Oxidative stability of soybean and corn oils enriched with Pluchea quitoc hydroalcoholic extract

2.1. Chemicals

Ethanol, methanol, glacial acetic acid, sodium acetate, acetonitrile (99.9%), caffeic acid (98.0%), ferulic acid (98.0%), gallic acid (97.5%), rosmarinic acid (98.0%), p-coumaric acid (98.0%), rutin (94%), quercetin (95.0%), campesterol (65%), β-sitosterol (95%), lupeol (94%), lupeol acetate (95.0%), 2,2-diphenyl-1-picrylhydrazyl (DPPH), (±) -6-Hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid (Trolox), ethyl benzothiazoline-6-sulfonic acid diammonium 2,2’-azino-bis (ABTS), sodium chloride, aluminum chloride, sodium acetate, sodium carbonate, potassium persulfate and butylhydroxyltoluene (BHT), were all purchased from Sigma-Aldrich, São Paulo - SP, Brazil. Additive-free vegetable oils were supplied by the company Cocamar, Maringá-PR, Brazil.

2.2. Plant material

The aerial parts of P. quitoc were collected in Ibarama-RS, Brazil, in January 2018. The collected material was dried in a place with low luminosity and humidity, at 35 ºC for 5 days, and maintained at room temperature for further processing. The voucher specimen was deposited in the Herbarium of the Faculty of Biological Sciences of the Federal University of Grande Dourados - UFGD (voucher no. 8507).

2.3. Preparation of hydroalcoholic extract

The P. quitoc aerial part (1.0 kg) was crushed and subjected to exhaustive extraction with ethanol/water (70/30, 2L) at room temperature and with occasional stirring. The extraction process was carried out over 10 days, and every 2 days in maceration, the hydroalcoholic extract was filtered and stored in amber bottles. The hydroalcoholic extract was obtained by removing the solvents in a rotary evaporator (50 °C) under vacuum. The drying was carried out at room temperature, 10 mL of the extract were deposited onto plates and kept for 48 hours. This process resulted in a solid, which was removed and conditioned.

2.4. Determination of the total phenolic content

The total phenolic content was determined according to the Folin-Ciocalteu’s reagent method (Djeridane et al., 2006Djeridane A, Yousfi M, Nadjemi B, Boutassouna D, Stocker P, Vidal N. 2006. Antioxidant activity of some Algerian medicinal plants extracts containing phenolic compounds. Food Chem. 97, 654-660. https://doi.org/10.1016/j.foodchem.2005.04.028 ). In a short period of time, 100 µL of the aqueous extract solution (1 mg·mL^-1) were added to 1000 µL of ultrapure water and 500 µL of the Folin-Ciocalteu’s reagent (1/10) in water. After 1 min 1500 µL of Na₂CO₃ (20% w/v) were added. The final mixture was shaken and incubated for 2 hours in the dark. The absorbance was read by a spectrophotometer (FENTO 700 PLUS) (λ=760 nm). Gallic acid (Sigma-Aldrich, USA) was used as standard at concentrations varying from 5 to 1000 μg·mL^-1, r² = 0.9992. The results were expressed in mg of gallic acid per g of dry weight of aqueous extract. All tests were performed in triplicate.

