1H NMR and rheological studies of the calcium induced gelation process in aqueous low methoxyl pectin solutions

The H-1 NMR relaxometry in combination with water proton spin-spin relaxation time measurements and rheometry have been applied to study the ionic gelation of 1% w/w aqueous low methoxyl pectin solution induced by divalent Ca2+ cations from a calcium chloride solution. The model-free approach to the analysis of H-1 NMR relaxometry data has been used to separate the information on the static (beta) and dynamic (<tau(c)>) behaviour of the systems tested. The H-1 NMR results confirm that the average mobility of both water and the pectin molecules is largely dependent on the concentration of the cross-linking agent. The character of this dependency (beta, <tau(c)> and T-2 vs. CaCl2 concentration) is consistent with the two-stage gelation process of low methoxyl pectin, in which the formation of strongly linked dimer associations (in the range of 0-2.5 mM CaCl2) is followed by the appearance of weak inter-dimer aggregations (for CaCl2 >= 3.5 mM). The presence of the weak gel structure for the sample with 3.5 mM CaCl2 has been confirmed by rheological measurements. Apart from that, the T-1 and T-2 relaxation times have been found to be highly sensitive to the syneresis phenomenon, which can be useful to monitor the low methoxyl pectin gel network stability.