Steady‐state and pulsed NMR studies of gelation in aqueous agarose

Abstract

SynopsisSteady-state and pulsed NMR techniques have been used to investigate molecular motion in sols and gels of agarose. In passing through the sol-gel transition, the molecular mobility of water molecules is reduced only by a small amount, whereas motion of the polymer chains is greatly attenuated. The results are discussed in terms of the network theory of gelation, with references to the role of water in the process and the nature of the "junction zones" between polymer chains. Tz and line-width measurements are dominated by exchange broadening. The effects of exchange rate and differences in relaxation time between the exchanging sites are discussed. The temperature hyst,eresis behavior of agarose gels ha8 been investigated and the effects of "ageing" correlated with changes in nuclear relaxation times. The synergistic increase in gel strength obtained on adding locust bean gum (LBG) to agarose has been investigated. The results indicate that LBG does not form double-helix junctions and may decrease rates of gelation by steric effects. At high agarose concentration, the LBG remains mainly in solution in interstitial water, but a t low agarose concentration, it is suggested that the LBG can link gel aggregates together into a self-supporting structure, producing a synergistic increase in gel strength. Comparisons have been made between the nature of the agarose-LBG interaction and agarose-cellulose interactions in biological systems.

Pulsed Low‐Resolution NMR Investigations of Protein Sols and Gels

“…This has already been observed for gelatin samples (Child and Pryce, 1972) and has been interpreted in terms of poor exchange capacities of the gelatin resulting in the observed relaxation rates being essentially governed by the motion of the bulk of the water molecules (Lelievre and Creamer, 1978). Only a slight difference was observed at 10.7 MHz between soybean isolates sols and gels (Mahdi, 1979).…”

NMR Spectroscopy of Red Seaweed Polysaccharides: Agars, Carrageenans, and Xylans

“…In accordance with these data the signal broadening in PMR spectrum of gel was explained by chemical exchange of protons between water molecules and hydroxy! groups of ordered agarose chains (Child et al 1970, Child and Pryce 1972, Ablett et al 1978, Indovina et al 1979.…”

Heat Stability of Proton Behaviors for Dietary Fiber in Water on Spin-Spin Relaxation Measured by 1H-NMR.

“…It is therefore rea sonable to consider that there are different pore-size dis tributions in agar before and after heat treatment. Although the T2 relaxation time is influenced by proton exchange processes and temperatures at the measuring points (8,9), it serves as a good parameter for gelation of agar polysaccharides (8)(9)(10)(11). Similar proton ex changes were considered to occur not only for agar in the water system but also for other dietary fiber sys tems, resulting in minor variations in proton relaxation behavior for sodium alginate-1, polydextrose, cellulose-3, alginic acid and so on.…”

“…The ton relaxation caused by heat treatment are explained to reflect the chain flexibility of agarose and its related polysaccharides (8,9). Lee et al, used non-linear trans verse magnetization decay to indicate the presence of several pore sizes in the system (7).…”