2.5. Evaluation of free radical scavenging activity

2.6. HPLC analysis of P. quitoc extract

The sample was solubilized in water: methanol (7:3 v:v) filtered through 0.45 µm ultrafilter. and evaluated in a LC analytical column (LC-6AD Shimadzu, Kyoto, Japan) with the assistance of a photodiode array detector (DAD) system which was monitored between wavelengths λ = 200-800 nm. In an LC analytical apparatus; the column was ODS HYPERSIL (C-18, 150 mm long x 4.6 mm diameter, Thermo Electron Corporation, United States). The flow rate and the injection volume were 1 mL·min^-1 and 20 µL, respectively. All chromatographic analyses were performed at 25 ºC. Eluent A was composed of a binary mobile phase of water with 6% acetic acid and 2 mM sodium acetate, and eluent B was composed of acetonitrile and the following gradient was applied: 0 min 5% B; 20 min 15% B; 30 min 60% B; and 40 min 100% B. Standard samples of gallic acid, ferulic acid, caffeic acid, ferric acid, rosmarinic acid, p-coumaric acid, rutin, quercetin, luteolin and apigenin were used (Sigma), prepared in methanol-water (7:3 v:v) in 1000 µg.mL^-1 concentration. The elution method developed for LC aimed at the identification of phenolic compounds with the assistance of the DAD scanning detector in the spectral range of 200-800 nm did not reveal interferences in the retention times. The absorption spectra and the retention time of the standards were the parameters for the identification and quantification of the compounds. Co-injection experiments, in which extracts and standard aliquots were mixed and diluted to a known volume, were also carried out to unequivocally identify the compounds. The calibration curves were determined by linear regression using LC. The linearity for standards was assessed for 10 concentration ranges. The average standard errors for the peak areas of replicated injections (n = 5) were less than 2%, thus showing good repeatability of the calibration curve. The respective coefficients of determination (r²) were 0.9994 for caffeic acid, ferulic acid and gallic acid and r² = 0.9996 for rutin and quercetin.

2.7. Chromatographic analysis by GC-MS

Sample preparation for GC-MS analysis was as follows; 100 mg of the P. quitoc extract were added to 1 mL of water and 1 mL of hexane and followed by separation of the hexane fraction. 2 mL of hexane were added to the aqueous fraction and the process was repeated again. These two hexane fractions were dried and suspended in 1.0 mL of hexane. For GC-MS analysis the solution was first filtered through a 0.45 µm ultrafilter.

To identify the compounds present in the sample, it was also evaluated by mass spectrometry (GC-MS). The GC-MS analysis was carried out on a GC-2010 Plus (Shimadzu, Kyoto, Japan), equipped with a mass spectrometry detector (GC-MS Ultra 2010), using LM-5 (5% phenyldimethylpolysiloxane), fitted with a capillary column of fused silica (15 m length x 0.2 mm id, and 0.2 µm-thick film). The analysis was performed under the following conditions: helium make up gas (99.999% and flow rate 1 mL·min^-1), 1 µL of injection volume, split ratio (1:20), initial oven temperature adjusted to 150 ºC and heating from 150 ºC to 280 ºC at 15 ºC·min^-1 and held at 280 ºC for 15 min. The injector temperature was 280 °C and the quadrupole detector temperature was 300 °C. An electron impact ionization voltage of 70 eV, a mass range of 45-600 nm m/z and a scanning interval of 0.3 s were the MS scanning parameters.

Compounds identification was accomplished by comparing the mass spectra obtained in the NIST21 and WILEY229 libraries. In some cases, reference compounds were co-chromatographed and the identification of the components was made by comparison of their retention times with standards. Standards for stigmasterol, campesterol, β-sitosterol, lupeol and lupeol acetate (Sigma) were prepared in hexane in the concentration of 1000 µg·mL^-1. The compound concentrations were determined by external calibration. The linearity of standards was assessed for 5 concentration ranges. The average standard errors for the peak areas of replicated injections (n = 5) was less than 2%, thus showing good repeatability of the calibration curve. The respective coefficients of determination (r²) were 0.9996 for stigmasterol, campesterol, β-sitosterol and lupeol and r² = 0.9994 for lupeol acetate.

2.8. Preparation of P. quitoc extract - enriched soybean and corn oils

Samples of soybean and corn oils were enriched with the P. quitoc hydroalcoholic extract to reach concentrations of 0.62, 1.25 and 2.5% (extract in oil). The extracts were added directly to the oils, followed by slow stirring for homogenization. Samples of soy and corn oils were also enriched with the synthetic antioxidant BHT at 0.02% (w/v) concentration (Brazil-Anvisa, 2005BRAZIL, ANVISA-National Health Surveillance Agency. Resolution nº 270, of September 22, 2005. Technical regulation for vegetable oils, vegetable fats and vegetable cream, Official Gazette of the Federative Republic of Brazil, Brasília, 2005.). BHT was added directly to the oils, followed by slow stirring until dissolved.

