Green Disinfestation Strategies for Different Stored Grains: Microwave, Infrared and Ultrasound Inactivation of the Red Flour Beetle (Tribolium castaneum Herbst)

Every year, a silent thief steals from the world's food supply. Long before a grain of wheat or a lentil reaches a kitchen, it is vulnerable to insects that live and breed inside stored harvests. Among the most persistent of these pests is the red flour beetle, a small insect that thrives in warehouses and silos, consuming grain and leaving behind waste that ruins the food's taste, smell, and safety. For decades, the standard defense against these pests has been chemical fumigation, a process that fills storage areas with toxic gases to kill the insects. However, this approach is becoming less viable. Many insect populations have developed resistance to these chemicals, and the residues left behind pose risks to human health and international trade. As the world seeks safer, cleaner ways to protect food, scientists are turning to physical forces—heat and sound—to do the job without poison.

In a recent study, researchers set out to test three specific physical methods against the red flour beetle across five different types of stored grains: wheat, bulgur, buckwheat, lentil, and chickpea. The team, based at Gaziantep University, wanted to see if they could kill the beetles using microwaves, infrared light, and ultrasound, while keeping the grain itself safe to eat. They did not just look at whether the insects died; they carefully measured how these treatments changed the grain's moisture, color, and temperature. The goal was to find a balance where the pest is eliminated, but the food remains high quality.

The first method tested was the microwave. This technology works by making water molecules inside the grain and the insect vibrate rapidly, generating heat from the inside out. The researchers exposed samples of grain containing beetles to microwave energy at different power levels, ranging from 360 to 900 watts, for short bursts of time between 10 and 40 seconds. The results were strikingly fast. At the highest power settings, the beetles died completely within 30 to 40 seconds. The heat generated was intense, raising the temperature of the grain by as much as 90 degrees Celsius in some cases. While this rapid heating was highly effective at killing the insects, it also caused the grain to lose some of its moisture. The researchers found that the longer the grain was exposed and the higher the power, the drier the grain became. Despite this drying effect, the visual appearance of the grain changed very little, suggesting that the food would still look acceptable to consumers.

The second approach involved infrared radiation, which heats the surface of objects much like the sun warms the skin. In this part of the study, the grain samples were placed under an infrared heater and exposed to temperatures of 50, 70, and 90 degrees Celsius for periods ranging from three to five minutes. Unlike the microwave, which heats the inside of the grain quickly, the infrared method warmed the surface more gradually. The beetles died at a rate that depended heavily on the temperature and the length of time they were exposed. At 90 degrees Celsius, the insects were completely eliminated after about four minutes. This method also dried out the grain, but the researchers observed that the color of the food remained remarkably stable. The grains did not turn brown or look burnt, even after the beetles were dead. This suggests that infrared heating could be a gentle way to disinfest grains that are sensitive to changes in appearance.

The third method was ultrasound, which uses high-frequency sound waves to create mechanical stress and tiny bubbles that can damage living cells. The researchers placed the grain and beetles in ziplock bags, which were then submerged in an ultrasonic bath at temperatures of 50, 70, and 90 degrees Celsius. This process took much longer than the other two methods, requiring exposure times between 10 and 30 minutes. At the lower temperature of 50 degrees, the ultrasound alone was not enough to kill the beetles, even after a long time. However, when the temperature was raised to 70 or 90 degrees, the combination of sound and heat became lethal, killing all the insects within 20 to 30 minutes. The advantage of this method was that it caused the least amount of change to the grain itself. The moisture content and color of the treated grains stayed very close to their original state, making it a promising option for preserving the quality of the food, provided the process can be made fast enough for large-scale use.

A key finding of the study was that not all grains reacted the same way to these treatments. The physical structure of the grain mattered. Smaller, lighter grains like buckwheat and lentil absorbed the energy from microwaves and infrared heaters more efficiently, leading to faster heating and quicker insect death. In contrast, denser grains like wheat and chickpea required longer exposure times to reach the same lethal temperatures. This difference highlights that a single setting cannot work for every type of food; the treatment must be tailored to the specific grain being protected.

The researchers also analyzed the relationship between the treatment conditions and the outcome. They found that for both microwaves and infrared heat, the temperature reached and the duration of exposure were the most critical factors in killing the beetles. There was a clear link between how hot the grain got and how many insects died. The study also confirmed that while these physical methods are effective, they do alter the grain slightly. Microwaves and infrared heat both reduced the moisture content, which could affect how the grain is stored or processed later. However, the changes in color were minimal across all methods, meaning the food would not look spoiled or unappealing.

Ultimately, the study demonstrates that it is possible to eliminate the red flour beetle without using chemical poisons. Each of the three methods tested offers a different set of trade-offs. Microwaves are the fastest, killing insects in seconds but risking rapid drying. Infrared heat provides a more even warming that preserves the look of the grain but requires a few minutes of exposure. Ultrasound is the gentlest on the grain's quality but takes the longest time and needs high temperatures to be effective. The researchers conclude that these technologies are viable alternatives to chemical fumigation, but their success depends on carefully adjusting the settings for each specific type of grain. By matching the right physical force to the right food, it may be possible to secure the world's grain supply against pests while keeping the food safe, clean, and free of chemical residues.