Effect of Ultrasonic Treatment on Pesticide Residue, Physical Quality and Microbial Availability of Water Spinach (Ipomoea aquatica Forssk.)

Among the concerns on the safety of consuming leafy vegetables which are usually eaten raw or minimally processed is presence of contaminants such as pesticide residues and microorganisms. This research studied the effect of ultrasonic treatment (40kHz frequency and 350W power output) for 1,7 and 15 minutes on residue level of a selected pesticide (diazinon), qualitative and quantitative changes in physical appearance and the microbial availability on water spinach (Ipomoea aquatica Forssk.). Diazinon residue level was quantified using QuEChERS extraction method followed by gas chromatography flame ionization detector (GC-FID) analysis. The changes in physical appearance were measured by image comparison, the total colour change and comparison of leaf firmness pre-treatment and post-treatment. Microbial availability was measured by total plate count (TPC). One sample was found to have traces of diazinon at 3.07 mg/kg, which exceeded the permitted maximum residual level (MRL) of 0.2 mg/kg according to Malaysia Food Regulations 1985, while no pesticide residue was detected on other samples treated with control and ultrasonic waves. Prolonging ultrasonic treatment beyond 7 minutes damaged water spinach leaves, thus affecting the physical appearance. Leaves treated with ultrasonic waves showed lower total colour change (ΔE) compared to control treatment. The firmness of water spinach leaves increases with increasing treatment duration with maximum increase obtained with 7 minutes’ ultrasonic treatment. Regardless of sonication time, ultrasonic treatment was found to be more efficient in reducing microbial availability on water spinach compared to soaking alone, with up to 1.82 log reduction after 15 minutes’ treatment. Ultrasonic treatment can be utilised as a decontamination method for water spinach and other leafy vegetables owing to its ability to remove pesticide residues and lowering microbial availability in 7 minutes’ sonication time while preserving the quality of water spinach.