Feasibility and interest of microwave heating for the disinsectisation of stored cereals – GRANIonde

Axis 1: Demonstrate the feasibility of electromagnetic pest control in a real environment by integrating the microwave equipment from the manufacturer SAIREM into a representative grain silo, located on the ARVALIS experimental platform. The grain handling system was adapted to accommodate a microwave column and its generator to perform a continuous flow treatment of grain, transferred from one storage bin to another, as would be done at a storage facility.

Axis 2: Achieve satisfactory insect eradication results on the main pest species encountered during storage, the rice weevil. Several target temperatures and temperature rise rates were tested to find the most appropriate settings to achieve complete insect eradication while optimizing the grain flow rate and minimizing the impact on the quality of the heated grain.

Axis 3: Characterize the evolution of seed quality for various potential end uses depending on the applied treatment. The treatment must preserve the qualities of the treated grains for both human consumption (milling, baking), animal feed (protein digestibility), and finally, the germination vigor of seeds. Otherwise, the use of such a control method would be limited and reserved for certain end uses.

Axis 4: Evaluate the economic cost of such a treatment, the potential industrial constraints for its adoption, and the environmental benefits/impacts. The energy consumption resulting from this treatment was measured and compared to that of heating grains using a gas dryer. Additionally, an estimate of greenhouse gas emissions was made and compared between these two methods of heating treatment. Finally, the regulatory constraints associated with the installation and use of microwaves in silos, potential barriers to their adoption, were studied through the involvement of storage organizations, potential users of this control solution, and the expertise of an organization specializing in hazard studies related to the use of industrial equipment in silos.

At the midpoint of the project, the results obtained confirm the potential of microwaves to eliminate insects from grain, as total eradication of rice weevil populations at various developmental stages was observed by heating the grains to 70°C at a flow rate of 1.4 or 2.1 t/h.

The monitoring of wheat characteristics before and after heating showed that the quality was preserved for various human and animal food uses. Indeed, no significant variation was observed in the physical parameters studied (protein content, rheological characteristics of the dough), except for a slight decrease in specific weight (-0.4 to -0.6 kg/hl) and moisture content of the grain (-0.3 to -0.6%). No impact on baking performance could be demonstrated. The germination energy of the heated wheat, determined at 120 hours (as in malting contracts), was preserved, even at the highest tested temperature (80°C).

Growth experiments conducted on young chickens revealed a slight effect on the animals' weight gain (-1% on the feed conversion ratio and -17g on live weight at 21 days). In vitro digestibility analyses will complement these initial observations.

The heating of grains using the pilot system incurred a cost of 7 to 8€/t to reach the target temperature of 70°C in the wheat, at the studied flow rates (1.4 and 2.1 t/h). This represents a savings of 2 to 3 €/t compared to insect control using a dryer.

This first series of experiments validates the feasibility of the method on a pilot system representative of real silo installations. The hazard study conducted showed that certain features and adaptations already made to the industrial microwave machine help avoid the fire and explosion risks typical of silos (target heating temperatures, confined tube, gravity flow, safety sensors on the generator and application column, ATEX protection).

The results obtained on soft wheat allowed the identification of the optimal treatment characteristics to achieve satisfactory insect control while minimizing potential impacts on the quality of the seeds for their various intended uses. Complementary studies are currently being conducted on barley, a cereal used in the brewing industry, for which the preservation of germination energy after heating is crucial. The behavior of this cereal when exposed to microwaves may differ from that of soft wheat.

A study on the storage of grains after microwave heating without immediate cooling is also underway to provide storage operators with references on the maximum waiting time before ventilating the heated batches (for the logistics of these grain).

Microwave heating is a physical process with various advantages for the disinsectisation of stored cereals (common wheat, durum wheat, barley, oats, corn, sorghum, triticale...). This technique allows temperature rises in a few seconds and an almost instantaneous transfer of energy. In a context of reduction of the use of phytopharmaceutical products, microwaves constitute an alternative lever of fight to control infestations in silos. Storage companies are looking for residue-free curative control methods, especially against the rice weevil (Sitophilus oryzae), a species with hidden forms frequently detected in French silos. The heat sensitivity of the main grain insect pests is well documented: populations decrease from 35°C and mortality from 45°C. Microwaves would allow to reach the same efficiency as the classical heating in dryer, more quickly and with a lesser impact on the quality of heated seeds and on their water content. Although the effectiveness of microwaves has already been demonstrated in the literature and by laboratory trials conducted by SAIREM and ARVALIS, there are no results available on the integration of microwave treatment equipment in a real field context. The impact of the treatment on the heated seeds needs to be characterized according to the various outlets (brewery, bread-making and animal feed).

This project aims at integrating a microwave column developed by SAIREM, in an environment representative of that of storage organisations, on the ARVALIS storage platform. Disinsectisations on cereals will be carried out in order to acquire technical and economic data to demonstrate the feasibility and interest of this alternative control lever. The industrial constraints of the silo will be taken into account to facilitate the adoption of this equipment by the targeted users.

Electromagnetic disinsectisation concerns farmers, collecting organisations (cooperatives, traders) and industrialists (millers) to secure their stocks and avoid rebates, returns of goods or losses. This material investment will allow to secure their margin. Storers will be mobilized as experts, to meet their expectations regarding this innovative solution.

Microwave heating is an alternative lever to contact insecticides in storage, this sector being the first possible source of savings in phytosanitary products for common wheat. This curative method without residues offers a complementary solution to the prophylactic methods already implemented against insects (ventilation and cleaning of grains) and is part of an integrated itinerary with a targeted use on infested stocks according to on-site monitoring. This solution contributes to preserving the health of both consumers and applicators. It is also in line with the issue of reducing the use of fossil energy, since it uses electrical energy and requires shorter exposure times than heating in a dryer. The energy cost of operating the machine and its environmental impact will be evaluated during the project, but are lower than those associated with conventional heating (-3.5 €/t).

ARVALIS is a technical institute that has the expertise, the rigor and the human resources to conduct grain conservation trials on its pilot storage platform in Boigneville. SAIREM is a small business specializing in the manufacture of industrial processes equipment, which also provides expertise in microwave processes. These partners have already carried out laboratory tests, the results of which motivate the continuation of work on the disinsectisation of stored cereals.