Quality evaluation of rapeseed oil in Chinese traditional stir‐frying

Abstract Canolol is a potential antioxidation ingredient in rapeseed oil. Rapeseed oil with two levels of canolol (528.9 vs. 250.5 mg/kg) was used for stir‐frying different foods (potatoes, tofu, and vegetables). Comprehensive evaluations indicated that the canolol content in high canolol rapeseed oil (HCR) and low canolol rapeseed oil (LCR) after stir‐frying were in the range of 187.8–237.7 and 45.6–96.4 mg/kg, respectively. The degradation rate of total phenol was 58.4% and 80.3% in HCR and LCR, respectively. The loss rates of α‐ and γ‐tocopherol were 24.5% and 47.6%, respectively. Phytosterol concentration decreased by 20% and trans‐fatty acid was not detected in either rapeseed oil. In addition, the peroxide value, anisidine value, and malondialdehyde content in HCR were lower than those in LCR. The oxidative stability index in HCR was longer, showing lower extent of deterioration. Rapeseed oil with high canolol content displayed good oxidation resistance due to significant positive correlation with oxidation induction time (p < .01).

Traditional Chinese food has a long history and attractive culture. The cooking methods in China are diverse, with abundant use of high temperature processing and multi-oil recipes. Among them, stir-frying is the most conventional and efficient cooking method used for preparing Chinese family cuisine. For the stir-frying process, edible oil is heated to high temperatures and mixed with other materials for a relatively short time (Cui, Hao, Liu, & Meng, 2017). Heating temperature and time differently affect the chemical composition and oxidation products of oils (Hosseini, Ghorbani, Meshginfar, & Mahoonak, 2016;Zribi et al., 2014). Vegetable oils are used for deep-frying for preparing Western country cuisine, which mainly includes potato and chicken (Abiona, Awojide, Anifowoshe, & Babalola, 2011;Shrestha, Stevens, et al., 2013;Wakako, Akiko, & Kaori, 2010). The effects of different cooking methods on the formation of oxidative products and trans-fatty acids profiles in various types of fish and chicken have been extensively investigated Neff, Bhavsar, Braekevelt, & Arts, 2014;Turkkan, Cakli, & Kilinc, 2008). However, unsaturated fatty acids (UFAs) can be converted into trans-fatty acids (TFAs) via thermal oxidative deterioration. Hence, cis and trans-fatty acid composition is an important index for evaluating edible oil. Studies have mainly focused on total polar compound content, cis and trans-fatty acid composition, oxidation characteristics, and other risk factors of fried edible oil after pan frying and deep-frying (Aladedunye & Przybylski, 2014;Felixa & Roman, 2009;Shinde & Gupta, 2014). The influence of four different cooking methods, namely, roasting, grilling, microwaving, and frying, on cooking loss, lipid oxidation, and volatile profile of foal meat was studied by Domínguez et al. (Domínguez, Gómez, Fonseca, & Lorenzo, 2014). Recently, TFAs content in edible vegetable oil also was evaluated based on the four types of Chinese cooking methods (vegetable salad, stir-frying, pan-frying, and deep-frying) (Cui et al., 2017).
The accumulation of high levels of bioactive compounds in daily diet is an important aspect to be considered while evaluating oil quality. With the rapidly increasing choices of different edible oils in the market, coupled with insufficient information on their appropriate use, selection of the best dietary oil is becoming an increasingly challenging task. To the best of our knowledge, studies discussing the effect of stir-frying on different processed food are limited. In this study, based on the traditional Chinese stir-frying method, rapeseed oil with two different canolol levels were used to evaluate various parameters such as fatty acid content, bioactive compound content, presence of primary and secondary oxidation products, and oxidative stability index (OSI). Our observations will provide basic data, which will assist in selecting rapeseed oil suitable for cooking.

| Sample preparation
High canolol rapeseed (HCR): Two hundred kilograms of rapeseed were heated in a commercial tunnel microwave oven (2,450 Hz, 55 KW) for 7 min and the whole transfer line was 10 m under the microwave machine. The microwaved rapeseeds were pressed using twin screw oil press machine (Wuhan, China), and the obtained rapeseed oil was degummed and deacidificated using physical adsorbent in order to get qualified frying oils. Low canolol rapeseed (LCR) was purchased from the factory with the hot pressing process. High canolol rapeseed (HCR) and low canolol rapeseed (LCR) were refined using physical methods.

