Applications of NMR in Food Science and Technology

Pre-saturation In this technique prior to data acquisition, a highly selective low-power rf pulse irradiates the solvent signals for 0.5 to 2 s to saturate them. No irradiation should occur during the data acquisition. This method relies on the phenomenon that nuclei which have equal populations in the ground and excited states are unable to relax and do not contribute to the FID after pulse irradiation. This is an effective pulse sequence of NOESY-type pre-saturation that consists of three 900 pulses RD - 900 - tx - 900 - tm - 90° - FID, where RD is the relaxation delay and t and tm are the presaturation times. 

Soft-pulse multiple irradiation In this method, pre-saturation is done using shaped pulses having a broader excitation profile. Therefore, it is a more suitable method for the suppression of multiplets. This technique is very effective, easy to apply and easy to implement within most NMR experiments. In aqueous solutions, however, slowly exchanging protons would be detectable due to the occurrence of transfer of saturation. In addition, the spins with resonances close to the solvent frequency will also be saturated. 

However, all these suppression techniques have a common disadvantage in that the desirable signals of interest lying under the solvent signal are also suppressed. 

A detailed analysis of the proton high field NMR spectra of tomato juice and pulp has recently been acquired [15]. The combination of suitable selective and two-dimensional techniques (J-resolved, COSY, TOCSY, DOSY, etc.) was used for 

NMR alone is insufficient to enable the full assignment of the beer spectra to be made. Application of Principal Component Analysis (PCA) to the spectral profiles of beers of differing type (ales and lagers) showed some distinction on the basis of the aliphatic and sugar compositions, whereas the PCA of the aromatic profiles 

---