Water mobility in heterogeneous systems as examined by (1)hydrogen, (2)hydrogen and (17)oxygen NMR

• Main Author: Vittadini, Elena
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1998
• Subjects: Food science|Microbiology
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9909231

The dynamic behavior of water molecules was analyzed in multicomponent systems by $\sp1$H, $\sp2$H and $\sp{17}$O Nuclear Magnetic Resonance. Systems chosen were culture media for microorganisms. For the case of a fast exchange, evaluation of results obtained from $\sp{17}$O NMR suggested that the "anisotropic, two correlation time model" (Halle and Wennestrom, 1981; Belton et al., 1991) might be an appropriate model. The correlation time for the slow relaxing water component ($\rm\tau\sp{s}\sb{bw}$) was higher in NaCl solutions than in BHI of same concentrations, suggesting a stronger interaction of the water with NaCl than with the BHI solids. P$\sb{\rm bw},$ or the population of bound water (fast and slow) increased with increasing solid content. Water molecular mobility was studied in solid and semisolid systems (gums, with and without mannitol; cellulose, with and without sorbose) by applying solid state $\sp1$H and $\sp2$H NMR. No observable glass transition as analyzed by DSC and/or DMA was observed in the polymers used. However, at the molecular level, NMR mobility was observed consistently. The molecular mobility, as monitored by T$\sb2$ relaxation time, and liquid (or mobile) signal intensity increased with increasing moisture content. The mobile fraction increased greatly from 10% mobile at approximately 3% water, to close to 100% mobile at $\sim$12-16% moisture (T$\sb2$ relaxation time from $\sp2$H NMR was 100-700 $\mu$sec) in the xanthan and locust bean gum mixtures. In the case of cellulose, solid state $\sp1$H and high resolution $\sp2$H NMR data at 2-16% moisture content (dry basis) indicated not only anisotropic reorientation but also a slow exchange within the NMR time frame. In the presence of water-soluble solutes, water mobility was primarily affected by the amount of dissolved solute present. This is due to the osmotic competition for water; more water in a liquid state solubilizing the solute was found to play a major role with a significant increase in T$\sb2$ relaxation time. In a diluted solution, NMR T$\sb2$ relaxation time was dependent slightly on solid concentration. The NMR water molecular mobility results were correlated with microbial survival and growth studies (done by other researchers), as compared with other parameters, such as water activity, water content and kinematic viscosity. In a fast exchange (liquid systems, BHI and NaCl), NMR mobility was less influential and a$\sb{\rm w}$ seemed to correlate well with bacterial growth (although a specific solute effect is expected). In a slow exchange (solid system, cellulose, or gums with added solute), mold germination and growth, as well as survival under extreme osmotic conditions, were found to be highly dependent on the molecular mobility of water. In such cases, a$\sb{\rm w}$ alone was a poor indicator. (Abstract shortened by UMI.)