Crambe seed oil: Extraction and reaction with dimethyl carbonate under pressurized conditions

J. of Supercritical Fluids 159 (2020) 104780

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Crambe seed oil: Extraction and reaction with dimethyl carbonate under pressurized conditions Caroline Portilho Trentini a , Bruna Tais Ferreira de Mello a , Vladimir Ferreira Cabral a , Camila da Silva a,b,∗ a b

Departamento De Engenharia Química, Universidade Estadual De Maringá (UEM), Av. Colombo 5790, 87020-900, Maringá, PR, Brazil Departamento De Tecnologia, Universidade Estadual De Maringá (UEM), Av. Angelo Moreira Da Fonseca 1800, 87506-370, Umuarama, PR, Brazil

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• PLE removed 45% of oil from crambe seeds.

• Higher extraction of phytosterols and tocopherols were obtained from PLE.

• Similar fatty acid and acylglycerols composition for both oils evaluated.

• Sequential process provided high ester yield (66.5%) and triglyceride conversion (98.5%).

a r t i c l e

i n f o

Article history: Received 17 October 2019 Received in revised form 3 January 2020 Accepted 29 January 2020 Available online 1 February 2020 Keywords: Crambe abyssinica Dimethyl carbonate Phytosterols Tocopherols Fatty acid methyl esters

a b s t r a c t In this study, the efficiency of dimethyl carbonate as a solvent in the pressurized liquid extraction (PLE) of crambe seed oil and also as the acyl acceptor in the transesterification reaction using the mixture obtained, simulating a sequential process, was investigated. In the PLE process, the effect of temperature (140–180 ◦ C) on the yield and quality of the oil was evaluated, and the results were compared to data obtained applying Soxhlet extraction. The reaction of the PLE mixture was conducted from 250 to 350 ◦ C. The results showed greater oil removal from the seeds under pressurized conditions, reaching a yield of ∼45%, which was higher than the yield obtained with Soxhlet extraction. The fatty acids and acylglycerols composition of the extracts obtained with the two methods were similar. However, the phytosterols and tocopherols contents of the oil obtained by PLE were around ∼62% and ∼574% higher, respectively, compared with the oil from the Soxhlet extraction. The best results in the reaction step for ester yield (66.5%) and triglyceride conversion (98.5%) were obtained at 300 ◦ C. © 2020 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author at: Departamento de Engenharia Química, Universidade Estadual de Maringá (UEM), Av. Colombo 5790, 87020-900, Maringá, PR, Brazil. E-mail address: [email protected] (C. da Silva). https://doi.org/10.1016/j.supflu.2020.104780 0896-8446/© 2020 Elsevier B.V. All rights reserved.

Crambe (Crambe abyssinica) seeds have a high content of oil (30–51 %) [1,2], which is mainly comprised of monounsaturated fatty acids, predominantly erucic acid (∼60%) [2,3], conferring a high oxidative stability [4], and active compounds, such as tocopherols and phytosterols, are also present [1,3,5].

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C. Portilho Trentini, B.T.F. de Mello, V. Ferreira Cabral et al. / J. of Supercritical Fluids 159 (2020) 104780

The high amount of erucic acid in crambe oil makes it an important raw material for industrial application, including the production of biodegradable plastics and lubricants [6]. In addition, it is appropriate for use in the biodiesel industry since it does not compete with oilseeds produced for use in food products. This is because the high amount of erucic acid has adverse effects on human health, directly affecting the heart, with a risk of the accumulation of triacylglycerides (TAG) in the myocardium [7]. Thus, crambe meets the main requirements for biodiesel production, and it is also a rustic, adaptable and edaphoclimatic plant [8], which contributes to making it competitive without affecting the food sector. Procedures to obtain vegetable oils using a solvent that acts in the extraction of the oil and as reagent for the synthesis of biodiesel have been studied and reported in the literature [3,9–11]. The solvents available include dimethyl carbonate (DMC), which is efficient in the extraction of the lipid fraction of oilseeds [10,12] and, besides allowing the simultaneous extraction and reaction, avoids the generation of glycerol as a co-product, according to the transesterification reaction scheme shown in Fig. 1. In this reaction, initially, triacylglycerides react with DMC to produce fatty acid methyl esters (FAMEs) and fatty acid glycerol carbonate monoesters as a co-product. These monoesters are miscible in the mixture of methyl esters and can remain in the biofuel without changing its main properties [13,14]. The fatty acid glycerol carbonate monoesters can then react with DMC to give a new coproduct, that is, glycerol dicarbonate (Fig. 1a). With the presence of water in the reaction medium, glycerol dicarbonate is decomposed into glycerol carbonate (Fig. 1b), which is a higher value-added product when compared to glycerol (generated in the reaction using alcohols) and can be applied in functionalized polymers, such as polyesters and polycarbonates, for use in biolubricants and personal care products [15]. Therefore, the application of this solvent can help to address the problem of excess glycerol in the market due to the production of biodiesel, which has reduced the commercial value of this byproduct, despite efforts to develop new chemical routes to find a destination for the surplus produced. Pressurized liquid extraction (PLE) is a technique that can be applied using dimethyl carbonate, aimed at obtaining an oil + solvent mixture that can be directly applied in the production of biodiesel, in a sequential process, reducing the costs associated with the separation process. PLE has been shown to be efficient in extracting the lipid fraction [3,16,17] and compounds such as carotenoids, flavonoids, phytosterols, tocopherols and tocotrienols [3,18–20], achieving better results than those obtained with conventional extraction methods [3]. The literature reports some of the advantages of using the PLE as extraction method and among these it is possible to mention the energy saving during the process, use of less solvent when compared to conventional technologies, use of extractors with less complex characteristics and fast extraction rate with possibility of adjusting the process variables in order to obtain greater method selectivity for a given fraction of compounds [21–23]. It is noteworthy that PLE is a technique with varied applications and besides the extraction of oils and bioactive compounds, it has been reported as a method of analysis of organic pollutants in environmental samples [24,25]. According to Vazquez-Roig and Picó [25], this technique provides repeatability and improved sample throughput in a shorter process time and can be substituted for traditional methods such as Soxhlet and supercritical fluid extraction. In PLE, an increase in temperature increases the solubility and diffusion of the analytes, reducing the viscosity and the surface tension of the solvent, leading to greater penetration into the pores of the matrix [26,27]. This is because a higher temperature favors the thermodynamic and transport properties. Rodrigues et al. [17]

