Vegetal material

J. dioica seeds, in a state of maturity and without physical damage were collected during the months of June and July of 2017 in the locality of Buenavistilla, Municipality of San José Iturbide, Guanajuato, Mexico, at an elevation of 2,100 m. The specimen was identified and deposited in the Herbario-Hortorio ¨Jorge Espinoza Salas¨ of the department of the Preparatoria Agrícola of the Autonomous University of Chapingo. Seeds were let dry in the sun, then cotyledons were obtained and stored at 4 °C for 24 h.

Morphological characterization of J. dioica seeds

A total of 30 seeds with pericarp were weighted in a digital weighting scale OHAUS CS200 (precision of 0.0001 g). Geometrical dimensions were determined as: length (L), width (W) and thickness (T) with a digital vernier TRUPER CALDI-6MP (precision 0.01 mm). In addition, statistical methods were used to analyze data of arithmetic diameter, geometrical diameter and sphericity of the seeds using the equations 1, 2 and 3:

Obtaining and characterizing J. dioica oil

J. dioica seeds (previously ground, 50 g) were packaged in filter paper cartridges and were submitted to extraction by using 150 mL of hexane by Soxhlet method for 18 h and by maceration method for 48 h. Oil and hexane mixture, previously dried with 20 g of anhydrous Na₂SO₄, was vacuum evaporated in a rotary evaporator brand Buchi model R-210 at a temperature of 50 °C. The process was performed in triplicate and oil yield was determined. Physico-chemical properties which was determined in oil were: viscosity, dynamic and kinematic (Viscometer Anton Parr SVM 3000) and acid and saponification indexes (^{AOAC, 1990}) and gross heating value was determined in a calorimeter brand Anton Parr model 6400.

Preparation and characterization of Biodiesel and mixtures of biodiesel-diesel

For obtaining biodiesel, firstly, a mixture of methanol with potassium hydroxide was prepared at a concentration of 0.028 g L^-1. J. dioica oil was added to this mixture in a proportion of 0.25:1 (v/p). The mixture was shaken and heated under reflux with methanol for 1 h. Once passed this time, it was transferred to a separation funnel and was let rest for 12 h, until observing two consistent phases in biodiesel and glycerin. Biodiesel (superior phase) was treated with citric acid at 0.1 % (two washings in proportion 1:1), after it was washed with hot water, finally it was dried with 20 g of anhydrous sodium sulphate and stored protected from the light at 4 °C until analysis. Fatty acids composition was determined by gas chromatography in an Agilent 6890 chromatograph equipped with a flame ionization detector and an AT-FAME column (30 m x 0.25 mm x 0.25 mm). Initial temperature of the oven was 170 °C (1 min), ramp 10 °C min^-1, final temperature 240 °C, injector and detector temperature 260 °C. Helium was used as a transporting gas, at a flow velocity of 1.8 mL min^-1. A mixture of FAME was used as a standard and retention times were used for the detection of the picks of samples. Fatty acids were estimated as the percentage of the total area of the picks of FAME (Figure 1).

Figure 1 Chromatogram from gas chromatography analysis of J. dioica seeds oil. 

Mixtures of biodiesel-diesel called B05, B10, B20, B30, B40 and B100 were prepared, where the number indicates the percentage of biodiesel in the mixture. Kinematic and dynamic viscosities and biodiesel densities and of each mixture were measured at atmospheric pressure, at 20 and 40 °C in an Anton Parr rotational Viscometer-densimeter Stabinger SVM 3000. Measurements of gross heating values of biodiesel and mixtures B10, B20, B30, B40 and B50 were performed in a calorimeter brand Anton Parr model 6 400, using approximatively 0.5 g of sample.

Morphological characterization of J. dioica seeds

J. dioica seeds were composed by a drupe, with a firm dark brown pericarp. Inside, a white seed could be found, which was divided into two cotyledons with an embryo in the middle. To improve the exploitation for obtaining biodiesel from seeds oil, it was necessary to establish physical (arithmetic diameter, geometrical diameter and sphericity), mechanical and chemical variables, which were useful for a better design and operation of the equipment, as well as for processes of oil extraction (^{Betancur-Prisco et al., 2014}). Physical characteristics of J. dioica seeds were presented in Table 1. Measures as length, width and thickness of seeds were significantly similar, resembling a sphere. In addition, the obtained value of sphericity of 91.82 % indicated a more spherical shape compared with J. curcas.

Content and characterization of oil and biodiesel of J. dioica seeds

The average yield of the extraction of J. dioica seed oil by Soxhlet method was of 55.8 ± 1.2 % and by maceration of 43.5 ± 0.7 %. Some physico-chemical parameters of crude oil and biodiesel obtained from J. dioica seeds were shown in Table 2. Extracted oil content was observed to be similar to other oleaginous crops; as castor oil plant, 64.84 % (^{Goytia-Jiménez et al., 2010}), Soy bean 35.5 % (^{Mosquera-Artamonov et al., 2016}) and even similar to other species from Jatropha genus, 44.4 % and 56.5 % (^{Pradhan et al., 2012}; ^{Prasad et al., 2012}).