| Stir-frying and oil extraction
The stir wok (diameter, 32 cm, Supor) was washed and wiped before the frying experiments. All ingredients and oils were matched according to the prescribed ratio. To minimize human error, the same operator performed the entire process. Before stir-frying, the oils were heated within 40 s.

| Fried shredded potato
Potato (700 g) was peeled, cut in pieces, and soaked in water for 3 min. Thirty milliliters oil sample (HCR and LCR, respectively for six parallels) were put into a stir wok. The shredded potato was put into stir wok when the temperature of oil was near its smoke point.
The potato in the wok was stirred for 3 min. After cooking, the shredded potato were collected and stored at −20°C until further analysis.

| Fried tofu
Tofu (600 g) was put into a stir wok with 30 ml oil sample (HCR and LCR with six parallel operations). During the process, 10 ml water was added gradually to prevent the tofu from burning. The content in the wok was stirred for 5 min. After cooking, the tofu were collected and stored at −20°C until further analysis.

| Fried vegetables
Chinese cabbage (600 g) was selected and cleaned. Chinese cabbage was put into a stir wok with 30 ml oil sample (HCR and LCR, each with six parallel operations). The content in the stir wok was stirred for 1 min. After cooking, the Chinese cabbage was collected and stored at −20°C until further analysis.
Parallel operations were conducted for six cycles, and three replicates were used for each material. At the same time, uncooked materials (potato, tofu, and Chinese cabbage) were collected and oil was extracted from them to eliminate the influence of bioactive ingredients in material oil. Oil was extracted from food and cooked oil according to Cui et al. (2017). Starting from oil heating, the temperature of the pan was monitored every 20 s using infrared detector.

| Determination of 2,6-dimethoxy-4vinylphenol (canolol) content
Rapeseed oil sample (0.5 g) was added into a 10 ml colorimetric tube, followed by the addition of 1.5 ml hexane and 1.5 ml methanol: water (ratio, 4:1). The mixture was vortexed for 5 min at 706 g and centrifuged for 10 min at 2,823 g. The supernatant was repeat extracted twice as described above. The mixed extracts were passed through a 0.45-µm filter. Three replicates were used per experiment. Canolol was quantified using UPLC analysis according to Yang et al. (2014).

| Determination of oxidative index
The peroxide value, p-anisidine value, and malonaldehyde content were measured according to the conventional method ISO 3960, ISO 6885-2006, andGB 5009.181-2009 standards, respectively.

| Determination of cis and trans-fatty acid composition
Methyl esterification of the cis-and trans-fatty acids was performed according to Zribi et al. (2014). Cis-and trans-fatty acid compositions were analyzed using the Agilent 7890N GC instrument coupled with a flame ionization detector (Agilent Technologies Inc.). A nonpolar fused-silica capillary column (HP-88, 100 m × 0.25 mm × 0.20 μm) was used for gas chromatography (GC) analysis. Helium was used as a carrier gas at a flow rate of 1.5 ml/min. A split ratio was set as 1:30. The temperature program was as follows: Initial temperature set at 120°C was on hold for 1 min, after which it was increased to 180°C at 20°C/min; next, the speed was set as 5°C/min, and finally, the column temperature was increased to 220°C and held for 5 min.
Both the detector and injector port temperatures were maintained at 260°C. The fatty acids were identified from retention time comparison versus mixed standards run under the same conditions. They were quantified according to percentage peak area ratio. The final results were expressed as the percentages of individual fatty acids.

| Determination of total phenol content
Rapeseed oil (0.5 g) was added into a 10 ml colorimetric tube, followed by the addition of 5 ml 95% methanol and 0.5 ml Folin's phenol reagent, and mixed for 3 min. One milliliter saturated Na 2 CO 3 solution was added, and the mixture was diluted to a constant volume of 10 ml using distilled water. The solution was stewed and allowed to react at room temperature for 60 min. The absorbance of the sample was measured at 765 nm. The result was expressed in sinapic acid equivalent (mg/100 g oil).

| Determination of tocopherols
Tocopherol content in rapeseed oils was determined according to Zhang et al. (Zhang et al., 2017). Different tocopherols were identified by comparing their retention times to those of α-, β-, γ-, and δ-tocopherol standards. The final result was expressed as mg/100 g.

