Preparation of rapeseed oil with superhigh canolol content and superior quality characteristics by steam explosion pretreatment technology

Abstract In this study, rapeseed was pretreated by steam explosion pretreatment technology and subsequently pressed to prepare rapeseed oil. GC, UPLC, and HPLC techniques were employed to analyze the quality characteristics of the rapeseed oil, including the canolol content and other quality characteristics. Additionally, the effect of steam explosion pretreatment technology on the canolol content of rapeseed oil was studied and the formation mechanism of canolol elucidated. The results revealed that when the steam explosion pressure reached 1.0 MPa, the canolol content of the tested oil increased from 41.21 to 2,168.69 mg/kg (52.63‐fold increase) and that sinapic acid played a significant role in the conversion of canolol. Thus, the sinapine was converted into the intermediate (sinapic acid) by hydrolysis, which in turn was transformed into canolol through decarboxylation. The instantaneous high‐energy environment generated by steam explosion pretreatment could intensify the hydrolysis and decarboxylation reactions of sinapine and sinapinic acid, thereby significantly increasing the canolol content of the oil. To prove the superiority of steam explosion pretreatment, we compared it with other pretreatment technologies, including traditional high‐temperature roasting and popular microwave pretreatment. The results revealed that rapeseed oil prepared by steam explosion pretreatment displayed the best quality characteristics. This study can be a reference for the preparation process of rapeseed oil with superhigh canolol content and superior quality characteristics using steam explosion pretreatment.

. Therefore, this polyphenol is a potentially important natural antioxidant that can be applied to healthcare products (Han et al., 2017). Unfortunately, although polyphenols are abundant in rapeseed, most remain in the rapeseed cake after oil production and only a small amount of polyphenols is transferred to the rapeseed oil (Zheng, Yang, Zhang, & Huang, 2017).
Moreover, when roasting the PN1 03/1i/14 line at 180°C for 15 min, the tocopherol content of the tested rapeseed oil increased by 17%-18%, while the plastochromanol-8 content increased approximately twofold. Wroniak et al. (2016) investigated the effects of the moisture contents (7 and 9%) and microwave radiation (800 W for 0, 3, and 7 min) on the microstructure and quality characteristics of two rapeseed varieties (Kana and Bakara). The results indicated that the tocopherol amounts and oxidative stabilities were significantly affected by the different microwave pretreatment conditions. Moreover, the canolol contents increased to 926.42 and 821.86 μg/g (56.69-and 48.43-fold increases), respectively, under 800 W microwave radiation for 7 min at 7% moisture level, while the fatty acid compositions were not significantly affected (p > .05).
To date, there are relatively few studies on the influence of steam explosion pretreatment technology on the canolol content and other quality characteristics of rapeseed oil. Steam explosion technology is a physicochemical pretreatment method that transforms thermal energy into mechanical energy and achieves the separation and structural changes of macromolecule substances in components through instantaneous pressure release and expansion under a high-temperature and pressure environment Shi, Li, Li, Cheng, & Zhu, 2019;Zhang et al., 2019). Consequently, this research performed steam explosion pretreatment experiments on rapeseed to analyze the formation mechanism of canolol and evaluate the quality characteristics, including the physicochemical properties, fatty acid profiles, phytochemical (tocopherol, phytosterol, and polyphenol) contents, and in vitro antioxidant activities. The findings of this research can serve as guidance for the industrial preparation of rapeseed oil of superhigh canolol content and are also of important reference significance for the development of processes that afford superior quality rapeseed oil.

