Possibility of using low-field nuclear magnetic resonance (LF-NMR) and differential scanning calorimetry (DSC) to monitor frying oil degradation

Abstract

In this work, chemical degradation of oil was determined as a function of frying time during intermittent deep-fat frying (i.e. 185 °C for 5 h per day for 12 days) and was correlated with the thermal characteristics as measured by Differential Scanning Calorimetry (DSC) and proton mobility by Low-Field Nuclear Magnetic Resonance (LF-NMR). Linear increases of FFAs and PV were observed as a function of frying time. FFAs and PV were strongly correlated with the melting enthalpy. The melting enthalpy of oil decreased exponentially, and the melting temperature remained nearly same until 30 h of frying time followed by a sharp increase. The relaxation times of rigid, semi-rigid and mobile protons (i.e. T2b, T21, and T22) decreased with the increase of frying time. The decrease of T21, and T22 were steeper as compared to the T2b. A strong correlation was observed between conventional chemical parameters (i.e. PV and FFAs), and T21 and T22. The populations of the three pools of protons as identified by Inverse Laplace Transformation and Gaussian distribution function showed the interchange of the protons with the chemical changes during frying. It is concluded that chemical free DSC and LF-NMR could be potential methods to monitor frying oil degradation.