| Determination of phytosterols
Oil samples were saponified according to the method described by Azadmard-Damirchi, Nemati, Hesari, Ansarin, and Fathi-Achachlouei (2010). The 0.03 g oil sample (exact to 0.0001 g) was mixed thoroughly with 3 ml of 2 mol/L KOH in 95% ethanol in a glass tube and shaken in a water bath at 90°C for 15 min. After cooling the tubes, 2 ml of water and 1.5 ml of hexane were added and mixed vigorously. The mixture was then centrifuged at 5,000 rpm for 5 min, and the hexane layer containing unsaponifiables was separated for further analysis. The phytosterols were quantified using GC analysis according to Yang et al. (2013). The GC conditions for phytosterol identification are as follows: DB-5 capillary column (30.0 m × 320 μm × 0.10 μm) was used for analysis; the injector temperature was 260°C; the temperature program was as follows: hold at 60°C for 1 min, increased at 40°C/min to 310°C, and maintained for 6 min; the carrier gas was helium with flow rate of 2 ml/min; and analysis was performed in a split ratio of 10:1.

| Oxidation induction time
The OSI of oil samples was evaluated using a Metrohm 743 Racimat apparatus (Metrohm Herisan, Switzerland). The oil samples (3.0 g) were heated at 110°C. This method is based on the hypothesis that continuous bubbling of air through the oil-containing sample at constant speed of 20 L/h can be used to measure the oxidation induction time of the oil. Results were expressed as hours.

| Statistical analysis
Analysis of variance (ANOVA) and Duncan's multiple range tests were performed using the SPSS (Statistical Product and Service Solutions) 19.0 software version. The ANOVA test was performed for all experimental runs to determine the significance at 95% confidence interval. All experiments were performed in quintuplicate.
The means and standard deviations for all values were computed in Origin 8.0.

| Characterization of primary and secondary oxidation products after stir-frying
The temperature was monitored and shown in Figure 1. The oils were heated from 32 ℃ to about 190°C in 40 s. The temperature reduced rapidly when the food items were put into stir wok and then increased in the whole stir-frying process. Peroxide value is an important indicator reflecting the primary oxidation products of oil. Table 1 shows that peroxide values ranged from 2.1 mmol/kg to 2.7 mmol/kg, 2.3 mmol/kg, and 2.3 mmol/kg in HCR from shredded potato, tofu, and vegetable, respectively, indicating a slightly increasing trend. Peroxide values ranged from 1.6 mmol/kg to 2.7 mmol/kg, 2.5 mmol/kg, and 1.9 mmol/kg in the LCR of the above cooked foods, respectively. The oxidative rancidity of oils and the deterioration of the products are affected not only by the saturation degree of the oil, but also by other factors, such as temperature, light, oxygen, metal ions, water activity, and antioxidants. The high temperature of deep-frying is sufficient to disrupt the C-C and C-H covalent bonds of the acyl skeleton, forming various lipid hydrocarbyl radicals, which initiates the chain reaction of free radical oxidation (Goyary, Kumar, & Nayak, 2015). However, after a short stir-frying, the peroxide value was less than 6 mmol/kg, which was within the range of the recommended standard for oil. The effect of quick stir-frying on peroxide value is limited.
Oil is further decomposed to produce a series of aldehydes, ketones, and acids (O'Keefe and Pike 2010). The p-anisidine value is an index of the nonvolatile aldehyde content in oil, formed from fatty acid hydroperoxides, especially 2-alkenals and 2, 4-dienals. p-anisidine value and malondialdehyde (MDA) content is measures of the oxidative state of edible oil. P-anisidine value and MDA content was lower in HCR than in LCR. Table 1 shows that p-anisidine value increased highly and significantly (p < .05) from 3.2 to 6.5 and from 5.5 to 16.2 for HCR and LCR, respectively. The extent of increase was different for HCR and LCR, which may be because of their initial levels in uncooked oils. P-Anisidine value of more than 10 indicates severe deterioration (Neff et al., 2014). The canolol, total phenol, and total phytosterol contents showed significant correlation with the p- respectively, after stir-frying (Table1). The increase for the LCR samples was higher than that for the HCR samples, which indicated that HCR suffered less oxidative deterioration after stir-frying. Lower p-anisidine value and MDA content in oil indicated lower extent of oxidation.