2.9. Rancimat test

The Rancimat test was used to assess the oxidative stability of the additive oils (with BHT and P. quitoc extract) and of the additive-free oils. The analyses were performed using a Rancimat apparatus (Metrohm, model 893, Herisau, Switzerland). The analyses were performed in a fixed amount of oil sample (3 g) at 110 °C and 10-L·h^-1 air flow (Tinello et al., 2018). The oxidative stability was expressed as the induction period (IP) corresponding to the time (h). The induction period was determined using the software provided by the equipment manufacturer (StabNet).

2.10. Schaal oven test and evaluation of acid value

The Schaal oven test, which aims to assess the effect of accelerated storage conditions on the oxidative stability of oils, was performed as described by Yang et al. (2016) in soybean and corn oils without any addition (C), with P. quitoc hydroalcoholic extract (1%, 2.5% and 5.0%), and with butylated hydroxytoluene (BHT) as synthetic antioxidant at 0.02% (w/w) concentration, which corresponds to the maximum level established by Codex Alimentarius (2019)Alimentarius C. 2019. General standard for food additives (Codex Stan 192-1995). http://www.fao.org/fao-who-codexalimentarius/codex-texts/dbs/gsfa/en/. (Accessed 18 September-2020).. In detail, the oil samples were accurately weighed (40 g ± 0.01 g) in beakers and stored in an oven at a constant temperature of 62 °C for 28 days. Every 7 days the samples were collected and submitted to analysis.

2.11. Statistical analysis

Results are presented and expressed in terms of mean and standard deviations. Significant differences were calculated using ANOVA in combination with the Duncan’s test with significance level of α=0.05, supplemented, when necessary, by the Tukey test with Statistic Software (version 7.0).

3.1. Characterization of the P. quitoc hydroethanolic extract

The hydroalcoholic extract of the P. quitoc aerial part was obtained st a yield of 10.2%. This yield can be considered good and comparable to the extracts obtained from from Schinus molle (fruits and leaves) (12 and 16%), Psidium firmum (11.5%) and lower than Rosmarinus officinalis (16-19%) (Klein et al., 2020Klein AFNV, Silva CAA, Fernandes, SSL, Nicolau CL, Cardoso CAL, Fiorucci AR, Simionatto E. 2020. Effect of leaf and fruit extracts of Schinus molle on oxidative stability of some vegetables oils under accelerated oxidation. Grasas Aceites 71, e363. https://doi.org/10.3989/gya.0456191; Peres et al., 2013Peres MTLP, Lopes JRR, Silva CB, Candido ACS, Simionatto E, Cabral MRP, Oliveira RM, Facco JT, Cardoso CAL, Simas PH. 2013. Phytotoxic and antioxidant activity of seven native fruits of Brazil. Acta Bot. Brasilica 27, 836-846. https://doi.org/10.1590/S0102-33062013000400024 , Wang et al., 2018Wang YZ, Fu SG, Wang SY, Yang DJ, Wu YHS, Chen YC. 2018. Effects of a natural antioxidant, polyphenol-rich rosemary (Rosmarinus officinalis L.) extract, on lipid stability of plant-derived omega-3 fatty-acid rich oil. LWT-Food Sci. Technol. 89, 210-216. https://doi.org/10.1016/j.lwt.2017.10.055 ). The extract was initially characterized following the Folin-Ciocalteau method for the presence of phenolic compounds attributed to the concentration of 314 mg GAE (gallic acid equivalent)/g of P. quitoc extract. This result serves to classify this plant as rich in phenolic compounds at levels close to the extracts widely known and already highlighted as having high contents of these compounds. Several works highlight the phenol content identified in peanut skin extracts, ginger (Zingiber officinale), turmeric (Curcuma longa) and rosemary, which have phenol contents close to those found in the P. quitoc hydroalcoholic extract (Franco et al., 2018Franco D, Rodríguez-Amado I, Agregan R, Munekata PES, Vazquez PA, Barba FJ, Lorenzo JM. 2018. Optimization of antioxidants extraction from peanut skin to prevent oxidative processes during soybean oil storage. LWT-Food Sci. Technol. 88, 1-8. https://doi.org/10.1016/j.lwt.2017.09.027 ; Tinello and Lante, 2020Tinello F, Lante A. 2020. Accelerated storage conditions effect on ginger- and turmeric-enriched soybean oils with comparing a synthetic antioxidant BHT. LWT - Food Sci. Technol. 131, 109797. https://doi.org/10.1016/j.lwt.2020.109797 ; Wang et al., 2018Wang YZ, Fu SG, Wang SY, Yang DJ, Wu YHS, Chen YC. 2018. Effects of a natural antioxidant, polyphenol-rich rosemary (Rosmarinus officinalis L.) extract, on lipid stability of plant-derived omega-3 fatty-acid rich oil. LWT-Food Sci. Technol. 89, 210-216. https://doi.org/10.1016/j.lwt.2017.10.055 ).

