Thermally reversible acid-induced gelation of low-methoxy pectin

Gelation of low-methoxy pectin (DE 31.1) on cooling under acidic conditions in the absence of Ca(2+) has been investigated by rheological measurements under low-amplitude oscillatory shear. The mechanical spectra obtained after 60 min at 5 degrees C showed a progressive increase in solid-like response (increasing G'; decreasing tan delta; increasing frequency-dependence of eta*) as the pH was reduced from 4.0 to 1.6, with formation of a critically crosslinked network at similar to pH 3.0 (for a polymer concentration of 3.0 wt%). By extrapolation from X-ray fibre diffraction analysis of pectic acid, it is suggested that crosslinking occurs by association of three-fold helices. At pH values between similar to 3.5 and similar to 2.5 there is no detectable thermal hysteresis between the sol-gel transition on cooling and gel-sol transition on heating, and both are accompanied by a sigmoidal change in optical rotation (attributed to formation and melting of three-fold order). Substantial hysteresis is, however, observed at lower and higher pH, and is attributed to extensive aggregation as electrostatic repulsion is suppressed (below similar to pH 2.5) and slow formation of intermolecular hydrogen bonds by protonated carboxyl groups (above similar to pH 3.5), respectively. The transition enthalpy from DSC heating scans has a maximum value of Delta H approximate to 11 J/g at similar to pH 3.0, but decreases sharply at lower and higher pH, with accompanying loss of a detectable transition in optical rotation. It is suggested that the chain conformation in solution at low pH is predominantly three-fold with, therefore, little conformational change on adoption of the ordered, intermolecular structure, whereas at high pH the solution conformation is predominantly two-fold, with only limited conversion to the three-fold (acid) form on cooling. (C) 2000 Elsevier Science Ltd. All rights reserved.