Field collection

The collection of seeds was carried out through field trips between the months of June 2014 to May 2015, considering the annual availability of the plants, in 17 municipalities in the states of Zacatecas, San Luis Potosi and Aguascalientes, Mexico. The following criteria were used for the selection of the species in the field: 1) abundance; 2) seed availability; 3) accessibility to the sites; and 4) use history.

The species and the characteristics of their location are shown in Table 1. The 19 species collected in the study area are: chicalote Argemone mexicana, calabacilla loca Cucurbita foetidissima, chilacayota Cucurbita ficifolia, melón hediondo Apodanthera undulata, aceitilla Bidens odorata, jara Leonotis nepetifolia, toloache Datura inoxia, cardo Datura ferox, cadillo Xanthium strumarium, toritos Proboscidea louisianica, mostacilla Brassica sp., saramago Eruca sativa, sangre de grado Jatropha dioica, huizache Acacia farnesiana, mezquite Prosopis glandulosa, gobernadora Larrea tridentata, maguey Agave sp., pirúl Schinus molle and lampote Helianthus annuus.

To estimate its productivity in the field, once the plant was located, the methodology proposed by ^{Mostacedo and Fredericksen (2000)} was followed, which recommends the sampling method by square plot of 1 m * 1 m for herbaceous plants, of 5 m * 5 m for creepers and 10 m * 10 m for shrubs and trees. In each of the plots, the coverage, density and frequency of plants, as well as the number of seeds per plant, of all the plants contained in the respective plot were estimated. The variables density and number of seeds per plant were used to estimate the potential yield of seed extrapolated to one hectare under natural conditions. For each collection site, the location and altitude coordinates were recorded using a GPS and georeferencing was performed using the Google Maps program.

Morphological characterization of seeds

The clean seeds were dried at 60 °C for a period of 18 h. Subsequently, the weight (g) of 100 seeds taken at random from each of the species considered in the study was recorded by means of an analytical balance (Vlab V300^®). To 10 seeds of each the species were measured length, width and thickness (cm) using a vernier and a tape measure, according to the methodology proposed by ^{Pérez et al. (2006)}.

Extraction and characterization of oils

The chemical extraction of oil was carried out using a fat and oil determinant (Soxtec System HT 1043), according to the technique applied by ^{Loredo (2012)}. All analyzes were performed in triplicate.

Once the oils were extracted, the determination of the following parameters was carried out, taking into consideration the Official Mexican Standards (NOM): refractive index (NMX-F-074-S 1981), saponification index (NMX-F-174-S-1981), acid number (NMX-F-101-1987), iodine number (NMX-F-408-S-1981) and peroxide index (NMX-F-154-1987). In this study, considering that each species produced a different oil content, and that in its analysis different sample quantities are required, the characterization was done in duplicate, depending on the amount of oil available from each species and was concentrated in the chemical aspect.

Infrared spectra of the samples extracted in the region 600 to 4 000 cm^-1 were also made in a Nicolet is50^® spectrophotometer by comparing the absorbance peaks of the signals obtained through the analysis of the functional groups present in each one of the oil samples. For this, oleic, linoleic, linolenic and palmitic fatty acid (Sigma-Aldrich^®) standards were used. The graphics were made through the Origin Pro 8 Program.

Statistic analysis

The variables obtained were calculated descriptive statistics parameters (means and standard deviation) and for potential seed and oil yield, analysis of variance was performed; the separation of means was performed by the Tukey test (p≤ 0.05) using the SAS program (Statiscal Analysis Software, V9.1). In addition, some principal components analysis (PCA) was carried out using the InfoStat/L program.

Potential seed and oil yield

The statistical analysis performed shows that there is a statistical difference for the potential seed yield and oil yield, depending on the vegetable species under study (p≤ 0.05). Agave sp., was superior and statistically different (DSH= 2457) to the rest of the species, registering a potential seed yield of 24 305 kg ha^-1. However, this represents a special case given that in large species of this type of plants maturation can occur between 10 and 25 years, while in small plants of this species can occur between four and five years (^{García-Mendoza, 2002}).

In the Figure 1 shows that four species registered potential seed yields greater than 1 000 kg ha^-1, but excel, in addition to Agave sp., A. mexicana, P. louisianica and D. inoxia. On the other hand, seven species presented seed yield values lower than 100 kg ha^-1, with L. tridentata standing out among them with only 0.8 kg ha^-1 despite having registered one of the highest densities. An important group of species were located with potential intermediate seed yields, with the exception of P. glandulosa, all of them are medium-low, herbaceous and creeping. Regarding B. odorata, it highlights that despite registering a high density per unit area and having a high number of seeds, it was not reflected in their potential yield due to their low weight. Similar behavior was observed with L. tridentata since its seeds are very light.