The extract was analyzed by HPLC, and initially detected the presence of seven peaks of greater intensity. With the use of standards and spectral analysis, it was possible to identify five compounds. Figure 1 shows the chromatographic profile of the extract and the identification of gallic acid (peak 1), caffeic acid (peak 2), ferulic acid (peak 3), rutin (peak 4) and quercetin (peak 5). It is worth noting that these phenolic compounds found in the P. quitoc extract have already been reported as good natural antioxidants (D’Andrea, 2015D’Andrea G. 2015. Quercetin: A flavonol with multifaceted therapeutic applications? Fitoterapia 106, 256-271. https://doi.org/10.1016/j.fitote.2015.09.018 ; Shahidi and Ambigaipalan, 2015Shahidi F, Ambigaipalan P. 2015. Phenolics and polyphenolics in foods, beverages and spices: Antioxidant activity and health effects - A review. J. Funct. Foods 18, 820-897. https://doi.org/10.1016/j.jff.2015.06.018 . Considerable research has been done with the objective of obtaining extracts enriched with classes of phenolic compounds such as flavonoids, and organic phenolic acids, among others (Taghvaei et al., 2014Taghvaei M, Jafari SM, Mahoonak AS, Nikoo AM, Rahmanian N, Hajitabar J, Meshginfar N. 2014. The effect of natural antioxidants extracted from plant and animal resources on the oxidative stability of soybean oil. LWT-Food Sci. Technol. 56, 124-130. https://doi.org/10.1016/j.lwt.2013.11.009 ).

The hydroalcoholic extract of P. quitoc was also subjected to extraction with hexane and after filtration, analyzed by gas chromatography (Figure 2). Through this analysis it was possible to identify and quantify five compounds: campesterol (22.0 mg·g^-1), stigmasterol (26.5 mg·g^-1), β-sitosterol (57.1 mg·g^-1), lupeol (47.4 mg·g^-1) and lupeol acetate (48.9 mg·g^-1).

