for improved recovery of essential oil and pectin from citrus waste
Abstract
BACKGROUND
Citrus essential oils (EO), particularly orange essential oil (OEO) and pectin, are high-value chemical products widely used across various industries, including pharmaceuticals, food and beverages, detergents, and cosmetics. These compounds are present in the residual by-products generated by juice production industries, and their recovery and valorization have been the subject of research for years. Methods ranging from traditional extraction processes to emerging technologies have been reported. This study presents a novel, sustainable method for utilizing citrus waste from a processing company in Mexico. It is based on a sequential treatment involving pulsed electric fields (PEF) and reactive extrusion on a twin-screw extruder.
RESULTS
The twin-screw extruder was modified through the integration of modular components and two side filters for the separation of OEO and pectin. With a brief PEF exposure of just 40 μs at a low field intensity of 1 kV cm−1, the extraction yield of OEO from the raw waste increased by approximately 50% compared to the untreated control. The quality of the extracted oil was confirmed using gas chromatography. Simultaneously, 43% of the pectin was recovered through the extruder's side streams.
CONCLUSION
The proposed sequential PEF and reactive extrusion treatment is a novel and competitive approach for the simultaneous extraction of essential oil and pectin from citrus industrial waste with competitive values as compared to reports in the literature. Further optimization of operating conditions and detailed characterization of the extracted pectin are recommended to enhance the process efficiency and product quality. © 2025 The Author(s). Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI).
INTRODUCTION
The extensive use of essential oils (EOs) in personal care, food and beverages, flavorings, pharmaceuticals, aromatherapy, perfumery, and detergents has led to a growing industry. This growth is largely driven by increasing demand for cosmetics and skincare products, alongside a consumer preference for natural and organic ingredients over synthetic alternatives.
Orange essential oil (OEO) has become a key ingredient in skin and hair care formulations due to its natural antiseptic and antibacterial properties. It is also known for its anti-inflammatory activity and excellent blending capabilities, making it an important component in various formulations across multiple industries.
In parallel with the rising demand for OEO, orange juice production has also seen consistent growth in recent years. In Mexico, the citrus industry is well established and is currently among the largest citrus producers globally, accounting for 4.6% of total global production. In 2021, Mexico became the fourth-largest orange producer in the world.1
Additionally, orange peels are rich in pectin – a valuable product widely marketed in large volumes. This versatile ingredient is extensively used in industries such as food, beverages, cosmetics, and pharmaceuticals.2 In the personal care and cosmetics sectors, citrus pectin has gained popularity for its ability to stabilize skin feel, support formulation, and balance pH levels. Furthermore, citrus pectin serves as a healthy, soluble fiber with probiotic properties that help regulate blood serum cholesterol.
Both OEO and pectin have significant commercial value. The global market for citrus EO was valued at $6.3 billion in 2018, with a projected growth rate of 6.8% from 2019 to 2025. In 2023, the orange oil (OEO) segment led the citrus oil market, capturing a 29.7% share.3 In contrast, the global pectin market reached a value of $117 billion in 2022 in the USA alone, with citrus pectin experiencing a compound annual growth rate of 5.1% between 2018 and 2022.4
However, industrial citrus processing generates substantial amounts of organic, biodegradable semi-solid waste – composed mainly of peels, pulp, and other fruit parts where EOs and pectin are concentrated. This waste is not always properly treated, resulting in environmental issues such as the production of leachates with high biological oxygen demand, which can contaminate aquifers and boost the proliferation of harmful fauna when stored in open air.
Given these environmental and economic factors, utilizing industrial orange residues to extract OEO and pectin – both of high commercial value – presents an attractive opportunity to enhance industrial efficiency and sustainability.5, 6
Several methods have been reported for extracting EO from citrus sources, including distillation, cold pressing, microwave-assisted distillation, and supercritical extraction.7 While these methods yield products of varying quality, they often involve high infrastructure and operational costs. Common approaches such as steam distillation and cold pressing tend to result in low yields or thermal degradation of key components.
