Pilot commercial treatment of bulk wheat with CO₂ for insect control: the need for gas recirculation

Abstract

Two 1900 t capacity, cylindrical steel bins were sealed to a high standard of gastightness (pressure decay from 1500 Pa to 750 Pa in greater than 220 sec. when full). After being filled with wheat, both bins were purged with an 80% carbon dioxide-20% air mixture. A mean CO2 content of more than 75% (v/v) was achieved using 1.91 t CO2 in bin 1 and 2.13 t CO2 in bin 2. Immediately after purging the atmosphere in bin 2 (but not bin 1) was recirculated from the base to the apex through an external duct using a small fan. The CO2 concentrations in this bin remained above 40% for 14 days, and those at the base and apex differed by less than 5% CO2 during recirculation. The CO2 concentration at the apex of bin 1 fell rapidly and was less than 20% 2 days after purging, while the CO2 level at the base was above 60%. Fourteen days after purging, the recirculation apparatus was transferred to bin 1, reducing the existing concentration differential of 40% CO2 to 4% in 4 days. The gas interchange rate between the bin under recirculation and the external atmosphere averaged from 2.4 to 4.0% day-1. The cost of the CO2 and its application was between 20-25 ct-1 of grain. A naturally occurring infestation of Rhyzopertha dominica and Tribolium castaneum in both bins was controlled by the CO2 treatment and the wheat was exported without need for further insecticidal treatment.