3.2. Evaluation of the antioxidant activity of hydroethanolic extract

Table 1 shows the results of the antioxidant activity against the DPPH and ABTS radicals, where it can be seen that the extract has high activity. The IC50 values for the extract were 13.2 µg·mL^-1 and 5.6 µg·mL^-1, in the tests with DPPH and ABTS radicals, respectively. According to the results, there is a variation in the IC50 values obtained by both methods. This variation has also been observed in other studies on the evaluation of the antioxidant properties of extracts using the radicals DPPH, ABTS and other methods (Franco et al., 2018Franco D, Rodríguez-Amado I, Agregan R, Munekata PES, Vazquez PA, Barba FJ, Lorenzo JM. 2018. Optimization of antioxidants extraction from peanut skin to prevent oxidative processes during soybean oil storage. LWT-Food Sci. Technol. 88, 1-8. https://doi.org/10.1016/j.lwt.2017.09.027 ; Pedro et al., 2018Pedro AC, Maurer JBB, Zawadzki-Baggio SF, Ávila S, Maciel GM, Haminiuk CWI. 2018. Bioactive compounds of organic goji berry (Lycium barbarum L.) prevents oxidative deterioration of soybean oil. Ind. Crops Prod. 112, 90-97. https://doi.org/10.1016/j.indcrop.2017.10.052 ; Tinello and Lante, 2020Tinello F, Lante A. 2020. Accelerated storage conditions effect on ginger- and turmeric-enriched soybean oils with comparing a synthetic antioxidant BHT. LWT - Food Sci. Technol. 131, 109797. https://doi.org/10.1016/j.lwt.2020.109797 ). The high activity of the extract is verified against the two radicals in comparison to the standard of synthetic antioxidant BHT and to the natural flavonoid rutin, whose extracts presented IC 50 values close to the values for these standard compounds. Another parameter analyzed for the antioxidant potential of P. quitoc extract was the determination of the Trolox Equivalent Antioxidant Capacity (TEAC). The TEAC results also showed that the extract had significant activity, obtaining the values of 1024.8 and 1943.43 mM Trolox/g of hydroalcoholic extract, for the tests with the radicals DPPH and ABTS, respectively. The extract showed higher TEAC compared to the flavonoid rutin using the ABTS method. This behavior can be related to a probable synergistic effect of the phenolic compounds present in the extract of P. quitoc, acting in the inhibition of the radical ABTS. The TEAC values obtained for the P. quitoc extract and standards can be compared to those obtained from green tea ethanolic extract (3028 mM Trolox·g^-1), green tea aqueous extract (1723 mM trolox·g^-1) and extracts obtained from Mangifera indica (2750 mM trolox·g^-1) (Leite et al., 2012Leite RP, Malta LG, Jorge MHA, Sousa IMO, Foglio MA, Dolder H. 2012. Antioxidant activity of ethanolic extract of Heteropterys aphrodisiaca, an endemic Brazilian plant - trolox equivalent antioxidant capacity assay (TEAC). Free Radic. Biol. Med. 53, S85-S86. https://doi.org/10.1016/j.freeradbiomed.2012.08.178 ; Sánchez-Camargo et al., 2020Sanchez-Camargo AP, Ballesteros-Vivas D, Buelvas-Puello VM, Martinez-Correa HA, Parada-Alfonso F, Cifuentes A, Ferreira SRS, Gutiérrez LF. 2020. Microwave-assisted extraction of phenolic compounds with antioxidant and anti-proliferative activities from supercritical CO2 pre-extracted mango peel as valorization strategy. LWT - Food Sci. Technol. 137, 110414. https://doi.org/10.1016/j.lwt.2020.110414 ). The results obtained in the present study are relevant to the data on antioxidant activity, demonstrating that P. quitoc extract has great capacity to eliminate radicals, suggesting that the extract could act as a preventive or blocker of the formation of reactive oxygen species chains, which could delay damage to certain systems and materials.

