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Effects of Planting and Processing Modes on the Degradation of Dithianon and Pyraclostrobin in Chinese Yam (Dioscorea spp.) Kaiwei Shi, Xujin Wu, Jingwei Ma, Junfeng Zhang, Ling Zhou, Hong Wang, and Li Li J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b03916 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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Journal of Agricultural and Food Chemistry

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Effects of Planting and Processing Modes on the Degradation of Dithianon and

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Pyraclostrobin in Chinese Yam (Dioscorea spp.)

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(Running Title: The Degradation of Dithianon and Pyraclostrobin in Yam)

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Kaiwei Shib, Xujin Wua,∗, Jingwei Maa, Junfeng Zhanga, Ling Zhoua, Hong Wanga, Li

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Lib,∗

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a

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Academy of Agricultural Sciences /Henan Key Laboratory of Grain Quality and

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Safety and Testing/ Laboratory of Quality Safety Risk Assessment for Agro-products

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(Zhengzhou), Ministry of Agriculture, Zhengzhou 450002, China

Institute of Quality Standard and Testing Technology for Agro-products, Henan

b

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State Key Laboratory of Integrated Management of Pest Insects and Rodents, Institute of Zoology, Chinese Academy of Sciences, Beijing 100101, China

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∗

Corresponding author: Xujin Wu, phone: 86-135-9803-0746. E-mail address:[email protected]

∗

Co-corresponding author: Li Li, phone: 86-138-1043-6839. E-mail address: [email protected] 1

ACS Paragon Plus Environment


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ABSTRACT

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The yam (Dioscorea spp.) is widely cultivated in China. The degradation of dithianon

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and pyraclostrobin in yams with different planting and processing treatments were

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investigated in this paper. An analytical method for two pesticides in yam and yam

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plant was developed, and recoveries were between 77% and 93%, with relative

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standard deviations from 0.8% to 7.4%, respectively. Based on this method, half-lives

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for plants grown on stakes versus plants grown without stakes were compared. The

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results indicated that the half-life for pesticides residues for plants grown on stakes

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versus plants grown without stakes differed as 6.7 versus 3.1 days for dithianon and

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5.4 versus 5.2 days for pyraclostrobin. Dithianon was significantly influenced by

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planting mode due to its low stability under sunlight. The processing factors of

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various processing treatments (hot-air-drying, vacuum-freeze-drying, microwave-

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vacuum-drying, infrared-drying, steaming and boiling) were all less than 1, indicating

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that those process can reduce residues of two pesticides at different level. Significant

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amounts of residues were removed during the boiling treatment, whereas the others

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showed less effect.

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KEYWORDS: degradation; dithianon; pyraclostrobin; planting mode; processing

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factor; yam

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INTRODUCTION Yam (Dioscorea spp.) is the fourth major root crop in the world after cassava,

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potatoes and sweet potatoes.1 In China, different yam species are not only a common

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food, but also a traditional Chinese medicine that have also been widely used for the

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treatment of diabetes, diarrhoea, asthma and other ailments.2-6 Yams are cultivated

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either on mounds or on ridges, and the growing plants can be staked or spread on the

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ground without stakes. Staked yams are more productive, however, the cost of stakes

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and labor to place them may render the practice uneconomical.7 Ground yams without

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stakes is still widely cultivated in rural areas in China.

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Fresh yams are consumed in different ways, such as boiled, steamed or baked

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products. However, fresh yams are perishable during storage. Consequently, the

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processed yam products occupy a considerable part for daily consumption. Drying is

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one of the most important preservation processes for yams to extend their storage life

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and improve the taste, including air-drying, freeze-drying, far-infrared radiation-

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assisted freeze-drying, and vacuum-drying.8-12

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During the growth cycle of yam, the crops are exposed to a wide range of pests

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and diseases. Yam anthracnose is the most important diseases and is caused by

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Colletotrichum gloeosporioides (Popoolaet al.2013; Raj et al. 2013; Katoh et al.2016),

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which can be effectively controlled by dithianon and pyraclostrobin.13-17 Dithianon

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(CAS 3347-22-6, Figure 1 (A)), 5, 10-dihydro-5, 10-dioxonaphtho-(2, 3-b)-1, 4-

