Original Research Article
Sinapic acid derivatives in microwave-pretreated rapeseeds and minor components in oils

https://doi.org/10.1016/j.jfca.2019.103394Get rights and content

Highlights

  • SP, SG, DSG, QSDG, SM, DDSG were the main SA derivatives in untreated rapeseed.

  • SG, DSG, SM and DDSG were regarded to be one of the precursor substances of canolol.

  • Tocopherols and sterols content increased after microwave pretreatment process.

  • Oils from microwaved rapeseed owned stronger antioxidant ability for canolol formation.

Abstract

Microwave pretreatment has been considered to improve the quality of rapeseed oil for canolol generation. Canolol is assumed to be formed by the decarboxylation of sinapic acid (SA). In this work, sinapic acid derivatives in 39 microwave-pretreated rapeseeds and their roles in the enhanced oxidative stability of rapeseed oils were investigated. The average contents of sinapic acid derivatives in rapeseeds, including (from high to low) sinapine, sinapoyl glucoside (SG), disinapoyl gentiobioside (DSG), quercetin-sinapoyl-di-hexosepentose (QSDG), sinapoyl malate (SM), disinapoyl glucoside (DDSG) and SA, were determined. After microwave pretreatment, the canolol content in rapeseed increased from nil to 6.16–76.1 mg/100 g, while sinapic acid derivatives contents decreased. The degradation rates of SG, DDSG, DSG, SM, SA and sinapine were 59.1%, 40.2%, 33.7%, 27.4%, 14.4% and 11.3%, respectively. There was no correlation relationship between sinapine and canolol. However, SG, DSG, SM and DDSG were regarded to be the precursor substances of canolol. Additionally, the tocopherols, sterols and canolol contents, along with the induction period (IP) of microwaved rapeseed oil increased by 3.79%, 10.0%, 76.8 times and 38.7%, respectively. There was a significant positive correlation between IP and canolol content. These findings clarify the precursor substances of canolol and provide a theoretical support for the development and utilization of canolol.

Introduction

Rapeseed (Brassica napus L.) is the most popular oilseed crop in China. Its production in 2018/2019 reached 13.1 million tons, which accounts for approximately 17.5% of global total (U.S. Department of Agriculture, Foreign Agricultural Service, 2019). Rapeseed oil is known as a natural nutritive for its reasonable fatty acid composition and rich minor components. In rapeseed oil, tocopherols contents range from 113 to 824 mg/kg in the forms of γ-(60–74%) and α-tocopherol (26–35%). Sterols contents in rapeseed oils vary from 432 to 1190 mg/100 g, of which β-sitosterol, campesterol and brassicasterol accounts for 45.7–67.4%, 22.6–45.5% and 5.8–16.0%, respectively (Dolde et al., 1999; Yang et al., 2013).

About phenolic compounds, rapeseed contains 10 times more than other oilseeds (Siger et al., 2008). These phenolic compounds exist in the form of free, esterified and insoluble state with concentrations of 0.60–2.62 g/kg, 5.70–15.20 g/kg and 0–1.05 g/kg, respectively (Alu’datt et al., 2017; Krygier et al., 1982). The main phenolic compounds in rapeseed are sinapic acid (SA) and its derivatives, comprising 95–99% of the total (Kozlowska et al., 1983; Liu et al., 2012). Free sinapic acid alone makes up 6–14% of the total phenolic compounds in rapeseed (Thiyam et al., 2006). Sinapine (SP), a choline ester of sinapic acid, is the principal sinapic acid derivative in rapeseed, representing 70–90% (Siger et al., 2008; Liu et al., 2012). Sinapoyl glucoside (SG) is the second most abundant phenolic compound, at 0.50–6.54 mg/g (Khattab et al., 2010). Other sinapic acid compounds, including disinapoyl gentiobioside (DGSS), disinapoyl glucoside, quercetin-sinapoyl-di-hexosepentose (QSDG) and sinapoyl malate (SM), were reported by Liu et al. (2012) listed profile and distribution of soluble and insoluble phenolics in 11 species Chinese rapeseed However, rapeseed cultivars are getting continuously upgrading in recent years, status of rapeseed cultivation also changed greatly. Quantitative data on the sinapic acid derivatives in current varieties of rapeseeds are rare.

