Introduction

Sweet pittosporum (Pittosporum undulatum) is native to eastern parts of Australia. It is a tree growing to 15 m tall with wavy (undulating) leaf edges and also has attractive fragrant flowers, sweet pittosporum [1]. It is known as an invader of many tropical and subtropical mountain forests and in warm temperate regions of both hemispheres and many islands. The fruits have diameter between 1 and 1.5 cm and are orange capsules and when ripe they became sticky and possess a strong odor [2]. This tree has been introduced widely as an ornamental plant to North America, southern Brazil, and also in Africa. In Brazil, this species was introduced for ornamental purposes and also used for afforestation purpose in public way, but because of its extremely rapid growth, it turned into invasive species [3, 4].

The composition of leaves of sweet pittosporum has been studied and analyzed, were has been found several terpenoids. Higuchi et al. relate the deacylsaponin and the A1-barrigenol 3-O-glycoside, while Mendes et al. extracted the essential oils of the fruits of this tree with methanol (MeOH), and the results show that there are two types of sequiterpenes, namely 4-guaiene-11-O-b-d-(30-angeloxy-60-deoxy)-glucopyranoside and 1(5)-guaiene-11-O-b-d-(30-angeloxy-60-deoxy)-glucopyranoside [5, 6]. Moreover, essential oils extracted from sweet pittosporum leafs were characterized in order to determine the volatiles chemical diversity in this species in Azorean islands, and one of the aims of this study was to determine the chemical composition to establish the volatiles products [7].

Therefore, the exploration of new sources of seed oils has presented a great advance due to the possibility to find new fatty acids with high levels of polyunsaturated with high concentrations of the long-chain fatty acids. Thus, the main objective of this work is to contribute to the study of oils present in seeds found in Brazil to show the chemical composition and thermal properties of this new oil. The present investigation reports the chromatographic characterization and thermogravimetric (TG/DTG) and calorimetric (DSC) evaluations of the oil extracted from fruits of sweet pittosporum (Pittosporum undulatum). Additionally, the thermal behavior and kinetic evaluations of this oil by TG curves were made with two different mass samples under nitrogen and oxygen purge gases.

Experimental

Materials

The sweet pittosporum (Pittosporum undulatum) was obtained from Ivaí city (25°00′39″S, 50°51′32″W), Paraná state, Brazil. Mature fruits were collected, cooled, and subsequently submitted to a manual pulp removal process and then dried at room temperature. The seeds were grounded in a domestic blender, homogenized and sieved (0.42 mm mesh) resulting in flour which was mixed with n-hexane (analytical reagent) in an extraction bath at ambient temperature to avoid the oxidation of the obtained oil. Posteriorly, the mixture was filtered in a qualitative paper filter (Whatman 125 mm, no. 1), and the n-hexane phases were removed and concentrated in a rotary evaporator.

Methods

The method described below and used in this work on the chromatographic analysis, thermal analysis, and kinetic evaluation was widely exposed in other work carried out. Thus, it should be noted that this is not a self-plagiarism but is aimed at the use of a standard methodology. Therefore, the use of standard analysis is a practical basis that allows to reproduce the results obtained by other research groups without the need to complicate the understanding of extraction procedures and analysis.

Lipid profile

The fatty acid profiles of the oils extracted from the flours were determined using the method of Folch et al. [8], followed by lipid methylation to produce fatty acid methyl esters according to Hartman and Lago [9]. The fatty acid determination was carried out on GC-2010 Gas Chromatograph Shimadzu, Tokyo, Japan, fitted with an SP2560-fused silica column (100 m × 0.25 mm I.D., film thickness × 0.20 µm by Supelco). The oven temperature was programmed to rise from 100 to 235 °C at the rate of 10 °C min−1 under the following conditions: carrier gas H2 (2.5 mL min−1), split ratio 50:1, flame ionization detector (FID) 250 °C. Thus, the peaks of fatty acids were identified by comparing the retention time of a standard mixture of fatty acid methyl esthers (37 FAME Mix 47,885, Supelco). The area of each peak was expressed as a percentage in the chromatogram.

