Contactless monitoring of food drying and freezing processes with millimeter waves

Millimeter (mm) waves are of interests to monitor the properties of interest (POIs) of food products which are opaque for optical and infrared frequencies as they not suffer from all the atomic and intermolecular absorption processes situated in the latter bands of the Electromagnetic (EM) spectrum. They also offer a higher penetration depth into water containing products than the IR domain. With respect to the microwave domain, they are less perturbed by ionic contributions of food products, hydration shell effects and finally they offer a higher spatial resolution compared to microwaves. In the considered mm-wave frequency range, the dipole moment of free water molecules plays a dominant role compared to the other constituents, hence the electromagnetic interaction depends mainly on the amount and phase state of free water present in food products. These features make mm-wave based sensors an appealing alternative for monitoring and quality control of food processes. However, the effectiveness of mm-wave sensing solutions has not yet been explored in the food industry. In this paper, we present a real time, contactless and non-destructive sensor methodology based on mm-waves to monitor food processes. An analytical model is developed to estimate the dielectric properties of food products with varying POIs and to establish the relation between the mm-wave sensor's output and the POIs. A drying and freezing process of potato slabs are monitored by measuring the transmission and reflection coefficients by means of mm-wave sensors. The theoretical and experimental results are compared and validated. It will be proven that mm-wave based sensors are a promising real time, contactless and non-destructive monitoring tool for food quality control.