Disinfestation of grain using hot-air dryers: Killing hidden infestations of grain weevils without damaging germination

Abstract

This project aimed to provide recommendations on using hot-air dryers to disinfest grain as an alternative to OP admixture or fumigation. To do so, the most heat tolerant stages and species of insects were identified by oven heating of infested grain. A model of their mortality at elevated temperatures was integrated with existing models of germination loss, incorporated into a simulation of hot-air dryer operation and used to study the conditions needed for disinfestation. Based on information from these models, a practical-scale trial was undertaken.

Based on the heat mortality results, exposures of 15, 30, 45 and 60 minutes would be required to kill 99.9% of the most heat resistant stages of the grain weevil at grain temperatures of 52, 50, 48 and 46°C in malting barley at 12% moisture content.

A "window of opportunity" has been established within which disinfestation can be achieved without damage to grain, as judged by germination of malting barley. Grain temperatures 16-18°C above those listed above would be required to cause a fall of 1% in germination in the same exposure time. The temperature window is widest at low grain moisture and with barley of high initial germination. A practical test showed that the predictions of the model gave the desired result - disinfestation of the grain weevil without grain damage - except for a few locations where insects unexpectedly survived, indicating cool spots in the grain bed. Temperature differences in the dryer are large enough to make achieving a target temperature with a margin of error of + or - some 5oC, much more difficult.

Simulation of commonly used dryer types used in continuous flow has shown that, in principle, it is possible to achieve disinfestation of the grain weevil without grain damage in a dryer where the temperatures and airflows are constant and uniform. In a continuous-flow grain dryer, an air temperature of 80oC in combination with a particular residence time was predicted by a validated simulation model to kill 99.9% of S. granarius and to cause a reduction in germination of barley of less than 1%. For a given level of insect mortality, increasing the drying air temperature increased the grain throughput and reduced moisture loss and energy cost. Therefore the optimum treatment would be to use as high an air temperature as limits to germination loss allow. To get the temperature and transit time correct, the discharge rate and drying air temperature would have to be selected prior to the run, based on a guide for the type of dryer, the grain species and moisture content.

Energy costs at 80ºC were typically in the range 0.50 -1.00 £/t of input grain. Cost of lost weight were in the range 0.65£/t when starting from 11% moisture content to 3.26£/t when drying from 16%.

Selection of temperature and treatment time is not simple. Guidance for the appropriate combinations of inlet air temperature and temperature at the exhaust side will be needed for various designs of dryer and grain moisture levels.

The recommended air temperature and residence times would, therefore, enable disinfestation from most free-living species such as the saw-toothed grain beetle. Because feed wheat quality is much less critical and temperatures of 100-120°C for 3h and 1h respectively are permissible without quality loss, disinfestation of feed wheat from grain weevil is feasible without risk to feed grain quality.

Disinfestation of grain weevil in recirculating-batch dryers is expected to be more reliable than in continuous-flow. Further work is needed to find the best operating conditions for such dryers to achieve disinfestation.