Thermal methodology

TG/DTG curve measurements were performed in a SDT 2960, from TA Instruments, with sample masses of 5 and 20 mg, both in platinum crucible with heating rates of de 5, 10, and 20 °C min−1. Nitrogen and oxygen were used as purge gas to perform the TG curves with the flow rate of 100 mL min−1. The phase transition evaluation was performed by DSC evaluation. The measurements were obtained from DSC1 Stare (Mettler Toledo) with a mass sample around 5 mg in the aluminum crucible. The samples were cooled from 100 to − 80 °C and subsequently heated from − 80 °C up to 100 °C, both at cooling/heating rate of 1 and 10 °C min−1 under nitrogen as a purge gas at the flow rate of 50 mL min−1. Furthermore, in the kinetic study, this work adopted the isoconversional method as a way to avoid the use of explicit kinetic models as mentioned in previous works [10,11,12,13,14,15]. Then, in this work, the kinetic parameters were obtained using the approximation to the integral temperature based on the Jacobi fraction, as proposed by Capela and Ribeiro [16].

Also, the ICTAC Kinetics Committee has recommended conditions to obtain kinetic data for thermoanalytical curves [17]. It is important to note that this Committee suggests three or more curves, because it has been considered that statistics require three minimal quantities of TG curves. We also consider being appropriate to use more than one sample mass and also purge gas to promote a better interpretation of kinetic data as much as possible, as carried out in others works, where was possible to see the results obtained with different conditions of analysis [18,19,20,21].

Results and discussion

Chromatography analysis and thermal cooling

The amount of oil extracted was about 13.8% by mass of the sample. At room temperature, the oil has a yellow color and characteristic odor, what is known in Brazil as the scent of incense. The lipid profile of this oil is shown in Table 1, where it is possible to see that this oil is predominantly composed of unsaturated fatty acids, representing around 96% of the total composition. The present analysis shows the presence of cis-11-eicosenoic (C20:1), eicosatrienoic, and docosahexaenoic acids which has not been reported by us earlier. Also, the results indicate the presence of cis-11-Eicosenoic acid, which is a fatty acid, also called as gondoic acid that is found in a variety of plant oils and seeds, as in the seed of the Simmondsia chinensis (jojoba) plant [22, 23]. However, the sweet pittosporum oil fatty acid can have in nature phytochemicals molecules, which can be a health hazard to humans, due the presence of trypsin, phenolics, lectin, among others, and therefore, this oil can be used for consuming, but after the elimination of the toxic antinutrients molecules [24, 25].

Table 1 Fatty acid composition of sweet pittosporum oil
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Based on the chain length and degree of unsaturation of the fatty acids, especially that fatty acids on the 3 and 6 positions have a great dietary importance in human foods, as suggested by Yamanaka et al. They consider the polyunsaturated fatty acids, having a carbon chain length of 18, 20, or 22 carbons, with 2 to 6 interrupted unsaturated bonds (cis configuration) [26, 27]. Nevertheless, many fatty acids were not found in significant quantities in the human body. As example, the eicosatrienoic acid (C20:9:n3), that was taken as a biochemical essential fatty acid, that had inhibitory effects on angiogenesis, shows in carried out study with cell culture system [28, 29], but however, your derivate (C20:3:n3) does not exist in animal tissues to any appreciable extent, according indicate by Hamazaki et al. [30].

The docosahexaenoic acid (DHA), with 22 carbons and six double bonds, is an omega-3 polyunsaturated fatty acid, which found in fish and marine oils. This fatty acid also is essential for normal brain growth and cognitive functions, and besides, been associated with neural development, with decreasing incidence of cardiovascular disease and other diseases [31, 32].

Essential fatty acids can be used as raw material for supplements, in functional food products fortified with lipids, which are currently available in the market to meet consumer demands or as beneficial to various materials such as creams, cosmetics, and therapeutic medicine [33]. Therefore, the evaluation of crystallization temperature and melting point of fatty acids is important. Due to the several compositions of fatty acids of this oil, in this work, the sample was subjected to a cooling and heating scans, with heating rates of 1 and 10 °C min−1. Figure 1 shows these evaluations, where it is possible to see the typical DSC curves with a sample mass of 5 mg. For analysis with 10 °C min−1, during the cooling, two thermal events can be observed: (1) the first sharp exothermic peak between − 18 and − 35 °C, (2) it is possible to see a change in base line, around − 37 and − 52 °C, which was attributed to the crystallization of the eicosatrienoic acid (C20:3n3) and docosahexaenoic (C22:6n3), respectively. Now, for analysis in cooling rate of 1 °C, it is possible to relate three events: (3) the first between 25 and 8 °C; (4) second between − 8 and − 29 °C; (5) third between − 29 and − 45 °C. The first event may be related to overlapping reactions, to both samples, oleic and cis-11-Eicosenoic acids, while the second and third are linked with the same suggestion made by the analysis of 10 °C min−1.

