1. Introduction
Wheat flour is one of the most common food ingredients in the world, and it occupies an important position in the food industry [
1,
2]. Pesticides are commonly used in wheat production and play a positive role in reducing crop yield losses caused by crop diseases, pests, and weeds [
3]. The general population is exposed to pesticides primarily through eating food contaminated with pesticides residues. Although these pesticides are developed to function with minimal impact on human health, long-term exposure to pesticide residues in food remains a major risk. Controlling pesticide residues in the supply chains of food, especially rice and wheat flour, is important, and the ability to predict their degradation under various environmental conditions and processing factors is vital to their control [
4]. Thus, in this research, the degradation profiles of commonly used pesticides in the supply chain of wheat flour, mainly including the storage and milling period of wheat flour, were studied.
The common pesticides in wheat include fungicides (e.g., carbendazim, chlorothanlonil, carboxin, cyproconazole, and difenoconazole), herbicides (e.g., bensulfuron methyl, carfentrazone-ethyl, dicamba, flupyrsulfuron methyl, and difenoconazole), insecticides (e.g., chlorpyrifos, triazophos, carbosulfan, deltamethrin, and esfenvalerate), plant growth regulators (e.g., ethephon, trinexapac, and chlormequat), of which five pesticides (carbendazim, bensulfuron methyl, triazophos, chlorpyrifos, and carbosulfan) are used more frequently in wheat cultivation. Pesticide residue problems often occur in wheat and wheat products [
5,
6,
7].
The commonly used methods for pesticide extraction and purification include dispersive liquid–liquid microextraction, solid-phase extraction, solid-phase microextraction, QuEChERS (Quick, Easy, Cheap, Effective, Rugged, and Safe) method [
8,
9,
10,
11]. Among them, the QuEChERS method is one of the most successful methods [
12], which is based on the principle of using absorbent filler to interact with impurities in the matrix to adsorb impurities and achieve the purpose of impurity removal. This method is fast, simple, economical, efficient, durable, and safe, which has been widely used in recent years for the study of pesticide residues in food [
13].
The commonly used methods for the detection of pesticides in food are gas chromatography, gas chromatography–tandem mass spectrometry, liquid chromatography, liquid chromatography–tandem mass spectrometry, and ultra-high performance liquid chromatography–tandem mass spectrometry (UPLC-MS/MS) [
14,
15,
16,
17]. Gas chromatography is incapable of analyzing pesticides with high polarity and poor thermal stability [
18]. Liquid chromatography–mass spectrometry is capable of detecting various pesticide residues, but its detection limit is often difficult to meet the requirements of pesticide residue detection [
19]. Tandem mass technique, especially triple quadrupole mass spectrometry, coupled with ultra-high-performance liquid chromatography, is more comprehensive, sensitive, stable, and of wide detection range for the quantification of pesticide residues [
20].
In this study, the degradation patterns of five typical pesticides during the storage of wheat and flour under various storage conditions were investigated, using QuEChERS method coupled with UPLC-MS/MS. The five pesticides included carbendazim, a broad-spectrum fungicide, bensulfuron methyl, an herbicide, triazophos, an organophosphorus acaricide, chlorpyrifos, a thiophosphate insecticide, and carbosulfan, a carbamate insecticide. A typical wheat supply chain includes the storage of wheat grain, the milling process, and the storage of wheat flour. The degradation profiles of these five pesticide residues, combined with their processing factors during wheat milling, will facilitate the model construction to predict the pesticide residue levels in the wheat supply chain.
2. Materials and Methods
2.1. Materials and Reagents
Wheat (Jimai 22) was provided by Crop Research Institute, Shandong Academy of Agricultural Sciences (Jinan, China). Flour (Fuqiang) was supplied by Beijing Guchuan Food Co., Ltd. (Beijing, China). The pesticide standards, including triphenyl phosphate (TPP), carbendazim, bensulfuron methyl, triazophos, chlorpyrifos, and carbosulfan (all 99% purity), were purchased from Accustandard Inc. (New Haven, CT, USA). The five pesticides (carbendazim, bensulfuron methyl, triazophos, chlorpyrifos, carbosulfan) chemical structures are shown in
Figure 1. Chromatographic-grade methanol and formic acid were purchased from Mreda (Beijing, China), and 0.22 µm nylon membrane filter was purchased from Tianjin jinteng Experimental Equipment Co., Ltd. (Tianjin, China). Waters BEH C
18 column (100 mm × 2.1 mm, 1.7 μm) was purchased from Waters (Milford, CT, USA). QuEChERS purifier and salt package were purchased from Beijing Dima Outai Science Technology Co., Ltd. (Beijing, China).
2.2. Standard Solutions
Stock solutions of each pesticide standard (100 mg/L) in the same volumetric flask were prepared with chromatographic grade methanol. The stock solutions were prepared with methanol as a mixed stock solution (2 mg/L) and stored at 4 °C in a refrigerator for use. A series of concentrations of working solutions (0.005, 0.020, 0.050, 0.200, 0.400, 1.000 mg/L) were obtained by sequentially diluting the mixed stock solution with wheat and flour blank matrix solution.