demonstrated that the enthalpy and entropy changes are positive, indicating that PLE is an endothermic process with randomness and irreversibility. In addition, the Gibbs free energy decreased with increasing temperature, demonstrating that the process is spontaneous. Similarly, increasing the temperature in PLE influenced the internal mass transfer coefficient, according to Jesus et al. [28], with a fixed pressure of 10 MPa. In general, higher internal mass transfer coefficients result in improved solute diffusion to the surface of the oleaginous matrix [29]. In addition, mass transfer in PLE is also favored at high pressures, facilitating cellular permeability and intermolecular interactions [30]. In the same context, technology using pressurized acyl acceptors is also widely reported in the literature on reaction processes for the production of fatty acid esters [31–36]. In reaction processes, a sub- or supercritical fluid demonstrates intermediate properties, that is, between those of the liquid and gas phases, resulting in a high reaction rate due to relatively low viscosity, high diffusivity and low surface tension. These characteristics also lead to greater solubility of the reaction mixture and thus improve the mass transfer [37,38]. The process under pressurized conditions allows the simultaneous reaction of triglycerides and FFA and can be applied to a wide variety of raw materials, while enabling the simplification of the purification process and separation of reaction products [39–41]. In the case of DMC, studies conducted to evaluate its use under these conditions are reported in the literature [42–45] and temperature was found to be the variable with the greatest influence on the reaction. The use of very low temperatures, often below the critical point (275 ◦ C and 4.63 MPa), results in very slow reaction rates due to poor solubility between the oil and DMC, with the formation of a biphasic solution [43,45]. Thus, temperatures in the range of 300–380 ◦ C were found to be the most suitable for the formation of esters with this solvent [39,42,43,45–48]. The objective of this study was to investigate the extraction of oil from crambe seeds using DMC under pressurized conditions, aimed at carrying out the transesterification reaction. The effect of temperature on the extraction kinetics and chemical composition of the oil (fatty acids, acylglycerols, phytosterols and tocopherols) was evaluated. Similar data were obtained applying Soxhlet extraction for comparison purposes. The PLE conditions which led to the maximum oil yield and compound removal were applied in the reactions conducted at different temperatures.

2. Materials and methods 2.1. Materials Crambe seeds (Crambe abyssinica H.) donated by the MS Foundation and dimethyl carbonate (Sigma-Aldrich, 99%) was used in the extractions. The seeds (with 3.52 ± 0.05% moisture) were prepared as reported by Santos et al. [1], to obtain particles with an average diameter of 0.92 ± 0.09 mm. The reagents and solvents used to determine the composition of fatty acids, acylglycerols, phytosterols and tocopherols contents were: sodium hydroxide (Anidrol), metanol (Anidrol, 95%), heptane (Anidrol), boron trifluoride (Sigma-Aldrich, 14% in methanol), methyl heptadecanoate (Sigma-Aldrich, >99%), monolein (SigmaAldrich, 98.0%), 1,3-diolein (Sigma-Aldrich, 99.9%), glyceryl trioleate (Sigma-Aldrich, 99.5%), oleic acid (Sigma-Aldrich, 99.0%), tricaprin (Sigma-Aldrich, 99%), the derivatizing agent N-methyl-N(trimethylsilyl)-trifluoroacetamide (MSTFA, Sigma-Aldrich, 98.5%), 5␣-colestane (Sigma-Aldrich), N,O-bis (trimethylsilyl) trifluoroacetamide (BSTFA, 1% TMCS, Fluka), ␣-,␥- and ␦-tocopherol standards (Sigma-Aldrich, >99.9%), isopropanol (JT Baker), methanol (JT Baker) and ultrapure water(Milli-Q).