Fig. 1
figure 1

DSC curves of sweet pittosporum oil under cooling and heating rate of 1 and 10 °C min−1, with sample mass of 5 mg in aluminum crucible under nitrogen purge gas

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During the heating stage, for 10 °C min−1, it is possible to observe two broad endothermic events between − 44 and − 24 °C and also between − 22 and 7 °C. Additionally, there is a weak third event between 38 and 48 °C. Now, with the analysis of 1 °C min−1, it is possible to evaluate that there were other reactions, namely (1) the first between − 50 and − 45 °C; (2) the second between − 39 and − 26 °C; (3) the third between − 26 and − 16 °C (exothermic peak); (4) the fourth between − 16 and 8 °C; finally, (5) the fifth between 39 and 52 °C. These reactions, except to that one that occurs with exotherm peak, may be related to the relaxation solid–liquid of different fatty acids present in this oil during heating, which can be suggested to the following order: Docosahexaenoic (first), Eicosatrienoic acid (second), Linoleic, Oleic, and cis-11-Eicosenoic acids with the fourth interval and Palmitic with the fifth interval. Generally, the solid–liquid transitions are complex reactions because the present fatty acids may have more than one phase transition, which hampers the interpretation of the DSC curves. Usually, polymorphism occurs in the following order, α, β′, and β, which is attributed to increased melting temperature, as described by us and also for other authors [10, 12, 13, 15, 34,35,36]. Nevertheless, according to Rao, the endothermic reactions are α and β transitions, while the exothermic reaction can be attributed to α transition to β, where a molecular rearrangement occurs during heating. Therefore, it was not possible to observe that this oil has an absolute value of the melting point, due the several mixtures of fatty acids. In addition, it is well known that unsaturated fatty acids have lower melting points than the saturated fatty acids. This fact occurs because the molecular geometries for saturated fatty acids that are relatively linear although with zig-zags, while that the introduction of one or more double bonds in the hydrocarbon chain results in one or more “bends” in the molecule. As the geometry of the double bond is almost always a cis configuration in natural fatty acids, the intermolecular interactions are much weaker than saturated molecules. Then, as a result, the melting points are much lower for unsaturated fatty acids, and in general, the melting point decreases with an increasing number of isolated cis-double bonds. Table 2 indicate the several melting points of some fatty acids, included the showed in this work, as shown by several authors [37,38,39]. It should be noted that the melting point of eicosatrienoic acid (C20:3n3) was not found in the literature and therefore is not displayed (this omitted) in Table 2.

Table 2 Configuration/position and melting points of fatty acids
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Thermal decomposition

The TG/DTG curves under nitrogen and oxygen purge gases with a mass of 5 and 20 mg and heating rate of 10 °C min−1 are showed in Fig. 2a, b, while Table 3 shows the mass losses. The thermal profile for 5 mg under oxygen indicates that the initial thermal decomposition occurs after 215 °C, with the DTG curve showed that thermal decomposition occurs in fourth stages, which occurs as overlapping reactions. The analyses with 20 mg (Fig. 2b), still under oxygen, show that there are three main reactions during thermal decomposition. The first stage occurs between 220 and 383 °C, the second stage occurs between 383 and 468 °C, which shows a sharp peak, and in end of the reaction it has overlapping reactions, and the third stage (468–574 °C) is due to the final residues of the material.

Fig. 2
figure 2

TG/DTG curves of sweet pittosporum oil under oxygen and nitrogen purge gas, with heating rates of 10 °C min−1, sample mass of 5 mg in platinum crucible

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Table 3 Mass losses under nitrogen and oxygen purge gases, (100 mL min−1), with heating rate of 20 °C min−1
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According to the DTG curves for analysis in nitrogen purge gas with sample of 5 mg, it can be seen that the thermal events start from 190 °C in a continuous reaction up to 341 °C, which was attributed to the volatiles molecules, specifically known in food analysis as “smoke point.” The same behavior also can be seen in DTG curve with the sample mass of 20 mg, but between interval of 190 and 318 °C, while the second stage occur between 317 and 493 °C, without the formation of carbonaceous residue. Besides, simultaneous TG–DTA analysis under oxygen purge gas, with sample mass of 5 and 20 mg and heating rates of 5, 10, and 20 °C, is shown in Fig. 3a, b, respectively. The result shows that the thermal behavior, individually, of 5 mg that does not there were changes in the profiles of curves, while that with the increase of the mass to 20 mg there was changes. As it is possible to see in DTA curves, the behavior of the oil shows variation among the heating rates, with a sudden combustion for heating of 20 °C min−1 in 303 °C, which can be observed in the alterations of the TG and DTA curves. In analysis of 5 mg and with heating rates of 5, 10, and 20 °C min−1 and for 5 and 20 °C min−1 of the sample of 20 mg, the DTA curves show that the thermal decomposition is an exothermic reaction (without combustion).

