High-sensitivity microwave sensor for buffalo milk fat rate and adulteration analysis with branch line coupler

In this study, an innovative microwave sensor was designed based on a Complementary Elliptical Electric-LC (CEE-LC) resonator structure compact with a Branch Line Coupler (BLC) that can determine milk fat content and perform adulteration analysis. Firstly, the fat content measurements of the milk used in the study were performed using a Lactoscan brand milk analyzer to determine the proposed sensor performance: buffalo milk (BM) was determined to have 12.19 % fat content, goat milk (GM) 5.02 %, Jersey milk (JM) 6.81 %, and Simmental milk (SM) 4.10 %. Then, adulterated milk samples were created using JM-BM and water-BM mixtures to perform adulteration analysis using the proposed sensor. In the final stage of sample preparation, a common dielectric probe was used to measure the permittivity and electrical conductivity of all milk samples in the 10 MHz to 20 GHz frequency range. The resonance frequency of the S₂₁, which provides transmission-zero (TZ) in the S-parameter of the proposed sensor, was determined to be 7.03 GHz. The CEE-LC resonator structure in the sensor was positioned on the ground surface projection of the transmission line between “port 1” and “port 2”. In the BM-water adulteration analysis, the resonance frequencies of the sensor at parameter S₂₁ were measured as 2.40 GHz for only BM sample, 2.335 GHz for BM with 20 % water mixture, 2.295 GHz for BM with 40 % water, 2.260 GHz for BM with 60 % water, 2.215 GHz for BM with 80 % water, and 2.105 GHz for only water. Similarly, the proposed sensor detected the resonance frequencies of the S₂₁ parameter in the adulteration analysis performed when JM mixed with the BM as 2.395 GHz for 20 % addition, 2.385 GHz for 40 % addition, 2.370 GHz for 60 % addition, 2.325 GHz for 80 % addition, and 2.32 GHz for pure JM. To increase the usability of the sensor in field applications, a portable measurement system was developed by integrating a voltage-controlled RF oscillator and a power detector. In DC voltage-based measurements, the output voltage increased from 1.23 V to 1.56 V as the milk fat content increased, indicating a correlation between the DC output voltage and fat content. In the adulteration analysis, it decreased from 1.55 V to 1.18 V as the water content in buffalo milk increased. During DC analyses, the 2.4 GHz resonant frequency of the BM, which has the TZ value for parameter S₂₁ in the proposed sensor, was used to determine fat content and adulteration. Literature comparisons revealed that the proposed system exhibits superior performance compared to existing methods, especially in detecting water adulteration in buffalo milk.