Household cooking evaluation for pesticide reduction in brown rice samples. Microwave cooking optimization

Four different household cooking methods of brown rice were preliminary evaluated for pesticide reduction. The best method was optimized using Response Surface Methodology. A chromatographic method was validated for the determination of five commonly used pesticides in Argentinean rice fields, in raw and cooked brown rice. Extraction was performed with a modified QuEChERS. The method was linear in the evaluated range (0.01 - 1 mg/kg) with r2 higher than 0.9938. Precision and recovery values were lower than 12.5% and 94.6 - 109.9%, respectively. The validated methodology was used for the analysis of 60 brown rice samples obtained in different supermarkets. The results showed the presence of dichlorvos in 19 samples and tebuconazole in 10 samples despite of been prohibited. Cooking with the right amount of water, with excess of water, pre-soaking rice and cooking with extra water, and microwave cooking methods were evaluated. All methods destroyed malathion and the microwave alone was the only method that completely eliminated dichlorvos too and reached the highest pesticide reduction for the other analytes. After the optimization, a method using 60% of the maximum microwave power (800 W) and cooking time of 33 min was used on brown rice samples getting a approximately 90% reduction of pirimiphos-methyl, tebuconazele and fenitrothion, and 100% reduction of malathion and dichlorvos.