C. Portilho Trentini, B.T.F. de Mello, V. Ferreira Cabral et al. / J. of Supercritical Fluids 159 (2020) 104780

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Fig. 1. Stoichiometric reaction scheme for the use of dimethyl carbonate as an acyl acceptor for the triglyceride conversion.

2.2. Methods 2.2.1. Oil extraction The classical Soxhlet extraction method was performed using 150 mL of DMC as solvent. The extraction time applied was 480 min and the temperature was kept constant (above the solvent reflux temperature, 90 ◦ C). The pressurized liquid extraction was conducted in a semicontinuous process, in a laboratory unit described in detail by Rodrigues et al. [17]. The extraction procedure is described by Mello et al. [3], that consisted of filling the extraction bed (made of 3/8 stainless steel tube with 30 cm length) with ∼5.5 g of crambe seeds, then the oven was heated to the test temperatures and methyl acetate was pumped at a flow rate of 1 mL min−1 until the pressure was reached 10 MPa, which was kept fixed throughout the extraction process. With the pressurized system, 30 min were counted as a static extraction period. Then the methyl acetate flow rate was changed to 3 mL min−1 starting dynamic extraction. The oil yield was calculated from Equation 1: Oil yield (%) =

 We  Ws

× 100

(1)

where We (g) is weight of extract and Ws (g) is weight of the seed used in the extractor. The apparent solubility of CSO in DMC was estimated from the initial slope of the curve obtained by mass of extracted CSO and mass of DMC used (g oil g DMC−1 ). 2.2.2. Oil characterization For the quantification of acylglycerols present in oil (monoglycerides, diglycerides and triglycerides) and free fatty acid

(FFA), 100 mg of sample was derivatized with MSTFA (15 min at room temperature) [49], and diluted in heptane. The solutions were injected (2 ␮L) into a gas chromatograph (Shimadzu, GC-2010 Plus) equipped with a capillary column Zebron ZB5HT (10 m × 0.32 mm × 0.10 m), flame ionization detector and on-column injector, conditions were those as described in Trentini et al. [50]. The identification of the compounds was performed by the injection of chromatographic standards of triolein, diolein, monolein and oleic acid. For the quantification of the compounds, calibration curves were constructed using standards, with concentrations of 0.06–3.05 mg mL−1 for triglycerides, (0.025–2.5) mg mL−1 for diglycerides, (0.025–2) mg mL−1 for monoglycerides and (0.05–2.5) mg mL−1 for FFA, both curves presenting coefficients of determination (R2 ) of >0.99. To determine the fatty acid composition, the samples were prepared according to the methodology of Gonzalez et al. [51], with use of ∼60 mg of sample, which were derivatized using methanolic solution of boron trifluoride (BF3 ). After the derivatization step, the samples were diluted in methyl heptadecanoate solution. Subsequently, samples were injected (1 ␮L), in duplicate, into a gas chromatograph (Shimadzu, GC-2010 Plus) with a flame ionization detector equipped with a capillary column Shimadzu Rtx–Wax (30 m x 0.32 mm x0.25 ␮m) applying the conditions described by Mello et al. [3]. The components present in the samples were identified through comparison with a FAME (fatty acid methyl esters) mixture (Supelco) and quantified using methyl heptadecanoate as the internal standard. Phytosterols content were evaluated in gas chromatograph coupled to mass spectrum (Shimadzu, GCMS – QP2010 SE) using capillary column Shimadzu SH-Rtx-5MS) 5% diphenyl/95% dimethyl polysiloxane, 30 m x 0.25 mm x 0.25 m). Oils were previ-

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ously derivatized using N,O-bis (trimethylsilyl) trifluoracetamide (BSTFA) for 30 min at 60 ◦ C. The conditions were those described by Santos et al. [1] and identification of phytosterols was carried out with using the NIST 14 Mass Spectrum Library. The ␣-, ␥- and ␦-tocopherols contents were determined by high performance liquid chromatograph (Shimadzu LC-20AT, coupled to a UV–VIS detector SPD-20 A) equipped with a column C18 Shimpack CLC-ODS M (25 cm × 4.6 mm, 5 ␮m). The method employed was that described by Freitas et al. [26]. Calibration curves for determination of the respective compounds were prepared at the concentrations of 0.5−12 mg L−1 , with their corresponding standards (␣-, ␥- and ␦-tocopherols) which showed regressions of R2 >0.99.

