The extraction rate of pesticide residues in water during the infusion process is dependent on the water solubility. The extraction of pesticide residues by boiling water can be regarded as a reversible equilibrium between adsorption and dissolution. The influence of chemical degradation is not important. When the water solubility is lower than 5 mg/kg, the extraction rate is 1-4%. Within the water solubility range of 10-150 mg/kg the extraction rate (Ri) is very sensitive to the water solubility (S); the relationship can be described as Ri = 59.8log S-42.5. When water solubility is higher than 170 mg/kg, the extraction rate is 90-100%.
This paper presents a discussion on residue behaviour of 17 pesticides on tea, on the basis of the research conducted during the last 20 years. Sunlight photolysis and growth dilution were the most important factors affecting pesticide persistence in the growing tea plant, while vapour pressure played an important role in loss of pesticide residues during tea processing. The percentage of pesticide extracted during the tea infusion process was highly related to water solubility. It is suggested that in studies on pesticide residue behaviour on tea, all the four factors should be considered integrally.
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