Effect of ultrasonic on deterioration of oil in microwave vacuum frying and prediction of frying oil quality based on low field nuclear magnetic resonance (LF-NMR)

•The combination of ultrasound in microwave vacuum frying system was investigated.•The ultrasonic combined with microwave vacuum frying can delay the deterioration of the frying oil.•Low field nuclear magnetic resonance (LF-NMR) was used to detect the quality of oil during frying.•Good correlations were found between the LF-NMR relaxation characteristics and the physicochemical properties. The ultrasound assisted microwave vacuum frying system was investigated to achieve higher physicochemical and quality attributes of frying oil. In this study, the effect of ultrasonic on peroxide value, acid value, carbonyl value, polar component, fatty acid, color, viscosity, dielectric properties and low field nuclear magnetic resonance (LF-NMR) relaxation characteristics of frying oil under microwave vacuum frying were analyzed; the principal component regression (PCR) analysis was used to establish a correlation model for predicting physicochemical properties through LF-NMR relaxation characteristics. The results indicated that the acid value, carbonyl value, polar component, color, viscosity, dielectric constant and saturated fatty acids of the oil samples increased with the prolongation of frying time, and the peroxide value increased first and then decreased with the extension of frying time. Furthermore the relaxation time of peak T21, T22 and T23 decreased gradually of frying oil, and the peak area ratio of S21 gradually increased as the frying time goes on. Good correlations were found between the LF-NMR relaxation characteristics and the physicochemical properties using PCR, and the analysis results are credible. In conclusion, compared with the microwave vacuum frying, the ultrasonic combined with microwave vacuum frying can delay the deterioration of the frying oil and the physicochemical properties of frying oil could be well predicted by the LF-NMR T2 relaxation characteristics during the frying process.