Experimental study of the effect of dielectric and physical properties on temperature distribution in fluids during continuous flow microwave heating

A Review and Perspective of Environmental Disinfection Technology Based on Microwave Irradiation

PURPOSE OF REVIEW

In the context of COVID-19 sweeping the world, the development of microbial disinfection methods in gas, liquid, and solid media has received widespread attention from researchers. As a disinfection technology that can adapt to different environmental media, microwave-assisted disinfection has the advantages of strong permeability, no secondary pollution, etc. The purpose of this review is to put forward new development requirements for future microwave disinfection strategies by summarizing current microwave disinfection methods and effects. From the perspective of the interaction mechanism of microwave and microorganisms, this review provides a development direction for more accurate and microscopic disinfection mechanism research.

RECENT FINDINGS

Compared to other traditional environmental disinfection techniques, microwave-assisted disinfection means have the advantages of being more destructive, free of secondary contamination, and thorough. Currently, researchers generally agree that the efficiency of microwave disinfection is the result of a combination of thermal and non-thermal effects. However, the performance of microwave disinfection shows the differences in the face of different environmental media as well as different types of microorganisms.

SUMMARY

This review highlights the inactivation mechanism of microwave-assisted disinfection techniques used in different scenarios. Suggestions for promoting the efficiency and overcoming the limitations of low energy utilization, complex reactor design, and inaccurate monitoring methods are proposed.

Continuous flow microwave heating and sterilization for liquid food

Continuous flow microwave sterilization for liquid food has advantages of a short time and high retention rate of nutrients. However, uneven microwave heating is the critical factor restricting the industrialization of microwave sterilization. This paper reviews the up-to-date research on the continuous flow microwave heating and the continuous flow microwave sterilization system for liquid food. The causes of the non-uniformity of continuous flow microwave heating are thoroughly discussed and the methods of improving the uniformity are proposed. Finally, the recommendations for future research of continuous flow microwave sterilization for liquid food are presented.

Research progress in fluid and semifluid microwave heating technology in food processing

Microwave is a form of electromagnetic radiation that has high penetration and heating efficiency in food processing. Uneven heating is the main problem of microwave processing, especially in solid foods. Fluid and semifluid media, which are good carriers in microwave processing, have uniform dielectric properties and good material fluidity. Herein, we review the development, application prospects, and limitations of microwave in fluid and semifluid food processing and the research progress in microwave heating with steam as carrier. The mixture of generated steam and tiny micro droplets from food material under the action of microwave can absorb microwave and transfer heat evenly, which effectively improves the uniformity of microwave heating. Due to the relatively uniform dielectric properties and consistent texture of fluid and semifluid food materials, uneven heating phenomenon during their microwave processing can be significantly inhibited. Based on the development of microwave heating technology and equipment design, the microbial inactivation and enzyme inhibition in fluid and semifluid food were improved and food product with better retention of nutrients and sensory profile were produced. Also, microwave radiation can be used to prepare the printing material or process the printed product for 3D food printing, which enhances the added value of 3D printed products and the personalization of food manufacturing. In future research, intelligent control technology can be applied in the microwave processing of fluid and semifluid food materials for various applications. Therefore, the processing conditions can be adjusted automatically.

Effects of shape and size of agar gels on heating uniformity during pulsed microwave treatment

Model gel systems with different shape (sphere, cylinder, and slab) and size (180 and 290 g) were prepared with agar (5%) and sucrose (5%). Dielectric constant (ε'), loss factor (ε"), thermophysical properties, and temperature distribution of the model system were measured. Each agar model system was immersed and suspended in water, and then, heated in a microwave oven with intermittent heating until the core temperature reached 50 °C. The ε' and ε" of agar gels decreased when frequency increased. The density and thermal conductivity values of the agar gels were 1033 kg/m(3) and 0.55 W/m °C, respectively. The temperature distribution of sphere, cylinder, and slab was different when similar power doses were applied. The slab reached 50 °C in less time (10 min) and showed a more uniform heating than spheres and cylinders in both sizes. Agar model systems of 180 g heated faster than those of 290 g. The coldest point was the center of the model systems in all studied cases. Shape and size are critical food factors that affect the heating uniformity during microwave heating processes.

Microwave-enhanced persulfate oxidation to treat mature landfill leachate

Microwave oxidation process (MOP) was evaluated for treatment of landfill leachate. Kinetics of persulfate oxidation in MOP, effects of pH and persulfate doses on fates of derivative organic acids, and the energy cost of MOP were evaluated. The results showed that total organic carbon (TOC) removal of 79.4%, color removal of 88.4%, and UV254 removal of 77.1% were reached at MOP 550 W/85 °C within 30 min. The kinetics of oxidation by MOP followed the first-order reaction. For a given persulfate dose, the reaction rate increased with the microwave power setting (775 W>550 W>325 W>128 W) with reaction rate constants ranging from 10(-5) to 10(-2) min(-1). The adverse effects on reaction rates under higher microwave power settings and high persulfate doses are plausibly caused by excessive persulfate oxidation and self-scavenging termination of free radicals. During the MOP treatment, TOC/COD ratio dropped with time and an 86.7% reduction in TOC/COD ratio after 120 min at pH 7. Oxalic acid was the major derivative and its concentrations were higher under acidic conditions. Malic, lactic, and acetic acids were formed and soon degraded, and the solution pH has an insignificant effect on their fates. The energy cost of MOP (USD$6.03/m(3)) is essentially similar to that of conventional heating oxidation (CHO) (USD$6.10/m(3)).

A study on microwave oxidation of landfill leachate--contributions of microwave-specific effects

Microwave oxidation process (MOP) was evaluated for treatment of landfill leachate. The experimental parameters include pH, temperature, oxidant doses, microwave power setting, and irradiation time. The study explored the microwave-specific effects of the MOP. The contributions of pure thermal, persulfate oxidation and microwave irradiation on TOC removal were quantified. It was then found the combinations of them were usually synergistic in MOP except two of them were antagonistic (128 W/85°C/1M Na(2)S(2)O(8) and 128 W/85°C/2M Na(2)S(2)O(8)). At the highest temperature tested (85°C) in this study, microwave irradiation may cause generation and termination of oxidizing radicals at adverse rates. The study also found that persulfate decayed rapidly in either MOP or conventional heating oxidation (CHO) treatment of landfill leachate.

Microwave Food Processing

Consumers prefer food products with enlarged shelf life, which are quick to prepare, healthy and fresh like. Traditional food processing methods are based on intensive heating and as a result cause quality loss. Minimal processing techniques such as microwaves approach consumer's demands to create fresh like products with enlarged shelf life. Microwave technology can be very useful for food processing, because products are heated directly instead of conventional heating by convection and conduction. This means a reduction of the total processing time, no overheating on the outside of the product, and preservation of the fresh product quality. This chapter presents a complete picture of current knowledge on application of microwave in food processing which has been used for different processes like blanching, sterilisation, thawing, drying and extraction of various products. In many cases combinations with microwaves gave the best results.