2.2.3. Reaction From the extraction mixture (CSO + DMC) obtained, the transesterification under pressurized conditions was conducted in experimental apparatus operated in continuous mode described in the work of Visioli et al. [52]. The apparatus is composed of a reaction mixture reservoir, mechanical stirrer, isocratic pump, one-way valve, heating oven, preheating tubular section, glass bead-filled reaction bed (4 mm diameter) and tubular reactor. The reaction process consisted of continuous pumping of the extraction mixture (CSO + DMC) to the reactors coupled to the heating furnace and subsequent pressurization of the system. After leaving the oven, the samples were cooled by the contact of the stainless steel tube with water cooled to 15 ◦ C by an thermostatic bath, and finally the samples were collected after two residence times of the mixture in the reaction zone to ensure steady state has been reached. The furnace temperature was monitored by thermocouples coupled to reactor and connected to a temperature indicator, the system pressure was accompanied by a pressure indicator and controlled by a control valve pressure. After sample collection, the unreacted DMC was removed from vacuum evaporator (Marconi, MA 120) and then oven (Marconi, MA035) at 100 ◦ C to remove remaining DMC until constant sample weight.

2.2.4. Reaction product analysis For determining fatty acid methyl esters (FAME), a 100 mg sample was diluted with 5 mL heptane, an aliquot of 150 ␮L was transferred to vial, adding 80 ␮L internal standard and 770 ␮L of heptane, it was then injected into a gas chromatograph (Shimadzu, GC-2010 Plus) under the analysis conditions specified by Trentini et al. [35]. FAME content were determined according to Trentini et al. [50] and FAME yield was calculated according to Eq. 2, and the sum of the acylglycerols from CSO was determined as the maximum FAME that can be produced from crambe oil. FAME yield (%) =



Esters content Total acylglycerols



× 100

(2)

For triglycerides (TG) conversion of the reaction samples, 50 mg of the sample was derivatized and prepared in 5 ml as quoted for oil, then a 300 ␮l aliquot was transferred to the assay line, adding 60 ␮L tricaprin (internal standard) and 640 ␮L heptane. The chromatographic conditions were the same for the quantification of acylglycerols present in the oil, described in section 2.2.2. TG content and TG conversion was calculated according to Eq.s 3 and 4, respectively. Triglyceride content(%) =

 A   C  PI TG API

×

CS

× 100

(3)

Fig. 2. Kinetics of the extraction of crambe seed oil using dimethyl carbonate as the solvent at 10 MPa and 140 ◦ C (䊏), 160 ◦ C (䊉) and 180 ◦ C ().

where ATG is the area of triglycerides, API is area of internal standard and CPI and CS are the concentration of the internal standard and the sample, respectively. Triglyceride conversion (%) =

 TG − TG  I F TGI

× 100

(4)

where TGI and TGF are the triglyceride content of the oil extraction sample and the reaction sample determined from the internal standard, respectively. To quantify the percentage of FAME decomposition, performed by gas chromatography, which relates the effect of reaction temperature on the reduction of fatty acids, which causes the formation of other compounds, the methodology proposed by Vieitez et al. [53]. 2.3. Statistical analysis All analyses were performed in duplicate, and the results are presented as mean values ± standard deviation. The data collected were subjected to analysis of variance (ANOVA) using the Microsoft® Excel 2010 software and Tukey’s test (p = 0.05), to evaluate differences between the results. 3. Results and discussion 3.1. Oil extraction In the conventional extraction of crambe oil with DMC in a Soxhlet extractor the oil yield reached 31.7 ± 0.09%. Tavares et al. [2] and Mello et al. [3] evaluated the extraction of crambe oil using this technique with n-hexane as the solvent and obtained 41.2% and 38.8% of oil, respectively. According to Cascant et al. [54] the dimethyl carbonate shows lower solubility than n-hexane in the extraction of lipids. The results for the oil yield obtained in the pressurized liquid extraction of the crambe oil with DMC, at 140, 160 and 180 ◦ C as a function of extraction time, are reported in Fig. 2. It can be observed that the increase in temperature from 140 ◦ C to 160 ◦ C raised the maximum oil yield from ∼ 40.48%–45.51%. With a further increase in the temperature to 180 ◦ C no increase in oil yield was observed (p > 0.05). The oil extraction occurred mainly in the initial 40 min, at all temperatures, when ∼70% of the oil in relation to the maximum was obtained. After ∼105 min of extraction at 160 and 180 ◦ C and ∼130 min at 140 ◦ C the yield becomes constant.

