Dielectric Measurements for Monitoring the Storage of Vegetable Oils

During the storage of vegetable oils, various chemical and physicochemical changes occur that can significantly affect oil quality and safety. These include autoxidation, the breakdown of triglycerides, and increased acidity. Several indicators and methods are known for monitoring these changes, such as measurements of viscosity and colour, and the determination of peroxide value and free fatty acid (FFA) concentration. Depending on the applied measuring frequency, dielectric parameters are considered sensitive to chemical, physical, and structural changes. Changes in the concentration of polar compounds (e.g. FFAs, aldehydes, peroxides, and water) also influence dielectric behavior. Dielectric measurements are non-destructive methods suitable for real-time detection, and therefore have great potential for application in vegetable oil quality monitoring.

In our research, the 16-month storage of olive, palm, sunflower, rapeseed, and pumpkin seed oils was investigated through the determination of peroxide value (using a Hanna HI83730 PV photometer), dielectric constant, and loss factor (using a Speag DAK 3.5 open-ended coaxial sensor in the 200–2400 MHz frequency range).

Our results show that the processing technology influences the dielectric behavior of oils, with certain processes (e.g. cold pressing and degumming) resulting in significant differences. Over the 16-month storage period, both the dielectric constant and the loss factor increased with time. The variation in dielectric parameters was most consistent in the frequency range of 200–700 MHz, while differences became non-significant at measuring frequencies above 2200 MHz. Based on our findings, a strong linear correlation (R > 0.9) was observed between the peroxide value and the dielectric constant in the 300–500 MHz range. In summary, the changes occurring in edible oils during storage can be effectively monitored by measuring dielectric parameters, if the measurement frequency is appropriately selected.