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dithiin-2, 3-dicarbonitrile, is a non-systemic fungicide used against a variety of foliar

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diseases.16 Pyraclostrobin (CAS 175013-18-0, Figure 1 (B)), methyl 2-[1-(4-

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chlorophenyl)pyrazol-3-yloxymethyl]-N-methoxycarbanilate, is a broad-spectrum

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foliar fungicide in the strobilurin chemical class.18 The combination of these two 3



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pesticides shows higher fungicidal activity and longer lifetimes than either alone.19 In

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previous studies, dithianon and pyraclostrobin have been determined in various fruits,

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vegetables and grain by high-performance liquid chromatography coupled with

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ultraviolet detector (HPLC-UV), high-performance liquid chromatography coupled

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with tandem mass spectrometry (HPLC-MS/MS) and gas chromatography coupled

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with time-of-flight mass spectrometry (GC-TOFMS).20-25

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Pesticide in yams might be concentrated or converted to more toxic metabolites

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during processing. The processing factors (PFs) are the ratio of residue levels in

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processed products and their respective raw products, which are important when

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evaluating the risk associated with intake of pesticide residues.26 The Joint

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FAO/WHO Meeting on Pesticide Residues (JMPR) evaluated food processing data on

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dithianon and pyraclostrobin residue behaviors where significant residues occur in

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plant or plant products that are processed into food.27 To the best of our knowledge,

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few papers have reported the degradation of dithianon and pyraclostrobin and PFs in

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yams during different processing procedure.

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Therefore, the objectives of this work were as follows: (1) to develop an

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analytical method for determining the residues of dithianon and pyraclostrobin in yam;

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(2) to assess the influence of different plant modes (ground and staked) on the

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dissipation of dithianon and pyraclostrobin in Chinese yam; and (3) to evaluate the

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degradation of dithianon and pyraclostrobin regarding PFs during different processing

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treatments, including hot-air-drying (HAD), vacuum-freeze-drying (VFD),

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microwave-vacuum-drying (MVD), infrared-drying (ID), steaming and boiling.

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MATERIALS AND METHODS

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Chemicals and Reagents 4


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Certified reference standards of dithianon (99.0%) and pyraclostrobin (99.0%)

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were provided by Dr.EhrenstorferGmbh (Augsburg, German). Water dispersible

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granules (WDGs) containing 12% dithianon and 4% pyraclostrobin was used.

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Acetonitrile, methanol, acetone and chloroform were of chromatography purity and

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purchased from Thermo Fisher Scientific Inc. (Waltham, USA). Hydrochloric acid

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and sodium chloride were of analytical grade and purchased from Beijing Chemical

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Reagent Co., Ltd. (Beijing, China). Primary secondary amine (PSA) was purchased

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from Bonna-Agela Technologies, Ltd. (Tianjin, China).

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Apparatus

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Several dryers used in this study are as follows: DHG-9070A dryer (Shanghai

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Keelrein Instruments Co., Ltd., Shanghai, China) for HAD; VirTis lyophilizer

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(Sentry2.0, SP Industries, Inc., PA, USA) for VFD; HWZ-2B drier (Hongyuan

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microwave equipment manufacturing Co., Ltd., Shanghai, China) for MVD; WS70-1

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drier (GongyiYuhua Instrument Co., Ltd., Zhengzhou, China) for ID. Extraction and

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purification of pretreatment were performed using XW-80A vortexer (Haimen

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QilinBei'erInstrument Manufacturing Co., Ltd., Jiangsu, China) and H-1650

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centrifuge (Xiangyi centrifuge instrument Co., Ltd, Hunan, China).

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Preparation of Standard Solutions

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Individual stock standard solutions (1000 mg L-1) were prepared by exact

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weighing of the solid pesticide and dissolved in 25 mL of acetonitrile. To determine

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the linearity a 100 mg L-1 stock solution was also prepared and working standard

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solutions (0.10, 0.25, 0.50, 1.00, 2.00, 5.00 mg L-1 for dithianon, 0.05, 0.10, 0.25, 0.50,

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1.00, 2.00 mg L-1 for pyraclostrobin) were prepared by diluting the stock solutions

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with acetonitrile. All standard solutions were stored at -10 °C before use. 5



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Field Trials The field trial was conducted in 2014 at Jiaozuo City, Henan Province, China.