Some plant extracts are already mentioned in the literature as promising natural antioxidants, verified by several methods. Plant extracts that can be prepared from herbs, leaves, vegetable residues and fruit residues stand out as important sources of phenolic compounds (Sharma et al., 2019Sharma S, Cheng SF, Bhattacharya B, Chakkaravarthi S. 2019. Efficacy of free and encapsulated natural antioxidants in oxidative stability of edible oil: Special emphasis on nanoemulsion-based encapsulation. Trends Food Sci. Tech. 91, 305-318. https://doi.org/10.1016/J.TIFS.2019.07.030 ). It is also intensively mentioned in the literature that the content of simple phenols and phenolic acid compounds is linked to the antioxidant effects of plant extracts (Bodoira et al., 2017Bodoira RM, Penci MC, Ribotta PD, Martínez ML. 2017. Chia (Salvia hispanica L.) oil stability: Study of the effect of natural antioxidants. LWT-Food Sci. Technol. 75, 107-113. https://doi.org/10.1016/j.lwt.2016.08.031 ).

This antioxidant action potential may be associated with the phenolic compounds present in the P. quitoc extract. The chromatographic analysis showed that the extract consisted of three groups of compounds, two of which were phenolic, flavonoids and phenolic acids. Many authors have already reported the ability of phenolic extracts, flavonoids and phenolic acids to eliminate ABTS and DPPH radicals (Zahran and Najafi, 2020Zahran H, Najafi Z. 2020. Enhanced stability of refined soybean oil enriched with phenolic compounds of olive leaves. Egypt. J. Chem. 63, 215 - 224. https://doi.org/10.21608/EJCHEM.2019.16592.2010 ; Wang et al., 2018Wang YZ, Fu SG, Wang SY, Yang DJ, Wu YHS, Chen YC. 2018. Effects of a natural antioxidant, polyphenol-rich rosemary (Rosmarinus officinalis L.) extract, on lipid stability of plant-derived omega-3 fatty-acid rich oil. LWT-Food Sci. Technol. 89, 210-216. https://doi.org/10.1016/j.lwt.2017.10.055 ; Klein et al., 2020Klein AFNV, Silva CAA, Fernandes, SSL, Nicolau CL, Cardoso CAL, Fiorucci AR, Simionatto E. 2020. Effect of leaf and fruit extracts of Schinus molle on oxidative stability of some vegetables oils under accelerated oxidation. Grasas Aceites 71, e363. https://doi.org/10.3989/gya.0456191). Among the phenolic compounds, flavonoid quercetin and ferulic acid were the most representative compounds of the P. quitoc extract with concentrations of 162.7 mg·g^-1 and 217.5 mg·g^-1, respectively. These compounds (quercetin and ferulic acid) are reported in the literature because both have important antioxidant effects, and present high activity (D’Andrea, 2015D’Andrea G. 2015. Quercetin: A flavonol with multifaceted therapeutic applications? Fitoterapia 106, 256-271. https://doi.org/10.1016/j.fitote.2015.09.018 ; Shahidi and Ambigaipalan, 2015Shahidi F, Ambigaipalan P. 2015. Phenolics and polyphenolics in foods, beverages and spices: Antioxidant activity and health effects - A review. J. Funct. Foods 18, 820-897. https://doi.org/10.1016/j.jff.2015.06.018 ). All other phenolic components (gallic acid, caffeic acid and rutin) of the P. quitoc hydroalcoholic extract are also highlighted for their important antioxidant effects (Shahidi and Ambigaipalan, 2015Shahidi F, Ambigaipalan P. 2015. Phenolics and polyphenolics in foods, beverages and spices: Antioxidant activity and health effects - A review. J. Funct. Foods 18, 820-897. https://doi.org/10.1016/j.jff.2015.06.018 ), which justify the high activity of this extract. The antioxidant potential of phenolic compounds depends on the number and arrangement of hydroxyl groups in the molecules. Phenolic substances, such as those found in the extract of P. quitoc, can act as antioxidants by donating hydrogen atoms to lipid radicals and produce lipid derivatives and phenoxyl radicals, which are more stable and have less potential for self-oxidation (Kiokias et al., 2008Kiokias S, Varzakas T, Oreopoulou V. 2008. In vitro activity of vitamins, flavonoids, and natural phenolic antioxidants against the oxidative deterioration of oil-based systems. Crit. Rev. Food Sci. Nutr. 48, 78-93. https://doi.org/10.1080/10408390601079975 ). The other compounds characterized are steroids (campesterol, stigmasterol, β-sitosterol) and triterpenoids (lupeol and lupeol acetate).