Figure 1 Relationship between potential seed yield and oil content. 

In X. strumarium its reduced yield could be associated with the presence of a low number of seeds (two seeds per fruit); however, it is a species to consider since its seed contains 20.4% oil. In the particular case of Brassica sp., its low yield could be the result of its low density and small seed size; however, it has a high percentage of oil (30.3%), which is interesting especially if one considers that it can be cultivated intensively in order to increase its seed yield.

In other cultivated oleaginous species, very contrasting results have been observed in terms of yields, especially considering that many of them are under experimental conditions. ^{Mazzani (2007)} points out that R. communis can produce between 350 and 700 kg ha^-1 when minimal care is provided and up to 1 250 kg ha^-1 in already technified sowings. In Cuba, when evaluating different accessions of the same species, yields of up to 4 398 kg ha^-1 were obtained (^{Machado et al., 2012}).

The results obtained for oil yield of the species studied are shown in Figure 1. A. mexicana was the species with the highest oil yield with 1 315 kg ha^-1, statistically different from the rest of the species (DSH= 773.2). A. mexicana was as competitive or better than some cash crops currently used in the production of biodiesel, such as soy Glycine max (335 kg ha^-1), sunflower Helianthus annuus (568 kg ha^-1), peanut Arachis hypogaea (712 kg ha^-1), rape Brassica napus (832 kg ha^-1), pinion Jatropha curcas (950 kg ha^-1), castor oil Ricinus communis (1 133 kg ha^-1), tung Aleurites fordii (1 204 kg ha^-1), according to ^{Martinez-Valencia et al. (2011}). A. undulata and P. lousianica showed interesting oil yields of 743 and 730 kg ha^-1.

These are derived from its large number of fruits, size and quantity seeds and especially its oil contents greater than 30%. However, their values are lower at 5 772 L ha^-1 obtained in sunflower by ^{INTA (1997)}, but close to those consigned by ^{Rebora and Gomez (2007)} for rapeseed, where in managed varieties 1 466 L ha^-1 were obtained. For its part, A. mexicana registered a potential oil yield of 1 315.9 kg ha^-1, a value much higher than that found by ^{Singh (2010)} of 190 L ha^-1, so it can be considered that, with proper management and high densities of plants, yields could increase. It is worth noting that this oil yield is even higher than that obtained in R. communis in Cuba (1 130.2 kg ha^-1) (^{Machado et al., 2012}).

In Figure 1 highlights the trend presented in C. ficifolia as it presents a combination of high seed yield and high oil content. On the contrary, the contrasts of the A. undulata species are observed to obtain a high content of oil combined with a low yield of seeds and the particular case of S. molle to generate a large number of seeds, but with a small percentage of oil.

Percentage of oil

In Figure 2, the percentages of oil obtained from the seeds of the species used are presented. It is highlighted that six species presented percentages higher than 30%, among which are: C. ficifolia (34.1), C. foetidissima (33.90), P. louisanica (33.6), A. undulata (31.40), J. diodica (32.86) and Brassica sp. (30.33). These values are generally lower than those reported by ^{Martinez-Valencia et al. (2011}) for oilseeds commercially cultivated as sunflower Helianthus annuus (45-55%), canola Brassica napa (40-44%), palm Elaeis guineensis (44-57%), coconut Cocus nucifera (65-75%), peanut Arachis hypogaea (48-50%) and safflower Carthamus tinctorius (35-40%); or the one reported by ^{Calderón et al. (2013)} of 51% for palm. The highest yields, on the other hand, are within the values observed by ^{Cruz et al. (2015)} for various accessions of Jatropha curcas, which ranged between 31 and 49%, as well as those mentioned by ^{Martínez-Valencia et al. (2011)} for this same plant ranging from 20 to 60% and for Ricinus communis (26-66%). However, they are superior to yields soybean Glycine max (18-20%) and cotton Gossypium hirsutum (18-25%), as reported by ^{Martínez-Valencia et al. (2011)}. In Figure 2, it is also seen that five of the species used obtained an oil percentage between 10 and 30% and six resulted with an oil percentage lower than 10%.

Figure 2 Percentage of oil obtained from the species of plants collected. 

