Determination of water self-diffusion coefficient in complex food products by low field 1H PFG-NMR: Comparison between the standard spin-echo sequence and the T1-weighted spin-echo sequence

[Departement_IRSTEA]GEAPA [TR1_IRSTEA]42 - ALITECH / IRMA

International audience

Inglés. In 1990, Van Den Enden et al. proposed a method for the determination of water droplet size distributions in emulsions using a pulsed field gradient nuclear magnetic resonance (PFG-NMR) T1-weighted stimulated-echo technique. This paper describes both the T1-weighted spin-echo sequence, an improved method based on this earlier work, and, the standard PFG spin-echo sequence. These two methods were compared for water self-diffusion coefficient measurement in the fatty protein concentrate sample used as a 'cheese model`. The transversal and longitudinal relaxation parameters T1 and T2 were determined according to the temperature and investigated for each sample; fat-free protein concentrate sample, pure anhydrous milk fat and fatty protein concentrate sample. The water self-diffusion in fat-free protein concentrate samples followed a linear behavior. Consequently, the water self-diffusion coefficient could be easily characterized for fat-free protein concentrate samples. However, it seemed more complicated to obtain accurate water self-diffusion in fatty protein concentrate samples since the diffusion-attenuation data were fitted by a bi-exponential function. This paper demonstrates that the implementation of the T1-weighted spin-echo sequence, using the different T1 properties of water and fat phases, allows the accurate determination of water self-diffusion coefficient in a food product. To minimize the contribution of the 1H nuclei in the fat phase on the NMR echo signal, the fat protons were selectively eliminated by an additional 180° pulse. This new method reduces the standard errors of diffusion data obtained with a basic spin echo technique, by a factor of ten. The effectiveness of the use of the T1-weigted spin-echo sequence to perform accurate water self-diffusion coefficients measurement in fatty products is thus demonstrated.