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Each treatment consisted of three replicate plots and a control plot. The area of each

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plot was 30 m2. A buffer area was also employed to separate each plot, according to

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“Guidelines on Pesticide Residue Trials”.28 The fields in this study were previously

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investigated and determined to be free of the pesticide.

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To investigate the dissipation of dithianon and pyraclostrobin in Chinese yam

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with or without stakes, the WDGs were sprayed once at 800 g a. i. ha-1(two times of

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the recommended dosage) on the surface of yam plant. Representative yam samples

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(about 1 kg) were collected at 2 hours and 1, 2, 3, 5, 7, 10, 14, 21, 28 and 35 days

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after the application. All samples were homogenized and stored at -20 °C for further

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analysis.

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The physicochemical properties of the soil are as follows: the soil type was lu

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soil; organic matter 1.2-1.6%; pH 6.5-7.5. The average annul precipitation was 552

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mm. The average annual temperature was 14.3 °C.

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Sample Preparation and Processing

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Pretreatments before Processing

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The whole tubers of yams sampled from the control plot were soaked in water

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with WDGs at dosage of 1.76 g a. i. L-1 for 30 min, then dried in the open air. For

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HAD, VFD, MVD and ID, the tubers were cut into slices 5 mm in thickness. For

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steaming and boiling, the tubers were cut into pieces (20 cm long). In this study, the

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samples in different processing steps were collected to determine the residue. The

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detailed processing procedures are as follows. 6


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Hot-air-drying (HAD)

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A DHG-9070A dryer was first allowed to run for an hour to stabilize at the

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desired temperature (35 °C) at constant air velocity (1 m s-1).The yam samples were

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then transferred into the drier and dried for 24 hours to constant weight.

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Vacuum-freeze-drying (VFD)

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The yam samples were previously spread on a plate and placed at -80 °C for 24

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hours, then freeze-dried with a VirTis lyophilizer. The pressure was set to 32 Pa. The

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temperature of the cold trap was -80 °C. The primary and secondary

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drying temperatures were 20 °C and 35 °C, respectively. The yam samples were dried

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for 12 hours to constant weight.

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Microwave vacuum-drying (MVD)

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The yam slices were spread in one layer on the tray of an HWZ-2B drier. The

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vacuum degree, drying temperature and power density were set at 0.95 MPa, 50 °C

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and 1.3 W g-1, respectively. The yam samples were dried for 10 hours to constant

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weight.

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Infrared-drying (ID)

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The yam slices were spread in one layer on the tray of a WS70-1 drier. The

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power was set at 500 W. The yam samples were dried for 24 hours to constant weight.

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Steaming and Boiling

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The yam pieces were steamed or boiled at 100 °C for 10 min. Extraction and Cleanup 7



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Extraction and Cleanup for Dithianon The homogenized yam sample (5.00 ± 0.01g) was weighed into a 50-mL

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centrifuge tube (Corning Inc., NY, USA). Next, 1 mL hydrochloric acid (37.2%, w/w),

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5 mL of water and 10 mL of acetonitrile were added in that order. The tube was

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vortexed vigorously for 1 min with vortexer. After the addition of 5 g sodium chloride,

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the tube was vortexed vigorously for another 0.5 min, allowed to stand for 20 min at

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room temperature, and finally centrifuged for 5 min at 5000 rpm. Subsequently, 2 mL

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of supernatant was transferred into a 2.5mL centrifuge tube containing sorbent (100

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mg anhydrous MgSO4 and 50 mg PSA). The tube was vortexed for 1 min, then

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centrifuged for 5 min at 4500 rpm with centrifuge. The resulting supernatant was

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filtered using a 0.22µm nylon syringe filter into a colored auto-sampler vial for

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analysis.

