Water Mobility in Foods

This chapter focuses on the measurement and usefulness of the molecular mobility of water in food materials as probed by nuclear magnetic resonance (NMR) techniques. Measurements obtained using NMR techniques can be made for virtually any food or ingredient. However, depending on the nature of these measurements, the training needed by a person obtaining such measurements varies widely. Both NMR and water activity are used to measure the mobility of water in food systems, although NMR reflects molecular-level rotational and translational mobility and aw reflects macroscopic-level translational mobility. The pulsed-field gradient stimulated echo (PGSTE) pulse sequence is attenuated mainly by longitudinal relaxation, which is usually much slower than the transverse relaxation for systems containing water plus solute(s), such as foods. Thus, the PGSTE pulse sequence is useful for measuring diffusion in viscous phases and for determination of droplet size distribution and pore size distribution.