Time-Domain NMR in Quality Control: More Advanced Methods

The classic application of time-domain (TD-)NMR in food science and industry are devoted to products containing either a dominating amount of water or fat with the aim to determine the concentration or the amount of a specific component. The analysis result is commonly the fat content, in some examples also the moisture content. This is due to the fact that in TD-NMR almost no spectral resolution is available and that the transverse and longitudinal NMR relaxation properties of fat and water molecules are not substantially different. Moreover, the selectivity in commonly known TD-NMR pulse sequences is too small to guarantee a clear separation of signals. The classical approach was to pre-dry the samples either for example by an oven, chemical reagents like CuSO₄ or via infrared or microwave drying processes. The disadvantage of the pre-drying is clearly that it is a time-consuming method requiring labor time and rendering the TD-NMR application a two-step approach. Therefore, more elaborate experiments have to be examined in order to address these application questions on food products. Due to the technical development of the TD-NMR spectrometer, especially of the electronic part in recent years, also more sophisticated experiments can be implemented and run for quality control in food science and industry. In this paper the physical background and selected examples of new applications will be shown.