C. Portilho Trentini, B.T.F. de Mello, V. Ferreira Cabral et al. / J. of Supercritical Fluids 159 (2020) 104780

The apparent solubility of crambe seed oil (CSO) in DMC presented values of 7.66 × 10−3 , 8.49 × 10−3 and 8.48 × 10-3 g oil per g DMC for temperatures of 140, 160 and 180 ◦ C, respectively. This parameter is obtained from the concentration of the saturated solution in equilibrium with the plant matrix. It relates to the amount of the solute that is freely accessible on the surface of the plant particles and is dissolved immediately, and this controls the extraction kinetics and affects the production cost [55]. An increase in the extraction temperature is associated with lower surface tension values, improving the solubility and diffusion rate of the solute in the solvent within the matrix, allowing a faster dissolution of the oil, thus increasing the extraction efficiency [28,56]. Other authors have reported that increasing the temperature in the extraction process favors higher oil yields. Balvardi et al. [57] conducted the extraction of Amygdalus scoparia oil using pressurized ethanol and observed an increase of ∼10% in oil yield when the temperature was increased from 100 to 150 ◦ C. Efthymiopoulos et al. [58] found that raising the temperature from 110 to 180 ◦ C resulted in a ∼20% increase in the oil yield obtained from coffee grounds using ethanol under pressurized conditions. Derwenskus et al. [21] evaluated the pressurized extraction of Chlorella vulgaris lipids using ethanol, ethyl acetate and n-hexane as solvents and observed that increasing the temperature from 100 to 150 ◦ C increased the lipid extraction yield by ∼18%, ∼33% and ∼35%, respectively, for the solvents studied. Mello et al. [3] performed the extraction of crambe oil with methyl acetate under pressurized conditions and reported an increase of ∼12% in oil yield with a temperature increase from 140 to 180 ◦ C. However, above a certain value, an increase in the extraction temperature does not influence the oil yield, as found by Castejón et al. [59] for the extraction of Echium plantagineum L. oil using ethanol at 150 and 200 ◦ C. In addition, it can compromise the oil quality, degrading the active compounds of interest present in the matrix under study [60,61], or promote the extraction of unwanted compounds together with the oil [62]. Moreover, the type of solvent involved in the extraction process may influence the observed effect of the temperature variable, as can be seen when comparing the data from this work with those obtained by Mello et al. [3]. Regarding the method evaluated, the extraction of crambe oil by PLE provided higher oil yields at the different temperatures evaluated (∼14%) than the Soxhlet extraction. This can be explained by the use of higher temperatures in PLE, as well as the application of pressure [26,27]. Conte et al. [63], Mello et al. [3] and Rudke et al. [64] performed the PLE at 100 bar and observed increases of 36, 14 and 16% in the oil yield for safflower, crambe seed and buriti peel, respectively, when compared to the results obtained with Soxhlet extraction. 3.2. Oil characterization The contents of acylglycerols (triglycerides, diglycerides, monoglycerides and FFA) and fatty acid composition of the oils obtained from the PLE extractions and Soxhlet extractor can be observed in Table 1. The proportions of triglycerides and diglycerides identified were similar for the PLE at 140 ◦ C and 160 ◦ C and the Soxhlet extraction. However, the values obtained with PLE at 180 ◦ C differed from those of the other extractions (p < 0.05). The use of high temperature may lead to the extraction of other compounds that can not be converted into esters, thus reducing the acylglycerols contents and consequently the convertibility of the extracted oil. A higher temperature, for instance, favors the extraction of polar components [65], such as phospholipids, carbohydrates and proteins [66,67]. Mello et al. [3] used PLE to extract crambe oil and reported that an increase in temperature led to a reduction in the convertibility of the oil, which is defined as the esti-

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mated maximum conversion that can be achieved with the oil [51], since it may contain several components that can not be converted to esters. The FFA contents of the oils obtained with the two extraction methods were similar, (around 2.3–2.7). Values of between 1.2% and 2.3% are reported in the literature for the crambe oil extracted by Soxhlet using n-hexane as the solvent [1,2,5]. The FFA content was above 0.5%, which is the limit for biodiesel production employing the conventional method with homogeneous alkaline catalysis [68,69]. Above this limit saponification can occur, leading to the consumption of the catalyst in the reaction medium, thus reducing the formation of esters [70]. The fatty acids of the oils were similar for the different extraction methods and temperatures used. The main fatty acids identified were erucic (∼59%), oleic (∼20%) and linoleic acid (∼6%). In a study reported in the literature, crambe seeds cultivated with different doses of phosphorus resulted in oil with 93.45% unsaturated fatty acids, with mean erucic and oleic acids contents of 54.44% and 16.02%, respectively [5]. Li et al. [71] reported that the fraction of erucic acid present in crambe oil was ∼56%, followed by oleic acid with ∼17%. Mello et al. [3] observed that the main fatty acids extracted by PLE using methyl acetate as the solvent were erucic (∼57%) and oleic acid (∼20%). The results for the identification and quantification of phytosterols and tocopherols in the CSO extracted with DMC are shown in Table 2. Phytosterols are present in the unsaponifiable part of plant lipids and the CSO had stigmasterol, campesterol and ␤-sitosterol in its composition, with ␤-sitosterol being the predominant phytosterol, as also reported by Mello et al. [3] and Aguiar et al. [5]. Mello et al. [3] reported higher amounts of phytosterols in crambe oil obtained from both Soxhlet extraction and PLE using methyl acetate as a solvent when compared to the amount obtained under the same conditions using DMC. The higher solubilization of phytosterols may be related to the solvent viscosity, since methyl acetate has lower viscosity (0.367 mPa s) [72] when compared to DMC (0.625 mPa s) [42]. The analysis of these points justifies to evaluate the concentration of these compounds in extracts obtained from the same vegetal matrix and different solvents. In addition, according to Hussain and Mohamed [73], higher amounts of total phytosterols can be obtained with increasing solvent polarity. However, DMC presents higher polarity than methyl acetate, with polarity index values of 3.9 [74] and 3.5 [75], respectively. In this regard, Outcalt and Laesecke [76] noted that although DMC is highly polar under the pressurized liquid conditions, this does not result in greater attraction of molecules, since the concentration of negative charges on the oxygen atoms causes greater repulsion between the DMC molecules, while the electrostatic attraction between the oppositely charged parts of the molecule contributes to their lesser interaction with other molecules. PLE conducted at 140 ◦ C showed a higher total phytosterols content and with the increase in temperature to 160 and 180 ◦ C the total phytosterols were reduced. However, the content obtained at 180 ◦ C is similar to that of the oil extracted by the Soxhlet method. In the PLE extraction, the degradation of the phytosterols could have occurred due to the high temperature applied, since the oxidation of these compounds can be influenced by several external factors. On comparing the data obtained at 180 ◦ C with that for the lowest temperature applied (140 ◦ C), there were reductions of 52% and 58% in the levels of campesterol and ␤-sistoterol, respectively, at the higher temperature. Rudzinska et al. [77] verified the effect of applying different temperatures for 1 h on the degradation of pure phytosterols and it was observed that increasing the temperature from 120 to 180 ◦ C reduced the contents of campesterol and ␤-sitosterol by 50%. Barriuso et al. [78] studied the thermal stability of sterols at 180 ◦ C and reported that those most susceptible to degradation are campesterol and ␤-sitosterol, with stigmasterol

