Limonene, the compound in essential oil of nutmeg displayed antioxidant effect in sunflower oil during the deep‐frying of Chinese Maye

Abstract The deep‐frying process for plenty of fried products using vegetable oils needs safe and effective antioxidants. In the present exploration, the nutmeg essential oil (NEO) was employed as a potential antioxidant for sunflower oil during the deep‐frying of Chinese Maye at 180°C for 30 hr. In the comparative study, the additions for NEO at 0.12 g/kg, TBHQ at 0.12 g/kg, BHA at 0.08 g/kg, and BHT at 0.08 g/kg to sunflower oil were able to obviously improve its oxidative stability during the deep‐frying process, and their antioxidant effects were in the relative order: TBHQ at 0.12 g/kg > NEO at 0.12 g/kg > BHA at 0.08 g/kg > BHT at 0.08 g/kg (p < .05). Besides, NEO at 0.12 g/kg could markedly ameliorate the sensory properties including flavor, taste, crispness, and overall acceptability of the fried products, Chinese Maye (p < .05 or p < .01). In addition, using antioxidant activity‐guided fractionation, three active compounds including limonene, terpinolene, and geranyl acetate were isolated from NEO. Among them, limonene was demonstrated to not only significantly increase the oxidative stability of sunflower oil in the deep‐frying process, but also significantly increase the sensory properties of the fried products, Chinese Maye (p < .05 or p < .01). Consequently, limonene could be employed as antioxidants in sunflower oil for the deep‐frying of Chinese Maye, and the sunflower oil flavored by NEO could be used as frying oils for its oxidative stability and unique flavor.

. However, the safety of these synthetic antioxidants has long been questioned because of their negative effects on human health, such as enlargement of the liver, and transformation of ingested materials into carcinogenic substances (Hwang, Winkler-Moser, & Liu, 2012). In the meantime, natural antioxidants (natural products/crude extracts) from spices and herbs have been used in the frying procedure, and they were considered to be safe (Wang, Liu, & Qin, 2017). For instance, the addition of a carotenoid extract from Lycium barbarum was able to improve the oxidative stability of extra-virgin olive oil during a long-term storage for 28 days at room temperature and a frying process for 180 min at 180 ± 4°C (Blasi et al., 2018;Montesano et al., 2019). Therefore, it is quite necessary and meaningful to search for natural antioxidants from herbs and spices.
As a leafy tree of height about 12 m when mature, Myristica fragrans Houtt. (Myristicaceae family) is important for plenty of beverages derived from its fruits and two spices used in sweet and savory cooking, mace, and nutmeg. Among them, mace is the aril, and nutmeg is the nut (Piras et al., 2012;Zhao, Wu, Wei, & Yang, 2019). Except spices, nutmeg has been used to treat nausea, anxiety, cholera, psychosis, diarrhea, flatulence, rheumatism, and stomach cramps in Chinese medicine for more than 1000 years (Acuña, Carcache, Matthew, & de Blanco, 2016). Additionally, in the continuous phytochemical investigations and pharmacological explorations, secondary metabolites including lignans, steroids, cyclobutanones, diphenylalkanes, phenylpropanediols, and essential oils have been isolated from M. fragrans, and natural products/crude extracts of M. fragrans have been demonstrated to display biological effects including antifungal, antimicrobial, anti-inflammatory, and hepatoprotective effects (Abourashed & El-Alfy, 2016).
Above all, the antioxidant potential of M. fragrans has been reported by researcher all over the world. For example, the 50% acetone extract and 80% methanol extract of M. fragrans leaves exhibited radical-scavenging effects against ABTS •+ , DPPH • , ORAC, and HO • radicals (Su et al., 2007). Interestingly, the essential oil of M. fragrans fruits revealed inhibitory effect against the generation for the primary and secondary oxidation products in mustard in the accelerated storage (Kapoor et al., 2013). Furthermore, the essential oil of M. fragrans was demonstrated to improve the oxidative and microbial stability of cooked sausage during the refrigerated storage (Šojić et al., 2015). Spontaneously, one hypothesis that nutmeg essential oil may be used as an effective natural antioxidant for the frying procedure of vegetable oils arose.
According to these considerations, the aims of the present study were to investigate the influences of nutmeg essential oil (NEO) on the oxidative stability of sunflower oil and the sensory properties of Chinese Maye, and search for the compounds in NEO that reveal antioxidant effect during the deep-frying procedure.

