Lethal effects and mechanism of infrared radiation on Sitophilus zeamais and Tribolium castaneum in rough rice

The objective of this study was to study the lethal effects of IR on Sitophilus zeamais and Tribolium castaneum in rough rice and its mechanism based on the differences of IR absorption of infrared radiation (IR) absorption characteristics between insects and rough rice. The eggs, larvae, pupae and adults insects (Sitophilus zeamais and Tribolium castaneum) and the No.5 Huaiyin japonica rough rice were used for this study. The IR absorption characteristics of adult Sitophilus zeamais, Tribolium castaneum, and the rough rice with moisture content of 21.1 ± 0.5% dry base were determined by ATR-FTIR spectra. For IR treatments, rough rice samples mixed with Sitophilus zeamais and Tribolium castaneum were heated by a laboratorial ceramic IR drying device. The tempering and non-tempering treatments were followed with IR heating for comparison study. The mortality of insects, moisture removal and milling quality of rough rice were further determined. To achieve the temperature distribution of the insects and rough rice after IR heating, insects were concentrated in a circle area surrounded by rough rice for determining with infrared thermal imager. Compared with rough rice, the adult Sitophilus zeamais and Tribolium castaneum achieved higher absorption of infrared radiation within the wavenumber of 1800 to 1308 cm−1. According to the Wien displacement law, the practical IR heating temperature beyond 300 °C could improve the heating pertinence for insects, and the corresponding IR intensity was beyond 2780 W/m2. The heated temperatures of adults were higher than that of rough rice based on the thermal images. The results of disinfestation experiments showed that the high insect mortality, heating rate and corresponding high moisture removal could be achieved by IR heating. Tempering process could improve the moisture removal and milling quality of rough rice. When rice samples were heated to 60.2 ± 0.5 °C under the IR intensity of 2780 W/m2 for 110 s, the Sitophilus zeamais and Tribolium castaneum were 100% killed. After tempering and natural cooling, good rice milling quality was well maintained and 3.97 percentage points of rice moisture was removed. Therefore, the simultaneous disinfestation and drying for rough rice can be achieved by IR heating followed by tempering and natural cooling.