Acid-Induced Gelation of Low-Methoxyl Pectins Studied by 1H NMR and Rheological Methods

Abstract

Various 1H nuclear magnetic resonance (NMR) techniques (1H NMR dispersion [NMRD], water proton spin–spin relaxation time and diffusion measurements) in combination with rheological measurements were applied to the analysis of the acid-induced gelation of 3% (w/w) aqueous low-methoxyl (LM) pectin solutions at 279 K. A decrease of the pH value of solutions from 5 to 2.6 leads to a slowdown in the dynamics of water molecules and to a substantial modification in the structure of the system studied. The most significant changes in the 1H NMRD and T 2 measurements were observed when pH was varied from 5 to 3, which reflected an increase in the stiffness of the pectin chains caused by nonionic associations and by an increase of water molecules that were trapped between the pectin chains in the gelled state. The results obtained by the rheological method are consistent with those of 1H NMR, indicating a solution-like mechanical response for the sample at pH 5 and a gel-like response at pH 3. Results of 1H NMR measurements have also shown an important role of aggregation processes of LM pectin molecules in the acid-induced gel network formation.