Essential oil extraction from onion using ethanol and CO₂ as an extraction fluid mixture

Introduction

For Reddy (2019) and Tongnuanchan & Benjakul (2014), the natural essential oils are aromatic compounds present in different parts of medicinal plants, for instance: flowers, leaves, bark, roots and fruits that are separated after hydro-distillation or steam distillation as continuous solvent extraction techniques. Essential oils are a combination of low molecular mass chemical compounds, which contain: alcohols, polyphenols, terpenoids, carbonyl compounds, aliphatic substances, and they have distinct aromas and possess biological properties.

Cavalcanti (2013) and Sharmeen et al. (2021) noted that essential oils are used and applied as additives in the cosmetic, pharmaceutical, food, textile and perfumery industries to incorporate appropriate functional characteristics, ensuring a wider range of organic functions. In fact, most natural essential oils incorporate several functions, with emphasis on their use as natural dyes, nutraceuticals, functional foods, preserving agents, flavorings and fragrances, medicines, vitamin supplements, chemical standards and perfumes, among others.

Essential oils incorporate vegetable oil and are known by having an elevated content of organic and natural substances when obtained from flowers, herbs, fruits and aromatic spices. The extraction procedures involve the use of an extractor and a conventional solvent or supercritical fluid, or both, known as a hybrid mixture, where the proportion can be adjusted to improve the extraction yield (Uwineza & Waśkiewicz (2020) and Chemat et al. (2019)).

The extraction procedure used directly reflects in the quality of the products obtained, mainly in relation to the degree of purity, and the recovery capacity. The concepts of diffusion and convection mass transference, based on interfacial equilibrium between solid–liquid or solid–gas phases, are used to assess the process efficiency and to define the appropriate extraction equipment.

Boucard & Serth (2005) evaluated the extraction process based on diffusion and convectional mass-transfer phenomena to identify procedures that could enhance the quantity of the products. Shakir (2018) observed that a higher extraction temperature could increment the interaction of solvent and matrix and also the diffusion mass transfer.

Danlami et al. (2014) observed that the fundamental of solvent extraction is based on the contact of solvent and the solid material, allowing the solid materials to be transferred to the liquid phase as soluble compounds. However, in the case of plants, this extraction occurs through concentration differences. In the moment when the mass transfer decreases by the concentration of active sites, the equilibrium is finally reached and therefore the mass transfer will no longer occur.

In recent decades, an increase in international demand for essential oils produced from plants has been observed. This has driven further research to develop new extraction techniques aimed at increasing the oil quality and reducing the production cost.

In general, essential oil extraction involves vapor flow in distillation, hydro-distillation or procedures based on organic solvents, mechanical extraction or supercritical fluids (generally carbon dioxide) to improve the mass transfer during the extraction (Cavalcanti, 2013).

Filippis (2013) studied supercritical extraction with CO₂ and noted the importance of the production and commercialization of essential oil globally, with a considerable variety of essential oils on the market (more than 90 types). Several important differences in the oils obtained arise from the characteristics of the process applied for the extraction.

In this context, hydro-distillation is characterized by direct contact between two phases, vapor and solid. When the vapor comes into contact with an aromatic plant, the essential oil is dragged into this phase and then recovered in a condensation system (Figure 1). In this process, to increase efficiency extraction, the processes parameters are controlled, mainly temperature, pressure, vapor flow rate and particle size of the solids. Optimizing the operational conditions in this way generally increases the extraction efficiency and yield. After this step, the essential oil produced can be purified to satisfy international standards (Golmohammadi et al., 2018).

Figure 1. Description of hydro-distillation process.

Process adapted from Kusuma & Mahfud (2017).

When oil extraction is applied in industrial processes, the particle size is reduced and the essential oil is extracted inside an extractor tank, using appropriate solvents. After this step, the liquid phase is passed to an evaporator to remove the solvent and recover the essential oil. These processes can be applied on an industrial scale only after the associated phenomena have been studied. The difference between the ebullition temperature of essential oil and solvents ensures rapid solvent removal, using a one-stage evaporation processes. The solvent can be recovered, recycled and re-used in same extraction process, leading to a reduced need for new solvents.

For Vieira de Melo et al. (2000), the conventional procedures for essential oils recovery from plant materials, as cited above, can present limitations due to instability of essential oils by heat and by residual organic solvent from extraction. Hence, the use of SCF for essential oils extraction is being evaluated as an alternative to traditional methods.

Supercritical CO₂ offers advantages because it is not toxic, not expensive, non-flammable and odor- and colorless. Also, the critical temperature is near to ambient temperature and it has low viscosity and high diffusivity in relation to liquids. Thus, it has transformed into a common solvent in the process of natural materials (Vieira de Melo et al., 2000)).

