Determination of the solid contents of fats by wide-line nuclear magnetic resonance: The signal of liquid oils

Abstract

In order to ascertain the sources of error in the determination of the solid fat content by wide-line nuclear magnetic resonance (NMR) an investigation has been made of the factors governing the amplitude of NMR signals of liquid oils measured with a Newport Quantity Analyzer. The varying saturation properties of the oils concerned were found to be of major importance. The spin-lattice relaxation times (T₁) of 16 oils were measured on a pulse spectrometer at 20, 40 and 60 C, and quantitatively related to the saturation phenomenon. The observed deviation from the theoretical linearity of the NMR signal with the inverse of temperature is explained by the strong dependence of T₁ on temperature and viscosity, and new relationships are derived for these parameters. Since the improvement of the signal to noise ratio of the NMR signal, obtained by increasing the RF level (H₁), increases the saturation as (H₁)², a compromise must be chosen between these two factors to obtain optimal measurement. The results are used to establish criteria for selecting two reference oils to be used in solid fat content determinations.