| Chemical analysis of NEO
First, NEO was extracted using our previous method (Wang et al., 2018(Wang et al., , 2019. Briefly, 20.0 kg of nutmeg was smashed into particles (diameter <1.0 mm) by a grinder at 1 × 10 4 r/min for 20 min (400Y, Boou Machinery Co., Ltd.), and these particles were averagely divided into 20 portions (each portion 1 kg). Each portion was put into a flask (4,000 ml) with 1,000 ml of water (w/v, 1:1), and a steam distillation apparatus (XH-2000, Xinhu Experimental Device Co., Ltd.) was employed to distill NEO at 120°C for 6 hr. When the distillation process completed, the distillate was dumped into a separating funnel (500 ml) filled with 200 ml of ethyl acetate. In the partition process, the water phase was removed, while the organic phase was reserved. Subsequently, the organic phase for the distillate of the 20 portions was combined to obtain the crude NEO. After dried over anhydrous Na 2 SO 4 , the purified NEO was gained. Soon afterward, it was poured into a dark brown bottle and stored in a refrigerator He was the carrier gas at a flow rate of 1.1 ml/min. The oven temperature was programmed at 50°C and held for 2 min, and increased to 300°C by 10°C/min and held for 3 min. The temperatures of injector and detector were set at 250°C and 280°C, respectively. For injection, 1 μl of NEO was injected with a split of 20:1. In addition, the MS range (m/z) was set at 40-450, and the chemical constituents of NEO were elucidated by searching the NIST database 2018 (http://webbo ok.nist.gov.).

| Selecting for the concentrations of antioxidants in frying
In order to obtain the optimal concentrations of antioxidants in vegetable oils during the frying process, the physical and chemical properties of vegetable oils containing the antioxidants should be measured after the frying process have been operating for at least 5 hr (Guo et al., 2016). According to Table 1, NEO, TBHQ, BHA, and BHT were directly added into sunflower oil at the concentrations of 0.04, 0.08, 0.12, 0.16 and 0.20 g/kg to prepare sunflower oil samples (each 10.0 kg), and a Control sample was prepared without any antioxidant as well. Before the frying process, all the sunflower oil samples were stored in brown bottles at 4°C. Herein, each sunflower oil sample was subjected to a frying process of 5 hr using a frying pan (L-101C, Demashi Net Technology Co., Ltd.) at 180°C. For the frying process, the frying process began when the temperature of sunflower oil samples reached 180°C, and the acidity value (AV), iodine value (IV), peroxide value (PV), and p-anisidine value (AnV) of all these sunflower oil samples were determined every hour in accordance with Chinese National Standards GB 5009.229-2016, GB/T 5532-2008, GB 5009.227-2016and GB/T 24304-2009

| Deep-frying of sunflower oil added by antioxidants
In the frying process above, the optimal concentrations of NEO, TBHQ, BHA, and BHT were found to be 0.12, 0.12, 0.08, and 0.08 g/ kg, respectively. Therefore, the concentrations of NEO, TBHQ, BHA, and BHT used in the deep-frying process were chosen as 0.12, 0.12, 0.08, and 0.08 g/kg, respectively.

| Sensory analysis for frying product, Chinese Maye
In the deep-frying process, the evaluation for the sensory properties of Chinese Maye including flavor, taste, crispness, and overall acceptance was performed every 6 hr using a 10-point hedonic scale ( themselves with the sensory analysis procedure. Moreover, the meaning of flavor, taste, crispness, and overall acceptance was explained to them to reduce their misunderstanding. In the evaluation session, the Chinese Maye were labeled with three-digit codes at random, and panelists received a maximum of five samples to evaluation.

| Deep-frying of sunflower oil added by active compounds from NEO
First, in order to search for the active compounds with antioxidant effects in NEO, the fractionation guided by antioxidant activity was carried out using the dot-blot test on TLC using silica gel Al plates (Adiani, Gupta, Chatterjee, Variyar, & Sharma, 2015). In brief, 10 μl of NEO was spotted on a piece of TLC plate, and then, the plate was eluted with the solvent system hexane-ethyl acetate (80:20). After drying, the plate was sprayed with 160 μM of methanol solution of DPPH, and the bright yellow spots on the purple background of the sprayed plate was considered as active compounds with inhibitory effect against DPPH • . For the fractionation of them, preparative TLC plates produced with silica gel Al were used to with the same solvent system, and the active compounds with the corresponding Rf values were eluted with ether. Finally, the active compounds, limonene, terpinolene, and geranyl acetate were isolated, and their chemical structures were identified by HRESIMS and 1 H-NMR.
Second, for the evaluation of the antioxidant effects of the active compounds in the deep-frying for Chinese Maye, limonene (0.12 g/kg × 14.5% = 17.4 mg/kg), terpinolene (0.12 g/kg × 8.4% = 10.1 mg/kg), and geranyl acetate (0.12 g/ kg × 25.9% = 31.1 mg/kg) were added into sunflower oil at their corresponding concentrations to obtain sunflower oil samples (each 10 kg, Table 2) LI-SFO, TE-SFO and GE-SFO, respectively, together with sunflower oil samples Control (nothing added, Control) and TBHQ (0.12 g/kg, TBHQ). Subsequently, the deep-frying process was performed and continued for 30 hr, and the values for TPC, TBA, CD, and CT of sunflower oil samples were determined every 6 hr as well. Meanwhile, the sensory analysis of Chinese Maye was carried out.

