Identification of Two Dielectric Relaxations in Oleic-Rich Oils Within the 50-900 MHz Range Using a Low-Cost Method

The dielectric spectrum of vegetable oils has been widely used to explore relationships with their physicochemical quality parameters. A Cole-Cole relaxation with a characteristic frequency around 200-300 MHz is commonly reported at 25 °C. More recently, the efficient heating of vegetable oils at 2450 MHz has been attributed to the relaxation of a glycerol component (10% by weight) in the triglycerides of vegetable oils. This study presents a method to identify more than one relaxation process using a NanoVNA directly connected to a 5-pin open-ended coaxial probe. The reflection coefficient S₁₁(f) was measured and transformed into the relative complex permittivity ε(f) using the Marsland transformation. The resulting spectra were analyzed with the Cole-Cole model and with the Maxwell model incorporating two Debye relaxations. The method was validated over the 50-900 MHz range using silicone oil, glycerol, and pure triglycerides. The static dielectric constant was obtained with high accuracy. For the first time, two relaxations were observed in oleic oils, including a low-frequency relaxation between 86 and 96 MHz, close to the 100 MHz relaxation of glycerol at 25 °C. Additionally, the low-cost equipment and the open-source software make the method accessible to laboratories with limited resources.