Mortality of Rhyzopertha dominica (F.) (Coleoptera: Bostrychidae) at grain temperatures ranging from 50°C to 60°C obtained at different rates of heating in a spouted bed

Abstract

Heat disinfestation of grain has the potential for high market acceptance. However, the technology would be more affordable if less energy was used during the process. The paper explores two interrelated ways in which this could be achieved. The first is to decrease typical grain treatment temperatures, but hold those temperatures without the addition of subsequent energy for a time sufficient for disinfestation. The second is to increase the rate of heating to induce physiological heat shock and thereby bring about faster insect mortality. Using a spouted bed, rates of mortality of the heat-tolerant species Rhyzopertha dominica were determined over a range of grain temperatures from 50°C to 60°C at 12% m.c. The grain temperatures were gained at four rates of heating obtained by using different air inlet temperatures from 80°C to 200°C. From the data, a probit model was developed to predict LT values for five arbitrary immature development stages. At LT 99.9 and above, immature stage 2 was generally found to be the most heat-tolerant stage, while stage 5 was generally the most susceptible. When the initial rate of heating was increased by increasing air inlet temperature from 80°C to 100°C, the time required for a given level of mortality was significantly decreased. While subsequent rate increases had limited further impact on increasing rates of mortality, there was an increase in the efficiency of heating. Furthermore, decreasing the target grain temperature and increasing the treatment period accounted for additional cost savings. For example, at the most rapid rate of heating, grain that reached 60°C required 0.73 min of heat soak for 99.9% mortality and cost a theoretical $2.72/t, while grain that reached 55°C required 23.62 min for the same level of mortality and cost $1.87/t. By 50°C, some 22 h were required, but the theoretical running cost was reduced to $1.25/t. Thus, within practical boundaries, increasing the air inlet temperature or rate of heating, and decreasing the grain target temperature, increases the efficiency of the system and reduces operation costs. With the system used here, obtaining grain at 50°C using air at 200°C was 75% efficient and 80% cheaper than obtaining grain at 60°C using air at 80°C, which was only 18% efficient. An assessment of possible impact on grain quality due to the various heat treatments was also carried out, both initially and after 6 months storage. Using several criteria of germination measurement as a quality indicator, no adverse effects were observed.