As an alternative, extractive extrusion is an innovative technique gaining attention for recovering EOs from citrus peels. This method combines mechanical pressing and solvent extraction (typically using water) in a continuous process, thereby improving efficiency and yield. Solvent removal through centrifugation, distillation, or evaporation produces the purified EO.8
Additionally, pulsed electric field (PEF) extraction is an emerging non-thermal technology that enhances the release of EOs from plant material. It works by applying short bursts (microseconds to milliseconds) of high-voltage electric pulses (typically 1–50 kV cm−1) in a conductive medium (usually water). This increases the permeability of plant cells – such as those in tomatoes, kiwis, or citrus peels – facilitating the extraction of bioactive compounds, including EOs.9, 10
This report proposes a practical, economical, and efficient method to promote a more sustainable citrus industry. The innovative process for extracting OEO and pectin involves two treatment stages that require minimal water and energy and offer short processing times. First, industrial citrus waste is subjected to a series of PEF treatments that weaken cellular structures to promote EO and pectin extraction. This technique has been validated by our research group for enhancing oil extraction from dry pecan nuts.11 In the second stage, the material is processed using a twin-screw extruder operating continuously. Previous results from our group have shown that the EO extracted from grapefruit residues using this method meets all commercial quality standards.8
EXPERIMENTAL
All reagents and compounds used in this study were purchased from commercial suppliers in Mexico. Waste orange peels were provided by Citrofrut (Monterrey, Nuevo León, Mexico). The PEF treatments were conducted using a pilot-scale unit (EPulsus-LPM1A-10 system), equipped with a 1 L treatment chamber and parallel electrodes (Fig. 1). This equipment is located at the facilities of the Instituto Tecnológico y de Estudios Superiores de Monterrey (ITESM), Campus Querétaro, Mexico.
The extraction process was carried out using a laboratory-scale twin-screw extruder (Evolum 25, Clextral, Firminy, France), located at the School of Chemistry, Universidad Nacional Autónoma de México (UNAM, Mexico City, Mexico). The extruder features two parallel, corotating shafts housed within a 100 cm cylindrical barrel composed of ten contiguous modules. The basic design and operation of this equipment have been previously described.8 In the initial phase of the project, a filter unit was integrated into module 4 to separate EO in the filtrate. Modifications made later to this arrangement for pectin recovery, along with their rationale, are detailed in the Results section. A control experiment processed in the extruder but not subjected to PEF was used for comparison.
The EO obtained was characterized using gas chromatography. The equipment (Ellutia GmbH & Co., Kassel, Germany) was fitted with a 30 m ZB-5 column (Phenomenex, Torrance, CA, USA) and a poly(5% diphenyl/95% dimethylsiloxane) stationary phase. Detection of volatile compounds in the oil was performed according to previously described protocols.8
The fibrous insoluble components (cellulose and lignin) and soluble fibers (hemicellulose and pectin) in both the raw material and the processed streams were analyzed using the acid detergent fiber (ADF) and neutral detergent fiber (NDF) methods.12, 13
Pectin quantification was based on previously reported methods for orange peel.14 This protocol was optimized during the project as follows: 300 mg of material was placed in a boiling flask equipped with a reflux condenser, followed by the addition of 30 mL of a 1.25% w/v aqueous citric acid solution to adjust the pH to 4. The mixture was boiled under magnetic stirring for 30 min. The solids were removed by vacuum filtration using a porous glass filter and discarded. The filtrate was transferred to a 125 mL flask, cooled for 30 min, and mixed with 50 mL of a 50% v/v cold ethanol solution. The flask was then stored in a refrigerator to induce pectin precipitation. The precipitate was filtered using a No. 80 stainless steel mesh, dried in an oven at 50 °C for 12 h, and then weighed.
RESULTS
Fresh orange peels, cut into 1 cm2 pieces, were dehydrated in a convection oven at 60 °C until they reached an equilibrium moisture content of 25% (wet basis) (Fig. 2).
The drying conditions used in this study were selected based on previous literature, as the dehydration of orange byproducts has been extensively investigated. For example, convective air drying at 60 °C required approximately 315 min to reduce the moisture content of orange peel to below 10%.15 Another study demonstrated that hot-air impingement drying at 65 °C was effective in preserving polyphenols in orange peel.16 Additionally, optimal conditions for drying in a forced convection oven were found to be 52 °C with an air flow rate of 2.0 m s−1, minimizing hardness, moisture content, and drying time to achieve water activity values below 0.6.17 These studies collectively provide valuable insights into the drying kinetics and physicochemical characteristics of orange peels, emphasizing the critical role of temperature and drying method in shaping the drying curve and preserving bioactive compounds.
Due to the seasonal availability of industrial raw materials associated with the harvest period, initial preliminary tests were conducted using red grapefruit peel. The dehydrated material was ground in a ball mill to enhance solid–liquid contact and then subjected to PEF treatment prior to further processing in the extruder.