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Table 1 Characterization of crambe seed oil obtained by PLE and Soxhlet extraction using dimethyl carbonate as solvent. PLE

Property

Monoglyceride (%) Diglycerides (%) Triglycerides (%) FFA (%) Total (%) Fatty acid composition (%) Palmitic Palmitoleic Stearic Oleic Linoleic Linolenic Arachidic Gadoleic Behenic Erucic Lignoceric Unidentified

Soxhlet

140 ◦ C

160 ◦ C

180 ◦ C

NI 1.72 ± 0.01a 92.90 ± 0.60a 2.44 ± 0.01a 97.06 ± 0.60a

NI 1.86 ± 0.04a 92.49 ± 0.53a 2.32 ± 0.01a 96.68 ± 0.50a

NI 3.03 ± 0.01b 88.01 ± 0.59b 2.29 ± 0.24a 93.33 ± 0.37b

NI 1.78 ± 0.13a 92.06 ± 0.56a 2.65 ± 0.02a 96.49 ± 0.66a

1.70 ± 0.21a 0.11 ± 0.03a 0.82 ± 0.01a 21.12 ± 1.05a 6.04 ± 0.41a 0.64 ± 0.29 a 2.39 ± 1.37 a 5.19 ± 0.14 a 1.20 ± 0.18 a 59.73 ± 0.57 a 0.69 ± 0.31 a 0.37 ± 0.04

1.59 ± 0.01a 0.09 ± 0.02 a 0.80 ± 0.03 a 20.51 ± 0.04a 6.34 ± 0.10a 0.73 ± 0.04 a 3.34 ± 0.03 a 5.63 ± 0.07 a 1.10 ± 0.04 a 58.86 ± 0.66a 0.67 ± 0.01 a 0.35 ± 0.01

1.82 ± 0.01a 0.12 ± 0.04 a 0.81 ± 0.02 a 20.77 ± 0.22a 6.41 ± 0.11a 0.72 ± 1.97 a 3.73 ± 2.13 a 5.45 ± 0.17 a 0.99 ± 0.05 a 58.33 ± 0.76 a 0.57 ± 0.05 a 0.39 ± 0.01

1.49 ± 0.01a 0.06 ± 0.04 a 0.71 ± 0.05 a 20.24 ± 0.18a 5.80 ± 0.09a 0.68 ± 0.07 a 3.33 ± 0.09 a 5.62 ± 0.15 a 1.00 ± 0.07 a 59.94 ± 0.65a 0.96 ± 0.01 a 0.33 ± 0.02

NI: Not identified. Means followed by same letters in the same row indicates no significant difference (p > 0.05).