| Experimental materials and pretreatment
Double low rapeseed (Brassicanapus) samples were collected from Wuhan Zhongyou Grand Science and Technology Industry Co., Ltd.
Steam explosion pretreatment was carried out on an XSS-QPD multifunctional air expander (Wuhan KINHE Food Machinery Co.,Ltd.). A certain amount of rapeseed was weighed, the moisture of rapeseed was adjusted to about 12%, when the water quality was balanced, rapeseed was put into the air puffing bin, and the heating temperature by constant temperature system was controlled (about 220°C). When the pressure of the silo reached the set pressure (0.4 MPa, 0.6 MPa, 0.8 MPa, 1.0 MPa, and 1.2 MPa), it was within a very short time (0. 0875s) to complete the pressure release to realize steam explosion .
A sealed microwave digestion instrument (maximum power, 4,800 W and frequency, 2,450 MHz; CEM Corporation) was employed to simulate the most common microwave pretreatment process currently in use. Some fresh rapeseed was accurately weighed, and the fresh rapeseed moisture was adjusted to 12%, when balanced, the rapeseed was pretreated under 800 W for 7 min at a frequency of 2,450 MHz and then cooled to room temperature.
Referring to the documentation (Siger et al., 2017), traditional high-temperature roasting pretreatment was performed on a GSCHseries multi-function heat-roasting machine (Henan Ruiguang Machinery Co., Ltd., China); some rapeseed were weighed; then, the temperature of the multi-function heat-roasting machine was set through the temperature control function; a short period of time (about 1 min) was stabilized after the roasting machine was heated to 180°C; and finally, the weighing rapeseed was roasted for 15 min, and then, it was cooled naturally to room temperature.

| Preparation of rapeseed oil
In view of the loss of water in rapeseed under different pretreatment conditions, the moisture content of rapeseed was controlled by adding distilled water to between 6% and 7%, then squeezed; at the same time, the pressing temperature was controlled within 65°C, and the oil phase after filtering was pressed rapeseed oil. What's more, rapeseed oil was weighed to calculate the oil yield.

| Oil yield contents
The oil yield contents were determined according to ISO659.2009, using analytical grade petroleum for gravimetric analysis in Soxhlet apparatus (B-811, Buchi Labortechnik AG) for 8 hr.
The oil yield content was calculated as: where Y is the oil yield content (%), M 1 and M 2 are the rapeseed oil and rapeseed masses, respectively (g), and X is the rapeseed oil content (%).

| Physicochemical properties
The physicochemical properties [acid (AV) and peroxide (POV) values] of the tested rapeseed oil samples were measured according to the AOCS official methods (Firestone, 2003).

| Canolol and other phenolic compounds
The contents of canolol and other phenolic compounds were determined according to a previously reported method.

| Phytosterols
The phytosterol contents were determined as follows: 10 ml of 2 mol/L KOH in ethanol was used to saponify 0.2 g (accuracy 0.0001 g) of the tested oil samples and 0.5 ml of 0.5 mg/ ml 5α-cholestane (internal standard) at 60°C for 60 min; the unsaponifiable compositions were obtained with hexane. The hexane layer was dried over anhydrous sodium sulfate and silylated using 100 µl N, O-bis (trimethylsilyl) trifluoroacetamide + 1% trimethylchlorosilane (BSTFA + TMCS) at 105°C for 15 min. The mixture was then dissolved in 1 ml hexane for further analysis on an Agilent 6890A gas chromatography system (Agilent) equipped with a DB-5HT column (30 m × 0.32 mm, 0.1 μm; Agilent). The nitrogen (carrier gas) flow rate was 2.0 ml/min, while the detector and injection temperatures were maintained at 320°C. The oven temperature was programmed as follows: an original temperature of 60°C for 1 min, increased to 310°C at 4°C /min and maintained at this temperature for 10 min. The split ratio was 25:1, and the injection volume was 1 μl.

| Tocopherols
The tocopherol contents in rapeseed oil were quantified using the AOCS Official Method Ce 8-89 with slight modifications. Briefly, 2 g (accuracy 0.0001 g) tested oil sample was weighed in a 25 ml volumetric flask, dissolved in a hexane layer, and then filtered through a 0.22 μm polytetrafluoroethylene filter. The treated samples (20 μl) were measured by high-performance liquid chromatography (HPLC; LC-20A, Shimadzu Corp.) on an SIL100A column (250 × 4.6 mm, 5 μm; GL Sciences Inc.). The flow rate of the mobile phase, which comprised a hexane-isopropanol mixture (99.5:0.5, v/v), was 1 ml/ min, and the α-and γ-tocopherol contents were analyzed at 292 and 298 nm, respectively.