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Extraction and Cleanup for Pyraclostrobin

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The homogenized yam or yam plant sample(20.0 g) was weighed into a 100-mL

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centrifuge tube (Taizhou Weierkang Medical Supplies Co., Ltd., Jiangsu, China), and

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5 mL of water and 40 mL of acetonitrile were added. The tube was then shaken in a

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mechanical shaker for 1 hour. After the addition of 5 g of sodium chloride, the tube

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was vortexed vigorously for 1 min and centrifuged for 5 min at 4500 rpm.

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Subsequently, 20 mL of supernatant was evaporated to dryness at 40 °C. For cleanup,

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the NH2solid-phase extraction (SPE) cartridge was conditioned with 5 mL of

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acetone/n-hexane (2:3, v/v). The concentrated extracts were dissolved in 5 mL of

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acetone/n-hexane (2:3, v/v),transferred to the cartridge, and then eluted with 15 mL of

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acetone/n-hexane (2:3, v/v). The eluent was collected and evaporated to near dryness

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at 40 °C. The extract sample was re-dissolved in 2 mL of acetonitrile and filtered

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using a 0.22-µm nylon syringe filter into an auto-sampler vial for analysis.

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Both the blanks and spiked samples were treated the same as the samples

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throughout the study process. Yam samples cultivated in control plots were used as

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field blanks. The blank samples were analyzed to ensure the absence of interference

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with retention times of the analytes. Blanks fortified with working solution mixtures

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of target pesticides at three concentration levels (0.5, 2 and 5 mg kg-1 for dithianon,

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0.05, 0.5 and 1 mg kg-1 for pyraclostrobin) were used for the and recovery experiment.

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All samples were analyzed in quintuplicate.

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HPLC Analysis

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HPLC determination was performed using a Waters HPLC system equipped with

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an E2695 separation module and 2998 photodiode array detector (Waters Corporation,

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MA, USA). SunFire C18 chromatographic column (4.6 mm×150 mm, 5 µm, Waters

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Corporation, MA, USA) was used. The injection volume was 20 µL. Mobile phase (A)

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was acetonitrile. and mobile phase (B) was ultra-pure water.

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HPLC Conditions for Dithianon

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The gradient elution program was as follows: 60% (A) from 0 to 10 min, then 80%

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(A) from 10 to 20 min, then 100% (A) from 20 to 25 min, then 60% (A) from 25 to

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25.1 min. The flow rate was 0.8 mLmin-1, and the column temperature was 30 °C. The

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detection wavelength for dithianon was set at 233 nm.

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HPLC Conditions for Pyraclostrobin

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The mobile phase of the isocratic elution was 70% (A) and 30% (B). The flow

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rate was 1 mLmin-1, and the column temperature was 25 °C. The detection

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wavelength for pyraclostrobin was set at 275 nm.

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Calculations

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Residue Levels in Fresh Yam

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The residue in this study was expressed as residue in fresh yam, calculated using

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the following equations (Eq. (1)-(2)):

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% =

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= × (1 − %) (2)

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where ω is the water content (%), WF is the fresh weight (g), WD is the dry weight (g),

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CF is the residue in fresh yam (mg kg-1), and CD is the residue in dry yam (mg kg-1).29

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Processing Factor

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× 100%(1)

Based on the effects on residue levels and the disposition of the residues in various processed products, the PFs are calculated as follows (Eq. (3)): ( ! "!#$ )

= %&%'(%())* (

! "!#$ )

(3)

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The ratios of residue levels in processed products and their respective raw

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products are called processing factors (PFs). A PF value 1 indicates

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the concentration effect of processing procedures.30,31

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Dissipation Kinetics 10


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The dissipation process follows first-order kinetics. The degradation rate

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constant and half-life were calculated using first-order kinetics equations as follows

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(Eq. 4): = + e- (4)

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where Ct is the concentration of pesticide residue (mg kg-1) at the time t (day), C0 is

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the initial concentration after application (mg kg-1), and k is the degradation rate

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constant (day-1). The half-life (t1/2) is defined as the time required for the pesticide

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residue level to fall to half of the initial residue level after application and was

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calculated from the k value:t1/2=ln 2/k.

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Statistical Analysis

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All experiments were performed at least three times. Data were statistically

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evaluated by one-way ANOVA analysis with SPSS base 20.0 software. When

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significant differences were found between groups (p

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