Methyl bromide sorption and residues on sultanas and raisins

Abstract

Methyl bromide is widely used as a fumigant for dried fruit, such as sultanas and raisins. Results from samples of unprocessed and processed (washed and oiled) Thompson seedless sultanas treated with methyl bromide showed that it was sorbed in a manner consistent with an initial physical sorption followed by a more gradual reaction phase. At 25 °C, 60% r.h. the reversible physical sorption approached equilibrium after about 8 h of exposure. Methyl bromide partitioned between the gas phase and the fruit, according to a simple partition coefficient, with an average ratio of 0.95 and 0.99 in unprocessed and processed fruit, respectively, at 25 °C and 60% r.h. The partition coefficient was constant for different filling ratios, but decreased with increase in temperature. Estimates for different varieties of unprocessed sultanas and raisins ranged from 0.36 to 1.11 at 25 °C and 60% r.h. Reaction of methyl bromide with sultanas followed simple pseudo first order kinetics, with a mean rate constant for reaction of 0.0096 and 0.0065 per hour for unprocessed and processed fruit, respectively, at 25 °C and 60% r.h. Similar values were found for a range of sultana and raisin varieties. The rate constant increased with an increase in both temperature and humidity, with the rate of increase not significantly different for unprocessed and processed fruit. Accumulation of bromide residues was found to be directly related to the number of fumigations and to the concentration of methyl bromide applied. Approximately 0.17 and 0.13 mg of bromide was added per kilogram of unprocessed and processed fruit, respectively (dry weight), for a 24 h exposure and for each unit of initial methyl bromide concentration expressed in g/m 3 in free air space. For a typical commercial dosage of 24 g/m 3, this corresponds to about 6 mg/kg bromide residue added per treatment for processed sultanas. About 82% of applied methyl bromide was released on airing after a 24 h exposure at 25 °C, 60% r.h. and 0.90 filling ratio. A model was developed to predict residue formation, concentration profile with time and proportion of methyl bromide released on airing after a typical fumigation of sultanas.