| Changes in fatty acid composition after stirfrying
Heating can significantly affect the chemical composition of oils. The presence of unsaturated fatty acids (UFAs) in vegetable oils, including monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs), is associated with decreased risk of coronary heart disease (Bendsen, Christensen, Bartels, & Astrup, 2011).
The most predominant fatty acids in uncooked rapeseed oil are palmitic acid (4.20% and 5.31%), oleic acid (58.97% and 58.22%), linoleic acid (17.35% and 20.79%), linolenic acid (8.18% and 7.65%), arachidonic acid (3.89% and 2.12%), and erucic acid (5.18% and 3.55%) ( 3.94% to 3.87%, respectively. In fact, linoleic acids are usually used as indicators of the extent of fat deterioration because it is more susceptible to oxidation than saturated fatty acids. Therefore, the C18:2/C16:0 ratio is also used to indicate the degree of oxidative deterioration in frying oil (Flakelar et al. 2017). Overall, our results indicate that the oxidation resistance was higher in HCR than in LCR.
TFAs are always used for evaluating the quality of frying oil. With the exception of cis fatty acid, TFAs were not detected after comparison with standards poststir-frying. TFAs were not detected in any sample ( Table 2). The TFAs concentration in frying oils can be determined from the extent of partial oil hydrogenation. The amounts of trans C18:1 in the fresh canola oils were less than the quantitative limit (QL). They were lesser than QL even after the tenth frying oper-

| Degradation of bioactive components in the rapeseed oils after stir-frying
Internal bioactive compounds or exogenous antioxidants are used to improve the thermal stability of oil during processing as they can TA B L E 1 Oxidation extent of rapeseed oils before and after stir-frying
inactivate pro-oxidant metals, scavenge free radicals, or quench singlet oxygen depending on their functional group. The total phenol and canolol level of rapeseed oil was increased by microwaving or roasting, as detected in the sample preparation obtained from an oil factory. Oil stability during food processing can be easily determined from the performance of a frying experiment and analysis of relevant parameters associated with the quality of the used frying oil. The initial content of total phenols in HCR and LCR oils was 188.3 mg/100 g and 111.4 mg/100 g, respectively (Figure 2).
After frying potatoes, the content of total phenols decreased to 88.9 mg/100 g and 23.0 mg/100 g, respectively, with a percentage of weight loss of 53.2 and 79.3%, respectively. The total polyphenols of HCR and LCR were reduced to 72.0 and 17.1 mg/100 g, respectively, during the frying of vegetables ( Figure 2) and the percentage weight loss were 61.7% and 84.6%, respectively. While frying tofu, the content of total phenols decreased to 74.4 and 25.9 mg/100 g, respectively, with a percentage weight loss of 60.4% and 76.7%.
Therefore, after frying three raw materials, the total phenol content of HCR was 70.5-88.9 mg/100 g, whereas for LCR it was of 17.1-25.9 mg/100 g. It is obvious that the total phenol concentration of HCR was significantly higher than that of LCR.
2,6-Dimethoxy-4-vinylsyringol is a newly identified phenolic compound in rapeseed oil that is formed by thermal decarboxylation of the sinapic acid naturally occurring in rapeseed seeds after roasting or microwaving . Thermal treatment of rape- There are four forms of tocopherols in rapeseed oil, including α-, β-, γ-, and δ-tocopherols. Tocopherols play an important role as antioxidants, as they prevent oxidation reactions by donating hydrogen atoms to peroxyl radicals (Normand, Eskin, & Przybylski, 2001;Saguy & Dana, 2003). Two types of tocopherols, α-tocopherol and γ-tocopherol, were detected in the two rapeseed oils. The main isomer of tocopherol in rapeseed oil is γ-tocopherol, which accounts for almost two-third of the total amount of tocopherol. These TA B L E 2 Fatty acid compositions of rapeseed oils before and after stir-frying    to 180°C, ranged from 450 mg/kg to 359 mg/kg and to 145 mg/kg after 10 cycles of heating (Barrera-Arellano et al., 2002). Compared to that after baking or boiling, the instability of tocopherols after stir-frying is possibly because of the heat-sensitive nature of the tocopherols (Saguy & Dana, 2003). The rates of degradation of α-tocopherols and γ-tocopherols were more than 40% during stir-frying of three types of dishes. The decrease in tocopherol content was accompanied by increase in MDA in LCR (p < .001); however, MDA levels did not correlate with the change in tocopherols in HCR.  during stir-frying is less, which may be due to its excellent stability at high temperature compared to that of canolol, phenols, and tocopherols. In addition, the phytosterols in rapeseed oil mainly include brassicasterol, campesterol, β-sitosterol, and Δ5-avenastrerol.
Among these, β-sitosterol is the most abundant, followed by campesterol, and Δ5-avenasterol is the least abundant in HCR and LCR.
β-Sitosterol loss was about 16% after stir-frying in both HCR and LCR. A similar degradation rate was observed in these two rapeseed oils. This may be attributed to the fact that many important components exhibit antioxidant activity during high temperature cooking.

