Assessment of the intermediate phase in milk fat by low-resolution nuclear magnetic resonance

The ratio of solid to liquid in fat, at a given temperature, is one of the most important physical quality control measurements used in the fat industry. The dilatometric method is tedious and time consuming. Differential scanning calorimetry (DSC) and low-resolution nuclear magnetic resonance (LR-NMR) techniques are widely used by control laboratories. However, DSC and LR-NMR do not give identical results below 20 °C, the difference being 41.8 points at 0 °C for milk fat between the NMR value (47.9% solid) and the DSC value (89.7%). A correction of DSC results obtained when taking into account a different melting enthalpy ΔH for the low and high melting point fraction only decreases the gap by 2.1 points. This difference in the SFI (solid fat index) value has been attributed to the presence of an amorphous phase seen as a solid by DSC and as a liquid by NMR. First, we justified the fitting of the FID signal of the liquid phase of an oil or of a milk fat at 20 °C and 5 °C by a bi-Gaussian function; this bi-Gaussian fitting can obviously be applied to any sort of sample containing a liquid and a solid phase, such as polymers. So, we have been able to show that at 5 °C, there is an intermediate phase between the liquid and the solid phases whose T2∗ is in the 50–250 μs range, but only for about 4% of the total signal. On the other hand, we have explained why the DSC method is not suitable for the measurement of SFI of complex mixtures such as milk fat. We propose new acquisition parameters to determine the SFI by LR-NMR.