Figure 3, elaborated with the first two main components, shows that the field variables that influence the percentage and content of oil are the density of plants and the weight of 100 seeds (g), since species such as: A. mexicana, D. inoxia, A. undulata, P. louisianica, C. ficifolia and C. foetidissima, are grouped, indicating a similar behavior of these species. In the case of higher species such as S. molle, A. farnesiana, L. tridentata and P. glandulosa are associated with a low frequency and a low oil content. Likewise, the tendency was observed that the plant height variable is negatively related to the oil content, since none of the tree species presented a high percentage of oil. Although it is also observed that the plant height variable shows a tendency to produce a higher potential seed yield. As for the components, the first two explains a total of 49% of the variability contained in the data.

Figure 3 Behavior of 19 plant species and main variables with the main components 1 and 2. Physical-chemical characterization of oils 

The quality and efficiency of biodiesel depend on the process and the quality of the oil generated by the crop; that is, oils with a low concentration of free fatty acids, high in monounsaturated fatty acids and without gums and impurities, among other physicochemical properties (^{Martínez-Valencia et al., 2011}). ^{Martínez-Sánchez et al. (2015)} point out that the most used chemical characteristics for the classification and determination of the commercial quality of the oils are: iodine index, saponification, peroxides and acidity, within the physical characteristics they emphasize the specific gravity, the refractive index, density and the melting point. The values of the indices of refraction, saponification, iodine, acidity and peroxides, of the oils contained in the species studied, are shown in Table 2.

The refractive index (IR) of the oils registered very homogeneous values, ranging from 1 473-1 487. The oil of R. communis, one of the most used oils in the production of biodiesel, has a value of 1.47 or very close (^{ASTM, 2008}). Some of the outstanding species that showed this characteristic were Datura ferox (1.47), Datura inoxia (1.472), A. mexicana (1.473) and C. foetidissima (1.475).

The saponification indexes (IS) registered in the oils of C. foetidissima, P. louisianica and Brassica sp., Are very similar to the values obtained for R. communis (174.6, 176.3 and 173.8 mg KOH g^-1) indicated by ^{Danlami et al. (2015)}, the oils of these species could be used in the production of biodiesel. It is worth noting that some of the oils obtained from the plants studied share characteristics with the sunflower, rapeseed and castor oil mentioned by ^{CORPODIB (2003)}. Also, in the cosmetics industry, ^{Ruiz and Huesa (1991)} observed IS values for Shea Butter (Butyruspermum parki) 180-190 mg KOH g^-1, these values are similar with C. foetidissima oils (180.02 mg KOH g^-1) and P. louisianica (182.83 mg KOH g^-1), which makes possible its potential use in the cosmetics industry.

^{Cruz et al. (2015)} obtained in J. curcas, one of the inputs most used in obtaining biodiesel, IS that ranged between 192 and 196 mg KOH g^-1, only P. louisianica approached these values.

In relation to the acidity index (AI), ^{Martínez-Valencia et al. (2011)} mention that values greater than 5% indicate that the oil contains a high amount of free fatty acids, generated by a high degree of hydrolysis, which negatively affects the efficiency of transesterification in the biodiesel production process. In Table 2, it can be seen that the oil of the species with AI of less than 5% are Cucurbita ficifolia (0.68%), Brassica sp. (3.13%), Eruca sativa (4.24%), Proboscidea louisianica (3.50%) and Xanthium strumarium (1.79%), which makes them suitable for the production of biodiesel without the need to carry out a pretreatment of the oil to neutralize the acids free fatty acids. The oil of the rest of the species presents AI greater than 5%, of which Datura ferox, Leonotis nepetifolia and Schinus molle had the highest values with 29.66, 36.17 and 26.80%, respectively.

Other oils used to make biodiesel such as soybean show an AI of 1.5% (^{Pinzi et al., 2009}), while those of oil palm (Elaeis guineensis), Mexican pinion (Jatropha curcas) and castor bean (Ricinus communis) present values for this index of 4.95, 3.19 and 1.77%, respectively (^{Martínez-Valencia et al., 2011}). On the other hand, ^{Lafargue-Pérez et al. (2012)} report values of 11.84% for J. curcas.

The iodine index (IY) of the oil in the species studied showed a strong oscillation between 23.35 and 141.26 g I₂ 100 g^-1 (Table 2). High values of IY indicate high degrees of unsaturation in the fatty acids of triglycerides, which causes, in the biodiesel made from them, a greater tendency to oxidize and low cetane numbers, which in turn results in a biodiesel unstable and of low combustion quality in diesel engines (^{Martinez-Valencia et al., 2011}). For that reason, the European standard EN-14214 establishes an upper limit for the IY of 120 g I₂ 100 g^-1. In that sense, the oils of A. undulata and C. foetidissima, with 123.8 and 141.26 g I₂ 100 g^-1, respectively, could produce a biodiesel that does not conform to the norm. However, oils with high IY can be used in the manufacture of cosmetics for the ease with which they are absorbed by the skin (^{Londoño et al., 2011}) or in the production of high quality food oils.