3.3. Soybean and corn oils enriched with hydroalcoholic extract: Analysis of oxidative stability by Rancimat and acid value

After the characterization of the P. quitoc hydroalcoholic extract and verification of potential antioxidant activity (against DPPH and ABTS radicals), the extract was evaluated for its activity in the inhibiting of lipid oxidation, once added to vegetable oils. The Rancimat analysis was performed at 110 °C and the induction period was evaluated until the end point of the samples’ stability. Different concentrations of the extract were added to soybean and corn oils and the effects of these oils enriched with the extracts were compared to the synthetic standard BHT. Table 4 presents the results of the induction period of soybean and corn oils added with different concentrations of P. quitoc hydroethanolic extract, with BHT, and without the addition of any antioxidants.

According to the results, considering antioxidant-free oils, it is noted that corn oil had greater oxidative stability in comparison to soybean oil, the difference being statistically evidenced, according to the Tukey test (p <0.05). It can be attributed to the differences in the oils’ compositions; corn oil has a higher content of saturated compounds and less content of unsaturated compounds in comparison to soybean oil, parameters that contribute to the greater stability of corn oil (Dweck and Sampaio, 2004Dweck J, Sampaio CMS. 2004. Analysis of the thermal decomposition of commercial vegetable oils in air by simultaneous TG/DTA. J. Therm. Anal. Calorim. 75, 385-391. https://doi.org/10.1023/B:JTAN.0000027124.96546.0f ). In oils enriched with extracts, it was observed that there was an increase in the induction period (IP), demonstrating the effectiveness of the extract in protecting the oil in relation to oxidative degradation. Analyzing corn oil specifically, the control sample showed a significant difference compared to the oil samples incorporated into the extract, with an increase in the induction period (IP) from 1.2 h, 1.4 h and 2.0 h to 1.0 2.5 and 5.0%, concentrations, respectively. As for soybean oil, the increases in induction periods in relation to the control sample were 0.7 h, 1.4 h and 1.7 h at concentrations of 1.0, 2.5 and 5.0%, respectively. Once the action of P. quitoc extract was compared to the synthetic antioxidant BHT, promising effects were also observed, since in both oils evaluated, the inhibition of oxidation was more effective with the use of the extract. The increase in the induction period (IP) of the oils added with BHT in relation to the control was 0.7 h and 0.4 h for corn and soybean oils, respectively; whereas with a concentration of 1% of extract, the oils presented increases in the induction period of 1.2 h and 0.7 h in relation to the control for corn and soybean oils, respectively. Therefore, oils already enriched with extract at a concentration of 1% performed better than the antioxidant BHT.

The acid value was selected as a parameter, with the objective of evaluating the stability of soybean and corn oil samples with or without antioxidants during storage at 62 °C for 28 days (Schall oven test). Acceptable acid values for vegetable oils must be below 0.6 mg KOH per g of oil, as established by the ANVISA (National Health Surveillance Agency) standard for vegetable oils (Brazil-Anvisa, 2005BRAZIL, ANVISA-National Health Surveillance Agency. Resolution nº 270, of September 22, 2005. Technical regulation for vegetable oils, vegetable fats and vegetable cream, Official Gazette of the Federative Republic of Brazil, Brasília, 2005.). Table 4 shows the results obtained from the determination of acid values. By comparing the acid values of oils without additives to the oils enriched with extracts, it was noted that the additive oils showed a beneficial effect in reducing acid values. According to the values, it appeared that all samples had an acid value below 0.6 mg KOH·g^-1 at the time of preparation (day zero), demonstrating that the addition of the extract to the oil samples did not affect this parameter. The tendency to increase the acid value of soybean and corn oils samples was slow in the first 14 days, while a sharp increase was observed until the end of thermal storage.

