A 23Na NMR study on the environment and mobility of sodium ions in complex food matrices and model systems

A high sodium diet increases the risk of mortality and high blood pressure morbidity. With the aim of reducing the sodium content, different food systems (liquid, semi-solid and solid) have been studied. Low-field NMR spectrometry (23 MHz) was only used for liquid systems (reported in a previous dissertation) due to the limitations of the low field and long deadtime. A Bruker 600 MHz NMR spectrometer with solution and solid probes has been used to determine the mobility and form of sodium in the other systems. Dowex resin results were different from those reported in the literature suggesting that a fraction of the sodium was behaving anisotropically. In the production of cheddar cheese, a certain time is required for the added salt to dissolve and be assimilated into the food matrix. This was detected in the experiments reported here. The sodium mobility behaviour also suggests that pools of free water are produced in cheeses matured for periods of months in which a fraction of the sodium is located. There is also a missing fraction of the sodium believed to be located in the solid matrix. In order to quantify the amount of ‘bound’ sodium in Dowex, the decay times T2s and T2f have been measured by a modified program (DQ-QE).The quantification procedure is still under development. Sodium in crystalline salts with different space groups present different signal line shapes. Higher rates of MAS minimise the quadrupolar interaction and provide information on the state of the sodium in the crystal. Crystalline and “amorphous” sodium in biscuits had sharp lines with little in the way of sideband manifolds indicating a symmetrical environment. Differences in particle size had no significant effect on the NMR spectrum. The decreasing water content resulted in the recrystallization of sodium salts in the 2%sodium chloride plain and shortbread biscuits. A large difference in T1values showed that crystalline sodium nuclei had much lower mobility than in a non-crystalline form in biscuits. MQMAS enabled the separation of spectra on the y-axis with theconventional spectrum on the x-axis. Accurate quantification of sodium proved difficult due to a complex combination of effects