Characterization of water mobility in dry and wetted roasted coffee using low-field proton nuclear magnetic resonance

Roasted and ground coffee was studied by low-field H-1 nuclear magnetic resonance at various water contents and temperatures. The spin-spin relaxation times (T-2) were measured with single pulse free induction decay (FID) and Carr-Purcell-Meiboom-Gill (CPMG) sequences. Four relaxing components were distinguished: the solid population was observed with FID sequence at T(2)s similar to 9 ps; the other three populations, measured with the CPMG sequence, corresponded to an apolax phase, the coffee oil, and two polar phases. The two polar populations, observed at T-2m similar to 6 ins and similar to 27 ms (for coffee with 50% water content at 90 degrees C) were attributed to water in cell wall polymers and in water filling cells lumen. The T2 values appeared relatively insensitive to temperature, showing an Arrhenius type evolution with no break due to an important structure change. Furthermore, the intensity of the mobile phase increased as a function of time up to similar to 10 min after wetting, providing information about water absorption dynamics in roasted coffee. (c) 2007 Elsevier Ltd. All rights reserved.