The extraction of compounds from solid matrices is associated with the internal mass transfer (dissolution and diffusion) and external mass transfer (solid particles). Thus, the solute diffused in the supercritical fluid-rich phase can be carried away by the bulk flow. Numerous authors have been published their attempts of model for these supercritical extraction process (Reverchon & Morone, 1997; Sovová, 1994; Sovová, 2005), and a broad discussion of modelling aspects is presented elsewhere (Capuzzo et al. (2013); Danlami et al. (2014); Khaw et al. (2017); Reverchon (1996); Vieira de Melo et al. (2000) and Lin et al. (2013)).

Essential oil extraction using high pressure or supercritical fluids (SCF) is an alternative from organic solvent extraction or steam distillation, and has been studied by different industries, especially in the food, pharmaceutical and cosmetic industries (Ferreira et al. (1999); Pinto et al. (1999) and Cao et al. (2007)).

Khaw et al. (2017) observed the influence of solubilization on fluid density. At a high pressure, SCF presents high density, allowing the dissolution of great quantities of organic compounds. When the density is reduced, those dissolved compounds can be recovered, and it can be performed by decreasing the pressure or increasing the temperature.

For the author cited below, by performing the separation process at lower temperatures it can avoid the degradation of compounds by decreasing the heat exposure, which occur in steam distillation.

Supercritical fluid extraction provides better extraction efficiency, increases the production rate and improves the product quality. These processes, generally hybrid, involve the use of supercritical fluid, compressed air and a co-solvent, which, in combination, can intensify the fluid bubbling in the solid mass and increase the extraction efficiency (Figure 2).

Figure 2. System used in supercritical extraction.

Process adapted from Borges et al. (2017).

To increase the extraction efficiency with use the supercritical fluids, the extraction process must be optimized, guaranteeing the best operational conditions; mainly the temperature, pressure, flow rate, residence time, solid material size, and ratio between the material and the flow rate of the fluid used as the extractive solvent (organic solvent, supercritical fluid and compressed air).

For Manjare et al. (2019), the low polarity of supercritical CO₂ is an important obstacle to its application, hindering the extraction of polar compounds. However, this could be surpassed by the addition of polar modifiers, as methanol or ethanol together with supercritical CO₂, therefore increasing its solution power.

Under these conditions, it is possible to increase mass transfer rate and improve the performance of the process, guaranteeing the applicability of this type of fluid as an alternative for the improved extraction in industrial processes (Huang et al. (2016) and Silva et al. (2004)).

De Barros et al. (2014) studied supercritical extraction using laboratory and pilot scale processes. Their parametric studies supported the mathematical models developed for industrial-scale extraction, guaranteeing extraction products with high quality and greater added value. The parameters studied highlight the most appropriate and highest-performing operating conditions, allowing extraction curves to be constructed, and those arising from different procedures can be compared.

Based on these considerations, this paper proposes the extraction of essential oil, using a hybrid solvent system, involving a conventional solvent (ethanol) and a supercritical fluid (CO₂). In this study, the operational parameters (temperature, pressure and solvent flow rate and mass used ratio) were controlled in order to determine the optimal condition, to increase the mass transfer and yield of the essential oil production process. The procedure used in this paper is the same used by Capuzzo et al. (2013).

Methods

In this study, for the experimental setup, an extractor system was built. The system involves the use of ethanol and supercritical fluid (CO₂), as a hybrid fluid mixture, based on supercritical concepts, and the essential oil was extracted from onion. The best operational conditions were determined, aimed at market efficiency, considering the supercritical flow rate, temperature and mass of onion used in the extraction. The turbulence phenomenon was evaluated in terms of the intensity of the solid-fluid mixture, considered the velocity of the ethanol-CO₂ mixture. The CO₂ was bubbled in ethanol in a distillation balloon, where the mixture received the thermal energy needed to increase the solid to fluid phase transferring. The effect of onion mass on the mass transfer efficiency was also investigated.

The onion sample used in this study was previously chopped and dried in a horizontal dryer with controlled circulating air speed and the air was heated using electrical resistance, in order to improve the efficiency during the extraction process. The results are represented as graphs showing the mass loss over time.

Experimental procedure

Raw material preparation. In the study reported herein, onion (Allium cepa) was used as the raw material, which was chopped into cubes to obtain an adequate size for essential oil extraction. The chopped onion was placed in a convective dryer, coupled to an analytical balance and temperature control sensors.

The mechanical ventilation system provided airflow and there was a humidity (water concentration) gradient between the air and the onion, to increase the mass transfer from solid particles to the air.

The operation parameters for drying, hot air and mass loss of raw material were controlled. The process was carried out to give an onion mass loss of 50%, with a temperature of 40±1.2ºC, hot air speed of approximately 1.7 m/s, extraction time for 240 min and average particle size of 4 mm.

Procedure for essential oil extraction. The experimental apparatus shown in Figure 3 was used to carry out the extraction of essential oil from the dried onion.

Figure 3. Experimental apparatus constructed for essential oil extraction.

