Chiang Mai Journal of Science

Determination of Organophosphate, Carbamate, and Pyrethroid Pesticide Residues in Vegetables from Hotpot and Shabu Sectors and Local Markets within Bangkok

1. INTRODUCTION

Pesticides are chemical compounds applied mainly in the agricultural sector to control pest populations, protect plants from harmful diseases or organisms, and enhance crop productivity throughout the plant growth cycle [1]. Although the usefulness of pesticides is apparent, the widespread use of these compounds has exposed humans to adverse health outcomes, ranging from acute toxicity, including nausea, skin irritation, eye pain, and respiratory distress [2], to chronic toxicity from prolonged exposure, such as disruption of thyroid function or cancer [3]. Moreover, the integrity of the relationship between abiotic and biotic components in ecosystems is impaired by the persistence of pesticides, which affects soil fertility, contaminates water sources, and leads to adverse consequences for the microbiome and wildlife [4].

More than 40% of Thailand’s land area is dedicated to agriculture, making the country a leading global supplier and exporter of rice, durian, and processed products like pet food. Consequently, pesticides are extensively utilized, with the Thai market supporting a vast diversity of types and quantities. Based on their chemical composition, these pesticides are primarily classified into four groups: organochlorines, organophosphates, carbamates, and pyrethroids [5]. Imports of agricultural substances containing these chemicals remained stable between 2019 and 2023; herbicides accounted for the largest share of these imports in Thailand, totaling approximately 90,000 metric tons [6]. Previous studies have shown that numerous pesticides were found contaminating various crops and vegetables, such as chlorpyrifos in school lunch boxes [7] and a group of pyrethroids (cyfluthrin, flumethrin, and cypermethrin) in coffee beans [8], which is consistent with the large quantity of pesticide use. Dietary exposure to these pesticide residues via vegetables could adversely affect human health, ranging from acute symptoms (such as headaches, nausea, and blurred vision) to chronic, severe diseases like cancer [26–27]. A prominent trend in the Thai food service sector is the prevalence of hotpot and suki-shabu restaurants, characterized by the tabletop boiling of raw ingredients by the consumer. With a market valuation of approximately 740 million USD (23 billion THB), this sector represents a significant pathway for potential daily exposure to pesticide residues via contaminated produce [9]. Despite its popularity, documentation concerning the pesticide contamination profiles of the ingredients specific to these restaurants is still unavailable.

Therefore, the aim of this study is to determine the level of pesticide contamination (organophosphates, carbamates, and pyrethroids) in raw vegetables from the hotpot and suki-shabu food sector in Bangkok, Thailand, using a screening process (certified test kits) and confirmatory testing (LC-MS/MS). The vegetables were also sampled from local markets for comparison. Additionally, the effect of boiling vegetables via the tabletop pot was simulated to assess whether pesticide residues could eventually lead to human oral exposure.

2. MATERIALS AND METHODS

2.1 Sample Collection and Screening Test Preparation
Five vegetables, including Chinese cabbage (Brassica rapa subsp. pekinensis), water spinach (Ipomoea aquatica), cabbage (Brassica oleracea var. capitata), baby corn (Zea mays L.), and carrot (Daucus carota var. sativa), were selected for analysis due to the high frequency of reported residues in Thai literature [22-23]. A total of 85 samples were obtained from 12 hotpot and suki-shabu restaurants (S01–S12) and 5 Type 1 (structured) markets (M01–M05) within the Ratchathewi district, Bangkok Metropolitan Area. Samples were selectively bought from the restaurants and markets that include all five vegetables. All samples underwent preliminary screening using two distinct colorimetric assays. The GT-test kit (30-minute version) was utilized to determine organophosphates (dicrotophos and EPN) and carbamates (carbofuran and methomyl), while the PY-test kit was used to detect pyrethroids (cypermethrin, cyfluthrin, cyhalothrin, deltamethrin, esfenvalerate, fenvalerate, and fenpropathrin). Both test kits were obtained from GT Trading, the developer and sole manufacturer of the GT and PY test kits. The sampling period spanned from December 2023 to April 2025. The vegetables were not subjected to washing prior to the screening test to determine the actual contamination levels present at the point of source.

The GT-test kit utilizes a cholinesterase inhibition technique to detect organophosphate and carbamate residues. This method is based on the principle that these pesticides act as cholinesterase inhibitors, effectively suppressing the activity of the cholinesterase (ChE) enzyme. Briefly, the edible portions of the samples were finely chopped and homogenized. A 5 g portion of each sample was transferred into a 30 mL polypropylene (PP) bottle, followed by the addition of 5 mL of Solvent-1. The mixture was shaken for 1 minute and allowed to stand for 10 minutes to facilitate residue extraction. One milliliter of the extract was then transferred to a glass test tube, and 1 mL of Solvent-2 was added to separate the residues. The tube was placed in a water bath (37^oC) and purged with air until the solvent had completely evaporated. Next, positive and negative controls were prepared by adding 0.25 mL of Solvent-2 into new glass test tubes and placing them in the water bath to ensure accuracy. A series of reagents were then added to each tube and mixed sequentially: GT-1 (0.5 mL, incubated for 10 min), GT-2 (0.25 mL for samples and negative control; 0.375 mL for positive control, incubated for 30 min), GT-3 (1 mL), GT-4 (0.5 mL), and GT-5 (0.5 mL). Finally, the color intensity of the reaction was visually inspected. Pesticide residues inhibit the cholinesterase enzyme (GT-1), preventing it from hydrolyzing the acetylcholine (GT-2). The concentration of unhydrolyzed acetylcholine determines the color depth, which reflects the levels of organophosphates and carbamates present in the vegetable samples (Figure 1). The test kit was validated for vegetables and fruits, demonstrating a sensitivity, specificity, and accuracy of 92.3%, 85.1%, and 87.1%, respectively. The detection limit for the assay was 0.05 mg/kg, calculated based on the insecticide trichlorfon.