The genus Sterculia comprises 300 species of trees that belong to the order Malvales, occurring mostly in tropical regions of the world. The most extensively studied among them is Sterculia foetida L., which is found from Eastern tropical Africa to North Australia (Mujumdar et al., 2000). Its leaves, roots and gum are used in herbal medicine as aperient, diuretic, anti-inflammatory, and as a central nervous system depressant (Chopra et al., 1992).

The species Sterculia mexicana (R. Br), locally known as “Castaño”, and Sterculia apetala (Jacq.), locally known as “Bellota”, “Castaña” or “Tepetaca”, are found in the tropical rain forests of southeastern Mexico in the states of Veracruz, Tabasco, Oaxaca and Chiapas (Penington and Sarukhán, 2005). They are exotic deciduous trees, 30–40 m in height, with horizontal branches grouped in the top of the tree in S. mexicana, or distributed over the tree trunk in S. apetala. The leaves are palmated-lobulated, 15–50 cm long, and located in groups at the end of the branches. The flowers are yellow with red or purple stripes. The fruits are brown (S. apetala) or redish (S. mexicana) follicles, 8–10 cm long, found in groups of 3–5, that contain 10–15 black seeds of 2.5×1.5 cm, each covered by orange stinging spicules. The seeds are boiled or roasted (to develop a nutlike or peanut butter-like aroma and taste) and used for human nutrition and as animal forage. In some areas, grounded seeds are used for chocolate flavoring (Benitez et al., 2004; Vazquez-Torres et al., 2010).

It has been reported that the oils extracted from plants from the order Malvales, Fabales and Sapindales naturally contain cyclopropene fatty acids (CPFA) which are unusual fatty acids containing a three member carbocyclic ring forming the center of their 18 and 17 carbon chains (Bao et al., 2003; Vickery, 1980). Sterculic acid [8-(2-octyl-1-cyclopropenyl) octanoic acid] (Figure 1A) and malvalic acid [7-(2-octyl-1-cyclopropenyl) heptanoic acid] (Figure 1B) are CPFA that contain a highly strained and reactive cyclopropene ring in their carbon chains. They are present in variable amounts in these plants, the highest content being reported for Sterculia foetida seeds oil, where the sterculic acid content is around 55% (Corl et al., 2001). In addition to its role as energy and carbon storage in seeds, it has been proposed that it may serve as a protective agent against fungal and insect attack (Schimid and Patterson, 1988). Moreover, various biological activities have been observed after sterculic acid administration in cell cultures and animal models, showing beneficial as well as adverse results. Some studies have demonstrated that sterculic acid provokes reductions in body weight and adiposity, improvements in glucose tolerance and attenuates hepatic inflammation (Gomez et al., 2003; Major et al., 2008; Ortinau et al., 2012), while others have suggested adverse effects on smooth muscle proliferation, reproductive function and growth, induction of hypercholesterolemia and liver damage, and carcinogenic effects when given at high doses to rats (>1%) (Mujumdar et al., 2000; Major et al., 2008; Lee et al., 1971; Look et al., 2004; Pawlowski et al., 1985). Some of these effects have been related to the ability of sterculic acid to inhibit stearoyl–CoA desaturase activity (SCD, also known as Δ9 desaturase), a central lipogenic enzyme catalyzing the synthesis of monounsaturated fatty acids, mainly oleate (C18:1) and palmitoleate (C16:1), by the O₂-dependent desaturation of palmitate (16:0) and stearate (18:0) (Gomez et al., 2003). Our group has found that the administration of S. mexicana seed oil to spontaneously hypertensive rats reduces body weight, adiposity, and improves blood pressure, adiponectin and triglyceride levels (Herrera-Meza et al., 2013).

Given the potential effects that the consumption of sterculic acid might have on lipid metabolism and the fact that there is no information about the fatty acid composition of Mexican Sterculiaceae, the purpose of this study was to determine the fatty acid profile and the physicochemical characteristics of S. apetala seed oil and to evaluate the effect of roasting on both aspects.

