1H and 23Na NMR Relaxation Studies of the NaCl/β‐Lactoglobulin System Equilibrated at Various Water Activities

Abstract

ABSTRACTNMR relaxation times (T1) of 1H and 23Na were determined at 8, 14, 60, MHz and at 14, 52 MHz, respectively, in the NaCl/β‐lactoglobulin system equilibrated at water activities ranging from 0.11 to 0.97. A two‐state model was used for both nuclei which allowed the calculation of proportions of bound and free species. It was shown that bound water and Na+ increased at medium water activities in the presence of increasing NaCl. The discrepancy with adsorption isotherms data was explained by (1) limitations of the mass balance equation used to calculate interacting salt by gravimetry and (2) the fact that NMR probed the totality of ions in the vicinity of the protein, whereas adsorption isotherms were indicative of sodium choride tightly bound to the protein. It was also shown that the protein and Na+ shared available water molecules at medium and low water activity.