| Statistical analysis
All of the experiments were carried out in duplicate. In the manuscript, unless otherwise indicated, the data are presented as mean values in test, and the data presented in figures and tables are presented as mean value ± standard deviations (SD). In statistical analysis, GraphPad Prism 6 (GraphPad Software) was employed to analyze the data by means of one-way analysis of variance, where p < .05 was identified as statistically significant, and p < .01 was identified as and highly significant.

| Chemical constituents of NEO
In the past few years, the essential oil of nutmeg was famous for its attractive fragrance, so that it has been extracted using nutmeg from area to area by means of different methods, and its chemical constitution has been frequently investigated using GC-MS (Bouchachia, Benkaci-Ali, Eppe, & Scholl, 2017;da Rocha Voris et al., 2018). As shown in Table 2, 26 compounds were identified, representing 98.0% of the essential oil in this study. The extraction yield of the NEO was 6.4 g/kg of dried nutmeg. The major compounds were geranyl acetate (25.9 g/100 g), limonene (14.5 g/100 g), caprylic acid (12.6 g/100 g), and trans-β-ocimene (10.5 g/100 g). The extraction yield and chemical composition of NEO were both quite different from these previous studies, where the extraction yields were 2.5 -5.0 g/kg of dried nutmeg, and the major compounds were considered to be sabinene

| Concentrations of the antioxidants for frying
Since the vegetable oils, frying temperature, or even frying pans that used during the frying process were different, the concentrations of antioxidants used in the frying process are also quite different, so that the selecting for concentrations of antioxidants is quite necessary (Bensmira, Jiang, Nsabimana, & Jian, 2007). As mentioned above, the physical-chemical properties of vegetable oils added by antioxidants during the frying process for at least 5 hr should be determined in details (Guo et al., 2016). As shown in Figure 1 BHA, and BHT were found when they were added at 0.12, 0.12, 0.08, and 0.08 g/kg, respectively. Therefore, this concentration of antioxidant was able to use to stabilize and retard frying oil oxidation, so that the concentrations of them used during the frying process were selected as 0.12, 0.12, 0.08, and 0.08 g/kg, respectively.

| Influences of antioxidants on sunflower oil in deep-frying
In the frying process, relating to the content of malondialdehyde (MDA) generated during the oxidation of vegetable oils, the TBA level is frequently employed as an indicator to measure the secondary products of them (Blasi et al., 2018;Montesano et al., 2019). TPC level is another indicator of much importance to evaluate the quality of vegetable oils.
Since these substances of vegetable oils are not volatile and represent the major reaction products at frying temperature, to measure the TPC level offers us a reliable parameter for evaluating the oxidative stability of frying oils (Mattäus, Pudel, Chen, Achary, & Thiyam-Holländer, 2014). The influence of antioxidant addition on the changes of TBA and TPC levels in sunflower oil samples during the deep-frying process is shown in Figure 2. During the whole frying process, the TBA and TPC levels of the Control sample were prominently (p < .05) higher than that  Values are expressed as means ± SD (n = 100). b As compared to control group at the same time: p < .05. and BHT-0.08 were 8. 8, 8.2, 10.8, and 12.5, and 5.4, 4.5, 5.5, and 5.7, respectively, implying the relative order of their antioxidant effects was TBHQ at 0.12 g/kg > NEO at 0.12 g/kg > BHA at 0.08 g/kg > BHT at 0.08 g/kg, too. The results also demonstrated that NEO at 0.12 g/kg was able to restrain the oxidation of sunflower oil during the deep-frying process, which was in agreement with the results of TBA and TPC levels mentioned above. For the second time, the possible reason of the phenomenon may be the existence of some nature compounds with antioxidant effects, which still remained in the sunflower oil at 180°C (Blasi et al., 2018;Cardoso-Ugarte et al., 2013;Montesano et al., 2019). The exploration herein is in agreement with the previous studies, and the antioxidative potential of the essential oil from nutmeg was confirmed again (Kapoor et al., 2013;Šojić et al., 2015).