Table 2 Phytosterol (PHY) and tocopherols (TOC) contents for crambe seed oil obtained by PLE and Soxhlet extraction using dimethyl carbonate as the solvent. Method

PLE Soxhlet

o

T ( C)

140 160 180 90

Phytosterol (mg of PHY per 100 g of oil)

Tocopherol (mg of TOC per 100 g oil)

Stigmasterol

Campesterol

␤-sistoterol

Total

␣

␥

␦

Total

37.41 ± 0.42 38.86 ± 2.31 45.70 ± 0.76 37.00 ± 1.25

88.71 ± 7.03 71.72 ± 0.07 45.94 ± 0.35 41.13 ± 3.87

146.38 ± 1.79 130.33 ± 5.41 84.27 ± 0.99 90.51 ± 2.11

272.50 ± 8.39a 240.91 ± 7.79b 175.91 ± 1.49c 168.64 ± 7.14c

3.63 ± 0.18 2.95 ± 0.37 2.46 ± 0.04 6.75 ± 0.14

127.71 ± 0.25 120.21 ± 0.26 125.93 ± 1.45 10.35 ± 0.35

11.65 ± 0.23 8.27 ± 0.13 6.14 ± 0.06 4.14 ± 0.40

142.99 ± 0.19a 131.43 ± 0.50b 134.53 ± 1.53b 21.24 ± 0.19c

Means followed by same letters indicates no significant difference (p > 0.05).

being less susceptible, which is consistent with the data reported in Table 2. It can be observed in Table 2 that there was a higher extraction of total phytosterols by PLE at 140 ◦ C, with an increase of ∼61% in relation to Soxhlet extraction. In the PLE extraction there is greater solubility of these compounds, since the combination of high pressure and temperature facilitates extraction by directing the solvent to the sample pores, resulting in better solvent penetration into the matrix, due to the lower viscosity of the solvent and its higher diffusivity, which increases the extraction efficiency [27,79]. The predominant tocopherol in the oils obtained by PLE and Soxhlet was ␥-tocopherol (∼90% and ∼49%, respectively). However, for both techniques the oils obtained contained ␣-tocopherol, ␦tocopherol and ␥-tocopherol in their composition, compounds also identified by Santos et al. [1] and Aguiar et al. [5] in CSO. The PLE at 140 ◦ C showed a higher total tocopherol value when compared to the other extraction conditions. Soxhlet extraction showed a low tocopherol content, with a decrease of ∼85% compared to the PLE at 140 ◦ C. An increase in the PLE temperature resulted in lower recovery of total tocopherols from the crambe seeds, as observed from the ␣- and ␦-tocopherols content, and these tocopherols may have been decomposed at the higher temperature. According to Réblová [80], ␣- and ␦-tocopherols lose their antioxidant activity above 150 ◦ C. Tocopherols are thermolabile compounds, so high temperatures can lead to their decomposition [81,82]. The stability of tocopherols (␣, ␤ + ␥ and ␦) in rice bran oil was evaluated by Bruscatto et al. [83], who exposed the oil to temperatures of 100, 140 and 180 ◦ C for 48 h. The authors reported low decomposition of tocopherols at 100 and 140 ◦ C, which showed similar values, however, at 180 ◦ C, increased degradation of tocopherols occurred. Similar result were obtained in this study, where the application of a temperature of 180 ◦ C reduced the concentration of tocopherols present in the CSO.

Tocopherols are compounds that contain polar and nonpolar functional groups and thus also have lipid solubility, which results in a balanced distribution of these antioxidants in the lipid-solvent extraction system [84]. Thus, when comparing the results of the present study with those reported by Mello et al. [3] it can be verify that the cited researchers obtained superior oil yields (>47% with PLE and ∼42% with Soxhlet extraction). This will be associated with the higher solubility of the tocopherols, which reached >40% applying the PLE at 180 ◦ C and 87% with Soxhlet extraction. 3.3. Data for the sequential biodiesel production process Table 3 shows the results obtained for the extractions performed at 140 and 160 ◦ C, considering data of compound concentration and oil yield. Although the highest concentration of phytosterols and tocopherols was obtained at 140 ◦ C (Table 2), the highest oil removal from crambe seeds was reported at 160 ◦ C (Fig. 2) and thus the amount extracted from these compounds were similar. Thus, 160 ◦ C is the best extraction temperature for obtaining the mixture (CSO + DMC) to be used to perform the transesterification reaction, due to the lower oil to DMC mass ratio and shorter extraction time. It is noteworthy that the presence of these compounds will give greater stability to the oil during its processing under pressurized conditions. 3.4. Transesterification reaction The transesterification reactions were performed at temperatures in the range of 250 to 350 ◦ C, with fixed residence time (15 min) and pressure (20 MPa). Fig. 3 shows the results obtained in terms of FAME yield, fatty acid decomposition and triglyceride (TG) conversion.

