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

Authors:

• Maria Dobies,
• S. Kuśmia,
• Stefan Jurga

Abstract

The ¹H 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 ¹H NMR relaxometry data has been used to sep arate the information on the static (β) and dynamic 〈τ_c〉 behaviour of the systems tested. The ¹H NMR results confirm that the average mobility of both water and the pectin molecules is largely dependent on the concentra tion of the cross-linking agent. The character of this dependency (β, 〈_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 CaCl ₂ ≥ 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 T₁ and T₂ 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.