The fate of labelled insecticide residues in food products. II. —The nature of methyl bromide residues in fumigated wheat

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AbstractThe chemical fate of methyl bromide absorbed by wheat under the conditions of fumigation has been studied. Whole‐wheat flour was exposed to 14C‐labelled methyl bromide. The fat, starch, gluten, and water‐soluble fractions were prepared from the exposed flour and their 14C‐content assayed. The gluten or protein fraction was responsible for some 80% of the decomposition of the absorbed fumigant. By measuring the 14C of the volatile products obtained on treating the gluten with sodium hydroxide or hydriodic acid under different conditions, it was shown that the decomposition of methyl bromide in gluten was due almost entirely to methylation with the formation of 50% of N‐methyl derivatives, 30% of dimethyl sulphonium derivatives, and of 20% of methoxyl and thiomethoxyl derivatives in about equal proportions. Similar results were obtained when gluten alone was exposed to the labelled fumigant. The production of free methanol in the flour by hydrolysis of the absorbed fumigant was about 10% or less. The rate of spontaneous decomposition of the dimethyl sulphonium compounds formed as a result of fumigation was estimated by using wheat which had been grown on 35S‐labelled sulphate.