Further development of heat-based methods for disinfesting flour mills

Abstract

Following detailed studies at two mills, (volumes 3078 and 6600 m³) a target temperature of 50^o C and a total heating period greater than 40 hours is recommended for heat treatments.  Commonly about half of this period will be required for the structure to approach target temperatures. Heating larger mills will take longer, and the scaling up of heating requirements may introduce other problems that could preclude the use of heat as a whole-site disinfestation strategy. 

The survival data contained within this report indicates that there were many areas within the mill structures where insects could be expected to survive.  This was due to design and construction material that prevented adequate temperatures being reached for a sufficient time. 

A thermal model was developed to predict heating times of different structural components. Heat was provided by 18 kW electric fan heaters deployed on each floor, and 2.2 kW ducted fan heaters to selectively heat wall floor joints. In the concrete basement areas significant heat sinks were found. Laboratory tests on three common mill pests, the red and confused flour beetles (Tribolium castaneum and T. confusum) and the Turkish flat grain beetle (Cryptolestes turcicus ) showed that treatment times of 7, 2, 0.5 and 0.3 hours are necessary to achieve kill at 47, 49, 51 and 53^oC respectively.  However, these have to be the temperatures actually experienced by the insects and do not take account of the shielding effect of local food residues, or building design.  An insect death rate model has been incorporated into the thermal model of structures to facilitate the prediction of killing times. 

Using data gathered from temperature monitoring, a treatment assessment programme has been developed to permit real time process monitoring.  This has incorporated the model for insect death rate within the structure, and so allows a prediction of insect mortality to show the progress of the treatment at each location.

The target temperatures were achieved for adequate periods within most machinery and air temperatures throughout the mills were raised to 50^oC within 10-20 hours.  In areas such as basement wall/floor joints, outside walls and windowsills, target temperatures were not achieved demonstrating that additional treatments such as spraying and inert dust application will be required as an adjunct to the heat treatment.  A combination of insecticidal spraying and, where possible, diatomaceous earth formulation was applied to such locations prior to heating in both mill trials.  It was apparent that mills with roller mills situated in the basement will experience difficulty in achieving a kill temperature within and beneath the machines.
The cost of heat treatments is likely to exceed those currently incurred for fumigation with methyl bromide.