FE 315-INSTRUMENTAL ANALYSIS

PULSED NUCLEAR MAGNETIC RESONANCE (p-NMR)

SOLID FAT CONTENT

The crystallinity of a fat at given temperature and time is characterised with the solid fat content (SFC). Atomic nuclei are spinning, resulting in a magnetic momentum. Brought in a magnetic field, the nuclei show precession with the so-called Lamor frequency around the lines of magnetic flux. During a NMR measurement, an electromagnetic pulse and such energy is sent through the sample. The spins get deflected. The magnetic momentum of all atomic nuclei are summed, inducing a signal in the receiving inductor. As soon as the energy drops the spins will relax to their original orientation. Two different spin relaxation behaviours can be investigated, the so-called T1- and T2- relaxations. T1-relaxation gives the time needed for the spins to come back to their original orientation. The T2-relaxation, also called spin-spin-relaxation, gives the decay of the the accumulated signal. The T1- and T2- relaxations are dependent on different material and the state of aggregation. The determination of the SFC for fats is possible using two different methods: the direct and the indirect method. Only the direct method is described here: For determining the SFC for fats by direct method, the signal of the solid and the liquid phase is measured and compared to each other. A calibration of the NMR apparatus is needed. K-, F- and O-values are calculated, where the K-value describes the slope of the solid signal domain. The F-value is used for the extrapolation of the signal to the y-axis, as the signal is not counted until a certain dead time. As the F-factor is dependent on the crystal polymorph and on the temperature, a systematic error occurs using the same calibration. For the value of the solid phase the signal measured after 11 μs, and for the value of the liquid phase the signal measured after 70 μs, is used and averaged from several scans.

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