| Fatty acids
The rapeseed oil samples were methylated with sodium methoxide and subsequently analyzed using an Agilent 7890A gas chromatography system (Agilent) equipped with an HP-INNOWAX capillary column (30 m × 0.32 mm, 0.25 μm; Agilent Corp.). The injector and detector temperatures were maintained at 250°C. The flow rate of the N 2 (carrier gas), with an 80:1 split ratio, was set to 1.5 ml/min. The oven temperature was programmed as follows: an original temperature of 210°C for 9 min, increased to 230°C at 20°C /min and maintained at this temperature for 10 min. The injection volume in this experiment was 1 μl. The analysis of the fatty acid composition in the treated oil samples was achieved by comparing the afforded retention times with those of standard fatty acids.

| Antioxidant capacity assays
Briefly, exactly 1.25 g rapeseed oil was placed in a 10 ml centrifuge tube. About 1.5 ml 80% methanol aqueous solution was then added with shaking (HA9-HMV multi-vortex mixer; Wuxi Voshin Instruments Manufacturing Co., Ltd.), at 4,863 g, for 5 min in the dark. The supernatant was carefully transferred into another tube, and the extract was separated with 1.5 ml 80% methanol aqueous solution. This process was repeated three times. were stated as micromolar Trolox equivalents per 100 g oil (μmol TE/100 g).

Fluorescence recovery after photobleaching (FRAP) method
A freshly prepared FRAP working solution (2.5 ml 10 mmol/L tripyridyl triazine (TPTZ) solution in 40 mmol/L HCl, 2.5 ml 20 mmol/L FeCl 3 , and 25 ml 0.1 mol/L acetate buffer; pH 3.6) was hatched at 37°C for 10 min. Next, 0.5 ml extract and 2 ml FRAP working solution were added to a 10 ml volumetric flask containing redistilled water. The mixture was maintained in the dark at 25°C for 20 min. The reagent blank was analyzed at 593 nm using a TU-1901 Dual Beam UV-vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd.). Trolox reagent was used as the standard for the calibration curve (FRAP = 0.0088 × C Trolox −0.0179; R 2 = .995).
The results were stated as micromolar Trolox equivalents per 100 g oil (μmol TE/100 g).

| Oxidation induction time (OIT)
According to the AOCS Cd 12b-92 method, the OITs of the rapeseed oil samples were determined on a Metrohm Rancimat 743 (Herisau). A 3 g sample was weighed and placed into a glass reaction tube, heated at 110°C, and then passed through 20 L/hr of dried and cleaned air. The volatile organic acid was collected in a measuring vessel comprising 50 ml distilled water. As the oxidation reaction progressed, the water conductivity was automatically measured and the results were reported in hours (hr).

| Data analyses
All the analyses were implemented in triplicate and presented as the means ± standard errors. Statistical analyses were performed using the SPSS program (SPSS 23.0 for Windows, SPSS Inc.).
Duncan's test at the 5% significance level (p < .05) and one-way analysis of variance (ANOVA) were used to determine the significance differences.

| Effects of steam explosion pretreatment technology on the canolol content of rapeseed oil
The influence of the steam explosion pressure on the canolol content of the tested rapeseed oil was illustrated in Figure 1. The results revealed that the canolol content increased with the extension of the explosion pressure range from 0.4 to 1.0 MPa (p < .05), where it reached a maximum (2,168.69 mg/kg). However, when the steam explosion pressure was further extended to 1.2 MPa, a decrease in the canolol content was observed. It was considered that sinapic acid and its derivatives were directly or indirectly pyrolyzed to canolol. However, canolol underwent thermal degradation and became combined with the protein with the extension of the explosion pressure to 1.2 MPa, thereby resulting in its partial decomposition (Wroniak et al., 2016;Yang et al., 2014). This was consistent with the results reported by Spielmeyer, who observed that the temperature had a significant effect on the canolol content of the tested rapeseed oil (Spielmeyer, Wagner, & Jahreis, 2009).

| Effects of three different pretreatment technologies on the canolol content of rapeseed, rapeseed oil, and rapeseed cake
The influences of different pretreatment technologies on the canolol content of rapeseed, rapeseed oil, and rapeseed cake were compared in  (Zago et al., 2015). This was reflected by the lowest residual canolol value observed in the steam-exploded rapeseed cake.