A total of nine PEF treatments were applied to 100 g samples of grapefruit peel, varying the electric field intensity (1.0, 3.0, and 5.6 kV cm−1) and treatment duration (40, 400, and 800 μs). Each treatment used monopolar pulses with a square wave form, a pulse width of 4 μs, and a frequency of 200 Hz. The full factorial design comprising these nine treatment combinations is presented in Table 1.
| Experiment number | Field intensity (kV cm−1) | Pulses | Pulse width (μs) | Treatment time (μs) | Frequency (Hz) |
|---|---|---|---|---|---|
| 1 | 5.6 | 100 | 4 | 400 | 200 |
| 2 | 3.0 | 100 | 4 | 400 | 200 |
| 3 | 5.6 | 10 | 4 | 40 | 200 |
| 4 | 1.0 | 100 | 4 | 400 | 200 |
| 5 | 3.0 | 200 | 4 | 800 | 200 |
| 6 | 1.0 | 10 | 4 | 40 | 200 |
| 7 | 5.6 | 200 | 4 | 800 | 200 |
| 8 | 3.0 | 10 | 4 | 40 | 200 |
| 9 | 1.0 | 200 | 4 | 800 | 200 |
Samples obtained from each PEF experiment were processed in the extruder at room temperature. A water/solids/oil emulsion was collected through a filter located in module 4 of the extruder and subsequently subjected to hydrodistillation. The EO yields obtained from grapefruit peels are presented in Fig. 3. In all cases, the extracted oil was transparent, with a slight yellow-orange tone, closely resembling the appearance of commercial grapefruit EO.
The results indicate that applying PEF for a short duration of 40 μs at 3.0 or 5.6 kV cm−1 increased the EO extraction yield from grapefruit peel by 55–83%. A similar enhancement was previously reported for orange peel using a field intensity of 5 kV cm−1 and the same treatment duration.10 These results reinforce the effectiveness of PEF pretreatment as a non-thermal, low-energy method to enhance EO extraction from citrus waste. Based on these findings, PEF pretreatments at 40 μs across the three field intensities were applied to industrial orange juice waste material, as orange is the primary product of interest for manufacturers and marketers. The subsequent OEO extraction process was carried out as previously described, and the results are presented in Fig. 4.
In this case, no significant impact of the electric field intensity on OEO extraction yield was observed across the range tested for this specific industrial byproduct.
In the next stage of the study, the industrial orange peel waste was supplied in a frozen state. This approach ensured consistent batch quality throughout the research and helped prevent degradation during transport and storage. A PEF treatment (1.0 kV cm−1, 40 μs) was applied to both 100 g and 200 g samples of frozen orange peel, followed by EO extraction via extrusion.
The results (Fig. 5) revealed a synergistic effect between freezing and PEF pretreatment. Compared to frozen control samples without PEF, the PEF-treated samples showed an increase in oil yield ranging from 36% to 48%. This improvement is likely due to the formation of ice crystals during freezing, which physically disrupt cell membranes. When combined with electroporation from PEF, this enhanced cell damage appears to facilitate mass transfer, significantly enhancing extraction efficiency.
Based on the results, the optimal sequence of steps to enhance oil yield was as follows: (i) freezing of orange peels to induce ice crystal formation; (ii) PEF treatment at 1 kV cm−1 for 40 μs using monopolar square wave pulses (4 μs width, 200 Hz); and (iii) extrusion under previously defined standard conditions, followed by recovery of EO.
The OEO recovered from the filter in module 4 was further purified by hydrodistillation. The retained solids from the filtration process, designated as solids 2 (refer to Fig. 7) were collected via coarse cloth filtration. The corresponding filtrate was identified as lixiviate 3. Both output streams were subsequently analyzed, and the results are discussed in later sections of this report.
To facilitate the extraction of soluble fiber in the form of pectin, a second filtration unit (filter 2) was added in module 9 (see Fig. 6). This configuration enables simultaneous dual extractive extrusion – for both EO and soluble fiber – within a continuous operation. To support this process, compressor–shear type screw elements were installed in module 10, creating a dynamic pressure plug that increased internal pressure and forced the suspension through the filter.
Based on our previous experience, a diluted citric acid solution was introduced into module 5 of the extruder, based on the concept that a weak acid reduces the pH of the output streams to approximately 4, which facilitates the subsequent alcoholic precipitation of pectin. Additionally, citric acid is industrially produced via fermentation by Aspergillus niger, which adds a sustainable aspect to the process by aligning with bio-based production principles.
With this setup, the suspension collected from filter 2 was strained using a coarse cloth to separate the solid and liquid fractions. The retained solids were designated as solids 4, while the filtrate was labeled lixiviate 5. Both streams were subsequently analyzed to determine their pectin content. The complete flow diagram of the integrated extrusion process, including the strategic positioning of both filtration modules and the resulting output streams, is presented in Fig. 7.