The high content of extracted oil from the seed indicated that it is a potentially useful source to be transformed in biofuel. However, this could be related to the origin and state of physiological maturity. In this case, seeds obtained after natural dehiscence of the fruits of this plant were used, presenting a dark brown pericarp, corresponding to approximatively 60 days of maturity.

The determination of gross heating value of a substance allows to estimate the quantity of energy released when it is burnt (^{Knothe, 2010}). Gross heating values obtained in crude oil and biodiesel presented, in both cases, quantities inferior to 39.500 MJ kg^-1, the established limit by International Standard ASTM D240, Method of standard test for gross heating value in hydrocarbons, by means of a calorimetric bomb for the use of biodiesel; so that both the crude oil and biodiesel obtained in this work can be considered suitable for their use in internal combustion engines.

The acid index allows to determine the percentage of fatty acids in an oil sample, in addition to evaluating whether the transesterification reaction will be satisfactorily performed. With the saponification index, an estimation of the length of fatty acid chains was obtained (^{Nielsen, 2010}), in addition, it indicated whether the oil was potentially useful for the elaboration of liquid soaps, as well as in the shampoo industry (^{Akbar et al., 2009}).

The obtained acid index (0.63 mg KOH g^-1 ± 0.03) was slightly higher than the established limit in European Union Standard EN 14214 for biodiesel production (0.0-0.5 mg KOH g^-1). Results of saponification corresponded to those obtained in other studies in species of Jatropha genus (^{Adebowale et al., 2006}), suggesting that the oil was potentially useful for biodiesel elaboration or for its direct use as a biofuel.

Kinematic viscosity is defined as the resistance of a fluid to movement, this characteristic has different behavior according to the molecular weight of the oil and the proportion of unsaturated fatty acid (^{Akbar et al., 2009}). This characteristic has a great impact on engine, causing high dragging in the injection pump, with high fuel pressures and volumes, especially operating at low temperatures (^{Pinzi et al., 2009}).

Comparison of Standards (EN 1424) of diesel and mixtures of biodiesel (ASTM D 6751) for properties of kinematic viscosity and density of mixtures prepared with biodiesel-diesel (B05, B10, B20, B30, B40 and B100) of Jatropha dioica. were shown in Table 3.

Results indicated that kinematic viscosity not lineally decreases, while temperature increases. High viscosities in fuels do not favor a good performance in engines, therefore specific characteristics are wanted, which do not affect the flow and fuel atomization (^{Akbar et al., 2009}). Kinematic viscosity of J. dioica decreases when increasing temperature from 20 ˚C to 90 ˚C (7.38 to 2.07 mm² s^-1). At 40 °C, biodiesel (B100; 4.57 mm² s^-1) was observed to cover requirements for EN 14214 Standard (3.5-5.0 mm² s^-1). In addition, mixtures B05, B10, B20, B30 and B40 of biodiesel-diesel are suitable for its use according to Standards established in ASTM D 6751-09 Standard (1.9-6.0 mm² s^-1). Data were close to those obtained by ^{Prasad et al., (2012)} who reported 5.14 mm² s^-1. The density of biodiesel-diesel mixtures had a lineal behaviour, corresponding every time more when increasing progressively diesel mixture and temperature up to 70 ˚C. ^{Gunstone (2004)} reported that the density of an oil decreases according to its molecular weight, but increases according to unsaturated fatty acids content. The density at 15 ˚C of biodiesel obtained from J. dioica (888.8 kg m^-3) achieved Standards established in EN 14214 Standard (860-900 kg m^-3) for biodiesel, making easier the way of managing different mixtures of biodiesel-diesel.

Fatty acids profile of J. dioica

The main percentages obtained in the analysis of gas chromatography and its comparison with others oleaginous species were observed in Table 4. The chromatogram used for obtaining data from the fatty acids composition by means of corresponding FAME retention time was shown in Figure 1.

The main fatty acids in J. dioica seeds were the polyunsaturated ones (60 %) and superior to saturated (15.4 %) and monounsaturated (24.36 %), in these last ones, linoleic, oleic and palmitic acids were found. These data were comparable to those determined in J. curcas by some other researchers (^{Akbar et al., 2009}; ^{Pradhan et al., 2012}). Palmitoleic, stearic and linolenic acids were present in small quantities. J. dioica contained a higher percentage of linoleic acid (57.80 %) regarding J. curcas (32.8 %), both Jatrophas were constituted by a lower quantity of palmitic acid regarding those of oil palm (Elaeis guineesis) (^{Syamsuddin et al., 2016}).

At an industrial scale, a disadvantage of J. dioica biodiesel is that it contained high polyunsaturated fatty acids levels (linoleic acid 57.80 %), which have negative effects when decreasing its stability and reducing cetane number (^{Pinzi et al., 2009}). In addition to be susceptible to the incidence of undesirable odors and flavors caused by the formation of aldehydes and alcohols caused by peroxides and hydroperoxides. However, an advantage is that it presented fluidity at low temperatures for its low content in saturated fatty acids (^{Bahandur et al., 2013}) as palmitic (9.31 %) and stearic (6.09 %) acids. Therefore, the use of biodiesel derived from oils of this species is potentially useful as a renewable energy source in cold climates.