The apparatus built for this study was comprised of three sections: a) section with fluid mixture of solvent and supercritical fluid (CO₂) heated to the temperature required to provide the best conditions for the extraction process; b) the extraction of essential oil section, allowing interaction between solid and fluid phases, where the mass transfer from solid to fluid mixture occurs; c) section for recovery of essential oil, where the extraction fluid mixture carries the essential oil through the heat exchanger to the expansion tank for recover, by condensation, of the fluid mixture, ethanol and essential oil.

The experiments were performed using the apparatus (Figure 3), applying the following procedure: a) the raw material is placed inside the extraction tank and the water in the condensation system is cooled. Also, during this step, the ethanol inside the balloon is heated; b) cooled water begins to recirculate in the condenser system and when the temperature in the balloon reaches that suitable for extraction, the bottle of the CO₂ valve was opened and the flow of supercritical fluid was bubbled into the ethanol to form the mixture which would pass to the extraction tank; c) the fluid mixture flows in contact with the solid phase and the mass transfer occurs. The flow then passes through the condenser to recover the ethanol and essential oil in the expansion tank.

The methodological sequence described herein was used to carry out the essential oil extraction using onion as the raw material, as well as the effect of operation parameters on the quality of the process was evaluated.

Table 1 show the operational conditions used in this study, based on an experimental plan to evaluate the parameters and their effect on the process efficiency, based on the essential oil extraction capacity.

Table 1. Operational conditions used to carry out the experiments.

Nº	Parameters	Exp. 1	Exp. 2	Exp. 3
1	Initial onion mass (g)	255.8	614.97	184.73
2	Pressure (mm Hg)	760.0	760.0	760.0
3	Temperature (ºC)	63.4	62.6	55.4
4	CO2 flow rate (L/min)	0.5	1.44	3.60
6	Ethanol mass in balloon (g)	480	481	478
7	Size of raw material particles (mm)	4.0	4.0	4.0

The performance of each experiment was evaluated based on Equation 1.

$$\text{η}(\%)=\frac{Massofoilextracted(g)}{Initialmassofbiomassintheextracted(g)}\ast 100(1)$$

The performance of each extraction essay was related to the operation parameters mainly the bubble intensity of CO₂ in ethanol, temperature and fluid mixture flow rate (measured at the inlet and outlet of the expansion tank).

Essential oil purification. The essential oil produced in the extraction process describe herein was purified using simple distillation and filtration, supported by technical norms. The procedure applied provides products with a high degree of purity, suitable for application as a raw material in other industrial processes.

Characterization of essential oil. The essential oil extracted and purified was characterized to determine the acid and refraction indexes, viscosity, and specific mass. The analysis procedures are described in the standard test method ASTM D 974 and the European standard EN14103.

Results and discussion

The results of the experimental assays described above are reported in this section along with a discussion, supported by data available in the literature, to better contribute to future scientific and technological developments.

Drying of raw material

Table 2 shows the results obtained from the drying of the raw material (onion) used in this study, based on experiments conducted using the procedure described above. The results show the mass loss over time, characterized by a loss of the water present in the structure of the raw material. The runs were conducted with control of the operation parameters.

Table 2. Results obtained for drying of raw material over time.

Nº	Time (min)	Mass (g)	Mass loss (%)	Evolution of mass (%)
0	0	1784.8	0	100
1	30	1543.5	11.46	88.53
2	60	1340.2	20.52	79.47
3	90	1161.3	27.72	72.27
4	120	1094.7	34.38	65.61
5	150	965.1	40.82	59.17
6	180	940	45.28	54.72
7	210	917.2	49.32	57.68
8	240	892.4	52.41	47.59

The data in Table 2 were obtained during the drying process (240 min) and the mass was measured at 30-min intervals. Globally, for this assay, a mass loss of 892.4g corresponds to 54.42% of the initial raw material mass. These data were used to construct the graphs in Figure 4, which show the mass loss during the drying time. The drying curve indicates that a greater mass was transferred from the solid to gas phase, occurred during first hour of drying, equivalent to 39.15% of the global mass loss. In the second hour, the mass loss represents 26.45% of the global mass, in the third hour 20.80% and in the last hour 13.5%.

Figure 4. Drying curve obtained from experimental data.

The decrease in water mass during drying, according to Figure 4, was associated with the mass transfer phenomenon, where the flow rate is directly related to the concentration gradients of water, between the solid and gas phases, associated with this process. In this process, mass transfer by convection at the solid surface predominates, and this is proportional to the solid area through the transfer coefficients. Equation 2 can thus describe the mass transfer, where the flow transference is related with global coefficient and the concentration gradient.

$$N_A=k_c\text{Δ}{C}_A(2)$$

Where N_A is the flow rate of water (kmol/m²s), k_c is the global coefficient and ΔC_A is the concentration difference between the solid particle and the airflow.

The mass loss rate was greatest during the first 180 min when approximately 65.60% of the total loss occurred. The mass transfer rates then decreased substantially leading to relative stability. The reduction in mass transfer rates is related to the drying time, which can be adjusted to guarantee the optimal condition for the essential oil extraction, as described herein.

Conclusions

We can conclude:

Data availability

All data underlying the results are available as part of the article and no additional source data are required.