The fatty acid composition of Sterculia apetala natural seed oil is shown in Table 1. The total ion current chromatogram of the FAMEs from Sterculia apetala (A) and Sterculia mexicana (B) is shown in Figure 2, and the respective mass spectra in Figure 3. The fatty acid profile was similar to those reported for S. foetida (Corl et al., 2001) and S. mexicana (Herrera-Meza et al., 2013) seed oils, so their fatty acid composition is included in tables for comparison purposes. As can be seen in Table 1, the major component in S. apetala and S. mexicana seed oil was sterculic acid (19:1cyclo), the highest content was found in S. apetala (56.3±0.91 vs 51.3±0.29 in S. mexicana, p < 0.02; [9% higher]). The other CPFA, malvalic acid (18:1cyclo), was present in a minor proportion (1.1±0.05 in S. mexicana and 1.3±0.06 in S. apetala). With respect to other fatty acids, the most abundant was palmitic acid (16:0) followed by oleic acid (18:1) and stearic acid (18:0). Minor but significant differences were found in oleic acid (C18:1) (8.8±0.09 in S. mexicana vs 9.5±0.16 in S. apetala, p < 0. 04) and linoleic acid (C18:2) (3.8±0.06 in S. mexicana vs 2.9±0.11 in S. apetala, p < 0.01). Palmitic acid (C16:0) and stearic acid (18:0) percentages were found similar in both species (Table 1). In comparison to other Sterculia species, S. apetala and S. mexicana showed a high content of sterculic acid (56.3% and 51.3% vs 11.3% in S. tormentosa, 30.2% in S. tragacantha, 5.3 in S. striata, 4% in S. alata, 5. 8% in S. guttata, 3.2% in S. villosa), while a relatively low content was found for malvalic acid (1.3% and 1.1% vs 5.8% in S. tormentosa, 5.1% in S. tragacantha, 2.3 in S. striata, 17.6% in S. alata, 2.1% in S. guttata, 2.5% in S. villosa) (Miralles et al., 1993; Nitao et al., 2008; Badami et al., 1980). Conversely, the fatty acid profiles were more like those found in S. foetida seed oil (Corl et al., 2001) (Table 2). With respect to CPFA, sterculic acid was found in similar proportions, although S. mexicana showed a significantly lower amount (56.3±0.91 in S. apetala, 51.3±0.29 in S. mexicana vs 55.8 in S. foetida, p < 0.03). S. mexicana and S. apetala showed a significantly lower amount of malvalic acid (18:1cyclo) (1.1±0.05 in S. mexicana [81% lower], 1.3±0.06 in S. apetala [80% lower] vs 6. 3 in S. foetida, p < 0.004). A higher content of stearic acid (18:0) (6.1±1.99 in S. apetala, 8.5±0.13 in S. mexicana vs 1.8 in S. foetida, p < 0.01) and oleic acid (C18:1) (9.5±0.16 in S. apetala, 8.8±0.09 in S. mexicana vs 5.0 in S. foetida, p < 0.01) were found in both Sterculia species, while the concentration of linoleic acid was lower than in S. foetida (C18:2) (2.9±0.11 in S. apetala, 3.8±0. 06 in S. mexicana vs 5.0 in S. foetida, p < 0.02) (Table 2).

The consumption of roasted Sterculia apetala seeds among the southeastern Mexican population is very common. Roasting is executed by a handmade process intended to develop a desirable flavor and aroma similar to peanut butter or nuts. We followed the procedure used by local people in order to analyze the effect of the roasting process on Sterculia apetala seed oil's fatty acid composition. It has been reported that heat destroys the inhibitory activity of sterculic acid over SCD (Gomez et al., 2003), and that vegetable edible oils containing CPFA need to be treated with high temperatures before consumption in order to avoid possible health problems (Bao et al., 2003). Results showed that roasting exerted important changes in the seed oil's fatty acid composition. Although sterculic acid was still the main component of the seed oil, its amount diminished by 36% with respect to the natural seed oil (35.9±0.12 in roasted seed oil vs 56.3±0.91 in natural seed oil, p < 0.001). The content of malvalic acid was found to have increased by 96% (2.55±0. 016 in roasted seed oil vs 1.2±0. 06 in natural seed oil, p < 0.001). We have found that the administration of natural seed oil from S. mexicana to spontaneously hypertensive rats provokes an inhibition of SCD activity (Herrera-Meza et al., 2013). Whether these modifications may cause a loss in the inhibitory activity of sterculic oil over SCD remains to be investigated. With the exception of stearic acid, the percentage of all the other fatty acids was increased after the roasting procedure (Table 3).

The physicochemical properties of S. apetala natural and roasted seed oil are shown in Table 4. The refractive index value of natural seed oil was similar to that reported for common oils such as soybean (1.4) (Bello et al., 2011), and was not modified by the roasting procedure. Specific gravity in the natural seed oil was found similar to other edible oils (Bello and Agg, 2012), and was increased after seed roasting. The iodine value is a measurement of the degree of instauration and reflects the susceptibility of oil to oxidation (Bello et al., 2011). The oil presented a low iodine value (45.7) which is in accordance with seed oil's fatty acid profile, mainly composed of saturated and monounsaturated fatty acids; it was found to have decreased after roasting (33.9). Since acid value is indicative of the content of free fatty acids in the oil, it is an important variable when considering the quality of oil: the lower the free fatty acids, the better the quality (Ibeto et al., 2012). According to the results, there are free fatty acids and triglycerides, methyl esters were obtained from both. S. apetala seed oil showed a very high acid value, both in natural (79.4 mg KOH·g⁻¹) and in toasted (87.8 mg KOH·g⁻¹) seed oil; however it could be minimized during refining of the oil. The saponification value is expressed as the number of milligrams of potassium hydroxide (KOH) required to saponify 1 g of sample (Nayak and Patel, 2010). The saponification values of natural and roasted seed oil were 140.4 and 159.3 which indicates that the oil contains high molecular weight fatty acids on average. In consequence, this oil is not suitable for soap manufacturing.

In conclusion, the cyclopropenoic fatty acids, malvalic and sterculic acids were identified in Sterculia apetala seed oil in high amounts, similar to those found in Sterculia mexicana and Sterculia foetida. The roasting process modified the fatty acid composition of the seed oil; in particular a decrease in the content of sterculic acid was observed. It is noteworthy that S. mexicana and S. apetala seeds are commonly used in some tropical areas in Mexico for human and animal nutrition, so epidemiological studies are needed to determine possible health effects on the population. To our knowledge, this is the first report of the fatty acid composition of S. apetala seed oil.