Fig. 3
figure 3

TG/DTA curves of sweet pittosporum oil under oxygen purge gas, with heating rates of 5, 10, and 20 °C min−1, sample mass of 5 mg (a) and 20 mg (b), both in platinum crucible

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Kinetic evaluation

Table 4 provides the average of the activation energy and also the linear correlation obtained for the two mass samples in oxygen and nitrogen purge gas and also the temperature intervals in DTG curves that were used for obtaining the kinetic data. The TG/DTG curves in nitrogen and oxygen gases are shown in Figs. 4, 5. Figure 4a, b, shows that the decomposition thermal of this oil occurs with three stages for 5 and 20 mg, which are indicated in Table 2 (only for heating rate of 10 °C min−1). For kinetic evaluation, the intervals used to both sample were excluded the initial interval of temperature and also the final intervals, according to Table 2. In fact, considering the effect of “smoke point”, as aforementioned, the use from these intervals to evaluation of the kinetic data, can bring deformations in the activation energy data as determine the exact range of the smoke point and the effective thermal decomposition is difficulted. Then, a good parameter should have high correlation coefficients (r), as shown in Table 2, which has a relationship between activation energy and conversion degree [40, 41]. These values are satisfactory for the sample mass of 5 and 20 mg under nitrogen gas, but showed decrease for oxygen gas with 5 mg, which attributed to the overlapping reactions, as verified in intervals used (Table 4) and also to see in Fig. 5a, b. Besides, for oxygen analysis with mass of 20 mg, the energy of activation was not obtained because the TG curve with heating of 20 °C min−1 has undergone a combustion process and due to this a temperature regression occurs when compared to curve with heating of 10 °C min−1.

Table 4 E a/k mol−1 and correlation coefficient (r) for the thermal decomposition stages
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Fig. 4
figure 4

TG/DTG curves of sweet pittosporum oil under nitrogen purge gas, with heating rates of 5, 10, and 20 °C min−1, sample mass of 5 mg (a) and 20 mg (b), both in platinum crucible

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Fig. 5
figure 5

TG/DTG curves of sweet pittosporum oil under oxygen purge gas, with heating rates of 5, 10, and 20 °C min−1, sample mass of 5 mg (a) and 20 mg (b), both in platinum crucible

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The relation between the activation energy versus conversion degree is shown in Fig. 6. As can be seen in the nitrogen purge gas that the kinetic behaviors are the same, while for analysis under oxygen purge gas, the kinetic value decreases. In previous papers, we consider that the oils can have different behaviors during heating due the oxidation of unsaturated fatty acids, with the formation of several products, which can produce alterations in kinetic behavior [13,14,15]. Therefore, the relationship between the activation energy and sample mass allows a better evaluation of the oils.

Fig. 6
figure 6

Activation energy vs conversion degree under nitrogen and oxygen purge gases

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Conclusions

The results of gas chromatographic analysis showed that this oil has a high amount of unsaturated fatty acids, which corresponds to around 96%. The extracted oil of sweet pittosporum seeds can be recommended for utilization as food, but after treatment for removing the less toxic antinutrients, and with special attention to eicosatrienoic acid (C20:3n3). It is necessary to carry out in vivo studies to confirm their harmlessness. The DSC curves under cooling/heating show there were two crystallization peaks and also others thermal asymmetric events, which were attributed to the presence of the several components of this oil.

The TG/DTG curves under oxidant and also in an inert purge gases allows monitoring the profiles of TG curves with two sample masses and several heating rates. In this work, also it was possible to verify that the activation energy under nitrogen gas is similar to both samples’ masses. The activation energy of this oil under oxygen purge with sample mass of 20 mg was not obtained due to combustion of this sample. Besides, the activation energy behavior with sample of 5 mg was lower than that in nitrogen purge, which was attributed to the reaction of the oil with the oxygen gas.