Extraction conditions and identification of volatile organic compounds from umbu pulp by HS-SPME/GC-MS

-despite the high social and economic importance of Spondias tuberosa Arruda, to the best of our knowledge there are very few detailed studies on the volatile compounds of this fruit popularly named umbu. Therefore, the aim of this study was to find the best extraction conditions by solid-phase microextraction in headspace mode (HS-SPME) to determine the volatile compounds from umbu fruit pulp by gas-chromatography coupled to mass spectrometry (GC-MS). The optimal conditions were obtained using 4 g of pulp and 0.2 NaCl/pulp (w/w), maintained for 10 min in incubation at 40 °C. The SPME-fiber was exposed for 20 min for extraction and then for 40 min for desorption. Thus, a total of 25 volatile compounds were detected and 16 were identified under these conditions, with 9 compounds being identified for the first time in the volatile fraction of umbu fruit pulp and two compounds were identified for the first time in the Spondias genus. The major chemical class was terpenes and esters, which together represent more than 90% of total chromatographic area.


Introduction
The Caatinga is one of the main Brazilian biomes and has stood out for its variety of species which present high importance for the population (FILIZOLA; SAMPAIO, 2015). Several authors have highlighted Spondias tuberosa among these species, popularly known as umbuzeiro, and its fruits are called umbu Although the species has some different characteristics within the ecosystem, their fruits are generally reported as having an exotic and sweet flavor, greenish color and a diameter of about 3 cm (DE LIMA; SILVA; DE OLIVEIRA, 2018;. The economic exploitation of S. tuberosa fruits has been carried out through extractivism by Brazilian semi-arid populations, but some umbuzeiro orchards have been established in different locations in the North of Minas Gerais, being considered a species undergoing domestication (DONATO et al., 2019;. Previous studies have been carried out with S. tuberosa aiming at better knowledge for preserving this species (DONATO et al., 2019;SALES et al., 2019;SANTOS et al., 2011). In this sense, the Empresa de Pesquisa Agropecuária de Minas Gerais (EPAMIG) has cultivated several S. tuberosa accessions for 20 years (MOREIRA et al., 2007). Nevertheless, there are few and insufficient studies on the volatile compounds of the fruits of S. Tuberosa (THOMAZINI et al., 1998 apud FRANCO;JANZANTTI, 2005;GALVÃO et al., 2010GALVÃO et al., , 2011. Although there are studies on the identification of this class of compounds in umbu fruits, the technique used for extracting volatile compounds involves heating and the use of solvents (THOMAZINI et al., 1998 apud FRANCO;JANZANTTI, 2005;GALVÃO et al., 2010GALVÃO et al., , 2011. It is important to remark that there are studies about the characterization of volatile compounds from fruits of other species of the Spondias genus by techniques without solvents and at close to ambient temperatures, but not of S. Tuberosa, one of the most important Brazilian semi-arid fruit (CEVA-ANTUNES et al., 2003TODISCO et al., 2014).
There are currently solvent free techniques which also do not implement temperature increases, and consequently avoid contamination and chemical transformations in the analytes, such as solid phase microextraction in the headspace mode (HS-SPME) which has been widely used for characterizing volatile compounds in fruits. Despite being a well-known technique and widespread in the scientific community, several studies have shown that the chemical composition of the volatile fraction depends on the conditions employed in the extraction method, and therefore optimizing the extraction parameters may lead to a greater content and number of substances (MEHTA et al., 2018;NONGONIERMA et al., 2006;SILVA et al., 2019).
Thus, the aim of this study was to optimize the extraction conditions by solid phase microextraction in the headspace mode (HS-SPME) for determining the volatile compounds from S. tuberosa fruit pulp by gaschromatography coupled to mass spectrometry (GC-MS). Gerais. The fruit pulps were separated from the seeds and peels and homogenized using a domestic mixer for 30 s. Then, NaCl was added and mixed again for 1 min. Aliquots of the material were then transferred to 20 mL vials, sealed, and analyzed by HS-SPME-GC-MS.

