Research progress in the microwave technologies for foodstuffs and agricultural products

Microwave heating is characterized by high speed and controllability, particularly with the strong potential for the thermal processing of food and agricultural products. This review was introduced on the current status of microwave heating applications, from the aspects of technical application, equipment development, and simulation research. The drying, sterilization, extraction and expansion were widely used the microwave heating. Mechanism analysis, process limitations, non-uniform heating and low energy utilization limited the microwave field, such as the polar molecules inside the material. It was lacking in the quantitative characterization of the uneven distribution of temperature and moisture inside the material during microwave heating, The stability of product quality was dominated by the full utilization of energy. The development trend of microwave heating was proposed for food and agricultural products. Microwave transmission and absorption were revealed within the heating chamber. The uniformity and quality of microwave heating greatly contributed to the intelligent industrial equipment of microwave processing. In the scientific, technical and equipment needs of microwave heating, the technological advantages were required to expand the application fields in foodstuffs and agricultural products. Microwave heating trends were proposed in the theoretical research and technological application. A strong reference was provided for the microwave technology in foodstuffs and agricultural products. The uneven microwave drying was attributed to the electromagnetic waves reflecting on the inner wall of the microwave cavity, thus forming a standing wave and an electric field that generated by the incident wave, and finally resulting in a different distribution of electric field intensity. When entering the material inside the microwave cavity, the intensity decayed exponentially, due to its dielectric properties. The state and physical properties of materials (including dielectric properties, thermal properties, size, shape, composition, and moisture content) depended mainly on the microwave volume heat generation and heat transfer, leading to the differences in temperature inside the materials. The combination of electric field distribution and material heat transfer capacity in the microwave cavity resulted in a significant distribution of cold and hot zones in microwave heating, leading to uneven heating and local burning. The interior of the material was covered with cold and hot spots during microwave heating. Cold spots led to insufficient microbial inactivation or bacterial growth in damp areas of the product, thus posing the potential microbial safety. Hot spots led to the deterioration of texture and quality, degradation of thermosensitive components, and a decrease in product quality. In response to the scientific issues, technical requirements, and equipment status of microwave heating in agricultural or food processing, some recommendations were proposed in the technological advantages and application fields. 1) Taking the dielectric and thermal properties of agricultural products or food as the mediators, the transmission, transformation, and distribution of microwave energy inside materials were analyzed to further elucidate the response mechanism on the polar molecules of target components under the microwave field. The theoretical basis was offered to expand the applications, such as targeted heating, non-thermal effects, and starch modification. 2) Effect of non-uniformity of microwave heating on food quality as the common issue, the demand level of microwave energy was investigated to meet the processing requirement in heating uniformity and food quality. The mathematical models of quality indicators were also established as the function of microwave heating parameters. The feedback control was proposed to achieve the control mode of microwave heating with an energy supply on demand. 3) The multiple fields were considered to integrate microwave heating with technology, such as electromagnetics, food chemistry, heat and mass transfer, and control theory. It is very necessary to explore the impact of microwave heating on food material quality. The equipment control strategies can be improved in the theory of food microwave thermal processing.