Microwave-Assisted Freeze-Drying with Frequency-Based Control Concepts via Solid-State Generators: A Simulative and Experimental Study

Abstract

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. Therefore, frequency-based control of MFD represents a promising approach for process control and intensification.

A Time-Saving Approach to Parameter Studies in Microwave-Assisted Freeze Drying

“…Frequency modulation was also utilized to increase energy efficiency, as indicated by temperature measurements [29,[31][32][33]. These findings support the results of Sickert et al [9], which revealed that frequency is an important process parameter in MFD.…”

“…Cuboids of tylose gel were used as a model product in this study. The samples were prepared as previously described [9]. Briefly, cuboids of 12.50 ± 0.05 g and approximately 25 mm × 25 mm × 20 mm in dimension were prepared in a composition of 76.23 wt% demineralized water, 23.77 wt% Tylose MH1000 (Kremer Pigmente, Aichstetten, Germany), and 1.00 wt% L-(+)-ascorbic acid (Carl Roth, Karlsruhe, Germany).…”

“…Freeze drying (FD) is an established method in process engineering, yielding products of a particularly high quality [1]. Recent work on FD covered microwave-assisted freeze drying (MFD) as an approach to process intensification [2][3][4][5][6][7][8][9][10]. In conventional freeze drying (CFD), energy is supplied through the mechanisms of thermal radiation, conduction, and convection [11].…”

“…In MFD, by contrast, electromagnetic energy dissipates into heat directly inside the product by interactions of the microwaves with the product as a dielectric [12]. During MFD, the absorption of microwave energy by interactions with bound water accounts for a major part of energy input, whereas frozen water absorbs energy to a minor extent only [9]. Microwave application offers a variety of advantages for drying, such as avoiding heat transfer resistances through the product surface [13], selective generation of heat in product areas containing large amounts of water [14], as well as short process durations and cost savings [15].…”

“…These dependencies of dielectric properties on drying progress and frequency pose a challenge for frequency control in MFD. In a previous study, however, Sickert et al [9] developed frequency-based control concepts in electromagnetic simulations and applied them successfully to MFD in experiments with an SSG. More homogeneous power dissipation and drying at a similar dissipated microwave power were observed for the application of multiple frequencies.…”