Thermally reversible acid-induced gelation of low-methoxy pectin

Abstract

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°C showed a progressive increase in solid-like response (increasing G′; decreasing tan δ; increasing frequency-dependence of η ∗) as the pH was reduced from 4.0 to 1.6, with formation of a critically crosslinked network at ∼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 ∼3.5 and ∼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 ∼pH 2.5) and slow formation of intermolecular hydrogen bonds by protonated carboxyl groups (above ∼pH 3.5), respectively. The transition enthalpy from DSC heating scans has a maximum value of ΔH≈11 J/g at ∼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.