Identification of dimethachlon metabolites and dissipation behavior, processing factor and risk assessment of dimethachlon in rapeseed

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Pesticides and their transformation products are found in raw agricultural and processed food products, posing risks to human health. Therefore, a thorough investigation of dimethachlon’s dissipation, metabolites, residue levels in crops, and changes during food processing is essential for an accurate dietary risk assessment. This study identified two metabolites, 4-(3,5-dichloroanilino)-4- oxobutanoic acid (DCBAA) and 3,5-dichloroaniline (3,5-DCA), formed from dimethachlon in crops under field conditions. A precise and sensitive analytical method was developed to detect dimethachlon, DCBAA, and 3,5-DCA in rapeseed pods, seeds, and oil. The half-lives of dimethachlon in rapeseed pods at two sites ranged from 1.61 to 1.67 days. During degradation, the maximum residue levels of DCBAA and 3,5-DCA in rapeseed pods reached 38.59 % and 3.47 % of the initial parent compound concentration, respectively. At harvest (day 10), final dimethachlon residues in rapeseed were 0.11 mg/kg and 0.27 mg/kg. Processing rapeseed into oil through sun-drying and stir-frying reduced dimethachlon residues, with processing factors (PF) of 0.69 and 0.64, respectively. However, pressing the seeds concentrated dimethachlon in the oil, with a PF of 1.92. Notably, DCBAA tended to concentrate during sun-drying but decreased during stir-frying and pressing. Supervised trials indicated that dimethachlon residues in rapeseed posed no long-term dietary risks.