2.3. Preparation of Positive Samples
According to the standard “Maximum Residue Limits of Pesticides in Food of National Food Safety Standard” (GB 2763-2021), and the limit of detection (LOD) and limit of quantification (LOQ) of the experiment, a mixed solution containing 10-fold maximum residue limit (MRL) of carbendazim, triazophos, and carbosulfan, 1-fold MRL of chlorpyrifos, and 20-fold MRL of bensulfuron methyl were prepared in methanol. Then, 5.0 g of each wheat and flour samples were sprayed with the above prepared mixed solution and left sealed for 24 h for storage experiments.
2.4. Control of Sample Storage Conditions
Different concentrations of glycerol were prepared and placed into closed drying containers to obtain environments with relative humidity of 50%, 60%, 70%, 80%, respectively. The wheat and flour samples sprayed with pesticides were placed in containers with different humidity control, and then each container was placed in incubators with constant temperature at 20 °C, 30 °C, 40 °C, and 50 °C, respectively. Different combinations of temperature and humidity were obtained.
2.5. Extraction and Purification of Samples
The extraction and purification methods for wheat and flour were based on the classical QuEChERS method with minor change [
21,
22,
23]. Wheat and flour samples were ground and weighed (5 ± 0.02 g) in a polypropylene centrifuge tube (50 mL), and TPP (100 μL) and pure water (10 mL) were added to mix, and the samples were left for 10 min to infiltrate. After immersion, samples were extracted by adding 10 mL methanol and salt packets (1.5 g CH
3COONa, 6 g anhydrous MgSO
4) and vortexed to mix [
24,
25,
26]. After centrifugation at 4000×
g for 10 min, the supernatant (6 mL) was collected in a centrifuge tube (10 mL). The samples were purified by adding a purifier (400 mg PSA, 400 mg C18, anhydrous MgSO
4) to the supernatant, and then vortexed and mixed. After centrifugation at 2000×
g for 10 min, the supernatant (2 mL) was aspirated into a new centrifuge tube (10 mL). After blowing nitrogen to near-dryness at 40 °C, 2 mL methanol was added to redissolve, and then vortexed and mixed. Finally, the purified extracts were filtered through a 0.22 µm nylon membrane filter to analyze with UPLC-MS/MS.
2.6. Conditions for the UPLC-MS/MS Analysis
The five pesticides were separated on a UPLC-MS/MS (Waters ACQUITY UPLC I-Class/Xevo TQ-S) (Waters, Milford, CT, USA) equipped with positive mode (ESI+) and Waters BEH C
18 column (100 mm × 2.1 mm, 1.7 μm). The mobile phase consisted of 0.1% formic acid in a mixed solvent of water (A) and acetonitrile (B). The gradient elution procedure was as follows: 10% B (0–1.5 min), 50% B (1.5–4 min), 90% B (4–10.5 min), and 10% B (10.5–13 min). The flow rate of the mobile phase was set at 0.4 mL/min, and the injection volume was 5 µL. The samples were measured by multiple reaction monitoring (MRM) in positive ion. The MRM parameters are shown in
Table 1. The parameters of MS detection were as follows: capillary voltage, 3.0 KV; ion source temperature, 150 °C; desolvation temperature, 500 °C; desolvation air flow, 800 L/h; and cone voltage, 35 V. The quantitative ion chromatograms for the five pesticides are shown in
Figure 2.
2.7. Data Analysis
The data of this study were collected under Masslynx 4.1 software (Waters Corp., Milford, MA, USA). Microsoft Excel 2020 software (Microsoft Corp., Redmond, WA, USA) was used for preliminary sorting of experimental data, and Minitab 17 software (Minitab Inc., State College, PA, USA) was used for response surface analysis. Origin 2019b software (OriginLab Corp., Northampton, MA, USA) was used to draw plots and calculate the Area Under Curve (AUC).
4. Conclusions
In this study, the five pesticides in wheat and flour were extracted and purified by the QuEChERS method and quantified by UPLC-MS/MS. The linear range, linear equation, LOD, LOQ, recovery rate, and precision of the method were investigated, and the results showed that the method was simple, rapid, with high accuracy and good applicability.
A quantitative model was constructed to predict the pesticide residue degradation during the storage of wheat and flour. The results showed that the R2 reached above 0.817 in wheat, and the R2 reached above 0.796 in flour, with good fitting effect. The model could be used to predict the degradation of pesticide residues at given time points of the wheat flour supply chain from wheat grain to the final product, i.e., wheat flour.
In flour and wheat, the five pesticide residues gradually decreased with the increase of storage time, and the degradation rate was faster in the early stage and slower in the later stage. The degradation rates of the five pesticides increased with increasing temperature and reached a peak at 50 °C. The degradation rates of the five pesticides increased with increasing relative humidity and reached a peak at 80%. Temperature may influence the volatility of pesticides and the rate of chemical reactions involved in the degradation process. Relative humidity may influence the hydrolysis reaction of pesticides and the growth and reproduction rate of microorganisms. Results showed that high temperature and high relative humidity accelerate the degradation of the five pesticides residues, and their degradation profiles and half-lives over temperature and relative humidity varied among pesticides.