| Influences of antioxidants on sensory properties of Chinese Maye
As known, if we want to evaluate the suitability of vegetable oils used in the frying process, the most important is the sensory analysis of the fried products. There is no doubt that the consumer will refuse to use the vegetable oils if the fried products are not attractive to them. After all, during the frying process, a remarkable part of the frying medium, vegetable oils, is taken up by the fried products to the extent that the sensory properties of the vegetable oils are able to strongly influence the sensory properties of the fried products (Taha et al., 2014;Tohma & Turan, 2015). The influences of NEO sensory properties of Chinese Maye including flavor, taste, crispness, and overall acceptability were shown in or p < .01), with the point 6.99, 6.29, 6.89, and 6.98 for its flavor, taste, crispness, and overall acceptability at 30 hr, respectively.
Meanwhile, the additions of TBHQ at 0.12 g/kg, BHA at 0.08 g/kg, and BHT at 0.08 g/kg to sunflower oil could not markedly influence the sensory properties of Chinese Maye (p > .05), in relative to that of the Control sunflower oil sample. The results exhibited that the sensory properties of Chinese Maye including flavor, taste, crispness, and overall acceptability could be improved by the addition of NEO at 0.12 g/kg. In fact, nutmeg has long been employed as one spice in the frying process in China, such as Maye and Youtiao in Henan province, China . Herein, NEO was found to increase the sensory properties of Chinese Maye, so that the aroma components of nutmeg were attributed to its essential oil, and the suitability of sunflower oil added by NEO at 0.12 g/kg was demonstrated as well.

| Influences of active compounds of NEO on sunflower oil and Chinese Maye
The antioxidant activity-guided fractionation of 5.0 NEO resulted in the isolation for three active compounds, 575.4 mg of limonene, 320.5 mg of terpinolene, and 1,002.1 mg of geranyl acetate, with their isolation yield 79.4%, 76.3%, and 77.4%, respectively. After Moreover, in relative to that of the Control sunflower oil sample, the addition of limonene at 17.4 mg/kg to sunflower oil could also dramatically inhibit the drop of the sensory properties of Chinese Maye from 12-30 hr, with the point 7.03, 6.33, 6.93, and 7.06 for flavor, taste, crispness, and overall acceptability at 30 hr (Table 4). In the meantime, the other two active compounds, terpinolene and geranyl acetate, did not reveal antioxidant effect and sensory potentiation (p > .05). Consequently, the antioxidant effect and sensory potentiation of NEO were attributed to limonene. The results were in accordance with the previous studies, where limonene was demonstrated to reduce the frequency of micronucleus and DNA damage induced using H 2 O 2 , prevent early lipidemic-oxidative stress-induced detrimental changes, and normalize the cardiovascular disease risk parameters by means of its antioxidant activity (Ahmad & Beg, 2013;Bacanlı et al., 2015).

| CON CLUS ION
In a word, NEO at 0.12 g/kg was able to not only prominently improve the oxidative stability of sunflower oil during the deep-frying process at 180°C for 30 hr, but also prominently increase the sensory properties including flavor, taste, crispness, and overall acceptability of fried product, Chinese Maye. Meanwhile, although the synthetic antioxidants such as TBHQ, BHA, and BHT could also exhibit observably antioxidant effects during the deep-frying process with the relative order: TBHQ at 0.12 g/kg > NEO at 0.12 g/kg > BHA at 0.08 g/kg > BHT at 0.08 g/kg, they could not ameliorate the sensory properties of Chinese Maye. Furthermore, the antioxidant effect and sensory improvement of the essential oil were attributed to limonene. Now, the function TA B L E 4 The influences of active compounds on flavor, taste, crispness, and overall acceptability of Chinese mechanism of limonene in the deep-frying process and the toxicology of the sunflower oil added by limonene at 17.1 mg/kg are being explored in our laboratory. Once these experiments finished, limonene will be used to substitute the synthetic antioxidants in the deep-frying process, and the sunflower oil flavored by NEO can be developed as flavored oils for deep-frying.

ACK N OWLED G M ENTS
The authors acknowledge financial support from the Scientific Research Fund of Henan University of Technology, Zhengzhou, China (2018QNJH20).

CO N FLI C T O F I NTE R E S T
None.

E TH I C A L A PPROVA L
This study does not involve any human or animal testing.