Extraction method
The extraction of volatile compounds was performed by the HS-SPME method using SPME-fiber of divinylbenzene/carboxen/polydimethylsiloxane (DVB/ CAR/PDMS) from Supelco (São Paulo, Brazil). The vials with the samples were inserted in a thermostatic bath, and SPME-fiber was subsequently inserted in the vials. Finally, the fiber was transferred to the gas-chromatograph injector. The volatile compound extraction conditions by HS-SPME were chosen through univariate analysis. For the choice of extraction parameters, the effect of different sample mass, m(NaCl)/m(fruit pulp) ratio, incubation time, extraction time using SPME-fiber, temperature and desorption time in the chromatography injector (values of parameters was present in Table 1) on the total chromatographic area and number of volatile compounds detected was evaluated. All procedures were performed in duplicate.

Analysis and identification of volatile compounds
After the extraction, the SPME-fiber was injected into the gas-chromatograph for compound desorption and kept in the equipment until the end of the chromatographic analysis. Static headspace analysis was performed using a PAL Syr HS 2.5 mL for combi-PAL. The chromatographic analyses were performed in a 7890A GC gas chromatograph coupled to 5975C mass spectrometer (Agilent Technologies). The capillary column was a DB-5 MS column of 30 m x 0.32 mm x 0.25 μm from Agilent technologies. The GC oven temperature was started at 60 °C, and then increased by 3 °C min -1 to 240 °C. Helium (99.9999% purity) was used as carrier gas at a flow rate of 1 mL min -1 . All samples were injected in splitless mode with the injector temperature at 220 °C. The interface temperature was 280 °C, and the acquisition mass range was 45-600 m/z. A standard hydrocarbon solution (C7-C30) was injected under the same conditions to calculate the Van den Dool and Kratz retention index (LRI) of the sample compounds. Identification of the volatile compounds was performed using mass spectra NIST 2.0 database and Van den Dool and Kratz retention index (LRI) (Van den Dool; Kratz, 1963).

Statistical analysis
The data were analyzed by one-way ANOVA with Tukey's test (P<0.05) using BioEstat 5.0 free software made available by the Instituto Mamirauá (INSTITUTO DE DESENVOLVIMENTO SUSNTETÁVEL MAMI-RAUÁ).

Results and Discussion
Optimization of the extraction conditions The first step was to evaluate the best sample mass for extracting volatile compounds from umbu pulp. The results obtained may be observed in Figure 1 and Table  1. The total chromatographic areas obtained were higher using 2 g and 4 g of sample. However, it was observed that 15, 15 and 16 volatile compounds were detected using 1, 2 and 4 g, respectively. Therefore, 4 g was selected as the better condition in this work. Despite having been the specific choice in this work, we emphasize that the use of a larger amount of sample does not necessarily improve the extraction of the compounds. The second step was to choose the best proportion between salt mass and pulp mass to be used in the analysis. The results obtained are shown in Figure 2 and Table 1. It was possible to observe that the proportion of 0.20 g of NaCl/g of pulp promoted a higher number of volatile compounds (17 compounds) and higher chromatographic area. Therefore, the addition 0.2 g of NaCl/g of pulp was chosen for the next step of this study. The causes of this effect have been well explained in previous works (ZHANG;YANG;PAWLISZYNZ, 1994;PROSEN;ZUPANCIC-KRALJ, 1999;PORTO-FIGUEIRA et al., 2018).