C. Portilho Trentini, B.T.F. de Mello, V. Ferreira Cabral et al. / J. of Supercritical Fluids 159 (2020) 104780

7

Table 3 Compilation of data obtained from the extraction of crambe seed oil (CSO) with pressurized dimethyl carbonate (DMC) as the solvent. Conditions

Oil yield (%)

Oil to DMC mass ratio

140 ◦ C and 135 min 160 ◦ C and 100 min

40.5 45.5

1:191 1:132

a

Mass of compounds of interest (mg)a Phytosterol

Tocopherol

110.36 109.61

57.91 60.36

(oil yield x concentration of the compounds of interest).

temperatures above 325 ◦ C, a considerable increase in the decomposition occurred and this behavior may be related to the beginning of the decomposition of saturated and unsaturated esters. According to Ilham and Saka [43] in transesterification with supercritical DMC, the saturated FAMEs are stable at 300 ◦ C, however, at 350 ◦ C they show instability and decomposition occurs. In addition to the thermal decomposition of fatty acid esters, which mainly involves isomerization, polymerization (Diels-Alder reaction) and pyrolysis mechanisms [88], glycerol dicarbonate decomposition may occur in the presence of water, with the formation of glycerol carbonate, methanol and CO2 . Studies on the decomposition products of a supercritical process using DMC are limited. The application of high temperatures may also result in DMC decomposition, and decarboxylation (> 200 ◦ C, 1 atm) can occur, with the formation of CO2 and dimethyl ether [89]. 4. Conclusions Fig. 3. Effect of temperature on the ester yield (––), triglyceride conversion (– 䊉–) and fatty acid decomposition (–䊏–) in the interesterification of the extraction mixture (crambe seed oil + dimethyl carbonate) at 20 MPa with an extraction time of 15 min.

The temperature that resulted in the highest FAME yield (66.5%) was 300 ◦ C, presenting low decomposition (∼3%) and ∼98.5% of TG conversion. As can be seen in Fig. 3, increasing the temperature from 250 to 300 ◦ C resulted in an increase in the values of the response variables, FAME yield and TG conversion. However, the reaction carried out at a temperature above 300 resulted in a decrease in the FAME yield, as it caused an increase in FAME decomposition, which reached 34% at 350 ◦ C. Increasing the temperature provides greater mutual solubility in the system, thereby improving the miscibility between oil and DMC [45]. Thus, the FAME yield is also high as a result of the greater reactivity between reagents [43]. It should also be noted that the activation energy decreases with the application of higher temperatures in the system, and the reaction is therefore completed in a shorter period of time [85]. In addition, there is greater molecular interaction, which facilitates the mass transfer [86]. In cases where the temperature of the DMC is close to or below the critical point (270 ◦ C), the DMC behavior assumes that of subcritical conditions, which may hinder the formation of FAME [46]. This finding is corroborated by studies reported in the literature where DMC was used as an acyl acceptor under pressurized conditions for FAME synthesis. Jung et al. [47] obtained FAME yields of 18.26% at 250 ◦ C and 41.85% at 300 ◦ C for avocado oil transesterification. Lamba et al. [87] reported an increase in the TAG to FAME conversion of ∼61% when the temperature was increased from 275 to 300 ◦ C. In the work of Ilham and Saka [42], changing the temperature from 270 to 300 ◦ C increased the FAME content by ∼77%. It is noteworthy that in these studies the authors performed a reaction mixture of commercial oil with DMC, unlike this study that used the extraction step product (oil + DMC) without any previous treatment in the oil. Increasing the temperature makes fatty acid esters more susceptible to thermal decomposition, and polyunsaturated esters are even more vulnerable to the effect of temperature [43,88]. At

From PLE it was possible to remove higher oil, tocopherol and phytosterol content from crambe seeds when compared to Soxhlet extraction, proving the advantages of the technique. The two techniques provided oils with similar acylglyceride compositions, indicating similar convertibility of the oils into esters. PLE at 160 ◦ C demonstrated better characteristics for application in the production of esters, with an oil yield higher than that obtained at 140 ◦ C and similar phytosterols and tocopherols contents. On evaluating the results of the reaction step, the temperature of 300 ◦ C was ideal considering the high drop in ester production and high decomposition at temperatures above this value. The process proposed in this study led to a 98.5% conversion of triglycerides (TG), FAME yield of 66.5%, values considered high since it was used crude oil obtained directly from the extraction step. The results obtained indicate that the sequential process has potential to be explored in biodiesel production, since it was possible to obtain a FAME mixture directly from the oilseed. Declaration of Competing Interest The authors declare that this is an original manuscript, not published in full or elsewhere, and not under consideration for publication in another journal and all co-author approves a submission to the Journal of Supercritical Fluids. Acknowledgement The authors are grateful to CNPq (304903/2016-7 and 153274/2018-2) for financial support and to the State University of Maringá. References [1] K.A. Santos, R.A. Bariccatti, L. Cardozo-Filho, R. Schneider, F. Palú, C. Silva, E.A. Silva, Extraction of crambe seed oil using subcritical propane: kinetics, characterization and modeling, J. Supercrit. Fluids 104 (2015) 54–61, http:// dx.doi.org/10.1016/j.supflu.2015.05.026. [2] G.R. Tavares, T.B. Massa, J.E. Gonc¸alves, C. Silva, W.D. Santos, Assessment of ultrasound-assisted extraction of crambe seed oil for biodiesel synthesis by

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