On the other hand, low values of IY indicate a greater presence of saturated fatty acids and a lower presence of polyunsaturates, which provides greater stability as they are less prone to oxidation and with higher cetane numbers; however, these saturated fatty acids tend to solidify at higher temperatures, so their use for biodiesel should be restricted to tropical regions, since in regions with cold climates, diesel engines may present starting problems (^{Martínez-Valencia et al., 2011}). The most desirable in the IY, is that they present low values, since this allows a greater stability of the oil in storage (^{Martínez-Sánchez et al., 2015}).

Oils with low values can also be used in personal hygiene products. The IY registered for oil from the African palm Elaeis guineensis is 52.29 g I₂ 100 g^-1 (^{Calderón et al., 2013}) and from 53 to 57 g I₂ 100 g^-1 (^{Martínez-Valencia et al., 2011}).

However, according to ^{Gunstone et al. (2004)} about 82% of the oil E. guineensis are saturated fatty acids, so it tends to solidify at low temperatures, which is why it is recommended to use biodiesel made from it only in hot climates. It could be inferred that the oils of Brassica sp., H. annuus, L. nepetifolia and X. strumarium, with IY of 29, 29, 31 and 23 g I₂ 100 g^-1, could produce a biodiesel with this problem. These same authors indicate values of 121-143, 96-117, 127-142, 96-101 and 85 g I₂ 100 g^-1, for soy, rapeseed, sunflower, mexican pinion and castor bean crops, respectively. On the other hand, ^{Armendariz et al. (2015}) mention, for biodiesel produced from R. communis, an IY of 85 g I₂ 100 g^-1, values similar to those of B. odorata, C. ficifolia, E. sativa and S. molle.

The iodine (IY) levels obtained in the oils of the species studied, due to their degree of unsaturation, can be considered as drying, semi-drying and non-drying. Driers, usually, are those that have a high IY (^{Bailey, 1984}) and can be used in the production of paints and varnishes. For this purpose the oils of the species C. foetidissima, J. dioica, D. inoxia, and P. louisianica could be used since they have IY of 141.26, 113.49, 114.95 and 116.65 g I₂ 100 g^-1, respectively. The non-drying oils include A. farnesiana, B. odorata, Brassica sp., C. ficifolia, E. sativa, H. annuus, L. nepetifolia, S. molle and X. strumarium. These oils by their characteristics under normal weather conditions do not solidify. Semi-dryers can be considered A. undulata, A. mexicana, D. inoxia, J. dioica and P. louisianica, with characteristics similar to commercial oils such as sesame and corn (^{Ortega and Vázquez, 1993}).

Peroxides are known as compounds of the primary decomposition of the oxidation of fats and oils (^{Gómez, 2010}), so that those values of peroxide index (IP) close to 0 meq kg^-1, are related to a level of low rancidity; that is to say, they oxidize slowly allowing the oil to retain its quality for a longer time, giving them an advantage for possible later use. A high IP value indicates the presence of oxidation; in this study the oils of the majority of the vegetable species studied had an IP less than 1. According to ^{Luna-Guevara and Guerrero-Beltrán (2012)} the IP and the AI are considered indicators of quality and freshness of the oils. For African palm oil, very high peroxide values of 31.3 meq kg^-1 were obtained (^{Calderón et al., 2013}).

FTIR spectrometry

Figure 4 shows the spectrum of palmitic, oleic, linoleic and linolenic fatty acid standards, determined. In this way, the purity pattern of the standards was confirmed and allowed to compare them with the oil samples of the species studied. It is observed, for example, that the palmitic one expresses in the fingerprint seven peaks located in the region 1 187-1 319 cm^-1 that differentiate it from the other fatty acids which did not present those characteristic peaks.

Figure 4 Infrared spectrum of fatty acids used as a standard. 

According to the spectrum of fatty acids and by comparison, the fatty acids present in each of the species studied were detected. Table 3 shows a predominance of oleic and palmitic acids in most species. In the case of C. foetidissima, A. undulata and J. dioica, linoleic and linolenic acids were recorded, which are important in the diet since the body does not synthesize them, which gives them a greater added value for possible use as edible oils (^{Koolman and Röhm, 2004}).

It is worth mentioning that oleic fatty acid (monounsaturated), besides being one of the main components of the main edible oils, can contribute to reduce the levels of glucose and cholesterol in the blood, as well as reduce the risk of cardiovascular diseases and obesity (^{Luna-Guevara and Guerrero-Beltrán, 2012}).