Recently, microwave technology is increasingly used in rapeseed processing. It can improve yield of rapeseed oil (Yang et al., 2012). Meanwhile, the content of phytosterol and tocopherol in rapeseed oil are also increased (Wroniak et al., 2016; Yang et al., 2012). Nonetheless, the greatest advantage of rapeseed microwave pretreatment is to canolol generation Canolol, which is associated with antioxidant and anti-mutagenic properties and gastric tumor inhibition, was first identified by Anna Koski et al. (2003) in refined rapeseed oil (Cao et al., 2008; Kuwahara et al., 2004). The oxidative stability of microwave-pretreated (800 W for 7 min, 145−147 °C) rapeseed oil was enhanced from 3.64-4.09 h up to 11.52-12.75 h due to canolol (Wroniak et al., 2016). It is assumed to form when SA is heat decarboxylated on exposure to microwave or heat treatment (Moldoveanu, 2010; Spielmeyer et al., 2009). After exposure to microwave irradiation (800 W for 7 min, 160 °C) the canolol content in rapeseed oil increased from 14.1 to 89.9 mg/100 g (Yang et al., 2014). Spielmeyer et al. (2009) also revealed that the maximum canolol content in microwave-pretreated (560 W for 7.5 min, 160 °C) rapeseed oil was 720 μg/g, or 120 times that of untreated rapeseed oil.

The content and distribution of sinapic acid derivatives are different according to different rapeseed varieties (Liu et al., 2012). The variety of rapeseed effects canolol content greatly. Wroniak et al. (2016) had found that canolol in Kana rapeseed oil (926 μg/g) was significantly higher than Bakara (822 μg/g) by the same microwave pretreatment (800 W for 7 min, 145−147 °C) but the reason was unknown. Sinapic acid derivatives were speculated to favor canolol formation during roasting (Shrestha et al., 2012), however, no further studies were conducted and the precursor substances of canolol were left unknown.

At present, studies tend to concentrate on the quantitative changes of sinapic acid and sinapine in microwaved rapeseed but pay little attention to other sinapic acid derivatives. Therefore, the relationship between canolol and sinapic acid derivatives remains to be investigated. This study was conducted using 39 typical varieties of rapeseed from the main production areas in China.

The objectives of this research were a) to evaluate the profile and distribution of sinapic acid and its derivatives in 39 rapeseed cultivars in China; b) to compare the variation of sinapic acid and its derivatives in rapeseed after the microwave pretreatment and explore potential precursor substances for canolol; and c) to evaluate the influence of microwave pretreatment on the amount of nutritional substances in rapeseed oils and their roles in enhancing oxidative stability.

Section snippets

Materials and reagents

Thirty-nine rapeseed cultivars (Brassica napus L.) were collected from the main production areas in China in 2018. The codes, names and origins of rapeseeds samples were listed in Table S1. Sinapic acid (SA, purity ≥ 99.9%), 5α-cholestane (purity ≥ 97%), β-carotene (purity ≥ 95%), α-tocopherol (purity ≥ 98%), γ-tocopherol (purity ≥ 96%), fatty acid methyl ester (FAME) qualitative mixtures (C4-C24) and N,O-Bis(trimethylsilyl) trifluoroacetamide with trimethylchlorosilane (BSTFA + TMCS, ≥ 98.5%,

Identification of sinapic acid and its derivatives in microwaved rapeseeds by UPLC-ESI-QTOF-MS/MS

Except for sinapine, the sinapic acid derivatives, including SG, DSG, SM, DGSS and QSDG were also found both in all cultivars and microwaved rapeseeds. The retention times from UPLC and the characteristic fragmentation information of the sinapic acid derivatives in rapeseed are presented in Fig. 1 and Table 1. The first peak could be assigned as sinapine, with a parent ion at m/z 310.1571 in the positive ion mode and daughter ions at m/z 251.0848, 207.0597 and 175.0341, which is consistent with

Conclusions

Sinapic acid and its derivatives in 39 rapeseed cultivars both in untreated and microwaved were analyzed. Microwave pretreatment led to the content increase of canolol, while other sinapic acid derivatives contents decreased for degradation. The degradation rates were influenced by their structures and heat stabilities. Sinapic acid derivatives, including SG, SM, DSG and DDSG were involved into the transformation of canolol. Sinapine was not the good precursor substance for canolol during

CRediT authorship contribution statement

Yanxia Cong: Conceptualization, Methodology, Investigation, Writing - original draft. Mingming Zheng: Software, Formal analysis, Data curation. Fenghong Huang: Resources, Writing - review & editing, Project administration, Funding acquisition. Changsheng Liu: Validation, Data curation, Funding acquisition. Chang Zheng: Writing - review & editing, Supervision.

Acknowledgments

This work was supported by the National Natural Science Foundation31772010, Central Public-interest Scientific Institution Basal Research Fund (No.Y2019LM12) and the Earmarked Fund for China Agriculture Research System (CARS-12).

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