Table 2 presents the results obtained using the integrated process configuration shown in Fig. 7. The data confirm an approximate 30% increase in OEO extraction yield from frozen orange peel pretreated with PEF, compared to the untreated control. These findings highlight the effectiveness of PEF as a pretreatment step, enhancing the efficiency of extractive extrusion for EO recovery.
It is important to note that these results correspond to an industrial residue containing all the material derived from the extraction of orange juice, with a high proportion of albedo material rich in lignin, cellulose and polysaccharides, seeds and other components. Other studies were carried out with non-stressed fresh orange peels with OEO values in the range of 6.2–1.16%.18
In various studies of extraction of EO from citrus peels, using hydrodistillation, yields ranging from 0.46% to 2.5% (v/w) have been reported, with processes at temperatures above 100 °C, and times of 1–4 h.19-25 The use of microwaves in combination with hydrodistillation has managed to reduce process times to 30 min and lower temperatures with yields ranging from 0.58% to 2.47%.26-29 However, the reduction of processing times and temperatures of the application of microwaves does not generate significant increases in the extraction performance of the EO. Other methods such as steam explosion can enhance EO yields from fresh citrus peels compared to traditional techniques like simple hydrodistillation or Soxhlet extraction.30 However, these methods are costly to scale up, operate in intermittent batches, and may lead to the loss of important functional properties due to exposure to high temperatures. It can be seen in Table 2 that the yield obtained by combining PEF with extrusion is 2.17% – a value close to the maximum yields reported in previous studies. When the orange peels are frozen prior to PEF and extrusion treatments, yields of 3.23% are achieved. These results reflect the relevance of the processes evaluated in this study to generate yields in the extraction of EO similar to or greater than those reported in other studies.
| Weight at feed 1 (g) | OEO after hydrodistillation (g) | Yield mg OEO g−1 residue (dry weight) | |
|---|---|---|---|
| Control | 760 | 1.65 | 2.17 |
| PEF | 978 | 3.16 | 3.23 |
To assess the quality of the obtained OEO, the volatile components were analyzed using gas chromatography. The resulting profiles, compared with those of OEO extracted from untreated frozen orange peels, are shown in Fig. 8. No significant changes in composition were observed, indicating minimal impact on the quality of the final product and preservation of its original characteristics during fractionation. Notably, the concentration of limonene – 94.8% in raw orange peels and 89.1% in those subjected to dual treatment (PEF + extrusion) – remains high – an important quality parameter since limonene is considered the primary component in citrus EOs. Another important finding is the absence of β-pinene – a compound known to cause skin irritation.
For the mass balance of the process, Table 3 present the solids content of the lateral streams (lixiviate 3, solids 2, and solids 4), as well as the final extrudate (stream 6). For the PEF-treated samples (978 g dry weight in feed 1), and the control samples not subjected to PEF (760 g dry weight in Feed 1) recovery of solids is practically complete.
| Filter 1 | Filter 2 | Final extrudate 6 | Total mass (g) | Total recovery | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Solids 2 (g) | Recovery | Lixiviate 3 (g) | Recovery | Solids 4 (g) | Recovery | Lixiviate 5 (g) | Recovery | Weight (g) | Recovery | |||
| Control 1 | 138 | 18% | 43 | 6% | 64 | 8% | 42 | 5% | 465 | 61% | 752 | 98% |
| PEF | 205 | 21% | 45 | 5% | 81 | 8% | 30 | 3% | 615 | 63% | 976 | 100% |
Table 4 presents the fiber content in the final extrudate (stream 6). The ADF values indicate that the insoluble fiber content (comprising cellulose and lignin) is similar for PEF sample and the control, indicating that the insoluble fiber is not modified during the extraction process. In terms of soluble fiber, since pectin is largely retained during the NDF treatment, a slight increase in soluble fiber content (pectin plus hemicellulose) was observed, as indicated by a 2% difference in the NDF–ADF value, a difference not observed for the control sample.
| Neutral and acid fiber at Final Extrudate 6 | ||
|---|---|---|
| NDF (%) | ADF (%) | |
| Control | 26.9 | 27.2 |
| PEF | 30.2 | 28.2 |
Finally, the material balance for pectin is presented in Table 5. Approximately 43% of the pectin was recovered in the side streams (solids 2, solids 4, and lixiviate 5), allowing for efficient separation via alcoholic precipitation. When accounting for the pectin present in the final extrudate (stream 6) – although it is mixed with a significant portion of insoluble fiber – the overall recovery approaches 100%.
