Limitations of microwave processing due to inhomogeneities of power input and energy absorption have been widely described. Over‐ and underheated product areas influence reproducibility, product quality, and possibly safety. Although a broad range of methods is available for temperature measurement and evaluation of time/temperature effects, none of them is sufficiently able to detect temperature differences and thermally induced effects within the product caused by inhomogeneous heating. The purpose of this review is to critically assess different methods of temperature measurement for their suitability for different microwave applications, namely metallic temperature sensors, thermal imaging, pyrometer measurement, fiber optic sensors, microwave radiometry, magnetic resonance imaging, liquid crystal thermography, thermal paper, and biological and chemical time‐temperature indicators. These methods are evaluated according to their advantages and limitations, method characteristics, and potential interference with the electric field. Special attention is given to spatial resolution, accuracy, handling, and purpose of measurement, that is, development work or online production control. Differences of methods and examples of practical application and failure in microwave‐assisted food processing are discussed with a special focus on microwave pasteurization and microwave‐assisted drying. Based on this assessment, it is suggested that infrared cameras for measuring temperature distribution at the product surface and partially inside the product in combination with a chemical time/temperature indicator (e.g., Maillard reaction, generating heat‐induced color variations, depending on local energy absorption) appear to be the most appropriate system for future practical application in microwave food process control, microwave system development, and product design. Reliable detection of inhomogeneous heating is a prerequisite to counteracte inhomogeneity by a targeted adjustment of process and product parameters in microwave applications.

Freeze-drying is a common process to extend the shelf life of food and bioactive substances. Its main drawback is the long drying time and associated high production costs. Microwaves can be applied to significantly shorten the process. This study investigates the effects of modulating the electromagnetic field in microwave-assisted freeze-drying (MFD). Control concepts based on microwave frequency are evaluated using electromagnetic simulations. The concepts are then applied to the first part of primary drying in a laboratory-scale system with solid-state generators. Targeted frequency modulation in the electromagnetic simulations enabled an increase in energy efficiency or heating homogeneity throughout MFD while having negligible effects on the power dissipation ratio between frozen and dried product areas. The simulations predicted the qualitative effects observed in the experimental proof of concept regarding energy efficiency and drying homogeneity. Additionally, shortened drying times were observed in the experiments with a targeted application of energy-efficient frequencies. However, differences occurred in the quantitative validation of the electromagnetic models for energy efficiency in dependence on frequency. Nevertheless, the models can be used for a time-efficient investigation of the qualitative effects of the control concepts. In summary, frequency-based control of MFD represents a promising approach for process control and intensification.

The pursuit of energy-efficient and uniform processing drives ongoing research in microwave-assisted freeze drying (MWFD). While microwave application is acknowledged for its potential to reduce energy consumption of freeze drying applications, it introduces new challenges to gentle processing, particularly in achieving uniformity of processing due to the inherent uneven microwave field distribution in the drying chamber. This study investigates the impact of maximum drying temperature (T_max) and microwave power input on energy consumption and uniformity in temperature-controlled MWFD. Experimental results reveal that shorter equilibration times associated with higher T_max significantly amplify the inhomogeneity of temperature distribution. Further, higher T_max was associated with a significant reduction in total energy demand of MWFD. Despite noticeable trends, microwave power input did not yield statistically significant differences in energy consumption or uniformity. The limited range of explored values, combined with the temperature-controlled nature of the process, may have rendered a potential influence of microwave power input negligible. This research elucidates the extent of inhomogeneity in MWFD, with implications for achieving uniform, gentle drying. It highlights the critical role of temperature control in MWFD. The study contributes to advancing the understanding of optimal processing in temperature-controlled MWFD.