Figure 2.
Total chromatographic area of volatile compounds using 0.050, 0.10 and 0.20 g of NaCl per gram of umbu fruit pulp. Different letters indicate significant differences (P < 0.05), according to the Tukey's test.
The third evaluated parameter was the temperature for extracting volatile compounds from umbu fruit pulp. The results obtained showed no significant differences in the total chromatographic area (Figure 3). It was observed that 15, 17 and 17 volatile compounds were detected using 30, 35 and 40 °C, respectively (Table 1). However, 40 °C promoted a minor experimental deviation in comparison with 35 °C. Therefore, the chosen temperature was 40 °C. In addition, the choice in this case also considered that the same temperatures have been used to determine volatile compounds in other fruit matrices. (GIANNETTI et al., 2017;MAMEDE et al., 2017;PORTO-FIGUEIRA et al., 2018). The fourth stage of this study was to evaluate the extraction time, meaning the SPME-fiber exposure time inside the headspace vial to adsorb the volatile compounds from the umbu fruit pulp. The results obtained may be observed in Figure 4 and in Table 1. It was observed that 20 min promoted a higher number of volatile compounds (18 compounds) than the extractions by 15 (17 compounds) and 10 min (16 compounds), as may be seen in Table 1. In addition, the total chromatogram area was higher using 20 min, as shown in Figure 4. Therefore, this extraction time was defined for the next steps in this study.
The fifth step of this study was to study the incubation time of volatile compounds from umbu fruit pulp, and the results obtained are shown in Figure 5 and in Table 1. The results revealed that the incubation time of 10 min did not promote a statistical difference in chromatographic area, but the number of detected compounds (17 compounds) was higher in relation to the other times, as may be seen in Figure 5 and in Table 1. Therefore, 10 min was chosen as the best incubation time. The last evaluated parameter was the retention time of the SPME-fiber inside the chromatographic injector for desorbing the volatile compounds. The results obtained from this experiment are presented in Figure 6 and in Table  1S (Supplementary material). They showed no significant differences in the total chromatographic area and number of volatile compounds detected (16 compounds), as may be seen in Figure 6 and Table 1. Although 1 min has been enough time to ensure satisfactory desorption (ARTHUR et al., 1992;PROSEN;ZUPANCIC-KRALJ, 1999), the permanence of the SPME-fiber for a longer period in the injector ensures the absence of interferences in the next analysis. Therefore, 40 min was chosen to desorb and condition the SPME-fiber during the analysis despite being a long time, thus avoiding contaminating an immediately following analysis. Figure 6. Total chromatographic area of volatile compounds from umbu fruit pulp using 1, 5 and 40 min retention time of the SPME-fiber inside the chromatographic injector. Different letters indicate significant differences (P < 0.05) according to the Tukey's test.

Volatile compounds from umbu fruit pulp
After completing the optimization of all these parameters, an analysis was performed on the optimized extraction conditions, and the total ion chromatogram obtained is shown in Figure 7.
A total of 25 volatile compounds were detected by GC-MS from umbu fruit pulp, and 16 compounds were identified, representing about 91.5% compound abundance and 64% of the compound number (Table 2). The relative chromatographic areas of the main chemical classes identified in the volatile compounds from umbu fruit pulp may be seen in Figure 8.   Figure 7. t Linear retention index on DB-5 (or equivalent) column from literature (AVSAR et al., 2004;BACCOURI et al., 2007;BALBONTÍN et al., 2007;DEHGHAN et al., 2007;FLAMINI et al., 2007); ti tentatively identified compound. Terpenes were the main class containing representing about 64% of total chromatographic area. These compounds are important for predominantly acting in plant protection and communication (GERSHENZON;DUDAREVA, 2007). The main compound in this class was α-pinene with ~20% of the total chromatographic area, also being the main compound of volatile compounds from umbu fruit pulp. Two other terpenes found in high quantities were limonene and β-ocimene with 19% and 15% of the total chromatographic area, respectively.
The second largest chemical class was esters, with ethyl butanoate with ~14.3%. These compounds have already been identified in other fruits (CHEN et al., 2006). The other chemical classes detected were aldehydes. Seven unidentified compounds which do not belong to any of these chemical classes were also observed in this study, representing 1.65% of the total chromatographic area.

Conclusions
The influence of pulp mass, salt/pulp ratio, temperature, incubation time, extraction time, and desorption time on the HS-SPME extraction were evaluated and the optimal conditions were chosen to extract volatile compounds from umbu fruit pulp. The chosen conditions were 4 g of pulp and 0.2 NaCl/pulp (w/w), maintained for 10 min in incubation at 40 °C, the extraction time was 20 min and the desorption time was 40 min. Thus, 25 volatile compounds were detected under these conditions, in which 9 compounds were identified for the first time in this fruit, and two compounds in the Spondias genus were characterized for the first time from vapour-phase extraction of umbu fruit pulp. Terpenes and esters were the predominant chemical classes in this volatile compound, together representing ~90% of the total chromatography area, of which the most predominant were α-pinene at ~20 %.