*4.2. Comparison of the Removal Efficiency of Each Pesticide Residue*

As mentioned earlier, ssamchoo had the lowest pesticide reduction among the five leafy vegetables, which might be due to the following two pesticides: imidacloprid (3.5–92.1%, average 31.8%) and thiamethoxam (0.2–92.0%, average 30.7%). Imidacloprid and thiamethoxam also had relatively low reductions in the perilla leaves and lettuce. For other pesticides, washing with running water was the most effective. However, imidacloprid and thiamethoxam had significantly higher removal efficiencies with boiling and blanching than those with other methods (Figure 2). This finding can be explained by the fact that an increase in temperature during heating affects the hydrolysis of pesticide compounds [33]. In addition, the removal patterns of the two pesticides were similar for the five leafy vegetables; this is due to the characteristics of each pesticide component. Imidacloprid and thiamethoxam are neonicotinoid-based insecticides, with higher water solubility compared to that of other pesticides, and are non-volatile substances with a log *P* value of <1 and high polarity.

The reductions in chlorfenapyr and lufenuron contents using the blanching and boiling methods tended to be lower than those of other pesticides (*p* < 0.05). In crown daisy, the residual amounts of chlorfenapyr after blanching were higher than their initial values. In addition, the residual level of lufenuron increased in perilla leaves and ssamchoo after boiling. Similarly, Yang et al. reported that the residual level of acetamiprid increased in green chilis after boiling and stir-frying (reduction: approximately 80%) [34]. This finding might be due to the concentration of pesticides as a result of moisture evaporation via heating in an open environment [35]. In a study by Lozowicka et al., the concentration of most pesticides was significantly reduced after 5 min of the boiling process used to prepare strawberry jam. However, pyrethroids (alpha-cypermethrin, deltamethrin,

and lambda-cyhalothrin) had a processing factor (PF) of 1 or more (when the PF is 1 or more, the residual pesticide amount is higher than the initial amount). The solubility of alpha-cypermethrin, deltamethrin, and lambda-cyhalothrin was low, at 0.004, 0.0002, and 0.005 mg/L, respectively [36]. The authors judged that the removal of pesticide residues during the heat treatment process would be influenced by the strong adsorption of the pesticides onto plant tissues and the solubility of the pesticides in water. Similarly, in this study, the residual amount of the pesticides chlorfenapyr and lufenuron was higher than the initial amount in the boiling process, and these pesticides had relatively low solubility (0.112 mg/L and 0.046 mg/L, respectively) compared to other pesticides (Table 1). Therefore, it is judged that the characteristics of the pesticides, such as the solubility as well as evaporation of water during the boiling process, may have influenced the residual amount.

Diniconazole had the lowest reduction after lufenuron and chlorfenapyr (*p* < 0.05). Diniconazole was characteristically and mainly detectable in ssamchoo [20]. Therefore, diniconazole must be removed from ssamchoo prior to consumption. When ssamchoo grows too large, it is less commercialized. Therefore, diniconazole is used to control the growth of ssamchoo to a moderate size. Diniconazole is a broad-spectrum triazole fungicide that acts as a plant growth regulator, decreasing the height and leaf area in bean plants when applied to roots [37,38]. In this study, the reduction of diniconazole contents in ssamchoo using different methods showed the following trend: running water (56.7%) and boiling (51.9%) > detergent (36.2%) > ultrasonic cleaning (31.6%) (*p* < 0.05). Overall, the reduction in pesticide residues in ssamchoo was low; washing with running water and boiling proved to be the most effective, especially for the removal of diniconazole from ssamchoo. The ultrasonic method and detergents led to the lowest reductions. Therefore, both washing and boiling are recommended for ssamchoo.

The reduction amounts of chlorfenapyr, diniconazole, indoxacarb, fludioxonil, pyraclostrobin, and lufenuron in all leafy vegetables were low after blanching and boiling (*p* < 0.05). Crown daisy and spinach are consumed as vegetables after blanching in boiling water or as soup after boiling for a long time. Therefore, crown daisy and spinach must be cooked after washing to sufficiently remove these pesticides.

In a previous study from 2005 to 2019, the MRL of leafy vegetables in South Korea was highly exceeded for diniconazole and lufenuron. The detection amount of diniconazole in ssamchoo and perilla leaves was 0.4–6.6 mg/kg and 0.7–2.4 mg/kg, and the MRL was 0.3 mg/kg. Furthermore, the detection amount exceeding the MRL (5.0 mg/kg) of lufenuron in spinach and crown daisy was 0.4–3.8 mg/kg and 0.3–2.1 mg/kg [20]. When running water was used for washing, the reduction of diniconazole from ssamchoo and perilla leaves (ssamchoo, 56.7%; perilla leaves, 81.8%) was highest (Table S1). The reduction of lufenuron in these two leafy vegetables (spinach, crown daisy) was also the highest when running water was employed as the washing method (spinach—77.9%, crown daisy—73.4%). In previous studies, leafy vegetables that do not meet the MRL were not harmful to health based on a risk assessment (risk index ranged from 0.001 to 7.6%) [20]. However, washing with running water before ingestion can reduce pesticide residues to levels below the MRL concentration.