| Relationship between OSI and bioactive compounds
Comparison of micronutrients showed that the canolol, total phenolic, and phytosterol contents in HCR were significantly higher than those in LCR before and after cooking different raw materials. The degradation rate indicated slight differences in oxidation stability among the three raw materials. Among bioactive compounds with antioxidant properties, the total phenolic content showed significant (p < .05) differences, with initial concentrations of 188.3 mg/100 g for HCR and 111.4 mg/100 g for LCR. Consequently, the oxidative stability, as determined using the Rancimat method, also differed significantly (p < .05), with 11.8 hr for HCR and 9.5 hr for LCR (Table 1) depends not only on their fatty acid composition, but also on the presence of fatty bioactive constituents, such as phytosterols and phenols (Rudzińska et al., 2005). Generally, HCR showed better cooking oxidation stability than LCR. This is similar to the results of a recent study showing that the canolol-rich extracts were able to retard the degradation rate of α-and γ-tocopherol during frying (Hosseini et al., 2016).
However, the results of correlation analysis were different in the two rapeseed oils (Table 4). In HCR samples, total phenols, canolol, total tocopherols, and phytosterols showed a significant positive relationship with OSI. Although total phenols, canolol, and phytosterols showed significant negative correlation with p-anisidine value, they showed significant correlation with MDA content in LCR. Bioactive compounds contribute significantly to oil oxidation stability. Anisidine is formed from fatty acid hydroperoxides, especially 2-alkenals and 2,4-dienals. However, MDA is a degradation product of polyunsaturated fatty acid peroxide.
The MDA in fried food products is mainly derived from oil oxidation. Longer duration of frying substantially increased MDA F I G U R E 4 α-tocopherols and γtocopherols level of rapeseed oils before and after stir-frying level at 170°C (Normand et al., 2001). In LCR, the total tocopherol content correlated significantly with MDA levels, but not in HCR.
Canolol possess stronger antioxidant properties than tocopherols (Shrestha, Stevens, et al., 2013). In the rapeseed oils with high canolol content, canolol as a major antioxidant plays an important role in frying-associated oxidation resistance. The good correlation with oxidative induction time illustrates its antioxidative character.

| CON CLUS ION
During stir-frying, plant oils are heated at high temperatures and mixed with food ingredients in a relatively short period of time.
Hence, it was necessary to evaluate the changes in rapeseed oil quality in conventional stir-frying style of cooking. In summary, oils with two different canolol contents, HCR and LCR, were studied with respect to oxidation stability. Results showed that oils with high micronutrient content manifested higher oxidation stability.
When frying different raw materials (vegetable, tufu, potato), the contents of total phenols, phytosterols, and 2,6-dimethoxy-4-vinylphenol in HCR were significantly higher than those in LCR, in addition to lower cooking degradation rates, indicating that HCR showed better antioxidant capacity during frying. Statistically significant differences (p < .05) between HCR and LCR were observed for bioactive compound concentrations and OSI. Therefore, highquality rapeseed oil is suitable for cooking different raw materials due to its good oxidation stability during cooking and high content of micronutrients in oil after stir-frying. We concluded that HCR oils are advantageous for Chinese stir-frying as they provide nutritional and health benefits.

ACK N OWLED G M ENTS
This research was supported by National Natural Science Foundation Research System (CARS-12).

CO N FLI C T O F I NTE R E S T
There is no conflict of interest in this paper.

E TH I C A L A PPROVA L
All authors were actively involved in the work leading to the manuscript and will hold themselves jointly and individually responsible for its content. There is no conflict of interest in this paper. Human or animal testing is unnecessary in our study.