The effect of boiling on the removal of persistent malathion residues fromstored grains

This study aimed at finding the fate of malathion in maize grains and beans stored in a tropical laboratory for 12 months at temperatures of 20-24 deg reesC, relative humidity of 42-80% and grain moisture contents of 11.7-12.5 % in maize grains and 14.1-15.3% in beans. Malathion and its degradation pr oducts malaoxon, malathion alpha -monocarboxylic acid and beta -monocarboxy lic acid were found in stored maize grains and beans treated with a mixture of pure radiolabelled malathion and 2% malathion dust after 12 months of s torage. A significant percentage of these residues were removed from the gr ains by cooking in boiling water. Though malathion and its polar metabolite s, malathion alpha- and malathion beta -monocarboxylic acids were completel y eliminated by boiling, malaoxon was still detected in quite high quantiti es in the solvent extracts of cooked beans and maize. Addition of NaCl to t he grains increased the rate of removal of the residues from both maize gra ins and beans by boiling water. The data obtained clearly showed a distinct ion in persistence, distribution and metabolism of malathion between the tw o types of crop, indicating also that more adverse windy, humid and hot tro pical conditions contributed to higher rates of metabolism and loss of the pesticide from the grains. The accuracy and ease of using radiolabelled mal athion in investigating its fate and behaviour in the two matrices is shown in this study. (C) 2001 Elsevier Science Ltd. All rights reserved.