Using magnetic levitation for density-based detection of cooking oils

Abstract

Adulterated cooking oils and the repeated use of frying cooking oils are harmful to human health. Current techniques for the quality control of cooking oils such as gas chromatography and high performance liquid chromatography usually require expensive facilities, and they are complicated to operate. This paper describes a simple technique that uses magnetic levitation (MagLev) to analyse cooking oils based on density. We have demonstrated the application of MagLev to detect the quality of cooking oil by simulating adulterated sesame oil using peanut oil and frying soybean oil for different times. We have also demonstrated the use of MagLev to differentiate secondhand cooking oil from certified cooking oil. MagLev provides a portable and inexpensive method for the on-site inspection of cooking oils, and it may be extended to many other applications in food safety, environmental monitoring, medical diagnosis, and so on. The advantages of high sensitivity, low cost, and convenience of operation make MagLev especially useful for in situ applications in resource-limited settings.

Fig. 1. Schematic representation of the experimental setup of magnetic levitation (MagLev). A container filled with MnCl₂ solution was placed between two indistinguishable NdFeB magnets (50 mm × 50 mm × 25 mm), aligned vertically, 45 mm apart, with like poles facing each other. The droplet of cooking oil was levitated in the MagLev device due to the balance of the gravitational force, and the magnetic force,

Fig. 2. Detection of mixed cooking oil using MagLev. (a) Photographs of the droplets of mixed oil of sesame oil and peanut oil with different proportions of sesame oil of 0%, 10%, 30%, 50%, 70%, 90%, and 100% levitated in 0.2 M MnCl₂ solution in the MagLev device. (b) Plots correlating the levitation height of the droplets of the mixed oil with the proportion of sesame oil in 0.2 M and 0.4 M MnCl₂ solution. Each data point is the mean of five analyses.

Fig. 3. Detection of heated purchased soybean oil using MagLev. (a) Photographs of the droplets of purchased soybean oil that was heated for 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, and 7 h levitated in the MagLev device. (b) Plot correlating the levitation height of the droplets of purchased soybean oil with the heating time.

Fig. 4. Detection of heated pure soybean oil using MagLev. (a) Photographs of the droplets of pure soybean oil that was heated for 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, and 7 h levitated in the MagLev device. (b) Plot correlating the levitation height of the droplets of pure soybean oil with the heating time. Each data point is the mean of five analyses.

Fig. 5. Bland–Altman plot for assessing the agreement between MagLev and the traditional method (mass divided by volume). Solid black line: mean difference; dashed black line: limits of agreement (from −1.96 SD to +1.96 SD).

Fig. 6. Detection of secondhand cooking oil using MagLev. Photographs of the droplets of (a) certified soybean oil and (b) secondhand cooking oil levitated in the MagLev device.