| Pectin weight (g dry solids) | |||
|---|---|---|---|
| Feed 1 | 308.1 | % Recovery | |
| Exit streams | Solids 2 | 77.9 | 25% |
| Lixiviate 5 | 22.1 | 7% | |
| Solids 4 | 30.5 | 10% | |
| Final extrudate 6 | 175.9 | 57% | |
| Total | 306.5 | 99% | |
Our results for pectin fractionation and recovery are promising when compared to previous reports. In a recent review on pectin extraction from various plant-based by-products, Macias-Frotto et al. summarized yields and properties of pectin obtained through acid and enzymatic extraction processes.31 Specifically, in acid extraction at pH levels near 2, with process times ranging from 1 to 3 h, yields from orange peel range between 10.19% and 40%.32, 33 For other citrus by-products, yields around 25% have been reported.34
Enzymatic extraction processes typically operate at 40–50 °C but require significantly longer times – up to 20 h – with reported yields ranging from 3.11%35 to 22.55%.36 In contrast, the pectin recovery achieved in the lateral streams of our study is notably higher.
Emerging techniques – such as ultrasound, microwave-assisted extraction, high hydrostatic pressure, ohmic heating, and the use of deep eutectic solvents – have shown potential to reduce extraction times in both acid and enzymatic processes while preserving key pectin characteristics such as galacturonic acid content and degree of esterification.31 In this study, the combination of PEF and extractive extrusion allowed for nearly total pectin recovery (see Table 5) if we consider its presence in all the exit streams in the extruder. Of particular interest are the high yields of soluble pectin, in the lateral streams (42%), which can be easily separated in a purer form by simple alcoholic precipitation, surpassing those previously reported in the literature.
The physicochemical characterization of the final purified and isolated pectin is still pending and will be essential for assessing potential applications of the proposed process.
DISCUSSION
The citrus juice industry, particularly orange juice processing, has been active for decades and continues to expand. One of its main by-products – orange peel – serves as a valuable source for two commercially significant compounds: OEO and pectin.
Although numerous separation and purification methods have been explored, reactive extraction stands out as an environmentally friendly approach aligned with the principles of green chemistry. It enables the production of high-quality products while minimizing energy and water consumption.8 The present study enhances this approach by integrating a PEF pretreatment, which disrupts cell structures in the waste tissue, thereby facilitating the extraction of both OEO and pectin during the extrusion process.
A key innovation in the proposed system is the incorporation of two strategically positioned filters within the extruder. These filters enable the recovery of EO and pectin from the intermediate suspension streams, in addition to the final extrudate. This design allows for nearly 100% recovery of the solids introduced into the system. The study utilized frozen industrial orange peel waste provided by a juice producer. Initial experiments, based on a factorial design, demonstrated that even a short PEF exposure of 40 μs at a low field intensity (1 kV cm−1) increased OEO extraction yield by approximately 50% compared to the untreated control. These yields surpass those reported in the literature. Gas chromatographic analysis confirmed that the extracted OEO contained a high concentration of limonene (~90%), with no detectable difference in quality compared to oil obtained from untreated material.
The material balance for pectin revealed a recovery rate of approximately 42% in its soluble form, distributed across the three lateral streams. This pectin can be further purified through alcohol-assisted precipitation. The result is especially promising when benchmarked against previously reported extraction yields: 10–40% using acids, 3–22% with enzymatic methods, 20.9% with ultrasound, and as low as 2.2% with water extraction.2
Further research is necessary to determine the optimal conditions and to perform comprehensive economic analyses that will support evaluations of the process's economic feasibility, sustainability potential, and integration with existing technologies. The physical and chemical characterization of the resulting pectin is currently in progress.
CONCLUSIONS
This study demonstrates the effectiveness of integrating PEF pretreatment with extractive extrusion for the recovery of high-value compounds from industrial orange peel waste. The process significantly improved OEO yield – by ~50% – and enabled nearly complete pectin recovery (~100%), with ~42% present in easily purifiable lateral streams. The addition of in-line filtration during extrusion further enhanced product separation and system efficiency. These results not only surpass conventional extraction methods in terms of yield and process time but also align with green chemistry principles, offering a scalable, low-energy alternative for valorizing citrus processing by-products in the food industries.
ACKNOWLEDGEMENTS
Financing of this project was provided by UNAM-TEC Consortium, Modalidad 2 ‘Nuevos Binomios’. Citrus residue was kindly provided by Citrofut SA in Nuevo Leon, Mexico. We appreciate Dr Viridiana Tejeda-Ortigoza for her participation during PEF experimental procedures. Open access funding provided by UNAM.
Open Research
DATA AVAILABILITY STATEMENT
Data are available upon reasonable request from the corresponding author.