Yang et al. (2014) pretreated rapeseed with microwave technology
under 800 W at different times followed by pressing to prepare the oil samples. The results manifested that microwave treatment dramatically affected the distribution and content of phenolic compounds in the rapeseeds and oil from them, and the formation of canolol was dramatically correlated with the loss of sinapic acid and sinapine (r = .997 and r = .952, respectively); furthermore, the canolol content increased to a maximum (sixfold increase) under 800 W for 7 min.    (Seçmeler, Üstündağ, Fernández-Bolaños 2018;Leskinen et al., 2017;Lopezlinares et al., 2015). These results well agreed with the results in this study, where compared with the other pretreatment methods, steam explosion pretreatment afforded rapeseed oil with the highest canolol content. sults revealed that the different pretreatments afforded significantly different quality characteristics (p < .05). Thus, after steam explosion pretreatment, the oil yield of the tested rapeseed oil increased from 69.26% to 80.11%, a value that was significantly higher than those observed for the roasted-(74.77%) and microwave-(75.47%) pretreated samples. This superior yield was attributed to steam expansion during the pressure release process, which led to more thorough destruction of the oil cells and thus a maximized oil yield (Niu et al., 2015).

| Effects of three different pretreatments on some quality characteristics of rapeseed oil
The AVs and POVs of the tested rapeseed oil samples slightly increased after all three pretreatment methods, owing to the long oxidation of rapeseed oil under high temperature (Xu et al., 2018).
However, although the AV and POV of the steam-explosion-pretreated rapeseed oil slightly increased after steam explosion pretreatment, these values were significantly lower than the limit standards. Moreover, these two values were markedly lower than those afforded by traditional high-temperature roasting and popular microwave pretreatment. This was attributed to the extremely short

TA B L E 4 Effects of different
pretreatments on the main fatty acid contents in rapeseed oil (%) completion time of the steam explosion pretreatment compared with those of the other two methods.
The influence of the three different pretreatment methods on the micronutrient contents of the respective pressed rapeseed oil samples was also illustrated in Table 3. The results manifested a marked difference in the phytosterol and tocopherol contents of the three tested rapeseed oil samples (p < .05). The highest phytosterol and tocopherol contents were observed in the rapeseed oil pretreated with steam explosion technology. These results were attributed to the instantaneous high-energy environment formed during the steam expansion process to accelerate fat accumulation, which was also more conducive to the dissolution of some fat-soluble micronutrient components (Liu & Chen, 2015). Thus, when the steam explosion pressure reached 1.0 MPa, the phytosterol and tocopherol contents were, respectively, 6,622.65 and 686.67 mg/kg, which were markedly superior to those afforded by the other two pretreatment methods.
The effects of the three pretreatments on the DPPH, FRAP, and OIT values of pressed rapeseed oil were also investigated ( Table 3).
The results indicated that all three pretreatment processes signifi-

| Effects of three different pretreatments on the main fatty acid contents in rapeseed oil
The effects of the three different pretreatment methods on the main fatty acid composition of rapeseed oil were compared in

| CON CLUS IONS
In this study, three pretreatment methods, namely traditional high-temperature roasting, popular microwave, and steam explosion pretreatments, were performed to prepare three dif-

ACK N OWLED G M ENTS
The authors would acknowledge the financial provided by Earmarked

CO N FLI C T O F I NTE R E S T
The authors declared that we had no any conflict of interest.

E TH I C A L A PPROVA L
The study did not include any animal or human tests. . Influence of rapeseed meal treatments on its total phenolic content and composition in sinapine, sinapic acid and