## 1 Introduction  
Dairy processing plants are susceptible to a variety of biofilm-forming microbes, posing a threat to product quality and safety. Common culprits include bacteria like Listeria monocytogenes, known for causing serious foodborne illness, as well as psychrotrophic bacteria that thrive in refrigerated environments and can contribute to spoilage [1]. Traditional methods for detecting biofilm in dairy processing plants rely on culturing techniques like plate counts and adenosine triphosphate (ATP) bioluminescence. However, these methods can be time consuming, require disposable equipment surfaces, and may not accurately reflect viable biofilm populations [2; 3]. Modern methods like Congo red agar (CRA) staining [4] offer a faster qualitative assessment of biofilm formation, but they still require sample collection and lack the sensitivity to detect early biofilm stages. While promising, the application of techniques like mass spectrometry for biofilm characterization in dairy settings still requires a lot more research [5]. Real-time non-invasive microwave sensing is a developing technology, which has been successfully used in various industrial applications including biofilm detection in the food processing industry [6], environmental monitoring of water pollutant concentrations and treated wastewater quality monitoring [7], water level measurements [8], measurement of the material moisture content [9], sensing the carbon emissions for continuous process monitoring of biogas plants [10], and in the healthcare industry. The healthcare applications include non-invasive real-time monitoring of glucose in diabetic patients [11; 12] and assessing the lipid concentration of biological cells as a means of detecting malignant neoplastic growth [13]. Moreover, microwave-assisted heavy oil production is also a developing area [13]. The principle of real-time monitoring using microwaves is based on the interaction of the electromagnetic waves with the material under test. The electromagnetic properties of the material, such as permittivity, can change the RF wave velocity, attenuation, or reflection.  
Microwave sensing could offer a revolutionary approach for biofilm detection in dairy plants. This rapid, non-invasive technique provides real-time monitoring, and thus it may enable early intervention before biofilms compromise product safety. By preventing contamination and costly shutdowns, microwave sensing could become a viable alternative for both food safety and production efficiency.  
Exploiting the transmitted (S21) and reflected (S11) microwave waves at discrete frequencies, biofilm sensors can leverage the sensitivity of conductive waveguide structures containing a material such as milk. Similarly, biofilm sensors can leverage the sensitivity of resonance frequencies in cavities as a result of changes in material permittivity. These cavity resonators exhibit a characteristic frequency heavily dependent on their geometric dimensions, conductivity, and the dielectric properties of the material within the cavity.  
This work proposes the use of RF waves for the real-time detection of bacterial biofilms in pipes and heat exchangers carrying milk. First, a brief background on the RF wave properties in waveguides and cavities is presented along with a discussion of the various proposed measurement (diagnostic) approaches. Then, the simulation and results are described in the context of a milk-filled cylindrical waveguide (pipe) and a rectangular cavity (pasteurizer).