The antioxidant-free oils showed the highest acid values, and presented increases in all analyses of the period for soybean and corn oils, reaching high values on day 28, at 9.50 and 14.26 mg KOH·g^-1, respectively. With the extract addition, the samples showed a less accelerated behavior in the variation in acid values, which also presented lower indexes in comparison to oils without additives, suggesting that the extracts contributed to the control of this parameter.

The highest extract concentrations added to the oils (5%) provided samples with less acidity at the end of the analysis period (day 28), showing better behavior than the BHT standard for corn oil, with an acid value of 2.72 mg KOH·g^-1; whereas for the oil with BHT standard the acid value was 4.75 mg KOH·g^-1. For soybean oil, samples with BHT and 5% extracts also showed similar acid values, with 5.43 mg KOH·g^-1 and 6.11 mg KOH·g^-1, respectively. According to the results obtained, soybean oil showed the highest acidity values at the end of the analysis period (day 28). The lipid composition of each oil can influence these acidity values. There are differences in the proportions of saturated/unsaturated compounds in soybean and corn oils, which are approximately 15%/84% and 18%/81%, respectively (Dweck and Sampaio, 2004Dweck J, Sampaio CMS. 2004. Analysis of the thermal decomposition of commercial vegetable oils in air by simultaneous TG/DTA. J. Therm. Anal. Calorim. 75, 385-391. https://doi.org/10.1023/B:JTAN.0000027124.96546.0f ). This higher proportion of unsaturated compounds in soybean oil may contribute to less oil stability, which may result in higher acidity values. In addition, the oils differ in terms of their contents of polyunsaturated compounds. Soy oil has approximately 58%, while corn oil has 45% (Dweck and Sampaio, 2004Dweck J, Sampaio CMS. 2004. Analysis of the thermal decomposition of commercial vegetable oils in air by simultaneous TG/DTA. J. Therm. Anal. Calorim. 75, 385-391. https://doi.org/10.1023/B:JTAN.0000027124.96546.0f ). Oils that contain a greater abundance of polyunsaturated fatty acids are vulnerable to oxidative degradation (Sharma et al., 2019Sharma S, Cheng SF, Bhattacharya B, Chakkaravarthi S. 2019. Efficacy of free and encapsulated natural antioxidants in oxidative stability of edible oil: Special emphasis on nanoemulsion-based encapsulation. Trends Food Sci. Tech. 91, 305-318. https://doi.org/10.1016/J.TIFS.2019.07.030 ).

Generally, there was an increase in the acid value with the time of exposure of the sample to heat. This behavior is in agreement with other studies found in the literature (Yildirim, 2009Yildirim G. 2009. Effect of Storage Time on Olive Oil Quality. Master’s Thesis, Izmir Institute of Technology, Izmir.; Souza et al., 2014Sousa ER, Pereira Jr AS, Silva GS, Marques ALB. 2014. Avaliação da Qualidade de Óleos de Origem Vegetal Oriundos de Frituras. Acta Tecnológica 9, 58-62.https://doi.org/10.35818/acta.v9i2.285 ; Souza el al., 2019Sousa ER, Frota CS, Costa CHC, Silva GS, Sampaio DG. 2019. Avaliação da Oxidação e de Parâmetros de Qualidade do Óleo de Babaçu por Espectroscopia no Infravermelho Médio com Transformada de Fourier (FTIR) e Calibração Multivariada. Rev. Virtual Quim. 11, 849-865. https://doi.org/10.21577/1984-6835.20190059 ). For the oils analyzed, the greatest variation in acid values occurred in the period of 21-28 days, both for oils with extracts or with BHT. In this period of analysis, it was observed that the oils added with the P. quitoc extract had significantly lower acid values than the oil which was free of antioxidants, demonstrating the positive effect of the extract in the control of acidity.