*4.1. Differences in Efficiency Based on the Type of Leafy Vegetable and Removal Method for the Reduction of Pesticide Residues*

In this study, the reduction range for five leafy vegetables was 40.6–67.4% (Table 4). Previous studies revealed the effectiveness of pesticide residue removal from fruits and fruiting vegetables. When tomatoes and cucumbers were washed for 1 min, over 83–100% of the pesticide residues (dimethoate and profenofos) were removed. When okra (*Abelmoschus esculentus*) was washed for 1 min, malathion was almost completely removed. Up to 70–100% of azoxystrobin, acrinathrin, and kresoxim-methyl were removed when zucchini was washed (intensive) [23,24]. In China, when the kumquat (*Citrus japonica*) was washed with tap water for 5 min, the average reduction of the 10 pesticides (chlorpyrifos, myclo-butanil, tebuconazole, bifenthrin, lambda-cyhalothrin, beta-cypermethrin, esfenvalerate, difenoconazole, imidacloprid, and acetamiprid) was 25%. Furthermore, when spinach was washed in the same manner, the average removal efficiency was 11%. The reduction of the pesticide amount in spinach was lower than that in kumquat for the ten pesticides (chlorpyrifos, myclobutanil, tebuconazole, bifenthrin, lambda-cyhalothrin, beta-cypermethrin, esfenvalerate, difenoconazole, acetamiprid, and imidacloprid) [25]. The removal efficiency of leafy vegetables was relatively lower than that of fruits, as they are consumed without peeling. Removing the skin of fruits is known to have the greatest pesticide removal efficiency [13]. Peeling mangoes has been reported to completely remove fenthion, dimethoate, cypermethrin, and fenvalerate [26]. Based on a comprehensive review of the results of this study and other studies, it is concluded that leafy vegetables tend to have lower pesticide reduction than fruits and fruiting vegetables.

Spinach washed with running water (87.8%) and ssamchoo treated with NaHCO3 (28.0%) showed a three-fold difference in reduction (Table 4). Overall, leafy vegetables (perilla leaves and lettuce; average reduction, 63.6%) with a large leaf surface area had a higher reduction than those with a small surface area (spinach and crown daisy; average reduction, 54.7%) (*p* < 0.05); this is because each removal method is considered to have high efficiency over a large area [27]. However, although ssamchoo is a leafy vegetable with broad leaves, such as lettuce and perilla leaves, it has a different aspect ratio (lowest reduction on average). This finding indicates that the reduction may differ depending on the area and surface characteristics (wax amounts on the cuticle, curvature of the surface, etc.) of each agricultural product, despite the similar shape of leafy vegetables. Ssamchoo was developed in 1998 through the interspecies hybridization of Chinese cabbage and cabbage. Accordingly, ssamchoo is a new vegetable with completely different ecological characteristics, shapes, and genetic compositions from Chinese cabbage or cabbage [28].

Washing is the most effective method for removing pesticide residues from agricultural products in households and commercial processing [17]. The removal efficiency of pesticide residues by washing is affected by various elements (including the nature of sample, such as thickness, type, wax amounts on the cuticle, and surface area; pesticide characteristics; retention time of various pesticide residues; and washing methods) [11,29–31]. Despite these various factors, washing is the most effective treatment for leafy vegetables. In a study by Kim, washing crown daisy with a commercial detergent (0.5%) was found to reduce pesticide residues by 71.3–87.9%, owing to their ability to dissolve non-polar pesticides. Furthermore, when washed with NaCl (1%), vinegar (5%), and charcoal (1%), the reductions were 80.4–87.3%, 76.9–89.0%, and 78.5–88.2%, respectively. However, the reduction was lower than that of washing with water (80.2–90.5%) [32].

The reductions caused by the other eight methods were lower than those caused by washing with running water. The reduction efficiencies of alkaline water, blanching, vinegar, NaHCO3, stagnant water, and ultrasonic cleaning were approximately 51.2–56.4%. The removal efficiency was in the following order: running water > boiling > vinegar, stagnant water, and NaHCO3 > detergent (*p* < 0.05). Washing with detergent led to the lowest removal efficiency (43.7 ± 14.5) among all methods (Table 4). Other studies reported results different from those found in this study. Washing cucumber with micron calcium solution was more effective than washing with tap water, alkaline electrolyzed water (pH 10.50 and 12.35), or sodium bicarbonate. Washing with micron calcium solution for 20 min caused a greater loss of ten pesticides in cucumber, leading to a removal efficiency of 60% on average [25]. This difference might be due to the differences in agricultural products, processing times, and pesticides.

In Asian food culture, many foods (vegetable soup, seasoned vegetables) are consumed following the boiling or blanching of leafy vegetables. Residual pesticides in agricultural products are considered non-residue, as they decompose upon heating and cooking. However, according to the results of this study, blanching and boiling caused 54.9% and 59.5% reductions, respectively, which were lower than those induced by washing (*p* < 0.05). This finding highlights the importance of cooking after sufficient washing of leafy vegetables.
